Photoluminescence and photoconductivity of ZnS:Mn2+ nanoparticles synthesized via co-precipitation method

Spectrochimica Acta Part A 76 (2010) 523–530

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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa

Photoluminescence and photoconductivity of ZnS:Mn2+ nanoparticles synthesized via co-precipitation method Ram Kripal a, Atul K. Gupta a,∗, Sheo K. Mishra b, Rajneesh K. Srivastava b, Avinash C. Pandey c, S.G. Prakash b a b c

EPR Laboratory, Department of Physics, University of Allahabad, Allahabad 211002, India Department of Electronics and Communication, University of Allahabad, Allahabad 211002, India Nanophospher Application Center, University of Allahabad, Allahabad 211002, India

a r t i c l e

i n f o

Article history: Received 1 December 2009 Received in revised form 6 April 2010 Accepted 15 April 2010 Keywords: X-ray diffraction Photoluminescence Strain Photoconductivity

a b s t r a c t Mn2+ doped ZnS nanoparticles are characterized using UV–vis, photoluminescence and photoconductivity studies. The size of Mn2+ doped ZnS NPs is estimated to be 2–4 nm by X-ray diffraction. UV–vis spectra show a blue shift in absorption edge as compared to bulk counterpart. Photoluminescence spectra indicate that orange luminescence varies with Mn2+ concentration. The Mn2+ doped ZnS nanoparticles are found to be photosensitive. The doping of Mn2+ ions improves the photosensitivity of the ZnS nanoparticles system. The time-resolved rise and decay of photocurrent indicate anomalous behavior during steady state illumination. © 2010 Elsevier B.V. All rights reserved.

1. Introduction The doped nanomaterials have been largely studied in recent years due to their widespread applications in various devices such as sensors, solar cells, lasers, photocatalysts, photodetectors, IR detectors, optical communication, colour television, flat panel display, phosphors, light emitting diodes, etc. [1–13]. ZnS is one of the important luminescent materials with the band gap of 3.7 eV. It is transparent in the visible spectral region having exciton binding energy of 40 meV. Induced sub-band gap transitions in ZnS occur at energies in the visible range that allows the optical detection of traps, radiative recombination centers and surface states. The emission properties of ZnS are frequently being used in solid-state photoluminescence (PL) and electroluminescence studies related with the mechanism of emission in semiconductors especially those doped with Mn2+ ions [14,15]. In addition, the small size and high optical activity of ZnS nanoparticles (NPs) make them interesting for optoelectronic applications operating in the ultraviolet region [16–19]. Several techniques have been used to synthesize the ZnS NPs such as auto-combustion, sol–gel, solid state reaction method, co-precipitation method, etc. In this work, Mn2+ doped ZnS NPs are synthesized by co-precipitation method as it is simple, inexpensive and more productive.

∗ Corresponding author. Tel.: +91 532 2470532; fax: +91 532 2460993. E-mail address: atulkumar [email protected] (A.K. Gupta). 1386-1425/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2010.04.018

Mn2+ doped ZnS NPs are mainly studied due to their luminescence properties. This is due to the fact that doped Mn2+ ions provide good defect states for the excited electrons. The excited electrons are transferred from the conduction band to the dlevels of Mn2+ through which the radiative de-excitation occurs via 4 T1 → 6 A1 transition. This is observed as the orange emission. Photoconductivity (PC) is a useful tool to study the properties of semiconductors. In semiconductors, PC generally arises due to generation of electron–hole pairs because of the interaction of photons with bound electrons of lattice atoms. The conductivity of material depends upon the carrier density and complex process of carrier generation, trapping, and recombination [20,21]. PC is also a function of temperature, applied field, intensity of light and energy of radiation. There are several studies on PC of doped CdS, PbS and ZnO nanomaterials [22–27]. In the present study, we report X-ray diffraction (XRD), UV–vis, PL and PC results of Mn2+ doped ZnS NPs. The objective is to see if there is any change in behavior with the change in concentration of Mn2+ ions in host ZnS NPs.

