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Highly efficient green light harvesting from Mg doped ZnO nanoparticles: Structural and optical studies
Journal of Alloys and Compounds 552 (2013) 208–212
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Highly efficient green light harvesting from Mg doped ZnO nanoparticles: Structural and optical studies Sarla Sharma a,⇑, Rishi Vyas b, Neha Sharma a, Vidyadhar Singh c, Arvind Singh d, Vanjula Kataria e, Bipin Kumar Gupta e, Y.K. Vijay a a
Department of Physics, University of Rajasthan, Jaipur 302055, India Department of Physics, Malaviya National Institute of Technology, Jaipur 302017, India Okinawa Institute of Science and Technology, Graduate University, Okinawa 9040495, Japan d Department of Physics, Institute of Chemical Technology, Mumbai 400 019, India e National Physical Laboratory (CSIR), New Delhi 110012, India b c
a r t i c l e
i n f o
Article history: Received 12 September 2012 Received in revised form 15 October 2012 Accepted 18 October 2012 Available online 26 October 2012 Keywords: Zinc oxide nanoparticles Magnesium-doped zinc oxide Mechanochemical processing Defects Emission properties
a b s t r a c t Highly efficient green light emission was observed from Mg doped ZnO nanoparticles synthesized via facile wet chemical route with an average particle size 15 nm. The XRD analysis confirmed the growth of wurtzite phase of ZnO nanoparticles. Moreover, the optical properties of these nanoparticles were investigated by different spectroscopic techniques. The resulted nanoparticles exhibit intense green emission peaking at 530 nm (2.34 eV) upon 325 nm (3.81 eV) excitation. The photoluminescence (PL) intensity of visible emission depends upon the doping concentration of Mg. The PL intensity was found maximum up to 4% doping of Mg, and beyond it exhibits a decrees in emission. Furthermore, by varying the band gap from 3.50 to 3.61 eV, the PL spectra showed a near band edge (NBE) emission at wavelength around 370 nm (3.35 eV) and a broad deep level emission in the visible region. The obtained highly luminescent green emission of ZnO nanoparticle would be an ultimate choice for next generation portable optoelectronics device materials. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Recently, wide band gap semiconductor nanocrystals have fascinated enormous interest among researchers for both their fundamental size dependent optoelectronic properties as well as wide range of display applications. ZnO is a versatile wide band gap semiconductor with large excitonic binding energy (60 meV) compared to GaN (25 meV) which make it a candid material for optoelectronic applications [1,2]. Among these wide band gap semiconductors, the incorporation of impurity is well known to tailor the electrical and optical properties [3–6]. The doping of ZnO with metal could modify its optical properties, especially, doping of ZnO with Cd/Mg may modulate the value of the band gap and may increase the UV luminescence intensity [7]. Among these dopants, it is interesting to become aware of the Cd incorporation reduces the band gap while Mg substitution leads to enhancement of band gap of ZnO even up to 4 eV [8]. Moreover, Mg2+ has ionic radius of 0.66 Å, which is very close to ionic radius of Zn2+ (0.74 Å). Therefore, substitution of Zn by Mg does not give rise to significant changes in lattice constants. It is possible to obtain a
⇑ Corresponding author. Tel.: +91 141 2702457; fax: +91 141 2711049. E-mail address: [email protected] (S. Sharma). 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.10.084
wide band gap Zn1 xMgxO NPs by doping ZnO with a suitable concentration of Mg as reported by several other research groups previously [9–12]. Therefore, doping with Cd/Mg could be better alternate for synthesizing band gap engineered ZnO hetero-structure for optoelectronics applications in the UV (ultraviolate)range [13]. Typically, ZnO shows the PL characteristics in the visible and UV regions. The UV emission at 370 nm causes due to radiative annihilation of excitons, while the visible green emissions in ZnO originates from radiative recombination involving intrinsic defect centre such as the oxygen vacancies, interstitial oxygen, Zn vacancies and interstitial Zn [14] or from dopants in the ZnO nanoparticle and greatly on the preparation method and growth conditions [15]. In order to compare with the established visible green emission from Tb3+ doped rare-earth oxide (inorganic host lattice) nanophosphor [16] to ZnO nanoparticle. It is interesting to observe that the broad green emission spectrum observed in case of ZnO nanoparticle mainly related to defect induced emission while Tb3+ doped rare-earth oxide nanophosphor emits sharp green emission which is mainly caused by f–f transition of rare-earth. Both materials are equally important for their use in different specific applications in optoelectronics devices. But particularly in some cases, ZnO nanoparticles broad spectrum with enhanced luminescence has their own advantages over rare-earth sharp emission for example; high quantum yield with broad emission
S. Sharma et al. / Journal of Alloys and Compounds 552 (2013) 208–212
is desirable in few display device applications that can easily achieved by such kind of materials through enhanced PL intensity with broad emission spectrum. In addition to this, the successful optoelectronic applications of Mg doped ZnO requires deep understanding of the optical properties. There are only few reports available in literature on defect related PL in Zn1 xMgxO as a consequence of Mg incorporation in ZnO [17]. Here, in present investigations, we focused on the structural/ microstructural and photoluminescence properties of Zn1 xMgxO NPs synthesized by wet chemical route. The as-synthesized pristine and Mg doped ZnO NPs size are in the range of 15–20 nm which eliminates the possibility of quantum confinement effects on the band gap [18]. Furthermore, the Mg was incorporated in ZnO lattice up to 5 wt.% without any evidence of segregation of Mg.
