Enano newsletter issue20-21
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The document is a special double issue of the e-nano newsletter focusing on the advancements in carbon nanotube (CNT) research, covering the state-of-the-art findings and potential applications in nanoelectronics, photonics, and energy. It details the various growth techniques for CNTs, such as chemical vapor deposition (CVD) and emphasizes the role of catalysts in the growth process. Additionally, the issue includes a comprehensive catalogue of nanoscience and nanotechnology companies in Spain, highlighting the importance of market research and development in this field.
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Carbon Nanotubes transistors, thin-film electrodes, network transistors, single
(Position Paper version 2) CNT transistors, thermal management, memory.
M.Mann Cambridge University, UK Optical applications
W.I.Milne Cambridge University, UK Absorbers, microlenses in LCs, optical antennae, lighting.
S Hofmann Cambridge University, UK
P.Boggild DTU Technical University of Denmark Electromechanical applications
J.McLaughlin University of Ulster NEMS (resonators), sensors, nanofluidics, bio-medical.
J.Robertson Cambridge University
G Pagona National Hellenic Research Foundation, Greece Energy applications
G.A.D. Briggs Oxford University Fuel cells, supercapacitors, batteries, solar cells.
P Hiralal Cambridge University, UK
M.de Souza Sheffield University Blue sky
K.B.K.Teo AIXTRON Spintronics, quantum computing, SET, ballistic transport.
K.Bo Mogensen TU Denmark
J.-C. P. Gabriel CEA, Nanoscience Program, Grenoble
(Formerly, Nanomix Inc. USA)
Y Zhang Cambridge University, UK
M Chhowalla Imperial College, London, UK
Z Durrani Imperial College, London, UK
T Wilkinson Cambridge University, UK
D Chu Cambridge University
S.Roche CEA-INAC, Grenoble
Robert Baptist CEA-LETI, France
P.Bachmann Philips Research Laboratories, Aachen
J.Dijon CEA, Grenoble
A.Lewalter Philips Research Laboratories, Aachen
Key Words
Fig 1. (top) A graphene sheet rolled up to obtain a single-walled CNT.
Growth (bottom) The map shows the different single-walled CNT configurations
possible. Were the graphene sheet to roll up in such a way that the atom
Carbon nanotubes, multiwall, singlewall, nanofibres (all at (0,0) would also be the atom at (6,6), then the CNT would be
the words in a and the subtopics), cap structure, metallic. Likewise, if the CNT rolled up so that the atom at (0,0) was also
catalysts, adhesion, mechanism, modelling. the atom at (6,5), the CNT would be semi-conducting. The small circles
denote semiconducting CNTs, the large circles denote quasi-metallic
CNTs, the squares denote metallic CNTs.
Post-growth Modification
Doping, & functionalization, dispersion and separation,
purification, annealing, cap opening/closing, 1. Introduction
graphitization.
There has been extensive research into the properties,
Properties/characterization synthesis and possible applications of carbon nanotubes
Defects, electron transport, phonons, thermal (CNTs) since they came to prominence following the
properties/conductivity, wetting, stiction, friction, Iijima paper [1] of 1991. Carbon nanotubes are
mechanical, chemical properties, optical, toxicity, composed of sp2 covalently-bonded carbon in which
structural properties, contacts. graphene walls are rolled up cylindrically to form tubes.
The ends can either be bonded to a secondary surface,
Electronic Applications not necessarily made of carbon, they can be capped by
Field emission (X-ray, Microwave, FEDs, Ionization, a hemisphere of sp2 carbon, with a fullerene-like
Electron microscopy), interconnects, vias, diodes, thin-film structure [2], or the CNT can be open with the ends
passivated (by hydrogen). In terms of electrical
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properties, single-walled CNTs can be either semiconducting carbon. The CVD method is by far the most widely
or metallic and this depends upon the way in which they roll employed method at present, both for bulk growth (for
up, as illustrated in Fig. 1 (page 5). use in composites for example) or for growth onto
surfaces (for use in electronics). This is despite the fact
Multi-walled CNTs are non-semiconducting (i.e. semi-metallic that the CNTs produced by this method are not the
like graphite) in nature. Their diameters range from 2 to best; laser ablation remains the best method for
500 nm, and their lengths range from 50 nm to a few producing reference SWNT samples of high structural
mm. Multi-walled CNTs contain several concentric, and electrical quality, but CNTs produced by this
coaxial graphene cylinders with interlayer spacings of method are often coated by a large amount of
~0.34 nm [3]. This is slightly larger than the single crystal amorphous carbon.
graphite spacing which is 0.335 nm. Studies have shown
that the inter-shell spacing can actually range from 0.34 Common to all growth methods and of key importance
to 0.39 nm, where the inter-shell spacing decreases with to CNT growth is the use of a nano-particle catalyst. The
increasing CNT diameter with a pronounced effect in understanding of the role of the catalyst and a detailed
smaller diameter CNTs (such as those smaller than 1 nm)
5 CNT growth mechanism is still incomplete. This hinders
as a result of the high curvature in the graphene sheet the refinement of current growth techniques, in particular
[4],[5]. As each cylinder has a different radius, it is with regard to growth selectivity and efficiency.
impossible to line the carbon atoms up within the sheets
as they do in crystalline graphite. Therefore, multi-walled Here, the progress in catalytic CVD of CNTs is
CNTs tend to exhibit properties of turbostratic graphite reviewed, which is widely used because it offers high
in which the layers are uncorrelated. For instance, in production yield and an ease of scale-up for both bulk
highly crystallized multi-walled CNTs, it has been shown production and localized growth on surfaces. The CVD
that if contacted externally, electric current is generally review is split into two parts:
conducted through only the outermost shell [6], though
Fujitsu have been able to contact the inner walls to 1. Fundamental understanding of the growth process,
measure CNTs with resistances 0.7 kΩ per multi-walled 2. State-of-the-art growth results.
CNT [7].
2.1 The catalytic CVD process
This position paper summarizes state-of-the-art research
in CNTs. It should be noted, however, that what is CNT growth occurs as a result of the exposure of
regarded as state-of-the-art is dependent upon the catalyst nano-particles to a gaseous carbon feedstock at
nature of the desired end-structure. The optimum elevated temperatures. CNTs are selectively seeded by
properties listed in section 7 cannot be incorporated
the catalyst (Fig. 2) [8]. This can give control over the
into one CNT. Therefore, a selection of the properties
position at which CNTs form by patterning the catalyst
listed is chosen for a specific desired end-application,
onto a substrate — vertical arrays being a prominent
which is controlled by the growth method, described
example [9].
first. Subsequently, there follows a description of the
possible electrical, electronic and photonic applications
of carbon nanotubes (excluding bulk material composite
applications), the types of CNTs employed and the
organizations or groups that are most proficient at
fabricating them.
2. Growth
CNTs can be grown by three main techniques, chemical
vapour deposition (CVD), arc discharge and laser
ablation. The latter two techniques developed out of Fig. 2: Environmental-TEM image sequence of Ni-catalyzed SWNT root
fullerene research and involve the condensation of growth recorded in 8×10-3 mbar C2H2 at 615°C and schematic ball-and stick
carbon atoms generated by the evaporation of solid model [8].
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In bulk CVD, a catalyst is injected into a hot furnace carbon film over the substrate and nanotubes, and for
with the feedstock and the nanotubes are recovered at this reason diluents/etchants are added, which can
the base. This is realised with either a fluidised bed [10], be hydrogen, argon, water vapour or ammonia (at
or direct injection with fast growth. low temperatures).
More relevant to electronics, however, is the surface-based The CNT growth process involves the dissociation of
growth of CNTs which has many aspects in common with the carbon precursor on the catalyst particle surface,
classical heterogeneous catalysis. There are three basic transport of carbon atoms through or over the
stages to the growth process: particle, and precipitation of the carbon as a growing
tube. All the carbon in the nanotube is incorporated via
1. A catalyst is deposited/patterned on the desired the catalyst particle [12] . In contrast to arc-discharge
surface, or laser-ablation, each catalyst particle typically forms
2. It is transformed into a series of nano-particles and/or only one nanotube with CVD. Therefore, the catalyst
its active phase is stabilised by some pre-treatment, and particle size controls the CNT diameter [8],[13],[14].
3. It is exposed to the growth atmosphere. Catalyst nanoparticles, or rather their atoms, have a
There are numerous deposition methods for the high surface mobility, hence a key challenge for elevated
catalyst, ranging from methods based on wet chemistry
CVD temperatures is to stabilize a narrow particle size
in atmosphere to physical vapour deposition in vacuum
distribution. This catalyst coarsening can be minimised
(Fig. 3).
by a suitable choice of support and deposition
conditions. A wider catalyst size distribution and larger
CNT growth is essentially based on the self-organisation
particle size leads to a loss of control of the nanotube
of carbon; control over the carbon structure requires
diameter and ultimately a loss of catalytic activity.
careful tuning of the growth parameters. A significant
challenge is the large parameter space to optimise which
includes: Important to CNT growth, however, are not only
support interactions and gas-induced catalyst dynamics
•The size and material of the catalyst, but also the restructuring of the catalyst due to the
•The nature of the support (or surface), presence of the growing nanostructure. A SWNT
•The constituents of the carbon feedstock, nucleates by lift-off of a carbon cap (Fig. 2) [8]. Cap
•The quantity and type of diluents /etchants used, stabilization and nanotube growth involve the dynamic
•The temperature of the annealing and subsequent reshaping of the catalyst nanocrystal itself. For a carbon
growth process nanofibre (CNF), the graphene layer stacking is
•The pressure of the reactor. determined by the successive elongation and
contraction of the catalyst nanoparticle [8],[15].
A CNT growth recipe is a suitable combination of these Generally, CNTs grow by either root or tip growth from
parameters. the catalyst particle, depending upon whether the
catalyst remains attached to the support or rides at the
The catalysts used to gain the highest CNT yield are tip of the growing nanostructure.
most commonly the 3d transition metals Fe, Co and
Ni on the oxides SiO2 or Al2O3. The feedstock is In the laser or arc methods, the catalyst particles are liquid,
commonly diluted C2H2, C2H4 or CH4, depending on at least during the initial stage. In many high temperature
the temperature range. For high temperature CVD bulk growth methods, the catalyst is also liquid. But in
(>800°C) methane is often used, whereas for low surface-bound CVD the catalyst can be either liquid or
temperature growth (<750°C) CVD recipes are often solid. There is no necessity for catalyst liquefaction, as
based on ethylene or the more reactive acetylene [11]. in-situ growth experiments at lower temperatures have
The carbon activity can thereby be regulated by shown that for Fe and Ni, the active state of the catalyst
diluting the feedstock. Of key importance is to is that of a crystalline metallic nanoparticle [8],[16]. Solid
avoid/minimise pyrolysis of the carbon precursor, to catalysts are expected to allow for more controlled
prevent the formation of a deleterious amorphous growth, especially with regard to potential chiral selectivity.
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catalyst pre-treatment is to reduce the catalyst
to its metallic state prior to or during carbon
feedgas exposure.
A catalyst prepared by PVD is transformed
into its active state as a nano-particle by a
pre-treatment [16] as shown in figure 3.
Catalyst can also be deposited from solution,
but this tends to lead to less control over
CNT dimensions. This causes the de-wetting
of the catalyst thin film into a series of
separated nano-particles to reduce their total
surface and interface energies. For Fe, this is
aided by the reduction process, where the
reduction in molecular volume from oxide to
Fig 3. Catalyst preparation by (a) PVD and (b) wet chemistry [17][18][19]. metal also creates nano-particles. The diameter
of the resulting nano-particles is directly
In-situ experiments have shown that the active state of proportional to the initial film thickness h. Assuming
the most efficient catalysts Fe, Co and Ni is the metallic volume conservation and a contact angle of 90°, this
rather than the oxide state [16]. Thus a key part of the means the particle diameter D, is given by
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D ~ 6h Equation 2.1.1 The question as to what makes a good CNT catalyst has
not been answered in sufficient detail. Fundamentally,
With a density, N, of CNT CVD growth requires a nano-particulate catalyst.
An interesting comparison is graphene CVD, whereby
N ~ 1/60h2 Equation 2.1.2 planar catalyst films with a large grain size are required.
However, the actual CVD exposure is often chosen to
Processing conditions can affect the CNT type and be very similar to SWNT growth. Ni and Fe films
diameter distribution [8],[14]. For some years, certain catalysing graphene CVD were shown to be metallic and
catalysts such as CoMoCAT or some plasma enhanced showed no signature of carbide formation. CNT growth
growth have produced preferential semiconducting is not exclusive to Ni, Fe and Co, but has been
SWNTs, although the process was unknown. Recently reported, albeit often at lower yield and/or at higher
the preferential growth of metallic SWNTs was reported temperatures, for a range of metals (such as Au, Pt, Ag,
by modifying the catalyst annealing atmosphere [20]. Pd, Cu, Re, Sn, Ta) and semiconductors (Si, Ge).
Nano-particle restructuring is driven by a minimization
of the surface free energy of the system and support “Catalyst-free” SWNT formation has been reported on the
interactions and adsorbate-induced effects are well Si face of hexagonal silicon carbide (6H-SiC) at temperatures
known from heterogeneous catalysis to affect this above 1 500°C [22] and “catalyst free” growth of arrays of
process. In particular for Al2O3 supported Fe, it has multiwall carbon nanostructures has been reported on
been shown that support interactions restrict the cylindrical pores of porous anodic aluminum oxide at 900°C
catalyst surface mobility, leading to a much narrower [23]. More recently, metal-catalyst-free CVD of SWNTs has
catalyst particle size distribution [21]. Moreover, these been reported on roughened Al2O3 and SiO2 at 900°C
support interactions give a higher CNT density and a [24][25][26] and SWNT nucleation has been observed from
vertical nanotube alignment due to proximity effects, diamond nanoparticles at 850°C [27]. The CNT growth
i.e. these support interactions trigger CNT forest mechanism for these “metal-catalyst-free” CVD processes
growth. This is summarized in figure 4. is largely unknown and currently very speculative. In
particular, the physical and the chemical state of the catalyst
during growth are often unknown and what is stated is only
the state before/after growth.
Recent in-situ XPS measurements showed that
nano-particulate zirconia during CNT nucleation at
moderate temperatures (~750°C) does not reduce
to metallic zirconium or zirconium carbide, i.e., the
nano-particulate oxide is the active phase. This indicates
that CNT formation can be mediated solely by a surface
reaction, i.e., bulk C permeability is not a necessity for
a CNT catalyst [28].
There are numerous methods by which nano-particulate
catalysts can be prepared (fig. 5). Physical vapour
deposition (PVD; evaporation, sputtering), aerosol
based techniques, colloids, (electro-)plating and wet
chemistry (salts) are among various methods used to
prepare catalyst for CNT CVD. Each preparation
method requires a different CVD pre-treatment to
form well defined catalyst clusters with a narrow size
Fig 4. Above, the dewetting on the catalyst depends on surface distribution. Furthermore, the catalyst preparation
interactions between the catalyst and support. Below, a general is linked to available patterning techniques. For PVD
comparison of typical CNT growth densities with nanotube diameter
catalysts, photo- or e-beam lithography offer most
(reflecting equs 2.1.1 and 2.1.2).
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A significant disadvantage of the arc
method and some bulk CVD processes
is that the product contains 10% of the
catalyst and other graphitic material
which must be removed by purification.
The purification using acid washing can
then add a factor of 10 to the total cost.
Consequently, the super-growth method,
which produces high aspect ratio CNTs
has a nanotube to catalyst ratio of 105,
which gives a very high purity. Therefore,
super-growth is the only surface-bound
Fig 5. Catalyst patterning techniques: (a) e-beam lithography, (b) electroplating, (c) FIB, (d)
contact printing [9]. method of producing high-purity bulk
nanotubes. This is critical for some
applications such as supercapacitors in
control, whereas laser interferometry and
which metal impurities have a significant effect.
nanosphere lithography are among alternative
patterning techniques which can be used to pattern
A key criterion for growth is to lower the temperature.
small feature sizes at a lower cost over large areas.
This is important for a number of electronic applications
Nanocontact printing and micro-fluidic techniques are
such as interconnects where the temperature should not
often used with catalyst colloids.
exceed 400°C, limited by the fact that it is a “back-end”
process.
2.2 State-of the art CNT growth results
The growth of CNTs depends on the desired The first method used to reduce the growth
application. For instance, certain field emission temperature was to use PECVD which is widely used to
applications require spaced arrays of CNTs. It is lower processing temperatures during growth [31].
preferable that the CNTs remain fixed and vertical. PECVD in nanotube growth was assumed to dissociate
Consequently, MWNTs are the best choice in this the carbon pre-cursor gas and allow the growth reaction
situation. The best controlled growth of spaced arrays of to occur at lower temperatures. However, it is now
MWNTs is as defined in 2001 using PECVD [29]. There realized that the key role of the plasma is to convert the
have also been recent developments to scale down the catalyst into an active nano-particulate, metallic form at
diameter and height of the CNTs, in order to increase a lower temperature. Therefore, so long as growth uses
the emission current density. acetylene, it can proceed at quite low temperatures.
Other applications such as vias, interconnects, heat Two other low temperature activation methods have
spreaders, super-capacitors or adhesive surfaces require been used. First, Corrias et al [32] and also AIXTRON
densely packed aligned nanotube arrays. This area was [33] use pre-heating or plasma pre-dissociation. A
revolutionized by the so-called super-growth method of second method used by S.R.P. Silva et al in Surrey is to
Hata et al [30]. This uses a Fe catalyst on Al2O3 support, use non-thermal light irradiation to heat the substrate
and thermal CVD at 700°C, in hydrogen-diluted surface.
ethylene gas. A small proportion (400 ppm) of water
vapour is added to the growth gas. The water is shown Growth on metals is required for applications such as
by EELS to act as a mild etchant for amorphous carbon interconnects. However, metallic supports often show
which ultimately starts to cover the catalyst and interactions with the gas atmosphere and catalyst, i.e.
terminate growth. This led to an era where mm to cm oxide, carbide and alloy formation is likely. Hence, CNT
high forests could be grown by many groups world-wide. CVD directly on a metal support requires careful
The nanotube density is controlled by the catalyst film calibration and often gives poor growth. The way around
thickness, as previously described. this is to use metals with small Al content which then
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preferentially oxidizes into an ultra-thin layer of Al2O3, extremely fine, black thread consisting of aligned CNTs
or to use metallic compounds such as CoSi2 or TiN. [36]. Nanocyl also produce purified single-walled
nanotubes [37].
The quality of grown carbon nanotubes is subjective,
since their quality depends on the structure required. 2.2.1.2 Horizontally aligned single-walled CNTs
Quality screening is a challenge on its own and the
detailed characterisation of as-grown CNT samples Horizontally-aligned SWNTs have been grown on
remains time-consuming and relies on a combination of epitaxial surfaces such as sapphire and quartz with
direct imaging methods (such as SEM, AFM, TEM, STM) varying densities. The growing CNTs follow the crystal
and indirect methods (such as Raman, PL, optical planes with a great degree of alignment. The process is
absorption, TGA, XPS, XRD). Some applications standard CVD but the substrate needs to be annealed
require high purity and crystallinity; others require tight for surface reconstruction before growth. Among the
dimensional control, whilst others might require high best, Tsuji’s group have grown on sapphire [38] and the
packing densities and/or alignment. Consequently, the Rogers group [39], who have grown on quartz (figure
state of the art depends on the type of structure and 6). The density is some way (x10) below that achieved
application required. in vertically aligned forests.
2.2.1 Single-walled growth Horizontal alignment can also be achieved by electrical fields,
gas flow, or liquid post-treatment. Hata and co-workers
2.2.1.1 State of the art for bulk single-walled dipped a vertical aligned forest in alcohol to get
growth alignment in plane samples by capillary forces when he
pulls up the substrate from the liquid [40].
The highest purity CNTs nucleate from catalysts in a
fluidized bed and are currently sold by Thomas Swan 2.2.1.3 Challenges for SWNTs
[34]. The process produces high-quality CNTs,
inexpensively in large quantities. Windle’s [35] group A key challenge for SWNTs still concerns control of
grow CNTs in a continuous flow furnace. The nanotubes chirality during growth. For applications such as
are created rapidly by injecting ethanol and ferrocene transistors, all grown CNTs need to be semiconducting
into a furnace at 1200°C. An aerogel then starts to stick (and preferably of identical chirality and diameter) whilst
to the cooler wall in the furnace to form fibres. A for interconnects, all CNTs need to be metallic. Control
spindle then winds the aerogel fibres into a thread, at of diameter is related to this issue.
several centimetres per second. The result is an
The second challenge is nanotube
density. For interconnects, a high
density (of ~1013 tubes/cm-3) is needed
if it is to replace copper. This is now
looking possible.
The yield of SWNTs grown with
templates is very low and must be
solved if it is to be seriously considered
as a method for growing SWNTs. Also,
for SWNT growth to be combined
with CMOS, the temperature needs to
be reduced to ~400°C.
To a certain extent the chirality
problem has been overcome by using
Fig 6. (a-d) CNTs grown along quartz crystal planes by the Rogers group [39]. (e) Horizontally
aligned CNTs grown by Dai’s group using field to align the CNTs [13].
devices based on random network of
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nanotubes instead. This approach was first brought to and Arkema [45] and Bayer [44] have made significant
light by Snow and co-workers in 2003 [41] although it contributions to up scaling CVD. Recently, AIXTRON
was patented by Nanomix in June 2002 [42]. [46] and Oxford Instruments [47] have begun to provide
large area PECVD capability. The leading universities in
2.2.2 Multi-walled CNTs Europe include Cambridge Univ, Dresden and EPFL.
Growth of MWNTs on large wafers (200mm) is now
Though Endo started the injection process, for bulk routinely done at various locations for microelectronics
growth, the best CNTs are again grown by Thomas Swan applications (see for example, images of CNTs grown in
(as a result of rigorous qualification by Raman and TEM) various CVD reactors at CEA-Grenoble in figure 7). The
and Windle’s group in Cambridge, though Hyperion aerosol-assisted CCVD process allowing the production
[43] are the leading suppliers of nanofibres using a similar of carpets of aligned nanotubes is produced at CEA-Saclay
process to Thomas Swan. So-called Endo-fibres 150 nm in the group of Martine Mayne (and can also be seen in
in diameter can also be purchased from Showa Denko. figure 7).
Bayer produce narrower “Baytubes” 5-20 nm in
Fig 7. Dense forest of Small diameter MWCNT from left to right: a) Patterned layer on a 200mm layer b) 50μm high forest on conductive layer of
TiN c) close view of the material with individual CNT making bundles of 60nm of diameter (courtesy of CEA-LITEN)
diameter [44], but these are impure. However, they are 3. Post growth modification
suitable for many uses because the metal is encapsulated
in the tube ends and not exposed. If no particular attention is paid to it, CVD generally produces
CNTs with numerous defects. Such defects are favoured:
2.2.2.1 Challenges for multi-walled CNTs (i) when low temperatures are being used for the
growth;
Some of the challenges for MWNT growth are identical (ii) when dopants such as nitrogen or boron are
to that of SWNT growth. Growth temperature needs to inserted;
be reduced if CNTs are to be employed in CMOS. (iii) when the growth process is allowed to continue
Raman spectra of MWNTs grown by CVD/PECVD at while the process is being ended (namely, when the
low temperatures show them to be highly defective. power is shutdown and the substrate allowed to cool
Post-annealing processes can increase graphitization, but whilst still in the presence of a carbon source often
these are typically at temperatures much higher than resulting in the deposition of amorphous carbon around
circuitry can withstand. There is also the question of the CNT).
contact resistance that is often quite high and variable.
This needs to be addressed with still further Many defects can be removed either by hydrogen or
improvements on dimensional control. ammonia plasma, or by a rapid thermal annealing
process which also increases the graphitization,
European Position: Europe led the way with conductivity andcontact of the CNT [48]. Hence,
research in arc deposition but commercialisation was careful monitoring of CVD parameters can lead to
limited. More recently Nanocyl [37], Thomas Swan [34], defect free carbon nanotubes [49]. Single defects can
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even be created, monitored and can serve as the [77],[78],[79],[80]. Alkali-metal atoms located outside or
point for a single functionalization [50][51]. inside the tube act as donor impurities [81],[82] while
halogen atoms, molecules, or chains act as acceptors
Various techniques have been employed to purify CNTs [65],[73],[83],[84]. Fullerenes or metallo-fullerenes,
grown by arc discharge and laser ablation. This is encapsulated inside CNTs, allow good structural stability
because the best samples are only 70% pure (using laser and have been used to tune the band gap and/or Fermi
ablation), with the remainder made up of amorphous level of the host tube [85],[86],[87],[88].
carbon, fullerenes, and catalyst particles surrounded by
shells of chemically resilient turbostatic carbon (TSG). “Doping” by physisorption of molecules, lies at the heart
CNTs are first dispersed by sonication [52]. The of a growing field of chemical sensors, but their stability
gas-phase method developed at the NASA Glenn and selectivity issues must be very carefully addressed.
Research Center to purify gram-scale quantities of
single-wall CNTs uses a modification of a gas-phase European Position: In Europe Maurizio Prato’s group
purification technique reported by Smalley and in Trieste are the most successful in this area.
others [53], by combining high-temperature oxidations
and repeated extractions with nitric and hydrochloric 5. Oxidation/Functionalization/tip opening
acid. This procedure significantly reduces the amount of
impurities such as residual catalyst, and non-nanotube CNTs can be oxidized by various means including
forms of carbon within the CNTs, increasing their treatment in acids, ozone and plasma oxidation [89].
stability significantly. Once the CNTs are separated, the Reflux in nitric acid not only purifies the nanotubes but
use of a centrifuge enables the isolation of certain at the same time introduces a variety of oxygen groups
chiralities of SWNTs, particularly (6,5) and (7,5) as [90] at the open ends and sidewalls, which strongly
shown by Hersam’s group at North Western University facilitates the separation of nanotube bundles into
[54]. This method seems to be the way forward for individual tubes and enhances their dispersibility. The
scalable chirality separation. tips of CNTs are more reactive than their sidewalls and
reflux in HNO3 has proven to open the nanotubes tips
European Position: The US lead the way in novel and introduce carboxylic groups at the open ends. In
techniques based on density differentiation but in particular, sonication under harsh conditions, in a
Europe, Krupke, Knappes and co-workers at Karlsruhe mixture of concentrated nitric and sulphuric acid
pioneered the dielectrophoresis method. Regarding effectively cuts the single walled nanotubes into small
industrial production, a trend is observed in which CNTs fragments and gives rise to the formation of small length
are produced and included in a polymer matrix in the (100 to 300 nm) open pipes. The oxidatively introduced
same process line hence reducing the risk of exposure carboxyl groups represent useful sites for further
to airborne nanotubes. modifications, as they enable the covalent coupling of
molecules through the creation of amide and ester
4. Doping bonds. It has also been shown that CNTs react with
ozone [91].
Conventional doping by substitution of external
impurity atoms in a semiconductor is unsuited to CNTs, The growth of VACNTs is important for many potential
since the presence of an external atom modifies the technological applications such as field emission
properties resulting from ideal symmetry in the CNT. cathodes, vertical interconnects, and biosensors. Both
Theoretically, substitutional doping by nitrogen (n-type) thermal chemical vapour deposition (TCVD) and
and boron (p-type) has been widely examined microwave plasma enhanced chemical vapour
[55],[56],[57],[58],[59],[60]. Adsorption of gases such as deposition (MPECVD) have shown promising results on
H2, O2, H2O, NH3, NO2 have been reported producing well-aligned CNT arrays. In thermal CVD the
[61],[62],[63],[64]. More appropriate doping strategies grown nanotubes are very closely packed, the growth
which conserve the mean free path of the charge rate is very high and topology is highly defective
carriers involve physisorption of alkali metal atoms compared to that of MPECVD grown CNTs. Various
[65],[66],[67],[68],[69],[70],[71],[72],[73] [74],[75],[76], plasma sources have been successfully used to clean,
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and open nanotube tips [92]. Low energy, high flux Grafting of biomolecules such as bovine serum albumine
plasma such as ECR plasma is a suitable technique for [1 [120], [121] or horse spleen ferritin [122], poly-Llysine,
19],
efficient cleaning, tip opening and produces less a polymer that promotes cell adhesion [123], [124],
damage. Streptavidin [125] and biotin at the carboxylic sites of
oxidized nanotubes [126] and polymers [127], [128],[129],
Open-capped CNTs, unless functionalized, can be [130], [131], [132] have been reported.
unstable structures because of dangling bonds. Cap (c) cycloadditions [133].
closing of open-capped structures often occurs during
field emission. De Jonge et al. [93] demonstrated this European Position: Haddon and co-workers in the
happens for currents as low as 80 nA per tube. US were early leaders and Carroll and co-workers in
Wake Forrest University applied functionalisation to
Several treatment methods such as chemical, devices. In UK Papakonstantinou et al at the University
electrochemical, polymer wrapping, and plasma of Ulster have demonstrated a variety of plasma based
treatment have been applied to functionalize the CNT routes as an alternative to chemical functionalisation. In
surface for specific applications including, catalysis, Europe Hirsch in Erlangen has made major contributions
bio/gas sensors, composites, drug delivery, field and Coleman and co-workers at TCD have furthered our
emission and cell scaffolds [94], [95], [96], [97], [98], knowledge in this area.
[99], [100], [101], [102], [103], [104]. Among these,
plasma-treatment has the advantages of retaining the 7. Properties/characterization
structural integrity of the nanotube, is environmentally
friendly and it provides the possibility of scaling up for The physical properties of carbon nanotubes depend on
commercial use. Also, reactions are much slower than a number of variables. These include, if the tube is
other chemical modification methods and can also multi-walled or single-walled, the diameter of the tube
provide a wide range of functional groups depending on and if we have a bundle or a rope, or just an individual
the plasma parameters. Papakonstantinou’s group has tube. The chirality of the tube is also important for single
shown that plasma functionalisation can preserve the walled nanotubes. Table 1 summarises the experimental
vertical alignment of CNT arrays. findings of several different properties of carbon
nanotubes, highlighting differences between different
Generally, the main approaches to functionalisation can types of tube.
be grouped into two categories:
Table 1: Summary of main properties of CNTs
(a) the covalent attachment of chemical groups through MECHANICAL PROPERTIES
reactions with the π-conjugated skeleton of the CNT; Young’s modulus of multi-walled CNTs ~0.8-1.3 Tpa [134],[135]
(b) the non-covalent adsorption or wrapping of various Young’s modulus of single-walled CNTs ~1-1.3 TPa [136],[137]
Tensile strength of single-walled nanotube ropes > 45 GPa [138]
functional molecules
Tensile strength of multi-walled nanotube ropes 1.72GPa [139]
Stiction ~10-7 N on 5 μm latex beads [140]
The covalent functionalization of SWCNTs is not limited Hydrophobicity of MWNT forest 26°[141] -161°[142] contact angle
to the chemistry of carboxylic acid. More elaborate
THERMAL PROPERTIES AT ROOM TEMPERATURE
methods have been developed to attach organic moieties Thermal conductivity of single-walled CNTs 1750-5800 WmK [143]
directly onto the nanotube sidewalls. These include: Thermal conductivity of multi-walled CNTs >3000 WmK [144]
ELECTRICAL PROPERTIES
(a) photoinduced addition of azide compounds; Typical resistivity of single- and multi-walled CNTs 10-8 - 10-6 Ωm [145]
Reports of fluorination [106],[107] chlorination [108], Typical maximum current density >108 A cm2 [146]
atomic hydrogen [109]; aryl groups [110], nitrenes, Quantized conductance, theoretical/measured (6.5 kΩ)-1/(12.9 kΩ)-1
per channel
carbenes, and radicals [111]; COOH [112],[113], NH2 ELECTRONIC PROPERTIES
[1 N-alkylidene amino groups [1 alkyl groups [1
14] 15]; 16] Single-walled CNT band gap
and aniline [117] amine and amide [118] have been Whose n=m, armchair 0 eV (metallic)
reported. Whose n-m is divisible by 3 <0.1eV (quasi metallic) [146]
Whose n-m is non-divisible by 3 0.4-2eV(semiconducting) [148],[149]
(b) Bingel reactions; Multi-walled CNT band gap ~0 eV (non-semiconducting)
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8. Electronic Applications 8.2 Transparent conductors/contacts.
Various applications for CNTs in the ICT field have As the use of ITO becomes ubiquitous and indium
been touted but in the near term only a few of these becomes more scarce and thence more expensive
seem feasible: their use as a thermal interface material there is an ongoing search for alternative transparent
has gained the most interest in the last two years, as conducting contact materials. Initiated at Nanomix
has their application to transparent conductors. Work [154], various groups worldwide including those of
on interconnects and vias continues whereas field Rinzler, Roth, Chhowalla and Grüner have worked to
emission has remained somewhat static. There follows replace indium tin oxide (ITO) in e.g. LCDs, touch
a list of applications which, in the authors’ opinions,
screens, and photovoltaic devices. Nantero Inc.
orders the interest attracted by industry in terms of
(Boston), Eikos Inc. of Franklin, Massachusetts and
investment.
