Acknowledgements. I thank colleagues at Berkeley, Cambridge and Durham for stimulating conversations; L. Moustakas for assisting with the use of image analysis software; R. Bouwens for providing code to manipulate stellar populations models; and D. Hogg for providing data in tabular form. This work has been supported by NASA grants.

No abstract

No abstract

The optical gap between dangling-bond states has been measured on a single-domain diamond C(111)-2 × 1 surface. The experimental result is 1.47 eV. Such a large surface gap (even larger if excitonic effects are considered) so far has not been obtained by state-of-the-art band calculations and its physical origin is still not elucidated. The experimental result suggests a strong electron-phonon interaction in this surface. The renormalization of the bare electronic transition energy by electron-phonon interaction provides a clue to solve the apparent disagreement between theory and experiment.

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.