Ring formation and the structural and electronic properties of tetrahedral amorphous carbon surfaces

In this paper, we report first-principles structural models of surfaces of tetrahedral amorphous carbon (ta-C). The topology and defect structure of the amorphous surfaces are analyzed at the atomistic level. We examine the transition of the local bonding environment from the bulk to the surface. Comparing the surface with the bulk, many more surface atoms are threefold coordinated and planar rings or chains are formed with the sp² bonds. This ``graphization'' character of the ta-C surface also significantly influences its electronic properties. Electrons are easily delocalized within these surface sp² rings/chains and some of the surface electronic eigenstates become extended along the surface through such rings/chains. The implications of surface graphization for the growth and surface conduction are briefly discussed.