Electronic structure of dense amorphous carbon

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 {π} and π* states in the {σ} 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 p_π orbitals and dangling bonds. The models do not support the presence of large aromatic ring structures. We find that embedding the local {π} electron systems of small clusters in the rigid sp³ network leads to reduced level splittings and is thus consistent with the observed small optical gaps {≤}2.5 eV.