Imaging ultrathin Al2O3 ®lms with scanning tunneling microscopy

Reproducible scanning tunneling microscopic (STM) images were obtained from ultrathin Al2O3 ®lms epitaxially grown on Re…0001†. Initially, the oxide ®lms grow two-dimensionally in a layer-by-layer fashion with well-ordered surface morphologies. As the oxide ®lm thickens to ca. 9 monolayer equivalents (MLE), the surface roughens and becomes more disordered yet still exhibits signi®cant long-range, hexagonal periodicity as indicated by low energy electron di€raction (LEED). Because of limited conductivity, ®lms thicker than ca. 9 MLE could not be imaged. Ó 2000 Elsevier Science B.V. All rights reserved. Due to its inertness, chemical stability and mechanical strength, Al2O3 is an important ceramic material in catalysis, coatings and microelectronics. In heterogeneous catalysis, aluminum oxide is often used for supporting small Ag clusters to promote partial oxidation of ethylene to ethylene oxide [1,2]. There has been recent renewed interest in the role of metal oxide supports in modifying the catalytic activity of nanosized metal clusters, particularly Au [1]. Despite the considerable e€orts to understand metal cluster/oxide support interactions that can give rise to increased catalytic activity and selectivity, much remains to be understood. Scanning tunneling microscopy (STM), with its ability to probe surface morphology at the atomic level, is a powerful technique for systematically studying surface structure that can a€ect its catalytic properties. This Letter is a report of the ®rst successful imaging of thin ®lm Al2O3 on a refractory support, Re…0001†, using STM. The STM imaged oxide ®lms were also characterized with X-ray photoelectron spectroscopy (XPS) and low energy electron di€raction (LEED). In its bulk form, Al2O3 is a wide band gap (Eg ˆ 8:7 eV [2]) insulator that cannot be made to conduct by doping. To enhance conductivity, thin ®lms (ca. 5±30 A thick) of Al2O3 were deposited on Re…0001† at 300 K using `hot' ®lament evaporation in ambient …1 10ÿ5 Torr) O2. These ®lms were suciently thin to eliminate charging via charged particle probes, yet substantive enough to retain the chemical and electronic properties characteristic of bulk Al2O3 [3±7]. Such thin ®lm preparations (extensive reviews of which are given elsewhere [8,9]) have been used to investigate model oxide systems with charged particle spectroscopies. Thin ®lms facilitate tunneling of 10 November 2000 Chemical Physics Letters 330 (2000) 226±230 www.elsevier.nl/locate/cplett * Corresponding author. Fax: +1-979-845-6822. E-mail address: [email protected] (D.W.