Paper EN+SE+NS+SS-MoA2
Response of Ultra-Thin Palladium Films to Hydrogen Exposure Studied by Low Energy Electron Microscopy and Diffraction

Monday, October 20, 2008, 2:20 pm, Room 203

The ability of Palladium to store hydrogen is well known. Nevertheless surface science studies of the exposure of Pd(111) samples to hydrogen have found only a dense hydrogen layer betwen the last palladium-palladium layers, with additional hydrogen going into the bulk as a solid solution.^1,2 On Pd films a few atomic layers thick on a substrate which does not accept substantial quantites of hydrogen such as Ru, it should be possible to saturate completely the Pd film, and thus study the formation of PdH hydride with surface science techniques. In particular, as there is a substantial lattice mismatch betwen Pd and the beta-PdH hydride, the incorporation of H in Pd-intersticial places should induce a significant expansion of the Pd layers. Although H itself is basically invisible to low energy electron diffraction, the change of the Pd-Pd distances should be easily resolvable by LEED.³ In this work, we present a low energy electron microscopy study of the hydrogren exposure of thin films of Pd on Ru(0001) and W(110). We characterize and study the growth and structure of Pd thin films. LEEM is a useful technique to monitor and control the growth of thin films in real space. Furthermore, a LEEM microscope can be used to acquire selected area diffraction (SAD-LEED) to determine the structure of such film.⁴ Our experiments reveals that Pd films 2 to 6 ML thick on Ru(0001) follow the fcc sequence with the Ru in-plane lattice spacing, and present a corresponding expansion in the out-of-plane lattice spacing. Thicker films on Ru(0001) relax to a bulk-like Pd in-plane spacing. Both thick films on Ru(0001) or W(110) show a slight expansion of the last interlayer spacing, in line with reported studies on the surface of bulk Pd. We follow the dosing of atomic H on the Pd films by means of real-time LEED IV curves acquired by LEEM. We have found preliminary indications that room and low temperature exposure produce a significant expansion in the interlayer distance for a 4 ML thick film. But this expansion is smaller that the one expected for a full PdH beta-hydride formation.

¹G.E. Gdowski, T.E. Felter and R.H. Stulen Surf. Sci. 181, L147, (1987)
²Sampyo Hong and Talat S. Rahman Phys. Rev. B 75, 155405 2007
³T. E. Felter, Eric C. Sowa, and M. A. Van Hove, Phys. Rev. B 40 num. 2, 891-899, (1989)
⁴J. de la Figuera, et al, Surf. .Sci. 600, L105 (2006)