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 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. Carbon nitride materials have been the focus of considerable experimental and theoretical attention since Cohen and co-workers proposed that ␤-C 3 N 4 , a carbon nitride material analogous to ␤-Si 3 N 4 , should have a hardness comparable to that of diamond. 1 Subsequent calculations have shown that other crystalline C 3 N 4 structures should have stabilities comparable to or greater than that of ␤-C 3 N 4 , 2,3 and that many of these structures should be quite hard ͑e.g., cubic C 3 N 4). In addition, the energetically most stable material, rhombohe-dral C 3 N 4 , which has a graphitelike structure, is expected to be quite soft. The local structural property that distinguishes potentially superhard and dense C 3 N 4 structures from low-density, softer material is the carbon coordination: hard materials require tetrahedral or sp 3-bonded carbon in the C 3 N 4 network, while sp 2-bonded carbon will lead to much softer materials. This requirement of having sp 3-bonded carbon in a hard carbon nitride is completely analogous to that in amorphous diamondlike carbon ͑DLC͒. 4 Extensive experimental effort has been placed on preparing carbon nitride materials using physical deposition and other techniques. 5–12 In general, the materials produced in these studies have been amorphous with nitrogen compositions less than 50%. 5–8 Early on our group 6 and others 7 reported the observation of small ␤-C 3 N 4 crystallites on the basis of electron diffraction data. Unfortunately, these studies and subsequent ones have not identified the composition or local carbon bonding in the diffracting crystallites, and thus we do not think that this diffraction data provides strong evidence for sp 3-bonded ␤-C 3 N 4. In support of this analysis, we note that all recent electron energy loss spectroscopy ͑EELS͒ studies, which provide an unambiguous determination of the local …