Undoped ZnS nanocrystals(NCs) with different precursor molar ratios of [S2-]/[Zn2+] are prepared by the chemical precipitation method.The structural and optical properties of the samples are characterized by the X-ray diffraction(XRD) spectra,photoluminescence(PL) spectra and PL decay spectra.The XRD analysis shows that the crystal quality of ZnS NCs becomes better and the grain size is larger at higher [S2-]/[Zn2+] ratios.The PL peaked at 430 nm decreases with the [S2-]/[Zn2+] ratio increasing,which is ascribed to the structure defects of NCs.A multi-exponential decay time curve with hundreds of picoseconds,several nanoseconds and tens of nanoseconds is obtained,which also shows a distinct and regular change with [S2-]/[Zn2+] ratio.It is indicated that the PL and emission decay properties of ZnS NCs mainly depend on the change of the defects number from different particle sizes.
