White Emission And X-Ray Luminescence Properties Of ZnS Fine Particles Synthesized By Hydrothermal Method

Based on the urgent requirement of white light emission (n) UV LEDs phosphors and X-ray nano-phosphors with high luminescent efficiency, this paper focuses on the syntheses and study on luminescence properties of ZnS white light and X-ray phosphors activated with different dopants. The major works are shown as follows:Spherical ZnS:Cu,Al nanocrystals with high luminescence intensity were synthesized by a hydrothermal method. The synthesized nanocrystals show single cubic structure and well dispersity with average grain size of about 15nm. The effects of the molar ratio of ns/nZn,nCu/nAland surfactant on the photoluminescence (PL) spectra of rinsed and unrinsed samples under 343nm excitation were investigated systematically. The results indicate that with the increase of ns/nZn the PL intensity increases remarkably, and the PL intensites of unrinsed samples are stronger than that of rinsed ones. Moreover, the PL intensities of unrinsed samples are increased more significantly under a low ratio of nS/nZn. The PL intensity has a relation to the surfactants, this suggests that surface state of nanomaterials can also affect the PL intensity. In our experiment, the optimal condition is the unrinsed sample under n(Cu/Zn)=0.0003, nS/nZn=3, n(Cu/Al)=0.5, and the bright green light can be observed with the naked eye in the daytime.The energy dispersive X-ray spectroscopy (EDX) and atomic absorption spectrometry were applied to the analysis of S, Zn and Cu content in the sample. The results proved that a large number of zinc vacancies exist and Cu is incorporation into the sample lattice. The excitation spectrum is broad. Under 370-410nm excitation the sample emits white light. The broad emission spectra are almost coincident with any excitation wavelength of between 370-410nm making them attractive as conversion phosphors for LED application and full-color fluorescent display devices. The emitted white light under 375nm excitation was found to be the result of blue, green, and orange emission bands. For nCu/nZn, nCu/nAl and nS/nZn molar ratios of 0.0003,2 and 3, respectively, the near blue white light can be observed with the naked eye in daylight.ZnS:Cu,Al, ZnS:Au,Cu and ZnS:Cu,Tm fine X-Ray phosphors were successfully synthesized using a hydrothermal method combined with heat treatment technology. The experimental results are shown as follows:1) For ZnS:Cu,Al, the PL and XEL spectra of all of the samples show a broad emission band. The maximum PL and X-ray excited luminescence (XEL) intensities are observed for sample with nCu/nZn=0.0003 and nCu/nAl=0.5. The XEL peak center is at about 470nm. In this condition, the XEL intensity of the sample which was hydrothermally treated at 200℃for 3h then annealed in Ar at 800℃for 1h is enhanced and the peak is located at 520nm. In the mean time, the XEL intensity is about eight times by comparison with that of the samples without annealing. The particles agglomerate size is about 200-500nm and roughly spherical particles show pure hexagonal structure. The Samples with highly luminescence efficiencies and the smaller size could enhance the resolution of imaging systems. By comparing the PL and XEL spectra, the luminescence mechanism and different excitation mechanism of PL and XEL and effect of annealing on XEL have been discussed.2) For ZnS:Au,Cu, the PL and XEL spectra of all of samples show a broad emission band. The maximum XEL intensity of sample directly synthesized by hydrothermal treatment is observed for sample with nCu/nZn=0.00003 and n(Cu/Au)=0.5. The XEL peak center is at about 459nm. In this condition, the strongest XEL emission is observed for the direct synthesized sample further annealing in Argon at 1000℃for 1h sample and the XEL peak was centered at about 445 and 513nm, respectively. In the meantime, the XEL intensity increased about ten times by comparison with directly synthesized without annealing. The particles agglomerate size is about 1-2μm and roughly spherical particles show pure hexagonal structure. In addition, with the increase of Cu2+ content, we observe the red shift of XEL spectra both in ZnS:Cu,Al and ZnS:Au,Cu without annealing samples.3) For ZnS:Cu,Tm, the effects of hydrothermal treatment time on the structure, morphology and XEL spectrum of ZnS:Cu,Tm nanocrystals were studied. It is found that with the prolonging of hydrothermal treatment time the particle size and crystallization are increased, while the intensity of XEL is decreased. The analyzed results show that the decrease of XEL intensity can be attributed to the surface state. Based on the above results, it is suggested that the long hydrothermal treatment time is not beneficial to the XEL intensity of ZnS:Cu,Tm nanocrystals, and the optimal treatment time is 12h. The maximum XEL intensity of sample directly synthesized by hydrothermal treatment is observed for nCu/nZn=0.0003 and nCu/nTm=0.5. the optimal annealing temperature is 900℃. After annealing at this temperature, the particle size is 200-600nm. the sample with pure hexagonal structure shows the stronger XEL spectrum with emission peaks at 453nm and 525nm, the XEL intensity increased about eleven times by comparison with directly synthesized without annealing.