Evidence for truncated octahedral structures in supported gold clusters

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Abstract
Gold clusters deposited by evaporation on a Mylar substrate in many superimposed layers have been studied by EXAFS (extended x-ray-absorption fine-structure) spectroscopy at T=28 K. For such clusters, the first shell signal of the EXAFS spectrum has been compared with the corresponding crystal data used for the phase and amplitude calibration. The simulation of the EXAFS spectra for different possible geometrical structures (cubo-octahedral, icosahedral, truncated octahedral) applying a curved wave method with a set of theoretically calculated parameters (nearest-neighbor distance, coordination, and Debye-Waller factor) provided reference curves for the experimental data. A complete analysis including the cumulants of the radial distribution, the dispersion of the cluster sizes given by electron microscopy, and a decomposition in partial contributions obtained by a minimization method, allowed us to determine that the dominant geometrical structure of the clusters is truncated octahedral. In contrast with recent theoretical calculations, we show that the interatomic distance of gold nanoparticles results indeed contracted, independently of the use of the cumulants in the data analysis.