- Email: [email protected]
Optical properties of undoped and cobalt doped ZnS nanophosphor
Materials Science in Semiconductor Processing 16 (2013) 467–471
Contents lists available at SciVerse ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Optical properties of undoped and cobalt doped ZnS nanophosphor C.S. Pathak a,n, M.K. Mandal a, V. Agarwala b a b
Department of Physics, National Institute of Technology, Durgapur 713209, India Department of Metallurgical and Materials Engineering, Indian Institute of Technology, Roorkee 247667, India
a r t i c l e i n f o
abstract
Available online 20 August 2012
In the present study, the optical properties of ZnS and cobalt (Co) doped ZnS nanoparticles were investigated at room temperature. ZnS and ZnS:Co nanophosphors were prepared through chemical route, namely the chemical precipitation method and the formation of the nanoparticles were confirmed by X-ray diffraction and field emission scanning electron microscope (FESEM). Band gap energy of the prepared samples is determined by using a UV–vis–NIR spectrophotometer. The photoluminescence property of ZnS and ZnS:Co sample is determined by fluorescence spectroscopy. The sizes of as prepared nanoparticles are found to be in the 8–9 nm range. The FESEM morphology shows the formation of nanostructure of ZnS samples. The value of optical band gap has been found to be in the range 4.30–4.03 eV. Room temperature photoluminescence (PL) spectrum of the undoped sample exhibits emission in the blue region with multiple peaks under UV excitation. On the other hand, Co2 þ doped ZnS samples show enhanced visible light emissions under the same UV excitation wavelength of 310 nm. & 2012 Elsevier Ltd. All rights reserved.
Keywords: Semiconductors nanoparticles Optical absorption Band gap Photoluminescence properties
1. Introduction Undoped and doped semiconductor nanoparticles have been studied extensively during the past two decades, exploring their size-dependent optical properties. ZnS is one of the first semiconductors discovered and it has novel diverse applications, such as light-emitting diodes, flat panel displays, infrared windows, optical sensors, optical coating etc. [1]. When ZnS is doped with a small amount of metallic ions, such as Co ions, it emits a light in the visible region. The emission properties of ZnS are frequently being used in photoluminescence (PL) and electroluminescence (EL), thermoluminescences (TL) and cathodoluminescence (CL) studies. Luminescent properties of ZnS can be controlled by using various dopants such as Ni2 þ , Fe2 þ , Mn2 þ , Ag2 þ and Cu2 þ etc. [2–6]. They not only give luminescence in various regions but also
n
Corresponding author. Tel.: þ91 9458990137. E-mail address: [email protected] (C.S. Pathak).
1369-8001/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.mssp.2012.07.009
improve the properties of ZnS. Many methods have been explored for the preparation of semiconductor nanoparticles such as the chemical method [7,8], the mechano– chemical method [9,10], the hydrothermal process [11], the sol–gel method [12], the electro-spinning technique [13], the ultrasonic radiation method [14], the colloidal chemical treatment method [15], the reverse micelle method [16], the solvothermal method [17] and the thermal evaporation method etc. [18,19]. In this work, we report the optical properties of the undoped and Co2 þ doped ZnS samples at room temperature. The size of the prepared ZnS and ZnS:Co samples are in the range of 8–9 nm as calculated from X-ray diffraction (XRD) peak broadening. The optical absorption properties show a blue shift of the absorption edge due to the quantum confinement effect. The value of optical band gap has been found to be in the range 4.30–4.03 eV. PL emission band from undoped ZnS nanoparticles are highly broaden with multiple peaks in the blue region. Peaks are found at 427, 448 and 485 nm. Co2 þ doped ZnS nanoparticles show enhanced PL emission peaks at 427, 448
468
C.S. Pathak et al. / Materials Science in Semiconductor Processing 16 (2013) 467–471
and 485 nm in the visible region when excited with 310 nm UV light.
methanol and then taken on a four sided polished quartz cuvette of path length 10 mm.
2. Experimental
4. Results and discussion
All the chemicals used are of AR grade (Merck), without further purification. We prepare 10 ml of 1 M solution of zinc nitrate and 10 ml of 1 M solution of sodium sulfide in distilled water. Zinc nitrate solution is first stirred using a magnetic stirrer up to 30 min, and then the solution of sodium sulfide is mixed with above solution drop wise and stirred up to 1 h. The precipitate is separated by centrifugation (Remi, PR 24) for 4 min at 5000 rpm and is washed with methanol several times to remove all unreacted reagents. The precipitate is then heat treated in air at 200 1C for 2 h for further measurements. ZnS with Co doped is prepared at room temperature by mixing calculated amounts of 10 ml of 1 M solution of zinc nitrate, 10 ml of 2 M solution of cobalt nitrate in distilled water stirred using a magnetic stirrer up to 30 min and followed by drop wise addition of 10 ml of 1 M solution of sodium sulfide. The mixture is vigorously stirred using a magnetic stirrer up to 1 h. The precipitate is separated from the reaction mixture by centrifugation using centrifuge (REMI, PR 24) for 4 min at 5000 rpm and is washed with methanol several times to remove all sodium particles. The wet precipitate is washed with methanol several times to remove unreacted reagents and then heat treated at 200 1C for 2 h for further measurements.
