A Study of Optical Properties of Cesium Halide CsX X Cl Br I Thin Films Deposited by Thermal Evaporation Method

Abstract

This thesis details my investigations on CsCI, CsBr and CsI thin ploycrystalline films grown on glass substrate by thermal evaporation. Though films are transperent with band gap in UV region, absorption peaks in the visible region were found. Systematic proof has been provided that these peaks are of Localised Surface Plasmon Resonance (LSPR). The thesis concludes with description of the evolution of grain structures in Cesium Halide films which results in appearance of LSPR peaks in UV-visible absorption spectra. The LSPR appear with time because the grain boundaries are receding and forcing the large grain to become smaller and spherical in shape in order to minimize the free surface energy of the system. The process is also assisted by the presence of point defects/color centers in Cesium Halide films. These defects which appear due to the absence of massive halide atoms from the lattice create residual tensile stresses within the lattice which results in defect diffusion. The defects migrate outwards towards the grain surface giving rise to Cesium metal clusters. The lattice mismatch appearing between the lattices at the grain boundary and the bulk giving rise to Core-shell structure further contributes to grain division. Along with this volume diffusion, a surface diffusion of Cesium takes place towards sites of faceted grain boundaries resulting in a necking phenomenon, appearing like bridges between the daughter Cesium Halide grains (grains appearing due to division of a single grain). Breaking away of the daughter Cesium Halide grains results in nano-rods that contribute to the SPR peak seen. newline