Effect of precursor molar ratio of [S2-]/[Zn2+] on the lifetime of ZnS nanocrystals

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.