Renormalization of the phonon spectrum in semiconducting single-walled carbon nanotubes studied by Raman spectroscopy

Das, Anindya ; Sood, A. K. (2009) Renormalization of the phonon spectrum in semiconducting single-walled carbon nanotubes studied by Raman spectroscopy Physical Review B: Condensed Matter and Materials Physics, 79 (23). 235429_1-235429_9. ISSN 1098-0121

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Official URL: http://prb.aps.org/abstract/PRB/v79/i23/e235429

Related URL: http://dx.doi.org/10.1103/PhysRevB.79.235429

Abstract

In situ Raman experiments together with transport measurements have been carried out in single-walled carbon nanotubes as a function of electrochemical top gate voltage (Vg). We have used the green laser (EL=2.41 eV), where the semiconducting nanotubes of diameter ~1.4 nm are in resonance condition. In semiconducting nanotubes, the G- and G+-mode frequencies increase by ~10 cm−1 for hole doping, the frequency shift of the G mode is larger compared to the G+ mode at the same gate voltage. However, for electron doping the shifts are much smaller: G upshifts by only ~2 cm−1 whereas the G+ does not shift. The transport measurements are used to quantify the Fermi-energy shift (EF) as a function of the gate voltage. The electron-hole asymmetry in G and G+ modes is quantitatively explained using nonadiabatic effects together with lattice relaxation contribution. The electron-phonon coupling matrix elements of transverse-optic (G) and longitudinal-optic (G+) modes explain why the G mode is more blueshifted compared to the G+ mode at the same Vg. The D and 2D bands have different doping dependence compared to the G+ and G bands. There is a large downshift in the frequency of the 2D band (~18 cm−1) and D (~10 cm−1) band for electron doping, whereas the 2D band remains constant for the hole doping but D upshifts by ~8 cm−1. The doping dependence of the overtone of the G bands (2G bands) shows behavior similar to the dependence of the G+ and G bands.

Item Type:Article
Source:Copyright of this article belongs to American Physical Society.
ID Code:50234
Deposited On:22 Jul 2011 14:03
Last Modified:22 Jul 2011 14:03

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