2. Experimental Mn2+ doped ZnS nanoparticles were prepared by simple co-precipitation method in aqueous medium. The chemicals zinc acetate [Zn(CH3 COO)2 ·2H2 O], manganese acetate [Mn(CH3 COO)2 ·4H2 O] and sodium sulfide [Na2 S] were purchased from E. Merck Ltd., Mumbai 400018, India. All the chemicals were of AR grade and used without further purification. For 5% Mn2+ dop-

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ture of the samples corresponding to JCPDS file no. 80-0020. There was no significant change (no lattice patterns corresponding to cubic MnS were observed) due to substitution of Mn2+ ions in ZnS lattice because Zn2+ (0.74 Å) and Mn2+ (0.67 Å) have approximately similar ionic radii. The average crystallite size is calculated from Scherer formula [28,29]: D=

Fig. 1. Photographs of ZnS NPs with different concentration of Mn2+ under UV light.

ing, 10 ml of 1.0 M Zinc acetate and 5 ml 0.1 M manganese acetate solution were diluted with 75 ml double distilled water. This was followed by drop wise addition of 10 ml of 1 M of Na2 S under vigorous stirring for 1 h. A white precipitate was obtained which was separated by centrifugation and washed several times with double distilled water and ethanol. The precipitate was dried under vacuum at 50 ◦ C overnight to get the powder sample. Using same method 0%, 10% and 20% Mn2+ (Mn/Zn = 0%, 10% and 20% in atomic ratio) doped ZnS NPs were also prepared. The samples were characterized by TEM, XRD, UV–vis, PL and PC studies. All the observations were taken at room temperature. The actual concentration of Mn2+ in ZnS NPs was determined by atomic absorption spectroscopy (AAS) analysis using ECIL atomic absorption spectrometer AAS4141. The XRD was done on Rigaku D/max-2200 pc diffractometer operated at 40 kV/20 mA using MoK␣1 radiation of wavelength 0.72 Å in wide-angle range from 10◦ to 35◦ . UV–vis spectra were recorded on PerkinElmer Lambda 35 UV–visible spectrophotometer (resolution, 0.1 nm and wavelength range, 225–600 nm). PL recordings were done on PerkinElmer LS 55 luminescence spectrometer with excitation wavelength of 325 nm. For the measurement of PC, a sandwich type cell was deposited on an area of about 4 cm2 with thickness about 0.05 cm. The conducting glass surface was kept in direct contact with the material. The cell was kept in a dark box. The upper surface of the cell was exposed by a UV lamp and the current in dark as well as under illumination was recorded using nanoammeter. 3. Results and discussion 3.1. Atomic absorption spectroscopy analysis The actual concentration of Mn2+ doped into ZnS NPs was determined using AAS analysis. It revealed that with the increase of the initial Mn2+ in the reaction solution, the actual concentrations of Mn2+ in the samples also increase but all are less than the initial concentrations. The actual concentrations of 0.40, 0.72, and 1.32 at.% correspond to the initial concentrations of 5, 10, and 20 at.%, respectively. Since we will discuss the effect of the actual Mn2+ concentration on the NPs, the Mn2+ concentration in the following text denotes the actual concentration in the samples and not the initial concentration in the synthesis. 3.2. XRD and UV–vis measurement Fig. 1 shows photographs of the ZnS:Mn NPs under 350 nm UV light. Samples show light orange to dark orange colour as doping concentration of Mn2+ increases. This establishes that Mn2+ is responsible for orange emission in ZnS:Mn NPs. Fig. 2(a) shows the XRD patterns of ZnS:Mn NPs with different concentration of Mn2+ (0%, 0.40%, 0.72% and 1.32%) at room temperature. The obtained peak positions (1 1 1), (2 2 0) and (3 1 1) indicate zinc blend struc-