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2. Experimental 2.1. Synthesis of Mg doped ZnO nanoparticles ZnO NPs were synthesized using a simple wet chemical route [19] which can be easily scale-up in large quantity. 1.2 M KOH solution in ethanol was drop wise added to 0.2 M solution of Zn(CH3COOH)22H2O in DMSO (Dimethyl Sulphoxide) at room temperature. For synthesizing Zn1 xMgxO NPs with different doping concentrations of Mg, different amounts of Mg (CH3COOH)24H2O were added to the aforesaid solution. The obtained precipitate containing Zn1 xMgxO NPs were separated by filtration and then washed at least 3–4 times with methanol. The precipitate was dried in air at room temperature and then kept in vacuum for 12 h to remove organic impurities. All the chemicals used in this process were purchased from the leading suppliers (Sigma–Aldrich) and were used without further purification. The yield of nanoparticle was found 85% after several statistical run by this method.
2.2. Analytical characterizations Gross structures of undoped and Mg doped ZnO NPs were studied in terms of the X-ray diffraction (XRD) patterns, recorded on a PAN analytical X’pert PRO-PW 3040 X-ray diffractometer with CuKa radiation (k = 1.5404 Å). Surface topology of the samples were investigated using Field Emission Scanning Electron Microscopic (FESEM) images with a microscope of Carl ZEISS-SUPRA 40 at 5 kV operating voltage. In situ chemical analysis of ZnO nanoparticles was done through EnergyDispersive X-ray Spectrum (EDS) analyzer at 20 kV operating voltage. Transmission Electron Microscopic (TEM) images were taken on a High resolution-TEM (JEOLJEM-2100) at an acceleration voltage of 200 kV. Optical absorption spectra of the samples were recorded using a Varian Carry 5000 UV–Visible (UV–Vis) spectrometer. The Photoluminescence (PL) and fluorescence images studied using Varian Eclipse Fluorescence Spectrophotometer (excitation wavelength of 325 nm) and JPK Bio-AFM (Nanowizard II), respectively.
3. Results and discussion 3.1. XRD study
Fig. 1. Powder X-ray diffraction pattern of 1% Mg doped ZnO. The observed and refined patterns are shown as the crossed markers (red color) and the top solid line (black color), respectively. The vertical markers denote the angles of calculated Bragg reflections. The lowest solid line represents the difference between the calculated and observed intensities. RBragg = 3.65 and v2 = 1.77. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 1 shows X-ray diffraction pattern and Reitveld analysis of the 1% Mg doped ZnO. Both the observed and refined patterns are shown in the Fig. 1 as crossed markers (red color) and the top solid line (black color) respectively. Vertical markers denote the angles of calculated Bragg reflections. The lowest solid line represents the difference between the calculated and observed intensities. The pattern gets indexed with hexagonal wurtzite structure of ZnO with no evidence of secondary or impurity phase. Fig. 2A shows the X-ray diffraction patterns for the Zn1 xMgxO (x = 0, 0.01, 0.02, 0.03, 0.04, 0.05). A significant shift (Fig. 2B) of
Fig. 2. (A) XRD patterns of ZnO NPs (a) undoped, (b) 1% Mg doped, (c) 2% Mg doped, (d) 3% Mg doped, (e) 4% Mg doped, and (f) 5% Mg doped, (B) Shifts in 2h among XRD patters corresponding to the diffraction planes (1 0 0), (0 0 2), (1 0 1) of ZnO nanoparticles (a) undoped (b) 1% Mg doped (c) 2% Mg doped (d) 3% Mg doped (e) 4% Mg doped, and (f) 5% Mg doped.
