Physical Synthesis and Study of Ag@CaF 2 Core@Shell Nanoparticles: Morphology and Tuning of Optical Properties

Pre-formed Ag nanoparticles (NPs) and Ag@CaF 2 core–shell NPs are physically synthesized using DC magnetron-based NP source and deposited on Si-SiO x wafers. The samples are prepared by co-depositing Ag nanoparticles and CaF 2 produced by an evaporation source, or by sequential deposition method, i.e., by depositing in a sequence a CaF 2 buffer layer, the Ag NPs generated by the NP source and a capping CaF 2 layer. The supported films are characterized by Scanning Electron Microscopy (SEM), X-ray Photoelectron Spectroscopy (XPS), and Surface Differential Reflectivity (SDR). SEM shows that Ag NPs deposited directly on Si-SiO x tend to diffuse and to agglomerate, affecting the size distribution of the nanostructures. The presence of a CaF 2 buffer layer between Ag and Si-SiO x limits this effect, while XPS reveals electrical charging, caused by the insulating nature of the CaF 2 continuous film. The surface plasmon resonance behavior for different samples is analyzed using SDR with p-polarized light. There is a clear evidence of a blue shift in the plasmon excitation due to the presence of CaF 2 on Si, which can represent a potential advantage for the technological applications in photovoltaics and optoelectronics.