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REPORT TITLE
CARBON NANOTUBES:
PYSICAL PROPERTIES & APPLICATIONS
COURSE NAME:
01NUWKI
CHEMISTRY-PHYSICS OF MATERIALS FOR NANOTECHNOLOGY
SUBMITTED TO:
PROF. GARRONE EDOARDO
SUBMITTED BY:
NADIA PARVEEN
MATRICULATION # 169026
Date: July 2, 2010
CONTENTS
1. Carbon
1.1 Bonding of Atoms in Carbon Materials
1.2 Carbon Nanotubes and Their Fundamental Parameters
1.2.1 Defect-Free Nanotube
1.2.2 Defective Nanotubes
2. Properties of Carbon Nanotubes
2.1 Electronic Properties
2.2 Optical and Optoelectronic Properties
2.3 Mechanical and Electromechanical Properties
2.4 Magnetic and Electromagnetic Properties
2.5 Chemical and Electrochemical Properties
2.5.1 Opening
2.5.2 Wetting and Filling
2.5.3 Adsorption and Charge Transfer
2.5.4 Chemical Doping, Intercalation, and Modification
2.6 Thermal and Thermoelectric Properties
3. Progress of Single-walled carbon nanotube research
4. References
1. Carbon
Carbon is the most versatile element in the periodic table, owing to the type, strength, and
number of bonds it can form with many different elements. The diversity of bonds and their
corresponding geometries enable the existence of structural isomers, geometric isomers, and
enantiomers. These are found in large, complex, and diverse structures. The last few years have
seen a large growth in the scientific interest in inorganic nanotubes and fullerene-like
nanoparticles. Numerous studies have been published in this area.
1.1. Bonding of Atoms in Carbon Materials
To understand the structure and properties of carbon materials, the bonding structure and
properties of carbon atoms are discussed first. A carbon atom has six electrons with two of them
filling the 1s orbital. The remaining four electrons fill the sp3 or sp
2 as well as the sp hybrid
orbital, responsible for bonding structures of diamond, graphite, nanotubes, or fullerenes, as
shown in Figure 1.
In diamond, the four valence electrons of each carbon occupy the sp3 hybrid orbital and
create four equivalent σ covalent bonds to connect four other carbons in the four tetrahedral
directions. This three-dimensional interlocking structure makes diamond the hardest known
material. Diamond also has a high index of refraction, which makes large diamond single
crystals gems. Diamond has unusually high thermal conductivity.
In graphite, three outer-shell electrons of each carbon atom occupy the planar sp2 hybrid
orbital to form three in-plane σ bonds with an out-of-plane orbital (bond). This makes a planar
hexagonal network. Van der Waals force holds sheets of hexagonal networks parallel with each
other with a spacing of 0.34 nm. Graphite is stronger in-plane than diamond. In addition, an out-
of-plane orbital or electron is distributed over a graphite plane and makes it more thermally
and electrically conductive. The interaction of the loose electron with light causes graphite to
appear black. The weak van der Waals interaction among graphite sheets makes graphite soft and
hence ideal as a lubricant because the sheets are easy to glide relative to each other.
A CNT can be viewed as a hollow cylinder formed by rolling graphite sheets. Bonding in
nanotubes is essentially sp2. However, the circular curvature will cause quantum confinement
and rehybridization in which three σ bonds are slightly out of plane; for compensation, the
orbital is more delocalized outside the tube. This makes nanotubes mechanically stronger,
electrically and thermally more conductive, and chemically and biologically more active than
graphite. In addition, they allow topological defects such as pentagons and heptagons to be
incorporated into the hexagonal network. For convention, we call a nanotube defect free if it is of
only hexagonal network and defective if it also contains topological defects such as pentagon and
heptagon or other chemical and structural defects.
