High-Pressure Oxidation Of Copper On Au(111)-A Route Toward Bulk-Like Cuprous Oxide Films

The low-pressure oxidation of copper is inefficient to grow oxide films of more than monolayer thickness. To overcome this limit, 10 monolayer Cu deposited on Au(111) has been oxidized in a high-pressure cell at 50 mbar O-2 and postannealed in 10(-4) mbar of oxygen afterward. The procedure results in bulk-like Cu2O films with (111) termination, as concluded from X-ray photoelectron spectroscopy and electron diffraction. At 650 K annealing temperature, a (root 3 x root 3) superstructure develops on the oxide surface, showing up as an array of trifold symmetric maxima in scanning tunneling microscopy. The maxima are assigned to Cu ad-trimers located in the center of Cu-O six rings of the bulk-cut Cu2O(111) surface. Electronically, the films exhibit a p-type conductance behavior, with the valence-band maximum located directly at the Fermi level. Defect states in the band gap point to the presence of subsurface Cu vacancies, in accordance with the p-type electronic response of the oxide. At 700 K annealing, a dewetting transition takes place, in which Cu2O(111) crystallites phase-separate from a Cu3O2 surface oxide. The latter resembles the Cu-O monolayer that develops upon low-pressure Cu-oxidation. Our high-pressure scheme represents a reliable pathway to produce bulk-like Cu2O films that are suitable for surface-science and catalytic studies at the atomic scale.