Electronic structure of dense amorphous carbon.

Abstract

The tight-binding method is used to calculate the electronic densities of states of various models of a-C. Both models generated using Tersoff's potential and quantum mechanically derived forces show \ensuremath{\pi} and ${\mathrm{\ensuremath{\pi}}}^{\mathrm{*}}$ states in the \ensuremath{\sigma} gap in agreement with experimental data. The former models, however, have an additional large peak at the Fermi level, which is shown to derive mainly from misoriented ${\mathit{p}}_{\mathrm{\ensuremath{\pi}}}$ orbitals and dangling bonds. The models do not support the presence of large aromatic ring structures. We find that embedding the local \ensuremath{\pi} electron systems of small clusters in the rigid ${\mathit{sp}}^{3}$ network leads to reduced level splittings and is thus consistent with the observed small optical gaps \ensuremath{\le}2.5 eV.