Fullerenes are made of 20 hexagons and 12 pentagons. The bonding is also sp2, although
once again mixed with sp3 character because of high curvature. The special bonded structures in
fullerene molecules have provided several surprises such as metal–insulator transition, unusual
magnetic correlations, very rich electronic and optical band structures and properties, chemical
functionalizations, and molecular packing. Because of these properties, fullerenes have been
widely exploited for electronic, magnetic, optical, chemical, biological, and medical applications.
(a) (b)
(c)
Figure 1 Bonding structures of diamond (a), graphite (b), nanotubes (c), and fullerenes (c).
1.2. Carbon Nanotubes and Their Fundamental Parameters
Nanotubes might be defected or defect-free. Both types of nanotubes have been paid
attention during the last two decades. In this section, the fundamental parameters for defected
and defect-free carbon nanotubes are summarized and the basic relations governing these
parameters are also given.
1.2.1. Defect-Free Nanotubes
There has been a tremendous amount of work studying defect-free nanotubes, including
single or multiwalled nanotubes (SWNTs or MWNTs). A SWNT is a hollow cylinder of a
graphite sheet whereas a MWNT is a group of coaxial SWNTs. SWNT was discovered in 1993,
2 years after the discovery of MWNT. They are often seen as straight or elastic bending
structures individually or in ropes by scanning electron microscopy (SEM), transmission electron
microscopy (TEM), atomic force microscopy (AFM), and scanning tunneling microscopy
(STM). In addition, electron diffraction (EDR), x-ray diffraction (XRD), Raman, and other
optical spectroscopy can also be used to study structural features of nanotubes.
A SWNT can be visualized as a hollow cylinder, formed by rolling over a graphite sheet.
It can be uniquely characterized by a vector C in terms of a set of two integers (n, m)
corresponding to graphite lattice vectors a1 and a2 (Figure 2),
C = na1+ ma2 (1.1)
Thus, the SWNT is constructed by rolling up the sheet such that the two end-points of the
vector C are superimposed. This tube is denoted as (n, m) tube with diameter given by
(1.2)
where a = |a1| = |a2| is lattice constant of graphite. The tubes with m = n are commonly referred
to as armchair tubes and m = 0 as zigzag tubes. Others are called chiral tubes in general with the
chiral angle, θ defined as that between the vector C and the zigzag direction a1,
(1.3)
θ ranges from 0 for zigzag (m = 0) and 30° for armchair (m=n) tubes. The lattice constant
and intertube spacing are required to generate a SWNT, SWNT bundle, and MWNT. These two
parameters vary with tube diameter or in radial direction. Most experimental measurements and
theoretical calculations agree that, on average, the C–C bond length dcc = 0.142 nm or a = |a1| =
|a2| = 0.246 nm and intertube spacing dtt = 0.34 nm. Thus, equations (1.1) to (1.3) can be used to
model various tube structures and interpret experimental observations.
Figure 2 Rolling of a graphite sheet along the chiral vector C = na1 + ma2 on the graphite to form a nanotube (n, m).
By rolling graphite sheet in different directions, two typical nanotubes can be obtained: zigzag (n, 0), armchair (m,
m) and chiral (n, m) where n>m>0 by definition.
Strain energy caused by forming a SWNT from a graphite sheet is proportional to 1/D per
tube or 1/D2 per atom. It is suggested that a SWNT should be at least 0.4 nm large to afford
strain energy and at most about 3.0 nm large to maintain tubular structure and prevent collapsing.
The smallest innermost tube in a MWNT was found to be as small as 0.4 nm whereas the
outermost tube in a MWNT can be as large as hundreds of nm. But, typically, MWNT diameter
is larger than 2 nm inside and smaller than 100 nm outside. A SWNT rope is formed usually
through a self-organization process in which van der Waals force holds individual SWNTs
together to form a triangle lattice with lattice constant of 0.34 nm.
The significance of the tube chirality (n, m) is its direct relation with the electronic
properties of a nanotube. STM can be used to measure tube geometry (d, θ)
be used to derive (n, m).
