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.

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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.

Nominally clean diamond surfaces have been shown to contain adsorbed foreign atoms on them. Some of the adsorbed atoms have also been known to lead to various surface modifications, a phenomenon that is important in diamond electronics. Establishing the identity and nature of the adsorbed atoms on these surfaces is therefore key to the successful application of diamond in various technological fields. In this work, X-ray photoelectron spectrometry (XPS) has been used to investigate the three low index diamond surfaces. The results show oxygen as the main foreign atom on these surfaces other than hydrogen, with varying fractions of oxygen monolayer coverage being observed. It is further established that the adsorbed oxygen exists in several bonding configurations, with the energy shifts associated with the various functional groups being determined.

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