Structural, thermal, magnetic and optical characterization of undoped nanocrystalline ZnS prepared by solid state reaction

Abstract

Nanocrystalline zinc blende and wurtzite ZnS phases with sulfur and/or zinc vacancies were obtained from a mechanically alloyed Zn50S50 powder mixture. Structural, thermal, magnetic, optical and photoacoustic studies were carried out using X-ray diffraction, transmission electron microscopy, differential scanning calorimetry, vibrating sample magnetometer, UV–Vis absorption, photoluminescence and photoacoustic spectroscopy techniques. The cubic zinc blende (ZnSZB) and hexagonal wurtzite (ZnSWZ) phases were nucleated in 3h of milling and remained until 10h when the milling process was stopped. The coexistence of these two phases was confirmed by high resolution transmission electron microscopy. X-ray diffraction measurements attested the structural stability of the sample milled for 10h and aged for eighteen months and of the sample milled 10h and annealed at 300°C and 600°C. Differential scanning calorimetry measurements showed the unreacted sulfur in molecular form (rings and/or chains). Magnetic behavior was observed for as-milled sample and Curie temperature was estimated at 430°C. Moreover, an irreversible behavior of magnetic properties was observed and correlated with changes on the structural vacancies densities. The UV–Vis absorption spectra and McLean analysis showed an optical band gap around 3.4eV and 3.9 for ZnSZB and ZnSWZ phases, respectively. The sample milled for 10h showed low blue photoemission intensity centered at 470nm and thermal diffusivity around 0.02cm2/s.