1.2.2. Defective Nanotubes
Besides defect-free nanotubes, experimentally observed structures also include the
capped, bent, branched (L, Y, and T), and helical MWNTs, and the bent, capped, and toroidal
SWNTs. Figure 3 shows TEM images of some of these structures. Most of these structures are
believed to have topological defects such as pentagons and heptagons incorporated in nanotube
of hexagonal network. Generally, most SWNTs are defect-free whereas MWNTs are relatively
more defective, containing either topological defects (pentagon-heptagon) or structural defects.
Figure 3 Representative TEM and AFM (insert) images of the individual SWNT bends. (a), (b) and (c) denote three
typical bend angles of 34°, 26°, and 18°, MWNT coils, and Y branches.
Many approaches have been developed to model nanotubes containing topological
defects because these structures present intratube heterojunction nanoelectronic devices. Han et
al. have developed a generic approach and a computer program to generate and model
configurations of bent, branched, toroidal, and capped nanotubes. In this approach, a single bend
or each bend in a branched, toroidal, or helical nanotube is considered to connect two types of
nanotubes with the topological defects (pentagon-heptagon pairs). The bend angle between two
connected nanotubes follows a simple topological relation:
(1.4)
where θ1 and θ2 are defined in equation (1.3). Figure 4 illustrates the approach to construct and
generate the model structure. Han et al. have modeled the experimentally observed 2-, 3-, and 4-
terminal; toroidal; and helical nanotubes using molecular dynamics simulations of the model
structures. The experimentally measured diameter of each tube and bend angle are used to derive
possible tube chirality. They found that a set of chiralities could be matched to fit the same
experimental parameters. For example, a 30° sharp bend can be connected by two nanotubes
satisfying:
m2 = n2 (m1 + 2n1) / (m1-n1) (1.5)
If n1 = 0, then m2 = n2. This indicates any zigzag tube (n1,0) can be connected with any
armchair tube (m2, n2) for a 30° bend. This bend can be, for example, (17,0)-(10,10), (17,1)-
(11,9), and (15,4)-(13.6). These isomers slightly differ energetically.
Figure 4 Construction of a SWNT bend junction (10,0)-(6,6). (a) and (b), two graphite sheets representing (10,0)
and (6,6) nanotubes are connected to form a 30° planar bend; (b) and (c), the planar bend is rolled over to form a 30°
tube bend; and (c) and (d), the 30° bend is relaxed to a 36° bend via a molecular dynamics simulation. The sj, mj,
and I between four broken lines represent the unit cells of two tubes and junction interface.
Topologically, a 0° and a 30° bend need only a pair of pentagon-heptagons. In the 0°
bend structure, this pair is fused together. In the 30° bend, the pentagon and heptagon reach the
maximum separation along the tube circumference. Between these two energy minimized
configurations, as bend angle decreases, the number of pentagon-heptagon pairs increase. For
example, the three and five pairs of pentagon-heptagons are required to form 26° and 18° bends,
respectively.
It is a simple matter to construct branched, toroidal, and helical nanotubes from bent
nanotubes through topological operation of fusion, rotation, and connection. When two or more
bends are fused and connected to form branched structures, pentagons may be eliminated with
only heptagons required for negative curvature. By Euler’s topological theorem, an n-branched
structure follows n = [(number of heptagons – number of pentagons) + 12]/6. Thus, to obtain 3-
or 4-branched structure, the minimum number of topological defects is 6 or 12 heptagons. In
addition, any number of pentagon-heptagon pairs is allowed, but this may cause extra energy.
2. Properties of Carbon Nanotubes
In the following section, the properties of defect-free nanotubes including (a) an
individual SWNT, (b) an individual MWNT, and sometimes (c) a SWNT rope will mainly be
discussed. There has been a great deal of work on defective, filmed, bundled, or arrayed SWNT
or MWNT samples. However, the measured properties, for example, in electrical and thermal
conductivity and elastic modulus can vary by several orders of magnitude from sample to
sample. This is mainly because defective structures in a MWNT and random orientation of
various nanotubes in film or bulk samples have yet to be characterized or specified and
correlated with the properties of interest, which are mostly one-dimensional.
