Correlation between the sp2-phase nanostructure and the physical properties of unhydrogenated carbon nitride

Abstract

This paper consists of an investigation of the structural arrangement of the sp2 phase in amorphous unhydrogenated carbon nitride (a-CNx) films and its effect on their physical properties. The a-CNx films (0.16<x1020∕cm3) and the EPR linewidth (of a few gausses) along with a decrease in nitrogen content. Visible Raman measurements indicate that these effects are accompanied by an increase in the degree of disorder of the sp2 phase, as inferred from the broadening and downshift of the G Raman band, and a reduction of the CN triple bond signal. The analysis of these results in terms of the structural configuration and bonding in the films, show that an enhancement of the connectivity of the sp2 phase in the layers, takes place when deposition is performed at high laser intensities. These structural modifications are strongly correlated to a decrease in the optical gap from 0.61 to 0.21 eV as well as to an increase of the hardness value of the films from 12 to 24 GPa. The transition from a reduced to an enhanced connectivity of the sp2 phase occurs when the nitrogen content decreases below 22 at. %, as a result of the detected reduction of the triply bonded CN species in the layers.