Resonant Raman spectroscopy of disordered, amorphous, and diamondlike carbon

The Raman spectra of a wide range of disordered and amorphous carbons have been measured under excitation from 785 to 229 nm. The dispersion of peak positions and intensities with excitation wavelength is used to understand the nature of resonant Raman scattering in carbon and how to derive the local bonding and disorder from the Raman spectra. The spectra show three basic features, the D and G around 1600 and 1350 cm^-1 for visible excitation and an extra T peak, for UV excitation, at 1060 cm^-1. The G peak, due to the stretching motion of sp^2 pairs, is a good indicator of disorder. It shows dispersion only in amorphous networks, with a dispersion rate proportional to the degree of disorder. Its shift well above 1600 cm^-1 under UV excitation indicates the presence of sp^2 chains. The dispersion of the D peak is strongest in ordered carbons. It shows little dispersion in amorphous carbon, so that in UV excitation it becomes like a density-of-states feature of vibrations of sp^2 ringlike structures. The intensity ratio I (D) /I (G) falls with increasing UV excitation in all forms of carbon, with a faster decrease in more ordered carbons, so that it is generally small for UV excitation. The T peak, due to sp^3 vibrations, only appears in UV Raman, lying around 1060 cm^-1 for H-free carbons and around 980 cm^-1 in hydrogenated carbons. In hydrogenated carbons, the sp^3C-Hₗ stretching modes around 2920 cm^-1 can be clearly detected for UV excitation. This assignment is confirmed by deuterium substitution.