A Model to Interpret the Raman Spectra of Disordered, Amorphous and Nanostructured Carbons

Abstract

Raman spectroscopy is a very popular, non-destructive tool for the structural characterisation of carbons. Raman scattering from carbons is always a resonant process, in which those configurations whose band gaps match the excitation energy are preferentially excited. The Raman spectra of carbons do not follow the vibration density of states, but consist of three basic features, the G and D peaks around 1600 and 1350 cm^-1 and an extra T peak, for UV excitation, at ~980-1060 cm^-1. TheRaman spectra at any wavelength depend on 1) clustering of the sp² phase, 2)bond length and bond angle disorder, 3) presence of sp² rings or chains, and 4) the sp²/sp³ ratio. It will be shown how the basic features of the Raman spectra vary by rationalising them within a three-stage model of order of carbons. It is shown how the three-stage model can account for the vast range of experimental data available for Raman experiments at any excitation wavelength. This model can also account for apparently contradictory trends reported in literature, since the clustering of the sp² phase and the sp³ to sp² conversion are separately treated.