Unidym Inc. (recently bought by Arrowhead) of Silicon
8.1 Thermal management Valley, California are also developing IP and
transparent, electrically conductive films of carbon
There is an increasing need to replace indium for nanotubes [155].
thermal interfaces in eg: CPUs, graphic processors and
(automotive) power transistors, as price and scarcity CNT films are substantially more mechanically robust
increase. Various companies and universities (such as than ITO films, potentially making them ideal for use
Ajayan’s group [150]) are working in this area but very in displays for computers, cell phones, PDAs and ATMs
little has been published. as well as in other plastic electronic applications. At
SID2008, the University of Stuttgart and Applied
As a thermal interface material for high brightness Nanotech presented the world's first 4-inch QVGA
LED’s, CNTs have been shown to outperform silver colour LCD display using CNT as the transparent
epoxy and other metal systems [151]. Fujitsu has also conductive film. The CNTs were deposited by spray
evaluated 15 micron tall CNT thermal bumps, bonded coating [156].
at 6 kg/cm2 between a GaN high performance
amplifier and an AlN substrate, to have a thermal
There is still a need to increase conductivity whilst
conductivity of 1400 W/mK [152]. It is believed that the
key advantage of CNTs is their compliance, which maintaining a sufficiently high (~95%) transparency
eliminates the problems of thermal mismatch between and for some applications, roughness is a problem.
surfaces as well as ensuring a good contact. Recent developments have addressed these issues.
First, the separation of metallic single walled nanotubes
By annealing the catalyst in different atmospheres of from semiconducting ones using scalable density
Ar/He/water, the catalyst shape can be controlled, gradient centrifugation has improved transparency and
leading to the preferential growth of up to 91% metallic conductance [157]. Second, the National Renewable
SWNTs [153]. Energy Laboratory in the US reported ultra-smooth
transparent and conducting SWNT thin film electrodes
Current problems in using CNTs are insufficient packing for organic solar cells which yielded efficiency values of
density and problems with graphitisation leading to a > 3.5%, the highest thus far for SWNT electrodes
reduction in conductivity. [158]. Despite this progress, the sheet resistance of
SWNT networks has remained at ~100 Ω/sq (about
European Position: Ajayan is the most notable
an order of magnitude higher than ITO) at a
contributor to this research together with the
transparency of 85%. Also, most recently it has been
contributions from Fujitsu above. Intel Europe are also
pointed out by Fanchini et al. [159] that CNT/Polymer
known to be working in this area. Very little work has
been published by them and other workers from the films are anisotropic and suffer from birefringent effects
EU. which may cause problems in some of its most useful
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Recent studies have also demonstrated that SWCNT
thin films can be used as conducting, transparent
electrodes for hole collection in OPV devices with
efficiencies between 1% and 2.5% confirming that they
are comparable to devices fabricated using ITO [160],
[161]. Note, however, that graphene based films are
very quickly gaining momentum as transparent
conductive electrodes and may very likely overtake the
performance of CNTs in this area [162].
Chemically derived graphene obtained from
reduction of graphene oxide is attractive because it is
Fig 8. Comparison between the various transparent Conducting Materials synthesized from graphite, a ubiquitous and
[166]. inexpensive mineral. The state-of-the-art reduced
graphene oxide films have yielded sheet resistances
potential application areas such as in Liquid Crystal of ~1 kΩ/sq at 85% transparency [163]. However,
Displays. Gruner summarizes the work in this area well chemical vapour deposition (CVD) of graphene on
(figure 8). copper and transfer onto desirable substrates have
Advertisement
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yielded sheet resistance values of 10—30 Ω/sq at
transparency values > 85% [164]. Workers at SKKU
have developed a process for synthesizing large-area
graphene films for use in touch screens [165]. Sony
are also working in this area.
These latest results suggest that CVD graphene can
meet or surpass the properties of ITO. The main
limitation of CVD graphene at present is that it can only Fig 9. Enhancement of a capacitor performance by enhancing its surface
be grown on copper and subsequently transferred. A area by using a ordered CNT backplane (a) Device structure (b)
general limitation of graphene is how to scale up the Comparison of capacitance of flat and CNT enhanced structures. Inset
shows SEM images of the respective CNT arrays. Reprinted with
process to produce a uniform film over a large area,
permission from (Jang et al). Copyright 2005, American Institute of Physics.
which is why CNT networks are the leaders in in the
area at present. CNTs are a natural candidate material for electrodes
in supercapacitors. Experimental devices replace the
European Position: US dominate this area activated carbon with CNTs. The surface area of
through the work of Eikos Inc, Nanomix Inc., Grüner carbon CNTs is not much greater than that of
and co-workers and Rinzler’s group in Florida. activated carbon, therefore energy densities are not
Chhowalla at Imperial College, London has now much improved. However they are mechanically
carried on this work and Roth in Stuttgart leads the arranged in a much more regular fashion, allowing a
way in Europe. lower internal resistance of the device and hence
higher power densities.
8.3 Supercapacitors and batteries
In the last few years, many examples have appeared in
8.3.1 Supercapacitors literature where different forms of CNTs have been
used in a multitude of ways to produce supercapacitor
Research efforts in the field are directed towards devices (see figure 10, page 20) . Thin films of SWNTs
combinations of materials with high dielectric constant can be deposited from solution, sprayed or inkjet
(for higher capacitance values) and high breakdown printed on any substrate with relative ease [169], [170].
strengths (for the ability to sustain higher voltages). Due to their conductive nature, these films can
Alternatively, the effective surface area can be enhanced substitute both the carbon electrode and the aluminium
by using nanomaterials. Jang et al. [167] demonstrated current collector, allowing for better charge
such a device by using an ordered array of CNTs and storage/device weight ratios. CNTs can also be grown
coating them with a dielectric and another conductor in an orderly fashion, as aligned forests for direct
(figure 9), demonstrating a 5x increase in capacitance per electron transport, higher packing density and direct
cm2. These structures open new possibilities for ultrahigh contact with electrodes. Capacitors from forests of both
integration density devices such as random access SWNTs [171] and MWNTs [172] show promising
memory and other nano-electric devices. behaviour, and the ever increasing control achievable
during growth allows for much fine tuning of devices.
Energy-storing electrochemical double-layer capacitors
(EDLCs) or supercapacitors have electrical parameters In more simple cases, CNTs are mixed into a low
between ordinary capacitors and batteries; they are used conductivity but very high surface area activated carbon
primarily as power sources or reserves. However, unlike electrode. Due to their high conductivity and aspect
batteries, electrodes in EDLCs, typically made of carbon, ratio, the resulting composite results in an appreciably
do not undergo any chemical reactions as in batteries; lower internal resistance [173]. A similar capacitive effect
they store the charge electrostatically using reversible results also from the very fast redox reactions that occur
adsorption of ions of the electrolyte onto active materials at the surface of some materials, known as
that are electrochemically stable [168]. pseudocapacitance. CNT forests are used as templates
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8.3.2 Batteries
The conductive properties and the high surface-to-volume
ratio make carbon nanotubes potentially useful as anode
materials [179] or as additives [180] in lithium-ion battery
systems. The CNTs give mechanical enhancement to the
electrodes, holding the graphite matrix together. They
also increase the conductivity and durability of the
battery, as well as increasing the area that can react with
the electrolyte. Sony produces the best CNT-enhanced
lithium-ion batteries.
The use of CNTs as scaffolds for other materials has
made the search for new electroactive materials wider
than ever because requirements for such materials are
now much relaxed. Low electronic conductivity, a low
diffusion coefficient for lithium, and poor structural
stability can all be compensated to some extent by
CNTs.
Recently, a so-called paper battery has been developed,
where CNT arrays bound with cellulose are used as
electrodes. The technology is cheap, the batteries are
flexible and no harmful chemicals are required [181].
Mass production of this technology is still some distance
away and the process is currently manual and laborious.
Cost of CNT production remains high and
Fig 10. Various carbon nanotube architectures which have been commercialisation is strongly dependent on this.
employed for supercapacitors. Left column shows the SEM image while
the right column shows the resulting cyclic voltammetry performance.
(a) SWCNT random thin film (b) Ordered aligned array of SWCNTs,
Overall, there are several potential advantages and
which have been compressed with liquid. Inset shows profile view. disadvantages associated with the development of CNT
Reprinted by permission from Macmillan Publishers Ltd: Nature based electrodes for lithium batteries.
Materials [v], copyright (2006) (c) An array of freestanding pre-grown
MWCNTs embedded in a cellulose matrix, resulting in a paper-like
electrode [176]. Copyright (copyright year) National Academy of Advantages include:
Sciences, U.S.A. (d) An array of MWCNTs used as a scaffold to hold low (i) better accommodation of the strain of lithium
conductivity but high capacitance MnO2. Reprinted with permission from insertion/removal, improving cycle life;
[177]. Copyright 2008 American Chemical Society.
(ii) higher electrode/electrolyte contact area leading to
on which to deposit high pseudocapacitance materials higher charge/discharge rates;
(e.g. oxides [174] or polymers [175]). Whether the use of (iii) short path lengths for electronic transport
CNTs will be possible at a reasonable cost is yet to be (permitting operation with low electronic conductivity
determined. or at higher power); and
(iv) short path lengths for Li+ transport (permitting
European Position: Main progress in this field is carried operation with low Li+ conductivity or higher power).
in the US (Ajayan group) and Japan. The leaders in Europe
are Beguin et al. [178] who have used multi-walled CNTs Disadvantages include:
mixed with polymers to create capacitance values of (i) an increase in undesirable electrode/electrolyte
100-330 F/g. Europe also has a strong industrial input reactions due to high surface area, leading to self-discharge,
in this area with companies such as Maxwell (formerly poor cycling and calendar life;
Montena, Switzerland). (ii) inferior packing of particles leading to lower volumetric
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energy densities unless special compaction methods are Problems include choice of catalyst, catalyst deposition,
developed; and depositing top contacts, increasing the packing density
(iii) potentially more complex synthesis. and reducing the overall resistivity. The growth also
needs optimization for back-end processing and must
Toward further integration and large scale production, be carried out at low enough temperatures so as not to
CNT-based negative electrodes for lithium micro-batteries damage CMOS. If SWNTs are to be employed, the
are being developed by a consortium led by CEA-LITEN, packing density of metallic tubes must be high enough
ST Microelectronics and Schneider [182]. to justify replacing metal interconnects. For MWNTs,
for a sufficient current density, internal walls must also
European Position: Although much of the innovation contribute to conduction. Neither has as yet been
has been carried out in the US and the Far East, in Europe achieved.
there are various groups notably in Germany contributing
in this area. European Position: Infineon identified vias as a
possible early application of CNTs in electronics, Intel in
8.4 Interconnects/vias the US evaluated spun-on CNTs for contacts but more
recently Fujitsu, Japan lead the way. In Europe, the
In order to achieve the current densities/conductivity TECHNOTUBES project which includes many partners
needed for applications in vias, dense arrays of CNTs from throughout Europe is focussing on this application
are required. Very dense arrays of nanotubes have been amongst others.
grown by chemical vapour deposition (CVD) by various
groups, following Fan et al. [183]. They are called forests, 8.5 Sensors
mats or vertically-aligned nanotube arrays. They are
usually multi-walled and grown from Ni, Co or Fe 8.5.1 Electronic Sensors
catalysts. It has been suggested that a nanotube density
of at least 1013 cm-2 is needed in order to produce the The use of CNTs for sensing is one of their most
required conductivity but recently Fujitsu have indicated interesting electronic applications. Both SWCNTs and
that 5x1012 cm-2 would be acceptable [184]. However MWCNTs, functionalised and unfunctionalised, have
growing such dense arrays in vias of high aspect ratio is been investigated as single nanotube and network
not so straightforward. Numerous groups worldwide devices. A vast number of prototypes and device
are trying to optimise the process including CEA but strategies have been demonstrated for gas [187],
Fujitsu [185] (see figure 11) have reported the most electrochemical and biological sensors [188], and so far
significant advances and have recently reported that field-effect based sensors have detected NO2
they have achieved a density of 9x1011cm-2. They have concentrations in the ppb range [189]. Ultrathin films of
also reported a resistivity of 379 μΩcm for a 2 μm SWNTs appear to be the most viable basis for an
diameter via. A Microwave CVD method was employed electrical sensor in terms of scaleability, and can be
to produce CNTs at temperatures compatible with fabricated through a number of different techniques
CMOS. However, much improvement is still required including dielectrophoresis [190], direct CVD growth
before these become a practical proposition. [191] and solution-based transfer [192], for instance
embedded in a polymer coating [193]. The review by N.
Sinha et al. [187] from 2006 covers carbon nanotube
sensors generally, while the recent reviews by Goldoni
et al [187] and Jacobs et al [188] overviews carbon
nanotube gas sensors and electrochemical sensors,
respectively.
However, there are still many other problems to
overcome in bringing this technology to market; high
Fig 11. Left, CNT vias grown in pores etched into silicon. Right, CNTs volume/quality manufacturing, the intrinsic variability of
grown in pores in silicon [186]. SWNT, functionalisation and cleaning/recycling. While
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very high sensitivity has been achieved, the results in
terms of selectivity are often less convincing, as the inert
carbon nanotubes are difficult to functionalise effectively.
As the energy bandgap, carrier mobility and also the
chemical reactivity depend on the diameter and chirality,
the variability must either be averaged out by a network
architecture or by sorting by chirality, which can be
done by ultracentrifuging; network TFT transistors with Fig.12: Left, structural cross-sectional layout of the sensing area of the
an on/off switching ratio above 104 have been chip. Right, microscopic images of carbon nanotubes grown locally on
demonstrated [195], [196]. Reproducibility of the CNT ultrathin membranes incorporating a tungsten heater [201].
growth, processing as well as variable behaviour once
pathology from the detection of biomarkers in the
integrated into a sensor can result in poor selectivity
breath.
and sensitivity. Resetting electronic nanotube sensors is
another issue; methods such as annealing or using a gate
Due to cost of R&D, commercialization of such sensors
voltage in a TFT architecture have been used [197].
are however very costly. For example Nanomix has
already raised more than $34 million and they have yet
In some devices, defects play a key role, in others, the
to deliver a significant, high volume product to the
source/drain metal-nanotube contact is key. Also, the
market although they are currently seeking FDA
nanotube-nanotube junction or even the amorphous
approval for their NO sensor, mentioned above, to be
carbon remaining on the nanotube can play a part in
used in the monitoring of asthma (Figure 13). Progress
the detection scheme. [198] Indeed, there are many
in these areas continues to be made globally.
possible sensing mechanisms, hence a fundamental
understanding of them is required to enable good
optimisation and reproducibility of the sensors.
However, this research area is very active, so further
progress is expected.
The US company Nanomix Inc was the first to put an
electronic device that integrated carbon nanotubes on
a silicon platform (in May 2005 they produced a
hydrogen sensor) on the market [199]. Since then,
Nanomix has taken out various other sensing patents
e.g. for carbon dioxide, nitrous oxide, glucose, DNA
Fig. 13: Preproduct picture of Nanomix’s NO sensor (with authorisation
detection etc. [200]. The next product to become
from YL Chang, Nanomix).
available should be a breath analyzer detecting NO as a
marker of asthma. More recently workers in Cambridge
European Position: Many companies and research
and Warwick University in collaboration with ETRI,
institutions in Europe are carrying out work in this area,
South Korea have integrated CNTs onto SOI substrates
with THALES, Dekker (Delft), being the most notable.
to produce smart gas sensors (see figure 12) [201]. The
CNTs have been locally grown on microheaters allowing
8.5.2 CNTs in Biotechnology and Medical Device
back end deposition at T~700°C without significantly
Research.
affecting the surrounding CMOS.
Nanomedicine, or the application of nanotechnology to
Other groups have been using a multi-functionalized
achieve breakthroughs in healthcare, exploits the
network of sensors combined with a principal
improved and often novel physical, chemical and
component analysis in order to enable pattern
biological properties of materials at the nanometer
recognition approach (artificial nose) [202]. Such an
scale. Nanomedicine has the potential to enable early
approach could enable early diagnosis of various
detection and prevention, and to essentially improve
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diagnosis treatment and follow-up of diseases. This of biocompatibility and nanotoxicity associated with the
section addresses the overall roadmaps associated with use, manufacture and purchase of CNTs in all forms.
nanomedicine and in particular identifies the role of Recent reviews point out that fullerenes have potentially
CNTs. Technologically, most CNT applications in biotech useful properties, and that several of the reports on
are in biosensing, lab-on-a-chip, drug screening and drug toxicity have used unrealistic conditions and doses [204].
delivery. The key issues associated with CNTs related to Nevertheless, the possible toxicity remains the single
nanomedicine closely resemble those mentioned in the largest barrier in terms of practical use of nanotubes for
previous section and are mainly related to the following drug delivery or cancer therapy; the nanotubes do not
issues: break down in the body, and are able to kill cells if not
effectively passivated. Since the carbon nanotubes will
•Some of the main challenges are linked to ultimately last longer than any passivation, and have
industrialisation. There is no conventional manufacturing been shown to aggregate in the lungs and brains of
method that creates low cost CNTs. Desirable animals, it is questionable whether or not carbon
properties are robustness, reproducibility, uniformity nanotube-based medicine will be used in clinical tests in
and purity. the near future. Furthermore, other very effective,
•Reproducible production relating to surface defects; nontoxic systems are available for drug delivery, such as
surface chemistry, size (height and diameter; protein-coded liposomes [205].
morphology; type etc.
•The ability to functionalise the surface with European Position: The area of carbon nanotube
appropriate chemistries. based sensors is very broad and diverse due to the
•The ability to produce arrays; periodicity; catalyst free- or many different physical quantities, applications and
tailored catalyst grown from self-assembly. environmental conditions for such devices. In terms of
•To produce lost cost routes to manufacture in the case gas and biological sensing, a substantial number of
of disposable or competitive devices. groups are pursuing high sensitivity gas or biological
•The ability to integrate into microfluidic systems; molecular sensing, better functionalisation/specificity as
CMOS circuitry or flexible substrate systems. well as scaleable fabrication methods, in US as well as in
Europe. While the research activities are spread all over
In the next ten years, the development of biosensors Europe, a few commercial players are active. Cambridge
and importantly, nanotechnology, will allow the design CMOS Sensors are developing sensor platforms based
and fabrication of miniaturised clinical laboratory on carbon nanotubes. There are strong activities in
analysers to a degree where it is possible to analyse many universities including ETH Zurich and Cambridge
several laboratory measurements at the bedside with as University but it is not possible to identify single leading
little as 3 μL of whole blood. The use of quantum dots; groups. In terms of nanotubes used inside humans, it is
self-assembly; multifunctional nanoparticles, nano-templates not likely that this will be possible on a short or even long
and nano-scale fabrication including nanoimprinting will term due to the unresolved toxicity issues.
have a major impact on the design and development of
much improved highly sensitive and rapid diagnostics; 8.6 Field emission
thus allowing accurate drug delivery integration.
Nanoenabled high throughput analysis will also reduce Because of their high—current-carrying capability,
the time it takes to bring a new drug delivery platform chemical inertness, physical strength and high aspect ratio,
to market. CNTs can be applied to many technologies requiring field
emission. Immediately after the growth, CNTs contain
CNTs have also been shown to favour neuronal growth impurities such as amorphous carbon, catalyst, residual
and reduce glial scar formation (gliosis). CNT-based growth gases as well as varying degrees of structural
electrodes have therefore been proposed for use in defects dependent on the growth parameters. The
implants to enable long term treatment of Alzheimer defects and impurities should be removed or passivated
disease using deep brain stimulation [203]. for optimum field emission properties. Treatments such
as plasma, IR and UV lasers and oxidative annealing can
A clear set of studies is required to resolve all the issue be used to clean the tips of the CNTs.
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Researchers have reported low threshold field (field CEA of France continue to fund research in this area,
required to emit electrons at a level of 1 μA/cm2) for with typical displays produced shown in figure 13.
various types of CNTs. However there is a debate about
the long term emission stability of CNTs. Open tips, 8.6.2 Microwave Generators
aligned CNTs and aligned CNTs embedded in PMMA
matrix have shown significant improvement in emission High power/frequency amplifiers for higher bandwidth,
stability as compared to the as-grown CNT samples. more channels and microwave links are increasingly using
Nanotubes can be used for single electron beam devices the 30 GHz and above frequency range. Attempts have
(such as field-emission scanning/ transmission electron been made to replace the thermionic cathode in a
microscopy) or multiple beam devices (like flat panel travelling wave tube (TWT) with a Spindt tip cathode
displays, or as light sources). The major field emission delivering the dc electron beam. However, the most
applications are listed below. effective way to reduce the size of a TWT is via direct
modulation of the e-beam, for example, in a triode
8.6.1 Field Emission Displays configuration using CNTs as the electron source.
Motorola in the early/mid 1990’s investigated the use of Thales, in collaboration with Cambridge University
carbon based materials for Field Emission Displays Engineering Department, have successfully demonstrated
including the use of diamond, DLC and CNTs a Class D (i.e. pulse mode/on-off) operation of a carbon
[206],[207]. More recently they have reported a CNT nanotube array cathode at 1.5 GHz, with an average
based Field Emission HDTV [208]. Over the last 10 current density of 1.3 A/cm2 and peak current density
years or so various companies including Philips, TECO of 12 A/cm2; these are compatible with travelling wave
Nanotech, ISE Electronics and especially Samsung tube amplifier requirements (>1A/cm2) [212]. Recently,
(SAIT) [209] have worked on the use of CNTs for TV they have also achieved 32 GHz direct modulation of a
applications. SAIT successfully produced demos of full carbon nanotube array cathode under Class A (i.e. sine
colour 39”diagonal TVs and this technology was wave) operation, with over 90% modulation depth [213].
transferred to Samsung SDI for production in the late Other advantages that carbon nanotube cathodes offer
2000s. However, no displays based on their technology
have reached the market.
Other work on Field Emission displays based on SEDs
(Surface-conduction Electron-emitter Displays) also took
place in the late 2000’s. Formerly a collaboration
between Toshiba and Canon, the displays utilise emission
from carbon but not CNTs [210], but these are yet to
appear commercially. Most recently, Sony have
announced a major investment in FEDs based on a
Spindt process. Teco Nanotech Co Ltd (a small company
based in Taiwan) also market three basic CNT-based
FEDs, the largest being an 8.9” diagonal [211].
Fig.14: (a) Schematics of the experimental setup with the cathode—grid
assembly (spacer thickness = 100 μm), the transparent and conductive
anode, and the 532 nm laser. (b) Top, left axis: emitted current as a
function of the applied voltage and absorbed optical powers. Bottom, right
axis: voltage drop ΔV as a function of the applied voltage and absorbed
optical powers. The dotted curve β = 465 is the Fowler-Nordheim fit for
the case where the p—i—n photodiode exhibits no voltage drop. (c)
Fig.13: Two stages of development of CNT FED at CEA. On the left: Photocurrent as a function of the absorbed optical power for a 2200 V
monochrome display with 350μm pixels, on the right: color video display cathode—grid applied voltage. The associated quantum efficiency
with 600μm pixels. On these display the non uniformity from pixel to calculated from the slope is ~10%. (d) Example of optical modulation of
pixel is 5% while it is 3% with LCD displays and 2% for CRT (courtesy the emission current from MWCNTs by pulsing the laser source at low
of CEA-LITEN). frequency (1 kHz) [215].
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include no heating requirement and the ability to turn 8.6.4 Backlighting
on or off instantly (for efficient operation). Xintek have
also been working on CNT-based microwave amplifiers Although their use in full colour FE-based TVs is still
for the US Air Force [214]. The main problem at present problematical, the use of CNTs as electron emitters in
is the limited modulation bandwidth associated with such FE-based backlight units for AMLCDs is still under
devices. However, Hudanski et al [215] reported the use investigation by various companies worldwide. Major
of photocathodes (shown in figure 14), which combine players in the TFT-LCD display industry, such as
semiconducting silicon p—i—n photodiodes with MWCNT Samsung, Corning and LG Electronics (LGE), are keen to
field emitters, which exhibited high quantum efficiency develop carbon-nanotube (CNT) backlight modules,
(~10%), significant current density (0.2 A cm−2) which with Taiwan-based backlight-module makers also
can be operated continuously or be optically modulated. interested in following suit [220]. In Korea Iljin also have
several years of experience in this area [221].
8.6.3 X-ray Instruments
In theory, CNT backlight modules have a lower
Oxford Instruments have worked together with NASA temperature, consume less power and are less expensive
on CNT-based X-ray sources that employ field emission to produce than traditional backlight modules. It is a good
as the electron source, rather than thermionic emission, candidate to eventually replace CCFL (cold cathode
which has much lower power efficiency [216]. Their fluorescent lamp) backlighting but has strong competition
application is targeted towards low-power use for a from LEDs, which could be much cheaper to produce
space mission to Mars (though high power would be and are already in the market place.
more preferable), once again because of their low
weight and fast response time. Oxford Instruments have The challenges are again to improve the lifetime of the
also developed and sold hand-held low power X-ray emitters and to reduce cost to be competitive with other
imagers which can be applied to medicine and for technologies.
diagnostics in circuit boards [217]. Zhou and co-workers
at Xintek (see figure 15) have developed a fast response, 8.6.5 Electron microscopy
sharp-focus X-ray tube with quick pulsation [218].
MoXtek have also produced similar devices [219]. Recent research has investigated whether the carbon
nanotube can act as an improved electron source for
Challenges for these devices are in achieving high power microscopy and how it compares to the other electron
with stability and reproducibility. sources available today. Various groups from FEI,
Cambridge University, EMPA, El-Mul etc have researched
the optimum way to produce CNTs for use in microscopy.
The CNTs act as a cold cathode source and the standard
manufacturing procedure is to add a single CNT to the tip
of a standard tungsten emitter. Growth, rather than
attachment is felt to be a better process [222]. Mann et
al. [223] used PECVD and describe how such a procedure
is scalable with the ability to grow a single CNT on each
W tip (shown in figure 16, see page 26). El Mul has
developed a silicon-based CNT microcathode in which
the CNT is grown in an etched pore [224].
The emission characteristics of the CNT have been found
to be extremely promising with the key parameters of
Fig.15: Left, the X-ray tube current versus the gate voltage measured with the process understood. Progress still needs to be made
the anode voltage fixed at 40 kV. It follows the classic Fowler—Nordheim to optimise reproducibility.
relation. The distance between the cathode and the gate is 150 μm. Right,
X-ray image of a normal mouse carcass (25 g) obtained using a CNT
source-based imaging system.
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8.6.6.2 Gauges/Sensors
The Physical Metrology Division, Korea Research
Institute of Standards and Science are using the field
emission effect from a carbon nanotube to characterize
a new type of technology for detecting low pressures. The
fabricated low pressure sensor is of a triode type,
consisting of a cathode (carbon nanotubes field emitter
Fig.16: Left, electron micrograph of a single CNT grown on a tungsten
tip. Note that the growth is aligned with the tungsten axis. Centre, a
arrays), a grid and a collector. Due to the excellent field
tungsten tip mounted in a suppressor module. Right, a CNT grown on emission characteristics of CNTs, it is possible to make a
a tungsten already mounted in the suppressor in situ. cost effective cold cathode type ionization gauge. For an
effective CNT cathode for both the sensor and gauge the
8.6.6 Ionization for Propulsion and Detection
researchers used the screen-printing method and also
controlled the collector and the grid potentials in order
8.6.6.1 Electric Propulsion
to obtain a high ionization current. They found that the
ratio of the ionization current to the CNT cathode current
Replacing hollow and filament cathodes with field
changes according to the pressure in the chamber [230].
emission (FE) cathodes could significantly improve the
scalability, power, and performance of some meso- and
8.6.6.3 Miniaturised gas ionization sensors using
microscale Electric Propulsion (EP) systems. There is
carbon nanotubes
considerable interest now in microscale spacecraft to
support robotic exploration of the solar system and
Ajayan et al. from the Rensselaer Polytechnic Institute have
characterize the near-Earth environment. The challenge
developed Ionization sensors by fingerprinting the
is to arrive at a working, miniature electric propulsion
ionization characteristics of distinct gases [231]. They
system which can operate at much lower power levels
report the fabrication and successful testing of ionization
than conventional electric propulsion hardware, and
microsensors featuring the electrical breakdown of a range
meets the unique mass, power, and size requirements of
of gases and gas mixtures at carbon nanotube tips. The
a microscale spacecraft.
sharp tips of the nanotubes generate very high electric
fields at relatively low voltages, lowering breakdown
Busek Company, Inc. (Natick, MA), has developed field
voltages several-fold in comparison to traditional electrodes,
emission cathodes (FECs) based on carbon nanotubes.
and thereby enabling compact, battery-powered and safe
The non-thermionic devices have onset voltages about
operation of such sensors.
an order of magnitude lower than devices that rely on
diamond or diamond-like carbon films.
The sensors show good sensitivity and selectivity, and are
unaffected by extraneous factors such as temperature,
Worcester Polytechnic Institute (WPI) includes the
humidity, and gas flow. As such, the devices offer several
programmes headed by Professors Blandino and
practical advantages over previously reported nanotube
Gatsonis. Blandino’s research is largely focused on the
sensor systems. The simple, low-cost, sensors described
study of colloid thrusters for small satellite propulsion,
here could be deployed for a variety of applications, such as
and in the development of novel, earth-orbiting
environmental monitoring, sensing in chemical processing
spacecraft formations [226]. The Gatsonis’ activity also
plants, and gas detection for counter-terrorism.
includes modelling of plasma micropropulsion [227].
McLaughlin and Maguire [232] at the University of Ulster
Groups from the Rutherford Appleton Laboratory [228] report the use of CNTs in order to decrease the turn-on
and Brunel University [229]are studying the field emission voltage associated with microplasmas and the
performance of macroscopically gated multi-walled enhancement of emission spectra associated with gas
carbon nanotubes for a spacecraft neutralizer. types. In particular the device focuses on mixed gas types
such as breath analysis and environmental monitoring. The
ability of low cost CNT structured electrodes is key to
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improving performances related to higher sensitivity and In theory the NRAM chip would replace two kinds of
specificity of gases such as NOx. Catalyst free growth memory. While cell phones, for example, use both flash
techniques have been reported using thermal CVD routes chips and SRAM or DRAM chips, NRAM would
and the study is also looking at the optimum CNT spacing perform both functions. However the memory market
and height required for short time ionisation or FE is oversupplied and they frequently have to be sold at a
applications to gas sensors. loss, making it difficult for any new technology to break
in. In addition, several other major companies are
The main driver at present is to improve the efficiency developing their own non-volatile memory technologies
which currently lies at around 1%. with PRAM perhaps the leading contender at present.
European Position: From a display viewpoint Europe PRAM, FRAM, MRAM and RRAM are all dominated by
were very much forerunners but then Samsung highly competitive, large companies. With Nantero’s
provided the more recent display drive. As regards relatively small size and long development time, market
work on sources for electron microscopy in Europe De penetration is a big issue.
Jonge and co-workers did some excellent work on
characterisation of single emitters as did Groning on CNT-NEMS have been suggested for high frequency
arrays of emitters. Thales in collaboration with several mechanical resonators and switches, where the low mass
universities has continued European interest in the and high stiffness can lead to GHz switching speeds and
design of high frequency CNT based sources. For X-ray resonance frequency. High frequency resonators are
sources Oxford Instruments led the way and more targeting RF electronics [234] and high-sensitivity mass
recently Xintek in the US and Philips in Europe has sensing [236]. Here the advantages provided by the
expanded the work. In backlighting as in Displays the outstanding material properties of CNT are possibly
Far East leads the way. The leaders in the FE based outweighed by the numerous technological and
propulsion area are in the US where the Jet Propulsion manufacturing challenges including controlled growth,
Laboratory Pasadena, Busek Co., Inc. and the durability of switches and scaled up production with a
Worcester Polytechnic Institute (WPI) lead the way. In sufficient repeatability and yield [237].
Europe the main groups are from the Rutherford
Appleton Laboratory, Brunel University, the University
of Cambridge, the University of Groningen, and the
University of Ulster. However, with the exception of
display technology, the field emission market size is
comparatively low when compared with the other ICT
applications described above.
8.7 NEMS switches, resonators and sensors
Jang et al [233] demonstrated novel non volatile and
volatile memory devices based on vertically aligned Fig. 17: Left, a schematic diagram showing a cross-section of a switch
fabricated by Jang et al.. Both contacts and catalyst were deposited with
MWCNTs (see figure 17). Nanoelectromechanical
e-beam lithography. Right, an electron micrograph showing the grown
switches with vertically and horizontally [234] aligned CNTs acting as a switch.
carbon nanotubes have been demonstrated. However,
Nantero are the market leaders in this area and have A significant effort has also been focused on CNT for
created multiple prototype devices, including an array of piezoresisitve applications, the advantage being a much
ten billion suspended nanotube junctions on a single larger gauge factor as compared to for instance silicon.
silicon wafer [235]. Nantero's design for NRAM™ While high-performance, high yield CNT based strain
involves the use of suspended nanotube junctions as gauges have been realised with wafer-level on-chip
memory bits, with the "up" position representing bit synthesis [238], the gauge factor’s dependence on
zero and the "down" position representing bit one. Bits chirality remains a very significant challenge in terms of
are switched between states through the application of commercialisation.
electrical fields.
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CNTs as high aspect ratio and supersharp tips for
Atomic Force Microscope remain a possibility; the high
stiffness and yield strength of MWNT allows more
slender tips to be used for probing deep trenches and
sidewalls [239], SWNT nanotubes present unparalled
sharpness and wear resistance [240], and bundles of
SWNT show enhanced surface potential imaging [241].
