ENHANCED PHOTOLUMINESCENCE PROPERTIES OF Mn 2+ DOPED ZnS NANOPARTICLES

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1 Chalcogenie Letters Vol. 8, No. 3 March 2011, p ENHANCED PHOTOLUMINESCENCE PROPERTIES OF Mn 2+ DOPED ZnS NANOPARTICLES C. S. PATHAK, M. K. MANDAL * Department of Physics, National Institute of Technology, Durgapur , Inia The room-temperature photoluminescence (PL) of manganese ope zinc sulphie (ZnS:Mn 2+ ) nanoparticles have been investigate. These Mn 2+ ope ZnS nanoparticles have been synthesize by chemical precipitation metho at room temperature. It has been foun that passivation with acrylic aci (AA) yiele higher photoluminescence intensity. So-prepare Mn 2+ ope ZnS nanoparticles then characterize by using X-ray iffraction (XRD), Scanning electron microscope (SEM), UV-Vis spectrophotometer. The size of the particle is foun to be in nm range. The value of ban gap energy has been foun to be in range ev. Room temperature photoluminescence (PL) spectrum of the unope sample exhibit a blue emission peake at 465 nm uner UV excitation whereas the Mn 2+ ope sample exhibits a yellow emission peake at 574 nm uner same UV excitation. (Receive February 4, 2011; accepte February 22, 2011) Keywors: ZnS, Nanoparticle, XRD, UV-Vis, Optical absorption, Ban gap. 1. Introuction Luminescent semiconuctor nano-crystals, especially II VI semiconuctors, have attracte great eal of attention in the past few ecaes ue to their unique properties an potential applications. The most extensively investigate ope semiconuctors are Mn ope ZnS or CS nano-crystals. The oping ions act as recombination centers for the excite electron hole pairs an result in strong an characteristic luminescence. This II VI compoun semiconuctor material has been stuie for a variety of applications, such as optical coating, electro-optic moulator, photoconuctors, fiel effect transistors, optical sensors, phosphors, an other light emitting materials. Zinc sulphie ope with transition metal ion is known to have efficient light emitting properties [1]. In ope compoun semiconuctors, in contrast to the unope semiconuctors, the impurity states can play a special role in affecting the electronic energy structures an transition probabilities [2]. Bhargava et.al. [3] reporte that Mn ope nanocrystalline semiconuctors yiel both high luminescent efficiencies an life time shortening. They have also shown that the quantum efficiency increases with ecreasing particle size. The objective of this work is to stuy the luminescence characteristics of Mn 2+ ope ZnS nanoparticles prepare by a surface passivation technique. To achieve the luminescence enhancement, Acrylic aci (AA) is use as a surface passivators after chemical precipitation of Mn 2+ ope ZnS nanoparticles at room temperature. AA is chosen as a surface passivation legen because it has a carboxylic group in its structure, which has potential to enhance PL. Mn 2+ ion sites enhance the photoluminescence of the system ue to the - transition through energy transfer from the S 2- vacancies to the AA when excite by UV light [4]. The size of the prepare ZnS an ZnS:Mn nano crystallites are calculate an it is foun in the range of nm. The PL emission takes place in the yellow region for Mn ope ZnS sample with a peak at 574 nm whereas the unope ZnS sample emits in the blue region. * Corresponing author: nitmkm@yahoo.co.in

