Luminescence Studies of (Zns)_1-X (Mnte)_X Nanophosphors Prepared by Wet Chemical Method

Luminescence Studies of (Zns)_1-X (Mnte)_X Nanophosphors Prepared by Wet Chemical Method

Dr Deepti Pateria School of Studies in Physics and

Astrophysics,

Pt. RaviShankar Shukla University, Raipur-492010 (C.G.), India

Dr Prabhakar Pateria Department of Chemistry

1. Rungta College of Engineering & Technology, Bhilai (C.G.)

   Abstract – The present paper reports the luminescence behavior of (ZnS) 1-x (MnTe) x nanophosphors prepared by wet chemical synthesis. (ZnS)1-x(MnTe)x nanophosphors give intense mechanoluminescence ,thermoluminescenceas as well photoluminescence. The structure investigated by X-ray

   diffraction patterns confirms the formation of sphalerite phase whose space group is found to be F4-3 m. From XRD measurement average size of particles was found of 11 nm. The TEM measurement indicates that the particle size is in the 9-13

   nm. Nanometre size phosphors are preferred in a number of applications not only due to their particle size but also due to smooth imaging of the stress. As the PL, ML, and TL intensity of (ZnS)1-x(MnTe)x phosphor is much higher as compared to ZnS: Mn phosphor, it can be used in lots of application like in biology, medicine, photonics and optoelectronics appear, damage sensor for structures ,ML based structural health monitoring, in various thermoluminescence applications like dosimeter etc. systems where ZnS:Mn phosphor is used presently. However, for higher values of x the luminescence intensity decreases because of the concentration quenching. Thus the Thermoluminescence (TL), Mechanoluminescence (ML) and Photoluminescence (PL) intensities are optimum for a particular value of x, that is, for x=0.05.

   Keywords Luminescence, (ZnS) 1-x(MnTe)x , Nanophosphors , Wet chemical method, II-IV semiconductors.

   1. INTRODUCTION

      Luminescence is the non-equilibrium phenomenon of excess emission over and above the thermal emission of a body, in which emission has a duration considerably exceeding the period of light oscillations. Although excess emission over and above the thermal background takes place in reflected and refracted light, Rayleigh and Raman scattering, and Cherenkov and laser radiations, they are not the phenomenon of luminescence as the emission duration in these phenomenon are of the order of the period of light oscillations. Whereas the intensity of incandescent emission increases with temperature, in most cases, the intensity of luminescence decreases with increasing temperature of materials. Luminescence may occur in crystalline and non- crystalline solids as well as in liquids and gases. Luminescence process involves at least three steps; (1) Energy absorption causing electronic excitation, (2) Storage of absorption energy and, (3) Emission of photon. Depending on the storage time, i.e., the time delay between the excitation and emission, luminescence can be classified

      in two categories. If the storage time is shorter than 10-8 sec, the process is termed as fluorescence, otherwise, it is phosphorescence. The interval 10-8 sec is chosen as it gives the lifetime of an atom in the excited stage, for which return to the ground state is accompanied by dipole radiation. [1- 5].

      Last two decades the luminescence of II-VI nanomaterials is attracting the interest of many workers [6]. Although photoluminescence [6-11] and electroluminescence [6,12-14] and mechanoluminescence [6, 15-17] of II-VI of semiconductors have been studied in detail, least studies on their TL have been made.Chen et al. [18, 19] have studied thermoluminescence of ZnS nanoparticles and they have reported that the TL intensity increases as the particle size is decreased. The significance of the present study are as follows (i) (ZnS)1-x(MnTe)x nanophosphors prepared by wet chemical synthesis give intense mechanoluminescence ,thermoluminescenceas, as well as photoluminescence.,(ii)the luminescence intensity of (ZnS)1_x(MnTe)x nanophosphor prepared by a wet chemical method is very high nearly 5 times more as compared to that of ZnS: Mn phosphor. Therefore, such phosphor may replace ZnS:Mn phosphor in its use in damage sensor and mechanoluminescence structure health monitoring system, and (iii) the nano metre size phosphors are preferred in a number of applications not only due to their particle size but also for smooth imaging of the stress distribution in solids. Thus, the present study provides a sufficient novel knowledge which are quite unknown till now. As the PL, ML, and TL intensity of (ZnS)1-x(MnTe)x phosphor is much higher as compared to ZnS: Mn phosphor, it can be used in lots of application like in biology, medicine, photonics and optoelectronics appear, damage sensor for structures ,ML based structural health monitoring, in various thermoluminescence applications like dosimeter etc. systems where ZnS:Mn phosphor is used presently.

