Surface Science Letters Preferential cluster nucleation on long-range superstructures on Al 2 O 3 =Ni 3 Al1 1 1) C. Becker * , K. von Bergmann, A. Rosenhahn, J. Schneider, K. Wandelt Institut f ur Physikalische und Theoretische Chemie, Universit at Bonn, Wegelerstrasse 12, 53115 Bonn, Germany Received 26 January 2001; accepted for publication 21 March 2001 Abstract The room temperature growth of metal clusters on well-ordered alumina ®lms on Ni 3 Al1 1 1) was investigated by scanning tunneling microscopy STM). The electronic superstructures that appear in the STM images of the bare oxide ®lms, constitute nucleation centers for the cluster growth. For Ag clusters this pinning operates only at low metal coverage, whereas higher coverages lead to large agglomerates of the clusters. This is attributed to the relatively weak bonding of Ag to the alumina ®lm. Conversely, in the case of Mn clusters the pinning persists even for moderate metal coverages owing to the much stronger bond between metal and oxide ®lm. Finally, at high metal coverage the pinning is lost, the Mn clusters do, however, not agglomerate. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Scanning tunneling microscopy; Nucleation; Manganese; Aluminum oxide; Clusters; Silver 1. Introduction The growth of alumina ®lms on various Ni/Al alloy single crystal surfaces has been investigated by a number of research groups in the past years. It has been found that well-ordered ®lms of sub- nm thickness can easily be grown on e.g. Ni 3 - Al1 1 1) [1±5], NiAl1 1 1) [6], and NiAl1 1 0) [7]. These ®lms are often regarded as suitable sub- strates for the preparation of model catalysts via metal deposition [8]. The growth and reactivity of a large variety of metals on Al 2 O 3 /NiAl1 1 0) has been studied [8]. These authors have found that for metals with a medium metal±support interac- tion such as Rh and Pd at low temperatures 90 K) point defects are the primary nucleation cen- ters, whereas line defects step edges and domain boundaries) play a more important role at higher temperatures. Recently we have demonstrated that thin alumina ®lms grown on Ni 3 Al1 1 1) at 1050 K are homogeneous and nearly free of domain boundaries [9]. Hence, the in¯uence of the struc- tural defects on the cluster growth does not appear to be eective in this case. Let us summarize the most signi®cant features of these alumina ®lms grown at 1050 K that are relevant for the present investigation. Fig. 1 shows an STM image of a well-ordered alumina ®lm [9]. The scan was started with a bias voltage of 2.2 V. After about one-third of the image the bias voltage was raised to 3.1 V. At a bias voltage of 2.2 V a 4.5 nm superstructure is visible that changes into a 2.6 nm superstructure Surface Science 486 2001) L443±L448 www.elsevier.nl/locate/susc * Corresponding author. Tel.: +49-228-733297; fax: +49-228- 732551. E-mail address: cbecker@uni-bonn.de C. Becker). 0039-6028/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII:S0039-602801)01052-4