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Synthesis, Characterization and Thermoluminescence Studies of (ZnS)1-x(MnTe)x Nanophosphors

Deepti Pateria, Jyostna Chauhan*

HOD Nanotechnology Rajiv Ganhi Techanical University, Bhopal (M.P.)

Abstract: The present paper reports the thermoluminescence (TL) of (ZnS) 1-x (MnTe) x nanophosphors which was prepared by wet chemical synthesis. (ZnS)1-x(MnTe)x nanophosphors give intense thermoluminescence. The structure investigated by X-ray diffraction patterns confirms the formation of sphalerite phase whose space group is found to be F3 m. From XRD mesurement 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. (ZnS) 1-x (MnTe) x is very use full material and can be used in various luminescence applications such as making of various sensors and thermoluminescence application like dosimetry, ete. Initially the TL intensity increases with increasing value of x because the number of luminescence centres increases. However, for higher values of x the TL intensity decreases because of the concentration quenching. Thus the TL, Mechanoluminescence (ML) and Photoluminescence (PL) intensities are optimum for a particular value of x, that is, for x=0.05. Thermoluminescence of (ZnS) 1-x (MnTe) x nanophosphor has not reported, till now. There are two peaks in thermoluminescence glow curves of in which the first peak lies at 105°C-100°C and the second peak lies at 183.5°C -178.5°C. The activation energies for frist and second peaks are found to be 0.45 eV and 0.75 eV, respectively.

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

Citation: Deepti Pateria, Jyostna Chauhan, Synthesis, Characterization and Thermoluminescence Studies of (ZnS)1-x(MnTe)x Nanophosphors. American Journal of Nanomaterials, 4 (3). 52-57. January 2016. doi:10.12691/ajn-4-3-1

1. Introduction

If an insulator or semiconductor is previously exposed to ionising radiation such as X-rays, β-particles or α-particles, or any other radiations and then heated, the energy stored in the phosphors as a result of irradiation process liberates in the form of visible light in addition to the normal thermal radiation. The additional visible light emitted during first heating of the material is called thermally stimulates luminescence (TSL) or simply thermoluminescence (TL). Reheating the phosphor immediately after the previous heating gives rise to only normal thermal emission. When an irradiated coloured crystal is heated, holes or electrons are set free from the traps (defect sites) and emission takes place when they recombine with charges of opposite sign. On the other hand, defect sites which release the carriers are known as traps. In contrast, the centres from where the thermal release of carrier, etc. are not possible, but where the probability of capture of a charge of opposite sign is appreciable, are called recombination centres. Thermoluminescence is a very active area of research because of its immense contribution in the fields of personnel and environmental dosimetry, dating of archaeological artefacts, sediments and study of defects in solids [1,2,3,4,5].

For the last two decades the luminescence of II-VI nanoparticles is attracting the interest of many workers [6]. Although photoluminescence [6-11] and electroluminescence [6,12,13,14] and mechanoluminescence [6,15,16,17] 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. TL may be produced by the recombination of carriers released from the surface states or defect sites by heating. Smaller particles have larger surface/volume ratio and more accessible carriers for TL. Also, the carrier recombination rate increases upon decreasing size due to the increase of the overlap between the electron and hole wave functions. These two effects may cause TL to increase with decreasing the size of particles. The investigation of TL may provide some useful information about the surface states that may explain the size dependence of the surface fluorescence. Zahedifer et al. [20] have observed a TL glow peak in ZnS:Mn nanoparticles at about 475 K, with its intensity depending on concentration of the Mn dopant in Zn. Activation energy of this glow peak was obtained to be about 0.6 eV. Yazici et al. [21] have reported the thermoluminescence properties of Cu-doped ZnS nanophosphors after β-irradiation at room temperature. The ZnS:Cu nanophosphors were prepared by spray pyrolysis method and the formation of the nanoparticles was confirmed by X-ray diffraction (XRD) and scanning electron microscope (SEM). It was observed that the shape of TL glow curve of this nanophosphor is not affected by the size of the particles during the growth of nanophosphors whereas its TL intensity was highly increased with a decrease in the size of particles. They have reported two broad peaks in the TL glow curve; one of which is centred at about 110 °C and the other at about 170 °C for a heating rate of 1 °C/s in the temperature range from RT to 350 °C. The TL emission spectrum of these materials has two main emission bands, namely the blue and green bands. Sharma et al. [22] have reported the TL and optical absorption spectra of ZnS:Mn nanoparticles, prepared by the chemical route in which merceptoethanol was used as a capping agent. They have reported that the TL intensity increases as the particle size is decreased. The shift in peak position was also seen with decreasing size of nanoparticles. The consistency of the size dependence of the TL with that of the surface fluorescence indicates that the TL may be related to the surface states. Singh et al. [23] have prepared small sizes of ZnS nanoparticles at low temperature and made two samples, one as-prepared (size~3 nm) and other heat-treated at 1073 K (size~32 nm). Even for higher dose of γ-radiation the as-prepared samples could not show the TL signal, but 1073 K heat-treated samples show the TL signals. This may be due to fact that smaller particles have larger surface area compared to bigger particles. The shape factor of the entire glow curves is nearly 0.48, the TL glow curves could be fitted with order of kinetics 1.5.

