SWCNT

There have been a considerable efforts at synthesis and purification of MWCNT for the measurements of its physical properties. The time is, however, gradually maturing toward its industrial application. As to SWCNT, it could not be efficiently obtained at first and, furthermore, both of its purification and physical-properties measurement were difficult. In 1996, it became that SWCNT could be efficiently synthesized [14,16] and, since then, it has become widely studied mainly from the scientific viewpoints. In what follows, the synthesis and purification of MWCNT and SWCNT are to be summarised itemisingly. 

Laser-ablation method shown in Fig. 3 was used when Cgo was first discovered in 1985 [15]. This method has also been applied for the synthesis of CNT, but length of MWCNT is much shorter than that by arc-discharge method [17]. Therefore, this method does not seem adequate to the synthesis of MWCNT. However, in the synthesis of SWCNT described later (Sec. 3.1.2), marvelously high yield has been obtained by this method. Hence, laser-ablation method has become another important technology in this respect. 

Fig. 7. TEM image of SWCNT growing radially from a La-carbide particles [10b].

Preparation research of SWCNT was also put forth by lijima and his co-worker [3]. The structure of SWCNT consists of an enrolled graphene to form a tube without seam. The length and diameter depend on the kinds of the metal catalyst used in the synthesis. The maximum length is several jim and the diameter varies from 1 to 3 nm. The thinnest diameter is about the same as that of Cgo (i.e., ca. 0.7 nm). The structure and characteristics of SWCNT are apparently different from those of MWCNT and rather near to fullerenes. Hence novel physical properties of SWCNT as the one-dimensional material between molecule and bulk are expected. On the other hand, the physical property of MWCNT is almost similar to that of graphite used as bulk [6c]. 

SWCNT is synthesized by almost the same method as that- for the synthesis of MWCNT. Remarkable difference lies in the fact that metallic catalyst is indispensable to the synthesis of fullerenes. The metal compounds used as the catalyst are listed in Table 2 [8]. 

Table 2. Metals and metal compounds catalysts for SWCNT synthesis (modified from ref. 8).

SWCNT is synthesized by co-evaporation of carbon and catalyst (mostly metals) in arc discharge. In early time, Fe [3], Co [4], Ni [8, 10] or rare-earth element [10] was employed as the catalyst (see Fig. 7). In these syntheses, however, the yield of SWCNT was quite low. In the improved method, the catalyst consisting of more than one element such as Co-Pt [12,13] or Ni-Y [14] is used to increase the yield of SWCNT (e.g., more than 75 % with Ni-Y [14]). 

Although laser-ablation method with pure carbon as the target only gives fullerenes, SWCNT can be obtained at high yield by mixing Co-Ni into the target carbon [16]. Isolation of thus synthesized SWCNT is rather of ease since the crude product is almost free of nanoparticle and amorphous carbon [39]. Such... 

SWCNT sample has widely been used for the physical-property measurements [40],... 

Very recently, it has been reported that SWCNT can be synthesized by decomposition of benzene with Fe catalyst [27]. It would be of most importance to establish the controllability of the diameter and the helical pitch in this kind of synthesis of SWCNT toward the development of novel kinds of electronic devices such as single molecule transistor [41]. It can be said that this field is full of dream. 

MWCNT was first discovered by arc-discharge method of pure carbon and successive discovery of SWCNT was also based on the same method in which carbon is co-evaporated with metallic element. Optimisation of such metallic catalyst has recently been performed. Although these electric arc methods can produce gram quantity of MWCNT and SWCNT, the raw product requires rather tedious purification process. 

The laser-ablation method can produce SWCNT under co-evaporation of metals like in the electric arc-discharge method. As metallic catalyst Fe, Co or Ni plays the important role and their combination or addition of the third element such as Y produces SWCNT in an efficient manner. But it is still difficult in the laser-ablation method to produce gram quantity of SWCNT. Nonetheless, remarkable progress in the research of physical properties has been achieved in thus synthesized SWCNT. 

Among the several known types of carbon fibres the discussion in this chapter is limited to the electric arc grown multi-walled carbon nanotubes (MWCNTs) as well as single-walled ones (SWCNTs). For MWCNT we restrict the discussion to the idealised coaxial cylinder model. For other models and other shapes we refer to the literature [1-6],... 

Diffraction patterns of well isolated SWCNT are difficult to obtain due to the small quantity of diffracting material present, and also due to the fact that such tubes almost exclusively occur as bundles (or ropes) of parallel tubes, kept together by van der Waals forces. 

Simulated SWCNT ED patterns will be presented below. Tbe most striking difference with tbe MWCNT ED patterns is tbe absence of tbe row of sharp oo.l reflexions. In tbe diffraction pattern of ropes there is still a row of sharp reflexions perpendicular to the rope axis but which now corresponds to the much larger interplanar distance caused by the lattice of the tubes in the rope. The ho.o type reflexions are moreover not only asymmetrically streaked but also considerably broadened as a consequence of the presence of tubes with different Hamada indices (Fig. 3). 

Fig. 3. (a) Diffraction pattern of a well formed rope (superlattice) of armchair-like tubes. Note the presence of superlattice spots in the inset (b). The broadening of the streaks of 1010 type reOexions is consistent with a model in which the SWCNTs have slightly different chiral angles. 

