Polarization properties, high-order Raman spectra, and frequency asymmetry between Stokes and anti-Stokes scattering of Raman modes in a graphite whisker

Abstract

The Raman spectra of a new type of graphite whiskers have been measured in the range of $150--7800{\mathrm{cm}}^{\ensuremath{-}1}.$ The intensity of the overtone $(2D)$ located at $\ensuremath{\sim}2700{\mathrm{cm}}^{\ensuremath{-}1}$ is found to be about 10 times stronger than that of the C-C stretching mode (G) at $1582{\mathrm{cm}}^{\ensuremath{-}1}.$ Because of the peculiar enhancement of the $2D$ mode, high-order Raman bands up to fifth order at $\ensuremath{\sim}7500{\mathrm{cm}}^{\ensuremath{-}1}$ have been observed. Polarized micro-Raman spectroscopy has been performed on an individual graphite whisker, and angular-dependent intensity measurements of all Raman modes in the $\mathrm{VV}$ and $\mathrm{HV}$ geometries are in agreement with the theoretical calculated results. Laser-energy-dependent dispersion effects and the frequency discrepancy of Raman modes between their Stokes and anti-Stokes lines in graphite whiskers are also carefully investigated. The energy dispersion of the D mode and G mode is very similar to that of highly oriented pyrolytic graphite (HOPG). In contrast to the Raman spectra of HOPG and other graphite materials, two laser-energy-dependent Raman lines are revealed in the low-frequency region of the Raman spectra of graphite whiskers, which are believed to be the resonantly enhanced phonons in the transverse-acoustic and longitudinal-acoustic phonon branches. Moreover, the obvious energy dispersion of the ${D}^{\ensuremath{'}}$ mode at \ensuremath{\sim}1620 ${\mathrm{cm}}^{\ensuremath{-}1}$ is observed in graphite whiskers. The results clearly reveal how strongly the peak parameters of Raman modes of graphite materials are dependent on their structural geometry. The Stokes and anti-Stokes scattering experiments show that the frequency discrepancy between the Stokes and anti-Stokes sides of a Raman mode in graphite materials is equal to the frequency value covered by the one-phonon energy of this Raman mode in its frequency versus laser energy curve, which is the product of the one-phonon energy of this mode $(E{\ensuremath{\omega}}_{s})$ and the value of its laser-energy dispersions $(\ensuremath{\partial}E{\ensuremath{\omega}}_{s}/\ensuremath{\partial}{\ensuremath{\varepsilon}}_{L}).$