XRD pattern of the prepared ZnS and ZnS:Co samples were obtained and these are shown in Fig. 1. This figure shows the three diffraction peaks at 2y values 28.61, 47.71 and 56.41. The peaks are appearing due to reflection from the (111), (220) and (311) planes of the cubic phase of the ZnS. The obtained peak positions correspond to zinc blended type patterns for both the samples. The XRD pattern of the prepared samples are well matched with the standard cubic ZnS (JCPDS Card no. 5-566). No other phases have been observed. ZnS nanoparticles have been grown with cubic phase. Sarkar et al. [20] and Yang et al. [21] have also observed the cubic phase. The average crystallite size of the ZnS nanoparticle as calculated by using Eq. (1) from the most intense peak is in the range of 8–9 nm. The crystallite size of undoped ZnS nanoparticles is 9 nm, whereas for 2% Co2 þ doped ZnS nanoparticle is found to be 8.2 nm. The XRD method is widely used for determining the average crystallite size of the particle and TEM is the best way to determine particle size by direct measurement. The broadening of the XRD pattern of the prepared undoped and Co doped ZnS sample takes place due to the nanocrystalline nature of the sample.The value of FWHM for ZnS:Co sample is 0.99741and for undoped ZnS sample, it is 0.91441. The FESEM image of 2% Co2 þ doped ZnS nanoparticles is shown in Fig. 2(a) and (b) shows its corresponding EDAX spectrum. From the FESEM images, the formation of nanoparticles is clearly observed but the actual size of the nanoparticles cannot be determined from the FESEM images as it is limited by the resolution of the used FESEM instrument. The EDAX spectrums confirmed the composition of ZnS:Co sample. Strong peaks of Zn and S are found in the EDAX spectrum and also detectable amounts of dopants indicate that impurity has doped into ZnS nanocrystallites. The optical absorption spectra have been observed by using a UV–vis–NIR (Varian, Cary 5000) spectrophotometer and the results are shown in Fig. 3. To measure
The samples were characterized by X-ray diffraction (XRD) using a D8 BRUKER AXS diffractometer with Cu Ka, operating at 40 kV and 30 mA. The average crystallite size of ZnS and Co doped ZnS nanoparticle was determined by X-ray line broadening by using the Scherrer equation as follows: D¼
0:9l bcosy
ð1Þ
where b is the full width at half maximum (FWHM), y is the angle of the peak maximum, and l is the Cu Ka wavelength ( ¼0.15406 nm). Field emission electron micrographs were obtained using a QUANTA FEI-200 with a field emission gun. Images were taken by fixing the sample powder on the aluminum stub by using a double sided adhesive carbon tape. The surface of these samples is coated with a very thin layer of gold (Au) by glow discharge sputtering before FESEM examination. Energydispersive analysis of X-Rays (EDAX) coupled with FESEM was used for the semi-quantitative investigation of the microstructure of the samples. The optical absorption spectra have been observed by the UV–vis-NIR spectrophotometer (Varian, Cary 5000) for measuring the absorption characteristics, the nanopowders were first dispersed in methanol and then taken on a quartz cuvette of path length 10 mm. The PL spectrum of the undoped and Co doped ZnS has been measured at room temperature using Hitachi F-2500 FL spectrophotometer. For measuring the PL intensity, the nanopowders were first dispersed in
1400 (111)
1200 (220)
1000
Intensity (au)
3. Characterization techniques
(311) ZnS:Co
800
600
400
200 ZnS
0 20
40
60
2θ (deg)
Fig. 1. X-ray diffraction (XRD) pattern of ZnS and ZnS:Co samples.
C.S. Pathak et al. / Materials Science in Semiconductor Processing 16 (2013) 467–471
469
the absorption characteristics, the synthesized ZnS and ZnS:Co nanopowders are first dispersed in methanol and then taken in a quartz cuvette. The characteristic absorption peaks due to undoped and Co2 þ doped ZnS nanoparticles appear in the wavelength range 340–245 nm. These peak positions reflect the band gap of nanoparticles and the synthesis ZnS nanoparticles have no absorption in the visible region (800–400 nm). The fundamental absorption, which corresponds to electron excitation from the valance band to conduction band, can be used to determine the value of the optical band gap of the synthesized ZnS nanoparticles. The relation between the incident photon energy (hn) and the absorption coefficient (a) is given by the following relation: ðahnÞ1=n ¼ AðhnEg Þ
Fig. 2. (a) FESEM image of ZnS:Co sample and (b) corresponding EDAX spectrum.
