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

• Nanocrystalline zinc blende and wurtzite ZnS phases produced by mechanical alloying.
• Sulfur and/or zinc vacancies in the ZnS as-milled.
• Magnetic and semiconductor behavior for the as-milled ZnS-10h.
• Irreversible demagnetization and Curie temperature above room temperature.
• Structural stability after annealing and aged of the ZnS-10h sample.

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 3 h of milling and remained until 10 h 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 10 h and aged for eighteen months and of the sample milled 10 h 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 3.4 eV and 3.9 for ZnSZB and ZnSWZ phases, respectively. The sample milled for 10 h showed low blue photoemission intensity centered at 470 nm and thermal diffusivity around 0.02 cm2/s.