Optical properties of CdS nanocrystalline thin films in the abrupt phase transition from zinc blende to wurtzite

A set of cadmium sulfide (CdS) thin films was grown on glass substrates by the chemical bath deposition technique at different bath temperatures (T_b). A microwave oven was used to heat the precursor aqueous solution employed to prepare the films in the 60–97 °C interval. The average crystallite size of the CdS films lies in the 7–20 nm range, calculated from X-ray diffraction data. The diffraction patterns reveal that the crystalline structure of CdS nanoparticles is cubic zinc blende (ZB) for 60 ≤ T_b ≤ 93 °C, hexagonal wurtzite (WZ) for 95 ≤ T_b ≤ 97 °C, and ZB-WZ mix of phases for the critical temperature T_b ≅ 94 °C (T_bc). The mixture of both phases is supported by Transmission Electron Microscopy. The CdS films show preferred orientation along (111) and (002) directions for ZB and WZ, respectively. The optical properties reveal significant changes at T_bc, namely, the energy band gap, photoluminescence emission, and refractive index. The photoluminescence results show an additional band at the critical phase transition temperature due to the presence of a high-density Cd interstitial/vacancies, produced by the mix of phases. Furthermore, high-energy transitions above the conduction band also exhibit splitting due to the phase transition.