Low-temperature synthesis of highly transparent carbon nitride thin films

Highly transparent carbon nitride thin films have been prepared on transparent plastic substrates at low temperatures by radio frequency reactive magnetron sputtering of a graphite target in the plasmas of pure nitrogen and nitrogen-hydrogen gas mixtures. The amorphous films thus obtained have been characterized for their optical transmittance by spectrophotometry, revealing a very high transparency in the visible region of the spectrum. The chemical composition and bonding states of a sample studied by x-ray photoelectron spectroscopy (XPS) showed that nitrogen has been predominately bonded to sp3 -hybridized carbon, in addition to a small amount of sp2 -hybridization. The elemental concentration was measured to be C = 51.1 at.%, N = 43.2 at.%, and O = 5.7 at.%. The N/C ratio for the N-sp3 C component is calculated to be 1.29, close to the expected stoichiometric value of 1.33 for -C3 N4 . Compared to the unhydrogenated C-N thin films, the hydrogenated samples exhibited increased optical transmittance, about 90% in the whole of the visible spectrum. Such transmittance characteristics of the hydrogenated C-N films closely matched those exhibited by the plastic substrate.