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Fig. 3. FESEM images of ZnO NPs (a) undoped, (b) 1% Mg doped, (c) 2% Mg doped, (d) 3% Mg doped, (e) 4% Mg doped, and (f) 5% Mg doped.
the diffraction peaks towards the higher angle side is observed in the doped specimens as compared to pristine ZnO. This could be attributed to the smaller ionic radius of Mg2+ as compared to Zn2+ [11,20–22] which in turn produce change in the value of lattice parameters. In addition to this no significant shifting is observed within the doped samples which can be attributed to insignificant increase in value of lattice parameter c [20,22]. For undoped ZnO nanoparticles, the lattice constants a and c are 3.232 and 5.210 Å, respectively which changes to 3.259 and 5.205 Å, respectively on doping. A series of diffraction patterns at 2h 31.7°, 34.4°, 36.2°, 47.5°, 56.5°, 62.8°, 66.3°, 67.9° and 69.0° corresponds to (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3), (2 0 0), (1 1 2), (2 0 1) planes of hexagonal structure of wurtzite ZnO. No impurity peaks related to Mg element or its compounds were found in the sample, which is suggestive of the presence of Mg in the form of impurity atoms. A slight decrease in broadening of the peaks, as shown in Fig. 2A and B, is observed with the increase in doping content which implies an increase in crystallite size with doping. This is in good agreement with our microstructure results. The particle size was calculated using Scherrer’s formula which was suggestive of increment in particle size (2– 3 nm) with Mg doping in ZnO NPs. The particle size for pure ZnO NPs was found 15 nm.
Fig. 4. EDS spectra of 4% Mg doped ZnO NPs.
ven distinct rings, namely (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3) and (1 1 2) are observed in the SAED pattern of pure ZnO NPs and identified as rings belonging to hcp-ZnO. These observations are consistent with XRD results.
3.2. FESEM and TEM studies Fig. 3A–F shows representative FESEM images of the samples in the series Zn1 xMgxO (0 6 x 6 0.05). The nanocrystalline powders form spherical aggregates with diameters of 50–100 nm. EDS analyzed of selective regions in the FESEM mode (Fig. 4) has the primary ZnKa, MgLa, and OKa peaks in a composition Zn39.1Mg3.9O57.0 for 4% Mg doped sample, with an AuKa peak from the coating used for SEM visualization. TEM image of pure ZnO NPs (Fig. 5A) shows the uniformity in particle size and nearly spherical shape. The particle size estimated from the TEM was 20 nm which is in good agreement with the particle size calculated from XRD spectrum. A computer simulated selected area electron diffraction (SAED) pattern for pure ZnO NPs (Fig. 5B) indicates that the NPs had a clear crystalline structure. Se-
3.3. UV–Vis study The UV–Vis spectra of pure and Mg doped ZnO NPs are shown in Fig. 6. We found a blue shift about 0.1 eV in the absorption peak with incorporation of Mg. The optical band gap energies determined from the obtained absorption spectra are varying from 3.50 to 3.61 eV for Zn1 xMgxO (x = 0–0.05), respectively. It is observed that the band gap energy of ZnO NPs increases with the increasing Mg concentration. This shift in energy band gap with doping concentration may be attributed to the incorporation of dopant atoms in contrast with the size effects. The size effects are known to negligibly influence the band structure of ZnO NPs for diameter greater than 7 nm as suggested by experimental
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Fig. 5. TEM image of undoped ZnO NPs with simulated SAED pattern.
Fig. 6. Absorption spectra of ZnO NPs (a) undoped, (b) 1% Mg doped, (c) 2% Mg doped, (d) 3% Mg doped, (e) 4% Mg doped, and (f) 5% Mg doped.
Fig. 7. PL spectra and images (inset) of ZnO NPs (a) undoped (b) 1% Mg doped, (c) 2% Mg doped, (d) 3% Mg doped, (e) 4% Mg doped, and (f) 5% Mg doped.
and theoretical studies on ZnO [23]. As seen from the XRD (Fig. 2B), there is a small increase (2–3 nm) in the particle size with doping concentration but this change is not significant to effect the band gap of ZnO NPs as compare to the change in band gap after Mg substitution.
