Ultramicroscopy 42-44 (1992) 1350-1354 North-Holland Behavior of small metal clusters on solid crystal surfaces Zili Ma, Changxin Zhu, Jian Shen and Shijin Pang Beijing Laboratory of Vacuum Physics, Chinese Academy of Sciences, P.O. Box 2724, Beijing 100080, China Received 12 August 1991 Association of single atoms into clusters and aggregation of small atomic clusters into a surface layer have been and are continued to be extensively studied as a tractable model system for the study of atomic processes and monolayer growth. In our experiments, the scanning tunneling microscope (STM) has been used to study some of the behavior of small Pt and Ni clusters on solid crystal surfaces. The samples have been prepared by controllable evaporation deposition on HOPG from Wollaston wires at room temperature in UHV. Here we present a series of STM images of transformation processes of small atomic clusters, which provide us a better understanding on how substrate surfaces dominate this process. The transforma- tion phenomena actually give rise to a complicated mechanism. STM images have been performed at room temperature. 1. Introduction Since metal clusters have many interesting properties which are different from bulk materi- als, the free clusters have been investigated ex- tensively by many techniques. Supported clusters have also been paid attention to because of their potential application as specific catalysts. An- other motivation for studying the behavior and morphology of small metal clusters on solid sin- gle-crystal surface is the desire to understand the physical mechanisms evolving in the initial stages of thin film growth. The early stages of thin film growth dominate the morphology of final film, so there have been many researches focused on the small supported clusters with a wide variety of analytical techniques. However, many of the most important fundamental questions still remain un- solved because of the limitations of analytical methods. With its high spatial-resolution ability to image special sites of surfaces in real space, the scanning tunneling microscope (STM) [1,2] is believed to be a particularly powerful tool suited for the investigation of the initial nucleation pro- cess of deposition where regular periodicity is not yet established, which is usually not suited for most conventional surface analytical techniques. A few groups have made attempts to image metallic clusters with STM [3-11]; several kinds of metallic clusters such as gold, silver, alu- minum, copper, palladium and platinum clusters, both 2D and 3D islands or clusters, were imaged by STM. The purpose of the present study is to enrich this research field by the STM images of small platinum and nickel clusters deposited by a superfine-metal-wire evaporation method, and some real-time STM observations of small metal- lic clusters' dynamic transformation processes. This paper presents the STM images of small platinum and nickel clusters supported on graph- ite surfaces on atomic scale, as well as a series of subsequent pictures about the behavior of small platinum clusters on a graphite substrate. The controllable metal evaporation was achieved us- ing a Wollaston-wire (a kind of superfine and high-purity metal wire with typical diameter less than a hundred micrometer) evaporator in the experiments. 2. Experiment Both the metal deposition and STM experi- ments were carried out in an ultra-high vacuum (UHV) chamber, in which a commercial STM set-up was mounted; the detailed description can 0304-3991/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved