Optical response from the ultraviolet to the far infrared and atomic force microscopy of Au implanted in CaF2

Abstract

Gold was implanted into randomly oriented single-crystal CaF2 hosts at doses of 1, 3, 6, and 10×1016 ions/cm2. The modifications of the host resulting from the ion implantation were characterized by measuring the optical response in the 0.0062–6.70 eV range and the changes in surface topography were investigated by atomic force microscopy (AFM). The implantation depth profiles were obtained by Rutherford backscattering measurements. In the as-implanted samples, a peak is observed at 2.33 eV in the electronic spectra which we attribute to the surface plasmon resonance of gold nanocrystals. The 1.24–0.099 eV range shows no significant difference compared to the spectra of the virgin host. However, the far-infrared spectra show a splitting in the longitudinal optical phonon and a decrease in the reflectivity of the transverse optical phonon as the ion dose increases. The surface topography changes from one where the scratch and dig features of the virgin substrate are clearly revealed in the constant force AFM measurements to one where the surface appears to be mottled by the ion implantation. Annealing the samples at 773 K brings about an increase in the surface plasmon intensity and a partial recovery of the intensity of the longitudinal optical phonon. Annealing the ion-implanted samples at temperatures significantly greater than 773 K leads to the development of craters on the CaF2 surface. The crater formation is expected to arise from a strain induced in the host by the growth of gold nanocrystals. Apparently, when the nanocrystals reach a certain size, a critical strain is induced in the crystal such that the primary channel for relaxation is spallation that leads to crater formation.