2.1. Electronic Properties
Electronic properties of nanotubes have received the greatest attention in nanotube
research and applications. Extremely small size and the highly symmetric structure allow for
remarkable quantum effects and electronic, magnetic, and lattice properties of the nanotubes.
Earlier theoretical calculations and later experimental measurements have confirmed many
extraordinary electronic properties, for example, the quantum wire feature of a SWNT, SWNT
bundle, and MWNT and the metallic and semiconducting characteristics of a SWNT.
When the graphite is rolled over to form a nanotube, a periodic boundary condition is
imposed along the tube circumference or the C direction. This condition quantizes the two-
dimensional wave vector k = (kx, ky) along this direction. The k satisfying k.C = 2πq is allowed
where q is an integer. This leads to the following condition at which metallic conductance
occurs:
(n – m) = 3q (1.6)
This suggests that one third of the tubes are metallic and two thirds are semiconducting. The
band gap for a semiconducting tube is give by
Eg = 2dccγ/D (1.7)
The derivation from graphite does not consider the curvature effect or σ-π
rehybridization. It is found that σ-π rehybridization can open up a small band gap (~0.02 eV) for
smaller (<1.5 nm) nonarmchair metallic tubes. A STM study indeed confirms such a small gap
for n – m = 3q SWNT. However, this effect is found to be very rapidly disappearing with the
tube diameter. In principle, only armchair tubes are intrinsically metallic. However, for most
discussions the metallic condition (n-m) = 3q and the band gap and structures predicted from
only the simplest π-orbital model have been accepted.
Intertube coupling needs to be considered when the results of a SWNT are used for a
SNWT rope or a MWNT. Calculations reveal interesting intertube coupling properties. The
intertube coupling induces a small band gap for certain metallic tubes but a reduced band gap by
40% for semiconducting tubes in a SWNT rope. Similar observations can be expected for a
MWNT as well, but the intertube coupling is relatively smaller because of bigger diameter in a
MWNT. For example, it is predicted that two metallic tubes (5,5) and (10,10) in a coaxial
MWNT can both open a small bang gap, but (10,10) and (15,15) tubes in a MWNT are found to
remain metallic because of less intertube coupling for larger tubes. All semiconducting tubes in a
MWNT tend to be semi-metallic just like graphite because of reduced band gap for large tubes
and hole-electron pairing for multiwall coupling. More experiments on individual MWNT
samples indeed show the dominating metallic or semimetallic nature of a MWNT while small
band gap was reported and attributed to presence of defects or an electric contact barrier. A
MWNT or a SWNT rope can be viewed as a parallel assembly of single SWNTs. The
conductance for a SWNT, a SWNT rope, or MWNT is given by
G = Go M = (2e2/h) M (1.8)
where Go = (2e2/h) is quantized conductance. M is an apparent number of conducting channels
including electron-electron coupling and intertube coupling effects in addition to intrinsic
channels. M = 2 for a perfect SWMT. M, however, is determined not only by the intrinsic
properties of a nanotube itself, but also by the intertube coupling as discussed above and the
scatters such as defects, impurities, structural distortions, coupling with substrate, and contacts.
Therefore, the experimentally measured conductance is much lower than the quantized value.
The resistivity of graphite varies remarkably depending on sample quality. As
temperature increases, it can decrease for disordered structures or increase for highly ordered
structures such as a single crystal. The room temperature in-plane resistivity of the highest
quality graphite is about 0.4 µΩm. In many measurements of SWNT ropes and MWNTs, the
resistivity is found to decrease with temperature, and the room temperature values are much
higher than 0.4 µΩm. This is mainly because nanotubes are randomly oriented in the sample.
When the measurement is carried out for the purified SWNT ropes or MWNTs aligned across
four electrodes, the result is consistently comparable with or lower than 0.4 µΩm.
The nanotube is a one-dimensional conductor and has to be aligned between two
electrodes for transport measurement. More theoretical attention has been paid to the electronic
properties of heterogeneous nanotubes, especially bent and branched structures.