CNT-HARP tips are either manufactured by growth Fig.18: Left, Experimental setup of the Er/Yb:glass laser. OC: output
[242] or direct manipulation [239],[240]. Recently, DTU, coupler; M1-M4: standard Bragg-mirrors; CNT-SAM: Saturable
absorber mirror based on carbon nanotubes; LD: pigtailed laser diode
the University of Cambridge and Oldenburg University for pumping the Er/Yb:glass (QX/Er, Kigre Inc., 4.8 mm path-length).
demonstrated multiple assemblies of MWNT on AFM Right, background-free autocorrelation. The solid line is a sech2 fit with
probes [243] using a microgripper-based approach to a corresponding FWHM pulse-duration of 68 fs [255].
automated robotic assembly [244]. A comprehensive
The major laser systems mode-locked by CNT saturable
overview is given by Wilson and Macpherson [240].
absorbers demonstrated so far (see figure 18) include
fibre lasers, waveguide lasers and solid-state lasers,
European Position: Nantero are the world leaders
generating sub-ps pulses in a spectral range between
but in Europe, one can cite the partners of the NanoRF
1070 and 1600 nm [255]. The shortest pulse of about
[245] European project as well as ETH, TU Denmark. A
68 fs was achieved with a solid state Er3+ glass laser by
collaboration between Cambridge Univ. Engineering,
using a CNT-polyimide composite [256]. Additionally,
Samsung and Thales is also ongoing.
amplified spontaneous emission noise suppression has
been demonstrated with CNT-based saturable
8.8 Saturable Absorbers
absorbers, showing great promise for this technology
for multi-channel, all-optical signal regeneration in fibre
The band gap of semiconducting CNTs depends on their
telecom systems [257].
diameter and chirality, i.e. the twist angle along the tube
axis[246]. Thus, by tuning the nanotube diameter it is easy
Challenges include justifying the research to industry
to provide optical absorption over a broad spectral range
due to the limited market potential.
[247]. Single-walled CNTs exhibit strong saturable absorption
nonlinearities, i.e. they become transparent under sufficiently
European Position: There are 5 major research groups
intense light and can be used for various photonic working on CNT saturable absorber applications around
applications e.g in switches, routers and to regenerate optical the world: Sakakibara at the National Institute for
signals, or form ultra-short laser pulses[248],[249],[250]. It is Advanced Industrial Science and Technology (AIST),
possible to achieve strong saturable absorption with CNTs Tsukuba, Japan, Maruyama and Yamashita at Tokyo
over a very broad spectral range (between 900 and 3000 nm University & Set in the Alnair Labs and Yoshida at Tohoku
[251]). CNTs also have sub-picosecond relaxation times and University. In Europe Dr. E. Obraztsova at the Institute for
are thus ideal for ultrafast photonics [252],[253]. CNT General Physics, Moscow, and Cambridge University
saturable absorbers can be produced by cheap wet Engineering Department are the major players.
chemistry and can be easily integrated into polymer
photonic systems. This makes a CNT-based saturable 8.9 Fuel Cells
absorber very attractive when compared to existing
technology, which utilises multiple quantum wells (MQW) Carbon nanotubes can be used to replace the porous carbon
semiconductor saturable absorbers and requires costly in electrode-bipolar plates in proton exchange membrane fuel
and complicated molecular beam epitaxial growth of cells, which are usually made of metal or graphite/carbon
multiple quantum wells plus a post-growth ion black. The CNTs increase the conductivity and surface area
implantation to reduce relaxation times[254]. Additionally, of the electrodes which means that the amount of platinum
MQW saturable absorbers can operate only between 800 catalyst required can be reduced [258].The state of the art in
and 2000 nm -a much narrower absorption bandwidth this area is the mixing of CNTs and platinum catalyst particles
than that available using CNTs. reported by Sun et al of Taiwan[259].
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Whilst CNTs reduce the amount of platinum required, it CNTs have also been used as a high surface area charge
is only a small percentage, which means that the cost of collecting scaffold for nanoparticles in several types of cells.
the fuel cell remains high. Also, CNTs are comparable in Photoconversion efficiencies of 1.5% and 1.3% have been
price to gold, meaning the saving is minimal. achieved with SWCNTs deposited in combination with light
harvesting CdS quantum dots and porphyrins, respectively
Nevertheless, vertically aligned MWCNTs can be used [264]. Other varieties of semiconductor particles including
as highly efficient fuel cell electrode material. Aligned CdSe and CdTe can induce charge-transfer processes under
CNTs electrode have a host of advantages in the visible light irradiation when attached to CNTs [265]. In
Polymer Electrolyte Membrane fuel cells (PEMFC) and dye-sensitized solar cells (DSSC), titanium dioxide coated
Direct Methanol fuel cells (DMFC), such as higher onto CNTs shows enhanced electron transport and
electrical conductivity, large surface area, possible higher increases the photoconversion efficiency [266].
gas permeability and higher hydrophobic surfaces
facilitating faster removal of water from the electrodes. Much work has been done to use SWCNT thin films in
In general, Pt, Pt/Ru nanoparticles are dispersed in PV as transparent conductive coatings to replace ITO
CNTs to obtain platinum/CNT-based electrocatalysts which has both a limited supply and a limited tolerance
[260]. However the large-scale market application of fuel to flexibility. Conductivity to transparency ratios are fast
cells will be difficult to realize if the expensive Pt-based approaching that of ITO. Barriers to commercialisation
electrocatalysts for oxygen reduction reactions (ORRs) are now more related to problems with the adhesion
cannot be replaced by other efficient, low-cost, and of the CNT film to the substrate.
stable electrodes. Recent results from Gong et al. [261]
have shown that N doped arrays of MWCNTs (acting Novel antennae effects, as well as improved charge
as a metal-free electrode), can provide superb catalytic collection and optical enhancement can be obtained in
activity for Oxygen Reduction Reaction (ORR). cells in which CNT growth is patterned. Zhou et al. [267]
recently demonstrated enhancement in amorphous
European Position: The leaders in this area are the silicon solar cells deposited onto a patterned array of
Taiwanese groups. The European leaders are linked to S. CNTs with spacing of the order of visible light
Roth (Max Planck Institute, Stuttgart). wavelengths.
8.10 Solar panels European Position: Although much of the work in
this area has been driven by Japan and the USA (with
CNTs have been utilised in solar cells in a number of Unidym Inc. which announced in February 2010 a joint
ways. Primarily they are used to enhance charge venture to market printable CNT electronics in Korea),
collection. Kymakis et al [262] dispersed CNTs in the significant input on both the incorporation of the CNTs
photoactive layer of organic solar cells to replace C60 as part of the active layer and in transparent contact
and benefit from the 1D structure. However, the power materials has been made across the European
efficiency of the devices remains low at 0.04% community. Commercial ventures are in place in both
suggesting incomplete exciton dissociation at low CNT the USA and Finland to commercialise CNT thin films.
concentrations. At higher concentrations, the CNTs
short-circuit the device. More recently, a polymer 8.11 Antennae
photovoltaic device from C60-modified SWCNTs and
P3HT has been fabricated [263]. P3HT, a conjugated Ren’ group at Boston College has demonstrated the use
polymer was added resulting in a power conversion of a single multi-walled CNT to act as an optical
efficiency of 0.57% under simulated solar irradiation antenna, whose response is fully consistent with
(95mWcm-2). An improved short circuit current density conventional radio antenna theory [268]. The antenna
was attributed to the addition of SWCNTs to the has a cylindrically symmetric radiation pattern and is
composite causing faster electron transport via the characterized by a multi-lobe pattern, which is most
network of SWCNTs. Hybrid CNT-polymer devices pronounced in the specular direction. Possible
however have shown so far only moderate applications for optical antennae include optical
performance. switching, power conversion and light transmission.
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One particular application is the “rectenna”, which is the emitters to stimulate phosphors has been reported by
light analogue of the crystal radio in which an antenna is various groups and the replacement of metallic filaments
attached to an ultrafast diode. This could lead to a new with carbon CNTs/Fibres has been investigated by
class of light demodulators for optoelectronic circuits, or groups mostly in China. Carbon nanotube bulbs made
to a new generation of highly efficient solar cells. from CNT strands and films have been fabricated and
their luminescent properties, including the lighting
The growth of microwave applications, such as mobile efficiency, voltage-current relation and thermal stability
phones, remote sensing and global navigation satellite have been investigated. The results show that a CNT
systems, etc, requires the development of materials with bulb has a comparable spectrum of visible light to a
a large tunability and very low loss in the microwave tungsten bulb and its average efficiency is 40% higher
frequency range (from GHz to sub-THz). This is than that of a tungsten filament at the same temperature
particularly important, as the intrinsic loss of the existing (1400―2300K) [269]. The nanotube filaments show
tunable dielectrics based on ferroelectric ceramics both resistance and thermal stability over a large
increases significantly when the frequency in use is above temperature region. No obvious damage was found on
a few GHz. Liquid crystals (LCs) have attracted much a nanotube bulb held at 2300 K for more than 24 hours
attention in recent years because of their low loss at in vacuum, but the cost needs to be significantly reduced
microwave frequencies. However, although the dielectric and the lifetime significantly increased for this to be
anisotropy/tenability of LCs is comparatively higher than considered seriously as an option.
other low loss materials, it is desirable to further increase
its value. This is particularly important for antennae and European Position: Most effort is located in the Far
phase arrays for beam steering in the applications such as East but Bonnard et al at EPFL have also contributed in
automobile forward looking radar and satellite up-links. this area.
At Cambridge, preliminary work shows that mixing CNTs
of suitable sizes with LCs can significantly enhance 8.13 Nanofluidics
dielectric anisotropy (see Fig. 19). Further measurement
confirms similar trend at much higher frequency ranges. The interest in taking advantage of the unique
properties of carbon CNTs in nanofluidic devices has
European Position: Early stages of research in the increased tremendously over the last couple of years.
Department of Engineering at Cambridge University, in The CNTs can either be used directly as a nanofluidic
collaboration with Technische Universität Darmstadt, channel in order to achieve extremely small and smooth
Queen Mary College, London, ALPS Electric, Japan, pores with enhanced flow properties [270] or be
Dow Corning, USA, and Nokia, Finland/UK. embedded into existing fluidic channels to take
advantage of their hydrophobic sorbent properties and
high surface-to-volume ratio for improving chemical
separation systems [271], [272].
By integrating vertically aligned CNTs into silicon nitride
[273] and polymer membranes [274],[275] respectively,
it has been possible to study the flow of liquids and
gases through the core of carbon nanotubes. The flow
rates were enhanced by several orders of magnitude,
Fig 19. Studies of pure and CNT loaded E7 using capacitor method,
switching with electric field: (a) 3GHz, sweeping from ε⊥ to ε||; (b)
compared to what would be expected from continuum
3GHz, sweeping from tanδ⊥ to tanδ||; (c) Δε in 1-4GHz range, switching hydrodynamic theory [266]. The reason for this is
with 0.5V/μm. believed to be due to the hydrophobic nature of the
inner carbon nanotube sidewall, together with the high
8.12 Lighting smoothness, which results in a weak interaction with
the water molecules, thereby enabling nearly frictionless
There is ongoing work on the use of CNTs for low flow through the core of the tubes. This effect
energy lighting applications. The use of CNTs as electron resembles transport through transmembrane protein
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pores, such as aquaporins, where water molecules line surface to avoid aggregation and to benefit from the high
up in a single file with very little interaction with the uniformity of the nanostructures.
sidewall.
European Position: Montena Components of
This application of CNTs is envisioned to result in novel Switzerland are in competition with Maxwell Technologies
ultrafiltration and size-based exclusion separation devices, in this area. DTU is also very active in the field.
since the pore sizes approach the size of ion channels in
cells [266]. The CNT membranes are, however, fabricated 8.14 Liquid crystal microlenses
by CVD and this application suffers from the lack of large
scale cost-effective CNT deposition equipment. Liquid crystals (LCs) are potentially a very exciting
technology for creating a real-time holographic three
In the last couple of years CNTs have also been dimensional (3D) display system. For the reproduction
investigated as a sorbent material for improving both the of a full 3D image, a fully complex hologram is the
resolution and sensitivity of chemical separations [271], ultimate solution, but it is very difficult to display using
[272]. This has been done by incorporating the nanotubes current technology such as Liquid Crystal (LC) over
in the stationary phase of mainly gas chromatography silicon (LCOS), as shown by the simple image in Figure
columns to take advantage of their high surface-to-volume 20(c) projected using a binary phase only LCOS device.
ratio and better thermal and mechanical stability
compared to organic phases, which make them ideal for A purely phase only hologram (or kinoform) is the best
especially temperature programmed separations for building 3D displays, however there are limits due to
[271],[272],[276]. The carbon nanotubes, in the form of the way in which a liquid crystal can be used to
powder, are hard to pack directly in columns due to their modulate a phase only hologram. A traditional pixel has
tendency for aggregation and hence channel blockage a top and bottom planar electrode creating a uniform
[271],[272], [277] so the CNTs have typically either been electric field. The device pixel as shown in Figure 20(a)
incorporated in a monolithic column [278] immobilized has a CNT in the centre which creates a non-uniform
on the inner channel wall [279] or deposited on the electric field profile [285]. This changes the way in which
surface of beads that subsequently were packed [280]. the LC switches and responds to the field.
A major complication of these methods, apart from the
fact that they are manual and very labour-intensive, is that
they rely on the necessity of forming uniform CNT
suspensions, which is difficult, since CNTs are insoluble
in aqueous solutions and most organic solvents [271]. It is
therefore typically required to either dynamically or
covalently modify the CNTs to avoid aggregation [272]. Fig 20. (a) A nanotube electrodes in a liquid crystal cell with an external
These problems can be overcome by direct growth of the fields applied. (b) Individual LC electrodes (top left are 1um pitch), (c) 3D
projected hologram image.
CNTs on a surface, in e.g. microfluidic channels
[281],[282],[283], so they are anchored to the channel
wall and therefore unable to form aggregates. This also Applications such as 3D holography require large densities
allows a much higher CNT concentration without of very fine pixels[286]. Current LCOS devices are limited
clogging the fluidic devices. Growing of CNTs in to pixels pitches of around 5um before alignment and
microfluidic systems has the additional benefit that fringing fields become a problem. Figure 20(b) shows a
lithography can be used for the pattern definition, which CNT electrode device fabricated at Cambridge with a
should make it possible to make much more uniform and pixel pitch of 1 μm. Due to the non-uniform field profile,
therefore more efficient columns [284]. the LC material clearly switches as single pixels. The
device as shown only switches as a single array of
A major limitation of this application is also the lack of electrodes and to make a hologram we need them to be
low cost CNT deposition equipment, since it is necessary individually addressed, hence we need an active backplane
to use vertically aligned CNTs that are attached to the such as that found in LCOS to grow them upon [287].
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European Position: The Engineering Department in 6000 S/m and a subthreshold swing of ~70 mV/decade
Cambridge as far as we know, are the only group in respectively [295]. Table 2 summarizes the work it this
Europe working in this area. area.
Table 2: A summary of the optimum properties obtained from single electron transistors
8.15 Transistors
8.15.1 Individual CNT-Based
Transistors
Arguably this has been the electronic
application on which most research has
focused. Martel et al. and Tans et al.
first reported a bottom gate individual
single walled carbon nanotube field
effect transistor (SWNT-FETs) with an
on/off ratio of ~105 and a mobility of
20 cm2/Vs in 1998 [288], [289].
Afterwards, Durkop et al. claimed a
mobility for bottom-gate SWNT-FET of
>105 cm2/Vs with a subthreshold swing ~100 mV/decade Several groups have also investigated vertical CNT-FETs
[290]. This mobility is still the highest reported for (wrap-around gate). Choi et al. reported the first vertical
bottom gate CNT-FETs thus far. Meanwhile, top gate MWNT-FET with a best conductance of 50 mS in 2003[296]
SWNT-FETs were also attracting attention since such a but this only works at low temperatures. Maschmann et al.
structure can be readily used for logic circuits. In 2002, demonstrate a vertical SWNT-FET in 2006 [297]. Their
Wind et al. first demonstrated a top gate SWNT-FET devices exhibited a good ohmic SWNT-metal contact, but
with an on/off ratio of ~106, a transconductance of the gate effect is not as efficient as either the top gate or
2300 S/m and a subthreshold swing of 130 mV/decade bottom gate SWNT-FETs. SWNT-FETs always exhibit p-type
[291]. Rosenblatt et al. and Minot et al. [293] using NaCl operation when contacted ohmically, but n-type SWNT-FETs
and KCl solutions as the top gate in SWNT-FETs are also needed for fabrication of logic circuits. Derycke et
showed a mobility of 1500 cm2/Vs, a subthreshold al. claimed both annealing (removal of oxygen) and doping
swing of ~80 mV/decade and an on/off ratio of 105. (e.g. potassium) can convert a p-type SWNT-FET into a
Yang et al. [294] showed a very high transconductance n-type and a logic inverter was demonstrated [298],[299].
of 1000 S/m in a top gate device (shown in figure 21, Javey et al. and Chen et al. reported that using different
together with a bottom gate device). Javey et al. also metal electrodes (e.g. Al) they could also obtain n-type
demonstrated high performance SWNT-FETs using high-k SWNT-FETs with a ring oscillator also fabricated
dielectric ZrO2 as the top gate insulator. Devices exhibited [300],[301].
a mobility of 3,000 cm2/Vs, a transconductance of
Challenges for the future
include controlling the chirality
and diameter, improving the
yield of working devices,
improving the reproducibility
of the contact, ensuring all
CNTs are semiconducting,
improving the uniformity of
the devices, controlling their
Fig. 21: Typical SWNT-FET transistor characteristics made at the University of Cambridge with different positioning, and developing a
contacts. Left, Pd makes and ohmic contact which results in p-type conduction. Centre, Ti contacts result in process that can be scaled up
strong ambipolar behaviour. Right, Al makes a Schottky contact which results in n-type conduction but with a to mass-production.
strong leakage current.
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European Position: The state-of-the-art transistors for on-off ratio and mobility (see figure 22). The group
(dependent on characteristics) are those produced by in Grenoble have also investigated this and have made a
the groups of Avouris at IBM and in Europe, Bourgoin small chip incorporating 75 such transistors.
at CEA, Saclay, Ecole Polytechnique and Dekker at Delft
University of Technology. The interest in the networks comes from the fact that
if the average nanotube length is small compared to the
8.15.2 Network CNTs distance between source and drain and more than one
tube is needed to make the connection, the probability
In order to overcome the various problems with individual of having an electrical path made only of metallic tubes
CNT transistors, numerous groups have concentrated on is ~(1/3)n where n is the number of tubes needed to
the production of transistors manufactured from CNT make the junction. Secondly, the on/off ratio increases
networks or even CNT/Polymer mixtures. since, even if two tubes are metallic, their contact is not
metallic. [306] Finally, even a single defect is enough to
In 2002, the first report (a patent) for transistors based open a bandgap in a metallic tube, turning it into a
on random networks of nanotubes and their use in semiconductor. [307] This means that controlling the
chemical sensors was produced by Nanomix Inc., [302] number of defects is an important challenge to
followed in 2003 by the disclosure of their integration overcome.
onto a 100 mm Si wafer. [303] The first public disclosure
was made in 2003 by Snow et al. [304] who The first transparent CNT based transistor made on a
demonstrated a SWNT thin film transistor with a flexible substrate was achieved by transferring a CNT
mobility of >10 cm2/Vs and a subthreshold swing of random network from a silicon substrate onto a
250mV/decade with an on/off ratio of 10. In 2007, polyimine polymer [308].
Kang et al. grew highly dense, perfectly aligned SWNT
arrays on a quartz substrate which were then 8.15.3 High frequency nanotube transistors
transferred to a flexible plastic substrate (PET). The
SWNT-FETs were fabricated on the PET substrate and The high carrier mobility of CNTs makes them potential
exhibited a mobility of 1000 cm2/Vs and a candidates for high frequency transistors. It was shown
transconductance of 3000 S/m [305] . The Rogers in 2004 that optimized carbon nanotube field effect
group has exhibited state-of-the-art network transistors transistors (CNTFETs) would have cut-off frequencies fT
above those of FETs built from any other semiconductor
[309]. For aggressively scaled devices working in the
ballistic regime, the intrinsic fT could reach the THz
range. These first projections were confirmed by
detailed calculations from several groups worldwide
[310], [311].
From an experimental point of view, measuring the high
frequency performances of CNTFETs is very challenging.
Fig. 22: (a) Transfer curves from a transistor that uses aligned arrays of
Indeed CNTs, when considered individually, have very
SWNTs transferred from a quartz growth substrate to a doped silicon high impedance (RON > 6.5 kΩ) whilst conventional high
substrate with a bilayer dielectric of epoxy (150 nm)/SiO2 (100 nm). frequency equipment is adapted for the 50Ω
The data correspond to measurements on the device before (open measurement range. In addition, due to the small size of
triangles) and after (open circles) an electrical breakdown process that
eliminates metallic transport pathways from source to drain. This process the CNT, parasitic contributions from the device
improves the on/off ratio by a factor of more than 10,000. (b) Optical structure tend to dominate the intrinsic contribution of
(inset) and SEM images of a transistor that uses interdigitated source and the CNT.
drain electrodes, in a bottom gate configuration with a gate dielectric of
HfO2 (10 nm) on a substrate and gate of Si. The width and length of the
channel are 93 mm and 10 μm, respectively. The box indicated by the To circumvent these problems, several groups proposed
dashed blue lines in the optical image inset delineates the region shown the use of mixing techniques and obtained indirect
in the SEM image [39]. Figure reproduced with permission from the indications of the high frequency operation of CNTFETs.
American Chemical Society.
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For example, IBM reached 580 MHz in 2004 [312], then [323], [324]. Most importantly, CNT-based network
Cornell University and Northrop Grumman transistors can be made compatible with printing
Corporation respectively reached 50 GHz [313] and technologies [324].
23 GHz [314]. Direct measurements of the HF
operation of a single nanotube CNTFETs are scarce. European Position: This is a small research field and
The most convincing result was obtained at ENS most of the activity is located in the USA (Univ. Illinois,
Paris in 2008 [315]. From direct measurements of gm Stanford, IBM). Main academic players in Europe are
and Cg up to 1.6 GHz, they obtained fT~50 GHz for CEA and IEM & Delft University. From an industrial
a 300 nm long channel. A convenient and powerful point of view one can cite Brewer Science (for the ink
way of directly measuring the full S-parameters formulation and deposition process) as well as
matrix of CNT based devices (which is a critical Northrop Grumman for circuitry developments.
requirement from a circuit design point of view),
consists in studying multiple nanotubes in parallel, thus 8.16 Hydrogen storage
reducing both the device impedance and the relative
impact of parasitics. This strategy was used by different CNTs have been suggested as potential candidates for
groups [316], [317], [318] [319], [320], [321], [322] to hydrogen storage. However, the reported hydrogen
demonstrate fT above 10 GHz. In particular, the uptake varies significantly from group to group, with
collaboration of two French groups from IEMN and CEA the mechanism not clearly understood. Current
achieved an intrinsic fT of 30 GHz in 2007 and 80 GHz in methods involve compressing the CNTs into pellets
2009. Interestingly, the latter result was only made possible which are then subjected to hydrogen at high
through the use of a high quality nanotube source from pressure. The target set by the US Department of
Northwestern University containing 99% semiconducting Energy is 6% by weight hydrogen by 2010. Whilst
nanotubes. The important results from the Roger’s group most groups have found hydrogen uptake to be in the
also originate from recent progress at the material level 1-2% region [325], amongst the highest reported are
(CVD growth of aligned CNTs in their case). Gundish et al. [326] at 3.7% and Dai’s group [327] at
5.1%. It should be noted that Hirschler and Roth
European Position: Rogers in the USA produces the found most reported values to be false [328], for
state of the art thin Film transistors and in Europe, apart example due to Ti take up during sonication.
from some preliminary work in Universities little seems
to be happening. From a more fundamental point of view, the average
adsorption energy of hydrogen on CNTs is not
8.15.4 High frequency flexible electronics significantly different from its value on amorphous
carbon. It is mainly the surface area which plays a
Even if they can reach high operating frequencies, the crucial role; e.g. 5.8% was achieved a long time ago
use of CNTFETs in conventional integrated circuits on super-high surface area activated carbon, a
remains unlikely in the near future. Indeed, the potential significantly cheaper material when compared to
gains in performances when compared with
CNTs. [329] Also, because the bond of hydrogen with
conventional semiconductors do not compensate for
silicon is weaker than that of carbon, it is much easier
the immense efforts required at the material level to
to get hydrogen out again.
solve the issues of selective placement and nanotube
variability. Conversely, when carbon nanotubes are
European Position: Over the last 10 or so years
compared with organic materials in the field of flexible
there have been numerous groups worldwide
electronics, the potential gains in performances are
working in this area; especially in the USA, Japan and
huge. Indeed, the low carrier mobility in organics
China. Europe too has made a significant investment,
(typically in the 10-3-10 cm2/Vs range) prevents their
notably through groups in Germany, France, Greece
use at very high frequencies. Flexible electronics with
CNTs has been studied since but only very recently at high (theoretical work) and the UK but still the DoE 6%
frequency. The first results are already very promising with target remains elusive. It is generally accepted that
operating frequencies in or close to the GHz range [316], absorptions of about 1% are practical [330]. Other
Cont. page 35
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From page 34
technologies involving other compounds such as 8.17.2 Quantum computing
carbohydrates are expected to be used instead.
Arrays of qubits have been created in the form of
8.17 Quantum computing endohedral fullerenes in SWNTs, to make so-called
peapods [337]. These structures have been modelled
8.17.1 Spintronics [338] and imaged [339]. The interactions between the
spins have been characterized by electron paramagnetic
Spin transport has been demonstrated over lengths of resonance, showing transitions from exchange
hundreds of nanometers in CNTs [331], and the limit may narrowing to spin-spin dephasing [340]. Theoretical
be much longer. The Kondo effect has been architectures have been developed for global control of
demonstrated [332], and Fano resonances have been qubits [341], [342]. The spin properties of N@C60 (or
found [333]. Spin blockade has been demonstrated in atomic hydrogen inside a C60) have been shown to make
double dot structures [334],[335]. With the development it one of the strongest candidates for condensed matter
of aberration corrected transmission electron quantum computing [343], [344]. Y@C82 can also be used
and typical relaxation and coherence times are shown in
microscopy at low voltage (80 kV), which minimises
figure 24. Quantum memories have been
knock-on damage, it has become possible to image the
demonstrated, in which information in the electron spin
actual piece of active material in a device [336], as shown
is transferred to the nuclear spin, and subsequently
in figure 23.
retrieved, with gate operation times of order 10 ns and
storage times in excess of 50 ms. The theoretical limit
for such memories is limited by twice the electron spin
flip time [345], and since this can exceed one second the
prospects are excellent. Entangled spins offer further
Fig. 23: A 20,3 chirality SWNT, observed in transmission electron
microscopy (aberration-corrected JEOL 2200MCO operating at 80 kV,
image courtesy of Dr Jamie Warner). Below the micrograph is an atomic
model to the left and an image simulation to the right, with a small Fig. 24: Y@C82 relaxation and coherence times as a function of
overlap also shown. temperature in deuterated toluene (circle red, T1 closed, T2 open) and
deuterated o-terphenyl (symbol black, T1 star, T2 cross) [347]. Insert:
Structural representation of a) d-toluene and b) o-terphenyl.
Problems to overcome include the production of
uniform, defect-free SWNTs, free from paramagnetic possibilities for other quantum technologies, such as
impurities, with a single chiral index, and fabrication of metrology and sensors [346].
reproducible devices with uniform contacts.
Problems to overcome include the development of the
European Position: Hitachi Cambridge Laboratory, technology for single spin read out in CNTs and the
and the Cavendish Laboratory Mark Buitelaar, in demonstration of entanglement using peapods.
collaboration with Andrew Briggs at Oxford are the
leaders in the field; Others include Delft (Leo European Position: Oxford leads the world in
Kouwenhoven) and the Niels Bohr Institute, peapods for quantum computing, in collaboration with
Copenhagen. Princeton (Steve Lyon), Nottingham (Andrei
Khlobystov), Cambridge (Charles Smith), EPFL (Laszlo
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Forro) and Peking (Lianmao Peng), There is also activity [358], [359], [360]. Single-electron memory operation,
in Berlin (Wolfgang Harneit), at L. Néel Institute in with charge storage on an Au nanoparticle and sensing
Grenoble and at CEMES-CNRS in Toulouse [348]. using a CNT FET, has now been demonstrated [361]. In
recent work, QDs have been induced along a SWCNT
8.17.3 CNT Single Electron Transistors wrapped with single-stranded DNA [362]. Furthermore,
a nanoscale resonator has recently been demonstrated
Single electron transistors (SETs) use the ‘Coulomb using a suspended CNT, where a single electron added
blockade’ effect to control charge at the one-electron to the CNT can be detected in a shift in the resonant
level, on a nanoscale conducting ‘island’ isolated by tunnel frequency [363].
barriers from source and drain electrodes. In these
devices, the total island capacitance C is small enough European Position: Considerable progress has been
such that the single-electron charging energy made in CNT based single-electron systems, in the USA,
Ec= e2/2C >>kBT at the measurement temperature T. A Japan and Europe. In particular, Europe is very strong in
very low current ‘Coulomb blockade’ region exists fundamental physics investigations in these systems, with a
around zero bias voltage in the Ids-Vds characteristics, number of novel device demonstrations. Early work on
where the charging energy prevents current flow. As the room-temperature CNT SETs has also occurred in Europe.
applied bias overcomes integer multiples of Ec, electrons
are added one by one to the island. In a SET, an additional 9. Conclusions
gate electrode is used to add/remove electrons from the
island. The Ids-Vgs characteristics show periodic single-electron CNTs have many unique and indeed useful properties
conductance oscillations, where each oscillation for applications in the ICT area. Research into CNTs at
corresponds to the addition of an electron. If C ~ 10-18 F or the university level will continue for at least the next
smaller, single-electron effects can occur at room several years especially into quantum effects and
temperature, raising the possibility of single-electron associated behaviour, as well-characterized, high-quality
memory and logic applications. For islands small enough such SWCNTs become more available. Although CNTs are
that the quantum confinement energy is also significant, the still being touted for various industrial applications,
much more investment is necessary for them to reach
device forms a quantum dot (QD), sometimes referred to
commercial viability. The USA and Japan lead in this
as an artificial atom. A combination of quantum confinement
development but Europe has made significant impact in
and single-electron charging effects are then observed. If the
many areas despite the fact that investment in Europe is
island in a SET is formed by a section of a SWCNT, then
but a fraction of that in the other major high-tech
electrons are confined in the axial as well as the
industrial zones. Consequently, partnership between
circumference direction.
higher education and industry could form the basis of
research in this enormous and diverse area for many
SET operation at cryogenic temperatures, with
years to come so that at least some of the many
conductance oscillations in the CNT Ids-Vgs characteristics,
applications of CNTs can be realised.
was demonstrated in the late 1990s in a single SWCNT
[349] and in ropes of SWCNTs [350]. More recently, SET
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[358] Sapmaz et al. Nano Lett. 6, 1350 (2006).
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[360] Steele et al. Nature Nanotechnology 4, 363
(2009). A new generation of UHV equipment resulting from the
combination of far field microscopy and of a miniaturised
[361] Marty et al. Small, 2, 110 (2006). set of near field microscopes is now emerging in research
and industry. The International Workshop on Atomic
[362] Li et al. Appl. Phys. Lett. 96, 023104 (2010). Scale Interconnection Machines will bring together
groups from all around the world who have designed
[363] Steele et al. Science 325, 1103 (2009). and started to utilise these new instruments.
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European Research Roadmap Security; Design Technologies; Semiconductor Process
for Nanoelectronics and Integration; and Equipment, Materials and
Francis BALESTRA Manufacturing.
IMEP-LAHC, Sinano Institute/Grenoble INP
Minatec-CNRS. 3 Parvis Louis Néel, BP 257, 38016 The projects in the field of nanoelectronics technology,
Grenoble, France. which is the focus of this paper, are thus funded by
CATRENE, ENIAC and FP programmes. In this respect,
The Nanoelectronics European Research Roadmap is the international roadmap proposes a minimum device
addressed focusing on the main European Programmes size starting from 45nm in 2010 to about 9nm in 2024
supporting the short, medium and long-term research (More Moore domain). The required performance
activities (CATRENE, ENIAC JTI/JU, Framework improvements for the end of the roadmap for high
Programme). The main challenges we are facing in the performance, low and ultra low power applications will
field of Nanoelectronic technology are summarized in lead to a substantial enlargement of the number of new
the More Moore (ultimate CMOS), More than Moore materials (thin and ultra-thin strain channels, high and very
(adding functionalities to CMOS) and Beyond-CMOS high k dielectrics, metallic source-drain, etc.), technologies
domains. The main objectives and some highlights of the (EUV, etc.) and device architectures (Ultra-Thin films,
Sinano Institute, an European association created for the Multi-gates, Multi-channels, etc.).
coordination of the efforts of the Academic Community
in the field of Nanoelectronics, and of the Nanosil and Complexity will also derive from diversity, with an
Nanofunction Networks of Excellence, devoted to the increasing number of functions integrated on CMOS
convergence of More Moore and Beyond-CMOS on one platforms as envisaged in the “More than Moore”
hand and of advanced More than Moore and approach. To learn how to combine CMOS with sensors,
Beyond-CMOS research activities on the other hand, actuators, MEMS, NEMS, RF components, biochips, high
are also outlined. voltage, imaging devices, photonics on Si, energy
harvesting and demonstrate their innovative benefits
1. European Nanoelectronics landscape requires enhancing multidisciplinary experiments by a
large research community.