The present paper reports the synthesis, characterization and thermoluminescence 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 850°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 Kα radiation (λ=1.54 Å and 2= 20⁰-70⁰) at room temperature. A spectrofluorometer (Shimadzu Spectroflurophotometer RF 5301PC) was used for photoluminescence (PL) measurement at room temperature.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 4 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 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 500 °C. Heater strip can be programmed to heat the sample from 1 °C/sec up to 40 °C /sec and a maximum temperature that can be attained was (allowed) 500°C. One heat absorbing glass and IR cut off filter was used, which allows only visible light and cuts-off IR radiation.

3. Results

Figure 1 shows the obtained XRD spectra for (ZnS)1-X(MnTe)X,x=0.05 powder sample. From this figure it is obvious that the sample is polycrystalline in nature. The XRD peak positions are compared with the standard ASTM Table and JCPDF data. In the present work the sample was found to be in spheralite structure having space group F3 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,x=0.05 powder samples exhibit sharp peaks at 2 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 Figure 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 Scherer’s formula which gave the average particle size 11 nm. The lattice parameter has been computed as to be 5.34 Å.

Figure 2(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.

Figure 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.

Figure 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°C-100°C and second peak intensity lies nearly 183.5°C -178.5°C. 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.

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

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

Figure 5 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. Near band edge emission of ZnS/ZnTe have not observed in our results. Reddy et al.[27] reported that Mn and Te co-doped ZnS nanoparticles exhibited intense red emission which is due to the energy transfer process, wherein Mn ions transfer energy to Te ions and the red emission occurs from Te ions through ³T₁ → ¹A₁ [28] transition. In the present nanoparticles of ZnS: (Mn, Te) the red emission may be due to Mn²⁺ surrounded by a mixture of S and Te ions or even by Te ions only.

Figure 6 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.

Figure 3. show the SEM image of (ZnS)1-X(MnTe)X,x=0.05, nanophosphor

Figure 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

Figure 5. Photoluminescence spectra of (ZnS) 1-x (MnTe)x x=.05 phosphors

Figure 6. Variation of TL and PL intensity with composition x in (ZnS) 1-x (MnTe)x

4. Discussion

In 1997, Chen et al. [18,19] and co-workers were the first to report the TL of a ZnS nanoparticle. They prepared the ZnS nanoparticles using Zn(NO₃)₂ and Na₂S as sources, and these nanoparticles were deposited on a quartz substrate for measurements. 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 200°C, were 1.81, 2.50, 2.74 and 3.01 nm, respectively.

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 proportionally with the surface 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. Figure 7 shows the schematic diagram for the size dependence of the surface states. It is to be noted that the separation between the electron–hole states (similar to the donor–acceptor 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.

Figure 7. 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

4.1. 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

[picture] | (1)

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

The Frequency Factor was determined using the following formula

[picture] | (2)

where s= Frequency Factor, = Heating Rate, E=trap depth, k= boltzman constant, and = 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.

5. Conclusions

The TL glow curve 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 TL intensity is maximum for x=0.05. Initially the TL intensity increases with increasing value of x because the number of luminescence centres increases however, for higher values of x the TL 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.

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