Raman Spectra from One Carbon Nanotube
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gle Nanotube Raman Spectroscopy The large density of electronic states for one-dimensional (1D) systems at van Hove singularities [see Fig. 1(b)] and the strong electron-phonon coupling in carbon nanotubes under resonance conditions allows observation of the Raman spectra from one individual single wall carbon nanotube [see Fig. 1(d)], when the incident or scattered photon is in resonance with an interband transition Eii between the i-th 1D van Hove singularities in the electron density of states of the valence and conduction bands. The enhancement of the Raman signal coming from the resonance between the excitation photon and these singularities in the joint density of states (JDOS) can be very large, so that in some cases the Raman intensity from one nanotube can be as large as that for the silicon substrate on which the nanotubes lie [see Fig. 1(e)], even though there is a 10 8 ratio of Si/C atoms within the laser excitation beam of 1 µm diameter [see inset to Fig. 1(d) where the radial breathing mode features for 3 different SWNTs are shown]. Such large enhancement factors arise from the highly 1D nature of the electronic density of states of carbon nanotubes with diameters less that ∼2 nm. Figure 1(d) shows that each feature in the Raman spectra of SWNT bundles can now be observed at the single nanotube level. These features include the non-dispersive radial breathing mode (RBM) which is not present in other sp2 carbons, and the non-dispersive G-band feature, which is also present in sp2 carbons and has many properties for nanotubes that are both similar to and distinct from the G-band features in other sp 2 carbons. The RBM and G-band processes are both first-order Raman processes, for which the resonance can be with either the incident or the scattered photon. Also seen in the Raman spectra in Fig. 1(d) are the highly dispersive disorder-induced D-band and its second harmonic G0 -band, for which the mode frequencies show a strong dependence on the laser excitation energy (for the G0 -band measured in SWNT bundles, ∂ωG0 /∂Elaser = 106 cm−1 /eV). Investigations of these spectral features at the single nanotube level reveal many interesting details about the dependence of each feature in Fig. 1(d) on nanotube diameter, chirality, metallic vs. semiconducting behavior. In addition, such phonon spectra provide an astonishingly sensitive probe of the unique electronic structure of single wall carbon nanotubes. Furthermore, study of the Raman spectra at the single nanotube level allows investigation of new physical phenomena, particularly phenomena about the resonance Raman effect that have never been observed before in any physical system. Raman spectroscopy is not normally a tool in solid state physics for the structural characterization of crystalline solids, but for the case of 1D carbon nanotubes, the observation of the Raman spectra from an individual nanotube can be used to provide a definitive identification of the nanotube structure through determination of its (n, m) indices. This is another remarkable and u
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