where A is the constant and Eg is the band gap energy of the material and the exponent n depends on the type of transition. For direct allowed transition n¼1/2, for indirect allowed transition n ¼2, for direct forbidden n ¼3/2 and for indirect forbidden n¼3. Direct band gap of the samples are calculated by plotting (ahn)2 versus hn and then extrapolating the straight portion of the curve on the hn axis at a ¼0 as shown in Fig. 4. The values of optical band gap are 4.30 eV and 4.03 eV respectively for undoped ZnS and 2% Co doped ZnS. The obtained values of the band gap of ZnS and Co doped ZnS nanoparticles are higher than that of the bulk value of ZnS (3.68 eV). For undoped ZnS nanocrystals, we obtain a band gap of 4.30 eV. The bulk band gap value of ZnS is 3.68 eV. This implies that the bandgap increases by 0.62 eV in the nanocrystal samples. On doping of 2% Co in the ZnS nanocrystals, we obtain a band gap of 4.03 eV; the band gap is changed by 0.35 eV. This blue shift of the band gap takes place because of the quantum confinement effect. Photoluminescence (PL) of ZnS and ZnS:Co samples are measured at room temperature using F-2500FL spectrophotometer and shown in Fig. 5. It is observed that PL emission band from undoped ZnS nanoparticles are highly broadening with multiple peaks and Co2 þ doped ZnS sample shows emission at the visible region, emission
4
40
ZnS ZnS:Co
ZnS ZnS:Co
35
3
30 25
(αh υ) (a.u.)
2
2
Absorbance (a.u.)
ð2Þ
1
20 15 10 5
0
0
300
400
500
600
700
800
Wavelength (nm)
Fig. 3. UV–vis–NIR absorption characteristics of the ZnS and ZnS:Co nanoparticles.
2
3
hυ (eV)
4
5
Fig. 4. Calculation of optical band gap from UV–vis–NIR absorption spectra.
470
C.S. Pathak et al. / Materials Science in Semiconductor Processing 16 (2013) 467–471
60
PL intensity (a.u.)
ZnS ZnS:Co 50
observed that PL emission band from undoped ZnS nanoparticles are highly broaden with multiple peaks in the blue region. Co2 þ doped ZnS nanoparticles show enhanced PL emission at 427, 448 and 485 nm in the visible region under UV excitation of 310 nm due to sensitizing nature of Co2 þ . The fluorescence efficiencies of the doped samples are higher than undoped sample due to the enhancement of the radiative recombination processes.
40
5. Conclusion
30 400
420
440 460 Wavelength (nm)
480
500
Fig. 5. PL emission from the ZnS and ZnS:Co nanoparticles dispersed in methanol under UV light excitations.
band with multiple peak maxima indicates the involvement of different luminescence centers in the radiative process. Sarkar et al. [20] reported that the PL emission intensity of ZnS:Co nanoparticles is 35 times larger than that of undoped ZnS nanoparticles. Yang et al. [21] reported that the emission wavelength is same as that of ZnS but the fluorescence intensity of Co2 þ doped ZnS is enhanced five times that of ZnS. Karar et al. [1] reported PL peak at 460 nm which was attributed to native acceptor levels. Warad et al. [22] reported the emission at around 424 nm in typical luminescence of undoped ZnS resulting from the transition of electrons from shallow states near the conduction band to sulfur vacancies present near the valence band. Gosh et al. [23] reported the emission peak at 450 nm which is due to the presence of sulfur vacancies in the lattice. Chen et al. [24] reported a PL emission peak at 445 nm from donor–acceptor pairs in undoped ZnS sample. In the Co doped ZnS nanoparticles, Co atoms act as electron trapping centers that result in nonradiative recombination. This means that the photoelectrons are transferred to the dopant level formed by Co doping, which become trapping centers compared to anion and cation vacancy defects [25]. Peng et al. [26] reported PL emission peak at 411 and 455 nm, which both can be attributed to the recombination of the defect sates of ZnS. Corrado et al. [27] reported PL emission peak at 440 nm. Yang et al. [28] reported the emission wavelength at 450 nm. Manzoor et al. [29] reported maximum PL emission peak at 434 nm. Sambasivam et al. [30] reported PL emission peak at 491 nm. In this article the emission wavelength of undoped ZnS is same as that of Co2 þ doped ZnS and the fluorescence intensity of Co2 þ doped ZnS nanoparticles is greater than undoped ZnS but it is not so large as reported earlier [20,21]. Generally, the PL intensity depends on the concentration of the dispersed ZnS nanoparticles in methanol. Since the same concentration of the undoped ZnS and Co2 þ doped ZnS solution is not accurately maintained for both the samples, so we cannot expect the distribution of PL intensity in regular manner. PL spectrum of ZnS and Co2 þ doped ZnS nanoparticles in this work is shown in Fig. 5. It is
ZnS and ZnS:Co nanoparticles of size 8–9 nm are synthesized at room temperature by the chemical precipitation method. This method is faster and with low cost chemical compounds, it is suitable for industrial large scale production. The band gap energy of the samples is found in the range 4.30–4.03 eV. It is found that the undoped sample exhibits PL emission in the blue region with multiple peaks under UV excitation of 310 nm whereas Co2 þ doped ZnS sample exhibits enhanced PL emission in the visible region.
