Amorphous carbon at low densities: An ab initio study

In this paper, we present new computer models of low-density amorphous carbon, and study the structural, electronic and vibrational properties all based upon plane-wave density functional methods. The static structure factor and real space pair-correlation function is in agreement with available experimental data. We observe chains of sp bonded carbon in the models, along with sp2 and sp3 structures in varying concentrations. These models provide atomistic insight into the microstructure of the system, delineating variation in bonding (sp2, sp3 and sp) preferences as a function of density. For these low densities, the vibrational density of states is computed for the first time, along with localization of modes and the specific heat with comparison to experiments. We contrast the amorphous three dimensional networks with amorphous graphene, which bears a striking similarity in the radial distribution functions, but shows a distinct signature in the vibrational density of states. The vibrational modes are analyzed and discussed, and animations of particularly interesting modes are provided as Supplementary Material. The modes are generally well extended, but are more complex than simple molecular pictures that are sometimes invoked, in some cases even revealing mixing among modes of different type.

Structure of low-density amorphous carbon from accurate ab initio computations. Left: 1.50 gm/cc, Right: 0.92 gm/cc. Red atoms are sp bonded, grey sp2 and blue sp3 (periodic boundary conditions are employed). Note the appearance of sp conformations, especially at the lower density.263