2.2. Optical and Optoelectronic Properties
Defect-free nanotubes, especially SWNTs, offer direct band gap and well-defined band
and subband structure, which is ideal for optical and optoelectronic applications. Optical spectra
have been established for individual SWNTs and ropes using resonant Raman, fluorescence, and
ultraviolet to the near infrared (UV-VIS-NIR) spectroscopies. In addition, electrically induced
optical emission and photoconductivity have been studied for individual SNWTs. Optical spectra
have been extensively used to determine the detailed composition of SWNT samples. Optical
and optoelectronic properties can be understood from the band structure or DOS of a SWNT.
Figure 5 includes tube curvature-induced s-p rehybridization effect with which only
armchair tubes (n=m) are truly metallic whereas others satisfying n-m = 3q are semi-metallic
with small band gap. The energy unit in Figure 5 is γ eV. Taking γ = 2.5 (low bound) and 3.0 eV
(high bound), the wavelength of a semiconducting tube (= hc/E) can vary from 300 to 3000 nm.
This suggests potential applications of semiconducting nanotubes in optical and optoelectronic
devices from blue lasers to IR detectors.
Figure 5 Energies for symmetric interband transitions in SWNTs as a function of their diameter.
Unlike conventional solid state optoelectronics, the semiconducting SWNT can emit light
from injecting electrons and holes from two contact electrodes, instead of doping. Electrical
control of the light emission of individual SWNTs allows detailed characterization of the optical
properties.
It is still very challenging to study the optical and optoelectronic properties of a single
nanotube. Extensive work has been carried out to establish the structure-assigned optical spectra
for identification of Raman-active, infrared-active photon modes from samples containing
different diameters and chiralities of nanotubes. In addition, the electronic and optical properties
of nanotubes are strongly coupled with mechanical, chemical (environmental), thermal, and
magnetic (radiation etc.) properties, as will be discussed in the following sections. This will
further complicate characterization of the nanotube structure and properties.
2.3. Mechanical and Electromechanical Properties
σ bonding is the strongest in nature, and thus a nanotube that is structured with all σ
bonding is regarded as the ultimate fiber with the strength in its tube axis. Both experimental
measurements and theoretical calculations agree that a nanotube is as stiff as or stiffer than
diamond with the highest Young’s modulus and tensile strength. Theoretical calculations are in
agreement with experiments on average. Experimental results show broad discrepancy,
especially for MWNTs, because MWNTs contain different amount of defects from different
growth approaches.
In general, various types of defect-free nanotubes are stronger than graphite. This is
mainly because the axial component of σ bonding is greatly increased when a graphite sheet is
rolled over to form a seamless cylindrical structure or a SWNT. Young’s modulus is independent
of tube chirality, but dependent on tube diameter. The highest value is from tube diameter
between 1 and 2 nm, about 1 TPa. When different diameters of SWNTs consist in a coaxial
MWNT, the Young’s modulus will take the highest value of a SWNT plus contributions from
coaxial intertube coupling or van der Waals force. On the other hand, when many SWNTs are
held together in a bundle or a rope, the weak van der Waal force induces a strong shearing
among the packed SWNTs. This does not increase but decreases the Young’s modulus.
The elastic response of a nanotube to deformation is also very remarkable. Most hard
materials fail with a strain of 1% or less due to propagation of dislocations and defects. Both
theory and experiment show that CNTs can sustain up to 15% tensile strain before fracture. Such
a high strain is attributed to an elastic buckling through which high stress is released.
The dependence of the electronic properties on the structure implies that mechanical
deformations can alter the band structure. This results in electromechanical effects. Therefore
nanotubes have remarkable both mechanical and electromechanical properties: stiffness,
strength, piezoresistance, the capability of electrostatic actuation, and few structural defects.
