Melt-quenched and as-deposited structures of amorphous selenium: a density functional/ molecular dynamics comparison

Molecular dynamics simulations using a density functional description of energies and forces have been carried out for a model of an as-deposited (AD) surface of amorphous selenium. The deposition model assumed the annealing (at 400 K) of layers of randomly located single atoms, followed by compression to the density used in earlier melt-quenched (MQ) simulations of amorphous Se, and by further annealing. The AD and MQ structures are predominantly twofold coordinated and similar, for example in the pair distribution functions, with notable differences: the AD structures have more defects (atoms with one and three neighbours), and the ring distributions differ. These differences are also reflected in the electronic structures of the AD and MQ samples, where the increased presence of defects in the former influences the Bader charges and the edge states of the band gap. The dominance of rings found in a previous simulation of AD structures is not found.