SWCNTs are imaged as two parallel lines with a separation equal to the tube diameter (Fig. 5). By image simulation it can be shown that under usual observation conditions the black lines correspond to graphene sheets seen edge on in MWCNT as well as in SWCNT tubes [7]. 

Fig. 5. Isolated SWCNT split off from a rope. The diffraction pattern produced by such a single tube is usually too weak to be recorded by present methods. The single graphene sheet in the walls is imaged as a dark line.

Fig. 7. High resolution images of ropes seen along their length axis. Note the hexagonal lattice of SWCNT s (Courtesy of A. Loiseau).

Assuming kinematical diffraction theory to be applicable to the weakly scattering CNTs, the diffraction space of SWCNT can be obtained in closed analytical form by the direct stepwise summation of the complex amplitudes of the scattered waves extended to all seattering centres, taking the phase differenees due to position into aeeount. 

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... 

A hexagonal lattice of identical SWCNT s leads in diffraction space to a 2D lattice of nodes at positions h +h2 2 A Bj = 27i5,y. Spots corresponding to such nodes are visible in Fig. 3. 

Fig. 11. Simulated diffraction space of a chiral (40, 5) SWCNT. (a) Normal incidence diffraction pattern with 2mm symmetry (b),(c),(d) and (e) four sections of diffraction space at the levels indicated by arrows. Note the absence of azimuthal dependence of the intensity. The radii of the dark circles are given by the zeros of the sums of Bessel functions [17].

Several sections of the diffraction space of a chiral SWCNT (40, 5) are reproduced in Fig. 11. In Fig. 11(a) the normal incidence pattern is shown note the 2mm symmetry. The sections = constant exhibit bright circles having radii corresponding to the maxima of the Bessel functions in Eq.(7). The absence of azimuthal dependence of the intensity is consistent with the point group symmetry of diffraction space, which reflects the symmetry of direct space i.e. the infinite chiral tube as well as the corresponding diffraction space exhibit a rotation axis of infinite multiplicity parallel to the tube axis. 

On the other hand, TED patterns can assign the fine structure. In general, the pattern includes two kinds of information. One is a series of strong reflexion spots with the indexes of (00/), 002, 004 and 006, and 101 from the side portions of MWCNTs as shown in Eig. 1(b). The indexes follow those of graphite. The TED pattern also includes the information from the top and bottom sheets in tube. The helieity would appear as a pair of arcs of 110 reflexions. In the case of nano-probed TED, several analyses in fine structures have been done for SWCNT to prove the dependence on the locations [11,12]. 

Fig, 6. EEL spectra of bundle of four SWCNTs, MWCNT and graphite in the energy ranges (a) from 0 to 45 eV (plasmon region) and (b) from 280 to 300 eV (carbon K-edge) (modified from ref. 14). 

Although CNTs showed similar EELS pattern in plasmon-loss and core-loss regions to graphite, SWCNT and fine MWCNT with a diameter less than 5 nm had different features. Furthermore, it has been found out that the angular-dependent EELS along the direction normal to the longitudinal axis of CNT shows stronger contribution from Jt electrons than [Pg.38]

In this chapter the results of detailed research on the realistic electronic structure of single-walled CNT (SWCNT) are summarised with explicit consideration of carbon-carbon bond-alternation patterns accompanied by the metal-insulator transition inherent in low-dimensional materials including CNT. Moreover, recent selective topics of electronic structures of CNT are also described. Throughout this chapter the terminology "CNT stands for SWCNT unless specially noted. 

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... 

An MWCNT has inner concentric tube(s) with smaller diameter(s) inside its hollow, and it is normally prepared in the carbon electrode of the arc-discharging method or by chemical vapour deposition method (see Chaps. 2 and 12). Influence of such inner tubes on the most outer layer in MWCNT is of interest with respect to electronic similarity of MWCNT and SWCNT. 

It has been referred to that SWCNT forms a bundle or rope by aggregating several hundreds of SWCNTs in parallel [22]. Intertube interaction in such... 

Electronic structures of SWCNT have been reviewed. It has been shown that armchair-structural tubes (a, a) could probably remain metallic after energetical stabilisation in connection with the metal-insulator transition but that zigzag (3a, 0) and helical-structural tubes (a, b) would change into semiconductive even if the condition 2a + b = 3N s satisfied. There would not be so much difference in the electronic structures between MWCNT and SWCNT and these can be regarded electronically similar at least in the zeroth order approximation. Doping to CNT with either Lewis acid or base would newly cause intriguing electronic properties including superconductivity. 

In the following sections, we first show the phonon dispersion relation of CNTs, and then the calculated results for the Raman intensity of a CNT are shown as a function of the polarisation direction. We also show the Raman calculation for a finite length of CNT, which is relevant to the intermediate frequency region. The enhancement of the Raman intensity is observed as a function of laser frequency when the laser excitation frequency is close to a frequency of high optical absorption, and this effect is called the resonant Raman effect. The observed Raman spectra of SWCNTs show resonant-Raman effects [5, 8], which will be given in the last section. 

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