Structural investigation of supported $\mathrm{Cu_{n}}$ clusters under vacuum and ambient air conditions using EXAFS spectroscopy

Extended X-ray absorption fine structure (EXAFS) measurements have been carried out to explore the size-dependent structure of deposited Cu$_{\eta}$ clusters using the element-specific K$_{\alpha}$ fluorescence signal of copper. In particular, Cu$_{55}$ and Cu$_{35}$ have been deposited on a thin natural silica layer of a Si(100) wafer and have been investigated under both ultra-high vacuum and ambient air conditions. Pieces of evidence of an icosahedral structure of Cu$_{55}$ and a densely-packed compact fcc-like geometry of Cu$_{35}$ are concluded from a comparison of the magnitude EXAFS spectra and FEFF simulations of the geometry of the corresponding free clusters. The exceedingly small radial distances as revealed from the radial distribution plots of the pristine clusters can be explained by a severe increase (∼100%) in the scattering phase shift with respect to that of copper bulk. After exposure to ambient atmosphere, the EXAFS signal of “magic” Cu$_{55}$ as well as that of Cu$_{35}$ exhibits oxidation of the clusters showing a nearest-neighbour radial distance peak which agrees with the Cu–O bond length in amorphous copper hydroxide