Micro-Nanoelectronics has been defined as one of the
5 Key Enabling Technologies (KET) for strengthening the Heterogeneous integration will also be needed in some
knowledge economy and a sustainable growth in the domains in order to obtain more functions/mm3.
strategic plan of the European Union (5 KETs: Special interests are in the fields of 3D integration,
Micro-Nanoelectronics, Nanotechnology, Photonics, interconnection, assembly & packaging.
Biotechnology, Advanced materials). Nanoelectronics
research activities in the EU are devoted to remain at Beyond-CMOS nanostructures (Nanowires, Tunnel FETs,
the forefront of state of the art innovation in the further Graphene devices, etc.) will also allow to push the limits
miniaturization and integration of nanoelectronic of Si integration down to nanometric dimension and to
devices while dramatically increasing their functionalities. develop new functionalities for the future Nanosystems.
Several Programmes, funded by the European In these research areas, there is a strong need to
Commission and the Members States, are supporting support and develop state-of-the-art Research
Nanoelectronics in the EU. CATRENE (EUREKA) [1] Infrastructures (RI) open to a large research community
and ENIAC Joint Technology Initiatives [2] projects are to overcome these formidable multidisciplinary challenges
devoted to technology driven and application driven for new generations of Nanoelectronic ICs. The proposed
short/medium term researches, respectively, and FP7 strategy in the EC is the development of a 3 levels
projects [3] focus on long-term research. In the infrastructure: i) Network of flexible RI, driven by the
framework of ENIAC/CATRENE, 5 applications Academic Community, for the study of basic properties,
oriented and 3 technology oriented domains have been test and validation of very innovative materials and devices
defined: Automotive and Transport; Communication; for long term nanoelectronics applications; ii) Pre-industrial
Energy Efficiency; Health and Ageing Society; Safety and RI, driven by the Institutes/Integration Centres, for
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medium term applications with technology implementation hand, and the merging of More than Moore and
and performance assessment on R&D equipments; iii) Beyond-CMOS, on the other hand, for developing
Industrial RI, driven by the Manufacturing Centres, for innovative nanoscale structures that can improve
short term applications with technology exploitation as performance and/or enable new functionalities in future
functional product, process optimization, yield, and terascale ICs and Nanosystems.
product reliability.
- perform training activities, University curricula,
The Sinano Institute and the Nanosil/Nanofunction Workshops to develop high competence levels in Europe.
Networks of Excellence, which are presented below, have
been launched these last years for the coordination of the - participate in roadmap definition.
European Academic Community and for the study of the
convergence of the More Moore, More than Moore and - play an important role in European structuring and
Beyond CMOS domains, respectively. These European programs, and strengthen the overall efficiency of the
consortiums are mainly performing long term European research in Nanoelectronics.
nanoelectronics research using flexible research
infrastructures (level i) of the previous structuring in strong The Sinano Institute has launched in 2008 and 2010 two
interaction with industrial and pre-industrial partners. FP7 Networks of Excellence which are described below.
2. The Sinano Institute 3. NANOSIL European Network of
Excellence
The Sinano Institute [4], launched in 2008 as a European
Academic and Scientific Association for Nanoelectronics, The FP7 Nanosil Network of Excellence (Grant agreement
gathers 18 laboratories from 10 European countries, n°216171), entitled “Silicon-based nanostructures and
representing the main partners from the Academic nanodevices for long-term nanoelectronics applications” [5]
Community in this field. It has been created after the FP6 has been launched in January 2008 for three years.
Sinano Network of Excellence, which has represented an
unprecedented collaboration in Europe in the field of It gathers 28 Partners from 11 European countries. The
Nanoelectronics. It is an open entity gathering the most main objectives of this NoE are to push the limits of Si
important flexible research infrastructures available in integration down to nanometric dimension. The Nanosil
Europe for long term Nanoelectronics research. partners are thus working on n+4 technology node and
beyond for:
The Sinano Institute has especially been created to:
- studying and validating new concepts, novel materials
- stablish a durable EU Network of researchers from the and technologies, innovative device architectures using
European Academic Community to form a distributed joint flexible platforms.
Centre of Excellence in the Nanoelectronics field.
- identifying the most promising topics for future
- carry out a role of representation and coordination of information and communication technologies and
the associated Organizations. updating roadmaps.
- explore the science and technology aspects for n+4 - overcoming the number of research challenges of
technology nodes and beyond using joint flexible ultimate CMOS and beyond-CMOS nanodevices in
technology, characterization and modeling platforms in order to speed up technological innovation for the
order to identify the most promising topics for future ICT. Nanoelectronics of the next 2-3 decades leading to the
possible integration of Si-based innovative CMOS and
- achieve activities centred on More Moore, Beyond emerging non-CMOS devices on one Si chip, which is a
CMOS and More than Moore fields — in this respect the strategic issue for the next IC generations.
SINANO Institute is particularly focusing on the
convergence of More Moore and Beyond-CMOS, on one Other important objectives are the following:
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Fig. 5. Numerical simulation of Tunnel FETs realized with various
architectures: A) Single gate SOI, Lg=100nm, 3nm SiO2, B) with additional
Fig. 4. Id(Vg) and Id(Vd) characteristics for Schottky barrier source-drain stress of 4GPa at source junction, C) with high k gate dielectrics, D) with
N- and P-channel MOSFETs with (DS) and without (w/o DS) dopant a double gate structure, E) oxide aligned to intrinsic region, F) for
segregation (DS P-channel: PtSi, BF2; DS N-channel: YbSi, As). Lg=30nm. Id(Vg), average subthreshold slope and minimum point slope
are shown.
devices, an enhancement of a factor of 2.5 of Id due to dopant segregation (DS) at the channel/source-drain
the increase in electron mobility is obtained compared interface induces a substantial increase of the driving
with unstrained nanowires (cross-section of NW current [7,8].
40×40nm2) [6].
Figure 5 shows the performance of Tunnel FET devices
In Figure 4 are plotted the transfer and output with various architectures. These transistors are very
characteristics of Schottky source-drain devices for N- interesting for reducing the off-state current for very
and P-channel MOSFETs. In both cases, the use of low power applications. One of the major challenge is
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activities, four main scientific and technological objectives
have been defined in the Nanofunction NoE:
i) Nanosensing with Si based nanowires:
- Exploring the use of Si-based nanowires for various
nanosensors with improved performance (sensitivity,
resolution, selectivity and response time).
- Nanowire and nanowire-FET fabrication.
- Multifunctional detection using nanowires.
- Demonstration of sensor arrays with Si based
nanowires as sensing element.
- Convergence of nanowires with microelectronics
substrate.
ii) Exploration of new materials, devices and technologies for
Energy Harvesting:
Fig. 6. Fabrication of ultra-dense Si Nanowire Networks by top-down
approach for vertical device with massively parallel NWs (10-15nm
diameter). - New materials and devices for mechanical energy
harvesting.
the possibility to obtain high drain current. The - New materials and devices for thermoelectric
substantial improvement of the electrical properties of energy harvesting.
TFETs is demonstrated below using short channel - New materials and device architecture for
double-gate SOI structures with strained at the source nanostructured solar cellsStorage (micro/nano-batteries),
junction and high-k gate dielectrics [9, 10]. power conversion and management in energy harvesting
systems.
In order to get a high driving current, a 3D integration
of Nanowires will be needed. This parallel integration is iii) Nanocoolers:
exemplified in Fig. 6. Vertical 3D nanowire structures
with very thin wire diameter (in the 10 nm range) and a - Development of Si-based very low Temperature
very high density (n=4 x1010 cm-2) is demonstrated [11]. coolers in order to obtain a local cooling of some devices
Possible applications of these 3D NW are in the field of or just the electrons in the devices using Si-based
ultimate integration of nanoMOSFETs, photovoltaics processing.
(improvement of light absorption), nanosensors or RF - Investigation of phonon transport at low
devices. temperature and in reduced dimensionality.
- Study of alternative nanostructures for thermal
The development of novel nanofunctionalities is the isolation (porous Si, nanowires, etc.).
purpose of the new FP7 Nanofunction NoE (2010-2013). - Integration of cooled detectors and read-out
electronics.
4. Nanofunction European Network
of Excellence iv) Exploration of new materials, devices and technologies
for RF applications:
The FP7 Nanofunction Network of Excellence (Grant
agreement n° 257375), entitled “Beyond CMOS - Exploration of the potential of nanowires and other
Nanodevices for Adding Functionalities to CMOS” [12] nanostructured materials (porous Si) as:
has been launched in September 2010 for three years.
It gathers 15 Partners from 10 European countries. * Substrate materials for reducing RF losses for
on-chip CMOS RF passives.
In addition to the integration and spreading of excellence * Materials for RF interconnects and nano-antennas.
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In Figure 7 are plotted an example the use of Nanowires References:
for thermoelectric applications. The thermal conductivity
of Si Nanowires is shown as a function of diameter and [1] www.catrene.org
incorporating nanoscale roughness. A very small thermal
conductivity is demonstrated emphasizing the extremely [2] www.eniac.eu
small phonon mean free path [13].
[3] http://cordis.europa.eu/fp7/home_en.html
[4] www.sinano.eu
[5] www.nanosil-noe.eu
[6] S.F. Feste, J. Knoch, S. Habicht, D. Buca, Q.T. Zhao, and
k [W m-1 K-1]
S. Mantl, Performance enhancement of uniaxially-tensile
strained Si NW-nFETs fabricated by lateral strain relaxation
of sSOI, Proc. ULIS, Glasgow, p. 109 (2009).
[7] Larrieu, G.; Dubois, E.; Valentin, R.; Breil, N.;
Danneville, F.; Dambrine, G.; Raskin, J.P.; Pesant, J.C.,
Low Temperature Implementation of Dopant-Segregated
Band-edge Metallic S/D junctions in Thin-Body SOI p-
MOSFETs, Proc. IEDM, p. 147 (2007).
Temperature [K]
[8] Larrieu, G., Yarekha, D. A., Dubois, E., Breil, N.,
and Faynot, O., Arsenic-Segregated Rare-Earth Silicide
Fig. 7. Thermal conductivity of Si NW vs Diameter incorporating Junctions: Reduction of Schottky Barrier and Integration
nanoscale roughness showing very low thermal conductivity and in Metallic n-MOSFETs on SOI; IEEE Electron Dev. Lett.,
extremely short phonon mean free path [13]. Dec. 2009, 1266-1268.
[9] K. Boucart et al, Proceedings ESSDERC’2009,
Athens, Greece.
Conclusions:
[10] Boucart, K., Riess, W., and Ionescu, A. M., Lateral
A summary of the Nanoelectronics European Strain Profile as Key Technology Booster for All-Silicon
Research Roadmap has been presented with the main Tunnel FETs; IEEE Elect. Dev. Lett., June 2009, 656-658.
European Programmes supporting the short, medium
and long-term research activities. The main challenges [11] X.L Han et al., Proc. Int. Conf. on Nanosc. and Tec.,
we are facing in the field of Nanoelectronic technology Beijing, Sept 2009.
have been summarized in the More Moore, advanced
More than Moore and Beyond-CMOS domains. [12] www.nanofunction.eu
The main objectives and some highlights of the Sinano
Institute, an European association created for the [13] Hochbaum, Nature 451, 163, 2008.
coordination of the efforts of the Academic
Community in the field of Nanoelectronics, and of the
Nanosil and Nanofunction Networks of Excellence,
devoted to the convergence of More Moore and
Beyond-CMOS on one hand and of More than Moore
and Beyond-CMOS research activities on the other
hand, have been outlined.
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Enano newsletter issue20-21
- 3. Dear Readers, C Contents This E-nano Newsletter special double issue 5 nanoICT research contains the updated version of the nanoICT Carbon Nanotubes position paper on Carbon Nanotubes (CNTs) W. I. Milne et al. summarising state-of-the-art research in this field 51 European Research Roadmap for as well as a description of the possible electrical, Nanoelectronics electronic and photonic applications of carbon F. Balestra. 58 nanoICT Conf Reports nanotubes, the types of CNTs employed and the Report nanoICT Graphene and organisations or groups that are most proficient Nanotubes Session - TNT2010 at fabricating them. S. Roche. 61 Phonons and Fluctuations Meeting In the second paper, the Nanoelectronics J. Ahopelto. European Research Roadmap is addressed 65 International Summer School Son et focusing on the main European Programmes Lumière supporting the short, medium and long-term C. M. Sotomayor Torres. research activities. This issue also contains a catalogue (insert), compiled by the Phantoms Foundation providing a general overview of the Nanoscience and Nanotechnology companies in Spain and in particular the E Editorial Information importance of this market research, product development, etc. We would like to thank all the authors who contributed to this issue as well as No. 20/21 the European Commission for the December 2010 January 2011 financial support (project nanoICT No. Published by 216165). Phantoms Foundation (Spain) Dr. Antonio Correia Editor - Phantoms Foundation Editor Dr. Antonio Correia [email protected] Assistant Editors Deadline for manuscript José Luis Roldán, Maite Fernández, Conchi Narros, submission Carmen Chacón and Viviana Estêvão Issue No. 22: April 30, 2011 Issue No. 23: June 30, 2011 2000 copies of this issue have been printed. Full color newsletter available at: Depósito legal/Legal Deposit: www.phantomsnet/Foundation/newsletters.php For any question please contact the editor at: M-43078-2005 [email protected] Impresión/Printing: Editorial Board Madripapel, S.A. Adriana Gil (Nanotec S.L., Spain), Christian Joachim (CEMES-CNRS, France), Ron Reifenberger (Purdue University, USA), Stephan Roche (ICN-CIN2, Spain), Juan José Saenz (UAM, Spain), Pedro A. Serena (ICMM-CSIC, Spain), Didier Tonneau (CNRS-CINaM Université de la Méditerranée, France) and Rainer Waser (Research Center Julich, Germany). 3
- 5. nanoICT research Carbon Nanotubes transistors, thin-film electrodes, network transistors, single (Position Paper version 2) CNT transistors, thermal management, memory. M.Mann Cambridge University, UK Optical applications W.I.Milne Cambridge University, UK Absorbers, microlenses in LCs, optical antennae, lighting. S Hofmann Cambridge University, UK P.Boggild DTU Technical University of Denmark Electromechanical applications J.McLaughlin University of Ulster NEMS (resonators), sensors, nanofluidics, bio-medical. J.Robertson Cambridge University G Pagona National Hellenic Research Foundation, Greece Energy applications G.A.D. Briggs Oxford University Fuel cells, supercapacitors, batteries, solar cells. P Hiralal Cambridge University, UK M.de Souza Sheffield University Blue sky K.B.K.Teo AIXTRON Spintronics, quantum computing, SET, ballistic transport. K.Bo Mogensen TU Denmark J.-C. P. Gabriel CEA, Nanoscience Program, Grenoble (Formerly, Nanomix Inc. USA) Y Zhang Cambridge University, UK M Chhowalla Imperial College, London, UK Z Durrani Imperial College, London, UK T Wilkinson Cambridge University, UK D Chu Cambridge University S.Roche CEA-INAC, Grenoble Robert Baptist CEA-LETI, France P.Bachmann Philips Research Laboratories, Aachen J.Dijon CEA, Grenoble A.Lewalter Philips Research Laboratories, Aachen Key Words Fig 1. (top) A graphene sheet rolled up to obtain a single-walled CNT. Growth (bottom) The map shows the different single-walled CNT configurations possible. Were the graphene sheet to roll up in such a way that the atom Carbon nanotubes, multiwall, singlewall, nanofibres (all at (0,0) would also be the atom at (6,6), then the CNT would be the words in a and the subtopics), cap structure, metallic. Likewise, if the CNT rolled up so that the atom at (0,0) was also catalysts, adhesion, mechanism, modelling. the atom at (6,5), the CNT would be semi-conducting. The small circles denote semiconducting CNTs, the large circles denote quasi-metallic CNTs, the squares denote metallic CNTs. Post-growth Modification Doping, & functionalization, dispersion and separation, purification, annealing, cap opening/closing, 1. Introduction graphitization. There has been extensive research into the properties, Properties/characterization synthesis and possible applications of carbon nanotubes Defects, electron transport, phonons, thermal (CNTs) since they came to prominence following the properties/conductivity, wetting, stiction, friction, Iijima paper [1] of 1991. Carbon nanotubes are mechanical, chemical properties, optical, toxicity, composed of sp2 covalently-bonded carbon in which structural properties, contacts. graphene walls are rolled up cylindrically to form tubes. The ends can either be bonded to a secondary surface, Electronic Applications not necessarily made of carbon, they can be capped by Field emission (X-ray, Microwave, FEDs, Ionization, a hemisphere of sp2 carbon, with a fullerene-like Electron microscopy), interconnects, vias, diodes, thin-film structure [2], or the CNT can be open with the ends passivated (by hydrogen). In terms of electrical 5
- 6. nanoICT research properties, single-walled CNTs can be either semiconducting carbon. The CVD method is by far the most widely or metallic and this depends upon the way in which they roll employed method at present, both for bulk growth (for up, as illustrated in Fig. 1 (page 5). use in composites for example) or for growth onto surfaces (for use in electronics). This is despite the fact Multi-walled CNTs are non-semiconducting (i.e. semi-metallic that the CNTs produced by this method are not the like graphite) in nature. Their diameters range from 2 to best; laser ablation remains the best method for 500 nm, and their lengths range from 50 nm to a few producing reference SWNT samples of high structural mm. Multi-walled CNTs contain several concentric, and electrical quality, but CNTs produced by this coaxial graphene cylinders with interlayer spacings of method are often coated by a large amount of ~0.34 nm [3]. This is slightly larger than the single crystal amorphous carbon. graphite spacing which is 0.335 nm. Studies have shown that the inter-shell spacing can actually range from 0.34 Common to all growth methods and of key importance to 0.39 nm, where the inter-shell spacing decreases with to CNT growth is the use of a nano-particle catalyst. The increasing CNT diameter with a pronounced effect in understanding of the role of the catalyst and a detailed smaller diameter CNTs (such as those smaller than 1 nm) 5 CNT growth mechanism is still incomplete. This hinders as a result of the high curvature in the graphene sheet the refinement of current growth techniques, in particular [4],[5]. As each cylinder has a different radius, it is with regard to growth selectivity and efficiency. impossible to line the carbon atoms up within the sheets as they do in crystalline graphite. Therefore, multi-walled Here, the progress in catalytic CVD of CNTs is CNTs tend to exhibit properties of turbostratic graphite reviewed, which is widely used because it offers high in which the layers are uncorrelated. For instance, in production yield and an ease of scale-up for both bulk highly crystallized multi-walled CNTs, it has been shown production and localized growth on surfaces. The CVD that if contacted externally, electric current is generally review is split into two parts: conducted through only the outermost shell [6], though Fujitsu have been able to contact the inner walls to 1. Fundamental understanding of the growth process, measure CNTs with resistances 0.7 kΩ per multi-walled 2. State-of-the-art growth results. CNT [7]. 2.1 The catalytic CVD process This position paper summarizes state-of-the-art research in CNTs. It should be noted, however, that what is CNT growth occurs as a result of the exposure of regarded as state-of-the-art is dependent upon the catalyst nano-particles to a gaseous carbon feedstock at nature of the desired end-structure. The optimum elevated temperatures. CNTs are selectively seeded by properties listed in section 7 cannot be incorporated the catalyst (Fig. 2) [8]. This can give control over the into one CNT. Therefore, a selection of the properties position at which CNTs form by patterning the catalyst listed is chosen for a specific desired end-application, onto a substrate — vertical arrays being a prominent which is controlled by the growth method, described example [9]. first. Subsequently, there follows a description of the possible electrical, electronic and photonic applications of carbon nanotubes (excluding bulk material composite applications), the types of CNTs employed and the organizations or groups that are most proficient at fabricating them. 2. Growth CNTs can be grown by three main techniques, chemical vapour deposition (CVD), arc discharge and laser ablation. The latter two techniques developed out of Fig. 2: Environmental-TEM image sequence of Ni-catalyzed SWNT root fullerene research and involve the condensation of growth recorded in 8×10-3 mbar C2H2 at 615°C and schematic ball-and stick carbon atoms generated by the evaporation of solid model [8]. 6
- 8. nanoICT research In bulk CVD, a catalyst is injected into a hot furnace carbon film over the substrate and nanotubes, and for with the feedstock and the nanotubes are recovered at this reason diluents/etchants are added, which can the base. This is realised with either a fluidised bed [10], be hydrogen, argon, water vapour or ammonia (at or direct injection with fast growth. low temperatures). More relevant to electronics, however, is the surface-based The CNT growth process involves the dissociation of growth of CNTs which has many aspects in common with the carbon precursor on the catalyst particle surface, classical heterogeneous catalysis. There are three basic transport of carbon atoms through or over the stages to the growth process: particle, and precipitation of the carbon as a growing tube. All the carbon in the nanotube is incorporated via 1. A catalyst is deposited/patterned on the desired the catalyst particle [12] . In contrast to arc-discharge surface, or laser-ablation, each catalyst particle typically forms 2. It is transformed into a series of nano-particles and/or only one nanotube with CVD. Therefore, the catalyst its active phase is stabilised by some pre-treatment, and particle size controls the CNT diameter [8],[13],[14]. 3. It is exposed to the growth atmosphere. Catalyst nanoparticles, or rather their atoms, have a There are numerous deposition methods for the high surface mobility, hence a key challenge for elevated catalyst, ranging from methods based on wet chemistry CVD temperatures is to stabilize a narrow particle size in atmosphere to physical vapour deposition in vacuum distribution. This catalyst coarsening can be minimised (Fig. 3). by a suitable choice of support and deposition conditions. A wider catalyst size distribution and larger CNT growth is essentially based on the self-organisation particle size leads to a loss of control of the nanotube of carbon; control over the carbon structure requires diameter and ultimately a loss of catalytic activity. careful tuning of the growth parameters. A significant challenge is the large parameter space to optimise which includes: Important to CNT growth, however, are not only support interactions and gas-induced catalyst dynamics •The size and material of the catalyst, but also the restructuring of the catalyst due to the •The nature of the support (or surface), presence of the growing nanostructure. A SWNT •The constituents of the carbon feedstock, nucleates by lift-off of a carbon cap (Fig. 2) [8]. Cap •The quantity and type of diluents /etchants used, stabilization and nanotube growth involve the dynamic •The temperature of the annealing and subsequent reshaping of the catalyst nanocrystal itself. For a carbon growth process nanofibre (CNF), the graphene layer stacking is •The pressure of the reactor. determined by the successive elongation and contraction of the catalyst nanoparticle [8],[15]. A CNT growth recipe is a suitable combination of these Generally, CNTs grow by either root or tip growth from parameters. the catalyst particle, depending upon whether the catalyst remains attached to the support or rides at the The catalysts used to gain the highest CNT yield are tip of the growing nanostructure. most commonly the 3d transition metals Fe, Co and Ni on the oxides SiO2 or Al2O3. The feedstock is In the laser or arc methods, the catalyst particles are liquid, commonly diluted C2H2, C2H4 or CH4, depending on at least during the initial stage. In many high temperature the temperature range. For high temperature CVD bulk growth methods, the catalyst is also liquid. But in (>800°C) methane is often used, whereas for low surface-bound CVD the catalyst can be either liquid or temperature growth (<750°C) CVD recipes are often solid. There is no necessity for catalyst liquefaction, as based on ethylene or the more reactive acetylene [11]. in-situ growth experiments at lower temperatures have The carbon activity can thereby be regulated by shown that for Fe and Ni, the active state of the catalyst diluting the feedstock. Of key importance is to is that of a crystalline metallic nanoparticle [8],[16]. Solid avoid/minimise pyrolysis of the carbon precursor, to catalysts are expected to allow for more controlled prevent the formation of a deleterious amorphous growth, especially with regard to potential chiral selectivity. 8
- 9. nanoICT research catalyst pre-treatment is to reduce the catalyst to its metallic state prior to or during carbon feedgas exposure. A catalyst prepared by PVD is transformed into its active state as a nano-particle by a pre-treatment [16] as shown in figure 3. Catalyst can also be deposited from solution, but this tends to lead to less control over CNT dimensions. This causes the de-wetting of the catalyst thin film into a series of separated nano-particles to reduce their total surface and interface energies. For Fe, this is aided by the reduction process, where the reduction in molecular volume from oxide to Fig 3. Catalyst preparation by (a) PVD and (b) wet chemistry [17][18][19]. metal also creates nano-particles. The diameter of the resulting nano-particles is directly In-situ experiments have shown that the active state of proportional to the initial film thickness h. Assuming the most efficient catalysts Fe, Co and Ni is the metallic volume conservation and a contact angle of 90°, this rather than the oxide state [16]. Thus a key part of the means the particle diameter D, is given by Advertisement 9
- 10. nanoICT research D ~ 6h Equation 2.1.1 The question as to what makes a good CNT catalyst has not been answered in sufficient detail. Fundamentally, With a density, N, of CNT CVD growth requires a nano-particulate catalyst. An interesting comparison is graphene CVD, whereby N ~ 1/60h2 Equation 2.1.2 planar catalyst films with a large grain size are required. However, the actual CVD exposure is often chosen to Processing conditions can affect the CNT type and be very similar to SWNT growth. Ni and Fe films diameter distribution [8],[14]. For some years, certain catalysing graphene CVD were shown to be metallic and catalysts such as CoMoCAT or some plasma enhanced showed no signature of carbide formation. CNT growth growth have produced preferential semiconducting is not exclusive to Ni, Fe and Co, but has been SWNTs, although the process was unknown. Recently reported, albeit often at lower yield and/or at higher the preferential growth of metallic SWNTs was reported temperatures, for a range of metals (such as Au, Pt, Ag, by modifying the catalyst annealing atmosphere [20]. Pd, Cu, Re, Sn, Ta) and semiconductors (Si, Ge). Nano-particle restructuring is driven by a minimization of the surface free energy of the system and support “Catalyst-free” SWNT formation has been reported on the interactions and adsorbate-induced effects are well Si face of hexagonal silicon carbide (6H-SiC) at temperatures known from heterogeneous catalysis to affect this above 1 500°C [22] and “catalyst free” growth of arrays of process. In particular for Al2O3 supported Fe, it has multiwall carbon nanostructures has been reported on been shown that support interactions restrict the cylindrical pores of porous anodic aluminum oxide at 900°C catalyst surface mobility, leading to a much narrower [23]. More recently, metal-catalyst-free CVD of SWNTs has catalyst particle size distribution [21]. Moreover, these been reported on roughened Al2O3 and SiO2 at 900°C support interactions give a higher CNT density and a [24][25][26] and SWNT nucleation has been observed from vertical nanotube alignment due to proximity effects, diamond nanoparticles at 850°C [27]. The CNT growth i.e. these support interactions trigger CNT forest mechanism for these “metal-catalyst-free” CVD processes growth. This is summarized in figure 4. is largely unknown and currently very speculative. In particular, the physical and the chemical state of the catalyst during growth are often unknown and what is stated is only the state before/after growth. Recent in-situ XPS measurements showed that nano-particulate zirconia during CNT nucleation at moderate temperatures (~750°C) does not reduce to metallic zirconium or zirconium carbide, i.e., the nano-particulate oxide is the active phase. This indicates that CNT formation can be mediated solely by a surface reaction, i.e., bulk C permeability is not a necessity for a CNT catalyst [28]. There are numerous methods by which nano-particulate catalysts can be prepared (fig. 5). Physical vapour deposition (PVD; evaporation, sputtering), aerosol based techniques, colloids, (electro-)plating and wet chemistry (salts) are among various methods used to prepare catalyst for CNT CVD. Each preparation method requires a different CVD pre-treatment to form well defined catalyst clusters with a narrow size Fig 4. Above, the dewetting on the catalyst depends on surface distribution. Furthermore, the catalyst preparation interactions between the catalyst and support. Below, a general is linked to available patterning techniques. For PVD comparison of typical CNT growth densities with nanotube diameter catalysts, photo- or e-beam lithography offer most (reflecting equs 2.1.1 and 2.1.2). 10
- 11. nanoICT research A significant disadvantage of the arc method and some bulk CVD processes is that the product contains 10% of the catalyst and other graphitic material which must be removed by purification. The purification using acid washing can then add a factor of 10 to the total cost. Consequently, the super-growth method, which produces high aspect ratio CNTs has a nanotube to catalyst ratio of 105, which gives a very high purity. Therefore, super-growth is the only surface-bound Fig 5. Catalyst patterning techniques: (a) e-beam lithography, (b) electroplating, (c) FIB, (d) contact printing [9]. method of producing high-purity bulk nanotubes. This is critical for some applications such as supercapacitors in control, whereas laser interferometry and which metal impurities have a significant effect. nanosphere lithography are among alternative patterning techniques which can be used to pattern A key criterion for growth is to lower the temperature. small feature sizes at a lower cost over large areas. This is important for a number of electronic applications Nanocontact printing and micro-fluidic techniques are such as interconnects where the temperature should not often used with catalyst colloids. exceed 400°C, limited by the fact that it is a “back-end” process. 2.2 State-of the art CNT growth results The growth of CNTs depends on the desired The first method used to reduce the growth application. For instance, certain field emission temperature was to use PECVD which is widely used to applications require spaced arrays of CNTs. It is lower processing temperatures during growth [31]. preferable that the CNTs remain fixed and vertical. PECVD in nanotube growth was assumed to dissociate Consequently, MWNTs are the best choice in this the carbon pre-cursor gas and allow the growth reaction situation. The best controlled growth of spaced arrays of to occur at lower temperatures. However, it is now MWNTs is as defined in 2001 using PECVD [29]. There realized that the key role of the plasma is to convert the have also been recent developments to scale down the catalyst into an active nano-particulate, metallic form at diameter and height of the CNTs, in order to increase a lower temperature. Therefore, so long as growth uses the emission current density. acetylene, it can proceed at quite low temperatures. Other applications such as vias, interconnects, heat Two other low temperature activation methods have spreaders, super-capacitors or adhesive surfaces require been used. First, Corrias et al [32] and also AIXTRON densely packed aligned nanotube arrays. This area was [33] use pre-heating or plasma pre-dissociation. A revolutionized by the so-called super-growth method of second method used by S.R.P. Silva et al in Surrey is to Hata et al [30]. This uses a Fe catalyst on Al2O3 support, use non-thermal light irradiation to heat the substrate and thermal CVD at 700°C, in hydrogen-diluted surface. ethylene gas. A small proportion (400 ppm) of water vapour is added to the growth gas. The water is shown Growth on metals is required for applications such as by EELS to act as a mild etchant for amorphous carbon interconnects. However, metallic supports often show which ultimately starts to cover the catalyst and interactions with the gas atmosphere and catalyst, i.e. terminate growth. This led to an era where mm to cm oxide, carbide and alloy formation is likely. Hence, CNT high forests could be grown by many groups world-wide. CVD directly on a metal support requires careful The nanotube density is controlled by the catalyst film calibration and often gives poor growth. The way around thickness, as previously described. this is to use metals with small Al content which then 11
- 12. nanoICT research preferentially oxidizes into an ultra-thin layer of Al2O3, extremely fine, black thread consisting of aligned CNTs or to use metallic compounds such as CoSi2 or TiN. [36]. Nanocyl also produce purified single-walled nanotubes [37]. The quality of grown carbon nanotubes is subjective, since their quality depends on the structure required. 2.2.1.2 Horizontally aligned single-walled CNTs Quality screening is a challenge on its own and the detailed characterisation of as-grown CNT samples Horizontally-aligned SWNTs have been grown on remains time-consuming and relies on a combination of epitaxial surfaces such as sapphire and quartz with direct imaging methods (such as SEM, AFM, TEM, STM) varying densities. The growing CNTs follow the crystal and indirect methods (such as Raman, PL, optical planes with a great degree of alignment. The process is absorption, TGA, XPS, XRD). Some applications standard CVD but the substrate needs to be annealed require high purity and crystallinity; others require tight for surface reconstruction before growth. Among the dimensional control, whilst others might require high best, Tsuji’s group have grown on sapphire [38] and the packing densities and/or alignment. Consequently, the Rogers group [39], who have grown on quartz (figure state of the art depends on the type of structure and 6). The density is some way (x10) below that achieved application required. in vertically aligned forests. 2.2.1 Single-walled growth Horizontal alignment can also be achieved by electrical fields, gas flow, or liquid post-treatment. Hata and co-workers 2.2.1.1 State of the art for bulk single-walled dipped a vertical aligned forest in alcohol to get growth alignment in plane samples by capillary forces when he pulls up the substrate from the liquid [40]. The highest purity CNTs nucleate from catalysts in a fluidized bed and are currently sold by Thomas Swan 2.2.1.3 Challenges for SWNTs [34]. The process produces high-quality CNTs, inexpensively in large quantities. Windle’s [35] group A key challenge for SWNTs still concerns control of grow CNTs in a continuous flow furnace. The nanotubes chirality during growth. For applications such as are created rapidly by injecting ethanol and ferrocene transistors, all grown CNTs need to be semiconducting into a furnace at 1200°C. An aerogel then starts to stick (and preferably of identical chirality and diameter) whilst to the cooler wall in the furnace to form fibres. A for interconnects, all CNTs need to be metallic. Control spindle then winds the aerogel fibres into a thread, at of diameter is related to this issue. several centimetres per second. The result is an The second challenge is nanotube density. For interconnects, a high density (of ~1013 tubes/cm-3) is needed if it is to replace copper. This is now looking possible. The yield of SWNTs grown with templates is very low and must be solved if it is to be seriously considered as a method for growing SWNTs. Also, for SWNT growth to be combined with CMOS, the temperature needs to be reduced to ~400°C. To a certain extent the chirality problem has been overcome by using Fig 6. (a-d) CNTs grown along quartz crystal planes by the Rogers group [39]. (e) Horizontally aligned CNTs grown by Dai’s group using field to align the CNTs [13]. devices based on random network of 12
