Deuterium plasma exposure on rhodium: Reflectivity monitoring and evidence of subsurface deuteride formation

The effects of low flux, low temperature deuterium plasma (LTP) exposure on nanocrystalline rhodium (Rh) films are investigated. The exposures do not cause any surface damage on the nanoscale and the specific electrical resistivity of the films remains invariant during exposures. However, the spectral reflectivity of Rh decreases during exposure and recovers very slowly during subsequent storage in high vacuum. This drop in the reflectivity can be associated with a formation of a subsurface rhodium deuteride (RhD _x , x ≤ 2), which has optical constants different to those of Rh. After air storage of the exposed samples, the Rh surface gets depopulated of deuterium adsorbates due to a catalytic reaction taking place between oxygen and deuterium, which results in a diffusion of the incorporated deuterium first to the surface and then into the air. Consequently, the reflectivity is rapidly recovered. Comparison of the ultraviolet photoelectron spectroscopy (UPS) measurements performed before and after plasma exposure reveals an increase in the work function which is attributed to deuterium adsorbates on the surface. Changes in the valence band structure were also observed with UPS measurements, lending support to the suggestion of subsurface RhD _x formation. Deuterium atoms in Rh are electron donors filling the 4d states above the Fermi level, thus reducing optical transitions.