down for 5% of Mg doping. In the UV region, around 350–400 nm (3.54–3.10 eV), a strong peak attributing towards the near band gap emission of ZnO was observed [29]. A broad green emission peak of the visible spectrum was observed near 530 nm (2.34 eV) and other part of broad spectrum covered to some extent in the yellow and red spectral regions respectively, which was resulted by the transition of excited luminescent centers from the deep level emission being usually accompanied by the presence of structural defects and impurities [26], the UV peak of the Mg-doped ZnO is closely related to the near band gap emission of ZnO. The green light emission can be assign to the transition between the photoexited holes and singly ionized oxygen vacancy or the anti-site oxygen and donor–acceptor complexes [29,18]. The band edge emission shifts to the blue with increasing concentration of Mg doping reflecting the change in the exciton energy seen in absorption spectra. As the concentration of Mg increases, the NBE emission peak shifts to higher energy region. This blue shift of NBE emission could be understood on the basis of Burstein–Moss effect [30]. ZnO is naturally n-type material in which Fermi level will be inside the conduction band when it is heavily doped. The absorption edge in this case should exhibit blue shift because the filled states would block thermal or optical excitations as suggested by Burstein [30]. The photoluminescence images shown in inset of Fig. 7 are in good agreement with the PL investigations. The fluorescence
3.4. PL studies Fig. 7 shows PL results of undoped and Mg doped ZnO NPs with different doping concentration. Near band emission and a broad deep level emission were observed for all the doped and undoped samples. Green emission in ZnO NPs is attributed to the zinc interstitials and oxygen vacancy [24] and also to singly ionized vacancies [25–28]. Yellow, orange–red emissions are attributed to the oxygen interstitials [24–27]. The PL spectra exhibit a broad emission peak with a maximum intensity in the green part of the visible spectrum (around 530 nm). This broad emission is mostly in the green and partly in the yellow and red spectral regions, which could be attributed to the transition of excited optical centers from the deep level to the valence level, such deep-level emission are usually accompanied by the presence of structural defects and impurities [18,28]. The intensity of broad deep level emission was found to be increased with doping concentration of Mg. An interesting observation is also found that the intensity of visible emission was increased up to 4% of Mg doping and beyond it fell
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intensity is found to be increasing with Mg concentration in ZnO and is highest for 4% Mg incorporation. The fluorescence Intensity further decreases for higher concentration of Mg in ZnO NPs. 4. Conclusions The ZnO NPs with different concentration of Mg doping were synthesized via wet simple chemical route which represents a noteworthy route for synthesizing Mg doped ZnO NPs with controlled size, uniform distribution and good structural and optical properties. XRD patterns showed formation of polycrystalline ZnO NPs with no evidence of Mg segregation. Only a small change in lattice parameters was evident with the increase in Mg concentration in ZnO nanoparticles from Reitveld refinement of XRD pattern of pristine and Mg-doped ZnO NPs. The energy band gap of the Mg doped ZnO NPs was found to be increasing with increase in dopant concentration, which coincides with the results from the PL measurements of Zn1 xMgxO NPs. The doping of 4% Mg content in ZnO NPs is found to enhance the green–yellow–red emission. As the concentration of Mg increases (>4%), the intensity of green emissions suppresses. The obtained strong green emission of ZnO nanoparticle could be a good alternative display materials for next generation portable smart optoelectronics devices. Acknowledgements S. Sharma thank to IUAC, New Delhi for financial support to carry out this work and also extend thank to Prof. D.C. Kothari, National Center for Nanosciences and Nanotechnology, University of Mumbai, for providing research facilities during this work. References [1] D.M. Bagnall, Y.F. Chen, Z. Zhu, T. Yao, S. Koyama, M.Y. Shen, T. Goto, Appl. Phys. Lett. 70 (1997) 2230.
