Phase change, tuning of optical and dielectric parameters in Bi/S-Se-Sb heterostructure film upon thermal annealing: An experimental and computational approach

The study is based on annealing-induced Bi diffusion into Se ₃₀S ₃₀Sb ₄₀ sites that bring modification in its linear-nonlinear optical, structural, and dielectric properties. The Bi/Se ₃₀S ₃₀Sb ₄₀ heterostructure film was annealed at 100,150,200 and 250 °C. The Bi ₂Se ₃ and Sb ₂Se ₃ phase formation was confirmed from the X-ray diffraction as well as from the Raman study. The morphology has been studied by the field emission scanning electron microscopy images with increased particle size upon annealing. The uniformity and homogeneity of the film were noticed from the energy dispersive X-ray analysis elemental mapping. The bandgap decreased from 1.461 eV (as-prepared Bi/Se ₃₀S ₃₀Sb ₄₀) to 1.258 eV (250 °C annealed) with an increment in static refractive index from 2.864 to 3.114 which was noticed from the optical study. The linear optical parameters were tuned with annealing conditions and such changes were understood from the concept of density of states and defects in the film. The increase in dielectric loss factor and decrease in quality factor with annealing is accompanied by the increase in both electronic and optical conductivity. The plasma frequency decreased and optical electronegativity reduced from 1.728 (as-prepared Bi/Se ₃₀S ₃₀Sb ₄₀) to 1.692 (250 °C annealed) with annealing. The value of 3rd-order nonlinear susceptibility and non-linear refractive index gets almost doubled (i.e. 1.838 × 10 ⁻¹¹ to 3.913 × 10 ⁻¹¹ esu and 2.419 × 10 ⁻¹⁰ to 4.735 × 10 ⁻¹⁰ esu respectively) with annealing with an increase in the nonlinear absorption coefficient. The hydrophilic nature of the films with annealing was observed from the surface wettability study. The computational study by density functional theory provided the necessary idea for structural and physical parameters of the films.