The atomic structure, stability and shape of supported nanometer-size metal clusters is a topic of much interest. Recent molecular-dynamics studies of metallic clusters have given new insights into the dynamics of cluster melting [1, 2] and quasi-melting [3]. Traditionally, the structure of a small metallic cluster resting on a substrate has been investigated using X-ray diffraction techniques or electron microscopy techniques such as high resolution transmission electron microscopy (HRTEM). Unfortunately, these techniques yield indirect evidence about the atomic structure. HRTEM often allows only the imaging of lattice fringes rather than individual atoms. Scanning probe microscopy (SPM) techniques such as scanning-tunneling microscopy (STM) and atomic force microscopy (AFM) have also been utilized [4, 5] but these studies have also lacked atomic resolution, as well as the time resolution necessary to observe structural changes which characterize the so-called quasi-melting phase of nanoscale clusters.
In light of these limitations, it would be advantageous to observe the structure and shape of supported clusters using techniques other than SPM and HRTEM. Ideally, any technique employed should be non-intrusive, possess atomic resolution, and have sufficient time resolution to resolve changes in cluster shape and structure. Field emission and field-ion emission seem like useful techniques to consider in this regard, since both techniques are well established and both techniques are compatible with the constraints outlined above. Progress in this regard has been made since Castro et al. developed techniques to deposit and image clusters on field emission tips.[4, 6, 7] Lin et al. have reported indirect evidence for structural fluctuations by studying the stability of the electron current emitted from a supported cluster in the presence of a high electric field.[8]
Field-ion microscopy (FIM) is particularly well suited to the study of nanoscale particle structure since it possesses true atomic resolution. FIM images are relatively easily simulated, allowing studies of the atomic structure of a sample under study.
In what follows, we investigate what information can be obtained from the simulation of field-ion images of nanometer-size clusters. The atomic structural information obtained in these simulations is illustrated by a number of examples. Comparisons between these simulations and an experimental FIM image of a nanometer-size Au cluster[4] is also carried out. We find this procedure provides structural and orientation information about an individual supported Au cluster and suggests the utility of this approach for further studies of nanometer-size clusters. These results indicate that future molecular dynamic simulations of cluster structure and dynamics should include a field-ion projection of the cluster's atoms. As demonstrated below, the field-ion image is one of the few techniques available that allows a detailed experimental check of cluster structure determined from molecular dynamic calculations.