Acknowledgments Authors are thankful to the reviewers for their valuable suggestions for the improvement of the paper. One of the authors (CSP) acknowledges MHRD, Government of India for the partial financial support and Indian Institute of Technology, Roorkee for providing XRD and FESEM facility. References [1] N. Karar, F. Singh, B.R. Mehta, Journal of Applied Physics 95 (2004) 656. ¨ Dong Xu, Duorong Yuan, Chunfeng Song, [2] Ping Yang, Mengkai Lu, Suwen Liu, Xiufeng Cheng, Optical Materials 24 (2003) 497–502. [3] C.S. Pathak, M.K. Mandal, Chalcogenide Letters 8 (2011) 147–153. [4] Andrea Klausch, Holger Althues, Christian Schrage, Paul Simon, Adam Szatkowski, Michael Bredol, Dieter Adam, Stefan Kaskel, Journal of Luminescence 130 (2010) 692–697. [5] P.H. Borse, N. Deshmukh, R.F. Shinde, S.K. Date, S.K. Kulkarni, Journal of Materials Science 34 (1999) 6087–6093. [6] A. Murugadoss, Arun Chattopadhyay, Bulletin of Materials Science 31 (3) (2008) 533–539. ¨ D. Xu, ¨ D. Yuan, J. Chang, G. Zhou, M. Pan, Applied [7] P. Yang, M. Lu, Physics A 74 (2002) 257–259. [8] C.S. Pathak, M.K. Mandal, Optoelectronic and Advanced Materials– Rapid Communications 5 (3) (2011) 211–214. [9] E. Ivanov, C. Suryanarayana, Journal of Materials Synthesis and Processing 8 (3/4) (2000). [10] P. Balaz, P. Pourghahramani, E. Dutkova, E. Turianicova, J. Kovac, A. Satka, Physica Status Solidi (C) 5 (12) (2008) 3756–3758. [11] Tran Thi Quynh Hoa, Ngo Duc The, Stephen McVitie, Nguyen Hoang Nam, Le Van Vu, Ta Dinh Canh, Nguyen Ngoc Long, Optical Materials 33 (2011) 308–314. [12] M.W. Wang, L.D. Sun, C.H. Liu, C.S. Liao, C.H. Yan, Chinese, Journal of Luminescence 20 (3) (1999) 247. [13] Yanbin Tong, Zijiang Jiang, Cheng Wang, Yi Xin, Zonghao Huang, Sidong Liu, Chunbo Li, Materials Letters 62 (2008) 3385–3387. [14] J.F. Xu, H.S. Jiw, Y.W. Tang, Du, Applied Physics A 66 (1998) 639. [15] N. Murase, R. Jagannathar, Y. Kanematsu, M. Watanable, A. Kurita, K. Hirata, T. Yazawa, T. Kushida, Journal of Physical Chemistry B 103 (1999) 754. [16] L.X. Cao, J.H. Zhang, S.L. Ren, S.H. Huang, Applied Physics Letters 80 (23) (2002) 4300.
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[17] Daixun Jiang, Lixin Cao, Ge Su, Hua Qu, Dake Sun, Applied Surface Science 253 (2007) 9330–9335. [18] Ramin Yousefi, A.K. Zak, Materials Science in Semiconductor Processing 14 (2011) 170–174. [19] Ramin Yousefi, M.R. Muhamad, A.K. Zak, Current Applied Physics 11 (2011) 767–770. [20] R. Sarkar, C.S. Tiwary, P. Kumbhakar, A.K. Mitra, Physica B 404 (2009) 3855–3858. ¨ D. Xu, ¨ D. Yuan, C. Song, G. Zhou, Journal of Physics [21] P. Yang, M. Lu, and Chemistry of Solids 62 (2001) 1181–1184. [22] H.C. Warad, S.C. Ghosh, B. Hemtanon, C. Thanachayanont, J. Dutta, Science and Technology of Advanced Materials 6 (2005) 296–301. [23] P.K. Ghosh, S.K.F. Ahmed, S. Jana, K.K. Chattopadhyay, Optical Materials 29 (2007) 1584–1590. [24] W. Chen, V.F. Aguekian, N. Vassiliev, A.Yu. Serov, N.G. Filosofov, Journal of Chemical Physics 123 (2005) 124707.
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[25] Chong Bi, Liqing Pan, Mei Xu, Jinhua Yin, Liangqiang Qin, Jiemin Liu, Hao Zhu, John Q. Xiao, Materials Chemistry and Physics 116 (2009) 363–367. [26] W.Q. Peng, G.W. Cong, S.C. Qu, Z.G. Wang, Optical Materials 29 (2006) 313–317. [27] Carley Corrado, Yu Jiang, Fadekemi Oba, Mike Kozina, Frank Bridges, Jin Z. Zhang, Journal of Physical Chemistry A 113 (2009) 3830–3839. ¨ D. Xu, ¨ D. Yuan, G. Zhou, Chemical Physics Letters [28] P. Yang, M. Lu, 336 (2001) 76–80. [29] K. Manzoor, S.R. Vadera, N. Kumar, T.R.N. Kutty, Materials Chemistry and Physics 82 (2003) 718–725. [30] S. Sambasivam, B. Sathyaseelan, D. Raja Reddy, B.K. Reddy, C.K. Jayasankar, Spectrochimica Acta Part A 71 (2008) 1503–1506.
