Probing the outermost layer of thin gold films by XPS and density functional theory

This work examines gold thin films of various thicknesses varying from 5 to over 100 nm along a 7 cm distance to investigate the effect on the surface sensitivity of the XPS Au4f signal. Increasing the relative intensity of the surface XPS signal can be useful for studying properties of the surface atoms in case of functionalized gold, and for shedding light on reactions occurring at the interface with the metal surface as well as adsorption geometry. The surface-to-bulk signal ratio increased with decreasing thickness for thicknesses greater than 15 nm. Below this value, the intensity of the surface signal decreased. The binding-energy shift of the surface component with respect to the bulk component was found to be in agreement with that calculated by density functional theory (DFT). The calculated shift was found to be essentially unaffected, whether or not final-state effects were included. KPFM was also performed to investigate correlations between the measured work function and the surface/bulk BE shift, revealing that the contact potential difference changes with thickness because of the influence of the substrate. Gradients were prepared by a novel evaporation method, by template stripping, and by photolithography, yielding stripes of increasing thickness in the same sample