Infrared spectra of carbon nitride films

Abstract The assignment of the vibrational modes in amorphous carbon nitride (CN) films is discussed by considering CN films deposited using a variety of methods. The infrared (IR) spectra of CN show three main absorption bands. In hydrogenated CN samples, CHx and NHx groups give rise to stretching vibrations at 3000 and 3400 cm−1, respectively. A weaker sharp band is observed approximately 2200 cm−1 due to CN–sp1 bonds. Finally, there is a broad band between 1000 and 2000 cm−1. It is usually stated that the effect of nitrogen into carbon films is to break the symmetry of the sp2 carbon bonds making the Raman ‘G’ (graphitic) and ‘D’ (disorder) modes IR active, so the broad band between 1000 and 2000 cm−1 is similar in both IR and Raman spectra. However, it is shown that nitrogen is not necessary to have significant IR activity in the 1000–2000 cm−1 region. Also, Raman spectroscopy in carbon is always a resonant process, so that the spectra depend on the excitation energy. Therefore, the similarity of the visible Raman and IR spectra of some CNs is generally a coincidence. We show that the IR broad band in the 1000–2000 cm−1 region is an electronic effect and is not due to activation of IR forbidden modes due to symmetry breaking. This explains the IR spectra not only of CN films but also of N-free amorphous carbon films and is related to the presence of the system of delocalized π bonds with increasing conjugation.