- 13. nanoICT research nanotubes instead. This approach was first brought to and Arkema [45] and Bayer [44] have made significant light by Snow and co-workers in 2003 [41] although it contributions to up scaling CVD. Recently, AIXTRON was patented by Nanomix in June 2002 [42]. [46] and Oxford Instruments [47] have begun to provide large area PECVD capability. The leading universities in 2.2.2 Multi-walled CNTs Europe include Cambridge Univ, Dresden and EPFL. Growth of MWNTs on large wafers (200mm) is now Though Endo started the injection process, for bulk routinely done at various locations for microelectronics growth, the best CNTs are again grown by Thomas Swan applications (see for example, images of CNTs grown in (as a result of rigorous qualification by Raman and TEM) various CVD reactors at CEA-Grenoble in figure 7). The and Windle’s group in Cambridge, though Hyperion aerosol-assisted CCVD process allowing the production [43] are the leading suppliers of nanofibres using a similar of carpets of aligned nanotubes is produced at CEA-Saclay process to Thomas Swan. So-called Endo-fibres 150 nm in the group of Martine Mayne (and can also be seen in in diameter can also be purchased from Showa Denko. figure 7). Bayer produce narrower “Baytubes” 5-20 nm in Fig 7. Dense forest of Small diameter MWCNT from left to right: a) Patterned layer on a 200mm layer b) 50μm high forest on conductive layer of TiN c) close view of the material with individual CNT making bundles of 60nm of diameter (courtesy of CEA-LITEN) diameter [44], but these are impure. However, they are 3. Post growth modification suitable for many uses because the metal is encapsulated in the tube ends and not exposed. If no particular attention is paid to it, CVD generally produces CNTs with numerous defects. Such defects are favoured: 2.2.2.1 Challenges for multi-walled CNTs (i) when low temperatures are being used for the growth; Some of the challenges for MWNT growth are identical (ii) when dopants such as nitrogen or boron are to that of SWNT growth. Growth temperature needs to inserted; be reduced if CNTs are to be employed in CMOS. (iii) when the growth process is allowed to continue Raman spectra of MWNTs grown by CVD/PECVD at while the process is being ended (namely, when the low temperatures show them to be highly defective. power is shutdown and the substrate allowed to cool Post-annealing processes can increase graphitization, but whilst still in the presence of a carbon source often these are typically at temperatures much higher than resulting in the deposition of amorphous carbon around circuitry can withstand. There is also the question of the CNT). contact resistance that is often quite high and variable. This needs to be addressed with still further Many defects can be removed either by hydrogen or improvements on dimensional control. ammonia plasma, or by a rapid thermal annealing process which also increases the graphitization, European Position: Europe led the way with conductivity andcontact of the CNT [48]. Hence, research in arc deposition but commercialisation was careful monitoring of CVD parameters can lead to limited. More recently Nanocyl [37], Thomas Swan [34], defect free carbon nanotubes [49]. Single defects can 13
- 15. nanoICT research even be created, monitored and can serve as the [77],[78],[79],[80]. Alkali-metal atoms located outside or point for a single functionalization [50][51]. inside the tube act as donor impurities [81],[82] while halogen atoms, molecules, or chains act as acceptors Various techniques have been employed to purify CNTs [65],[73],[83],[84]. Fullerenes or metallo-fullerenes, grown by arc discharge and laser ablation. This is encapsulated inside CNTs, allow good structural stability because the best samples are only 70% pure (using laser and have been used to tune the band gap and/or Fermi ablation), with the remainder made up of amorphous level of the host tube [85],[86],[87],[88]. carbon, fullerenes, and catalyst particles surrounded by shells of chemically resilient turbostatic carbon (TSG). “Doping” by physisorption of molecules, lies at the heart CNTs are first dispersed by sonication [52]. The of a growing field of chemical sensors, but their stability gas-phase method developed at the NASA Glenn and selectivity issues must be very carefully addressed. Research Center to purify gram-scale quantities of single-wall CNTs uses a modification of a gas-phase European Position: In Europe Maurizio Prato’s group purification technique reported by Smalley and in Trieste are the most successful in this area. others [53], by combining high-temperature oxidations and repeated extractions with nitric and hydrochloric 5. Oxidation/Functionalization/tip opening acid. This procedure significantly reduces the amount of impurities such as residual catalyst, and non-nanotube CNTs can be oxidized by various means including forms of carbon within the CNTs, increasing their treatment in acids, ozone and plasma oxidation [89]. stability significantly. Once the CNTs are separated, the Reflux in nitric acid not only purifies the nanotubes but use of a centrifuge enables the isolation of certain at the same time introduces a variety of oxygen groups chiralities of SWNTs, particularly (6,5) and (7,5) as [90] at the open ends and sidewalls, which strongly shown by Hersam’s group at North Western University facilitates the separation of nanotube bundles into [54]. This method seems to be the way forward for individual tubes and enhances their dispersibility. The scalable chirality separation. tips of CNTs are more reactive than their sidewalls and reflux in HNO3 has proven to open the nanotubes tips European Position: The US lead the way in novel and introduce carboxylic groups at the open ends. In techniques based on density differentiation but in particular, sonication under harsh conditions, in a Europe, Krupke, Knappes and co-workers at Karlsruhe mixture of concentrated nitric and sulphuric acid pioneered the dielectrophoresis method. Regarding effectively cuts the single walled nanotubes into small industrial production, a trend is observed in which CNTs fragments and gives rise to the formation of small length are produced and included in a polymer matrix in the (100 to 300 nm) open pipes. The oxidatively introduced same process line hence reducing the risk of exposure carboxyl groups represent useful sites for further to airborne nanotubes. modifications, as they enable the covalent coupling of molecules through the creation of amide and ester 4. Doping bonds. It has also been shown that CNTs react with ozone [91]. Conventional doping by substitution of external impurity atoms in a semiconductor is unsuited to CNTs, The growth of VACNTs is important for many potential since the presence of an external atom modifies the technological applications such as field emission properties resulting from ideal symmetry in the CNT. cathodes, vertical interconnects, and biosensors. Both Theoretically, substitutional doping by nitrogen (n-type) thermal chemical vapour deposition (TCVD) and and boron (p-type) has been widely examined microwave plasma enhanced chemical vapour [55],[56],[57],[58],[59],[60]. Adsorption of gases such as deposition (MPECVD) have shown promising results on H2, O2, H2O, NH3, NO2 have been reported producing well-aligned CNT arrays. In thermal CVD the [61],[62],[63],[64]. More appropriate doping strategies grown nanotubes are very closely packed, the growth which conserve the mean free path of the charge rate is very high and topology is highly defective carriers involve physisorption of alkali metal atoms compared to that of MPECVD grown CNTs. Various [65],[66],[67],[68],[69],[70],[71],[72],[73] [74],[75],[76], plasma sources have been successfully used to clean, 15
- 16. nanoICT research and open nanotube tips [92]. Low energy, high flux Grafting of biomolecules such as bovine serum albumine plasma such as ECR plasma is a suitable technique for [1 [120], [121] or horse spleen ferritin [122], poly-Llysine, 19], efficient cleaning, tip opening and produces less a polymer that promotes cell adhesion [123], [124], damage. Streptavidin [125] and biotin at the carboxylic sites of oxidized nanotubes [126] and polymers [127], [128],[129], Open-capped CNTs, unless functionalized, can be [130], [131], [132] have been reported. unstable structures because of dangling bonds. Cap (c) cycloadditions [133]. closing of open-capped structures often occurs during field emission. De Jonge et al. [93] demonstrated this European Position: Haddon and co-workers in the happens for currents as low as 80 nA per tube. US were early leaders and Carroll and co-workers in Wake Forrest University applied functionalisation to Several treatment methods such as chemical, devices. In UK Papakonstantinou et al at the University electrochemical, polymer wrapping, and plasma of Ulster have demonstrated a variety of plasma based treatment have been applied to functionalize the CNT routes as an alternative to chemical functionalisation. In surface for specific applications including, catalysis, Europe Hirsch in Erlangen has made major contributions bio/gas sensors, composites, drug delivery, field and Coleman and co-workers at TCD have furthered our emission and cell scaffolds [94], [95], [96], [97], [98], knowledge in this area. [99], [100], [101], [102], [103], [104]. Among these, plasma-treatment has the advantages of retaining the 7. Properties/characterization structural integrity of the nanotube, is environmentally friendly and it provides the possibility of scaling up for The physical properties of carbon nanotubes depend on commercial use. Also, reactions are much slower than a number of variables. These include, if the tube is other chemical modification methods and can also multi-walled or single-walled, the diameter of the tube provide a wide range of functional groups depending on and if we have a bundle or a rope, or just an individual the plasma parameters. Papakonstantinou’s group has tube. The chirality of the tube is also important for single shown that plasma functionalisation can preserve the walled nanotubes. Table 1 summarises the experimental vertical alignment of CNT arrays. findings of several different properties of carbon nanotubes, highlighting differences between different Generally, the main approaches to functionalisation can types of tube. be grouped into two categories: Table 1: Summary of main properties of CNTs (a) the covalent attachment of chemical groups through MECHANICAL PROPERTIES reactions with the π-conjugated skeleton of the CNT; Young’s modulus of multi-walled CNTs ~0.8-1.3 Tpa [134],[135] (b) the non-covalent adsorption or wrapping of various Young’s modulus of single-walled CNTs ~1-1.3 TPa [136],[137] Tensile strength of single-walled nanotube ropes > 45 GPa [138] functional molecules Tensile strength of multi-walled nanotube ropes 1.72GPa [139] Stiction ~10-7 N on 5 μm latex beads [140] The covalent functionalization of SWCNTs is not limited Hydrophobicity of MWNT forest 26°[141] -161°[142] contact angle to the chemistry of carboxylic acid. More elaborate THERMAL PROPERTIES AT ROOM TEMPERATURE methods have been developed to attach organic moieties Thermal conductivity of single-walled CNTs 1750-5800 WmK [143] directly onto the nanotube sidewalls. These include: Thermal conductivity of multi-walled CNTs >3000 WmK [144] ELECTRICAL PROPERTIES (a) photoinduced addition of azide compounds; Typical resistivity of single- and multi-walled CNTs 10-8 - 10-6 Ωm [145] Reports of fluorination [106],[107] chlorination [108], Typical maximum current density >108 A cm2 [146] atomic hydrogen [109]; aryl groups [110], nitrenes, Quantized conductance, theoretical/measured (6.5 kΩ)-1/(12.9 kΩ)-1 per channel carbenes, and radicals [111]; COOH [112],[113], NH2 ELECTRONIC PROPERTIES [1 N-alkylidene amino groups [1 alkyl groups [1 14] 15]; 16] Single-walled CNT band gap and aniline [117] amine and amide [118] have been Whose n=m, armchair 0 eV (metallic) reported. Whose n-m is divisible by 3 <0.1eV (quasi metallic) [146] Whose n-m is non-divisible by 3 0.4-2eV(semiconducting) [148],[149] (b) Bingel reactions; Multi-walled CNT band gap ~0 eV (non-semiconducting) 16
- 17. nanoICT research 8. Electronic Applications 8.2 Transparent conductors/contacts. Various applications for CNTs in the ICT field have As the use of ITO becomes ubiquitous and indium been touted but in the near term only a few of these becomes more scarce and thence more expensive seem feasible: their use as a thermal interface material there is an ongoing search for alternative transparent has gained the most interest in the last two years, as conducting contact materials. Initiated at Nanomix has their application to transparent conductors. Work [154], various groups worldwide including those of on interconnects and vias continues whereas field Rinzler, Roth, Chhowalla and Grüner have worked to emission has remained somewhat static. There follows replace indium tin oxide (ITO) in e.g. LCDs, touch a list of applications which, in the authors’ opinions, screens, and photovoltaic devices. Nantero Inc. orders the interest attracted by industry in terms of (Boston), Eikos Inc. of Franklin, Massachusetts and investment. Unidym Inc. (recently bought by Arrowhead) of Silicon 8.1 Thermal management Valley, California are also developing IP and transparent, electrically conductive films of carbon There is an increasing need to replace indium for nanotubes [155]. thermal interfaces in eg: CPUs, graphic processors and (automotive) power transistors, as price and scarcity CNT films are substantially more mechanically robust increase. Various companies and universities (such as than ITO films, potentially making them ideal for use Ajayan’s group [150]) are working in this area but very in displays for computers, cell phones, PDAs and ATMs little has been published. as well as in other plastic electronic applications. At SID2008, the University of Stuttgart and Applied As a thermal interface material for high brightness Nanotech presented the world's first 4-inch QVGA LED’s, CNTs have been shown to outperform silver colour LCD display using CNT as the transparent epoxy and other metal systems [151]. Fujitsu has also conductive film. The CNTs were deposited by spray evaluated 15 micron tall CNT thermal bumps, bonded coating [156]. at 6 kg/cm2 between a GaN high performance amplifier and an AlN substrate, to have a thermal There is still a need to increase conductivity whilst conductivity of 1400 W/mK [152]. It is believed that the key advantage of CNTs is their compliance, which maintaining a sufficiently high (~95%) transparency eliminates the problems of thermal mismatch between and for some applications, roughness is a problem. surfaces as well as ensuring a good contact. Recent developments have addressed these issues. First, the separation of metallic single walled nanotubes By annealing the catalyst in different atmospheres of from semiconducting ones using scalable density Ar/He/water, the catalyst shape can be controlled, gradient centrifugation has improved transparency and leading to the preferential growth of up to 91% metallic conductance [157]. Second, the National Renewable SWNTs [153]. Energy Laboratory in the US reported ultra-smooth transparent and conducting SWNT thin film electrodes Current problems in using CNTs are insufficient packing for organic solar cells which yielded efficiency values of density and problems with graphitisation leading to a > 3.5%, the highest thus far for SWNT electrodes reduction in conductivity. [158]. Despite this progress, the sheet resistance of SWNT networks has remained at ~100 Ω/sq (about European Position: Ajayan is the most notable an order of magnitude higher than ITO) at a contributor to this research together with the transparency of 85%. Also, most recently it has been contributions from Fujitsu above. Intel Europe are also pointed out by Fanchini et al. [159] that CNT/Polymer known to be working in this area. Very little work has been published by them and other workers from the films are anisotropic and suffer from birefringent effects EU. which may cause problems in some of its most useful 17
- 18. nanoICT research Recent studies have also demonstrated that SWCNT thin films can be used as conducting, transparent electrodes for hole collection in OPV devices with efficiencies between 1% and 2.5% confirming that they are comparable to devices fabricated using ITO [160], [161]. Note, however, that graphene based films are very quickly gaining momentum as transparent conductive electrodes and may very likely overtake the performance of CNTs in this area [162]. Chemically derived graphene obtained from reduction of graphene oxide is attractive because it is Fig 8. Comparison between the various transparent Conducting Materials synthesized from graphite, a ubiquitous and [166]. inexpensive mineral. The state-of-the-art reduced graphene oxide films have yielded sheet resistances potential application areas such as in Liquid Crystal of ~1 kΩ/sq at 85% transparency [163]. However, Displays. Gruner summarizes the work in this area well chemical vapour deposition (CVD) of graphene on (figure 8). copper and transfer onto desirable substrates have Advertisement 18
- 19. nanoICT research yielded sheet resistance values of 10—30 Ω/sq at transparency values > 85% [164]. Workers at SKKU have developed a process for synthesizing large-area graphene films for use in touch screens [165]. Sony are also working in this area. These latest results suggest that CVD graphene can meet or surpass the properties of ITO. The main limitation of CVD graphene at present is that it can only Fig 9. Enhancement of a capacitor performance by enhancing its surface be grown on copper and subsequently transferred. A area by using a ordered CNT backplane (a) Device structure (b) general limitation of graphene is how to scale up the Comparison of capacitance of flat and CNT enhanced structures. Inset shows SEM images of the respective CNT arrays. Reprinted with process to produce a uniform film over a large area, permission from (Jang et al). Copyright 2005, American Institute of Physics. which is why CNT networks are the leaders in in the area at present. CNTs are a natural candidate material for electrodes in supercapacitors. Experimental devices replace the European Position: US dominate this area activated carbon with CNTs. The surface area of through the work of Eikos Inc, Nanomix Inc., Grüner carbon CNTs is not much greater than that of and co-workers and Rinzler’s group in Florida. activated carbon, therefore energy densities are not Chhowalla at Imperial College, London has now much improved. However they are mechanically carried on this work and Roth in Stuttgart leads the arranged in a much more regular fashion, allowing a way in Europe. lower internal resistance of the device and hence higher power densities. 8.3 Supercapacitors and batteries In the last few years, many examples have appeared in 8.3.1 Supercapacitors literature where different forms of CNTs have been used in a multitude of ways to produce supercapacitor Research efforts in the field are directed towards devices (see figure 10, page 20) . Thin films of SWNTs combinations of materials with high dielectric constant can be deposited from solution, sprayed or inkjet (for higher capacitance values) and high breakdown printed on any substrate with relative ease [169], [170]. strengths (for the ability to sustain higher voltages). Due to their conductive nature, these films can Alternatively, the effective surface area can be enhanced substitute both the carbon electrode and the aluminium by using nanomaterials. Jang et al. [167] demonstrated current collector, allowing for better charge such a device by using an ordered array of CNTs and storage/device weight ratios. CNTs can also be grown coating them with a dielectric and another conductor in an orderly fashion, as aligned forests for direct (figure 9), demonstrating a 5x increase in capacitance per electron transport, higher packing density and direct cm2. These structures open new possibilities for ultrahigh contact with electrodes. Capacitors from forests of both integration density devices such as random access SWNTs [171] and MWNTs [172] show promising memory and other nano-electric devices. behaviour, and the ever increasing control achievable during growth allows for much fine tuning of devices. Energy-storing electrochemical double-layer capacitors (EDLCs) or supercapacitors have electrical parameters In more simple cases, CNTs are mixed into a low between ordinary capacitors and batteries; they are used conductivity but very high surface area activated carbon primarily as power sources or reserves. However, unlike electrode. Due to their high conductivity and aspect batteries, electrodes in EDLCs, typically made of carbon, ratio, the resulting composite results in an appreciably do not undergo any chemical reactions as in batteries; lower internal resistance [173]. A similar capacitive effect they store the charge electrostatically using reversible results also from the very fast redox reactions that occur adsorption of ions of the electrolyte onto active materials at the surface of some materials, known as that are electrochemically stable [168]. pseudocapacitance. CNT forests are used as templates 19
- 20. nanoICT research 8.3.2 Batteries The conductive properties and the high surface-to-volume ratio make carbon nanotubes potentially useful as anode materials [179] or as additives [180] in lithium-ion battery systems. The CNTs give mechanical enhancement to the electrodes, holding the graphite matrix together. They also increase the conductivity and durability of the battery, as well as increasing the area that can react with the electrolyte. Sony produces the best CNT-enhanced lithium-ion batteries. The use of CNTs as scaffolds for other materials has made the search for new electroactive materials wider than ever because requirements for such materials are now much relaxed. Low electronic conductivity, a low diffusion coefficient for lithium, and poor structural stability can all be compensated to some extent by CNTs. Recently, a so-called paper battery has been developed, where CNT arrays bound with cellulose are used as electrodes. The technology is cheap, the batteries are flexible and no harmful chemicals are required [181]. Mass production of this technology is still some distance away and the process is currently manual and laborious. Cost of CNT production remains high and Fig 10. Various carbon nanotube architectures which have been commercialisation is strongly dependent on this. employed for supercapacitors. Left column shows the SEM image while the right column shows the resulting cyclic voltammetry performance. (a) SWCNT random thin film (b) Ordered aligned array of SWCNTs, Overall, there are several potential advantages and which have been compressed with liquid. Inset shows profile view. disadvantages associated with the development of CNT Reprinted by permission from Macmillan Publishers Ltd: Nature based electrodes for lithium batteries. Materials [v], copyright (2006) (c) An array of freestanding pre-grown MWCNTs embedded in a cellulose matrix, resulting in a paper-like electrode [176]. Copyright (copyright year) National Academy of Advantages include: Sciences, U.S.A. (d) An array of MWCNTs used as a scaffold to hold low (i) better accommodation of the strain of lithium conductivity but high capacitance MnO2. Reprinted with permission from insertion/removal, improving cycle life; [177]. Copyright 2008 American Chemical Society. (ii) higher electrode/electrolyte contact area leading to on which to deposit high pseudocapacitance materials higher charge/discharge rates; (e.g. oxides [174] or polymers [175]). Whether the use of (iii) short path lengths for electronic transport CNTs will be possible at a reasonable cost is yet to be (permitting operation with low electronic conductivity determined. or at higher power); and (iv) short path lengths for Li+ transport (permitting European Position: Main progress in this field is carried operation with low Li+ conductivity or higher power). in the US (Ajayan group) and Japan. The leaders in Europe are Beguin et al. [178] who have used multi-walled CNTs Disadvantages include: mixed with polymers to create capacitance values of (i) an increase in undesirable electrode/electrolyte 100-330 F/g. Europe also has a strong industrial input reactions due to high surface area, leading to self-discharge, in this area with companies such as Maxwell (formerly poor cycling and calendar life; Montena, Switzerland). (ii) inferior packing of particles leading to lower volumetric 20
- 21. nanoICT research energy densities unless special compaction methods are Problems include choice of catalyst, catalyst deposition, developed; and depositing top contacts, increasing the packing density (iii) potentially more complex synthesis. and reducing the overall resistivity. The growth also needs optimization for back-end processing and must Toward further integration and large scale production, be carried out at low enough temperatures so as not to CNT-based negative electrodes for lithium micro-batteries damage CMOS. If SWNTs are to be employed, the are being developed by a consortium led by CEA-LITEN, packing density of metallic tubes must be high enough ST Microelectronics and Schneider [182]. to justify replacing metal interconnects. For MWNTs, for a sufficient current density, internal walls must also European Position: Although much of the innovation contribute to conduction. Neither has as yet been has been carried out in the US and the Far East, in Europe achieved. there are various groups notably in Germany contributing in this area. European Position: Infineon identified vias as a possible early application of CNTs in electronics, Intel in 8.4 Interconnects/vias the US evaluated spun-on CNTs for contacts but more recently Fujitsu, Japan lead the way. In Europe, the In order to achieve the current densities/conductivity TECHNOTUBES project which includes many partners needed for applications in vias, dense arrays of CNTs from throughout Europe is focussing on this application are required. Very dense arrays of nanotubes have been amongst others. grown by chemical vapour deposition (CVD) by various groups, following Fan et al. [183]. They are called forests, 8.5 Sensors mats or vertically-aligned nanotube arrays. They are usually multi-walled and grown from Ni, Co or Fe 8.5.1 Electronic Sensors catalysts. It has been suggested that a nanotube density of at least 1013 cm-2 is needed in order to produce the The use of CNTs for sensing is one of their most required conductivity but recently Fujitsu have indicated interesting electronic applications. Both SWCNTs and that 5x1012 cm-2 would be acceptable [184]. However MWCNTs, functionalised and unfunctionalised, have growing such dense arrays in vias of high aspect ratio is been investigated as single nanotube and network not so straightforward. Numerous groups worldwide devices. A vast number of prototypes and device are trying to optimise the process including CEA but strategies have been demonstrated for gas [187], Fujitsu [185] (see figure 11) have reported the most electrochemical and biological sensors [188], and so far significant advances and have recently reported that field-effect based sensors have detected NO2 they have achieved a density of 9x1011cm-2. They have concentrations in the ppb range [189]. Ultrathin films of also reported a resistivity of 379 μΩcm for a 2 μm SWNTs appear to be the most viable basis for an diameter via. A Microwave CVD method was employed electrical sensor in terms of scaleability, and can be to produce CNTs at temperatures compatible with fabricated through a number of different techniques CMOS. However, much improvement is still required including dielectrophoresis [190], direct CVD growth before these become a practical proposition. [191] and solution-based transfer [192], for instance embedded in a polymer coating [193]. The review by N. Sinha et al. [187] from 2006 covers carbon nanotube sensors generally, while the recent reviews by Goldoni et al [187] and Jacobs et al [188] overviews carbon nanotube gas sensors and electrochemical sensors, respectively. However, there are still many other problems to overcome in bringing this technology to market; high Fig 11. Left, CNT vias grown in pores etched into silicon. Right, CNTs volume/quality manufacturing, the intrinsic variability of grown in pores in silicon [186]. SWNT, functionalisation and cleaning/recycling. While 21
- 22. nanoICT research very high sensitivity has been achieved, the results in terms of selectivity are often less convincing, as the inert carbon nanotubes are difficult to functionalise effectively. As the energy bandgap, carrier mobility and also the chemical reactivity depend on the diameter and chirality, the variability must either be averaged out by a network architecture or by sorting by chirality, which can be done by ultracentrifuging; network TFT transistors with Fig.12: Left, structural cross-sectional layout of the sensing area of the an on/off switching ratio above 104 have been chip. Right, microscopic images of carbon nanotubes grown locally on demonstrated [195], [196]. Reproducibility of the CNT ultrathin membranes incorporating a tungsten heater [201]. growth, processing as well as variable behaviour once pathology from the detection of biomarkers in the integrated into a sensor can result in poor selectivity breath. and sensitivity. Resetting electronic nanotube sensors is another issue; methods such as annealing or using a gate Due to cost of R&D, commercialization of such sensors voltage in a TFT architecture have been used [197]. are however very costly. For example Nanomix has already raised more than $34 million and they have yet In some devices, defects play a key role, in others, the to deliver a significant, high volume product to the source/drain metal-nanotube contact is key. Also, the market although they are currently seeking FDA nanotube-nanotube junction or even the amorphous approval for their NO sensor, mentioned above, to be carbon remaining on the nanotube can play a part in used in the monitoring of asthma (Figure 13). Progress the detection scheme. [198] Indeed, there are many in these areas continues to be made globally. possible sensing mechanisms, hence a fundamental understanding of them is required to enable good optimisation and reproducibility of the sensors. However, this research area is very active, so further progress is expected. The US company Nanomix Inc was the first to put an electronic device that integrated carbon nanotubes on a silicon platform (in May 2005 they produced a hydrogen sensor) on the market [199]. Since then, Nanomix has taken out various other sensing patents e.g. for carbon dioxide, nitrous oxide, glucose, DNA Fig. 13: Preproduct picture of Nanomix’s NO sensor (with authorisation detection etc. [200]. The next product to become from YL Chang, Nanomix). available should be a breath analyzer detecting NO as a marker of asthma. More recently workers in Cambridge European Position: Many companies and research and Warwick University in collaboration with ETRI, institutions in Europe are carrying out work in this area, South Korea have integrated CNTs onto SOI substrates with THALES, Dekker (Delft), being the most notable. to produce smart gas sensors (see figure 12) [201]. The CNTs have been locally grown on microheaters allowing 8.5.2 CNTs in Biotechnology and Medical Device back end deposition at T~700°C without significantly Research. affecting the surrounding CMOS. Nanomedicine, or the application of nanotechnology to Other groups have been using a multi-functionalized achieve breakthroughs in healthcare, exploits the network of sensors combined with a principal improved and often novel physical, chemical and component analysis in order to enable pattern biological properties of materials at the nanometer recognition approach (artificial nose) [202]. Such an scale. Nanomedicine has the potential to enable early approach could enable early diagnosis of various detection and prevention, and to essentially improve 22
- 23. nanoICT research diagnosis treatment and follow-up of diseases. This of biocompatibility and nanotoxicity associated with the section addresses the overall roadmaps associated with use, manufacture and purchase of CNTs in all forms. nanomedicine and in particular identifies the role of Recent reviews point out that fullerenes have potentially CNTs. Technologically, most CNT applications in biotech useful properties, and that several of the reports on are in biosensing, lab-on-a-chip, drug screening and drug toxicity have used unrealistic conditions and doses [204]. delivery. The key issues associated with CNTs related to Nevertheless, the possible toxicity remains the single nanomedicine closely resemble those mentioned in the largest barrier in terms of practical use of nanotubes for previous section and are mainly related to the following drug delivery or cancer therapy; the nanotubes do not issues: break down in the body, and are able to kill cells if not effectively passivated. Since the carbon nanotubes will •Some of the main challenges are linked to ultimately last longer than any passivation, and have industrialisation. There is no conventional manufacturing been shown to aggregate in the lungs and brains of method that creates low cost CNTs. Desirable animals, it is questionable whether or not carbon properties are robustness, reproducibility, uniformity nanotube-based medicine will be used in clinical tests in and purity. the near future. Furthermore, other very effective, •Reproducible production relating to surface defects; nontoxic systems are available for drug delivery, such as surface chemistry, size (height and diameter; protein-coded liposomes [205]. morphology; type etc. •The ability to functionalise the surface with European Position: The area of carbon nanotube appropriate chemistries. based sensors is very broad and diverse due to the •The ability to produce arrays; periodicity; catalyst free- or many different physical quantities, applications and tailored catalyst grown from self-assembly. environmental conditions for such devices. In terms of •To produce lost cost routes to manufacture in the case gas and biological sensing, a substantial number of of disposable or competitive devices. groups are pursuing high sensitivity gas or biological •The ability to integrate into microfluidic systems; molecular sensing, better functionalisation/specificity as CMOS circuitry or flexible substrate systems. well as scaleable fabrication methods, in US as well as in Europe. While the research activities are spread all over In the next ten years, the development of biosensors Europe, a few commercial players are active. Cambridge and importantly, nanotechnology, will allow the design CMOS Sensors are developing sensor platforms based and fabrication of miniaturised clinical laboratory on carbon nanotubes. There are strong activities in analysers to a degree where it is possible to analyse many universities including ETH Zurich and Cambridge several laboratory measurements at the bedside with as University but it is not possible to identify single leading little as 3 μL of whole blood. The use of quantum dots; groups. In terms of nanotubes used inside humans, it is self-assembly; multifunctional nanoparticles, nano-templates not likely that this will be possible on a short or even long and nano-scale fabrication including nanoimprinting will term due to the unresolved toxicity issues. have a major impact on the design and development of much improved highly sensitive and rapid diagnostics; 8.6 Field emission thus allowing accurate drug delivery integration. Nanoenabled high throughput analysis will also reduce Because of their high—current-carrying capability, the time it takes to bring a new drug delivery platform chemical inertness, physical strength and high aspect ratio, to market. CNTs can be applied to many technologies requiring field emission. Immediately after the growth, CNTs contain CNTs have also been shown to favour neuronal growth impurities such as amorphous carbon, catalyst, residual and reduce glial scar formation (gliosis). CNT-based growth gases as well as varying degrees of structural electrodes have therefore been proposed for use in defects dependent on the growth parameters. The implants to enable long term treatment of Alzheimer defects and impurities should be removed or passivated disease using deep brain stimulation [203]. for optimum field emission properties. Treatments such as plasma, IR and UV lasers and oxidative annealing can A clear set of studies is required to resolve all the issue be used to clean the tips of the CNTs. 23
