Tunable bandgap and blue emission of ZnS nanoparticles induced by controlled S vacancies

Poly(vinylpyrrolidone) capped ZnS semiconductor nanoparticles with a controlled amount of S vacancies have been synthesized through the one step hydrothermal method at low temperatures by varying the concentration of thiourea, the sulfur reagent. The hexagonal wurtzite structure of ZnS with particle like morphology has been identified from powder X-ray diffraction, field emission scanning electron microscopy, and transmission electron microscopy. The formation of compound ZnS and elemental purity has been studied from energy dispersive spectra. The role of S vacancy, the influence of lattice strain and surface defect on the emission, as well as the variation of emission intensity with different excitation wavelengths have been studied using a photoluminescence spectrometer. With an increase in the excitation wavelength, a variation intensity and a red shift in the peak position due to different contributions of free excitonic emissions and its phonon replicas have been noticed and discussed in detail. Raman spectroscopy confirms the formation of ZnS nanoparticles; the shift and broadening in Raman spectra have been studied in detail. The spectral shift and the tunable bandgap of ZnS with S vacancy have been explained on the basis of crystallinity, crystallite size, particle size, lattice strain, and defects on the surface.