ZnO Nanostars Decorated with Mass-Selected Au Clusters for Photoluminescence.

Hybrid nanostructures combining semiconductor materials and noble metal clusters of atoms (nanoparticles) are of high interest in the energy sector and catalysis, with the idea of tuning the physicochemical properties of the system toward the desired performance. The design of this type of complex system requires the appropriate selection of the material combination to optimize the desired properties. However, less attention has been devoted to the effect of cluster size. In this work, we investigate the size and density effects for mass-selected monometallic Au clusters decorating ZnO-based nanostars. The Au clusters were prepared with narrow control of their size, in terms of atoms per cluster, via cluster deposition in a vacuum and mass selection with a cluster beam source. We study the coupling of ZnO nanostars with deposited Au N (N = 55, 147, and 309) clusters. We exploit transmission electron microscopy and Rutherford backscattering spectrometry for the structural characterization and for the determination of Au cluster density, obtaining 3.43 × 1012, 4.55 × 1011, and 7.98 × 1010 clusters/cm2 for samples decorated with Au clusters containing 55, 147, and 309 atoms, respectively. Moreover, we highlight the formation of a Schottky junction by performing photoluminescence investigations. We find distinctive changes in the behavior of the visible and UV emission as a function of the cluster size and density on the ZnO-based nanostars, identifying an increase of the photoluminescence efficiency with the decrease of the cluster dimension. Our findings indicate the enormous potential that a proper selection of cluster size offers in the fabrication of nanocomposite materials with precise electronic properties.