Thermal stimulated structural transformation of cubic Zn0.78Cd0.22S nanoparticles to ZnO nano-hexagons: Tailoring of optical band gap and emission spectra for optoelectronic implementations

Abstract

Ternary alloys of Zn0.78Cd0.22S nanoparticles (NPs) are synthesized via the facile co-precipitation technique. The as-synthesized sample exhibits a zincblende-type cubic phase with an average crystalline size of 2 nm. Thermal annealing and UV irradiation are utilized as post-treatment processes for tailoring the optical properties of Zn0.78Cd0.22S NPs. The as-synthesized sample exhibits a stable cubic phase up to 400 °C, during which partial phase transformation to a hexagonal structure is observed at a higher temperature of 500 °C. The oxidation of Zn0.78Cd0.22S NPs to mixed oxide phases with the majority of ZnO begins at 600 °C which induces morphology transformation to a relatively large nano-hexagon with a single crystalline domain size. The increase of the annealing temperature is accompanied by a decrease of Zn0.78Cd0.22S NPs optical band gap (Eg) due to the weakness of size confinement as well as the formation of localized states below the mobility band edges. The brief UV irradiation results in the increase of Eg whereas a further increase in exposure time is accompanied by a reduction of Eg. The photoluminescence (PL) spectrum of the as-synthesized sample covers a wide spectral range from UV to visible. Thermal annealing has a slight effect on the PL emission at high excitation energy, whereas low excitation energy reveals higher sensitivity to deep-state emission. Thermal oxidation incorporates a high concentration of oxygen-related defects that induce strong enhancement in the green emission at the expense of UV emission. This indicates the thermal-induced bleaching of shallow trapping sites and the evolution of other deep trapping or recombination sites.