      The present paper reports the luminescence studies of (ZnS)1-x(MnTe)x nanophosphors prepared by wet chemical method.

      .

   2. EXPERIMENTAL

      For the present investigation the (ZnS)1-x(MnTe)x nanophosphors were synthesized by wet chemical process. In the synthesis of (ZnS)1-x(MnTe)x we used

      Zn(CH3COO)2.2H2O (A.R. Himedia Laboratories., 99.5%), Mn(CH3COO)2.4H2O (AR fine-chem Limited., 99.5%), Na2S9H2O (Flakes, Himedia Laboratories Pvt. Ltd.) and TeO2 (Himedia Laboratories Pvt. Ltd.97.0%) as the starting materials. Firstly, a certain molar proposition of Zn (CH3COO) 2.2H2O and Mn (CH3COO)2.4H2O (Mn/Zn molar

      ratio = 5%) were dissolved in distilled water at room temperature with continuous stirring (solution 1). Similarly, Na2S.9H2O and TeO2 (Te/S molar ratio= 5%) were dissolved in distilled water with continuous stirring (solution 2). Then solution 1 and solution 2 were mixed together with continuous stirring whereby we found the precipitates of the material. Then the resulting precipitates were washed with distilled water many times. After the washing we separated the precipitates by centrifugation and then the precipitates were dried in vacuum. Finally, the dried precipitates were mixed with activated charcoal, and then the precipitates were fired at 850o C for 10 h. The powder phosphor obtained was used for the TL measurements.

      The crystal-structure was determined by X-ray diffraction (XRD) (Buker D2-Phaser) analysis using Cu K radiation (=1.54 Ã… and 20= 200-700) at room temperature. A spectrofluorometer (Shimadzu Spectroflurophotometer RF 5301PC) was used for photoluminescence (PL) measurement at room temperature. In the present investigation ML in (ZnS)1-x(MnTe)x phosphors was excited using impulsive technique. The phosphors were used without any irradiation such as UV, X-ray, rays, etc. In this experiment, inside the sample holder there is a transparent Lucite plate below the guiding cylinder. The sample of 4 mg was placed on the upper surface of the lucide plate. Then the sample was covered with a thin aluminums foil then it fixed using an adhesive tape. So it prevents the scattering of crystallite fragments while the impact of the load or piston onto the sample is applied. The ML was induced impulsively by applying a load onto the sample from a fixed height. By using RCA-931A photomultiplier tube (PMT) which placed just below the Lucite plate the luminescence intensity was measured.The themoluminescence measuring equipment is known as TLD reader. The experimental set up used for the present study was consists of a built-in arrangement for heating the sample, a photomultiplier tube, a high voltage power supply, a computer and a computer programme to read the TL glow curve.For the TL measurement the samples of mg each were exposed to UV- light for different times such as 10 min, 25 min, and 20 min, 15 min. Then the sample was heated at a fixed heating rate of 10OC per sec. Photomultiplier tube was used to detect individual emitted photons. The photomultiplier was preceded by optical filters to select the wavelength range of interest from the particular sample material. A microcomputer controls the heating rate and highest heating temperature and collects the data from room temperature up to 5000 C. Heater strip can be programmed to heat the sample from 10 C/sec up to 400 C

      /sec and a maximum temperature that can be attained was (allowed) 5000 C. One heat absorbing glass and IR cut off filter was used, which allows only visible light and cuts-off IR radiation.

   3. RESULTS

      Fig. 1 shows the XRD obtained in the present work for (ZnS)1-x(MnTe)x powder samples. From this figure it is obvious that samples of all the compositions are polycrystalline in nature. The peaks of these XRDs for all samples are indexed using the standard ASTM Table and

      JCPDF data. In the present work all the samples were found to be in spheralite structure haveing space group were F4-3

      m .Rao et al. [24, 25] have reported wurtzite structure while Toriyi et al. [26] have reported spheralite and wurtzite structure for ZnSMnTe system.

      The XRD spectra of (ZnS)1-x(MnTe)x powder samples exhibit sharp peaks at 20 values equal to 28.910, 33.900, 48.110, 57.100, 59.890, 70.400, 77.830, 80.250 , 89.820

      ,96.980, and 96.980, which correspond to diffraction from (111), (200), (220), (311), (222), (400), (331), (420), (422),

      and (333) planes, respectively. It can be very clearly seen from Fig 1 that the XRD pattern shows broadening in the peak. Nanomaterials have small particle size and this causes broadening in their diffraction peak. The broadening of the peak is due to a small number of crystal planes. The broadening in turn causes a loss of intensity in the signal of their diffraction patterns. Strain factors as well as broadening due to the instrument could also contribute to the broadening of the peaks. The average crystallite size was calculated using the Scherers formula which gave the average particle size 11 nm. The lattice parameter has been computed as to be 5.34 Ã….