Contents lists available at SciVerse ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Optical properties of undoped and cobalt doped ZnS nanophosphor C.S. Pathak a,n, M.K. Mandal a, V. Agarwala b a b
Department of Physics, National Institute of Technology, Durgapur 713209, India Department of Metallurgical and Materials Engineering, Indian Institute of Technology, Roorkee 247667, India
a r t i c l e i n f o
abstract
Available online 20 August 2012
In the present study, the optical properties of ZnS and cobalt (Co) doped ZnS nanoparticles were investigated at room temperature. ZnS and ZnS:Co nanophosphors were prepared through chemical route, namely the chemical precipitation method and the formation of the nanoparticles were confirmed by X-ray diffraction and field emission scanning electron microscope (FESEM). Band gap energy of the prepared samples is determined by using a UV–vis–NIR spectrophotometer. The photoluminescence property of ZnS and ZnS:Co sample is determined by fluorescence spectroscopy. The sizes of as prepared nanoparticles are found to be in the 8–9 nm range. The FESEM morphology shows the formation of nanostructure of ZnS samples. The value of optical band gap has been found to be in the range 4.30–4.03 eV. Room temperature photoluminescence (PL) spectrum of the undoped sample exhibits emission in the blue region with multiple peaks under UV excitation. On the other hand, Co2 þ doped ZnS samples show enhanced visible light emissions under the same UV excitation wavelength of 310 nm. & 2012 Elsevier Ltd. All rights reserved.
Keywords: Semiconductors nanoparticles Optical absorption Band gap Photoluminescence properties
1. Introduction Undoped and doped semiconductor nanoparticles have been studied extensively during the past two decades, exploring their size-dependent optical properties. ZnS is one of the first semiconductors discovered and it has novel diverse applications, such as light-emitting diodes, flat panel displays, infrared windows, optical sensors, optical coating etc. [1]. When ZnS is doped with a small amount of metallic ions, such as Co ions, it emits a light in the visible region. The emission properties of ZnS are frequently being used in photoluminescence (PL) and electroluminescence (EL), thermoluminescences (TL) and cathodoluminescence (CL) studies. Luminescent properties of ZnS can be controlled by using various dopants such as Ni2 þ , Fe2 þ , Mn2 þ , Ag2 þ and Cu2 þ etc. [2–6]. They not only give luminescence in various regions but also
n
Corresponding author. Tel.: þ91 9458990137. E-mail address: [email protected] (C.S. Pathak).
1369-8001/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.mssp.2012.07.009
improve the properties of ZnS. Many methods have been explored for the preparation of semiconductor nanoparticles such as the chemical method [7,8], the mechano– chemical method [9,10], the hydrothermal process [11], the sol–gel method [12], the electro-spinning technique [13], the ultrasonic radiation method [14], the colloidal chemical treatment method [15], the reverse micelle method [16], the solvothermal method [17] and the thermal evaporation method etc. [18,19]. In this work, we report the optical properties of the undoped and Co2 þ doped ZnS samples at room temperature. The size of the prepared ZnS and ZnS:Co samples are in the range of 8–9 nm as calculated from X-ray diffraction (XRD) peak broadening. The optical absorption properties show a blue shift of the absorption edge due to the quantum confinement effect. The value of optical band gap has been found to be in the range 4.30–4.03 eV. PL emission band from undoped ZnS nanoparticles are highly broaden with multiple peaks in the blue region. Peaks are found at 427, 448 and 485 nm. Co2 þ doped ZnS nanoparticles show enhanced PL emission peaks at 427, 448
468
C.S. Pathak et al. / Materials Science in Semiconductor Processing 16 (2013) 467–471
and 485 nm in the visible region when excited with 310 nm UV light.
methanol and then taken on a four sided polished quartz cuvette of path length 10 mm.
2. Experimental
4. Results and discussion
All the chemicals used are of AR grade (Merck), without further purification. We prepare 10 ml of 1 M solution of zinc nitrate and 10 ml of 1 M solution of sodium sulfide in distilled water. Zinc nitrate solution is first stirred using a magnetic stirrer up to 30 min, and then the solution of sodium sulfide is mixed with above solution drop wise and stirred up to 1 h. The precipitate is separated by centrifugation (Remi, PR 24) for 4 min at 5000 rpm and is washed with methanol several times to remove all unreacted reagents. The precipitate is then heat treated in air at 200 1C for 2 h for further measurements. ZnS with Co doped is prepared at room temperature by mixing calculated amounts of 10 ml of 1 M solution of zinc nitrate, 10 ml of 2 M solution of cobalt nitrate in distilled water stirred using a magnetic stirrer up to 30 min and followed by drop wise addition of 10 ml of 1 M solution of sodium sulfide. The mixture is vigorously stirred using a magnetic stirrer up to 1 h. The precipitate is separated from the reaction mixture by centrifugation using centrifuge (REMI, PR 24) for 4 min at 5000 rpm and is washed with methanol several times to remove all sodium particles. The wet precipitate is washed with methanol several times to remove unreacted reagents and then heat treated at 200 1C for 2 h for further measurements.