- 24. nanoICT research Researchers have reported low threshold field (field CEA of France continue to fund research in this area, required to emit electrons at a level of 1 μA/cm2) for with typical displays produced shown in figure 13. various types of CNTs. However there is a debate about the long term emission stability of CNTs. Open tips, 8.6.2 Microwave Generators aligned CNTs and aligned CNTs embedded in PMMA matrix have shown significant improvement in emission High power/frequency amplifiers for higher bandwidth, stability as compared to the as-grown CNT samples. more channels and microwave links are increasingly using Nanotubes can be used for single electron beam devices the 30 GHz and above frequency range. Attempts have (such as field-emission scanning/ transmission electron been made to replace the thermionic cathode in a microscopy) or multiple beam devices (like flat panel travelling wave tube (TWT) with a Spindt tip cathode displays, or as light sources). The major field emission delivering the dc electron beam. However, the most applications are listed below. effective way to reduce the size of a TWT is via direct modulation of the e-beam, for example, in a triode 8.6.1 Field Emission Displays configuration using CNTs as the electron source. Motorola in the early/mid 1990’s investigated the use of Thales, in collaboration with Cambridge University carbon based materials for Field Emission Displays Engineering Department, have successfully demonstrated including the use of diamond, DLC and CNTs a Class D (i.e. pulse mode/on-off) operation of a carbon [206],[207]. More recently they have reported a CNT nanotube array cathode at 1.5 GHz, with an average based Field Emission HDTV [208]. Over the last 10 current density of 1.3 A/cm2 and peak current density years or so various companies including Philips, TECO of 12 A/cm2; these are compatible with travelling wave Nanotech, ISE Electronics and especially Samsung tube amplifier requirements (>1A/cm2) [212]. Recently, (SAIT) [209] have worked on the use of CNTs for TV they have also achieved 32 GHz direct modulation of a applications. SAIT successfully produced demos of full carbon nanotube array cathode under Class A (i.e. sine colour 39”diagonal TVs and this technology was wave) operation, with over 90% modulation depth [213]. transferred to Samsung SDI for production in the late Other advantages that carbon nanotube cathodes offer 2000s. However, no displays based on their technology have reached the market. Other work on Field Emission displays based on SEDs (Surface-conduction Electron-emitter Displays) also took place in the late 2000’s. Formerly a collaboration between Toshiba and Canon, the displays utilise emission from carbon but not CNTs [210], but these are yet to appear commercially. Most recently, Sony have announced a major investment in FEDs based on a Spindt process. Teco Nanotech Co Ltd (a small company based in Taiwan) also market three basic CNT-based FEDs, the largest being an 8.9” diagonal [211]. Fig.14: (a) Schematics of the experimental setup with the cathode—grid assembly (spacer thickness = 100 μm), the transparent and conductive anode, and the 532 nm laser. (b) Top, left axis: emitted current as a function of the applied voltage and absorbed optical powers. Bottom, right axis: voltage drop ΔV as a function of the applied voltage and absorbed optical powers. The dotted curve β = 465 is the Fowler-Nordheim fit for the case where the p—i—n photodiode exhibits no voltage drop. (c) Fig.13: Two stages of development of CNT FED at CEA. On the left: Photocurrent as a function of the absorbed optical power for a 2200 V monochrome display with 350μm pixels, on the right: color video display cathode—grid applied voltage. The associated quantum efficiency with 600μm pixels. On these display the non uniformity from pixel to calculated from the slope is ~10%. (d) Example of optical modulation of pixel is 5% while it is 3% with LCD displays and 2% for CRT (courtesy the emission current from MWCNTs by pulsing the laser source at low of CEA-LITEN). frequency (1 kHz) [215]. 24
- 25. nanoICT research include no heating requirement and the ability to turn 8.6.4 Backlighting on or off instantly (for efficient operation). Xintek have also been working on CNT-based microwave amplifiers Although their use in full colour FE-based TVs is still for the US Air Force [214]. The main problem at present problematical, the use of CNTs as electron emitters in is the limited modulation bandwidth associated with such FE-based backlight units for AMLCDs is still under devices. However, Hudanski et al [215] reported the use investigation by various companies worldwide. Major of photocathodes (shown in figure 14), which combine players in the TFT-LCD display industry, such as semiconducting silicon p—i—n photodiodes with MWCNT Samsung, Corning and LG Electronics (LGE), are keen to field emitters, which exhibited high quantum efficiency develop carbon-nanotube (CNT) backlight modules, (~10%), significant current density (0.2 A cm−2) which with Taiwan-based backlight-module makers also can be operated continuously or be optically modulated. interested in following suit [220]. In Korea Iljin also have several years of experience in this area [221]. 8.6.3 X-ray Instruments In theory, CNT backlight modules have a lower Oxford Instruments have worked together with NASA temperature, consume less power and are less expensive on CNT-based X-ray sources that employ field emission to produce than traditional backlight modules. It is a good as the electron source, rather than thermionic emission, candidate to eventually replace CCFL (cold cathode which has much lower power efficiency [216]. Their fluorescent lamp) backlighting but has strong competition application is targeted towards low-power use for a from LEDs, which could be much cheaper to produce space mission to Mars (though high power would be and are already in the market place. more preferable), once again because of their low weight and fast response time. Oxford Instruments have The challenges are again to improve the lifetime of the also developed and sold hand-held low power X-ray emitters and to reduce cost to be competitive with other imagers which can be applied to medicine and for technologies. diagnostics in circuit boards [217]. Zhou and co-workers at Xintek (see figure 15) have developed a fast response, 8.6.5 Electron microscopy sharp-focus X-ray tube with quick pulsation [218]. MoXtek have also produced similar devices [219]. Recent research has investigated whether the carbon nanotube can act as an improved electron source for Challenges for these devices are in achieving high power microscopy and how it compares to the other electron with stability and reproducibility. sources available today. Various groups from FEI, Cambridge University, EMPA, El-Mul etc have researched the optimum way to produce CNTs for use in microscopy. The CNTs act as a cold cathode source and the standard manufacturing procedure is to add a single CNT to the tip of a standard tungsten emitter. Growth, rather than attachment is felt to be a better process [222]. Mann et al. [223] used PECVD and describe how such a procedure is scalable with the ability to grow a single CNT on each W tip (shown in figure 16, see page 26). El Mul has developed a silicon-based CNT microcathode in which the CNT is grown in an etched pore [224]. The emission characteristics of the CNT have been found to be extremely promising with the key parameters of Fig.15: Left, the X-ray tube current versus the gate voltage measured with the process understood. Progress still needs to be made the anode voltage fixed at 40 kV. It follows the classic Fowler—Nordheim to optimise reproducibility. relation. The distance between the cathode and the gate is 150 μm. Right, X-ray image of a normal mouse carcass (25 g) obtained using a CNT source-based imaging system. 25
- 26. nanoICT research 8.6.6.2 Gauges/Sensors The Physical Metrology Division, Korea Research Institute of Standards and Science are using the field emission effect from a carbon nanotube to characterize a new type of technology for detecting low pressures. The fabricated low pressure sensor is of a triode type, consisting of a cathode (carbon nanotubes field emitter Fig.16: Left, electron micrograph of a single CNT grown on a tungsten tip. Note that the growth is aligned with the tungsten axis. Centre, a arrays), a grid and a collector. Due to the excellent field tungsten tip mounted in a suppressor module. Right, a CNT grown on emission characteristics of CNTs, it is possible to make a a tungsten already mounted in the suppressor in situ. cost effective cold cathode type ionization gauge. For an effective CNT cathode for both the sensor and gauge the 8.6.6 Ionization for Propulsion and Detection researchers used the screen-printing method and also controlled the collector and the grid potentials in order 8.6.6.1 Electric Propulsion to obtain a high ionization current. They found that the ratio of the ionization current to the CNT cathode current Replacing hollow and filament cathodes with field changes according to the pressure in the chamber [230]. emission (FE) cathodes could significantly improve the scalability, power, and performance of some meso- and 8.6.6.3 Miniaturised gas ionization sensors using microscale Electric Propulsion (EP) systems. There is carbon nanotubes considerable interest now in microscale spacecraft to support robotic exploration of the solar system and Ajayan et al. from the Rensselaer Polytechnic Institute have characterize the near-Earth environment. The challenge developed Ionization sensors by fingerprinting the is to arrive at a working, miniature electric propulsion ionization characteristics of distinct gases [231]. They system which can operate at much lower power levels report the fabrication and successful testing of ionization than conventional electric propulsion hardware, and microsensors featuring the electrical breakdown of a range meets the unique mass, power, and size requirements of of gases and gas mixtures at carbon nanotube tips. The a microscale spacecraft. sharp tips of the nanotubes generate very high electric fields at relatively low voltages, lowering breakdown Busek Company, Inc. (Natick, MA), has developed field voltages several-fold in comparison to traditional electrodes, emission cathodes (FECs) based on carbon nanotubes. and thereby enabling compact, battery-powered and safe The non-thermionic devices have onset voltages about operation of such sensors. an order of magnitude lower than devices that rely on diamond or diamond-like carbon films. The sensors show good sensitivity and selectivity, and are unaffected by extraneous factors such as temperature, Worcester Polytechnic Institute (WPI) includes the humidity, and gas flow. As such, the devices offer several programmes headed by Professors Blandino and practical advantages over previously reported nanotube Gatsonis. Blandino’s research is largely focused on the sensor systems. The simple, low-cost, sensors described study of colloid thrusters for small satellite propulsion, here could be deployed for a variety of applications, such as and in the development of novel, earth-orbiting environmental monitoring, sensing in chemical processing spacecraft formations [226]. The Gatsonis’ activity also plants, and gas detection for counter-terrorism. includes modelling of plasma micropropulsion [227]. McLaughlin and Maguire [232] at the University of Ulster Groups from the Rutherford Appleton Laboratory [228] report the use of CNTs in order to decrease the turn-on and Brunel University [229]are studying the field emission voltage associated with microplasmas and the performance of macroscopically gated multi-walled enhancement of emission spectra associated with gas carbon nanotubes for a spacecraft neutralizer. types. In particular the device focuses on mixed gas types such as breath analysis and environmental monitoring. The ability of low cost CNT structured electrodes is key to 26
- 27. nanoICT research improving performances related to higher sensitivity and In theory the NRAM chip would replace two kinds of specificity of gases such as NOx. Catalyst free growth memory. While cell phones, for example, use both flash techniques have been reported using thermal CVD routes chips and SRAM or DRAM chips, NRAM would and the study is also looking at the optimum CNT spacing perform both functions. However the memory market and height required for short time ionisation or FE is oversupplied and they frequently have to be sold at a applications to gas sensors. loss, making it difficult for any new technology to break in. In addition, several other major companies are The main driver at present is to improve the efficiency developing their own non-volatile memory technologies which currently lies at around 1%. with PRAM perhaps the leading contender at present. European Position: From a display viewpoint Europe PRAM, FRAM, MRAM and RRAM are all dominated by were very much forerunners but then Samsung highly competitive, large companies. With Nantero’s provided the more recent display drive. As regards relatively small size and long development time, market work on sources for electron microscopy in Europe De penetration is a big issue. Jonge and co-workers did some excellent work on characterisation of single emitters as did Groning on CNT-NEMS have been suggested for high frequency arrays of emitters. Thales in collaboration with several mechanical resonators and switches, where the low mass universities has continued European interest in the and high stiffness can lead to GHz switching speeds and design of high frequency CNT based sources. For X-ray resonance frequency. High frequency resonators are sources Oxford Instruments led the way and more targeting RF electronics [234] and high-sensitivity mass recently Xintek in the US and Philips in Europe has sensing [236]. Here the advantages provided by the expanded the work. In backlighting as in Displays the outstanding material properties of CNT are possibly Far East leads the way. The leaders in the FE based outweighed by the numerous technological and propulsion area are in the US where the Jet Propulsion manufacturing challenges including controlled growth, Laboratory Pasadena, Busek Co., Inc. and the durability of switches and scaled up production with a Worcester Polytechnic Institute (WPI) lead the way. In sufficient repeatability and yield [237]. Europe the main groups are from the Rutherford Appleton Laboratory, Brunel University, the University of Cambridge, the University of Groningen, and the University of Ulster. However, with the exception of display technology, the field emission market size is comparatively low when compared with the other ICT applications described above. 8.7 NEMS switches, resonators and sensors Jang et al [233] demonstrated novel non volatile and volatile memory devices based on vertically aligned Fig. 17: Left, a schematic diagram showing a cross-section of a switch fabricated by Jang et al.. Both contacts and catalyst were deposited with MWCNTs (see figure 17). Nanoelectromechanical e-beam lithography. Right, an electron micrograph showing the grown switches with vertically and horizontally [234] aligned CNTs acting as a switch. carbon nanotubes have been demonstrated. However, Nantero are the market leaders in this area and have A significant effort has also been focused on CNT for created multiple prototype devices, including an array of piezoresisitve applications, the advantage being a much ten billion suspended nanotube junctions on a single larger gauge factor as compared to for instance silicon. silicon wafer [235]. Nantero's design for NRAM™ While high-performance, high yield CNT based strain involves the use of suspended nanotube junctions as gauges have been realised with wafer-level on-chip memory bits, with the "up" position representing bit synthesis [238], the gauge factor’s dependence on zero and the "down" position representing bit one. Bits chirality remains a very significant challenge in terms of are switched between states through the application of commercialisation. electrical fields. 27
- 28. nanoICT research CNTs as high aspect ratio and supersharp tips for Atomic Force Microscope remain a possibility; the high stiffness and yield strength of MWNT allows more slender tips to be used for probing deep trenches and sidewalls [239], SWNT nanotubes present unparalled sharpness and wear resistance [240], and bundles of SWNT show enhanced surface potential imaging [241]. CNT-HARP tips are either manufactured by growth Fig.18: Left, Experimental setup of the Er/Yb:glass laser. OC: output [242] or direct manipulation [239],[240]. Recently, DTU, coupler; M1-M4: standard Bragg-mirrors; CNT-SAM: Saturable absorber mirror based on carbon nanotubes; LD: pigtailed laser diode the University of Cambridge and Oldenburg University for pumping the Er/Yb:glass (QX/Er, Kigre Inc., 4.8 mm path-length). demonstrated multiple assemblies of MWNT on AFM Right, background-free autocorrelation. The solid line is a sech2 fit with probes [243] using a microgripper-based approach to a corresponding FWHM pulse-duration of 68 fs [255]. automated robotic assembly [244]. A comprehensive The major laser systems mode-locked by CNT saturable overview is given by Wilson and Macpherson [240]. absorbers demonstrated so far (see figure 18) include fibre lasers, waveguide lasers and solid-state lasers, European Position: Nantero are the world leaders generating sub-ps pulses in a spectral range between but in Europe, one can cite the partners of the NanoRF 1070 and 1600 nm [255]. The shortest pulse of about [245] European project as well as ETH, TU Denmark. A 68 fs was achieved with a solid state Er3+ glass laser by collaboration between Cambridge Univ. Engineering, using a CNT-polyimide composite [256]. Additionally, Samsung and Thales is also ongoing. amplified spontaneous emission noise suppression has been demonstrated with CNT-based saturable 8.8 Saturable Absorbers absorbers, showing great promise for this technology for multi-channel, all-optical signal regeneration in fibre The band gap of semiconducting CNTs depends on their telecom systems [257]. diameter and chirality, i.e. the twist angle along the tube axis[246]. Thus, by tuning the nanotube diameter it is easy Challenges include justifying the research to industry to provide optical absorption over a broad spectral range due to the limited market potential. [247]. Single-walled CNTs exhibit strong saturable absorption nonlinearities, i.e. they become transparent under sufficiently European Position: There are 5 major research groups intense light and can be used for various photonic working on CNT saturable absorber applications around applications e.g in switches, routers and to regenerate optical the world: Sakakibara at the National Institute for signals, or form ultra-short laser pulses[248],[249],[250]. It is Advanced Industrial Science and Technology (AIST), possible to achieve strong saturable absorption with CNTs Tsukuba, Japan, Maruyama and Yamashita at Tokyo over a very broad spectral range (between 900 and 3000 nm University & Set in the Alnair Labs and Yoshida at Tohoku [251]). CNTs also have sub-picosecond relaxation times and University. In Europe Dr. E. Obraztsova at the Institute for are thus ideal for ultrafast photonics [252],[253]. CNT General Physics, Moscow, and Cambridge University saturable absorbers can be produced by cheap wet Engineering Department are the major players. chemistry and can be easily integrated into polymer photonic systems. This makes a CNT-based saturable 8.9 Fuel Cells absorber very attractive when compared to existing technology, which utilises multiple quantum wells (MQW) Carbon nanotubes can be used to replace the porous carbon semiconductor saturable absorbers and requires costly in electrode-bipolar plates in proton exchange membrane fuel and complicated molecular beam epitaxial growth of cells, which are usually made of metal or graphite/carbon multiple quantum wells plus a post-growth ion black. The CNTs increase the conductivity and surface area implantation to reduce relaxation times[254]. Additionally, of the electrodes which means that the amount of platinum MQW saturable absorbers can operate only between 800 catalyst required can be reduced [258].The state of the art in and 2000 nm -a much narrower absorption bandwidth this area is the mixing of CNTs and platinum catalyst particles than that available using CNTs. reported by Sun et al of Taiwan[259]. 28
- 29. nanoICT research Whilst CNTs reduce the amount of platinum required, it CNTs have also been used as a high surface area charge is only a small percentage, which means that the cost of collecting scaffold for nanoparticles in several types of cells. the fuel cell remains high. Also, CNTs are comparable in Photoconversion efficiencies of 1.5% and 1.3% have been price to gold, meaning the saving is minimal. achieved with SWCNTs deposited in combination with light harvesting CdS quantum dots and porphyrins, respectively Nevertheless, vertically aligned MWCNTs can be used [264]. Other varieties of semiconductor particles including as highly efficient fuel cell electrode material. Aligned CdSe and CdTe can induce charge-transfer processes under CNTs electrode have a host of advantages in the visible light irradiation when attached to CNTs [265]. In Polymer Electrolyte Membrane fuel cells (PEMFC) and dye-sensitized solar cells (DSSC), titanium dioxide coated Direct Methanol fuel cells (DMFC), such as higher onto CNTs shows enhanced electron transport and electrical conductivity, large surface area, possible higher increases the photoconversion efficiency [266]. gas permeability and higher hydrophobic surfaces facilitating faster removal of water from the electrodes. Much work has been done to use SWCNT thin films in In general, Pt, Pt/Ru nanoparticles are dispersed in PV as transparent conductive coatings to replace ITO CNTs to obtain platinum/CNT-based electrocatalysts which has both a limited supply and a limited tolerance [260]. However the large-scale market application of fuel to flexibility. Conductivity to transparency ratios are fast cells will be difficult to realize if the expensive Pt-based approaching that of ITO. Barriers to commercialisation electrocatalysts for oxygen reduction reactions (ORRs) are now more related to problems with the adhesion cannot be replaced by other efficient, low-cost, and of the CNT film to the substrate. stable electrodes. Recent results from Gong et al. [261] have shown that N doped arrays of MWCNTs (acting Novel antennae effects, as well as improved charge as a metal-free electrode), can provide superb catalytic collection and optical enhancement can be obtained in activity for Oxygen Reduction Reaction (ORR). cells in which CNT growth is patterned. Zhou et al. [267] recently demonstrated enhancement in amorphous European Position: The leaders in this area are the silicon solar cells deposited onto a patterned array of Taiwanese groups. The European leaders are linked to S. CNTs with spacing of the order of visible light Roth (Max Planck Institute, Stuttgart). wavelengths. 8.10 Solar panels European Position: Although much of the work in this area has been driven by Japan and the USA (with CNTs have been utilised in solar cells in a number of Unidym Inc. which announced in February 2010 a joint ways. Primarily they are used to enhance charge venture to market printable CNT electronics in Korea), collection. Kymakis et al [262] dispersed CNTs in the significant input on both the incorporation of the CNTs photoactive layer of organic solar cells to replace C60 as part of the active layer and in transparent contact and benefit from the 1D structure. However, the power materials has been made across the European efficiency of the devices remains low at 0.04% community. Commercial ventures are in place in both suggesting incomplete exciton dissociation at low CNT the USA and Finland to commercialise CNT thin films. concentrations. At higher concentrations, the CNTs short-circuit the device. More recently, a polymer 8.11 Antennae photovoltaic device from C60-modified SWCNTs and P3HT has been fabricated [263]. P3HT, a conjugated Ren’ group at Boston College has demonstrated the use polymer was added resulting in a power conversion of a single multi-walled CNT to act as an optical efficiency of 0.57% under simulated solar irradiation antenna, whose response is fully consistent with (95mWcm-2). An improved short circuit current density conventional radio antenna theory [268]. The antenna was attributed to the addition of SWCNTs to the has a cylindrically symmetric radiation pattern and is composite causing faster electron transport via the characterized by a multi-lobe pattern, which is most network of SWCNTs. Hybrid CNT-polymer devices pronounced in the specular direction. Possible however have shown so far only moderate applications for optical antennae include optical performance. switching, power conversion and light transmission. 29
- 30. nanoICT research One particular application is the “rectenna”, which is the emitters to stimulate phosphors has been reported by light analogue of the crystal radio in which an antenna is various groups and the replacement of metallic filaments attached to an ultrafast diode. This could lead to a new with carbon CNTs/Fibres has been investigated by class of light demodulators for optoelectronic circuits, or groups mostly in China. Carbon nanotube bulbs made to a new generation of highly efficient solar cells. from CNT strands and films have been fabricated and their luminescent properties, including the lighting The growth of microwave applications, such as mobile efficiency, voltage-current relation and thermal stability phones, remote sensing and global navigation satellite have been investigated. The results show that a CNT systems, etc, requires the development of materials with bulb has a comparable spectrum of visible light to a a large tunability and very low loss in the microwave tungsten bulb and its average efficiency is 40% higher frequency range (from GHz to sub-THz). This is than that of a tungsten filament at the same temperature particularly important, as the intrinsic loss of the existing (1400―2300K) [269]. The nanotube filaments show tunable dielectrics based on ferroelectric ceramics both resistance and thermal stability over a large increases significantly when the frequency in use is above temperature region. No obvious damage was found on a few GHz. Liquid crystals (LCs) have attracted much a nanotube bulb held at 2300 K for more than 24 hours attention in recent years because of their low loss at in vacuum, but the cost needs to be significantly reduced microwave frequencies. However, although the dielectric and the lifetime significantly increased for this to be anisotropy/tenability of LCs is comparatively higher than considered seriously as an option. other low loss materials, it is desirable to further increase its value. This is particularly important for antennae and European Position: Most effort is located in the Far phase arrays for beam steering in the applications such as East but Bonnard et al at EPFL have also contributed in automobile forward looking radar and satellite up-links. this area. At Cambridge, preliminary work shows that mixing CNTs of suitable sizes with LCs can significantly enhance 8.13 Nanofluidics dielectric anisotropy (see Fig. 19). Further measurement confirms similar trend at much higher frequency ranges. The interest in taking advantage of the unique properties of carbon CNTs in nanofluidic devices has European Position: Early stages of research in the increased tremendously over the last couple of years. Department of Engineering at Cambridge University, in The CNTs can either be used directly as a nanofluidic collaboration with Technische Universität Darmstadt, channel in order to achieve extremely small and smooth Queen Mary College, London, ALPS Electric, Japan, pores with enhanced flow properties [270] or be Dow Corning, USA, and Nokia, Finland/UK. embedded into existing fluidic channels to take advantage of their hydrophobic sorbent properties and high surface-to-volume ratio for improving chemical separation systems [271], [272]. By integrating vertically aligned CNTs into silicon nitride [273] and polymer membranes [274],[275] respectively, it has been possible to study the flow of liquids and gases through the core of carbon nanotubes. The flow rates were enhanced by several orders of magnitude, Fig 19. Studies of pure and CNT loaded E7 using capacitor method, switching with electric field: (a) 3GHz, sweeping from ε⊥ to ε||; (b) compared to what would be expected from continuum 3GHz, sweeping from tanδ⊥ to tanδ||; (c) Δε in 1-4GHz range, switching hydrodynamic theory [266]. The reason for this is with 0.5V/μm. believed to be due to the hydrophobic nature of the inner carbon nanotube sidewall, together with the high 8.12 Lighting smoothness, which results in a weak interaction with the water molecules, thereby enabling nearly frictionless There is ongoing work on the use of CNTs for low flow through the core of the tubes. This effect energy lighting applications. The use of CNTs as electron resembles transport through transmembrane protein 30
- 31. nanoICT research pores, such as aquaporins, where water molecules line surface to avoid aggregation and to benefit from the high up in a single file with very little interaction with the uniformity of the nanostructures. sidewall. European Position: Montena Components of This application of CNTs is envisioned to result in novel Switzerland are in competition with Maxwell Technologies ultrafiltration and size-based exclusion separation devices, in this area. DTU is also very active in the field. since the pore sizes approach the size of ion channels in cells [266]. The CNT membranes are, however, fabricated 8.14 Liquid crystal microlenses by CVD and this application suffers from the lack of large scale cost-effective CNT deposition equipment. Liquid crystals (LCs) are potentially a very exciting technology for creating a real-time holographic three In the last couple of years CNTs have also been dimensional (3D) display system. For the reproduction investigated as a sorbent material for improving both the of a full 3D image, a fully complex hologram is the resolution and sensitivity of chemical separations [271], ultimate solution, but it is very difficult to display using [272]. This has been done by incorporating the nanotubes current technology such as Liquid Crystal (LC) over in the stationary phase of mainly gas chromatography silicon (LCOS), as shown by the simple image in Figure columns to take advantage of their high surface-to-volume 20(c) projected using a binary phase only LCOS device. ratio and better thermal and mechanical stability compared to organic phases, which make them ideal for A purely phase only hologram (or kinoform) is the best especially temperature programmed separations for building 3D displays, however there are limits due to [271],[272],[276]. The carbon nanotubes, in the form of the way in which a liquid crystal can be used to powder, are hard to pack directly in columns due to their modulate a phase only hologram. A traditional pixel has tendency for aggregation and hence channel blockage a top and bottom planar electrode creating a uniform [271],[272], [277] so the CNTs have typically either been electric field. The device pixel as shown in Figure 20(a) incorporated in a monolithic column [278] immobilized has a CNT in the centre which creates a non-uniform on the inner channel wall [279] or deposited on the electric field profile [285]. This changes the way in which surface of beads that subsequently were packed [280]. the LC switches and responds to the field. A major complication of these methods, apart from the fact that they are manual and very labour-intensive, is that they rely on the necessity of forming uniform CNT suspensions, which is difficult, since CNTs are insoluble in aqueous solutions and most organic solvents [271]. It is therefore typically required to either dynamically or covalently modify the CNTs to avoid aggregation [272]. Fig 20. (a) A nanotube electrodes in a liquid crystal cell with an external These problems can be overcome by direct growth of the fields applied. (b) Individual LC electrodes (top left are 1um pitch), (c) 3D projected hologram image. CNTs on a surface, in e.g. microfluidic channels [281],[282],[283], so they are anchored to the channel wall and therefore unable to form aggregates. This also Applications such as 3D holography require large densities allows a much higher CNT concentration without of very fine pixels[286]. Current LCOS devices are limited clogging the fluidic devices. Growing of CNTs in to pixels pitches of around 5um before alignment and microfluidic systems has the additional benefit that fringing fields become a problem. Figure 20(b) shows a lithography can be used for the pattern definition, which CNT electrode device fabricated at Cambridge with a should make it possible to make much more uniform and pixel pitch of 1 μm. Due to the non-uniform field profile, therefore more efficient columns [284]. the LC material clearly switches as single pixels. The device as shown only switches as a single array of A major limitation of this application is also the lack of electrodes and to make a hologram we need them to be low cost CNT deposition equipment, since it is necessary individually addressed, hence we need an active backplane to use vertically aligned CNTs that are attached to the such as that found in LCOS to grow them upon [287]. 31
- 32. nanoICT research European Position: The Engineering Department in 6000 S/m and a subthreshold swing of ~70 mV/decade Cambridge as far as we know, are the only group in respectively [295]. Table 2 summarizes the work it this Europe working in this area. area. Table 2: A summary of the optimum properties obtained from single electron transistors 8.15 Transistors 8.15.1 Individual CNT-Based Transistors Arguably this has been the electronic application on which most research has focused. Martel et al. and Tans et al. first reported a bottom gate individual single walled carbon nanotube field effect transistor (SWNT-FETs) with an on/off ratio of ~105 and a mobility of 20 cm2/Vs in 1998 [288], [289]. Afterwards, Durkop et al. claimed a mobility for bottom-gate SWNT-FET of >105 cm2/Vs with a subthreshold swing ~100 mV/decade Several groups have also investigated vertical CNT-FETs [290]. This mobility is still the highest reported for (wrap-around gate). Choi et al. reported the first vertical bottom gate CNT-FETs thus far. Meanwhile, top gate MWNT-FET with a best conductance of 50 mS in 2003[296] SWNT-FETs were also attracting attention since such a but this only works at low temperatures. Maschmann et al. structure can be readily used for logic circuits. In 2002, demonstrate a vertical SWNT-FET in 2006 [297]. Their Wind et al. first demonstrated a top gate SWNT-FET devices exhibited a good ohmic SWNT-metal contact, but with an on/off ratio of ~106, a transconductance of the gate effect is not as efficient as either the top gate or 2300 S/m and a subthreshold swing of 130 mV/decade bottom gate SWNT-FETs. SWNT-FETs always exhibit p-type [291]. Rosenblatt et al. and Minot et al. [293] using NaCl operation when contacted ohmically, but n-type SWNT-FETs and KCl solutions as the top gate in SWNT-FETs are also needed for fabrication of logic circuits. Derycke et showed a mobility of 1500 cm2/Vs, a subthreshold al. claimed both annealing (removal of oxygen) and doping swing of ~80 mV/decade and an on/off ratio of 105. (e.g. potassium) can convert a p-type SWNT-FET into a Yang et al. [294] showed a very high transconductance n-type and a logic inverter was demonstrated [298],[299]. of 1000 S/m in a top gate device (shown in figure 21, Javey et al. and Chen et al. reported that using different together with a bottom gate device). Javey et al. also metal electrodes (e.g. Al) they could also obtain n-type demonstrated high performance SWNT-FETs using high-k SWNT-FETs with a ring oscillator also fabricated dielectric ZrO2 as the top gate insulator. Devices exhibited [300],[301]. a mobility of 3,000 cm2/Vs, a transconductance of Challenges for the future include controlling the chirality and diameter, improving the yield of working devices, improving the reproducibility of the contact, ensuring all CNTs are semiconducting, improving the uniformity of the devices, controlling their Fig. 21: Typical SWNT-FET transistor characteristics made at the University of Cambridge with different positioning, and developing a contacts. Left, Pd makes and ohmic contact which results in p-type conduction. Centre, Ti contacts result in process that can be scaled up strong ambipolar behaviour. Right, Al makes a Schottky contact which results in n-type conduction but with a to mass-production. strong leakage current. 32
