Molybdenum deposition on TiO2 (110) surfaces with different stoichiometries

The deposition of ultra thin molybdenum films has been carried out on three different TiO2 surfaces: a stoichiometric and flat one obtained after anne aling, a non stoichiometric and rough surface made by Ar+ bombardment and a stoichiometric and rough surface obtained by oxygen bombardment. Whatever the substrate preparation, in situ AES and XPS studies and ex situ AFM and RHEED characterizations have revealed a Stranski-Krastanov growth mode: the completion of three monolayers followed by island growth is observed in an y case. The three monolayers are composed of amorphous molybdenum oxide wit h a molybdenum oxidation state between III and IV. The oxidation of the mol ybdenum layers generates a reduction of the substrate with the formation of Ti3+ and Ti2+ and induces a reconstruction of the surface: during the form ation of the molybdenum oxide layers the roughness of the surface strongly decreases. After the growth of the three layers, the surface is flat whatev er the initial roughness. Then, the molybdenum atoms can diffuse on the sur face and generate clusters. The resulting islands are metallic (BC structur e) but without preferential orientation. (C) 1999 Elsevier Science B.V. All rights reserved.