XRD pattern of the prepared ZnS and ZnS:Co samples were obtained and these are shown in Fig. 1. This figure shows the three diffraction peaks at 2y values 28.61, 47.71 and 56.41. The peaks are appearing due to reflection from the (111), (220) and (311) planes of the cubic phase of the ZnS. The obtained peak positions correspond to zinc blended type patterns for both the samples. The XRD pattern of the prepared samples are well matched with the standard cubic ZnS (JCPDS Card no. 5-566). No other phases have been observed. ZnS nanoparticles have been grown with cubic phase. Sarkar et al. [20] and Yang et al. [21] have also observed the cubic phase. The average crystallite size of the ZnS nanoparticle as calculated by using Eq. (1) from the most intense peak is in the range of 8–9 nm. The crystallite size of undoped ZnS nanoparticles is 9 nm, whereas for 2% Co2 þ doped ZnS nanoparticle is found to be 8.2 nm. The XRD method is widely used for determining the average crystallite size of the particle and TEM is the best way to determine particle size by direct measurement. The broadening of the XRD pattern of the prepared undoped and Co doped ZnS sample takes place due to the nanocrystalline nature of the sample.The value of FWHM for ZnS:Co sample is 0.99741and for undoped ZnS sample, it is 0.91441. The FESEM image of 2% Co2 þ doped ZnS nanoparticles is shown in Fig. 2(a) and (b) shows its corresponding EDAX spectrum. From the FESEM images, the formation of nanoparticles is clearly observed but the actual size of the nanoparticles cannot be determined from the FESEM images as it is limited by the resolution of the used FESEM instrument. The EDAX spectrums confirmed the composition of ZnS:Co sample. Strong peaks of Zn and S are found in the EDAX spectrum and also detectable amounts of dopants indicate that impurity has doped into ZnS nanocrystallites. The optical absorption spectra have been observed by using a UV–vis–NIR (Varian, Cary 5000) spectrophotometer and the results are shown in Fig. 3. To measure
The samples were characterized by X-ray diffraction (XRD) using a D8 BRUKER AXS diffractometer with Cu Ka, operating at 40 kV and 30 mA. The average crystallite size of ZnS and Co doped ZnS nanoparticle was determined by X-ray line broadening by using the Scherrer equation as follows: D¼
0:9l bcosy
ð1Þ
where b is the full width at half maximum (FWHM), y is the angle of the peak maximum, and l is the Cu Ka wavelength ( ¼0.15406 nm). Field emission electron micrographs were obtained using a QUANTA FEI-200 with a field emission gun. Images were taken by fixing the sample powder on the aluminum stub by using a double sided adhesive carbon tape. The surface of these samples is coated with a very thin layer of gold (Au) by glow discharge sputtering before FESEM examination. Energydispersive analysis of X-Rays (EDAX) coupled with FESEM was used for the semi-quantitative investigation of the microstructure of the samples. The optical absorption spectra have been observed by the UV–vis-NIR spectrophotometer (Varian, Cary 5000) for measuring the absorption characteristics, the nanopowders were first dispersed in methanol and then taken on a quartz cuvette of path length 10 mm. The PL spectrum of the undoped and Co doped ZnS has been measured at room temperature using Hitachi F-2500 FL spectrophotometer. For measuring the PL intensity, the nanopowders were first dispersed in
1400 (111)
1200 (220)
1000
Intensity (au)
3. Characterization techniques
(311) ZnS:Co
800
600
400
200 ZnS
0 20
40
60
2θ (deg)
Fig. 1. X-ray diffraction (XRD) pattern of ZnS and ZnS:Co samples.
C.S. Pathak et al. / Materials Science in Semiconductor Processing 16 (2013) 467–471
469
the absorption characteristics, the synthesized ZnS and ZnS:Co nanopowders are first dispersed in methanol and then taken in a quartz cuvette. The characteristic absorption peaks due to undoped and Co2 þ doped ZnS nanoparticles appear in the wavelength range 340–245 nm. These peak positions reflect the band gap of nanoparticles and the synthesis ZnS nanoparticles have no absorption in the visible region (800–400 nm). The fundamental absorption, which corresponds to electron excitation from the valance band to conduction band, can be used to determine the value of the optical band gap of the synthesized ZnS nanoparticles. The relation between the incident photon energy (hn) and the absorption coefficient (a) is given by the following relation: ðahnÞ1=n ¼ AðhnEg Þ
Fig. 2. (a) FESEM image of ZnS:Co sample and (b) corresponding EDAX spectrum.