- 33. nanoICT research European Position: The state-of-the-art transistors for on-off ratio and mobility (see figure 22). The group (dependent on characteristics) are those produced by in Grenoble have also investigated this and have made a the groups of Avouris at IBM and in Europe, Bourgoin small chip incorporating 75 such transistors. at CEA, Saclay, Ecole Polytechnique and Dekker at Delft University of Technology. The interest in the networks comes from the fact that if the average nanotube length is small compared to the 8.15.2 Network CNTs distance between source and drain and more than one tube is needed to make the connection, the probability In order to overcome the various problems with individual of having an electrical path made only of metallic tubes CNT transistors, numerous groups have concentrated on is ~(1/3)n where n is the number of tubes needed to the production of transistors manufactured from CNT make the junction. Secondly, the on/off ratio increases networks or even CNT/Polymer mixtures. since, even if two tubes are metallic, their contact is not metallic. [306] Finally, even a single defect is enough to In 2002, the first report (a patent) for transistors based open a bandgap in a metallic tube, turning it into a on random networks of nanotubes and their use in semiconductor. [307] This means that controlling the chemical sensors was produced by Nanomix Inc., [302] number of defects is an important challenge to followed in 2003 by the disclosure of their integration overcome. onto a 100 mm Si wafer. [303] The first public disclosure was made in 2003 by Snow et al. [304] who The first transparent CNT based transistor made on a demonstrated a SWNT thin film transistor with a flexible substrate was achieved by transferring a CNT mobility of >10 cm2/Vs and a subthreshold swing of random network from a silicon substrate onto a 250mV/decade with an on/off ratio of 10. In 2007, polyimine polymer [308]. Kang et al. grew highly dense, perfectly aligned SWNT arrays on a quartz substrate which were then 8.15.3 High frequency nanotube transistors transferred to a flexible plastic substrate (PET). The SWNT-FETs were fabricated on the PET substrate and The high carrier mobility of CNTs makes them potential exhibited a mobility of 1000 cm2/Vs and a candidates for high frequency transistors. It was shown transconductance of 3000 S/m [305] . The Rogers in 2004 that optimized carbon nanotube field effect group has exhibited state-of-the-art network transistors transistors (CNTFETs) would have cut-off frequencies fT above those of FETs built from any other semiconductor [309]. For aggressively scaled devices working in the ballistic regime, the intrinsic fT could reach the THz range. These first projections were confirmed by detailed calculations from several groups worldwide [310], [311]. From an experimental point of view, measuring the high frequency performances of CNTFETs is very challenging. Fig. 22: (a) Transfer curves from a transistor that uses aligned arrays of Indeed CNTs, when considered individually, have very SWNTs transferred from a quartz growth substrate to a doped silicon high impedance (RON > 6.5 kΩ) whilst conventional high substrate with a bilayer dielectric of epoxy (150 nm)/SiO2 (100 nm). frequency equipment is adapted for the 50Ω The data correspond to measurements on the device before (open measurement range. In addition, due to the small size of triangles) and after (open circles) an electrical breakdown process that eliminates metallic transport pathways from source to drain. This process the CNT, parasitic contributions from the device improves the on/off ratio by a factor of more than 10,000. (b) Optical structure tend to dominate the intrinsic contribution of (inset) and SEM images of a transistor that uses interdigitated source and the CNT. drain electrodes, in a bottom gate configuration with a gate dielectric of HfO2 (10 nm) on a substrate and gate of Si. The width and length of the channel are 93 mm and 10 μm, respectively. The box indicated by the To circumvent these problems, several groups proposed dashed blue lines in the optical image inset delineates the region shown the use of mixing techniques and obtained indirect in the SEM image [39]. Figure reproduced with permission from the indications of the high frequency operation of CNTFETs. American Chemical Society. 33
- 34. nanoICT research For example, IBM reached 580 MHz in 2004 [312], then [323], [324]. Most importantly, CNT-based network Cornell University and Northrop Grumman transistors can be made compatible with printing Corporation respectively reached 50 GHz [313] and technologies [324]. 23 GHz [314]. Direct measurements of the HF operation of a single nanotube CNTFETs are scarce. European Position: This is a small research field and The most convincing result was obtained at ENS most of the activity is located in the USA (Univ. Illinois, Paris in 2008 [315]. From direct measurements of gm Stanford, IBM). Main academic players in Europe are and Cg up to 1.6 GHz, they obtained fT~50 GHz for CEA and IEM & Delft University. From an industrial a 300 nm long channel. A convenient and powerful point of view one can cite Brewer Science (for the ink way of directly measuring the full S-parameters formulation and deposition process) as well as matrix of CNT based devices (which is a critical Northrop Grumman for circuitry developments. requirement from a circuit design point of view), consists in studying multiple nanotubes in parallel, thus 8.16 Hydrogen storage reducing both the device impedance and the relative impact of parasitics. This strategy was used by different CNTs have been suggested as potential candidates for groups [316], [317], [318] [319], [320], [321], [322] to hydrogen storage. However, the reported hydrogen demonstrate fT above 10 GHz. In particular, the uptake varies significantly from group to group, with collaboration of two French groups from IEMN and CEA the mechanism not clearly understood. Current achieved an intrinsic fT of 30 GHz in 2007 and 80 GHz in methods involve compressing the CNTs into pellets 2009. Interestingly, the latter result was only made possible which are then subjected to hydrogen at high through the use of a high quality nanotube source from pressure. The target set by the US Department of Northwestern University containing 99% semiconducting Energy is 6% by weight hydrogen by 2010. Whilst nanotubes. The important results from the Roger’s group most groups have found hydrogen uptake to be in the also originate from recent progress at the material level 1-2% region [325], amongst the highest reported are (CVD growth of aligned CNTs in their case). Gundish et al. [326] at 3.7% and Dai’s group [327] at 5.1%. It should be noted that Hirschler and Roth European Position: Rogers in the USA produces the found most reported values to be false [328], for state of the art thin Film transistors and in Europe, apart example due to Ti take up during sonication. from some preliminary work in Universities little seems to be happening. From a more fundamental point of view, the average adsorption energy of hydrogen on CNTs is not 8.15.4 High frequency flexible electronics significantly different from its value on amorphous carbon. It is mainly the surface area which plays a Even if they can reach high operating frequencies, the crucial role; e.g. 5.8% was achieved a long time ago use of CNTFETs in conventional integrated circuits on super-high surface area activated carbon, a remains unlikely in the near future. Indeed, the potential significantly cheaper material when compared to gains in performances when compared with CNTs. [329] Also, because the bond of hydrogen with conventional semiconductors do not compensate for silicon is weaker than that of carbon, it is much easier the immense efforts required at the material level to to get hydrogen out again. solve the issues of selective placement and nanotube variability. Conversely, when carbon nanotubes are European Position: Over the last 10 or so years compared with organic materials in the field of flexible there have been numerous groups worldwide electronics, the potential gains in performances are working in this area; especially in the USA, Japan and huge. Indeed, the low carrier mobility in organics China. Europe too has made a significant investment, (typically in the 10-3-10 cm2/Vs range) prevents their notably through groups in Germany, France, Greece use at very high frequencies. Flexible electronics with CNTs has been studied since but only very recently at high (theoretical work) and the UK but still the DoE 6% frequency. The first results are already very promising with target remains elusive. It is generally accepted that operating frequencies in or close to the GHz range [316], absorptions of about 1% are practical [330]. Other Cont. page 35 34
- 35. Catalogue of Nanoscience & Nanotechnology Companies in Spain This catalogue, compiled by the Phantoms Foundation The Phantoms Foundation is also coordinator of the (coordinator of the Spanish Nanotechnology action Spanish Nanotechnology Plan funded by ICEX (Spanish plan funded by ICEX), provides a general overview of Institute for Foreign Trade, www.icex.es) under the the Nanoscience and Nanotechnology companies in program España, Technology for Life, to enhance the Spain and in particular the importance of this market promotion in foreign markets of Spain’s more Innovative research, product development, etc. and leading industrial technologies and products in Note: only those contacted companies which provided order to: their details are listed. 1. Represent the Scientific, Technological and Innovative Edited and Coordinated by agents of the country as a whole. 2. Foster relationships with other markets/countries. 3. Promote country culture of innovation. 4. Better integrate the Spanish “Science - Technology - Company - Society” system in other countries. 5. Generate and develop scientific and technological knowledge. The Phantoms Foundation based in Madrid, Spain, 6. Improve competitiveness and contribute to the focuses its activities on Nanoscience and economic and social development of Spain. Nanotechnology (N&N) and is now a key actor in structuring and fostering European Excellence and Funded by enhancing collaborations in these fields. The Phantoms Foundation, a non-profit organisation, gives high level management profile to National and European scientific projects (among others, the COST Bio-Inspired nanotechnologies, ICT-FET Integrated Project AtMol, ICT/FET nanoICT Coordination Action, EU/NMP nanomagma project, NanoCode project under the Programme Capacities, in the area Science in Society FP7…) and provides an innovative platform for dissemination, transfer and transformation of basic nanoscience knowledge, strengthening interdisciplinary The Spanish Institute for Foreign Trade ("Instituto research in nanoscience and nanotechnology and Español de Comercio Exterior”) is the Spanish catalysing collaboration among international research Government agency serving Spanish companies to groups. promote their exports and facilitate their international expansion, assisted by the network of Spanish Embassy’s The Foundation also works in close collaboration with Economic and Commercial Offices and, within Spain, Spanish and European Governmental Institutions to by the Regional and Territorial Offices. It is part of the provide focused reports on N&N related research Spanish Ministry of Industry, Tourism and Trade areas (infrastructure needs, emerging research, etc.). ("Ministerio de Industria, Turismo y Comercio"). The NanoSpain Network (coordinated by the Phantoms Foundation and the Spanish National Research Council, Contact details CSIC) scheme aims to promote Spanish science and research through a multi-national networking action and Phantoms Foundation to stimulate commercial Nanotechnology applications. Calle Alfonso Gomez 17 NanoSpain involves about 310 research groups and 28037 Madrid (Spain) companies and more than 2000 researchers. www.phantomsnet.net
- 36. N&N Companies in Spain ACCIONA INFRAESTRUCTURAS Address C/ Valportillo Segunda, 8 - 28108 Alcobendas - Madrid - Spain WEB site www.acciona-infraestructuras.es Contact person Jose Antonio Sánchez Rojo e-mail [email protected] Phone +34 917 912 020 Main Research Areas Nanocoatings • Nanocomposites Created in 1997 No. of employees in R&D 152 % Nanoscience and Nanotechnology (R&D) 9,8 No. of Patents 6 National and 2 European ACTIVERY BIOTECH S. L. Address Avda Carlos III, 36, 1º dcha -31003 Pamplona - Navarra (Spain) WEB site www.activery.com Contact person Carles Ventosa e-mail [email protected] Phone +34 935 947 011 Main Research Areas Drug Delivery • Nanomedicine Created in 2003 No. of employees in R&D 5 % Nanoscience and Nanotechnology (R&D) 50 No. of Patents 6
- 37. N&N Companies in Spain AGROINDUSTRIAL KIMITEC Ctra. del Alicún, 369. Edificio Natalia 2º B Address 04721 El Parador de Roquetas de Mar - Almería WEB site www.kimitec.es Contact person Felix García e-mail [email protected] Phone +34 950 366 241 Main Research Areas NanoBio • Nanochemistry Created in 2010 No. of employees in R&D 15 % Nanoscience and Nanotechnology (R&D) 2 AIRBUS OPERATIONS S. L. Address Pº John Lennon, s/n - 28906 Madrid (Spain) WEB site www.airbus.com Contact person Tamara Blanco e-mail [email protected] Phone +34 916 242 573 Main Research Areas Nanocomposites • Nanotubes Created in 2000 No. of employees in R&D ≈3000 % Nanoscience and Nanotechnology (R&D) <5
- 38. N&N Companies in Spain ARAGONESA DE COMPONENTES PASIVOS S. A. Address Apdo. de correos 43 - 50500 Tarazona -Zaragoza (Spain) WEB site www.acptechnologies.com Contact person Luis José Ortíz e-mail [email protected] Phone +34 976 643 063 Main Research Areas Nanomaterials • Nanoparticles Created in 1988 No. of employees in R&D 2 % Nanoscience and Nanotechnology (R&D) 3 No. of Patents 1 APPLIED RESEARCH USING OMIC SCIENCES S.L. Address Travessera de Gràcia, 108, Entl. 08012 Barcelona WEB site www.aromics.es Contact person Carmen Plasencia e-mail [email protected] Phone +34 934 407 302 Main Research Areas NanoBio Created in 2005 No. of employees in R&D 5 % Nanoscience and Nanotechnology (R&D) 15
- 39. N&N Companies in Spain ASOCIACIÓN DE LA INDUSTRIA NAVARRA Address C/ San Cosme y San Damian, s/n - Navarra (Spain) WEB site www.ain.es Contact person Rafael Rodríguez e-mail [email protected] Phone +34 948 421 101 Energy • Manufacturing • Nanocoatings • Nanomaterials Main Research Areas Nanophotonics • Nanotubes Created in 1963 No. of employees in R&D 125 % Nanoscience and Nanotechnology (R&D) 10 ATOS ORIGIN Address C/ Albarracín, 25 - Madrid (Spain) WEB site www.atosorigin.eu Contact person Manuel M. Pérez e-mail [email protected] Phone +34 912 149 331 Modelling/Simulation/Software • Nanomedicine Main Research Areas Nanoparticles • Project Management Created in 1987 No. of employees in R&D 300 % Nanoscience and Nanotechnology (R&D) 3
- 40. N&N Companies in Spain AVANZARÉ INNOVACIÓN TECNOLÓGICA S. L. Address C/ Antonio de Nebrija, 8 -26006 Logroño (Spain) WEB site www.avanzare.es Contact person Julio Gómez e-mail [email protected] Phone +34 941 587 027 Graphene • Nanocomposites • Nanomaterials • Nanoparticles Main Research Areas Nanosensors Created in 2005 No. of employees in R&D 24 % Nanoscience and Nanotechnology (R&D) 100 No. of Patents 12 BIOKER RESEARCH S. L. Address Pol. de Olloniego, 22A, Nave 5 - 33660 Oviedo - Asturias (Spain) WEB site www.bioker.com Contact person Claudia Álvarez e-mail [email protected] Phone +34 985 761 141 Main Research Areas NanoBio • Nanomaterials • Nanoparticles Created in 2005 No. of employees in R&D 2 % Nanoscience and Nanotechnology (R&D) 100 No. of Patents 3
- 41. N&N Companies in Spain CIDETE INGENIEROS S. L. Address C/ Anselmo Clave, 98 - 08800 Barcelona (Spain) WEB site www.arrakis.es/~cidete/ Contact person Germán Noriega e-mail [email protected] Phone +34 938 157 003 Nanoelectronic/Molecular Electronic • Nanomaterials Main Research Areas Nanosensors Created in 2001 No. of employees in R&D 4 % Nanoscience and Nanotechnology (R&D) 60 DATAPIXEL S. L. Address Ronda Sta. Eulália, 37 Pol. Ind. de Pallejà, 1. 08780 Barcelona (Spain) WEB site www.datapixel.com Contact person Antonio Ventura-Traveset e-mail [email protected] Phone +34 93 663 1838 Manufacturing • Modelling/Simulation/Software Main Research Areas Nanometrology • Nanosensors Created in 1999 No. of employees in R&D 7 % Nanoscience and Nanotechnology (R&D) 20 No. of Patents 1
- 42. N&N Companies in Spain DOLMAR INNOVA S. L. Address CEAD. Paraje Micalanda, s/n - 26221 Gimileo - La Rioja (Spain) WEB site www.grupodolmar.es Contact person Mariano Fernández e-mail [email protected] Phone +34 941 303 730 Main Research Areas NanoBio • Nanoparticles • Nanosensors Created in 1992 No. of employees in R&D 8 % Nanoscience and Nanotechnology (R&D) 15 - 20 DROPSENS S. L. Av. Julian Clavería, s/n - Edif. Severo Ochoa Address 33006 Oviedo - Asturias (Spain) WEB site www.dropsens.com Contact person David Hernández e-mail [email protected] Phone +34 653 525 278 Main Research Areas NanoBio • Nanochemistry • Nanomaterials • Nanosensors Created in 2006 No. of employees in R&D 3 % Nanoscience and Nanotechnology (R&D) 40
- 43. N&N Companies in Spain DYNASOL ELASTOMEROS Address Paseo de la Castellana, 280, 1ª - 28046 Madrid (Spain) WEB site www.dynasolelastomers.com Contact person Jose María Cuervo e-mail [email protected] Phone +34 913 488 388 Main Research Areas Nanocomposites • Nanomaterials Created in 1999 No. of employees in R&D > 150 ENDOR NANOTECHNOLOGIES Address Baldiri Reixac, 15 - 08028 Barcelona (Spain) WEB site www.endornanotech.com Contact person Marc Ramis e-mail [email protected] Phone +34 934 020 468 Main Research Areas Drug delivery • Nanomedicine Created in 2007 No. of employees in R&D 7 % Nanoscience and Nanotechnology (R&D) 100 No. of Patents 1
- 44. N&N Companies in Spain FUNDACIÓN PHANTOMS Address C/ Alfonso Gómez, 17, 2nd loft 16 - 28037 Madrid (Spain) WEB site www.phantomsnet.net Contact person Antonio Correia e-mail [email protected] Phone +34 911 402 144 Main Research Areas Project Management Created in 2002 No. of employees in I+D 8 % Nanoscience and Nanotechnology (R&D) 100 GRAPHENEA Address Tolosa Hiribidea, 76. 20018 Donostia - San Sebastián (Spain) WEB site www.graphenea.com Contact person Amaia Zurutuza e-mail [email protected] Phone +34 943 574 052 Main Research Areas Graphene • Nanochemistry • Nanomaterials Created in 2010 No. of employees in R&D 3 % Nanoscience and Nanotechnology (R&D) 75
- 45. N&N Companies in Spain GRUPO ANTOLIN - INGENIERÍA S. A. Address Carretera Madrid-Irun Km 244,8 - 09007 Burgos (Spain) WEB site www.grupoantolin.com Contact person Cesar Merino e-mail [email protected] Phone +34 947 477 700 Carbon nanofibres • Nanocomposites Main Research Areas Nanoelectronic/Molecular Electronic • Nanosensors Created in 1999 No. of employees in R&D 11 No. of Patents 4 INGENIATRICS TECNOLOGÍAS S. L. Address Camino Mozarabe, 41 - 41900 Camas - Sevilla (Spain) WEB site www.ingeniatrics.com Contact person Joaquín Gómez e-mail [email protected] Phone +34 954 081 214 Main Research Areas Drug delivery • NanoBio • Nanomedicine Created in 2003 No. of employees in R&D 16 % Nanoscience and Nanotechnology (R&D) 100 No. of Patents 50
- 46. N&N Companies in Spain INNOVATEC SENSORIZACIÓN Y COMUNICACIÓN S. L. Address Avenida de Elche, 3 Bajo - 03801 Alcoi (Alicante) WEB site www.innovatecsc.com Contact person Francisco Ibáñez e-mail [email protected] Phone +34 965 548 285 Main Research Areas Nanoelectronic/Molecular Electronic • Nanomaterials Created in 2006 No. of employees in R&D 8 % Nanoscience and Nanotechnology (R&D) 90 No. of Patents 4 INTERQUÍMICA Address San Francisco, 11 - 26370 Navarrete (La Rioja) WEB site www.interquimica.org Contact person Marta Pérez e-mail [email protected] Phone +34 941 265 276 Graphene • NanoBio • Nanoclays • Nanocomposites Main Research Areas Nanomaterials • Nanomedicine • Nanoparticles • Nanosensors Created in 2005 No. of employees in R&D 14 % Nanoscience and Nanotechnology (R&D) 90
- 47. N&N Companies in Spain LABORATORIOS ALPHASIP Address Ceei Aragón. María de Luna, 11, Nave 13 - 50018 Zaragoza (Spain) WEB site www.alphasip.es Contact person Miguel A. Roncalés e-mail [email protected] Phone +34 626 004 107 Main Research Areas Nanomedicine • Nanotubes • Nanowires Created in 2009 No. of employees in R&D 6 % Nanoscience and Nanotechnology (R&D) 90 No. of Patents 120 LABORATORIOS ARGENOL S. L. Autovía de Logroño Km. 7,4. Address Polígono Europa II, Nave 1, 50011 Zaragoza (Spain) WEB site www.laboratorios-argenol.com Contact person Ivana Ascaso e-mail [email protected] Phone +34 976 336 266 Main Research Areas NanoBio • Nanoparticles Created in 2005 No. of employees in R&D 3 % Nanoscience and Nanotechnology (R&D) 50 No. of Patents 1
- 48. N&N Companies in Spain LAIMAT SOLUCIONES CIENTÍFICO TÉCNICOS Address PTS Edif. BIC. Av. Innovación, 1 - 18100 Armilla - Granada (Spain) WEB site www.laimat.com Contact person Mercedes Fernández Valmayor e-mail [email protected] Phone +34 958 750 951 Main Research Areas Drug delivery • Encapsulation • Microsensors • Nanomedicine Created in 2006 No. of employees in R&D 6 % Nanoscience and Nanotechnology (R&D) 80 MECWINS S. L. Address C/ Santiago Grisolía, 2 - 28760 Tres Cantos - Madrid (Spain) WEB site www.mecwins.com Contact person Óscar Ahumada e-mail [email protected] Phone +34 918 049 064 Main Research Areas NanoBio • Nanosensors Created in 2008 No. of employees in R&D 6 % Nanoscience and Nanotechnology (R&D) 100 No. of Patents 3
- 49. N&N Companies in Spain NANOBIOMATTERS INDUSTRIES S. L. Parque Tecnológico. Louis Pasteur, 11, Nave 5-6 Address 46980 Paterna - Valencia (Spain) WEB site www.nanobiomatters.com Contact person Javier Vilaplana e-mail [email protected] Phone +34 961 318 628 NanoBio • Nanoclay • Nanocoating • Nanofabrication Main Research Areas Nanocomposites Created in 2004 No. of employees in R&D 20 % Nanoscience and Nanotechnology (R&D) 100 No. of Patents 10 NANOGAP SUB-NM-POWDER S. A. Polígono Industrial Novo Milladoiro. Address C/ Xesta, 78 - A2 - 15895 Milladoiro - A Coruña ( Spain) WEB site www.nanogap.es Contact person Tatiana López e-mail [email protected] Phone +34 981 523 897 Main Research Areas Nanomaterials • Nanomedicine Created in 2006 No. of employees in R&D 7 % Nanoscience and Nanotechnology (R&D) 35 No. of Patents 6
- 52. N&N Companies in Spain NANOIMMUNOTECH S. L. Plaza de Fernando Conde Montero - Ríos, 9 Address 36201 Vigo - Ponteve dra (Spain) WEB site http://nanoimmunotech.es/ Contact person Christian Sánchez-Espinel e-mail [email protected] Phone +34 986 812 625 Main Research Areas Nanomedicine • Nanotoxicology Created in 2010 No. of employees in R&D 1 % Nanoscience and Nanotechnology (R&D) 100 No. of Patents 7 NANOINNOVA TECHNOLOGIES S. L. C/ Faraday, 7 Parque Científico de Madrid. Address Campus de Cantoblanco - 28049 Madrid (Spain) WEB site www.nanoinnova.com Contact person Rafael Ferrito e-mail [email protected] Phone +34 911 880 756 Main Research Areas Graphene • Nanosensors • Nanotubes Created in 2010 No. of employees in R&D 2 % Nanoscience and Nanotechnology (R&D) 100
- 53. N&N Companies in Spain NANORIOJA S. L. Address C/ Jardines, 5. Pol. Lentiscares - 26370 Navarrete - La Rioja (Spain) WEB site www.nanorioja.es Contact person Alberto Díez e-mail [email protected] Phone +34 941 411 422 Main Research Areas Graphene • Nanocomposites • Nanomaterials Created in 2008 No. of employees in R&D 3 % Nanoscience and Nanotechnology (R&D) 100 No. of Patents 1 NANOTEC ELECTRÓNICA S. L. Centro Empresarial Euronova 3. Ronda de Poniente, 12 Address Planta 2ª, Oficina C - 28760 Tres Cantos - Madrid (Spain) WEB site www.nanotec.es Contact person Adriana Gil e-mail [email protected] Phone +34 918 043 347 Main Research Areas SPM Created in 1998 No. of employees in R&D 18 % Nanoscience and Nanotechnology (R&D) 45
- 54. N&N Companies in Spain NANOTECNOLOGÍA SPAIN S. L. Address C/ de la Cruz 13 Bajos - 07800 Eivissa - Balears (Spain) WEB site www.ntc-spain.com Contact person Adam Prats e-mail [email protected] Phone +34 971 198 472 Main Research Areas Nanocomposites Created in 2004 No. of employees in R&D 8 % Nanoscience and Nanotechnology (R&D) 100 No. of Patents 1 NANOTEX (SOLUTEX GROUP) Parque Empresarial Omega. Avda. de Barajas, 24 Address Edificio Gamma. 28108 Alcobendas - Madrid (Spain) WEB site www.solutex.es Contact person Saray Morrondo e-mail [email protected] Phone +34 918 060 477 Drug delivery • Manufacturing • NanoBio • Nanochemistry Main Research Areas Nanofabrication • Nanomagnetism/Spintronics • Nanomaterials Nanomedicine • Nanoparticles Created in 2004 No. of employees in R&D 100 No. of Patents 6
- 55. N&N Companies in Spain NANOZAR S. L. Address C/ Miguel Luesma Castán, 4 - 50018 Zaragoza (Spain) WEB site www.nanozar.com Contact person Pere Castell e-mail [email protected] Phone +34 976 733 977 Main Research Areas Nanocoatings • Nanocomposites • Nanotubes Created in 2005 No. of employees in R&D 2 % Nanoscience and Nanotechnology (R&D) 100 NEOKER S. L. Address Pol. Ind. Milladoiro, Xesta 78 A1 - 15895 Ames - A Coruña (Spain) WEB site www.neoker.org Contact person Carmen Cerecedo e-mail [email protected] Phone +34 685 476 828 Main Research Areas Nanocomposites • Nanomaterials Created in 2008 No. of employees in R&D 8 % Nanoscience and Nanotechnology (R&D) 80 No. of Patents 4
- 56. N&N Companies in Spain NLAB DRUG DELIVERY PTA. Av. Juan López de Peñalver, 21 29590 Campanillas Address Málaga (Spain) WEB site www.nlabdrugdelivery.com Contact person Enrique Llaudet e-mail [email protected] Phone +34 665 176 305 Main Research Areas Drug delivery • Encapsulation • Nanomaterials • Nanomedicine Created in 2010 No. of employees in R&D 4 (2011) % Nanoscience and Nanotechnology (R&D) 80 No. of Patents 4 OPERÓN S. A. Address Camino del Plano, 19 - 50410 Cuarte de Huerva - Zaragoza (Spain) WEB site www.operon.es Contact person Manu Villacampa e-mail [email protected] Phone +34 976 503 597 Main Research Areas NanoBio • Nanoparticles Created in 1996 No. of employees in R&D 8 % Nanoscience and Nanotechnology (R&D) 35 No. of Patents 1
- 57. N&N Companies in Spain RAMEN S. A. Address C/ Sambara, 33 - 28027 Madrid (Spain) WEB site www.ioner.net Contact person Eladio Montoya e-mail [email protected] Phone +34 914 044 575 Main Research Areas Nanoparticles Created in 1958 No. of employees in R&D 34 % Nanoscience and Nanotechnology (R&D) 30 No. of Patents 5 REPSOL YPF (DIRECCIÓN DE TECNOLOGÍA) Address A-5 Km. 18 - 28935 Móstoles - Madrid (Spain) WEB site www.repsol.com Contact person Luisa María Fraga e-mail [email protected] Phone +34 913 487 653 Main Research Areas Energy • NanoBio • Nanocomposites • Nanomaterials Created in 2000 No. of employees in R&D 11 % Nanoscience and Nanotechnology (R&D) < 10 No. of Patents 2
- 58. N&N Companies in Spain SENSIA S. L. Address Industrialdea. Pab-1, A-Gunea - 20159 Asteasu - Gipuzkoa (Spain) WEB site www.sensia.es Contact person Iban Larroulet e-mail [email protected] Phone +34 918 049 622 Main Research Areas Nanosensors Created in 2004 No. of employees in R&D 1 % Nanoscience and Nanotechnology (R&D) 35 SGENIA S. L. Address C/ Chile, 4 - 28290 Las Rozas de Madrid - Madrid (Spain) WEB site www.sgenia.com Contact person María Moreno e-mail [email protected] Phone +34 916 306 388 Main Research Areas Energy • Modelling/Simulation/Software • Nanosensors Created in 2003 No. of employees in R&D 4 % Nanoscience and Nanotechnology (R&D) 10
- 59. N&N Companies in Spain SINATEC S. L. Marie Curie Annex Building, Campus of Rabanales Address University of Cordoba - 14071 Cordoba (Spain) WEB site www.sinatec.es Contact person Bartolomé Simonet e-mail [email protected] Phone +34 957 218 562 Main Research Areas Nanocomposites • Nanomaterials • Nanotubes Created in 2007 No. of employees in R&D 7 % Nanoscience and Nanotechnology (R&D) 60 TAMAG IBERICA S. L. Address Plaza de Armerias, 2, esc. izq. 1A, 20011 San Sebastián WEB site www.tamagiberica.com Contact person Arkady Zhukov e-mail [email protected] Phone +34 619 163 930 Main Research Areas Microsensors • Microwires Created in 2000 No. of employees in R&D 1 % Nanoscience and Nanotechnology (R&D) 100 No. of Patents 2
- 60. N&N Companies in Spain TECNOLOGÍA NAVARRA DE NANOPRODUCTOS S. L. (TECNAN) Address Área Induntrial “Perguita” A1 - 31210 Los Arcos - Navarra (Spain) WEB site www.tecnan-nanomat.es Contact person Germán Medina e-mail [email protected] Phone +34 948 640 318 Main Research Areas Nanocoatings • Nanomaterials • Nanoparticles Created in 2008 No. of employees in R&D 3 % Nanoscience and Nanotechnology (R&D) 35 THREELLOP NANOTECHNOLOGY Address C/ Puig,10 2B - 46980 Paterna - Valencia (Spain) WEB site www.threellop.com Contact person José Daniel Llopis e-mail [email protected] Main Research Areas NanoBio • Modelling/Simulation/Software Created in 2007 No. of employees in R&D 4 % Nanoscience and Nanotechnology (R&D) 100 No. of Patents 20
- 61. N&N Companies in Spain TOLSA S. A. Address Ctra. Vallecas - Mejorada del Campo, Km. 1,6 - 28031 Madrid (Spain) WEB site www.tolsa.com Contact person Julio Santarén e-mail [email protected] Phone +34 913 606 900 Main Research Areas Nanoclays • Nanocomposites • Nanomaterials • Nanoparticles Created in 1957 (Nanoscience and Nanotechnology: 2002) No. of employees in R&D 7 % Nanoscience and Nanotechnology (R&D) 30 No. of Patents 1 TORRECID S. A. Address Ptda. Torreta, s/n — 12110 Alcora. Apdo. 18 - Castellón (Spain) WEB site www.torrecid.com Contact person Carlos Concepción e-mail [email protected] Phone +34 964 630 900 Main Research Areas Nanocoatings • Nanomaterials • Nanoparticles Created in 1963 No. of employees in R&D 9 No. of Patents 4
- 62. N&N Companies in Spain TRIMEK S. A. Address Pol. Ind. Islarra. Camino de la Yesera, 2. — 01 Zuia - Álava (Spain) 139 WEB site www.trimek.com Contact person Fernando Larena e-mail [email protected] Phone +34 945 430 718 Main Research Areas Manufacturing • Nanometrology Created in 1993 No. of employees in R&D 11 % Nanoscience and Nanotechnology (R&D) 10 UNIMETRIK S. A. San Blas, 11. Lautadako Industrialdea Address Pol. Industrial de Gojain - 01170 Legutiano - Álava (Spain) WEB site www.unimetrik.es Contact person Borja de la Maza e-mail [email protected] Phone +34 945 465 800 Main Research Areas Manufacturing • Nanometrology • Nanosensors • SPM Created in 1997 No. of employees in R&D 10 % Nanoscience and Nanotechnology (R&D) 20 No. of Patents 2
- 63. N&N Companies in Spain YFLOW SISTEMAS Y DESARROLLOS S. L. C/ Marie Curie, 4. Parque Tecnológico de Andalucía Address 29590 Málaga (Spain) WEB site www.yflow.com Contact person David Galán e-mail [email protected] Phone +34 952 020 370 Main Research Areas Encapsulation • Nanocoatings • Nanocomposites • Nanoparticles Created in 2001 No. of employees in R&D 6 % Nanoscience and Nanotechnology (R&D) 80 No. of Patents 6 ZF BIOLABS Address Ronda de Valdecarrizo, 41B - 28760 Tres Cantos - Madrid (Spain) WEB site www.zfbiolabs.com Contact person Erika Sela e-mail [email protected] Phone +34 918 049 020 Main Research Areas NanoBio • Nanotoxicology Created in 2003 No. of employees in R&D 5 % Nanoscience and Nanotechnology (R&D) 10
- 67. nanoICT research From page 34 technologies involving other compounds such as 8.17.2 Quantum computing carbohydrates are expected to be used instead. Arrays of qubits have been created in the form of 8.17 Quantum computing endohedral fullerenes in SWNTs, to make so-called peapods [337]. These structures have been modelled 8.17.1 Spintronics [338] and imaged [339]. The interactions between the spins have been characterized by electron paramagnetic Spin transport has been demonstrated over lengths of resonance, showing transitions from exchange hundreds of nanometers in CNTs [331], and the limit may narrowing to spin-spin dephasing [340]. Theoretical be much longer. The Kondo effect has been architectures have been developed for global control of demonstrated [332], and Fano resonances have been qubits [341], [342]. The spin properties of N@C60 (or found [333]. Spin blockade has been demonstrated in atomic hydrogen inside a C60) have been shown to make double dot structures [334],[335]. With the development it one of the strongest candidates for condensed matter of aberration corrected transmission electron quantum computing [343], [344]. Y@C82 can also be used and typical relaxation and coherence times are shown in microscopy at low voltage (80 kV), which minimises figure 24. Quantum memories have been knock-on damage, it has become possible to image the demonstrated, in which information in the electron spin actual piece of active material in a device [336], as shown is transferred to the nuclear spin, and subsequently in figure 23. retrieved, with gate operation times of order 10 ns and storage times in excess of 50 ms. The theoretical limit for such memories is limited by twice the electron spin flip time [345], and since this can exceed one second the prospects are excellent. Entangled spins offer further Fig. 23: A 20,3 chirality SWNT, observed in transmission electron microscopy (aberration-corrected JEOL 2200MCO operating at 80 kV, image courtesy of Dr Jamie Warner). Below the micrograph is an atomic model to the left and an image simulation to the right, with a small Fig. 24: Y@C82 relaxation and coherence times as a function of overlap also shown. temperature in deuterated toluene (circle red, T1 closed, T2 open) and deuterated o-terphenyl (symbol black, T1 star, T2 cross) [347]. Insert: Structural representation of a) d-toluene and b) o-terphenyl. Problems to overcome include the production of uniform, defect-free SWNTs, free from paramagnetic possibilities for other quantum technologies, such as impurities, with a single chiral index, and fabrication of metrology and sensors [346]. reproducible devices with uniform contacts. Problems to overcome include the development of the European Position: Hitachi Cambridge Laboratory, technology for single spin read out in CNTs and the and the Cavendish Laboratory Mark Buitelaar, in demonstration of entanglement using peapods. collaboration with Andrew Briggs at Oxford are the leaders in the field; Others include Delft (Leo European Position: Oxford leads the world in Kouwenhoven) and the Niels Bohr Institute, peapods for quantum computing, in collaboration with Copenhagen. Princeton (Steve Lyon), Nottingham (Andrei Khlobystov), Cambridge (Charles Smith), EPFL (Laszlo 35
- 68. nanoICT research Forro) and Peking (Lianmao Peng), There is also activity [358], [359], [360]. Single-electron memory operation, in Berlin (Wolfgang Harneit), at L. Néel Institute in with charge storage on an Au nanoparticle and sensing Grenoble and at CEMES-CNRS in Toulouse [348]. using a CNT FET, has now been demonstrated [361]. In recent work, QDs have been induced along a SWCNT 8.17.3 CNT Single Electron Transistors wrapped with single-stranded DNA [362]. Furthermore, a nanoscale resonator has recently been demonstrated Single electron transistors (SETs) use the ‘Coulomb using a suspended CNT, where a single electron added blockade’ effect to control charge at the one-electron to the CNT can be detected in a shift in the resonant level, on a nanoscale conducting ‘island’ isolated by tunnel frequency [363]. barriers from source and drain electrodes. In these devices, the total island capacitance C is small enough European Position: Considerable progress has been such that the single-electron charging energy made in CNT based single-electron systems, in the USA, Ec= e2/2C >>kBT at the measurement temperature T. A Japan and Europe. In particular, Europe is very strong in very low current ‘Coulomb blockade’ region exists fundamental physics investigations in these systems, with a around zero bias voltage in the Ids-Vds characteristics, number of novel device demonstrations. Early work on where the charging energy prevents current flow. As the room-temperature CNT SETs has also occurred in Europe. applied bias overcomes integer multiples of Ec, electrons are added one by one to the island. In a SET, an additional 9. Conclusions gate electrode is used to add/remove electrons from the island. The Ids-Vgs characteristics show periodic single-electron CNTs have many unique and indeed useful properties conductance oscillations, where each oscillation for applications in the ICT area. Research into CNTs at corresponds to the addition of an electron. If C ~ 10-18 F or the university level will continue for at least the next smaller, single-electron effects can occur at room several years especially into quantum effects and temperature, raising the possibility of single-electron associated behaviour, as well-characterized, high-quality memory and logic applications. For islands small enough such SWCNTs become more available. Although CNTs are that the quantum confinement energy is also significant, the still being touted for various industrial applications, much more investment is necessary for them to reach device forms a quantum dot (QD), sometimes referred to commercial viability. The USA and Japan lead in this as an artificial atom. A combination of quantum confinement development but Europe has made significant impact in and single-electron charging effects are then observed. If the many areas despite the fact that investment in Europe is island in a SET is formed by a section of a SWCNT, then but a fraction of that in the other major high-tech electrons are confined in the axial as well as the industrial zones. Consequently, partnership between circumference direction. higher education and industry could form the basis of research in this enormous and diverse area for many SET operation at cryogenic temperatures, with years to come so that at least some of the many conductance oscillations in the CNT Ids-Vgs characteristics, applications of CNTs can be realised. was demonstrated in the late 1990s in a single SWCNT [349] and in ropes of SWCNTs [350]. More recently, SET References operation at room temperature has been demonstrated in a number of works [351], [352], [353], [354]. Here, [1] S. Ijima, Nature 354, 56 (1991). islands as small as ~10 nm [352] have been defined along single SWCNTs [351], in MWNTs [353], and in CNT [2] Niels de Jonge and Jean-Marc Bonard, Phil. Trans. bundles [352] using nicks or kinks defined by AFM, or by R. Soc. Lond. A 362, 2239—2266 (2004). chemical modification. The addition energy in these devices may be as large as ~120 meV [351]. QDs may [3] Y. Saito, T Yoshikawa, S. Bandow, M. Tomita and T. also be defined along a nanotube by variations in the Hayashi, Phys. Rev. B 48, 1907 (1993). doping type [355],[356]. In investigations of the physics of electron transport through QDs, coupled QD [4] M. Endo, K. Takeuchi, T. Hiraoka, T. Furuta, T. Kasai, behaviour at low temperature, controlled by multiple X. Sun, C.H. Kiang, and M.S. Dresselhaus, J. Phys. gates, has been observed in a number of works[357], Chem. Solids 58, 1707 (1997). 36