These properties provide the building blocks for motion detection and actuation, novel memory
architectures, nanoscale precision manipulation, low-friction bearings, and even oscillators. The
unique mechanical and electromechanical properties of nanotubes may well find application in
the emerging field of nanoelectromechanical systems (NEMS).
There has not been much effort studying the electromechanical properties of SWNT
bundles and MWNTs. Intertube coupling may play a larger role in electromechanical properties
as it does for Young’s modulus and tensile strength.
2.4. Magnetic and Electromagnetic Properties
Similar to mechanical and electromechanical properties, magnetic and electromagnetic
properties of CNTs are also of great interest. The magnetic properties are studied with electron
spin resonance (ESR), which is very important in understanding electronic properties, for
example, for graphite and conjugated materials. Once again, there is a large discrepancy from
different experimental measurements, especially in transport properties, because of sample
quality and alignment whereas qualitatively they agree with theoretical calculations.
Magnetic properties such as anisotropic g-factor and susceptibility of nanotubes are likely
to be similar to those for graphite while some unusual properties may also exist. Indeed, it is
found from ESR that the average observed g-value and spin susceptibility in MWNTs are only
slightly lower than those for graphite. Some interesting properties are also found from ESR
studies of Pauli behavior, for example, aligned MWNTs are metallic or semimetallic.
It can also be expected that CNTs would have remarkable electrical response to a
magnetic field. Indeed, both experiment and theory confirm the metal-insulator transition and
band gap change whereas transport again is an intriguing issue. The band gap of nanotube under
uniform magnetic filed parallel to the tube axis is given by:
For metallic tubes of n – m = 3q
Eg=Ego β, 0<β<3/2
Eg=Ego│3-β│, 3/2< β<3
For semiconducting tubes
Eg=Ego│1-β│, 0< β<3/2
Eg=Ego│2-β│, 3/2< β<3
These relations predict a metal-insulator transition and band gap change for
semiconductor tubes under magnetic field parallel to tube axis. This is similar to electrical
response of nanotubes to mechanical deformation. Similar response can also be observed when
magnetic field or strain field is perpendicular to tube axis. A major feature from the theory is that
the band gap change is oscillatory and that the semiconducting and metallic nature of nanotubes
can be altered by applying a magnetic field or strain field. This is called Aharonov-Bohm effect
in magnetic field case.
2.5. Chemical and Electrochemical Properties
Small radius, large specific surfaces and σ-π rehybridization make CNTs very attractive
in chemical and biological applications because of their strong sensitivity to chemical or
environmental interactions. These, however, also present challenges in characterization and
understanding of other properties. The chemical properties of interest include opening, wetting,
filling, adsorption, charge transfer, doping, intercalation, etc. Applications include chemical and
biological separation, purification, sensing and detection, energy storage, and electronics.
2.5.1. Opening
The nanotube end is more reactive than the sidewall because of the presence of pentagons
or metallic catalysts sitting on the opened ends and greater curvature. Many approaches have
been used to open nanotube ends, including, for example, vapor phase oxidation, plasma etching,
and chemical reaction using acids such as HNO3. The opened end is terminated with different
functional groups such as carboxyl, etc., as shown in Figure 6. The opening is required for many
applications as described below.
Figure 6 Possible chemical groups at opened nanotube ends.
2.5.2. Wetting and Filling
Nanotubes are hydrophobic and do not show wetting behavior for most aqueous solvents.
It is reported that various organic solvents, HNO3, S, Cs, Rb, Se, and various oxides such as Pb
and Bi2O2 can wet nanotubes. A nanotube provides a capillary pressure proportional to (1/D).
Therefore, these wetting agents can be driven to fill inside the nanotube by the capillary pressure.
It is also likely to fill nonwetting agents inside a nanotube by applying a pressure that is higher
than the capillary pressure. An effective alternative is to use wetting agents such as HNO3 to
assist filling of nonwetting agents inside the nanotube.
2.5.3. Adsorption and Charge Transfer
Enhanced molecular adsorption and charge transfer can be expected for nanotubes.