where A is the constant and Eg is the band gap energy of the material and the exponent n depends on the type of transition. For direct allowed transition n¼1/2, for indirect allowed transition n ¼2, for direct forbidden n ¼3/2 and for indirect forbidden n¼3. Direct band gap of the samples are calculated by plotting (ahn)2 versus hn and then extrapolating the straight portion of the curve on the hn axis at a ¼0 as shown in Fig. 4. The values of optical band gap are 4.30 eV and 4.03 eV respectively for undoped ZnS and 2% Co doped ZnS. The obtained values of the band gap of ZnS and Co doped ZnS nanoparticles are higher than that of the bulk value of ZnS (3.68 eV). For undoped ZnS nanocrystals, we obtain a band gap of 4.30 eV. The bulk band gap value of ZnS is 3.68 eV. This implies that the bandgap increases by 0.62 eV in the nanocrystal samples. On doping of 2% Co in the ZnS nanocrystals, we obtain a band gap of 4.03 eV; the band gap is changed by 0.35 eV. This blue shift of the band gap takes place because of the quantum confinement effect. Photoluminescence (PL) of ZnS and ZnS:Co samples are measured at room temperature using F-2500FL spectrophotometer and shown in Fig. 5. It is observed that PL emission band from undoped ZnS nanoparticles are highly broadening with multiple peaks and Co2 þ doped ZnS sample shows emission at the visible region, emission
4
40
ZnS ZnS:Co
ZnS ZnS:Co
35
3
30 25
(αh υ) (a.u.)
2
2
Absorbance (a.u.)
ð2Þ
1
20 15 10 5
0
0
300
400
500
600
700
800
Wavelength (nm)
Fig. 3. UV–vis–NIR absorption characteristics of the ZnS and ZnS:Co nanoparticles.
2
3
hυ (eV)
4
5
Fig. 4. Calculation of optical band gap from UV–vis–NIR absorption spectra.
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60
PL intensity (a.u.)
ZnS ZnS:Co 50
observed that PL emission band from undoped ZnS nanoparticles are highly broaden with multiple peaks in the blue region. Co2 þ doped ZnS nanoparticles show enhanced PL emission at 427, 448 and 485 nm in the visible region under UV excitation of 310 nm due to sensitizing nature of Co2 þ . The fluorescence efficiencies of the doped samples are higher than undoped sample due to the enhancement of the radiative recombination processes.
40
5. Conclusion
30 400
420
440 460 Wavelength (nm)
480
500
Fig. 5. PL emission from the ZnS and ZnS:Co nanoparticles dispersed in methanol under UV light excitations.
band with multiple peak maxima indicates the involvement of different luminescence centers in the radiative process. Sarkar et al. [20] reported that the PL emission intensity of ZnS:Co nanoparticles is 35 times larger than that of undoped ZnS nanoparticles. Yang et al. [21] reported that the emission wavelength is same as that of ZnS but the fluorescence intensity of Co2 þ doped ZnS is enhanced five times that of ZnS. Karar et al. [1] reported PL peak at 460 nm which was attributed to native acceptor levels. Warad et al. [22] reported the emission at around 424 nm in typical luminescence of undoped ZnS resulting from the transition of electrons from shallow states near the conduction band to sulfur vacancies present near the valence band. Gosh et al. [23] reported the emission peak at 450 nm which is due to the presence of sulfur vacancies in the lattice. Chen et al. [24] reported a PL emission peak at 445 nm from donor–acceptor pairs in undoped ZnS sample. In the Co doped ZnS nanoparticles, Co atoms act as electron trapping centers that result in nonradiative recombination. This means that the photoelectrons are transferred to the dopant level formed by Co doping, which become trapping centers compared to anion and cation vacancy defects [25]. Peng et al. [26] reported PL emission peak at 411 and 455 nm, which both can be attributed to the recombination of the defect sates of ZnS. Corrado et al. [27] reported PL emission peak at 440 nm. Yang et al. [28] reported the emission wavelength at 450 nm. Manzoor et al. [29] reported maximum PL emission peak at 434 nm. Sambasivam et al. [30] reported PL emission peak at 491 nm. In this article the emission wavelength of undoped ZnS is same as that of Co2 þ doped ZnS and the fluorescence intensity of Co2 þ doped ZnS nanoparticles is greater than undoped ZnS but it is not so large as reported earlier [20,21]. Generally, the PL intensity depends on the concentration of the dispersed ZnS nanoparticles in methanol. Since the same concentration of the undoped ZnS and Co2 þ doped ZnS solution is not accurately maintained for both the samples, so we cannot expect the distribution of PL intensity in regular manner. PL spectrum of ZnS and Co2 þ doped ZnS nanoparticles in this work is shown in Fig. 5. It is
ZnS and ZnS:Co nanoparticles of size 8–9 nm are synthesized at room temperature by the chemical precipitation method. This method is faster and with low cost chemical compounds, it is suitable for industrial large scale production. The band gap energy of the samples is found in the range 4.30–4.03 eV. It is found that the undoped sample exhibits PL emission in the blue region with multiple peaks under UV excitation of 310 nm whereas Co2 þ doped ZnS sample exhibits enhanced PL emission in the visible region.
