No 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.

Hexagonal array of Cu@CuO core–shell nanoparticles (NPs) on the a-C:H thin film was prepared by codeposition of RF-sputtering and RF-PECVD. The trace of hexagonal NPs supperlattice was recognized by AFM image and XRD result. On the basis of localized surface plasmon resonance (LSPR) of core–shell NPs, the prepared array detected a low flow rate of CO gas at room temperature. XPS results indicate that the surface of Cu@CuO core–shell NPs have no chemical reaction with CO molecule. The physical absorption of CO molecule on the surface of Cu@CuO core–shell NPs increases the LSPR absorbance and causes a red shift in LSPR wavelength. These experimental results are in agreement with Mie theory studying provided that CO absorption on the core–shell increases the dielectric coefficient of the shell. The prepared Cu@CuO core–shell NPs show an average shift of 10–2 nm per molecule absorption, which can lead to detection of a single CO molecule per NP.