Nitrogen-driven sp3 to sp2 transformation 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 sp(3)-bonded to sp(2)-bonded carbon and a density decrease from 3.3 to 2.1 g/cm(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 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.