Optical and chemical stability of sputtered-Au nanoparticles and film in ambient environment

Gold sputtered on quartz can exhibit particle- or film-like morphologies. For Au coverage less than equivalent thickness of 3 nm, well-dispersed Au nanoparticles (Au NPs) forms. Depending on size, the colour of these samples, which are associated with surface plasmon resonance (SPR) excitation, can appear pink and/or blue. Exposing these samples to an ambient environment oxidizes these NPs and the degree of oxidation is more significant with smaller NPs. Smaller NPs have larger surface-to-volume ratio and a greater number of under-coordinated surface Au atoms, thus making these samples more susceptible to oxidation. The oxidation process is accompanied by a redistribution of the valence-band electrons where the 5d and 6s electrons become less delocalized for metallic bonding and the appearance of a valence band maxima at ~ 2 eV results. As the oxidation proceeds, the initial SPR peaks were also blue-shifted. As the amount of sputtered Au increases, a labyrinth-like network of coalesced Au-particles forms before the film-like morphology eventually emerges. In comparisons to Au-NPs, these samples appear more bulk-like and thus do not exhibit SPR nor show any significant degree of oxidation in the ambient air environment.