Nitrogen-drivensp3tosp2transformation in carbon nitride materials

The coordination of carbon as a function of nitrogen concentration in energetically deposited carbon nitride films has been systematically studied. A structural transformation from primarily ${\mathrm{sp}}^3$-bonded to ${\mathrm{sp}}^2$-bonded carbon and a density decrease from 3.3 to $2.1\hspace{0.5em}{\mathrm{g}/\mathrm{c}\mathrm{m}}^3$ were observed as the nitrogen concentration increases from 11 to 17 %. Calculations on nitrogen-substituted carbon clusters indicate that there is a preference to form ${\mathrm{sp}}^2$-bonded carbon when the nitrogen concentration is larger than 12%. The implications for these results to the synthesis of superhard carbon nitride materials are discussed.