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Figure 5. STM images of atomically resolved carbon nanotubes. For each nanotube the apparent angle between hexagon rows and the tube axis is indicated. The 1-nm bar indicates the scale for all four images. (a), (b) Two chiral nanotubes with small chiral angles. (c), (d) Two chiral nanotubes with large chiral angles near 30°. Reprinted with permission from [49], L. C. Venema et al., Phys. Rev. B 61, 2991 (2000). © 2000, American Physical Society.

Figure 6. (a) Upper panel: Comparison between the DOS obtained from a STS experiment and a tight binding calculation for a (13, 7) and (12, 6) SWNT Lower panel: Comparison between the DOS obtained from a STS experiment and a tight binding calculation for a (10, 0) SWNT. Reprinted with permission from [46], T. W. Odom et al., J. Phys.: Condens. Matter 14, R145 (2002). © 2002, Institute of Physics Publishing.

exhibited relatively good agreement with the calculated (10, 0) DOS below Fermi level, but a poorer agreement above this level (Fig. 6). The difference was also attributed to curvature-induced hybridization, since the n-only DOS calculation does not include n/a and n* /a* mixing due to the curvature of the nanotube. This hybridization of n/a orbitals is believed to produce more pronounced effects on the conduction band than in the valence band and hence this could explain the observed deviations [46, 58].

STS has opened the possibility of performing detailed studies of the electronic structures and perturbation-like quantum effects. For example, the effect of the finite size of the nanotube was studied by experimentally obtaining the DOS of shortened SWNTs [60, 61]. In other studies, the influence of Coulomb charging was also analyzed by STS [62-64]. At the same time more practical issues such as the effect of bending, sidewall functionalization, and how the presence of an intermolecular junction between two SWNT modifies their DOS have been investigated by STM/STS [65, 66]. For instance, studies of atomically resolved STM images have allowed the identification of intermolecular junctions (IMJ) between two different types of SWNTs. Figure 7 shows one of these results. The upper and lower portions of a SWNT with appreciably dissimilar chiral angles are shown. Based on the chiral angle and the SWNT diameter, the (n, m) indices of the upper and lower regions were assigned as (21, -2) (semiconductor) and (22, -5) (metallic), respectively. The STS experiments provided spec-troscopic evidence for the overall structure showing that

Figure 7. (a) Atomically resolved STM image of a SWNT containing a junction indicated by the white arrow. (b) Models chosen to describe the junction. Model I has three separated 5/7 pairs, and model II has two isolated 5/7 pairs and one 5/7-7/5 pair. The solid black spheres highlight the atoms forming the 5/7 defects. (c) Solid line: spatially resolved dl/dV data acquired at the positions indicated the by six symbols in (a). Thick dashed line: calculated local DOS for model I. Thin dashed line: calculated local DOS for model II. Reprinted with permission from [66], M. Ouyang et al., Acc. Chem. Res. 35, 1018 (2002). © 2002, American Chemical Society.

Figure 7. (a) Atomically resolved STM image of a SWNT containing a junction indicated by the white arrow. (b) Models chosen to describe the junction. Model I has three separated 5/7 pairs, and model II has two isolated 5/7 pairs and one 5/7-7/5 pair. The solid black spheres highlight the atoms forming the 5/7 defects. (c) Solid line: spatially resolved dl/dV data acquired at the positions indicated the by six symbols in (a). Thick dashed line: calculated local DOS for model I. Thin dashed line: calculated local DOS for model II. Reprinted with permission from [66], M. Ouyang et al., Acc. Chem. Res. 35, 1018 (2002). © 2002, American Chemical Society.

the upper and lower portions corresponded to a semiconductor and metal, respectively. Indeed, as the data presented in Figure 7c show, this junction has a very sharp metal-semiconductor interface where the semiconducting gap between the van Hove singularities decays across the junction into the metallic segment within <1 nm, whereas the distinct spectroscopic features of the metallic nanotube seem to decay more rapidly across the junction interface. The authors modeled the interface in order to compare the calculated and experimentally measured DOS of this junction. Two possible structural models are represented in Figure 7b. The local DOS obtained from a tight binding calculation for model I agreed well with the measured local DOS across the IMJ interface. This result suggests that model I is a reasonable representation of the junction.

Kelly et al. [67] published a report on how STM can provide evidence for sidewall functionalization. The authors showed images of SWNTs whose sidewalls were chemically modified by a two-step synthesis in which they were first fluorinated and then butylated. Noticeable "narrow dark bands" around the walls of the SWNTs that were exposed to aggressive fluorination conditions were observed by STM. Based on these observations and on semiempirical calculations, the authors proposed a sidewall-functionalization mechanism. Unfortunately in this case the results were not supported by tunneling spectroscopy data and seem not to be conclusive. However, this was one of the few reports showing how STM images can provide information on the chemistry of SWNTs.

The big disadvantage of electron microscopy and spec-troscopy in this context is that one can never be sure that the observed image is truly representative of the bulk SWNT sample. Consequently a number of bulk sensitive methods that provide information regarding the quality, diameter distribution, and structural properties of a given sample have been employed. These methods comprise Raman spectroscopy, optical absorption, temperature programmed oxidation, and diffraction techniques.

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