Stability, chemical bonding, and vibrational properties of amorphous carbon at different mass densities

Abstract

We investigate correlations between the atomic-scale structure and the global electronic and vibrational properties in amorphous carbon versus mass density. The model structures have been generated by applying different annealing regimes using a density-functional-based nonorthogonal tight-binding molecular dynamics. The stability of the amorphous modifications and the calculated vibrational density of states (VDOS) are strongly affected by the density and the annealing sequences, altering the chemical composition, the sp/${\mathit{sp}}^2$/${\mathit{sp}}^3$ clustering, the structure, and related physical properties. A mass density of 3.0 g/${\mathrm{cm}}^3$ is confirmed as a magic density favoring the formation of most stable a-C modifications having lowest defect densities and maximum band gap. By projecting out different hybrid-fractional VDOS and analyzing the localization behavior we identify the spectral signatures for chemically different bonded species that may be used for comparison with related experimental work. At low density the vibrational spectra are reminiscent of graphite and clearly indicate a softening of modes due to chainlike segments and rings within a low-connectivity network. Opposite, the spectra at high density become more compact in range and rigid shifting the low-frequency bound to higher values developing a characteristic half-sphere shape. At all densities a set of localized modes at high frequency represents signs of the embedding of undercoordinated atoms in a rigid higher-coordinated environment.