Structural and optical properties of chemically deposited thin films of quantum-sized bismuth(III) sulfide

Quantum-sized bismuth(III) sulfide thin films were prepared using chemical deposition technique, based on controlled homogeneous precipitation reaction in acidic aqueous system giving colloidal particles. According to XRD study, the deposited thin film material is the pure orthorhombic modification of bismuth(III) sulfide with stibnite type of structure, crystallizing in the Pbnm  (D2h16) space group. Using the technique of X-ray line broadening, the estimated average crystal radius of as-deposited studied thin film material is 9.0 nm. Upon thermal treatment (for 10 min at 250 °C), which led to stronger coalescence and crystal growth, this value increases to 24.2 nm. Evolution of the lattice parameters, lattice strain, dislocation density and optical properties of the films upon thermal treatment is followed and discussed. The strain relaxation upon thermal treatment occurs quite anisotropically. The optical band gap energy was determined using the Fermi's golden rule for fundamental interband electronic transitions in case of a parabolic energy structure. The blue shifted band gap energy of as-deposited thin film compared to the corresponding bulk value results from size quantization characteristics of obtained nanocrystalline particles in thin film form. The manifested 3D confinement effects were analyzed within the framework of the effective mass approximation model. On the basis of the currently presented experimental data and the effective mass approximation, an estimation of the hole effective mass in the case of the studied material was made. The decrease of the magnitude of the absorption coefficient upon thermal treatment was also analyzed, along with the possible interdot electronic coupling in the case of as-deposited bismuth sulfide thin films.