Symmetry breaking in nitrogen-doped amorphous carbon: Infrared observation of the Raman-active G and D bands

We report the preparation of hard nitrogenated amorphous carbon films doped with as much as 20 at. % nitrogen. The nitrogen groups created by doping do not dramatically affect the film structure, but are found to break symmetry in ${\mathrm{sp}}^2$ domains causing the Raman-active G (‘‘graphitic’’) and D (‘‘disordered’’) bands to become ir active. Elemental analysis of the films together with the infrared spectra shows that about one in six of the carbons are replaced by nitrogen. Also observed in the infrared spectrum are nitrile (and possibly isonitrile) groups which demonstrate for the first time the existence of sp carbon in amorphous carbon films.