Ynb

Figure 18. Fitting of the second electronic transition between the second pair of van Hove singularities using Eq. (8); the solid lines are the experimental data (background subtracted) and the dashed lines are the simulation results obtained by using three different SWNT chiral angle distributions (30° stands for the armchair and 0° for the zigzag nano-tube). Reprinted with permission from [81], X. Liu et al., Phys. Rev. B 66, 045411 (2002). © 2002, American Physical Society.

called to attention by some authors that corrections for the diameter distribution of the SWNTs due to trigonal warping effects need to be considered [142].

Another successful application of optical absorption spec-troscopy has been the testing of the electronic structure of SWNTs in the presence of doping agents. For example, it was found that charge transfer induced by either halogen or alkali metal doping causes strong variations in the visible to near infrared range of the absorption spectra [145-147]. An example is presented in Figure 19, in which the absorption spectra of doped SWNTs of «-type (Cs) and p-type (Br) are reported for different carbon/dopant ratios. It is interesting to notice the disappearance of the absorption bands at 0.7, 1.2 (assigned to semiconducting SWNT), and 1.8 eV (assigned to metallic SWNT). Simultaneous electrochemical measurements showed a concomitant decrease in electrical resistance upon doping. The authors attributed this decrease to electron depletion or filling in specific bands of semiconducting or metallic SWNT. They also proposed an amphoteric doping behavior of SWNTs and claimed that the doping process occurs via charge transfer following a specific sequence. Initially, the transition at 0.7 eV is affected, then that one at 1.2 eV, and finally the feature at 1.8 eV The extent of the modification depends on the concentration of the dopant [148].

The doping of SWNTs has opened the possibility of tuning the Fermi level of SWNTs by exposing the nanotubes to molecules of different redox potentials. In principle, by choosing an adequate dopant, selective filling or depletion of the density of states can be accomplished. Therefore, the modification of the conducting nature of SWNTs is now a possibility [149]. Optical absorption appears to be the most adequate tool to probe the modifications of the SWNT electronic bands that would occur upon charge transfer.

Figure 19. Optical absorption spectra of doped SWNTs for the case of «-type (upper set) and p-type (lower set). CDx represents the stoichom-etry of dopant with respect to carbon (C: carbon, D: dopant, x: their ratio). The asterisks indicate absorption due to experimental artifacts. Reprinted with permission from [148], R. Jacquemin et al., Synth. Met. 115, 283 (2000). © 2000, Elsevier Science.

Figure 19. Optical absorption spectra of doped SWNTs for the case of «-type (upper set) and p-type (lower set). CDx represents the stoichom-etry of dopant with respect to carbon (C: carbon, D: dopant, x: their ratio). The asterisks indicate absorption due to experimental artifacts. Reprinted with permission from [148], R. Jacquemin et al., Synth. Met. 115, 283 (2000). © 2000, Elsevier Science.

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