First-principle study of nitrogen incorporation in amorphous carbon

First-principle calculations using quantum-mechanical density functional theory are carried out to study nitrogen incorporation in amorphous carbon, in which the structural models from liquid quench containing 64 atoms are introduced. The properties simulated for N incorporated amorphous carbon are in agreement with the available experimental results. The topological and electronic properties for nitrogen incorporation structures with various densities are investigated, and it is found that the bonding configuration of nitrogen atoms strongly depends on the density and the nitrogen concentration in the network. The simulations provide a qualitative support for a low nitrogen doping efficiency observed in the experiment since no true doping (N atoms substitutionally occupy C sites, leading to a donor level below conduction band mobility edge Ec) exists in any cases studied. For the tetrahedral amorphous carbon (at density of 2.9 g/cm3) with a low concentration of nitrogen (1.6 at% and 3.2 at%), the incorporated N atoms are found to adopt the auto-compensated sites or to be in threefold coordination. In particular, a new threefold C defect is found, which is introduced by nitrogen auto-compensation. Nitrogen incorporated amorphous carbon (2.0 g/cm3) leads to the formation of a graphite-like structure with twofold nitrogen coordination.