[2] K. Tang, G.K.L. Wong, P. Yu, M. Kawasaki, A. Ohtomo, H. Koinuma, Y. Segawa, Appl. Phys. Lett. 72 (1998) 3270. [3] B.E. Sernelius, K.F. Berggren, Z.C. Jin, I. Hamberg, C.G. Granqvist, Phys. Rev. B 37 (1998) 10244–10248. [4] C. Bundesmann, N. Ashkenov, M. Schubert, D. Spemann, T. Butz, E.M. Kaidashev, M. Lorenzand, M. Grundmann, Appl. Phys. Lett. 83 (2003) 1974. [5] G. Sanon, R. Rup, A. Mansingh, Phys. Rev. B 44 (1991) 5672–5680. [6] X.M. Cheng, C.L. Chien, J. Appl. Phys. 93 (2003). [7] T. Gruber, C. Kirchner, R. Kling, F. Reuss, A. Waag, F. Bertram, D. Forster, J. Christen, M. Schreck, Appl. Phys. Lett. 83 (2003) 3290. [8] M.K. Yadav, M. Ghosh, R. Biswas, A.K. Raychaudhuri, A. Mookerjee, S. Datta, Phys. Rev. B 76 (2007) 195450. [9] H. Li, Y. Zhang, X. Pan, H. Zhang, T. Wang, E. Xie, J. Nanopart. Res. 11 (2009) 917–921. [10] F.K. Shan, B.I. Kim, G.X. Liu, Z.F. Liu, J.Y. Sohn, W.J. Lee, B.C. Shin, Y.S. Yu, J. Appl. Phys. 95 (2004). [11] Y.J. Lin, P.H. Wu, C.L. Tsai, C.J. Liu, Z.R. Lin, H.C. Chang, C.T. Lee, J. Appl. Phys. 103 (2008). [12] W.Q. Peng, S.C. Qu, G.W. Cong, Z.G. Wang, Appl. Phys. Lett. 88 (2006). [13] H. Cao, Y.G. Zhao, H.C. Ong, S.T. Ho, J.Y. Dai, J.Y. Wu, R.P.H. Chang, Appl. Phys. Lett. 73 (1998). [14] B.K. Gupta, V. Grover, Govind, V. Shanker, Nanotechnology 21 (2010) 475701. 6pp. [15] S. Fujihara, Y. Ogawa, A. Kasai, Chem. Mater. 16 (2004) 2965–2968. [16] A.K. Parchur, A.I. Prasad, A.A. Ansari, S.B. Rai, R.S. Ningthoujam, Dalton Trans. 41 (2012) 1103–1145. [17] M. Trunk, V. Venkatachalapathy, A. Galeckas, A.Yu Kuznetsov, Appl. Phys. Lett. 97 (2010). [18] R. Viswanatha, S. Sapra, B. Satpati, P.V. Satyam, B.N. Dev, D.D. Sarma, J. Mater. Chem. 14 (2004) 661–668. [19] S. Sharma, R. Vyas, S. Shrivastava, Y.K. Vijay, Physica B 406 (2011) 3230–3233. [20] K. Matsumura, A. Ohnishi, M. Sasaki, T. Kakuta, M. Kurihara, M. Sakamoto, Jpn. J. Appl. Phys. 46 (2007) 1432–1436. [21] A. Ohtomo, M. Kawasaki, T. Koida, K. Masubuchi, H. Koinuma, Y. Sakurai, Y. Yoshida, T. Yasuda, Y. Segawa, Appl. Phys. Lett. 72 (1998). [22] D. Zhao, Y. Liu, D. Shen, Y. Lu, J. Zhang, X. Fan, J. Appl. Phys. 90 (2001). [23] E.A. Meulenkamp, J. Phys. Chem. B 102 (1998) 5566–5572. [24] X. Liu, X. Wu, H. Cao, R.P.H. Chang, J. Appl. Phys. 95 (2004). [25] H.T. Ng, B. Chen, J. Li, J. Han, M. Meyyappan, Appl. Phys. Lett. 82 (2003). [26] L.R. Singh, R.S. Ningthoujam, S.D. Singh, J. Alloys Comp. 487 (2009) 466–471. [27] L.E. Greene, M. Law, J. Goldberger, F. Kim, J.C. Johnson, Y. Zhang, R.J. Saykally, P. Yang, Angew. Chem. Int. Ed. 42 (2003) 3031–3034. [28] P. Zu, Z.K. Tang, G.K.L. Wong, M. Kawasaki, A. Ohtomoand, H. Koinuma, Y. Segawa, Solid State Commun. 103 (1997) 459–463. [29] T.M. Hammad, J.K. Salem, J. Nanopart. Res. 13 (2011) 2205–2212. [30] E. Burstein, Phys. Rev. 93 (1954) 632–633.
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Highly efficient green light harvesting from Mg doped ZnO nanoparticles: Structural and optical studies Sarla Sharma a,⇑, Rishi Vyas b, Neha Sharma a, Vidyadhar Singh c, Arvind Singh d, Vanjula Kataria e, Bipin Kumar Gupta e, Y.K. Vijay a a
Department of Physics, University of Rajasthan, Jaipur 302055, India Department of Physics, Malaviya National Institute of Technology, Jaipur 302017, India Okinawa Institute of Science and Technology, Graduate University, Okinawa 9040495, Japan d Department of Physics, Institute of Chemical Technology, Mumbai 400 019, India e National Physical Laboratory (CSIR), New Delhi 110012, India b c
a r t i c l e
i n f o
Article history: Received 12 September 2012 Received in revised form 15 October 2012 Accepted 18 October 2012 Available online 26 October 2012 Keywords: Zinc oxide nanoparticles Magnesium-doped zinc oxide Mechanochemical processing Defects Emission properties
a b s t r a c t Highly efficient green light emission was observed from Mg doped ZnO nanoparticles synthesized via facile wet chemical route with an average particle size 15 nm. The XRD analysis confirmed the growth of wurtzite phase of ZnO nanoparticles. Moreover, the optical properties of these nanoparticles were investigated by different spectroscopic techniques. The resulted nanoparticles exhibit intense green emission peaking at 530 nm (2.34 eV) upon 325 nm (3.81 eV) excitation. The photoluminescence (PL) intensity of visible emission depends upon the doping concentration of Mg. The PL intensity was found maximum up to 4% doping of Mg, and beyond it exhibits a decrees in emission. Furthermore, by varying the band gap from 3.50 to 3.61 eV, the PL spectra showed a near band edge (NBE) emission at wavelength around 370 nm (3.35 eV) and a broad deep level emission in the visible region. The obtained highly luminescent green emission of ZnO nanoparticle would be an ultimate choice for next generation portable optoelectronics device materials. