Study of surface plasmon resonance of Cu@Cu2O core–shell nanoparticles by Mie theory

Abstract

Cu@Cu2O core–shell nanoparticles on a-C : H thin films are prepared by co-deposition of RF-sputtering and RF-PECVD. Samples with different copper concentrations are grown. The copper content of films increases with reduction in initial pressure and rises with increasing RF power. When the Cu/C ratio reaches 0.5, the surface plasmon resonance (SPR) peak that is a signature of the formation of Cu nanoparticles appears in visible spectra of these films. X-ray photoelectron spectroscopy (XPS) characterization indicates that the surface of the copper nanoparticles oxidizes when they are exposed to air. The results are indicative that the shell of the nanoparticle is mainly the Cu2O phase that is stabilized with CuO covering a thin layer on the Cu2O shell. Using Mie theory for the SPR peak in visible spectra, the size of the copper cores and copper oxide shell, the dielectric constant of shells and the plasma frequency are estimated. The Mie theory estimation indicates that the SPR peak is damped by decreasing the size of copper cores. The SPR peak appears in visible spectra for a copper core with a diameter larger than 2 nm. This empirical result confirms the reported calculation result. The shift in the energy of the SPR peak follows the shift in the plasma frequency with respect to the reported value for copper bulk. The dielectric constant of the combined Cu2O shells and the CuO cover layer is larger than the reported value for Cu2O nanoparticles and increases with increasing CuO/Cu2O ratio.