We use liner-scaling tight-binding molecular dynamics to generate three struc- tural models of bulk amorphous carbon with different atomic density. Amorphous carbon surfaces are then obtained by imposing tensile strain on these computer generated networks until fracture occurs. Our results show that for a given density, the formation energy of surfaces obtained with different tensile strains differ by only a few 10-l eV/atom and their structural properties are qualitatively similar. The presence of sp sites at the surface is observed at all densities, but with different values of the concentration. The surface thicknesses obtained in our simulations agree with experimental data. Furthermore we find that surface roughness increases with the amount of graphitic component in the bulk sample. The same trends of the macro-scopic properties are obtained when using a two-center tight-binding Hamiltonian, an environmental dependent one, and first principles calculations.
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