Interpretation of infrared and Raman spectra of amorphous carbon nitrides

A general framework for the interpretation of infrared and Raman spectra of amorphous carbon nitrides is presented. In the first part of this paper we examine the infrared spectra. The peaks around 1350 and 1550 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ found in the infrared spectrum of amorphous carbon nitride or hydrogenated and hydrogen-free amorphous carbon are shown to originate from the large dynamic charge of the more delocalized \ensuremath{\pi} bonding which occurs in more ${\mathrm{sp}}^{2}$ bonded networks. The IR absorption decreases strongly when the \ensuremath{\pi} bonding becomes localized, as in tetrahedral amorphous carbon. Isotopic substitution is used to assign the modes to $\mathrm{C}=\mathrm{C}$ skeleton modes, even those modes around 1600 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ which become strongly enhanced by the presence of hydrogen. The infrared spectrum of carbon nitride may resemble the Raman spectrum at some excitation energy, but the infrared activity does not primarily result from nitrogen breaking the symmetry. In the second part we examine the Raman spectra. A general model is presented for the interpretation of the Raman spectra of amorphous carbon nitrides measured at any excitation energy. The Raman spectra can be explained in terms of an amorphous carbon based model, without need of extra peaks due to CN, NN, or NH modes. We classify amorphous carbon nitride films in four classes, according to the corresponding N-free film: $a\ensuremath{-}\mathrm{C}:\mathrm{N},$ $a\ensuremath{-}\mathrm{C}:\mathrm{H}:\mathrm{N},$ $ta\ensuremath{-}\mathrm{C}:\mathrm{H}:\mathrm{N},$ and $ta\ensuremath{-}\mathrm{C}:\mathrm{N}.$ We analyze a wide variety of samples for the four classes and present the Raman spectra as a function of N content, ${\mathrm{sp}}^{3}$ content, and band gap. In all cases, a multiwavelength Raman study allows a direct correlation of the Raman parameters with the N content, which is not generally possible for single wavelength excitation. The G peak dispersion emerges as a most informative parameter for Raman analysis. UV Raman enhances the ${\mathrm{sp}}^{1}$ CN peak, which is usually too faint to be seen in visible excitation. As for N-free samples, UV Raman also enhances the C-C ${\mathrm{sp}}^{3}$ bonds vibrations, allowing the ${\mathrm{sp}}^{3}$ content to be quantified.