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- 83. nanoICT research European Research Roadmap Security; Design Technologies; Semiconductor Process for Nanoelectronics and Integration; and Equipment, Materials and Francis BALESTRA Manufacturing. IMEP-LAHC, Sinano Institute/Grenoble INP Minatec-CNRS. 3 Parvis Louis Néel, BP 257, 38016 The projects in the field of nanoelectronics technology, Grenoble, France. which is the focus of this paper, are thus funded by CATRENE, ENIAC and FP programmes. In this respect, The Nanoelectronics European Research Roadmap is the international roadmap proposes a minimum device addressed focusing on the main European Programmes size starting from 45nm in 2010 to about 9nm in 2024 supporting the short, medium and long-term research (More Moore domain). The required performance activities (CATRENE, ENIAC JTI/JU, Framework improvements for the end of the roadmap for high Programme). The main challenges we are facing in the performance, low and ultra low power applications will field of Nanoelectronic technology are summarized in lead to a substantial enlargement of the number of new the More Moore (ultimate CMOS), More than Moore materials (thin and ultra-thin strain channels, high and very (adding functionalities to CMOS) and Beyond-CMOS high k dielectrics, metallic source-drain, etc.), technologies domains. The main objectives and some highlights of the (EUV, etc.) and device architectures (Ultra-Thin films, Sinano Institute, an European association created for the Multi-gates, Multi-channels, etc.). coordination of the efforts of the Academic Community in the field of Nanoelectronics, and of the Nanosil and Complexity will also derive from diversity, with an Nanofunction Networks of Excellence, devoted to the increasing number of functions integrated on CMOS convergence of More Moore and Beyond-CMOS on one platforms as envisaged in the “More than Moore” hand and of advanced More than Moore and approach. To learn how to combine CMOS with sensors, Beyond-CMOS research activities on the other hand, actuators, MEMS, NEMS, RF components, biochips, high are also outlined. voltage, imaging devices, photonics on Si, energy harvesting and demonstrate their innovative benefits 1. European Nanoelectronics landscape requires enhancing multidisciplinary experiments by a large research community. Micro-Nanoelectronics has been defined as one of the 5 Key Enabling Technologies (KET) for strengthening the Heterogeneous integration will also be needed in some knowledge economy and a sustainable growth in the domains in order to obtain more functions/mm3. strategic plan of the European Union (5 KETs: Special interests are in the fields of 3D integration, Micro-Nanoelectronics, Nanotechnology, Photonics, interconnection, assembly & packaging. Biotechnology, Advanced materials). Nanoelectronics research activities in the EU are devoted to remain at Beyond-CMOS nanostructures (Nanowires, Tunnel FETs, the forefront of state of the art innovation in the further Graphene devices, etc.) will also allow to push the limits miniaturization and integration of nanoelectronic of Si integration down to nanometric dimension and to devices while dramatically increasing their functionalities. develop new functionalities for the future Nanosystems. Several Programmes, funded by the European In these research areas, there is a strong need to Commission and the Members States, are supporting support and develop state-of-the-art Research Nanoelectronics in the EU. CATRENE (EUREKA) [1] Infrastructures (RI) open to a large research community and ENIAC Joint Technology Initiatives [2] projects are to overcome these formidable multidisciplinary challenges devoted to technology driven and application driven for new generations of Nanoelectronic ICs. The proposed short/medium term researches, respectively, and FP7 strategy in the EC is the development of a 3 levels projects [3] focus on long-term research. In the infrastructure: i) Network of flexible RI, driven by the framework of ENIAC/CATRENE, 5 applications Academic Community, for the study of basic properties, oriented and 3 technology oriented domains have been test and validation of very innovative materials and devices defined: Automotive and Transport; Communication; for long term nanoelectronics applications; ii) Pre-industrial Energy Efficiency; Health and Ageing Society; Safety and RI, driven by the Institutes/Integration Centres, for 51
- 84. nanoICT research medium term applications with technology implementation hand, and the merging of More than Moore and and performance assessment on R&D equipments; iii) Beyond-CMOS, on the other hand, for developing Industrial RI, driven by the Manufacturing Centres, for innovative nanoscale structures that can improve short term applications with technology exploitation as performance and/or enable new functionalities in future functional product, process optimization, yield, and terascale ICs and Nanosystems. product reliability. - perform training activities, University curricula, The Sinano Institute and the Nanosil/Nanofunction Workshops to develop high competence levels in Europe. Networks of Excellence, which are presented below, have been launched these last years for the coordination of the - participate in roadmap definition. European Academic Community and for the study of the convergence of the More Moore, More than Moore and - play an important role in European structuring and Beyond CMOS domains, respectively. These European programs, and strengthen the overall efficiency of the consortiums are mainly performing long term European research in Nanoelectronics. nanoelectronics research using flexible research infrastructures (level i) of the previous structuring in strong The Sinano Institute has launched in 2008 and 2010 two interaction with industrial and pre-industrial partners. FP7 Networks of Excellence which are described below. 2. The Sinano Institute 3. NANOSIL European Network of Excellence The Sinano Institute [4], launched in 2008 as a European Academic and Scientific Association for Nanoelectronics, The FP7 Nanosil Network of Excellence (Grant agreement gathers 18 laboratories from 10 European countries, n°216171), entitled “Silicon-based nanostructures and representing the main partners from the Academic nanodevices for long-term nanoelectronics applications” [5] Community in this field. It has been created after the FP6 has been launched in January 2008 for three years. Sinano Network of Excellence, which has represented an unprecedented collaboration in Europe in the field of It gathers 28 Partners from 11 European countries. The Nanoelectronics. It is an open entity gathering the most main objectives of this NoE are to push the limits of Si important flexible research infrastructures available in integration down to nanometric dimension. The Nanosil Europe for long term Nanoelectronics research. partners are thus working on n+4 technology node and beyond for: The Sinano Institute has especially been created to: - studying and validating new concepts, novel materials - stablish a durable EU Network of researchers from the and technologies, innovative device architectures using European Academic Community to form a distributed joint flexible platforms. Centre of Excellence in the Nanoelectronics field. - identifying the most promising topics for future - carry out a role of representation and coordination of information and communication technologies and the associated Organizations. updating roadmaps. - explore the science and technology aspects for n+4 - overcoming the number of research challenges of technology nodes and beyond using joint flexible ultimate CMOS and beyond-CMOS nanodevices in technology, characterization and modeling platforms in order to speed up technological innovation for the order to identify the most promising topics for future ICT. Nanoelectronics of the next 2-3 decades leading to the possible integration of Si-based innovative CMOS and - achieve activities centred on More Moore, Beyond emerging non-CMOS devices on one Si chip, which is a CMOS and More than Moore fields — in this respect the strategic issue for the next IC generations. SINANO Institute is particularly focusing on the convergence of More Moore and Beyond-CMOS, on one Other important objectives are the following: 52
- 85. nanoICT research * Perform training and dissemination activities, organize integrated on CMOS platforms in order to overcome Conferences and Workshops in order to develop high some possible CMOS limitations. competence levels in Europe; In figure 2 we present a possible architecture together * Strengthen interaction between the Academic/Scientific with some new materials which could be needed at the Community and the European Industry; end of the nanoelectronics roadmap. * Establish close links with other European (STREPs, etc.) and National projects in order to enhance the overall efficiency of the European research in Nanoelectronics; * Act as a cluster of projects, existing at the beginning or new ones to be proposed, providing they are sufficiently forward-looking; * Prepare the path for future industrial applications in the field of communications, computing, consumer electronics, health, environment. Fig. 2. Double-gate (or multi-gate) Silicon-On-Insulator MOS transistor with ultra-thin strained semiconductors channel, high k/metal gate stacks, silicide/Schottky source/drain. In the Nanosil NoE, we are working on the following research projects for the study and validation of innovative materials and nanodevice architectures for future CMOS (More Moore) and Beyond-CMOS components: i) More Moore: Fig 1. IC evolution in the past and next decades. - Appraisal of new channel materials for end of CMOS Figure 1 shows the IC evolution following Moore’s law era with improved transport parameters —carrier since the 60’s and the perspectives for the next 30 mobility and velocity, in order to boost the driving years. current Ion and the performance of CMOS ICs (sSOI, sSiGeOI and sGeOI, various channel orientations, etc.). In the sub-20nm gate length range, alternative CMOS devices using new architectures and integrating - Routes to realization of Schottky barrier contacts for innovative materials (ultra-thin Si, Ge or III-V films on end of CMOS era for reducing the source-drain access insulator, double-gate, FinFET or Gate-All-Around resistance and improving Ion (covering a wide spectrum structures, multi-channels MOSFETs, etc.) will be of silicide materials and dopants for the realization of necessary in order to get the needed performance dopant-segregated metallic junction; integration of such planned by the ITRS roadmap for high performance, junctions on strained and unstrained layers on insulator low and ultra-low power applications. In the sub-10nm in n/pMOS). range, Beyond-CMOS nanodevices (Nanowires realized by top-down or bottom-up processes, Carbon - Identification and appraisal of gate stack electronics, Tunnel FETs, etc.) will certainly be used and materials/combinations for post 22nm/HfSiO era with 53
- 86. nanoICT research Fig. 3. 50nm sSOI with 1Gpa biaxial tensile stress is used as starting material, (b) Uniaxial tensile strain obtained by lateral strain relaxation of patterned structures, (c)Transfer Id(Vg) characteristics of 2 Nanowire FET, one fabricated on SOI and one on uniaxial sSOI (tox=5nm, Lg=3μm). The inset shows the Id/gm1/2 plot for the devices, its slope being related to carrier mobility. chemical stability and low trap density in order to limit Other important research activities in the NoE are the tunnel leakage current Ioff through the gate for end dealing with the development of new modelling of CMOS era (with a product “permittivity. energy band approaches and characterization techniques: offset” ⇒ k . ΔE > 70). * Development and comparison of semi-classical ii) Beyond CMOS: (Deterministic and Monte Carlo techniques) and full-quantum transport treatment (NEGF, Wigner-Boltzmann approach). - Evaluation of the prospects of 1D nanowires for the post CMOS era (with strain, low Schottky barrier * Validation of new physically based and compact models contacts, high k/metal gate stacks, parallel nanowires, for thin-body, multi-gate MOSFETs, Nanowires, etc. junctionless nanowires, etc.). * Understanding of mobility and interface effects, - Investigation of the prospects for carbon structures - driving and off-state currents, variability in especially graphene, and their technological potential. nanoMOSFETs and innovative device architectures by - Assessment of the performance of new nanoelectronic combining modelling and characterization efforts. switches: Impact ionisation (IMOS), tunnelling FET, NEM-FET, ferroelectric gate in order to determine if Finally, the support and development of the flexible they can form the basis of new MOS device functionality research infrastructures (Joint Processing Platform, Joint with very low subthreshold swing, extremely low Vdd Characterization and Modelling Platform) also operation, acceptable Ion/Ioff ratio with small off-currents constitutes an important goal of the joint activities. and ultimately small standby power. Some highlights of the recent results obtained in the - Investigation of routes for producing high densities framework of Nanosil are shown below. (>1012cm-2) of nanodevices (nanowires, nanodots) by templated self-assembly, and assessment of their Fig. 3 exemplifies the possible increase of the drain technological potential and CMOS compatibility. current in the case of uniaxially strained nanowire 54
- 87. nanoICT research Fig. 5. Numerical simulation of Tunnel FETs realized with various architectures: A) Single gate SOI, Lg=100nm, 3nm SiO2, B) with additional Fig. 4. Id(Vg) and Id(Vd) characteristics for Schottky barrier source-drain stress of 4GPa at source junction, C) with high k gate dielectrics, D) with N- and P-channel MOSFETs with (DS) and without (w/o DS) dopant a double gate structure, E) oxide aligned to intrinsic region, F) for segregation (DS P-channel: PtSi, BF2; DS N-channel: YbSi, As). Lg=30nm. Id(Vg), average subthreshold slope and minimum point slope are shown. devices, an enhancement of a factor of 2.5 of Id due to dopant segregation (DS) at the channel/source-drain the increase in electron mobility is obtained compared interface induces a substantial increase of the driving with unstrained nanowires (cross-section of NW current [7,8]. 40×40nm2) [6]. Figure 5 shows the performance of Tunnel FET devices In Figure 4 are plotted the transfer and output with various architectures. These transistors are very characteristics of Schottky source-drain devices for N- interesting for reducing the off-state current for very and P-channel MOSFETs. In both cases, the use of low power applications. One of the major challenge is 55
- 88. nanoICT research activities, four main scientific and technological objectives have been defined in the Nanofunction NoE: i) Nanosensing with Si based nanowires: - Exploring the use of Si-based nanowires for various nanosensors with improved performance (sensitivity, resolution, selectivity and response time). - Nanowire and nanowire-FET fabrication. - Multifunctional detection using nanowires. - Demonstration of sensor arrays with Si based nanowires as sensing element. - Convergence of nanowires with microelectronics substrate. ii) Exploration of new materials, devices and technologies for Energy Harvesting: Fig. 6. Fabrication of ultra-dense Si Nanowire Networks by top-down approach for vertical device with massively parallel NWs (10-15nm diameter). - New materials and devices for mechanical energy harvesting. the possibility to obtain high drain current. The - New materials and devices for thermoelectric substantial improvement of the electrical properties of energy harvesting. TFETs is demonstrated below using short channel - New materials and device architecture for double-gate SOI structures with strained at the source nanostructured solar cellsStorage (micro/nano-batteries), junction and high-k gate dielectrics [9, 10]. power conversion and management in energy harvesting systems. In order to get a high driving current, a 3D integration of Nanowires will be needed. This parallel integration is iii) Nanocoolers: exemplified in Fig. 6. Vertical 3D nanowire structures with very thin wire diameter (in the 10 nm range) and a - Development of Si-based very low Temperature very high density (n=4 x1010 cm-2) is demonstrated [11]. coolers in order to obtain a local cooling of some devices Possible applications of these 3D NW are in the field of or just the electrons in the devices using Si-based ultimate integration of nanoMOSFETs, photovoltaics processing. (improvement of light absorption), nanosensors or RF - Investigation of phonon transport at low devices. temperature and in reduced dimensionality. - Study of alternative nanostructures for thermal The development of novel nanofunctionalities is the isolation (porous Si, nanowires, etc.). purpose of the new FP7 Nanofunction NoE (2010-2013). - Integration of cooled detectors and read-out electronics. 4. Nanofunction European Network of Excellence iv) Exploration of new materials, devices and technologies for RF applications: The FP7 Nanofunction Network of Excellence (Grant agreement n° 257375), entitled “Beyond CMOS - Exploration of the potential of nanowires and other Nanodevices for Adding Functionalities to CMOS” [12] nanostructured materials (porous Si) as: has been launched in September 2010 for three years. It gathers 15 Partners from 10 European countries. * Substrate materials for reducing RF losses for on-chip CMOS RF passives. In addition to the integration and spreading of excellence * Materials for RF interconnects and nano-antennas. 56
- 89. nanoICT research In Figure 7 are plotted an example the use of Nanowires References: for thermoelectric applications. The thermal conductivity of Si Nanowires is shown as a function of diameter and [1] www.catrene.org incorporating nanoscale roughness. A very small thermal conductivity is demonstrated emphasizing the extremely [2] www.eniac.eu small phonon mean free path [13]. [3] http://cordis.europa.eu/fp7/home_en.html [4] www.sinano.eu [5] www.nanosil-noe.eu [6] S.F. Feste, J. Knoch, S. Habicht, D. Buca, Q.T. Zhao, and k [W m-1 K-1] S. Mantl, Performance enhancement of uniaxially-tensile strained Si NW-nFETs fabricated by lateral strain relaxation of sSOI, Proc. ULIS, Glasgow, p. 109 (2009). [7] Larrieu, G.; Dubois, E.; Valentin, R.; Breil, N.; Danneville, F.; Dambrine, G.; Raskin, J.P.; Pesant, J.C., Low Temperature Implementation of Dopant-Segregated Band-edge Metallic S/D junctions in Thin-Body SOI p- MOSFETs, Proc. IEDM, p. 147 (2007). Temperature [K] [8] Larrieu, G., Yarekha, D. A., Dubois, E., Breil, N., and Faynot, O., Arsenic-Segregated Rare-Earth Silicide Fig. 7. Thermal conductivity of Si NW vs Diameter incorporating Junctions: Reduction of Schottky Barrier and Integration nanoscale roughness showing very low thermal conductivity and in Metallic n-MOSFETs on SOI; IEEE Electron Dev. Lett., extremely short phonon mean free path [13]. Dec. 2009, 1266-1268. [9] K. Boucart et al, Proceedings ESSDERC’2009, Athens, Greece. Conclusions: [10] Boucart, K., Riess, W., and Ionescu, A. M., Lateral A summary of the Nanoelectronics European Strain Profile as Key Technology Booster for All-Silicon Research Roadmap has been presented with the main Tunnel FETs; IEEE Elect. Dev. Lett., June 2009, 656-658. European Programmes supporting the short, medium and long-term research activities. The main challenges [11] X.L Han et al., Proc. Int. Conf. on Nanosc. and Tec., we are facing in the field of Nanoelectronic technology Beijing, Sept 2009. have been summarized in the More Moore, advanced More than Moore and Beyond-CMOS domains. [12] www.nanofunction.eu The main objectives and some highlights of the Sinano Institute, an European association created for the [13] Hochbaum, Nature 451, 163, 2008. coordination of the efforts of the Academic Community in the field of Nanoelectronics, and of the Nanosil and Nanofunction Networks of Excellence, devoted to the convergence of More Moore and Beyond-CMOS on one hand and of More than Moore and Beyond-CMOS research activities on the other hand, have been outlined. 57
- 90. nanoICT Conf Report Report nanoICT Graphene and probes for graphene, nanotube and -wire CVD Nanotubes Session - TNT2010 (Stephan Hofmann from University of Cambridge, UK). Prof. Mauricio Terrones (Carlos III Univ of Madrid, Spain & Exotic Nanocarbon Research Center, Shinshu Univ, Japan) has then presented the challenges and opportunities in using defect engineering to produce new types of nanotubes and graphene based applications and devices, whereas Adelina Ilie (University of Bath, United Kingdom) has explained the mechanism of Two sessions “Nanotubes & Graphene” have been symmetry breaking and on-tube modulated surface sponsored by the nanoICT Coordinated Action in potentials in hybrids of Single-Walled Carbon Nanotubes collaboration with GDRI (France). These sessions have with encapsulated inorganic nanostructures. presented recent advances on the electronic and transport properties of carbon nanotubes and graphene- The potential of graphene for thermoelectrics applications based materials, as well as related devices and has been then discussed theoretically by Haldun perspectives. The sessions have been chaired by Prof. S. Sevincli (Dresden University of Technology, Germany), Roche from the Catalan Institut of Nanotechnology and who has also proposed the use of defect engineering to CIN2. One important fact has been the participation of enhance the thermoelectric figure of merit in disordered Prof. Andre Geim from the University of Manchester carbon systems, by strongly suppressing thermal in UK, who was awarded the 2010 Physics Nobel prize conduction, while maintaining good electrical properties. for the discovery of Graphene just a few weeks after his Additionally, Koji Ishibashi (RIKEN, Japan) has participation to TNT. Prof. Geim has given an outstanding proposed several options to design Carbon nanotubes and lecture about "Graphene: Status and Prospects" in which graphenes as building blocks of nanodevices, based on he has first introduced the field to the large audience of electron-electron interactions. TNT, and he then addressed the most challenging current research directions, including the growth strategies, the New applications have been reviewed by Marion use of their exceptional physical properties such as giant Cranney (Institut de Sciences des Materiaux de mobilities or optical properties. New features of graphene Mulhouse, France) on the design of superlattices of such as the creation of considerable pseudo-magnetic resonators on monolayer graphene created by fields as large as 300 Tesla have been shown to be intercalated gold nanoclusters, or concerning optical triggered by strain fields. Prof. Geim has also explained properties by Matthew Cole (University of Cambridge, the great excitement concerning the merging between UK) on horizontally aligned carbon nanotube networks. chemistry and nanoelectronics, as well as perspectives for The route toward the production of highly conductive, high-frequency devices or ultrafast photodetectors. flexible & controllably transparent electrodes, field emitters and infra-red sensors for the design of Fast and In addition to this spectacular lecture, the scientific wavelength selective photoresponse from QD/CNT program of these two sessions have been of high level, hybrid has been presented by Chang-Soo Han (Korea including presentations from Germany, France, Japan, Institute of Machinery & Materials/Nano-Mechanics, Korea, with speakers ranging from Universities to Korea). industries such as THALES. Two talks have focused on the growth processes of carbon nanotubes and graphene, Finally the use of graphene in near future applications has been including the development of new ceramic catalysts discussed by Jong-Hyun Ahn (Sungkyunkwan University, (Mark H. Rümmeli from IFW Dresden, Germany) or Korea) who has presented his work on high-performance, the use of advanced HRTEM to understand (and further flexible graphene field effect transistors on Plastic substrates. control) the crystal growth on the nano-scale: in-situ This work supported by the Korean industry SAMSUNG 58
- 91. nanoICT Conf Report clearly evidences the great potential of graphene for Advertisement replacing ITO in all transparent electrodes applications. One also underlines the very interesting talk by Paolo Bondavalli (Thales Research and Technology, France) who has discussed the mass production of Gas Sensor based on carbon nanotubes based-FETs fabricated using an original dynamic air-brush technique for SWCNTs deposition. Again, industries (here THALES) shows interest in this new material (graphene) giving its wide spectrum of potential applications that range from high-frequency devices, to nanosensors or spintronics devices. During these sessions about 100 persons were attending the talks, and the sessions were receiving large interest with questions and debate. One concludes by pinpointing the importance of nanoICT support, which through its contribution to the working groups on carbon nanotubes and graphene, allows to enhance the visibility of European research excellence. The participation of the 2010 Physics Nobel prize (Prof. A.K. Geim from Manchester) was also a genuine success of this session. It profiles the high level of TNT2010 sessions on carbon nanotubes and graphene, and the relevance of nanoICT in supporting these networking as well as outreach and dissemination activities. 59
- 93. nanoICT Conf Report Phonons and Fluctuations Meeting, The very interesting second part of the meeting covered Paris, 8-9 November, 2010 topics ranging from statistical physics to phonon Organised by S. Volz (CNRS), J. Ahopelto (VTT) and C. transport to energy harvesting. Massimiliano Esposito Sotomayor Torres (ICN). (University of Brussels) explained the very basics of stochastic thermodynamics in small devices and the Background efficiency to extract power from fluctuating systems. Adrian Bachtold (ICN) talked about nenomechanical The field of nanophononics and thermal management is oscillations and Bernard Perrin (Institut des becoming very active and this has been recognised also by NanoSciences de Paris) described the experimental the European Commission which has allocated resources work on ultrafast energy relaxation in solids by pump to advance the research in this field in Europe. There are and probe techniques. already projects that tackle these issues, for example NANOPACK, TAIPOX, NANOPOWER, GREEN-Si and Phonon and heat transport in nanostructures and across SINAPS, among others. In the large FET-Flagship initiative interfaces was discussed in several talks. Olivier several of the proposed projects concentrate on energy Bourgeous (Néel Institute, CNRS) talked about thermal related issues. In the US the Semiconductor Research conductance of silicon nanowires at low temperatures. Council has included phonon engineering and a related He showed that the thermal conductance can be topic of out of equilibrium operation among the top five drastically decreased by adding meanders into the wires research needs in the near future for extended CMOS to hinder the transmission of ballistic phonons. Sebastian and Beyond CMOS devices. Volz (UPR, CNRS) discussed theoretically thermal resistances and phonon relaxation times, and showed In France there is an active network on nanophononics theoretical and experimental results on near-field led by Prof. Volz, consisting of several laboratories. The radiative heat transfer at nanoscale. He also showed that Coordination Action nanoICT established a Working fluctuations can be used to move particles in microfluidic Group Nanophononics, led by Prof. Ahopelto, in the systems. Philippe Ben-Abdallah (University of Nantes) spring 2010 in order to bring together the groups aiming gave a talk on near-field coupling by surface polaritons to understand and control the behaviour of phonons in and the potential to enhance tunnelling by applying solids, at interfaces, in composites and at molecular metamaterials. Javier Goicochea (IBM) justified in his level. The Paris meeting was a joint effort between these talk the research on thermal management by addressing two networks targeting to create a collaboration forum the heat dissipation in transistors, microprocessors and for teams active in the field. by data centers. He has modelled heat conduction and dissipation using molecular dynamics calculations at Program and Highlights solid-solid, solid-fluid and fluid-nanoparticle interfaces. The message was that thermal design is becoming very The program (attached in the end) included two parts, important for architectures and devices. Wolfgang first part consisted of greetings from the Commission, Rosenstiel from University of Tübingen tackled the short reports on two important recent meetings and an consequences of scaling both at transistor and at update regarding the Flagship initiative status, and the architecture level, showing that power management is second part concentrated on scientific issues related to one of the most important problems to be solved for nanophononics. Ralph Stübner from the European ICs. Commission gave a talk on possibilities for future projects in the field of nanoelectronics, related energy Energy harvesting was addressed by Luca Gammaitoni issues and nanophononics in Future Emerging (University of Perugia), who talked about noise driven Technologies part of the FP7 ICT priority. The focus in ICT and stressed the importance of understanding the international meetings reported (EU-NSF Workshop non-equilibrium phenomena and by Natalio Mingo on Nanotechnology and International Workshop on the (LITEN, CEA), who talked about nanophononics for Future of Information Processing Technologies) was on thermoelectricity. Here nanoparticle embedded energy issues in CMOS based nanoelectronics and materials have a great promise for high ZT because of Beyond CMOS approaches. For example, in the next highly reduced thermal conductivity. edition of the ITRS Roadmap there will be a Chapter on benchmarking of a variety of non-CMOS devices based In addition to the talks, a poster session with about on power-delay properties. 20 posters was included in the meeting. A get 61
- 94. nanoICT Conf Report together event was arranged in the Monday evening, tasks is to compile a Position Paper on Nanophononics. 8th of November. The plans also include arrangement of second Phonons and Fluctuations meeting to be held in 201 either late in 1, Participants the spring or early in the fall to further amalgamate the research on nanophononics in Europe. More than 50 scientists attended the meeting with the majority coming from academia and research institutes. Phonons and Fluctuations meeting 9th The number of attendees is very high taking into November 2010 account the a little short notice for the meeting. This again reflects the interest of and importance for the Venue: research community in Europe. Langevin Amphitheater Ecole Supérieure de Physique et de Chimie Follow-up Industrielle de Paris - 15 rue Vauquelin 75005 Paris - FRANCE The target of this meeting was to gather the groups and www.espci.fr/contact/plan-acces individuals working in the field of nanophononics, fluctuations and thermal management at nanoscale AIM: To bring the phonon and fluctuations communities together and initiate the formation of a coherent research together seeking convergence of partners of different community active on nanophonononics and related issues. projects, networks, conference series, such as NANOICT, The activity in this field is currently increasing in Europe. the CNRS-network, NANO-TEC, NANOFUNCTION, The biennial school “Son et Lumière” was arranged for the NANOPOWER, ZEROPOWER, the school series Son et third time in Cargèse this year, a NiPS summer school on Lumiere, etc, in order to address topics of general “Energy Harvesting at micro and nanoscale” was arranged concern, discuss the research trends and applications of, in August in Umbria, the ICREA Workshop on Phonon for example, thermal management on the nano scale, low Engineering was held in May and NiPS Workshop on Noise energy ICT, nanophononics, the role of noise in ICT in dynamical systems at the micro and nanoscale was held research and emerging design issues in future ICT. The in August. The activity in the nanophononics research is workshop will have a combined character of touching base increasing fast and it would be beneficial to establish a on the state of the art and vision in these fields broader forum for discussions and to avoid fragmentation. One possibility worth to consider is to establish a Organisers: Coordination Action on nanophononics as follow-up to the Sebastian Volz, Jouni Ahopelto and Cliva M. Sotomayor WG Nanophononics. This would support the recently Torres established Coordination Action ZEROPOWER on energy Email: [email protected] harvesting, led by Prof. Gammaitoni, by addressing the fundamental issues more widely. One of the near future Support: 62
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- 97. nanoICT Conf Report International Summer School Son et This active field of research is currently accelerated Lumière: phononics and photonicd at thanks to new developments in experimental methods nanoscale. (31st august-11th september and advanced setup. The control of coherent phonon in 2010, IESC Cargese, France) nanostructured material is for example well achieved by the use of femtosecond laser sources. New insight in http://sonetlumiere2010.univ-lemans.fr/ nanophononics is currently brought by the recent development of ultrafast X-ray pulse beamlines allowing Organizing committee: time-resolved studies of phonons dynamics at the atomic scale. New powerful THz sources should - Pascal Ruello, Professor Laboratoire de Physique de provide in a near future new routes for coupling directly l'Etat Condensé UMR 6087 CNRS-Université du Maine, light with vibronic state of the matter. Nevertheless, France. improvements are still necessary to increase sensitivity of probing the phonon in matter and to enhance - Adnen Mlayah, Professor Centre d'Elaboration des efficiency of photon-phonon, electron-phonon coupling Matériaux et Etudes Structurales CEMES UMR efficiencies for realistic manipulation of phonon in CNRS- Université Paul Sabatier, Toulouse, France. nanostructures. All these exciting potentialities become possible because of the subnanometric precision of - Clivia M Sotomayor-Torres, Professor Catalan Institute nanostructures fabrication achieved by the most of Nanotechnology and ICREA, Barcelone, Spain. advanced nanotechnologies. Desired superlattices, nanocavities or phononic-photonic crystals with well - Antony Kent, Professor School of Physics and controlled elastic and refractive index can now be Astronomy, University of Nottingham, United Kingdom. processed. - E. Ferré, Délégation Bretagne - Pays de la Loire CNRS, The aim of this school was to put together different France. scientific communities working on acoustic excitations in solids, mainly from the optical and the acoustical point of Scientific committee: view, communities which usually attend separate conferences and often do not share a common language. • J. Dijkhuis, Pays-Bas • B. Jusserand, France • B. Perrin, France • A. Ivanov, Scotland The 3rd edition of the International Summer School Son • T. Dekorsy, Germany • V. Gusev, France et Lumière : phononics and photonics at nanoscale has • F. Vallée, France • H. Maris, USA put together 60 attendees among them 16 • A. Fainstein, Argentina • O. Wright, Japan international lecturers and 4 organizers. On the total, there were 25 french people and 35 foreign people. Summary of the topics and the evaluation Among the other 40 attendees, 28 were PhD student. of the school: This large proportion of students attendance confirm once again the relevance of this international school The Son et Lumière (SEL) School is a School of physics among a very competitive research area. dealing with nanophononics and nanophotonics. Nanophononics and nanophotonics are promising fields According to the evaluation done after the school, more which aims at understanding and controlling the than 90% of the attendees are favourable for the future properties of phonon, photon and their interactions in edition which will be held in 2012. The results of the nanostructures. Since the spatial confinement deeply evaluation indicate also that the high level and high modifies the properties of electron, phonon and quality of the lectures were greatly appreciated even if it photon, new phonon-photon and phonon-phonon has been noted that some of them were too technical. interactions schemes require to be clarified and We will then pay more attention next time to plan more elucidated. The coherent control and tailoring of high basic lectures as introduction in the first week of the frequency phonon (GHz-THz) and photon spectrums school. It has been also noted that more time should be should pave the way of innovative functionalities in the offered to student to present their research. During this family of acousto-optic, opto-acoustic devices, acoustic school more than 30 posters were presented during two nanocavities, phononic-photonic based acousto-optic half days. This posters session time could then be modulators, etc. extended again for the next edition of the SEL School. 65
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