Effect of temperature-dependent phase transformation on UV detection properties of zinc sulfide nanocrystals

Abstract

In this work, design and optimization of ultraviolet sensors based on ZnS nanoparticles with the focus on the effect of annealing temperature on optical properties of zinc sulfide were carried out. Synthesis of ZnS nanoparticles was performed using a chemical deposition method and annealing process was followed at 300 °C, 500 °C and, 700 °C under flow of argon. Scanning electron microscopy indicated that by increasing the annealing temperature, the particle size of ZnS nanostructures is increased. Differential thermal analysis showed that the temperature range of phase transformation in ZnS nanostructures is at 300 °C–450 °C. X-ray diffractometry demonstrated a phase change from zinc blende to hexagonal wurtzite at the temperature of 500 °C. The UV-visible spectra of the ZnS samples indicated an increase in light absorption of ZnS nanostructures regarding to the phase transformation at 500 °C. Photoluminescence (PL) study also demonstrated a slight blue-shift in PL peak position of the sample that was annealed at 500 °C which is attributed to the higher optical band gap of this sample. Moreover, the intensity ratio of UV to visible peak is the highest for this sample showing an enhancement in optical properties of ZnS nanostructures which is suitable for fabrication of visible-blind UV detectors. For studying the influence of annealing temperature on optoelectrical properties of UV sensors based on ZnS nanostructures, metal-semiconductor-metal devices were fabricated using deposition of interdigitated Schottky contacts on different ZnS samples. The suitable optical and structural properties of the optimum sample including increasing the electron-hole pairs under UV illumination was manifested in optoelectrical characteristics of ultraviolet detectors based on ZnS sample which was annealed at 500 °C. For this sample, the sensitivity under UV radiation was enhanced by 6.9 times compared to the other ZnS samples.