Determination of bonding in diamond-like carbon by Raman spectroscopy

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. Any mixture of sp3, sp2 and sp1 carbon atoms always has a gap between 0 and 5.5 eV, and this energy range matches that of IR-vis-UV Raman spectrometers. The Raman spectra of carbons do not follow the vibration density of states, but consist of three basic features, the G and D peaks at approximately 1600 and 1350 cm−1 and an extra T peak, for UV excitation, at ∼980–1060 cm−1. We propose to rationalise the vast range of experimental data available in literature at any excitation wavelength by a simple model, which considers the main factors influencing the Raman spectra. The great advantages of multi-wavelength Raman spectroscopy will be clarified by a series of examples. In particular we show how it can be used to probe the structural changes induced by annealing and by nitrogen introduction. UV Raman spectroscopy also probes heteropolar σ bonds in a complementary way to infrared spectroscopy. We demonstrate the direct detection of CH vibrations in hydrogenated DLC samples, SiH and SiC vibrations in amorphous silicon and amorphous silicon–carbon alloys and the easier probe of CN sp bonds in amorphous carbon nitrides.