Preparation by wet chemical method and some of properties of ZnS and ZnS : Mn2+ nano materials

Journal of Science of Hanoi National University of Education Natural sciences, Volume 52, Number 4, 2007, pp. 72- 76 preparation by wet chemical method and some of properties of ZnS and ZnS : Mn2+ nano materials Tran Minh Thi, Nguyen Minh Thuy, Pham Kim Tuyen Faculty of Physics, Hanoi University of Education 1 Introductions Zinc sulphide (ZnS) is important II-VI semiconducting material with a wide direct band gap of 3.65 eV in the bulk [1]. It has potential applications in optoelectronic devices such as blue emitting diodes [2], electroluminescent devices and photovoltaic cells [3]. Semicon- ductors nanoparticles have quantum size effects and non-linear optical characteristics as well as high quantum efficiency, making them next-generation materials that can be ap- plied in various optoelectronic devices. Semiconductor nanoparticles have attracted much attention because of their novel electric and optical properties originating from surface and quantum confinement effects. Manganese- doped materials represent a class of phosphors that have already found their way into applications. The transition within the 3d5 configuration of the divalent man- ganese ion (Mn+2) has been studied extensively and its orange- yellow luminescence in ZnS is well documented. This luminescence was also observed in nanocrystalline ZnS :Mn2+[4] and applications have already been suggested [5]. Different types of Mn2+ centers are present in nanocrystalline ZnS :Mn2+[6]. The orange luminescence originates fromMn2+ ions on Zn2+ sites, where the Mn2+ are tetrahedral coordinated by S2−. Many articles have reported on factors influencing the quantum efficiency of nanoparticles ZnS : Mn2+ [6,7]. The luminescence quantum efficiency is important for the potential use of nanocrystalline ZnS : Mn2+ in light emitting devices where a high luminescence quantum efficiency is required. 2 Experimental ZnS and ZnS:MnS nanoparticles are prepared by wet-chemical method. This method was used chemical substances Zn(CH3COO)2.2H2O,Mn(CH3COO)2.4H2O andNa2S.9H2O and mix CH3OH : H2O. Firstly, 0,1 mol Zn(CH3COO)2.2H2O was dissolved in the buffer acetate CH3COOH (pH = 3.5), solution contained 0,1 mol Na2S was added drop by drop in a reaction vessel. A level of pH plays an important role in the precipitate of ZnS and ZnS : Mn2+. The 72 Preparation by wet chemical method reactions were happened as follows: Zn(CH3COO)2 +Na2S → ZnS ↓ +2CH3COONa Mn(CH3COO)2 +Na2S → MnS ↓ +2CH3COONa The theoretical calculation shows that pH = 3.5 may be choosen to make precipitate of ZnS and ZnS :Mn2+ in the mixed solution, without precipitating of Zn(OH)2. This solution was constantly mixed by a homogenny during the entire process. The precipitate was separated by centrifugation at 2500 rpm and rinsed by mixer CH3OH : H2O(1 : 1ratio) in several times. Then all the rinsed samples were dried in low pressure (10 mmHg) at 40oC for 48 hours. The ZnS : Mn2+ samples were produced with correlative concentration of Mn2+: 0.0 at%; 0.25at%; 0.75at%; 1at%, 1.5at%; 2.5at%, by wet-chemical method. For an qualitative analysis of ZnS, we used optical measurement at pH = 6.0 and test substances of blue methylthimol with maximum absorb wavelength λ = 592nm. The results of analysis show that the content of ZnS in samples achieved > 97% of volume The structure and crystallinity were characterized and analyzed by X- ray diffraction (SIEMENS D5005). The photoluminescence spectra were recorded with a by fluorescence spectrophotometer HP340-LP370 using laser that has excitation wavelength 325nm, at room temperature. Photoluminescence spectral range was chosen 360-880 nm. The average particle size was measured by scanning electron microscope (SEM) 3 Results and discussion Figure 1 shows the XRD patterns of ZnS:Mn2+ powders with correlative concen- tration of Mn2+: 0.0 at %, 1.0 at%. The samples ZnS-M5 (0%) and ZnS-M13 (0%) were equally concentration of Mn2+, but they are dried at different temperatures. 