A spectroscopic study of the negative electron affinity of cesium oxide-coated diamond (111) and theoretical calculation of the surface density-of-states on oxygenated diamond (111)

The modification of the electron affinity of clean and oxygenated C(111) with cesium has been studied using ultra-violet photoelectron spectroscopy. Oxygenated C(111) shows strong valence emission features at 4.2 eV attributable to CO bonding orbital. Adsorption of cesium on the oxygenated diamond results in the formation of cesium oxide features at 4.2 eV and 8 eV and the condition of negative electron affinity (NEA). The cesium oxide adlayer is thermally stable to 500 degreesC and the NEA condition is not removed even at saturated O exposures, suggesting that cesium oxide-modified C(111) may act as a good photocathode. In order to evaluate the effect of oxygen atom adsorption on the surface states of C(111) 2 x 1 surface, the density of states of the O:C(111) surface adopting two different oxygen binding configurations have been calculated using the periodic density functional theory. The surface gap states on clean C(111) 2 x 1 surface are passivated by the adsorption of 1/2 monolayer oxygen in a bridging 'epoxy' fashion (C-O-C) across the carbon in the Pandey chain. At 1 monolayer oxygen coverage, the 2 x 1 reconstruction is lifted and an 'on-top' carbonyl (CO) binding mode results. Layered-re solved partial DOS calculation reveals that the surface band gap is closed with the adoption of the CO bonding configuration due to the introduction of quasi-continuous surface band states. (C) 2002 Elsevier Science B.V. All rights reserved.