Excitation, Temperature, and Structural Dependence of Second-Order Raman Modes in Single-Wall Carbon Nanotubes

Abstract

In this work, we characterize second-order Raman modes between 1650 and 2000 cm−1 of small-diameter single-wall carbon nanotubes (SWNTs). Raman spectra of single nanotubes are acquired by confocal Raman microscopy. Besides the frequently observed overtone M band and combination iTOLA modes, an additional mode involving combination of the radial breathing mode (RBM) with the C−C stretching mode (G band) is clearly identified in small-diameter semiconducting nanotubes. Excitation wavelength-dependent Raman spectroscopy of the same nanotubes shows that the M band and iTOLA mode frequencies of a single nanotube are insensitive to excitation energy, which is in contrast to the dispersive behavior observed in ensemble measurement. Some single tubes display unusual Raman spectra with enhanced second-order modes, attributable to the resonance effect associated with the exciton transitions of the nanotubes. It is also discovered that the relative intensity of the M band and iTOLA band depends on the chirality and family type of a nanotube. Low-temperature Raman spectroscopy is used to reveal the fine structure of these second-order Raman features. The observed characteristics of these second-order Raman modes thus provide useful information on the chirality and electronic structure of a nanotube.