2 Experimental 2.1. Sample Preparation The important step in the stuies of nanoparticle is their synthesis. We have use here chemical precipitation metho to synthesize the samples at room temperature using Zn(CH 3 COO) 2.2H 2 O, Mn(CH 3 COO) 2.4H 2 O, an Na 2 S.9H 2 O as source materials. The chemical precipitation metho is the most popular technique because of its several avantages like easy haneling, simpler an with low-cost chemical compouns an it is suitable for inustrial large scale prouction [5]. We prepare 50 ml solution by mixing 2.195gm Zn(CH 3 COO) 2.2H 2 O an 0.049gm. Mn(CH 3 COO) 2.4H 2 O with stirring at room temperature for 15 min. Then 50 ml aqueous solution of gm Na 2 S.9H 2 O has been ae to the earlier solution rop by rop an mixe for 25 min with vigorous stirring resulting ull white colloial solution. After that appropriate amounts of acrylic aci are ae an mixe for 20 min. The power sample was then separate from the solvent an washe with methanol several times, after wash it is oven rie for 24 hours at 50 0 C. After rying the precipitate it is crushe in to fine power with the help of mortar an pestle. The power obtaine is then characterize Characterization techniques The size of all the samples are etermine by Phillips Expert Pro X-ray iffractometer with CuKα raiation (λ= nm). XRD ata are collecte over the range at room temperature. The particle size is calculate by using the Scherrer formula. The morphology of the nanoparticles is etermine using scanning electron microscopy (FESEM; QUANTA 3D FEG). Absorption spectra of the samples isperse in methanol are stuie with the help of U-3010 Spectrophotometer. The PL spectrum of the ZnS an ZnS:Mn nanoparticle has been measure at room temperature using Hitachi F-2500FL Spectrophotometer. 3. Results an iscussion 3.1. X-ray iffraction pattern The XRD patterns of ZnS:Mn nanoparticle samples are shown in Fig. 1. The crystalline size can be calculate with the help of Scherrer s equation mention below 0.9λ D = (1) β cosθ Where D is the mean grain size, λ is the X-ray wavelength (for CuKα raiation, λ = nm), θ is the iffraction angle an β is full with at half maximum. The grain size of the unope ZnS an 2% Mn ope ZnS nanoparticle as calculate by using Eq.(1) is in range of nm. Figure 1 shows the three iffraction peaks at 2θ values equalto , an which appears ue to reflection from the (111), (220) an (311) planes of the cubic phase of the ZnS. The obtaine peak positions correspon to zinc blene type patterns for all the samples. The XRD pattern of the nanocrystal is well matche with the Stanar cubic ZnS [6]. The broaening of the XRD pattern of the prepare ZnS:Mn sample takes place ue to the nanocrystalline nature of the sample. In this investigation we foun that with increasing amount of AA particle size is ecreasing as shown in Table - I.

3 149 Intensity (a.u.) c e b a (111) a - Unope ZnS b- 2% Mn Dope ZnS c- 2% Mn Dope ZnS with 0.1 mol AA - 2% Mn Dope ZnS with 0.5 mol AA e - 2% Mn Dope ZnS with 0.75 mol AA (220 ) (311) θ (eg.) Fig. 1. XRD pattern of ZnS an ZnS:Mn nanoparticle SEM analyses Fig. 2 shows the FESEM image of the unope ZnS an 2% Mn ope ZnS nanoparticles. The actual size of the nanoparticle can not be etermine from the FESEM images as it is limite by the resolution of the use FESEM instrument. a b c e Fig. 2. FESEM image of (a) unope ZnS, (b) 2% Mn ope ZnS, (c) 2% Mn ope ZnS with 0.1 mol AA, () 2% Mn ope ZnS with 0.5mol AA an (e) 2% Mn ope ZnS with 0.75 mol AA Optical absorption an ban gap The optical absorption spectra have been observe by using Hitachi U-3010 UV-Visible spectrophotometer an the results are shown in Fig. 3. To measure the absorption characteristics, the synthesize ZnS nanopowers are first isperse in methanol an then taken on a quartz

4 150 cuvette. From the optical absorption spectera of Mn 2+ ope ZnS nanoparticles it is clear that with increasing acrylic aci peak shifts towars shorter wavelength. Optical absorption spectera of Mn 2+ ope ZnS nanoparticles shows that the peak appears in the range of nm. These peak positions reflect the ban gap of nanoparticles an the synthesis ZnS nanoparticles have no absorption in the visible region ( nm). The relation between the incient photon energy (hν) an the absorption coefficient (α) is given by the following relation 1 ( αhν ) n = A( hν E ) g (2) E g Where A is constant an is the ban gap energy of the material an the exponent n epens on the type of transition. For irect allowe transition n = 1 2, for inirect allowe transition n = 2, for irect forbien n = 3 2 an for inirect forbien n = 3. Direct ban gap of the samples are calculate by plotting (αhν) 2 verses hν an then extrapolating the straight portion of the curve on hν axis at α = 0 as shown in Fig. 4. The ban gap energy of the samples is foun in the range ev as inicate in Table- I. The obtaine values of the ban gap of ZnS an Mn ope ZnS nanoparticles are higher than that of the bulk value of ZnS (3.68 ev). This blue shift of the ban gap takes place because of the quantum confinement effect [3]. Absorbance (a.u.) 2 1 a b c e a - Unope ZnS b- 2% Mn Dope ZnS c- 2% Mn Dope ZnS with 0.1 mol AA - 2% Mn Dope ZnS with 0.5 mol AA e - 2% Mn Dope ZnS with 0.75 mol AA Wavelength (nm) Fig. 3. Absorption spectrum of ZnS an Mn 2+ ope ZnS nanoparticles.

