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 sp
3, sp
2 and sp
1 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
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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
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H vibrations in hydrogenated DLC samples, Si
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H and Si
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C vibrations in amorphous silicon and amorphous silicon–carbon alloys and the easier probe
of CN sp bonds in amorphous carbon nitrides.