SCANNING-TUNNELING-MICROSCOPY-ENHANCED OXIDATION OF URANIUM DEPOSITED ON THE BASAL-PLANE OF GRAPHITE

Uranium metal was deposited onto the basal plane of graphite by pulsed excimer-laser ablation in a UHV system equipped with Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), and scanning tunneling microscopy (STM). Low deposition coverages imaged by in-situ STM show the formation of clusters of varying sizes on the surface. Thicker films (estimated to be 20-30 monolayers) were also investigated by STM in-situ and in air immediately after removal from the UHV system. Imaging in air was found to be possible for up to one hour, even though the expected oxidation rate at room temperature should lead to the complete oxidation of a uranium film to UO2 in a few minutes. Films estimated to be thicker than 50 monolayers, however, were not imageable at all, suggesting that thin films consisting of clusters exhibit reactivities that are different from bulk samples. STM-enhanced oxidation in air was observed for thin films if the tip was held at a negative potential with respect to the sample. Possible mechanisms of enhancement are discussed.