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

Das, Anindya and Sood, AK (2009) Renormalization of the phonon spectrum in semiconducting single-walled carbon nanotubes studied by Raman spectroscopy. In: Physical Review B, 79 (23). pp. 1-9.

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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: Journal Article
Publication: Physical Review B
Publisher: American Physical Society
Additional Information: The copyright of this article belongs to American Physical Society.
Keywords: carbon nanotubes; electrochemistry; electron-phonon interactions; elemental semiconductors; Fermi level; Raman spectra; semiconductor doping; semiconductor nanotubes
Department/Centre: Division of Physical & Mathematical Sciences > Physics
Date Deposited: 15 Dec 2009 08:28
Last Modified: 19 Sep 2010 05:38
URI: http://eprints.iisc.ac.in/id/eprint/21638

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