CARBON NANOTUBES: PYSICAL PROPERTIES & APPLICATIONS

Transcription

1 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 # Date: July 2, 2010

2 CONTENTS 1. Carbon 1.1 Bonding of Atoms in Carbon Materials 1.2 Carbon Nanotubes and Their Fundamental Parameters Defect-Free Nanotube 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 Opening Wetting and Filling Adsorption and Charge Transfer Chemical Doping, Intercalation, and Modification 2.6 Thermal and Thermoelectric Properties 3. Progress of Single-walled carbon nanotube research 4. References

3 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 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 sp 3 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 sp 2 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 outof-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 sp 2. 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

4 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 sp 2, although once again mixed with sp 3 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) 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 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

5 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 a 1 and a 2 (Figure 2), C = na 1 + ma 2 (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 = a 1 = a 2 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 a 1, (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 d cc = nm or a = a 1 = a 2 = nm and intertube spacing d tt = 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 = na 1 + ma 2 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.

6 Strain energy caused by forming a SWNT from a graphite sheet is proportional to 1/D per tube or 1/D 2 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) 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

7 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 θ 1 and θ 2 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: m 2 = n 2 (m 1 + 2n 1 ) / (m 1 -n 1 ) (1.5) If n 1 = 0, then m 2 = n 2. This indicates any zigzag tube (n 1,0) can be connected with any armchair tube (m 2, n 2 ) 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

8 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 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 twodimensional wave vector k = (kx, ky) along this direction. The k satisfying k.c = 2πq is allowed

9 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 E g = 2d cc γ/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 = G o M = (2e 2 /h) M (1.8) where G o = (2e 2 /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

10 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 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.

11 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 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.

12 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 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.

13 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 E g =E go β, 0<β<3/2 E g =E go 3-β, 3/2< β<3 For semiconducting tubes E g =E go 1-β, 0< β<3/2 E g =E go 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 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.

14 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 HNO 3. 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 Wetting and Filling Nanotubes are hydrophobic and do not show wetting behavior for most aqueous solvents. It is reported that various organic solvents, HNO 3, S, Cs, Rb, Se, and various oxides such as Pb and Bi 2 O 2 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 HNO 3 to assist filling of nonwetting agents inside the nanotube 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,

15 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: C 8 N 2 O 2 C l2, O 2, C 6 H 12, C 6 H 6, NO 2, H 2 O, NH 3, CH 4, CO 2, N 2, H 2, and Ar 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, C 6 H 12. 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 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.

16 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

17 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 laserexcited 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

18 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)