Synthesis, characterization and luminescent mechanism of ZnS:Mn ²+ nanophosphor

[摘要] ZnS:Mn2+ is commercially used in field emission displays (FEDs) and biological imaging ofbrain tumors. This study was done to determine the luminescent mechanism of both bulk andnano sized ZnS:Mn2+.Luminescent zinc sulphate doped with manganese (ZnS:Mn2+) nanoparticles were synthesizedvia a chemical precipitation method. These nanoparticles were embedded in an amorphous silica(SiO2) matrix by a sol-gel process. The prepared nanocomposite materials were then crushed intopowders, sieved and annealed at 600 °C in air. The morphology of the samples was determinedby scanning electron microscopy (SEM) and the chemical composition was analyzed by energydispersive x-ray spectroscopy (EDS). The crystal structure, morphology and particle sizes ofZnS:Mn2+ and SiO2-ZnS:Mn2+ nanoparticles were determined with x-ray diffraction (XRD) andtransmission electron spectroscopy (TEM). Both the cubic zincblende crystal structure for ZnSand the hexagonal wurtzite crystal structure for ZnO were found. The particle sizes for the unannealedsamples estimated from the XRD peaks and the TEM images were 2 �?4 nm indiameter.Absorption measurements were performed on the ZnS:Mn2+ samples. All the samples wereabsorbing in the UV range between 280 - 340 nm. The band gap of the samples was obtainedfrom the absorption data and it was found to be 4.1 ± 0.2 eV. It is blue-shifted from that of bulkparticles by 0.4 eV. This blue-shift can be attributed to quantum confinement effects in thecrystal. The mean particle radius was also obtained from the absorption data and it was found tobe 1.5 ± 0.1 nm. This corresponds well to the values obtained from XRD and TEM.The ZnS:Mn2+ and SiO2-ZnS:Mn2+ powders were irradiated with a 325 nm (He-Cd) laser and a15W Xenon flash lamp for photoluminescence (PL) measurements. Two emission peaks at 450nm (blue) and 600 nm (orange) were observed. The excitation peak was blue shifted from 340nm to ~ 300 nm. This blue-shift can be attributed to the increase in the band gap of thenanoparticles caused by quantum confinement effects. A proposed luminescent mechanism forZnS, ZnS:Mn2+ and ZnO is discussed. The blue emission (450 nm) associated with ZnS can beattributed to the hole trapping and recombination with electrons by defect states (zinc or sulphurvacancies) in ZnS. The orange emission at 600 nm for nano particles can be attributed to the 4T1 �?A1 transitions of Mn2+ ions. These transitions are explained in terms of the Tanabe-Sugano diagrams for the d5 level, the Russell Saunders coupling scheme and the Ligand fieldtheory. 6A1 is the ground state of Mn2+, while 4T1 is one of the excited states. For the annealedsamples a broad peak with a maximum at 550 nm (green) was observed. In the case of ZnO theemission is due to hole capturing and recombination with electrons by defect states.Commercial ZnS:Mn2+ powder were subjected to 2keV electron beam irradiation in a vacuumchamber at a pressure of 1 x 10-8 Torr for 24 hours. The cathodoluminescence (CL) intensity wasmeasured with a S200/PC2000/USB2000/HR2000 spectrometer and it showed an emission peakat ~ 600 nm. This emission is attributed to the 4T1-6A1 transitions of Mn2+ ions. Changes in thechemical composition of the surface together with the corresponding changes in the CL intensitywere investigated using Auger electron spectroscopy (AES), the CL spectrometer and a residualgas analyzer. The data showed a decrease in sulphur and carbon on the surface of the sample,while there was an increase in oxygen. The CL intensity decreased simultaneously with thedecrease of the sulphur Auger peak-to-peak height. This may be due to the formation of volatileSOx and a non-luminescent ZnO or ZnSO4 layer on the surface according to the electronstimulated surface chemical reaction (ESSCR) degradation mechanism.

[发布机构] University of the Free State