Synthesis, characterization and luminescent mechanism of ZnS:Mn²+ nanophosphor
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ZnS:Mn2+ is commercially used in field emission displays (FEDs) and biological imaging of brain tumors. This study was done to determine the luminescent mechanism of both bulk and nano sized ZnS:Mn2+. Luminescent zinc sulphate doped with manganese (ZnS:Mn2+) nanoparticles were synthesized via 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 into powders, sieved and annealed at 600 °C in air. The morphology of the samples was determined by scanning electron microscopy (SEM) and the chemical composition was analyzed by energy dispersive x-ray spectroscopy (EDS). The crystal structure, morphology and particle sizes of ZnS:Mn2+ and SiO2-ZnS:Mn2+ nanoparticles were determined with x-ray diffraction (XRD) and transmission electron spectroscopy (TEM). Both the cubic zincblende crystal structure for ZnS and the hexagonal wurtzite crystal structure for ZnO were found. The particle sizes for the unannealed samples estimated from the XRD peaks and the TEM images were 2 – 4 nm in diameter. Absorption measurements were performed on the ZnS:Mn2+ samples. All the samples were absorbing in the UV range between 280 - 340 nm. The band gap of the samples was obtained from the absorption data and it was found to be 4.1 ± 0.2 eV. It is blue-shifted from that of bulk particles by 0.4 eV. This blue-shift can be attributed to quantum confinement effects in the crystal. The mean particle radius was also obtained from the absorption data and it was found to be 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 a 15W Xenon flash lamp for photoluminescence (PL) measurements. Two emission peaks at 450 nm (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 the nanoparticles caused by quantum confinement effects. A proposed luminescent mechanism for ZnS, ZnS:Mn2+ and ZnO is discussed. The blue emission (450 nm) associated with ZnS can be attributed to the hole trapping and recombination with electrons by defect states (zinc or sulphur vacancies) in ZnS. The orange emission at 600 nm for nano particles can be attributed to the 4T1 →6A1 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 field theory. 6A1 is the ground state of Mn2+, while 4T1 is one of the excited states. For the annealed samples a broad peak with a maximum at 550 nm (green) was observed. In the case of ZnO the emission 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 vacuum chamber at a pressure of 1 x 10-8 Torr for 24 hours. The cathodoluminescence (CL) intensity was measured with a S200/PC2000/USB2000/HR2000 spectrometer and it showed an emission peak at ~ 600 nm. This emission is attributed to the 4T1-6A1 transitions of Mn2+ ions. Changes in the chemical composition of the surface together with the corresponding changes in the CL intensity were investigated using Auger electron spectroscopy (AES), the CL spectrometer and a residual gas 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 the decrease of the sulphur Auger peak-to-peak height. This may be due to the formation of volatile SOx and a non-luminescent ZnO or ZnSO4 layer on the surface according to the electron stimulated surface chemical reaction (ESSCR) degradation mechanism.