SWCNT individualization

Because the first reports on CNT-ceramic composites date only from 1998, and because only a few teams have worked so far on these novel materials, it could be argued that we are at the infancy of the development of a new class of composite materials. Researches on these materials depend firstly on a better knowledge of the CNTs by their users. Depending on their microstructural characteristics (SWCNTs, individual or in ropes, MWCNTs, diameter, length, number of walls), but also on the synthesis methods which have been used, the properties of CNTs may greatly vary. Notably, the treatments involved in the control of the surface properties and reactivity of the CNTs need to be optimized for a particular form of CNTs synthesized by a particular method. 

Direct contact of the dispersed SWCNTs and the conductive polymer is favorable, because this presumably decreases the non-contact resistivity between the CNTs by the formation of conductive bridges between adjacent CNTs in the CNT network. The high affinity of conjugated polymers for CNTs through it-it electronic interactions ensures a close conductive polymer-CNT contact. Applying these conductive polymers to a water-based system requires the conductive polymer to have a surfactant-like nature. Water-soluble polythiophenes have been used to disperse SWCNTs in water, but no quantitative information with regard to the level of SWCNT individualization was provided. Polymeric surfactants like poly(styrene sulfonate] (PSS] have been reported to effectively disperse CNTs in water. PSS is also used in the preparation of an aqueous dispersion (latex] of the conductive polymer poly(3,4-... 

The diffraction space of ropes of parallel SWCNT can similarly be computed by summing the complex amplitudes of the individual SWCNTs taking into account the relative phase shifts resulting from the lattice arrangement at... 

We have reviewed the electronic properties of CNTs probed by magnetic measurements. MW- and SWCNTs can individually be produced, however, the parameters of CNTs are uncontrollable, such as diameter, length, chirality and so on, at the present stage. Since the features of CNTs may depend on the synthesis and purification methods, some different experimental observation on CNT properties has been reported. It is important, however, that most of papers have clarified metallic CNTs are actually present in both MW- and SWCNTs. The characteristic of CESR of SWCNTs is different from that on non-annealed MWCNTs, but rather similar to that on annealed multi-walled ones. The relationship of the electronic properties between SW- and MWCNTs has not yet been fully understood. The accurate control in parameter of CNTs is necessary in order to discuss more details of CNTs in future. 

We will discuss below the reeent experimental observations relative to the eleetrieal resistivity and magnetoresistance of individual and bundles of MWCNTs. It is interesting to note however that the ideal transport experiment, i.e., a measurement on a well eharacterised SWCNT at the atomic scale, though this is nowadays within reaeh. Nonetheless, with time the measurements performed tended gradually eloser to these ideal eonditions. Indeed, in order to interpret quantitatively the eleetronie properties of CNTs, one must eombine theoretieal studies with the synthesis of well defined samples, which structural parameters have been precisely determined, and direet electrical measurements on the same sample. 

Electrical resistivity measurements have also been performed on individual SWCNT and on bundles of SWCNT. In the latter case thermoelectric power measurements have been carried out very recently (cf. Sec. 5.3.2). 

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]. 

Fig. 3. Derivatives of the tunnelling current of individual SWCNTs obtained by the STS measurement. Different features are clearly seen in the spectra of nos. 1-4 and nos. 5-7 [25].

Fig. 4. Calculated density of states for two zigzag individual SWCNTs with (a) semiconducting (10, 0) and (b) metallic (9, 0) configurations. Tight-binding approximation was used for the calculation [6].

Fig. 5. Relation,ship between observed band gap and the diameter of individual SWCNTs. Closed and open circles indicate the data from refs. 25 and 26, respectively. The data are fitted with the equation, E =2yac cld, where the nearest-neighbour transfer integral yis 2.7 eV and 2.,5 eV for linear and broken lines, respectively.

Fig. 6. The image of the nanodevice using an individual SWCNT (modified from ref., 50).

As described above, metallic CNTs are of great interest because they possess molecular orbitals which are highly delocalised. However, metallic CNTs are very difficult to use in actual devices because they require very low temperatures to control their carrier transfer. On the contrary, even at room temperature, the nonlinear /-V jas curve and the effective gate voltage dependence have been presented by using individual semiconducting SWCNTs [29]. 

Fig. 9./-Tbias curve of an individual semiconducting SWCNT with different gale voltages measured at room temperature [29].

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]. 

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 conductive properties of SWCNTs were predicted to depend on the helicity and the diameter of the nanotube [112, 145]. Nanotubes can behave either as metals or semiconductors depending upon how the tube is rolled up. The armchair nanotubes are metallic whereas the rest of them are semiconductive. The conductance through carbon nanotube junctions is highly dependent on the CNT/metal contact [146]. The first measurement of conductance on CNTs was made on a metallic nanotube connected between two Pt electrodes on top of a Si/Si02 substrate and it was observed that individual metallic SWCNTs behave as quantum wires [147]. A third electrode placed nearby was used as a gate electrode, but the conductance had a minor dependence on the gate voltage for metallic nanotubes at room temperature. The conductance of metallic nanotubes surpasses the best known metals because the... 

Scheme 1.6 Preparation of carboxamide junctions between SWCNTs (a) end-to-end junctions and (b) sidewall-to-end junction between individual tubes.

---