SWCNT conductivity

The adsorption of gas molecules near the dopant induces a large charge transfer which shifts the Fermi level of the SWCNT and modifies the doped-SWCNT conductivity to a considerable degree. Increasing the dopant concentration increases the number of active sites and the sensor becomes progressively more sensitive (Hamadanian et /.,2011). 

Ultimately, the quality of the CNT will play the most influential role on the conductive nature of the final composite. A substantial increase in ultimate conductivity [beyond the percolation threshold] with a decrease in melt-viscosity was reported for systems in which the inherent SWCNT conductivity was much lower than those used in the composites presented here. The higher inherent conductivity of the CNTs used in the present study could explain the absence of a large increase in ultimate conductivity with decreased melt viscosity. Differences in ultimate conductivities have been shown to vary widely across different CNT batches, thus making comparisons between different systems inaccurate. ... 

Although it is required to refine the above condition I in actuality, this rather simple but impressive prediction seems to have much stimulated the experiments on the electrical-conductivity measurement and the related solid-state properties in spite of technological difficulties in purification of the CNT sample and in direct measurement of its electrical conductivity (see Chap. 10). For instance, for MWCNT, a direct conductivity measurement has proved the existence of metallic sample [7]. The electron spin resonance (ESR) (see Chap. 8) [8] and the C nuclear magnetic resonance (NMR) [9] measurements have also proved that MWCNT can show metallic property based on the Pauli susceptibility and Korringa-like relation, respectively. On the other hand, existence of semiconductive MWCNT sample has also been shown by the ESR measurement [ 10], For SWCNT, a combination of direct electrical conductivity and the ESR measurements has confirmed the metallic property of the sample employed therein [11]. More recently, bandgap values of several SWCNT... 

Quantum effects are observed in the Raman spectra of SWCNTs through the resonant Raman enhancement process, which is seen experimentally by measuring the Raman spectra at a number of laser excitation energies. Resonant enhancement in the Raman scattering intensity from CNTs occurs when the laser excitation energy corresponds to an electronic transition between the sharp features (i.e., (E - ,)" type singularities at energy ,) in the ID electronic DOS of the valence and conduction bands of the carbon CNT. 

SWCNTs have been produced by carbon arc discharge and laser ablation of graphite rods. In each case, a small amount of transition metals is added to the carbon target as a catalyst. Therefore the ferromagnetic catalysts resided in the sample. The residual catalyst particles are responsible for a very broad ESR line near g=2 with a linewidth about 400 G, which obscures the expected conduction electron response from SWCNTs. 

After briefly introducing the main electronic features of CNTs (Sec. 2) and some general aspects of electronic conduction and transmission (Sec.. 1), we will show how complex electrical measurements to perform on such tiny entities are (Sec. 4). Then we will present the main experimental results obtained on the electrical resistivity of MWCNT and SWCNT and the very recent data relative to the thermopower of SWCNT bundles (Sec. 5). We will also discuss the effect of intercalation on the electrical resistivity of SWCNT bundles (Sec. 6). Finally, we will present some potential applications (Sec. 7). 

An SWCNT a few microns long with electrical contacts deposited via nanolithography techniques is an ideal system to study single-molecule transistor effects, in which an electrode (Vga/e) close to the conducting CNT is used to modulate the conductance [33]. 

For SWCNT bundles [35], ID intercalation would occur between the CNTs columns as it is the case for jxilyacetylene. Intercalation either by acceptors (Fig. 6) or donors (Fig. 7) increases the electrical conductivity as expected, however the effect is less pronounced than in bulk graphite [34]. 

In conclusion, wc have shown the interesting information which one can get from electrical resistivity measurements on SWCNT and MWCNT and the exciting applications which can be derived. MWCNTs behave as an ultimate carbon fibre revealing specific 2D quantum transport features at low temperatures weak localisation and universal conductance fluctuations. SWCNTs behave as pure quantum wires which, if limited in length, reduce to quantum dots. Thus, each type of CNT has its own features which are strongly dependent on the dimensionality of the electronic gas. We have also briefly discussed the very recent experimental results obtained on the thermopower of SWCNT bundles and the effect of intercalation on the electrical resistivity of these systems. 

Fig. 1. (a) Comparison of normalised electrical conductivity of individual MWCNTs (Langer 96 [17], Ebbesen [18]) and bundles of MWCNTs (Langer 94 [19], Song [20]). (b) Temperature dependence of resistivity of different forms (ropes and mats) of SWCNTs [21], and chemically doped conducting polymers, PAc (FeClj-doped polyacetylene [22]) and PAni (camphor sulfonic acid-doped polyaniline [2. ]) [24]. 

II is noted that much higher resolution is expected when an individual SWCNT is used. By taking advantages of the high conductivity of CNTs, STM images have also been obtained using a CNT probe [35]. 

Nanoscale diameter, hollow structure, high conductivity, mechanical strength and inertness of CNTs are also very useful for macroscale applications. MWCNTs have been more widely used for macroscopic applications since they are more readily available in larger quantities and are more stable than SWCNTs. 

Figure 9.15 shows how the electric conductivity of SWCNT-filled composites increases with SWCNT concentration (Park et al., 2002). The solid curve is the fit to the experimental data using the classical percolation power law. a = o0(K- F) ... 

Kilbride et al. (2002 Andriotis et al., 2003) measured the alternating current (ac) and direct current (dc) conductivities of polymer-SWCNT composite thin films such as PMPV and polyvinylalcohol (PVA), the result showed that the ac conductivity... 

CNTs own excellent thermal conductivity. Aligned bundles of SWCNTs show a thermal conductivity of >200 W/m K at room temperature (Biercuk et al., 2002). 

Biomolecules-functionalized CNTs can result in characteristic electric conductivity changes of CNTs (Hou et al., 2003), which may be developed into specific biosensor for ultrasensitive detection of biomolecules such as DNA molecules, bacteria and vims, etc. We also observed that oligo DNA-filled SWCNTs appeared as characteristic Electric Resistance peaks as shown in Fig. 9.22, which also may be used as biosensor to detect biomolecules or sequence DNA sequences. 

The thermal conductivity of individual SWCNTs was calculated to be as large as 6600 Wm K"1 in axial direction, but only 1.52 Wm 1K 1 perpendicular to its axis... 

The thermal conductivity of suspended graphene has been calculated by measuring the frequency shift of the G-band in the Raman spectrum with varying laser power. These measurements yielded a value for thermal conductivity of 4840 5300 W m 1 K 1 [23], better than that of SWCNTs, with the exception of crystalline ropes of nanotubes, which gave values up to 5800 W m 1 K 1 [24]. Even when deposited on a substrate, the measured thermal conductivity is 600 W m 1 K 1 [25], higher than in commonly used heat dissipation materials such as copper and silver. 

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