Surface Geometry and Chemistry of Hydrothermally Synthesized Single Crystal Thorium Dioxide.

摘要

The surface chemistry and geometry of hydrothermally grown, single crystal ThO2 was studied using X-ray Photoemission Spectroscopy (XPS) and Atomic Force Microscopy (AFM). The crystal was studied before and after sputter etching with Ar+, heating up to 675 K, and dehydration with dry nitrogen. The hydrothermal growth technique incorporated Cs and F into the near surface bulk. Heating to 675 K drove off the F, but did not the Cs. Sputtering with Ar+ at 75?? cleaned the crystal and created a more uniform surface geometry. The Th 4f peak resolution, as measured by XPS improved by a factor of 3, the surface roughness, as measured by AFM, reduced by a factor of 2, and the error in adhesion force measurements at different location on the crystal reduced by a factor of 2. The surface of ThO2 becomes more metallic with heating. X-rays, used in XPS, created 7.3 eV of charging on the ThO2 crystal surface at room temperature. Metallic Th 4f peaks appeared in the XPS spectrum at 675 K, while the charging was reduced. The as-grown crystal, the adhesion force strength by metal was Au>In>Ni>Ti. Post cleaning the adhesion force change to In>Au~Ni>Ti. Interface energy computations confirm that the Au metal adhesion decreased by half for the cleaned crystal. Based on these results, indium is the best choice of metal for application of contacts to the ThO2 single crystal used in this research.