Acknowledgments Authors are thankful to the reviewers for their valuable suggestions for the improvement of the paper. One of the authors (CSP) acknowledges MHRD, Government of India for the partial financial support and Indian Institute of Technology, Roorkee for providing XRD and FESEM facility. References [1] N. Karar, F. Singh, B.R. Mehta, Journal of Applied Physics 95 (2004) 656. ¨ Dong Xu, Duorong Yuan, Chunfeng Song, [2] Ping Yang, Mengkai Lu, Suwen Liu, Xiufeng Cheng, Optical Materials 24 (2003) 497–502. [3] C.S. Pathak, M.K. Mandal, Chalcogenide Letters 8 (2011) 147–153. [4] Andrea Klausch, Holger Althues, Christian Schrage, Paul Simon, Adam Szatkowski, Michael Bredol, Dieter Adam, Stefan Kaskel, Journal of Luminescence 130 (2010) 692–697. [5] P.H. Borse, N. Deshmukh, R.F. Shinde, S.K. Date, S.K. Kulkarni, Journal of Materials Science 34 (1999) 6087–6093. [6] A. Murugadoss, Arun Chattopadhyay, Bulletin of Materials Science 31 (3) (2008) 533–539. ¨ D. Xu, ¨ D. Yuan, J. Chang, G. Zhou, M. Pan, Applied [7] P. Yang, M. Lu, Physics A 74 (2002) 257–259. [8] C.S. Pathak, M.K. Mandal, Optoelectronic and Advanced Materials– Rapid Communications 5 (3) (2011) 211–214. [9] E. Ivanov, C. Suryanarayana, Journal of Materials Synthesis and Processing 8 (3/4) (2000). [10] P. Balaz, P. Pourghahramani, E. Dutkova, E. Turianicova, J. Kovac, A. Satka, Physica Status Solidi (C) 5 (12) (2008) 3756–3758. [11] Tran Thi Quynh Hoa, Ngo Duc The, Stephen McVitie, Nguyen Hoang Nam, Le Van Vu, Ta Dinh Canh, Nguyen Ngoc Long, Optical Materials 33 (2011) 308–314. [12] M.W. Wang, L.D. Sun, C.H. Liu, C.S. Liao, C.H. Yan, Chinese, Journal of Luminescence 20 (3) (1999) 247. [13] Yanbin Tong, Zijiang Jiang, Cheng Wang, Yi Xin, Zonghao Huang, Sidong Liu, Chunbo Li, Materials Letters 62 (2008) 3385–3387. [14] J.F. Xu, H.S. Jiw, Y.W. Tang, Du, Applied Physics A 66 (1998) 639. [15] N. Murase, R. Jagannathar, Y. Kanematsu, M. Watanable, A. Kurita, K. Hirata, T. Yazawa, T. Kushida, Journal of Physical Chemistry B 103 (1999) 754. [16] L.X. Cao, J.H. Zhang, S.L. Ren, S.H. Huang, Applied Physics Letters 80 (23) (2002) 4300.
C.S. Pathak et al. / Materials Science in Semiconductor Processing 16 (2013) 467–471
[17] Daixun Jiang, Lixin Cao, Ge Su, Hua Qu, Dake Sun, Applied Surface Science 253 (2007) 9330–9335. [18] Ramin Yousefi, A.K. Zak, Materials Science in Semiconductor Processing 14 (2011) 170–174. [19] Ramin Yousefi, M.R. Muhamad, A.K. Zak, Current Applied Physics 11 (2011) 767–770. [20] R. Sarkar, C.S. Tiwary, P. Kumbhakar, A.K. Mitra, Physica B 404 (2009) 3855–3858. ¨ D. Xu, ¨ D. Yuan, C. Song, G. Zhou, Journal of Physics [21] P. Yang, M. Lu, and Chemistry of Solids 62 (2001) 1181–1184. [22] H.C. Warad, S.C. Ghosh, B. Hemtanon, C. Thanachayanont, J. Dutta, Science and Technology of Advanced Materials 6 (2005) 296–301. [23] P.K. Ghosh, S.K.F. Ahmed, S. Jana, K.K. Chattopadhyay, Optical Materials 29 (2007) 1584–1590. [24] W. Chen, V.F. Aguekian, N. Vassiliev, A.Yu. Serov, N.G. Filosofov, Journal of Chemical Physics 123 (2005) 124707.
471
[25] Chong Bi, Liqing Pan, Mei Xu, Jinhua Yin, Liangqiang Qin, Jiemin Liu, Hao Zhu, John Q. Xiao, Materials Chemistry and Physics 116 (2009) 363–367. [26] W.Q. Peng, G.W. Cong, S.C. Qu, Z.G. Wang, Optical Materials 29 (2006) 313–317. [27] Carley Corrado, Yu Jiang, Fadekemi Oba, Mike Kozina, Frank Bridges, Jin Z. Zhang, Journal of Physical Chemistry A 113 (2009) 3830–3839. ¨ D. Xu, ¨ D. Yuan, G. Zhou, Chemical Physics Letters [28] P. Yang, M. Lu, 336 (2001) 76–80. [29] K. Manzoor, S.R. Vadera, N. Kumar, T.R.N. Kutty, Materials Chemistry and Physics 82 (2003) 718–725. [30] S. Sambasivam, B. Sathyaseelan, D. Raja Reddy, B.K. Reddy, C.K. Jayasankar, Spectrochimica Acta Part A 71 (2008) 1503–1506.