Visible to infrared luminescence from a 28-atom gold cluster

The luminescence properties of chemically prepared gold nanoclusters, each composed of a 28-atom core and a glutathione (GSH) adsorbate layer consisting of 16 molecules, were investigated. These clusters show a distinct absorption onset at 1.3 eV corresponding to the opening of an electronic gap within the conduction band (HOMO-LUMO gap). Here we report on the radiative properties of these molecular-like gold clusters. By using a combination of different detectors with sensitivities in the visible to the infrared (2.0-0.8 eV), a broad luminescence extending over this entire spectral range was observed. Our results further suggest that the luminescence can be separated into two bands with maxima around 1.5 and 1.15 eV indicating that radiative recombination between the ground state and two distinctively different excited states takes place. The origin of the observed luminescence bands is discussed using a solid state as well as a molecular model for the electronic structure and relaxation of the clusters. The total quantum yield of the luminescence as measured at ambient temperature was approximated to be about (3.5 ± 1.0) × 10^-3.