      Fig2 (a,b) show the TEM images of (ZnS)1- X(MnTe)X,x=0.05, nanophosphor. The sizes of nanoparticles are found to lie in the range 9 nm to 13 nm.This is comparable to obtained from XRD measurement.

      Fig 3 show the SEM image of (ZnS)1- X(MnTe)X,x=0.05, nanophosphor. The average grain size estimated from SEM image verifies that the phosphor sizes are in nanometer range.

      Fig 4 shows the TL glow curves for (ZnS)1- X(MnTe)X nanophosphor for different compositions of x. In this sample the first peak intensities lies nearly 105 oC-100 0C and second peak intensity lies nearly 183.5 0C -178.5 0C.

      The curves I, II, III, and IV corresponds the UV irradiation with 10 min, 25 min, 20 min, 15 min. It is seen that peak temperature first increases than decreases by increasing UV exposure time. It is seen that initially the TL intensity increases with ultraviolet exposure time and then it start decreasing with further increase in the ultraviolet exposure time. The primary resion for the decrease of the TL intensity with longer duration of UV exposure is the increase in temperature of the phosphor caused by the heat from ultraviolet lamp.

      Fig 5 shows the ML response to a mechanical impact for (ZnS)1-x (MnTe)x phosphor. In this case, the ML was excited by dropping a load of 400 gm, in which the dropping heights were changed to different values. It is found that the ML intensity rise with the increase of the impact velocity of the load on to phosphors. It is seen that when a load of a fixed mass was dropped on to the phosphor of a given mass from a known height, then an ML pulse is produced, in which at the beginning ML intensity rises linearly with time, attains a optimum value and then it

      decreases with time. The peak of the ML intensity Im increases linearly with the impact velocity vo = (2gh)1/2, where g is acceleration because of gravity and h is the height from which the load was dropped. No significant changes were found in the time tm corresponding to the peak of ML intensity versus time, with increasing value of the impact velocity vo.

      Fig.6 shows the PL spectra of (ZnS)1-x (MnTe)x,x=.05 phosphors. It is seen that the peak of the PL spectra occurs at 644 nm. For the measurement the wavelength of light used for excitation was 300 nm.

      Fig. 7 Illustrates the dependence of TL and PL intensity in (ZnS)1-x (MnTe)x,x=.05 phosphors on the value of

      x. It is evident that the initially the ML and PL intensity increas with x, attain an optimum value for x=0.05 then they decrease with further increase in x.

      Fig 8 Illustrates the dependence of ML and PL intensity in (ZnS)1-x (MnTe)x,x=.05 phosphors on the value of x.

   4. DISCUSSION

      . The average sizes of ZnS nanoparticles were estimated from the Debey-Scherrer formula using XRD. The size of the nanoparticles prepared at room temperature, 50, 100, and 2000 C, were 1.81, 2.50, 2.74 and 3.01 nm,

      respectively.

      The photoluminescence spectra emission peak was centered at 643 and 644 nm; the emission is in the red

      states. As the surface states increases with decrease in the size of the particle, nanoparticles with smaller size causes the increase in the TL efficiency. Furthermore, the wave functions of electrons and holes are effectively overlapped in nanoparticles, and this may also cause increase in their recombination probability, A. Because of these two factors, the TL of small nanoparticles is expected to be more than that of the bulk. Fig. 8 shows the schematic diagram for the size dependence of the surface states. It is to be noted that the separation between the electronhole states (similar to the donoracceptor pairs) increases with the decreasing size of the nanoparticles because the trap-depth does not change much upon decreasing size, while the bandgap increases .In the case of (ZnS) 1-x (MnTe) x phosphors also the trap depth should not change significantly with increasing size of the nano-phosphors.

      Calculation of Trap Depth and Frequency Factor

      The phosphor was given a UV irradiation using UV source for 10 min, 25 min, 20 min, and 15 min. Every time 2 mg of weighted phosphor was taken for TL measurements. We have calcutaled the trapping parameters like trap depth (E) and escape frequency factor (s) for glow peaks obtained under ultraviolet excitation.