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Recently, wide band gap semiconductor nanocrystals have fascinated enormous interest among researchers for both their fundamental size dependent optoelectronic properties as well as wide range of display applications. ZnO is a versatile wide band gap semiconductor with large excitonic binding energy (60 meV) compared to GaN (25 meV) which make it a candid material for optoelectronic applications [1,2]. Among these wide band gap semiconductors, the incorporation of impurity is well known to tailor the electrical and optical properties [3–6]. The doping of ZnO with metal could modify its optical properties, especially, doping of ZnO with Cd/Mg may modulate the value of the band gap and may increase the UV luminescence intensity [7]. Among these dopants, it is interesting to become aware of the Cd incorporation reduces the band gap while Mg substitution leads to enhancement of band gap of ZnO even up to 4 eV [8]. Moreover, Mg2+ has ionic radius of 0.66 Å, which is very close to ionic radius of Zn2+ (0.74 Å). Therefore, substitution of Zn by Mg does not give rise to significant changes in lattice constants. It is possible to obtain a
⇑ Corresponding author. Tel.: +91 141 2702457; fax: +91 141 2711049. E-mail address: [email protected] (S. Sharma). 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.10.084
wide band gap Zn1 xMgxO NPs by doping ZnO with a suitable concentration of Mg as reported by several other research groups previously [9–12]. Therefore, doping with Cd/Mg could be better alternate for synthesizing band gap engineered ZnO hetero-structure for optoelectronics applications in the UV (ultraviolate)range [13]. Typically, ZnO shows the PL characteristics in the visible and UV regions. The UV emission at 370 nm causes due to radiative annihilation of excitons, while the visible green emissions in ZnO originates from radiative recombination involving intrinsic defect centre such as the oxygen vacancies, interstitial oxygen, Zn vacancies and interstitial Zn [14] or from dopants in the ZnO nanoparticle and greatly on the preparation method and growth conditions [15]. In order to compare with the established visible green emission from Tb3+ doped rare-earth oxide (inorganic host lattice) nanophosphor [16] to ZnO nanoparticle. It is interesting to observe that the broad green emission spectrum observed in case of ZnO nanoparticle mainly related to defect induced emission while Tb3+ doped rare-earth oxide nanophosphor emits sharp green emission which is mainly caused by f–f transition of rare-earth. Both materials are equally important for their use in different specific applications in optoelectronics devices. But particularly in some cases, ZnO nanoparticles broad spectrum with enhanced luminescence has their own advantages over rare-earth sharp emission for example; high quantum yield with broad emission
S. Sharma et al. / Journal of Alloys and Compounds 552 (2013) 208–212
is desirable in few display device applications that can easily achieved by such kind of materials through enhanced PL intensity with broad emission spectrum. In addition to this, the successful optoelectronic applications of Mg doped ZnO requires deep understanding of the optical properties. There are only few reports available in literature on defect related PL in Zn1 xMgxO as a consequence of Mg incorporation in ZnO [17]. Here, in present investigations, we focused on the structural/ microstructural and photoluminescence properties of Zn1 xMgxO NPs synthesized by wet chemical route. The as-synthesized pristine and Mg doped ZnO NPs size are in the range of 15–20 nm which eliminates the possibility of quantum confinement effects on the band gap [18]. Furthermore, the Mg was incorporated in ZnO lattice up to 5 wt.% without any evidence of segregation of Mg.
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2. Experimental 2.1. Synthesis of Mg doped ZnO nanoparticles ZnO NPs were synthesized using a simple wet chemical route [19] which can be easily scale-up in large quantity. 1.2 M KOH solution in ethanol was drop wise added to 0.2 M solution of Zn(CH3COOH)22H2O in DMSO (Dimethyl Sulphoxide) at room temperature. For synthesizing Zn1 xMgxO NPs with different doping concentrations of Mg, different amounts of Mg (CH3COOH)24H2O were added to the aforesaid solution. The obtained precipitate containing Zn1 xMgxO NPs were separated by filtration and then washed at least 3–4 times with methanol. The precipitate was dried in air at room temperature and then kept in vacuum for 12 h to remove organic impurities. All the chemicals used in this process were purchased from the leading suppliers (Sigma–Aldrich) and were used without further purification. The yield of nanoparticle was found 85% after several statistical run by this method.
