Synthesis and Optical Properties of Dithiol-Linked ZnO/Gold Nanoparticle Composites

Semiconductor-metal nanocomposites are being pursued for use as a new generation of light emitters and photovoltaic devices. A convenient method for attaching gold nanoparticles (AuNPs) onto zinc oxide nanorods with variable surface densities is described that consists simply of mixing suspensions of monolayer-protected AuNPs and the nanorods in the presence of a dithiol. One end of the dithiol linker bonds to AuNPs via a ligand place-exchange reaction, and the other end attaches to ZnO via Zn-S bonding. The nanocomposites have been characterized by UV-vis absorbance, photoluminescence, and Raman spectroscopies. Attachment of the AuNPs affects the ZnO absorbance and photoluminescence (PL) spectra, resulting in blue shifts of the absorbance and UV excitonic emission peaks. Ultraviolet photoelectron spectroscopy has also been performed on the nanocomposite, zinc oxide nanorod, and gold nanoparticle samples, and an energy level diagram has been constructed. The PL and absorbance shifts are ascribed to the Burstein-Moss effect in which photogenerated electrons accumulate on nearby gold nanoparticles, transfer to the ZnO conduction band, and cause band-gap widening.