      The trap depth was determined using the following

      eqation

      2kT2

      region. The peak at 643nm and 644 nm is attributed to Mn

      E = m

      o

      … (1)

      4T1 (4G) 6 A1 (6S) transition. The red emission by the (ZnS)1-X(MnTe)X nanophosphor is mainly due to Te impurity in addition to Mn in ZnS lattice. Thus the present work shows that present of Te helps the energy transfer from yellow (ZnS : Mn) to (ZnS)1-X(MnTe)X red emitted centers. The present study indicates that these materials are good

      where E=trap depth, k= boltzman constant, T=temperature, o= T2 Tm, T2 and is the temperature to higher temperature side of full- width half- maxima.

      The Frequency Factor was determined using the following formula

      E

      candidates for use in display in the red region.The PL (photoluminescence) ,ML (Mechanoluminescence), and TL

      s = /3

      2 e kTm

      Tm

      … (2)

      (Thermoluminescence) intensity are optimum for x=0.05 in (ZnS)1-x(MnTe)x phosphor because of the concentration quenching..

      The thermoluminescence intensity I of nanoparticles can be expressed by the formula, I=-dm/dt=mnA, where m and n are the density of holes and electrons for recombination, respectively; and A is the carrier recombination probability. In fact, with increase in the content of the surface states, holes and electrons of the particle become more accessible for the TL recombination, i.e., the m and n increase prportionally with the surface

      where s= Frequency Factor, /3 = Heating Rate, E=trap depth, k= boltzman constant, and Tm= peak temperature,

      Table 1 shows the first and second peak position of

      nanophosphor for different values of x. The frequency factor, activation energy of first and second peak is given in table 1

      Table 1: Values of peak temperature, Activation Energy (E) and Frequency Factor (s) of first and second peak for (ZnS) 1-x(MnTe)X for different composition of x.

      S.N.	phosphors	Ist Peak Temp (oC)	Activation Energy E1 (eV)	Frequeny Factor s1 (sec -1)	IInd Peak Temp (oC)	Activation energy E2 (eV)	Frequeny Factor s2 (sec -1)
      1.	(ZnS)1x(MnTe)X,x=0.02	105	0.4636	4.9 x 10 11	183.5	0.7626	9.9 x 10 13
      2.	(ZnS)1x(MnTe)X,x=0.05	104	0.4612	4.7 x 10 11	182.5	0.7592	9.5 x 10 13
      3.	(ZnS)1x(MnTe)X,x=0.10	103	0.4580	4.5 x 10 11	181.5	0.7559	9.3 x 10 13
      4.	(ZnS)1x(MnTe)X,x=0.15	102	0.4563	4.4 x 10 11	180.5	0.7526	8.5 x 10 13
      5.	(ZnS)1x(MnTe)X,x=0.20	101	0.4539	4.2 x 10 11	179.5	0.7493	8.2 x 10 13
      6.	(ZnS)1x(MnTe)X,x=0.25	100	0.4515	4.1 x 10 11	178.5	0. 7460	7.9 x 10 13

   5. CONCLUSIONS

The various glow curve (TL, ML, PL ) of (ZnS) 1-x (MnTe) x nanophosphors having average size 11 nm has been studied for various composition of x. It is found that the luminescence intensity is maximum for x=0.05. Initially the Ml ,TL,and PL, intensity increases with increasing value of x because the number of luminescence centres increases however, for higher values of x the intensity decreases because of the concentration quenching. Thus the TL, ML and PL intensities are optimum for particular value of x that is for x=0.05. The activation energy of first and second peaks for all the samples was calculated and they are found to be first 0.45 eV and second 0.75 eV, respectively. The frequency factor is also calculated of first and second peak and it was found to be in the range of 4.4 x10 11 and 8.8 x 10 13 sec-1 respectively.

REFERENCES

1. S.W.S. Mckveever, thermoluminescence of solids, Cambridge University press, (1988).

2. Claudio Furetta, Hand book of thermoluminescence World

   Scientific Pub. Co. Inc. (2003).

3. M.Oberhofer and A.Scharmann, Applied thermoluminescence dosimetry, edited by M.Oberhofer and A.Scharmann, published by adam higher Ltd. Techno house, Redchiffney, Bristel (1981).

4. D.R.Vij, Luminescence of solids, Edited by D.R. Vij, Plennum press, New York and London (1998).

5. Stuart James Fleming, Thermoluminescence techniques in archaeology, Clarendon Press, (1979).

6. B.P. Chandra, V.K. Chandra, Piyush Jha, Luminescence of II-VI semiconductor nanoparticle. Sol.

   Stat. Phenomenon, 222 (2015) 1.

7. P. Yang, M. Lü , D. Xü, D. Yuan and G. Zhon, Catalytic growth and photoluminescence properties of semiconductor single-crystal ZnS nanowires. Chemical Physics Lett. 336 (2001)76.

