Fano Lineshape and Phonon Softening in Single Isolated Metallic Carbon Nanotubes

Abstract

Evolution of G-band modes of single metallic carbon nanotubes with the Fermi level shift is examined by simultaneous Raman and electron transport studies. Narrow Lorentzian line shape and upshifted frequencies are observed near the van Hove singularities. However, all G modes soften and broaden at the band crossing point. The concurrent appearance of an asymmetric Fano line shape at this point indicates that phonon-continuum coupling is intrinsic to single metallic tubes. The apparent Lorentzian line shapes of as-synthesized metallic tubes are induced by O2 adsorption causing the Fermi level shift.

Fundamental optical processes in armchair carbon nanotubes

“…27,87,91,[137][138][139][140] The absence of the LO phonon feature for armchair tubes is consistent with some theoretical studies (e.g., References 27 and 145), i.e., it is not Raman active in armchair SWCNTs (although this still does not eliminate the presence of a Kohn anomalyonly its Raman activity), while it is inconsistent with more recent theories. 90,146 In conclusion, using resonant Raman measurements of macroscopic ensembles of ν = 0 enriched SWCNT samples, we have demonstrated that the appearance of a broad G − peak is due to the presence of non-armchair "metallic" tubes.…”

“…Earlier theoretical studies described this broad feature as a Breit-Wigner-Fano line shape due to the coupling of phonons with an electronic continuum 81 or low-frequency plasmons, 136 but there is now growing consensus that the broad G − peak is a frequency-softened and broadened longitudinal optical (LO) phonon feature, a hallmark of Kohn anomalies. 27,87,91,[137][138][139][140] Through either scenario, this broad G − peak has conventionally been known to be a "metallic" feature, indicating the presence of metallic tubes.…”

“…Data taken on freestanding, single-tubes in references, 77,87,141 where (n,m) is assigned independently by other means, as compared to the more commonly used substrate-supported samples is mainly the source of the variability in the interpretation of the G − peak. This inconsistency most likely stems from the sensitivity of Gband phonons to doping and charge transfer from intentional gating 87,139 and environmental effects (common to substrate-supported single SWCNTs) that can induce a localization of electron density in the nanotube π electron cloud 142,143 and which ultimately modify SWCNT band structure. An excellent example of this sensitivity to environment is demonstrated by Shim et al 143 where Raman spectra of multiple examples of substrate-supported (12,6) resulted in widely differing G-band structure with regards to G − intensity, line shape and line width.…”

Kohn anomalies in graphene nanoribbons

“…[22][23][24][25][26][27] Recently, it has been shown that the frequencies and spectral widths of the ⌫ point optical phonons ͑called the G band in Raman spectra͒ depend on the position of the Fermi energy E F and the chirality of the metallic SWNT. [28][29][30][31] The Fermienergy dependence of the Raman spectra can be used to determine the position of the Fermi energy, and the chirality dependence of the Raman spectra provides detailed information on the electronic properties near the Fermi energy of metallic SWNTs. These dependences originate from the fact that the conduction electrons of a metal partly screen the electronic field of the ionic lattice.…”

Экспериментальное Исследование Динамики Решетки Индивидуальных Полупроводниковых Двустенных Углеродных Нанотрубок: Тангенциальные G-Моды

“…В зависимо-сти от направления колебаний G-моды разделяют на продольные (LO) и поперечные (TO). Известно, что характерные особенности G-мод (профиль линии, часто-та, полуширина, относительная интенсивность) крайне чувствительны, с одной стороны, к собственным струк-турным параметром ОУНТ (диаметру d, хиральному углу θ, типу проводимости), а с другой стороны, к различным внешним воздействиям: к допированию [5], напряжению и давлению [6,7], и изменению в окруже-нии нанотрубки [8]. Следует отметить, что подробная информация о G-модах была получена в основном из измерений спектров КРС, выполненных на простран-ственно изолированных индивидуальных структурно-идентифицированных ОУНТ [9].…”