73 TRAN MINH THI, NGUYEN MINH THUY, PHAM KIM TUYEN Order Sample d (111) a (Ao) 1 ZnS M5 (0%) 3.110 5.387 2 ZnS M13 (0%) 3.104 5.376 3 ZnS:Mn 1% 3.098 5.366 Table1: Parameters d, a of samples ZnS M5 (0%), ZnS M13 (0%), ZnS:Mn 1% We can see that ZnS nanoparticles present the sphalerite phase and they have good crystallinity. The locations, widths as well as intensities of the XRD peaks are nearly the same for all samples. The table 1 shows the lattices of samples are slightly decreased and seemed invariable at different dried temperatures and concentration of Mn2+. The average crystalline sizes are calculated by the width of the XRD peak at (111) plane in the light of Scherre's equation . The average crystalline sizes of samples are about 25 nm. Figure 2 shows SEM images of ZnS : Mn2+ sample (1at%Mn2+) . It is clear that the grains in sample are not uniform. The diameters of the grains are mainly distributed in the range of Figure 3 shows the room temperature photoluminescence of pure ZnS and ZnS : Mn2+ (the concentration of Mn : 0.0%; 0, 25%; 0, 5% and 1% correspond with d, c, b and a curves). Figure 4 shows the room temperature photoluminescence of ZnS : Mn2+0.25at%. In this sample we observe one band centered at 500 nm. The PL of ZnS : Mn2+0.25at% is analysed using a Gaussian fit into two peaks centered at 450 and 592 nm, respectively. This emission at 592 nm may be due to de-excitation of Mn2+ ion in the ZnS, but intensity of excitation is small because concentration ofMn2+ is very little in this sample. 74 Preparation by wet chemical method This result PL of ZnS : Mn2+2.5at% in figure 5 shows the intensity of peak at 592 is stronger. [8] shows the luminescence at 460 nm caused by a de-excitation of Mn2+ ion in the ZnS matrixrelated from the transition. As showed in fig. 3 the luminescence of samples are appeared at 460 nm position. It is felt that luminescence at this wavelength is indeed only native impurity related. [9] has noted that the emission peak centered at 465 nm. It is attributed to the self-activated emission caused by Zn vacancies in the lattice. The position of this PL peak remaines unchanged in position with increasing Mn concentration. We suppose that the emission peak centere at 460 nm is attributed to the self-activated emission The PL peak observed at near 600nm for Mn doped samples was interpreted due to an indirect excitation, i.e., an excitation into the excited state of the host matrix ZnS, followed by an energy transfer from the host to the Mn2+ ions and its subsequent de-excitation leading to luminescence. This peak also did not show any systematic shift in position. It is associated with the transition of Mn2+ in ZnS. In figure 4 we can see that the PL intensity increases with increasing Mn concentration. The enhanced PL may be due to Mn2+ assisted luminescence in the ZnS host; this evidenced an incorporation of Mn2+ in ZnS. 4 Conclusion We have successfully prepared Zn1−xMnxS powder (x = 0 ÷ 2.5%) by wet-chemical method. XRD patterns indicate that the samples are single. The average particle size was calculated to be about 25 nm. The room temperature PL of samples growed with increasing Mn concentration. The form of the spectra suggested the presence of two Gaussian peaks (460 and 600 nm respectively). The observed orange luminescence originates from Mn+2 ions on Zn+2 sites, where the Mn+2 is tetrahedral coordinated by S2− this evidenced an incorporation of Mn2+ in ZnS. The enhanced PL and orange emission of Mn doped samples show an application of these materials. 75 TRAN MINH THI, NGUYEN MINH THUY, PHAM KIM TUYEN 5 Acknowledgements The authors would like to thank Hanoi University of Education. This work was supported by the Natural Science Council of Vietnam. T i li»u [1] .D. 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