Impact of low dose of gamma irradiation on thin films of Bismuth Tri-iodide.

Abstract

Thin films of Bismuth Tri-iodide BiI3 were synthesized on a glass substrate by the thermal vacuum deposition technique. Films were exposed to gamma radiations of two doses 10Gy and 50Gy. Structural and optical properties of films were studied through X-ray Diffraction and UV spectroscopy both before and after the gamma irradiation. The effect of gamma radiations on microstructure parameters i.e. crystallite size and the micro-strain were studied qualitatively and quantitively. The observed increase in crystallite size and decrease in micro-strain is due to localized heating of crystallites which results in the merging of small crystallites and their assembly leading to the formation of larger crystallites as evident from X-ray Diffraction after gamma irradiation. Effect on the optical constants namely refractive index and extinction coefficient for different doses was also enumerated to throw light on the change in optical density of this material on gamma irradiation. It is observed that the refractive index n increases with an increase in the doses of gamma irradiation. The increase in optical density could be attributed either to the displacement of atoms from lattice sites to the interstitial sites or to the self-assembly behavior of the crystallites on irradiation. The absorption coefficient follows Urbach's empirical relation and exhibits exponential dependency on photon energy close to the band edge. Calculating Urbach's energy provides insight into the emergence of localized states at the energy gap's edges.