0.9 ˇ cos 

(1)

where D is the crystallite size,  is the wavelength of radiation used,  is the Bragg angle and ˇ is the full-width at half-maximum (FWHM) measured in radian. The average crystallite size of the samples lie in the range of 2–4 nm. The peak broadening indicates nanocrystalline nature and distribution of local strain in the crystal structure arising from defects like dislocation, etc. [30,31]. Fig. 2(b) shows the Williamson–Hall plot of different ZnS:Mn NPs. The strain and grain size of the samples were calculated by Williamson–Hall (W–H) method. According to this method, the FWHM may be expressed in terms of strain (ε) and particle size (D) by the equation [32]: ˇ cos  ε sin  K + = D  

(2)

where K (0.9) is crystallite shape constant and other parameters have the same meaning as in Eq. (1). The strain ε is estimated from the slope of the line and the crystallite size from the intersection with the vertical axis (Fig. 2(b)). The strain of as prepared ZnS:Mn NPs are found to be −0.102, −0.0988, −0.0797, and −0.067 with 0%, 0.40%, 0.72% and 1.32% Mn2+ concentrations, respectively. Here negative sign indicates compressive strain in ZnS:Mn NPs. The particle sizes obtained at zero strain are 1.9 nm, 2.0 nm, 2.3 nm and 2.5 nm with 0%, 0.40%, 0.72%, and 1.32% Mn2+ concentrations, respectively. The lattice parameters of cubic zinc blend are calculated by the formula: 1 dh2 k l

=

h2 + k 2 + l 2 a2

(3)

where ‘a’ is lattice parameter, dh k l is the interplaner separation corresponding to Miller indices h, k, and l. The calculated average value of lattice parameter for prepared ZnS:Mn NPs is 0.5308 nm. Fig. 3(a and b) shows the TEM image of the ZnS:Mn (1.32%). It is observed that these NPs are agglomerated with diameters of 8–12 nm. Fig. 3(c) gives the corresponding selected area electron diffraction (SEAD) pattern, which indicates that, ZnS:Mn NP is polycrystalline. The high-resolution transmission electron microscopic (HRTEM) image of the sample in Fig. 3(d) exhibits a clear lattice spacing of 0.31 nm, which matches with the distance of the (1 1 1) plane of zinc blend ZnS [33]. Fig. 4 shows the UV–vis spectra of different concentrations of Mn2+ doped ZnS NPs at room temperature. The as prepared ZnS:Mn NPs were dispersed homogeneously in water to record UV–vis absorption spectra. The optical absorption spectra show absorption peaks which are blue shifted as compared to bulk ZnS (336 nm) [34,35]. The observed blue shift in the absorption edge is reflection of the band gap increase as compared to bulk (3.7 eV) owing to quantum confinement effect [36–38]. The spectra of ZnS containing 0%, 0.40%, 0.72% and 1.32% Mn2+ ions have the peak at 300, 303, 305 and 306 nm, respectively. The absorption peak positions are found to be slightly red shifted with increase in Mn2+ concentration. Wang et al. [39] also observed similar red shift in CdS host nanocrystal with increasing Mn concentration. The reason behind the shift in UV–vis spectra and no shift in XRD pattern after doping of Mn2+ in ZnS is as follows. The XRD corresponds to the light reflected from the planes where the periodic arrangement of the atoms is perfect and continuous [28]. As doping of Mn2+ does not appreciably disturb the plane, no change in XRD pattern is obtained; whereas the

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Fig. 2. (a) XRD pattern of ZnS:Mn NPs at room temperature. (b) Williamson–Hall plot of ZnS:Mn NPs. (c) Plot of strain vs. concentration of Mn2+ in ZnS NPs.