5 151 (a.u.) (αhυ) Unope ZnS 2% Mn Dope ZnS 2% Mn Dope ZnS with 0.1 mol AA 2% Mn Dope ZnS with 0.5 mol AA 2% Mn Dope ZnS with 0.75 mol AA hυ (ev) Fig. 4.Calculation of optical ban gap from the UV-Vis absorption spectra. Table- I: Particle size an corresponing ban gap energy of the synthesis nanomaterials. Sample Name Particle size (nm) Ban gap energy (ev) Unope ZnS % Mn ope ZnS % Mn ope ZnS with 0.1 mol AA % Mn ope ZnS with 0.5 mol AA % Mn ope ZnS with 0.75 mol AA Photoluminescence stuy of ZnS:Mn 2+ nanoparticles Photoluminescence (PL) of the ZnS:Mn samples are measure at room temperature using F-2500FL Spectrophotometer. The PL spectrum of Mn 2+ ope ZnS nanoparticles are shown in Fig. 5. The PL spectrum shows the efficient emission of yellow color light with peak at 574 nm ue to the excitation wavelength of 260 nm. Whereas the unope ZnS sample shows the blue emission. The oping of Mn 2+ in host ZnS prouce energy levels between the conuction ban an valence ban of the ZnS, leas to the emission of yellow colour light from Mn 2+ ope ZnS nanoparticle. Lu et al. [7] reporte that the PL intensity of Mn 2+ ope ZnS nanocrystals passivate by MPTS was enhance by a 30-fol, in comparison with the samples without MPTS passivation. ZnS:Mn nanocrystals are foun to have increase photoluminescence efficiency with a very short lifetime of the transition associate with the magnetic impurity Mn 2+ [8]. For small particles like the ZnS:Mn 2+ nanoparticles, majority of the Mn 2+ ions are at the near-surface sites an occupy axial or lower symmetry sites [9]. The PL spectrum shows that the aition of AA enhances the PL intensities in comparison of the sample without aition of AA.This is ue to the reuction of surface efects. The yellow emission originates from a transition between the 4 T 1 excite state an 6 A 1 groun state of Mn 2+ ion. The PL enhancement can be explaine by a passivation of the surface efects on nanoparticle. Non raiative recombination occurs through surface efects, the raiative probability through the Mn 2+ centres ecreases with increasing surface efects. So it is expecte that an enhancement of the raiative probability will be obtaine

6 152 by elimination of the surface efects through surface passivation. In the presence of AA as a surface passivator the surface efects on Mn 2+ ope ZnS nanoparticles are passivate an eliminate. The carboxylic groups of surface moifying agents play an important role in enhancing the PL intensity [7]. PL intensity (a.u.) 7000 e a - Unope ZnS b- 2% Mn Dope ZnS c- 2% Mn Dope ZnS with 0.1 mol AA % Mn Dope ZnS with 0.5 mol AA e- 2% Mn Dope ZnS with 0.75 mol AA c b a Wavelength(nm) Fig. 5. PL emission spectrum of the ZnS an ZnS:Mn nanoparticles. 4. Conclusions Manganese ope zinc sulphie (ZnS:Mn 2+ ) nanoparticles are prepare using acrylic aci as a surface passivator by chemical metho at room temperature. The XRD measurements yiele the particle size in the range of nm. It has been observe that with the increasing acrylic aci concentration the absorption peak of the Zns:Mn shifts towars shorter wavelength. The ban gap energy of the samples is foun in the range eV. Room temperature photoluminescence (PL) spectrum of the unope sample exhibit a blue emission peake at 465 nm uner UV excitation whereas the Mn 2+ ope sample exhibits a yellow emission peake at 574 nm uner same UV excitation. The enhance PL intensity is ue to AA, this is achieve by eliminating the surface efects, in which the carboxylic group present in surface passivator plays an important role. Acknowlegements The authors acknowlege to Dr. S. S. Panja, Department of Chemistry, NIT Durgapur for proviing PL lab facilities. The authors also woul like to acknowlege Dr. S. Basu, Dept. of Physics, NIT Durgapur for the fruitful iscussion uring the entire work. One of the authors (Mr. C. S. Pathak) acknowleges to MHRD, Govt. of Inia for the financial support.

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