Atomic and electronic structure of tetrahedral amorphous carbon surfaces from density functional theory: Properties and simulation strategies

We present a computational study of tetrahedral amorphous carbon surfaces within density functional theory. The samples studied contain of the order of 250–300 atoms with densities in the range 2.78–3.12 g/cm3. Bulk samples within and below that range are also studied as reference. The morphological features are characterized by calculating local density and sp2/sp3 fraction, and with a new tool based on the analysis of the topology of the 4-atom sp2 chains found in the structures. This allows to establish that sp2 clustering occurs predominantly with the formation of olefinic chains, with a smaller amount of aromatic-ring structures formed. This preference for olefinic chains is present both in bulk and reconstructed surfaces. The degree of localization of the electronic states is obtained with a participation ratio analysis, showing a direct relation with density and sp2 fraction. The position of the surfaces Fermi level is calculated about 4 eV below the vacuum level, a result that appears to be almost independent of sample density. Finally, a passivation scheme is presented that allows to reduce the overall number of atoms in the surfaces without compromising the level of description of their properties, optimizing them in terms of the associated computational cost.