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Fig. 3. TEM image, SEAD pattern and HRTEM image on ZnS:Mn (1.32%) NPs.

sample, which absorbs UV–vis light, is an aggregate of metal ions spread homogeneously and therefore shows peaks corresponding to Mn2+ ions [40]. On increasing the doping concentration of Mn2+ , optical band gap reduces from 4.14 to 4.06 eV. This small change in band gap suggests that there is direct energy transfer between the semiconductor-excited states and the 3d levels of Mn2+ ions [41]. The band gap energies of 0%, 0.40%, 0.72% and 1.32% ZnS:Mn NPs are found as 4.14, 4.10, 4.08, and 4.06 eV, respectively. The quantum confinement effect allows one to tune the emission and excitation wavelengths of NPs by tuning particle size d(E). The particle size is given as [42]: d(E) =

√ 0.32 − 2.9 E − 3.49 3.50 − E

(4)

where E is the band gap in eV and d(E) is the diameter of the NPs in nm. The strain, particle size calculated by W–H plot and calculated by band gap energy of ZnS NPs with different concentration of Mn2+ ions are given in Table 1. The particle sizes obtained by different techniques are found to be different. This may be due to the reason that the XRD or UV–vis spectroscopy is only sensitive to the crystalline core size and detects the size of defect-free domains while the size from TEM can be bigger, if inner aggregate interfaces cannot be recognized [28,43]. For TEM, the particle size distribution overestimates larger and aggregated particles and therefore suffers from aggregation problems of samples due to the drying step in sample preparation. However, the results obtained here are consistent with those reported by Dieckmann et al. [43]. 3.3. Photoluminescence measurements Fig. 5(a) shows PL spectra of Mn2+ doped and undoped ZnS NPs. The spectra were recorded at the excitation wavelength of 325 nm. For all the samples, the blue green emissions centered at 417, 446, 480 and 520 nm arise due to interstitial sulfur (IS ) lattice defects, interstitial zinc (IZn ) lattice defects, sulfur vacancies (VS ) and zinc vacancies (VZn ), respectively [4,11,44–48]. Since sulfur ions have larger ionic radii (1.7 Å) than that of zinc ions (0.74 Å), interstitial sulfur produces more strain in the ZnS lattice and thus the electron levels due to this site will have smaller binding energy. Therefore, Table 1 The comparison of particle sizes, band gaps and strains of ZnS NPs with different concentration of Mn2+ .

Fig. 4. UV–vis optical absorption spectra of ZnS:Mn NPs at room temperature.

ZnS:Mn

Size (Å) (by W–H plot)

Size (Å) (by band gap)

Band gap (eV)

Strain

0% Mn 0.40% Mn 0.72% Mn 1.32% Mn

1.9 2.0 2.3 2.5

3.1 3.2 3.3 3.4

4.14 4.10 4.08 4.06

0.1020 0.0988 0.0797 0.0670

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Fig. 5. (a) Photoluminescence spectra of ZnS:Mn NPs at room temperature. (b) Schematic diagram of energy levels in ZnS:Mn NPs corresponding to photoluminescence spectra.

interstitial sulfur energy levels must be closer to valence band than the interstitial zinc energy levels to the conduction band. Similarly sulfur vacancy states are closer to conduction band edge than zinc vacancies states to the valence band edge, therefore peak at 417 nm is assigned to interstitial sulfur, 446 nm is assigned to zinc interstitial, 480 nm is assigned to sulfur vacancies and 520 nm is assigned to zinc vacancies defect states [34]. Sharma and Bhatti [49] also observed similar emission peaks, at 410, 432, due to defect levels. For the ZnS NPs doped with Mn2+ , an orange peak centered at 589 nm is observed in addition to blue green emissions, which arises due to 4 T1 –6 A1 transition within the 3d shell of Mn2+ . The intensity of this orange emission increases as Mn2+ concentration increases at the expense of blue green emissions. Karar et al. [34] also observed an increase in intensity of orange emission up to 20% (initial concentration) of Mn2+ ions into ZnS host nanocrystals and after that the intensity decreases for 30–40% Mn2+ concentration. The presence of orange emission indicates that Mn2+ has been incorporated into the ZnS NPs. According to Bhargava et al. [10,11], when Mn2+ ions substitute Zn2+ cation sites in ZnS lattice, the mixing of s–p electrons of host ZnS into the 3d electrons of Mn2+ causes strong hybridization and makes the forbidden transition of 4 T –6 A partially allowed, this yields orange emission at 589 nm 1 1 [45]. A schematic diagram of various emissions corresponding to PL spectra is shown in Fig. 5(b). 3.4. Photoconductivity measurements The experimental setup for the photoconductivity measurement is shown in Fig. 6(a). The schematic diagram of

