Electronic structure of molecular-capped gold nanoparticles from X-ray spectroscopy studies: Implications for coulomb blockade, luminescence and non-Fermi behavior

We report an X-ray spectroscopy study of two thiol (tiopronin)-capped gold nanoparticles of an average size of ∼3.0 and ∼1.5 nm with distinctly metallic and non-metallic behavior, respectively, as evident from their UV–visible absorption and luminescence. X-ray photoemission spectroscopy with a charge neutralization scheme confirms the respective nature of these nanoparticles and permits the alignment of the top of the valence band of the non-conducting nanoparticle to the Fermi level. The Au 5d band shows a particle size-dependent narrowing in quasi Au 5d5/2 and 5d3/2 spin–orbit splitting and bandwidth relative to Au metal, a behavior characteristic of reduction in the coordination number of Au on average. Using the Au 4f binding energy shift and the Au L3-edge threshold energy shift between Au metal and the non-metallic Au nanoparticle, we estimated a value of ∼1.7±0.3 eV for the pseudo-gap in the ∼1.5 nm Au nanoparticles. This value is in good agreement with the luminescence maximum of 1.63 eV. This behavior is attributed to the interplay of surface and quantum size effects.