Nanocluster structure deduced from AC-STEM images coupled to theoretical modelling

Abstract

Determining the atomic structure of nanoclusters is a challenging task and a critical one for understanding their chemical and physical properties. Recently, the high resolution aberration corrected scanning transmission electron microscope (AC-STEM) technique has provided valuable information about such systems, but the analysis of the experimental images has typically been qualitative rather than quantitative. A method is presented for detailed analsis of AC-STEM images combined with theoretical modelling to extract atomic coordinates. An objective function formed by a linear combination of a fit to the two-dimensional AC-STEM image plus an estimate of the cluster's energy for adding information about the third dimension is used in a global optimization algorithm to extract the atomic coordinates. The method is illustrated by analyzing model images generated for the Garzón structure of the Au 55 cluster, which is a metastable structure for the embedded atom method (EAM) potential function used here to estimate the total energy. As the method does not rely on the alignment of atom rows in the AC-STEM image, the partially disordered chiral structure of the Au 55 can successfully be determined even when a significant level of noise is added to the images.

Contrasting motif preferences of platinum and gold nanoclusters between 55 and 309 atoms

“…The calculated cluster structures presented here can be compared with experimental observations of structural features in Au and Pt clusters. The lowest energy structure of Au 55 found here is in good agreement with previous computational [17][18][19]31 and experimental 16 studies that identify the lowest energy isomers of Au 55 to be a family of distorted, chiral clusters. The exact structure of the global minimum depends highly on the theoretical description of the atomic interactions (see ESI † for detailed discussion).…”

“…27,28 A variety of calculations have been performed for both Au and Pt clusters, from survey calculations over a large cluster size range using an empirical potential 29 to detailed global optimisations at a few selected sizes. 17,18,30,31 Many studies have provided support for the notion of a crossover (either gradual or abrupt) from Ih to Dh to FCC as the cluster size is increased. 20,29,[32][33][34][35][36] However, it should be noted that both Dh and FCC have been shown to be competitive structures over a wide range of sizes; 29,37 recently, it has been found that Dh and FCC motif preference continues to oscillate even at sizes up to 4000 atoms.…”