527

Fig. 6. (a) Experimental arrangement for the measurement of photocurrent. (b) Schematic diagram of photoconductive cell. (c) Plot of photocurrent vs. concentration of Mn2+ in ZnS NPs.

photoconducting cell is shown in Fig. 6(b). The variation of photocurrent with different Mn2+ concentrations is shown in Fig. 6(c). ZnS doped with 0.40% Mn2+ is found to be more photosensitive as compared to other samples. Shionoya has also reported similar trend of change of photosensitivity in ZnS doped copper and manganese with variation of Mn concentration keeping Cu concentration constant [50]. 3.5. Field dependence of photocurrent and dark-current Fig. 7(a) shows variation of dark current with applied voltage for un-doped as well as Mn2+ doped ZnS NPs with different doping percentages. Fig. 7(b) shows similar variation on an ln–ln scale for power law verification. The ln(Idc ) vs. ln(V) plots are straight lines having different slopes and can be expressed by power law, I ∝ Vr, where ‘r’ represents slope or exponent of a straight line segment. The variation of dark-current (Idc ) with the applied voltage is found to be super-linear for un-doped as well as Mn2+ doped ZnS NPs with different concentrations. The variation of photogenerated current Ipc (Ipc = Itotal − Idark ) with applied voltage is shown in Fig. 8(a) where as the same is drawn on ln–ln scale in Fig. 8(b). The ln–ln plots are found to be straight lines having more than one segment with varying slopes for all the ZnS:Mn systems. The undoped ZnS NPs exhibit sub-linear variation at lower voltages whereas at higher voltages the behavior is super-linear. The Mn2+ doped ZnS NPs exhibit super-linear behavior at lower voltages as well as at higher voltages. Superlinear behavior suggests that some carriers are injected into the

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Fig. 7. (a) Variation of dark-current with applied voltage in ZnS:Mn NPs. (b) ln–ln plots of dark-current vs. applied voltage in ZnS:Mn NPs.

Fig. 8. (a) Variation of photocurrent with applied voltage in ZnS:Mn NPs. (b) ln–ln plots of photocurrent vs. applied voltage in ZnS:Mn NPs.

sample from the electrode side [51]. The sub-linear and super linear behavior may be explained based on class-I and class-II states [52–54]. 3.6. Time-resolved photocurrent measurements Fig. 9 shows the time-resolved rise and decay photocurrent spectra for undoped as well as Mn2+ doped ZnS NPs. The PC sample is first illuminated for 30 min under UV light of 366 nm with 30 V dc bias then light is switched off. Under steady state illumination, all the samples exhibit anomalous behavior. Such anomalous behavior was reported in ZnO nanowire, Co-doped ZnO nanobelts, TiO2 nanocrystalline ZnS single crystals, etc. [55–59]. In the present study, all the samples show an increase in photocurrent during illumination up to few hundred seconds and attain a maximum value. Zheng et al. have obtained similar result [60] by illuminating the sample for few seconds. Moreover, the photocurrent decreases exponentially during steady illumination for a longwhile and gets stabilized. When illumination is terminated, the photocurrent further decreases exponentially and attains a constant value. Bera and Basak [58] have reported the similar anomalous behavior in ZnO nanowire. The rise and decay of photocurrent during light illumination may be attributed to adsorption and desorption processes [60]. The adsorbed O2 molecules are desorbed

Fig. 9. Time-resolved rise and decay of photocurrent in ZnS:Mn NPs.