8. R.N. Bhargava and D. Gallangher, Optical properties of manganese- doped nanocrystals of ZnS. Physical Review Letters 72(1994) 416.

9. H. Chander, A Review on Synthesis of Nanophosphors Future Luminescent Materials Material Science and Engineering, R49 (2005)113.

10. X. Fang, T.Zhai, U.K.Gautum, L.Li, L.Wu, Y.Bando, D.Golberg, ZnS nanostructures: from synthesis to applications Progress in Materials Science, 56(2011)175-287.

11. K. Manzoor, S.R. Vadera, N. Kumar, T.R.N. Kutty, Energy transfer from organic surface adsorbate-polyvinyl pyrrolidone molecules to luminescent centers in ZnS nanocrystalsSolid State Communication, 129 (2004) 469.

12. H. Yang, S. Santra, and P.H. Holloway, Nanobiomaterials Handbook Journal of Nanoscience and Nanotechanology 5(2005)1364.

13. Y. Yang, J.M. Huang, S.Y. Liu, J.C. Shen, Macromolecules Containing Metal and Metal-Like Elements.J.Mater.Chem. 7(1997)131.

14. K. Manzoor, S.R. Vadera, N. Kumar, T.R.N. Kutty, Inorganic Nanoparticles: Synthesis, Applications, and Perspectives Applied Physics Letters 84(2004) 284.

15. B.P. Chandra, C.N. Xu, H. Yamada, X.G. Zheng, Intense visible luminescencefrom Nd-doped yttrium oxysulfide. Journal of Luminescnce, 130 (2010)442.

16. C.N. Xu, T. Watanabe, M. Akiyama, X.G. Zheng, Elastico- mechanoluminescence in

17. CaZr(PO4)2:Eu2+ with multiple trap levels Appl.Phys.Lett. 74(1999)1236.

18. B.P. Chandra, V. K. Chandra, Piyush Jha, Piezoelectrically-induced trap-depth reduction model of elastico-mechanoluminescent materials Physica B 461(2015) 38.

19. W. Chen, Z. Wang, Z. Lin, and L. Lin, Advanced Material J. Appl. Phys. 82 (1997a) 3111,156-159.

20. ] W. Chen, Z. Wang, Z. Lin, and L. Lin, Ap p l . Ph ys. Advan ced Materi a l s Lett. 82( 1997) 3111, 156- 1 59 .

21. ] M. Rao, D. R. Red d y, B. K. Red d y, C.N. Xu, Int en se red m echanoluminescenc e from ( Zn S) 1- x (M n Te) x Ph ys Lett. A 372 ( 2008) 4122 4126 .

22. ] M.Rao, R.P Vija ya lakshmi , D.RRedd y, B. K R ed d y. EPR and su scep tibilit y s t udies on (ZnS) 1 -x( M nTe)x powders Sp ect rochimi ca Act. A 69 ( 2008) 688 691.

23. ] T. Tori yi, Y. Ad achi, H. Ya ma da,Y. Imai and C. N. Xu , Enhancemen t of m echanoluminesc enc e from Zn S : Mn,Te b y Wet P r oc ess Key Engg. Mat. 388( 2009) 301 – 3 04 .

Figure Captions

Fig. 1 XRD of (ZnS)1-X(MnTe)X,x=0.05.

Fig. 2(a,b) show the TEM image of (ZnS)1-X(MnTe)X,x=0.05, nanophosphor. Fig. 3 show the SEM image of (ZnS)1-X(MnTe)X,x=0.05, nanophosphor.

Fig. 4 The TL glow curves for (ZnS)1-X(MnTe)X for different composition of x. The curves I, II, III, and IV corresponds the UV irradiation with 10 min, 25 min, 20 min, 15 min.

Fig 5 Time dependence the ML intensity of (ZnS)1-x (MnTe)x,for various composition of x (curves I,II and III correspond to the impact velocity 98, 140, and 221cm/s respectively

Fig. 6 Photoluminescence spectra of (ZnS) 1-x (MnTe)x x=.05 phosphors.

Fig.7 Variation of TL and PL intensity with composition x in (ZnS) 1-x (MnTe)x .

Fig .8 Variation of ML and PL intensity with composition x in (ZnS) 1-x (MnTe)x .Fig. 9 A schematic model for the size dependence of surface states in semiconductor nanoparticles. (a) (b) and (c) correspond to large size, medium size and small size of nano particle.

Fig 1

Fig 2(a,b)

3(a)

3(b)

Fig 3

Fig :4

Fig :5

Fig :5

Fig :6

Fig :7

Fig:8

Fig: 9