An investigation of carbon nitride

Abstract

This thesis employs experimental and theoretical methods to characterise carbon nitride solids and proposes a generalstructural model for amorphous carbon nitride (a-C:N). It finds that a-C:N deposited by several methods is essentially identical, with similar bonding environments for carbon and nitrogen atoms. Using evidence from several techniques, the saturation of nitrogen in an sp2 carbon matrix is discussed. The experimental studies on a range of carbon nitride solids show no evidence for a crystalline form of carbon nitride. In addition to the experimental characterisation of a-C:N, ab initio molecular dynamics were used to investigate bonding and structure in carbon nitride. These simulations show that the most common form of nitrogen bonding was three-fold sites with a lone pair of electrons. Two-fold nitrogen sites were also found in agreement with experimental findings. An increase of nitrogen in a-C:N decreases the sp3-carbon fraction, but this is not localised on the nitrogen and the effect is most severe at high densities. A simulation of a low density/high nitrogen content network shows that the nitrogen saturation seen experimentally may be due to the formation of N2 dimers and C-N molecules which are easily driven out of the structure. The ab initio simulations also explore the nature of charged nitrogen and carbon sites in a-C:N. An analysis based on Wannier Function centres provided further information about the bonding and allowed for a detailed classification of these sites. The removal of electrons from the networks caused structural changes that could explain the two-state conductivity in ta-C:N memory devices. Finally, a theoretical study of the electron energy-loss near-edge structure (ELNES) calculated using multiple scattering theory is presented. The calculated ELNES of diamond, graphite and boron, silicon and carbon nitride structures compare well to experiment and supports the experimental finding that no crystalline carbon nitride had (or has) been produced. These ELNES calculations will however, provide a means of identifying crystalline beta-C3N4 should it be synthesised.