Well-Ordered Molybdenum Oxide Layers on Au(111): Preparation and Properties

MoO3 layers on Au(111) were prepared via oxidation of molybdenum at elevated temperature in an atmosphere of 50 mbar of O-2. Three different types of oxide structures were identified. Up to monolayer oxide coverage a structure with a c(4 x 2) unit cell relative to the Au(111) unit cell forms. This structure was previously identified as being similar to a monolayer of alpha-MoO3.(1) At larger coverages of up to two layers an oxide with a 11.6 angstrom x 5 angstrom rectangular unit cell appears. Further increase in the coverage leads to the occurrence of crystallites of regular alpha-MoO3 with a very small density of defects. These crystallites grow with the (010) plane parallel to the substrate surface and with random azimuthal orientation leading to rings in the LEED pattern With increasing layer thickness the crystallites start to coalesce until finally a closed film forms. These layers sublimate at temperatures between about 670 and 770 K with the MoO3 aggregates sublimating at lower temperature than the bilayer and monolayer films.