Structural investigation of supported Cun clusters under vacuum and ambient air conditions using EXAFS spectroscopy

Abstract

Extended X-ray absorption fine structure (EXAFS) measurements have been carried out to explore the size-dependent structure of deposited Cu_n clusters using the element-specific K_α fluorescence signal of copper. In particular, Cu₅₅ and Cu₃₅ 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 Cu₅₅ and a densely-packed compact fcc-like geometry of Cu₃₅ 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” Cu₅₅ as well as that of Cu₃₅ exhibits oxidation of the clusters showing a nearest-neighbour radial distance peak which agrees with the Cu–O bond length in amorphous copper hydroxide.