Oxygen adsorption on the (1×1) and (2×1) reconstructed C(111) surfaces: a density functional theory study.

The structural and electronic properties, stability, optimum coverage and workfunction of oxygen atoms at different sites on the (1×1) unreconstructed and the (2×1) reconstructed C(111) surfaces have been investigated using density functional theory. Oxygen atoms prefer on-top sites on the C(111)-(1×1) surface, with an optimum coverage of 1/3 monolayers (ML), while on the (2×1) reconstructed surface, bridge sites are preferred with a coverage of 1/2 ML. With increasing oxygen coverage greater than 1/3 ML on the (1×1) surface, a repulsive interaction develops between the oxygen atoms, while for the (2×1) surface such a repulsive interaction occurs for coverages greater than 0.5 ML. For both surfaces, the workfunction initially increases with oxygen adsorption relative to that of each of the clean surfaces, reaching about ∼6 eV and then decreasing slightly at full monolayer coverage. Minimal buckling of the upper π-bonded chains and no dimerization of the clean (2×1) reconstructed surface was observed. An average valence band width of ∼21 eV occurs, and a surface state associated with the (2×1) surface reconstruction was established at ∼-2.5 eV. In addition, O 2s states were established at around -21 eV for the (1×1) surface and at ∼-22.5 eV on the (2×1) surface. These corresponded to similarly located C 2s states at -21.25 eV for both (1×1) and (2×1) surfaces. O 2p states were observed at the Fermi level, ∼-1.25, -2.5, -4.0, and ∼-7.5 eV on the (1×1) surface, and at ∼-2.5 eV, between -4 and -5 eV and at ∼-7.5 eV on the (2×1) surface.

Interaction of diamond (111)-(1 × 1) and (2 × 1) surfaces with OH: a first principles study.

The properties of hydroxyl groups on C(111)-(1 × 1) and reconstructed (2 × 1) surfaces at different sites and for various coverages are investigated using density functional theory. Out of the adsorption sites considered, i.e. face centred cubic, hexagonal close packed, on-top and bridge sites, the on-top site is the most stable for OH on the C(111)-(1 × 1) surface for all coverages. On the reconstructed (2 × 1) surface the on-top site is the preferred configuration. Adsorption of OH was not stable however at any site on the reconstructed C(111)-(2 × 1) relative to the (1 × 1) surface; thus adsorption of OH leads to the de-reconstruction of the former surface. Both the 0.5 and 1 monolayer (ML) coverages were able to lift the (2 × 1) surface reconstruction. Repulsion between the OH adsorbates on the (1 × 1) surface sets in for coverages greater than 0.5 ML. A general decrease in the work function with increasing OH coverage was observed on both the (1 × 1) and (2 × 1) surfaces relative to the values of their respective clean surfaces. Regarding the electronic structure, O 2p states on the reconstructed (2 × 1) surface are observed at around - 21, - 8.75 , - 5 and - 2.5 eV, while O 2s states are present at - 22.5 eV. On the (1 × 1) surface (for 0.33 ML in the on-top site), O 2p states occurred between - 8 and - 9 eV, - 5 and - 4 eV and at around - 2.5 eV. O 2s states are established between - 22.5 and - 21 eV. The valence band width is 21 eV, and a hybrid 2s/2p state that is characteristic of diamond is located at about 12.5 eV below the valence band minimum.