Fabrication of two-dimensional arrays of nanometer-size clusters with the atomic force microscope

The weak binding of preformed metallic clusters to atomically smooth substrates has presented an impediment to the study of supported nanometer-size clusters using scanning probe techniques. The sweeping of supported clusters on flat substrates by a scanning probe tip has been identified as a problem. Unless special precautions are taken, only those clusters pinned at defect sites are routinely observed in scanning probe studies of nanometer-size clusters. Recently, Schaefer et al. have shown that by using an atomic force microscope in the noncontact mode, clusters deposited on flat substrates from a cluster beam can be imaged in their as-deposited positions. Utilizing this breakthrough, Mahoney et al. have recently reported a systematic study of cluster–substrate interactions using a wide variety of atomically flat substrates and have provided a rank ordering of substrates based on their ability to immobilize Au clusters. In what follows we take advantage of these developments and show how the weak binding of clusters to substrates allows the assembly of nanometer-size clusters at room temperature in a predetermined pattern utilizing the vector control of the atomic force tip. Gold clusters used in this study were produced in the gas phase using a multiple expansion cluster source ~MECS!. As described elsewhere, the MECS is a gas aggregation cluster source in which both controlled cluster growth via accretion of single atoms or via cluster–cluster aggregation can be promoted. The Au clusters used here were annealed at 1400 K and are expected to be single fcc crystals having roughly the shape of a truncated octahedron. The atomic force microscope ~AFM! used here is a custom-built instrument capable of operating in the constant force ~contact!, constant height, and noncontact ~attractive! modes, and has been described elsewhere. Detection of the cantilever displacement was done using a laser deflection method. The output of a bicell position sensitive detector was digitized by a 68030 CPU based computer system similar to that described previously. The computer was used to handle all data acquisition and system control. For the purposes of this study, there are two important features of the AFM apparatus. First, the AFM was mounted inside a small stainless-steel chamber which could be evacu-