Strong adsorption and charge transfer of oxygen to CNTs have been experimentally observed at
room temperature. The gas adsorption and charge transfer capability are functions of sites and
gas molecules. The site on which a gas molecule can adsorb includes interstitial in tube bundles,
groove above the gap between two neighboring tubes, nanopore inside a tube, and surface of a
single tube. The adsorption and charge transfer capability is found to follow a decreasing order:
Sites: Interstitial, groove, nanopore, and surface.
Gas: C8N2O2Cl2, O2, C6H12, C6H6, NO2, H2O, NH3, CH4, CO2, N2, H2, and Ar.
2.5.4. Chemical Doping, Intercalation, and Modification
The substitutional doping with B and N dopants was pursued to make nanotubes p- and
n-types. However, molecular adsorption as discussed above provides a simple, noncovalent
doping approach to turn nanotubes into p-type with oxygen or water adsorption or n-type with,
for example, C6H12. On the other hand, intercalation of the alkali metals with nanotubes is used
for enhanced metallic conductivity or halogens with nanotubes for charge- or energy-storage
applications. Experimental observation and theoretical calculations show that these intercalating
agents mainly enter intertube spaces or defects on nanotubes for enhanced electrochemical
capability for charge transfer and storage.
In fact, nanotubes as electrode materials show enhanced electrochemical capability. The
reduction and oxidation reactions that occur at the electrodes produce a flow of electrons that
generate a signal for chemical and biological detection and store energy. In battery applications,
conventional graphite, or other electrodes can reversibly store one lithium ion for every six
carbon atoms. Experiments reveal an electrical storage capacity approximately double that of
graphite. Theoretical studies show that the tube’s open ends facilitate the diffusion of lithium
atoms into interstitial sites. However, their nanoscale dimension provides unique electrochemical
properties in greatly improved sensitivity and speed in chemical and biological sensor
applications.
2.6. Thermal and Thermoelectric Properties
Graphite and diamond show extraordinary heat capacity and thermal conductivity. It can
be expected that nanotubes have similar thermal properties at room and elevated temperatures
but unusual behavior at low temperatures because of the effects of phonon quantization.
Experimental results on MWNTs show a temperature-dependent specific heat, which is
consistent with weak interlayer coupling, although different measurements show slightly
different temperature dependencies. When T >100 K, an SWNT, SWNT bundle, and MWNT all
follow or are close to specific heat relation of graphite. However, at lower temperatures, CNTs
show quantum confinement effects.
The thermal conductivity of both SWNTs and MWNTs should reflect the on-tube phonon
structure, regardless of intertube coupling. Measurements of the thermal conductivity of bulk
samples show graphite- like behavior for MWNTs but quite different behavior for SWNTs.
Thermal conductivity is onedimensional for nanotubes like electrical conductivity.
Theoretical calculations and experimental measurements showed that the thermal
conductivity for a SWNT ropes and MWNTs at room temperature could vary between 1800 and
6000 W/mK. The thermoelectric power, defined by TEP = ΔV/ΔT in which V is thermoelectric
voltage and T is temperature, is of great interest in understanding transport due to its extreme
sensitivity to the change of electronic structure at the Fermi level. TEP for a single metallic or
semiconducting tube follows linear temperature dependence with positive and negative slope,
respectively, for p- and n-doped tube. Thermoelectric properties vary significantly from sample
to sample for filmed and bundled SNWTs and MWNTs.
3. Progress of Single-walled carbon nanotube research
SWNTs are a distinctive class of molecules that exhibit unique properties. Since the
discovery of carbon nanotubes (CNTs), numerous ideas for applications have arose in a wide
variety of scientific disciplines, including (1) electronics (wires, transistors, switches,
interconnects, memory storage devices); (2) opto-electronics (light-emitting diodes, lasers);
(3) sensors; (4) field emission devices (displays, scanning and electron probes/microscopes);
(5) batteries/fuel cells; (6) fibers, reinforced composites; (7) medicine/biology (fluorescent
markers for cancer treatment, biological labels, drug delivery carriers); (8) catalysis; and (9)
gas storage. This section presents a brief description of some of the most significant findings.