2.2. Analytical characterizations Gross structures of undoped and Mg doped ZnO NPs were studied in terms of the X-ray diffraction (XRD) patterns, recorded on a PAN analytical X’pert PRO-PW 3040 X-ray diffractometer with CuKa radiation (k = 1.5404 Å). Surface topology of the samples were investigated using Field Emission Scanning Electron Microscopic (FESEM) images with a microscope of Carl ZEISS-SUPRA 40 at 5 kV operating voltage. In situ chemical analysis of ZnO nanoparticles was done through EnergyDispersive X-ray Spectrum (EDS) analyzer at 20 kV operating voltage. Transmission Electron Microscopic (TEM) images were taken on a High resolution-TEM (JEOLJEM-2100) at an acceleration voltage of 200 kV. Optical absorption spectra of the samples were recorded using a Varian Carry 5000 UV–Visible (UV–Vis) spectrometer. The Photoluminescence (PL) and fluorescence images studied using Varian Eclipse Fluorescence Spectrophotometer (excitation wavelength of 325 nm) and JPK Bio-AFM (Nanowizard II), respectively.
3. Results and discussion 3.1. XRD study
Fig. 1. Powder X-ray diffraction pattern of 1% Mg doped ZnO. The observed and refined patterns are shown as the crossed markers (red color) and the top solid line (black color), respectively. The vertical markers denote the angles of calculated Bragg reflections. The lowest solid line represents the difference between the calculated and observed intensities. RBragg = 3.65 and v2 = 1.77. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 1 shows X-ray diffraction pattern and Reitveld analysis of the 1% Mg doped ZnO. Both the observed and refined patterns are shown in the Fig. 1 as crossed markers (red color) and the top solid line (black color) respectively. Vertical markers denote the angles of calculated Bragg reflections. The lowest solid line represents the difference between the calculated and observed intensities. The pattern gets indexed with hexagonal wurtzite structure of ZnO with no evidence of secondary or impurity phase. Fig. 2A shows the X-ray diffraction patterns for the Zn1 xMgxO (x = 0, 0.01, 0.02, 0.03, 0.04, 0.05). A significant shift (Fig. 2B) of
Fig. 2. (A) XRD patterns of ZnO NPs (a) undoped, (b) 1% Mg doped, (c) 2% Mg doped, (d) 3% Mg doped, (e) 4% Mg doped, and (f) 5% Mg doped, (B) Shifts in 2h among XRD patters corresponding to the diffraction planes (1 0 0), (0 0 2), (1 0 1) of ZnO nanoparticles (a) undoped (b) 1% Mg doped (c) 2% Mg doped (d) 3% Mg doped (e) 4% Mg doped, and (f) 5% Mg doped.
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Fig. 3. FESEM images of ZnO NPs (a) undoped, (b) 1% Mg doped, (c) 2% Mg doped, (d) 3% Mg doped, (e) 4% Mg doped, and (f) 5% Mg doped.
the diffraction peaks towards the higher angle side is observed in the doped specimens as compared to pristine ZnO. This could be attributed to the smaller ionic radius of Mg2+ as compared to Zn2+ [11,20–22] which in turn produce change in the value of lattice parameters. In addition to this no significant shifting is observed within the doped samples which can be attributed to insignificant increase in value of lattice parameter c [20,22]. For undoped ZnO nanoparticles, the lattice constants a and c are 3.232 and 5.210 Å, respectively which changes to 3.259 and 5.205 Å, respectively on doping. A series of diffraction patterns at 2h 31.7°, 34.4°, 36.2°, 47.5°, 56.5°, 62.8°, 66.3°, 67.9° and 69.0° corresponds to (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3), (2 0 0), (1 1 2), (2 0 1) planes of hexagonal structure of wurtzite ZnO. No impurity peaks related to Mg element or its compounds were found in the sample, which is suggestive of the presence of Mg in the form of impurity atoms. A slight decrease in broadening of the peaks, as shown in Fig. 2A and B, is observed with the increase in doping content which implies an increase in crystallite size with doping. This is in good agreement with our microstructure results. The particle size was calculated using Scherrer’s formula which was suggestive of increment in particle size (2– 3 nm) with Mg doping in ZnO NPs. The particle size for pure ZnO NPs was found 15 nm.
Fig. 4. EDS spectra of 4% Mg doped ZnO NPs.
ven distinct rings, namely (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3) and (1 1 2) are observed in the SAED pattern of pure ZnO NPs and identified as rings belonging to hcp-ZnO. These observations are consistent with XRD results.
