Regensburg 2007 – wissenschaftliches Programm

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O: Fachverband Oberflächenphysik

O 43: Oxides and Insulators: Adsorption II

O 43.4: Vortrag

Mittwoch, 28. März 2007, 16:30–16:45, H42

Water on ZnO(1120): An UPS Study — •Joachim Wider and Christian Pettenkofer — Hahn-Meitner-Institut, Glienicker Strasse 100, 14109 Berlin

In order to understand the growth process of epitaxically grown MOMBE ZnO films on Al₂O₃, we have studied the adsorption of H₂O on a ZnO(1120) single crystal. Furthermore it is controversially discussed, whether water adsorbs molecularly or dissociatively on oxide surfaces [1, 2]. Our UV photoemission experiments show that at 95 K water adsorbs both, molecularly and dissociatively on ZnO(1120). At increasing exposures, the fraction of dissociatively adsorbed water remains constant, whereas the molecularly adsorbed water forms a ice multilayer. In a second experiment, we annealed the water exposed surface. Between 160 and 180 K, the ice multilayer desorbs, and a layer consisting of H₂O and OH remains on the surface. At 395 K, all adsorbats are desorbed. The adsorption of 20 L H₂O results in a band bending of 0.9 eV. Subsequent annealing decreases band bending. Upon adsorption, the workfunction is constant within 0.1 eV. Annealing the sample yields a sudden incease of the work function at 100 K of 0.2 eV, followed by decrease at 160, and a slow increase between 180 and 340 K of 0.3 eV. At 395 K, the work function has the same value as the clean cooled sample.

[1] G. Zwicker, K. Jacobi, Surf. Sci. 131 (1983) 179.

[2] B. Meyer, D. Marx, O. Dolub, U. Diebold, M. Kunat, D. Langenberg, C.Wöll, Angew. Chem. Int. Ed. 43 (2004) 6642.