Preparation and luminescence properties of ZnS:Cu nanocrystals

ZnS is a wide direct band gap of II-VI compound, which is a widely used matrix in various luminescent materials. ZnS materials doped with different rare metal ions and transition metal ions (Mn, Cu, Ag) formed by different techniques have been studied widely in recent years. Compared to other ions-doped ZnS, Cu-doped ZnS is rarely studied by researchers, especially as photoluminescence nanoparticles. This paper introduces a method of hydrothermal used to prepare ZnS:Cu nanocrystals. And on this base, the effections of reaction time and the mole ratio of zinc and sulfur on the luminescence properties of ZnS:Cu nanocrystals are investigated. X-ray diffractometer and transmission electron microscope are used to characterize the phase and the surface morphology of samples. The results show that ZnS:Cu nanocrystals have cubic zinc blende crystal structure, and the size of these spheroidal particles is in a range of 1 to 6nm. At room temperature, the luminescence properties are investigated with PL spectra and PLE spectra. In PL spectra, emission peaks are at about 542 nm, which may be attributed to a transition from shallow donor state of ZnS to the t2level of Cu in ZnS band gap. In PLE spectra, excitation peaks are at about 350 nm, which are attributed to the near-band-edge absorption of ZnS matrix. Luminescence intensity increases, and then decreases with the mole ratio of zinc and sulfur increasing. When the mole ratio of Zinc and Sulfur is 2:1, luminescence intensity touches its maximum. When excitation wavelength is 350 nm, ZnS:Cu nanocrystals give off green band emissions from 510-542 nm, according to the extend of reaction time. And the highest intensity appears in the sample which has reacted for 8 h. Following this method, ZnS:Cu nanocrystals giving green luminescence can be prepared. It can be deduced that shallow donor state of ZnS:Cu nanocrystals is concerned with sulfur vacancies. The numbers and the levels of sulfur vacancies are affected by the reaction time and the mole ratio of Zinc and Sulfur.