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

Abstract

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.

First-Principles Calculations of Stable Geometric Configuration and Thermodynamic Parameters of Cadmium Sulfide Thin-Film Condensates

“…Comparison of the values of Gibbs energies ΔG for sphalerite and wurtzite modifications indicates a transition from cubic to hexagonal modification at 320.74 K. It should be noted that the data [44] indicate the existence of both phases at a temperature of 367.15 K, so the ab initio methods quite accurately determines the temperature of the phase transition.…”