Phase transformation from cubic ZnS to hexagonal ZnO by thermal annealing

Abstract

We have investigated the mechanism of phase transformation from ZnS to hexagonal ZnO by hightemperature thermal annealing. The ZnS thin films were grown on Si (001) substrate by thermal evaporation system using ZnS powder as source material. The grown films were annealed at different temperatures and characterized by X-ray diffraction (XRD), photoluminescence (PL), four-point probe, scanning electron microscope (SEM) and energy dispersive X-ray diffraction (EDX). The results demonstrated that as-deposited ZnS film has mixed phases but high-temperature annealing leads to transition from ZnS to ZnO. The observed result can be explained as a twostep process: (1) high-energy O atoms replaced S atoms in lattice during annealing process, and (2) S atoms diffused into substrate and/or diffused out of the sample. The dissociation energy of ZnS calculated from the Arrhenius plot of 1000/T versus log (resistivity) was found to be 3.1 eV. PL spectra of as-grown sample exhibits a characteristic green emission at 2.4 eV of ZnS but annealed samples consist of band-to-band and defect emission of ZnO at 3.29 eV and 2.5 eV respectively. SEM and EDX measurements were additionally performed to strengthen the argument.

Synthesis and Characterization of Multi-Angular Branched ZnO Microstructures and Peculiar Nanopushpins Obtained by Thermal Treatment from Compacted ZnS Powder

“…Above ~1000 °C the stable phase is wurtzite, and below this temperature zincblende is stable, however, the mutual transformation is very slow, leaving wurtzite phase metastable at room temperature [19]. In the case of nanostructures, it has been reported that the transition to occur at a lower temperature [20,21]. Preliminary inspection of the XRD patterns revealed no diffraction peaks that could be attributed to other phases than the cubic ZnS-type (JCPDS Card no.…”

“…The growth process can be explained two-step process: (i) formation of ZnO nanoparticles and (ii) axial growth of ZnO nanoparticles to form nanostructures. The detailed growth process is as follows during annealing process: in the first step, Zn-S bonds are broken and oxygen atoms from the atmosphere get high enough energy; in the second step, the oxygen atoms replaced the sulphur atom in the lattice and S to form Zn-O bonds [21]. As a wurtzite-structured metal oxide, ZnO belongs to the P63mc space group and has a highly anisotropic structure along the c-axis.…”