R. Kripal et al. / Spectrochimica Acta Part A 76 (2010) 523–530 Table 2 The comparison of decay time, trap depth and probability of escape of an electron from trap per second of ZnS NPs with different concentration of Mn2+ . ZnS systems

Decay time  d (s)

Trap depth E (eV)

ZnS ZnS:Mn (0.40%) ZnS:Mn (0.72%) ZnS:Mn (1.32%)

30 76 101 118

0.68 0.72 0.69 0.68

P

3.7. Trap depth calculation Trap depth can be calculated from decay curves. The decay of photocurrent can be expressed as

 −t  

where I0 is the steady current for time-resolved decay,  is time constant and t is the time. From time-resolved rise and decay of photocurrent spectra, the trap depths are calculated by peeling off the decay portion of the curves into the possible number of exponentials and governed by exponential law of the current amplitude is given by the equation I = I0 exp(−pt) [54], where I0 is the current at the time when light is switched off, I is photocurrent at any instant of time, and the p(=S exp(−E/kT)), is the probability of escape of an electron from trap per second. The trap depth (E) is calculated using following equation:



E = kT loge S − loge

loge (I0 /I) t

ity. Trap depths for un-doped as well as Mn2+ doped ZnS NPs are calculated as 0.68–0.72 eV. Acknowledgements

27 × 10−4 65 × 10−5 24 × 10−4 30 × 10−4

and photocurrent rises initially. At the same time, O2 molecules are easily re-adsorbed causing a decrease in photocurrent by trapping some electrons [61]. When the light is terminated, the electrons recombine with holes and are captured by re-adsorbed O2 molecules causing an exponential decay in the current [58,56]. The photoconductivity rise and decay curves are also governed by trap levels and recombination centers lying in the forbidden region of the photoconductor, so these curves are used to know the nature and distribution of traps and recombination centers [62].

I = I0 exp

529



(5)

where E denotes trap depth, k is Boltzman constant (1.381 × 10−23 J/K), T is absolute temperature and S is attempt to escape frequency of the order of 109 at room temperature. The trap depth, decay time response and the value of p for all the ZnS NP systems are calculated and listed in Table 2. 4. Conclusions

The authors are thankful to the Head, Department of Physics for providing departmental facilities. The authors are also thankful to Prof. M.C. Chattopadhay, Head, Department of Chemistry, University of Allahabad for providing AAS facility. The author Atul K. Gupta is thankful to the University Grants Commission for granting Junior Research Fellowship. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37]

The ZnS:Mn nanoparticles were synthesized by co-precipitation method. The average particle size of ZnS:Mn NPs were estimated using different techniques. With increase of concentration of Mn2+ ions, the absorption peaks shift towards longer wavelength side. The band gap of ZnS:Mn NPs are found to increase for all the samples as compared to bulk ZnS which may be attributed to quantum confinement. The PL spectra of un-doped as well as Mn2+ doped ZnS nanoparticles exhibit blue green emissions which may be attributed to interstitial sulfur (IS ) lattice defects, interstitial zinc (IZn ) lattice defect, sulfur vacancies (VS ) and zinc vacancies (VZn ) emissions, respectively. PL spectra of Mn2+ doped ZnS NPs show an orange emission centered at 589 nm which arises due to 4 T1 –6 A1 transition within the 3d shell of Mn2+ . The intensity of orange peak is found to increase with Mn2+ concentration. PC study shows an anomalous behavior of photocurrent in ZnS:Mn NPs. ZnS NPs doped with 0.40% Mn2+ are found to exhibit maximum photosensitiv-

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