PHOTOEMISSION AND LOW-ENERGY-ELECTRON-DIFFRACTION STUDY OF CLEAN AND OXYGEN-DOSED CU2O (111) AND (100) SURFACES

The geometric and electronic structure of clean and oxygen-dosed Cu2O single-crystal surfaces was studied with x-ray and ultraviolet photoelectron (UPS) spectroscopies and low-energy electron diffraction. The nonpolar (111) surface can be prepared in a nearly stoichiometric (1X1) form by ion bombardment and annealing in vacuum. Oxygen adsorbs molecularly on the stoichiometric (111) surface at 300 K, but adsorbs dissociatively on a defective (111) surface prepared by ion bombardment. For the polar Cu2O(100) face it was possible to prepare a reconstructed, Cu-terminated surface with a (3 square-root 2 X square-root 2)R45-degrees periodicity by ion bombardment and annealing in vacuum. Preparation of an unreconstructed, (1X1), O-terminated (100) surface was possible by large (10(9)-L) oxygen exposures. UPS investigations of the O-terminated (100) surface suggest a mixture of incorporated (i.e., lattice) oxygen and adsorbed atomic oxygen (i.e., adatoms) in the terminating layer. The annealing behavior of the Cu2O(100) surface was history dependent. Early in the sample history, bulk lattice oxygen diffused to the surface at temperatures above 800 K giving domains of (square-root 2 X square-root 2)R45-degrees periodicity associated with half a terminating layer of oxygen atoms. After repeated ion bombardment and annealing cycles, heating above 800 K gave only a Cu-terminated surface, apparently because of a depletion of bulk lattice oxygen.