Size-induced transition-temperature reduction in nanoparticles of ZnS

X-ray-diffraction studies of nanometer-sized particles of zinc sulfide show a significant reduction in the zinc-blende-to-wurtzite phase transition temperature, as compared to the bulk value. Five nanoparticle samples were annealed in vacuum at temperatures increasing from room temperature (23 °C) to 500 °C. Post-anneal analyses revealed an increase of crystallite size, accompanied by a partial transformation from the cubic, zinc-blende structure to the hexagonal, wurtzite structure at temperatures as low as 400 °C. This is significantly less than accepted bulk transition temperature of 1020 °C. The particles also show some lattice distortion with decreasing particle size and there is a monotonic reduction in the specific volume of about 2.3% as the particle size decreases from about 240 to about 30 Å.