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

The properties of hydroxyl groups on C(111)-(1 x 1) and reconstructed (2 x 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 x 1) surface for all coverages. On the reconstructed (2 x 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 x 1) relative to the (1 x 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 x 1) surface reconstruction. Repulsion between the OH adsorbates on the (1 x 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 x 1) and (2 x 1) surfaces relative to the values of their respective clean surfaces. Regarding the electronic structure, O 2p states on the reconstructed (2 x 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 x 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.