In computer chip circuits, transistors and wires are produced by lithography. Smaller
and cheaper circuitry may be feasible from using molecular nanostructures. CNTs as quasi
one-dimensional (1D) molecular nanostructures are perfect applicants for nanoscale
transistors or wires. Additionally, because CNTs can be both metallic and semiconducting,
an all-nanotube electronic device can be envisioned. In this case, metallic CNTs could act as
high current carrying local interconnects, while semiconductoring CNTs would form the
active devices.
Fibers and yarns are among the most promising forms for using nanotubes on a
macroscopic scale, mainly because, in analogy to high-performance polymer fibers, they
allow nanotubes to be aligned and then weaved into textile structures or used as cables. The
fibers and ribbons produced had an elastic modulus 10 times higher than the modulus of
high-quality bucky paper. These fibers show rather good alignment and can be tied into knots
without breaking.
AFM evolved to be one of the most important tools for analyzing surfaces, with the
use of CNTs as tips an advancement regarding lateral resolution. The huge aspect ratio
allows investigation of samples with deep holes or trenches. Furthermore, due to their
elasticity, CNTs allow more gentle investigations of surfaces than standard tips. In 1995, the
first example of carbon nanotubes as scanning probe tips was reported.
Because of their intrinsic optical properties, nanotubes have been considered potential
candidates for drug delivery carriers. The capped ends of nanotubes may be opened up by
oxidation, allowing for the insertion of molecules of interest inside the nanotube. IR laser-
excited photoconductivity was observed for a semiconducting SWNT within an ambipolar field
effect transistor device, which suggests that a semiconducting SWNT can be used for a polarized
IR photo detector in which the photocurrent is nearly a linear function of IR intensity. In
contrast, the same device can be also used for optoelectronic devices such as a light emitter in
which emission of wavelength of 1500 nm is induced electronically.
CNTs also have potential for use in energy applications. For heterogenic catalysis,
activated coal is very often used as a catalyst carrier substance because it has a high specific
surface. Using CNTs as a carrier substance has the advantage that the morphology and the
chemical composition of the CNTs are better defined; therefore the covalent connection of
the catalyst is better controlled. The potential of using CNTs as catalyst supports has already
been investigated. An industrial interest exists in the area of fuel cell electrodes or supported
catalysts for fluid phase reactions. The strong capillarity of CNTs due to their tubular shape,
together with their high surface/weight ratio, make CNTs ideal for gas adsorption, and hence
for fuel cell applications. There is great interest in small and lightweight hydrogen storage
materials.
The novel electronic properties of nanotubes have attracted great interest in applications
of nanotubes in nanoelectronics. Much of the effort to date has been made in using individual
semiconductor SWNTs for transistors, memories, and logic devices. The striking feature of these
nanoelectronic devices is higher mobility and stronger field effect. In addition, nanotube
junctions such as sharp bends and T and Y branches have been studied as nanoelectronic devices.
Despite all the progress made on various uses for CNTs, a great deal of research is
still focused on fundamental problems that inhibit the use of CNTs for applications. For
many applications, the availability of ensembles of CNTs with uniform diameters, length,
and electronic properties is important. To date there is no existing CNT synthesis method
that sufficiently allows the control over length, diameter, or the electronic properties of the
CNTs. Chemical vapor deposition (CVD) is the most controllable method for producing
CNTs suitable for mass production and large-area deposition.
4. References
Michael J. O’Connell. Carbon Nanotubes: Properties and Applications, (CRC, London-
2006)
M. Meyyappan. Carbon Nanotubes: Science and Applications, (CRC, London-2005)
R. Saito, G. Dresselhaus, M. S. Dresselhaus. Physical Properties of Carbon Nanotubes,
(ICP, London-1998)
A. Jorio, G Dresselhaus, M. S. Dresselhaus. Carbon Nanotubes, (Springer, New York-
2008)
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