3.2. FESEM and TEM studies Fig. 3A–F shows representative FESEM images of the samples in the series Zn1 xMgxO (0 6 x 6 0.05). The nanocrystalline powders form spherical aggregates with diameters of 50–100 nm. EDS analyzed of selective regions in the FESEM mode (Fig. 4) has the primary ZnKa, MgLa, and OKa peaks in a composition Zn39.1Mg3.9O57.0 for 4% Mg doped sample, with an AuKa peak from the coating used for SEM visualization. TEM image of pure ZnO NPs (Fig. 5A) shows the uniformity in particle size and nearly spherical shape. The particle size estimated from the TEM was 20 nm which is in good agreement with the particle size calculated from XRD spectrum. A computer simulated selected area electron diffraction (SAED) pattern for pure ZnO NPs (Fig. 5B) indicates that the NPs had a clear crystalline structure. Se-
3.3. UV–Vis study The UV–Vis spectra of pure and Mg doped ZnO NPs are shown in Fig. 6. We found a blue shift about 0.1 eV in the absorption peak with incorporation of Mg. The optical band gap energies determined from the obtained absorption spectra are varying from 3.50 to 3.61 eV for Zn1 xMgxO (x = 0–0.05), respectively. It is observed that the band gap energy of ZnO NPs increases with the increasing Mg concentration. This shift in energy band gap with doping concentration may be attributed to the incorporation of dopant atoms in contrast with the size effects. The size effects are known to negligibly influence the band structure of ZnO NPs for diameter greater than 7 nm as suggested by experimental
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Fig. 5. TEM image of undoped ZnO NPs with simulated SAED pattern.
Fig. 6. Absorption spectra of ZnO NPs (a) undoped, (b) 1% Mg doped, (c) 2% Mg doped, (d) 3% Mg doped, (e) 4% Mg doped, and (f) 5% Mg doped.
Fig. 7. PL spectra and images (inset) of ZnO NPs (a) undoped (b) 1% Mg doped, (c) 2% Mg doped, (d) 3% Mg doped, (e) 4% Mg doped, and (f) 5% Mg doped.
and theoretical studies on ZnO [23]. As seen from the XRD (Fig. 2B), there is a small increase (2–3 nm) in the particle size with doping concentration but this change is not significant to effect the band gap of ZnO NPs as compare to the change in band gap after Mg substitution.
down for 5% of Mg doping. In the UV region, around 350–400 nm (3.54–3.10 eV), a strong peak attributing towards the near band gap emission of ZnO was observed [29]. A broad green emission peak of the visible spectrum was observed near 530 nm (2.34 eV) and other part of broad spectrum covered to some extent in the yellow and red spectral regions respectively, which was resulted by the transition of excited luminescent centers from the deep level emission being usually accompanied by the presence of structural defects and impurities [26], the UV peak of the Mg-doped ZnO is closely related to the near band gap emission of ZnO. The green light emission can be assign to the transition between the photoexited holes and singly ionized oxygen vacancy or the anti-site oxygen and donor–acceptor complexes [29,18]. The band edge emission shifts to the blue with increasing concentration of Mg doping reflecting the change in the exciton energy seen in absorption spectra. As the concentration of Mg increases, the NBE emission peak shifts to higher energy region. This blue shift of NBE emission could be understood on the basis of Burstein–Moss effect [30]. ZnO is naturally n-type material in which Fermi level will be inside the conduction band when it is heavily doped. The absorption edge in this case should exhibit blue shift because the filled states would block thermal or optical excitations as suggested by Burstein [30]. The photoluminescence images shown in inset of Fig. 7 are in good agreement with the PL investigations. The fluorescence
3.4. PL studies Fig. 7 shows PL results of undoped and Mg doped ZnO NPs with different doping concentration. Near band emission and a broad deep level emission were observed for all the doped and undoped samples. Green emission in ZnO NPs is attributed to the zinc interstitials and oxygen vacancy [24] and also to singly ionized vacancies [25–28]. Yellow, orange–red emissions are attributed to the oxygen interstitials [24–27]. The PL spectra exhibit a broad emission peak with a maximum intensity in the green part of the visible spectrum (around 530 nm). This broad emission is mostly in the green and partly in the yellow and red spectral regions, which could be attributed to the transition of excited optical centers from the deep level to the valence level, such deep-level emission are usually accompanied by the presence of structural defects and impurities [18,28]. The intensity of broad deep level emission was found to be increased with doping concentration of Mg. An interesting observation is also found that the intensity of visible emission was increased up to 4% of Mg doping and beyond it fell
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intensity is found to be increasing with Mg concentration in ZnO and is highest for 4% Mg incorporation. The fluorescence Intensity further decreases for higher concentration of Mg in ZnO NPs. 4. Conclusions The ZnO NPs with different concentration of Mg doping were synthesized via wet simple chemical route which represents a noteworthy route for synthesizing Mg doped ZnO NPs with controlled size, uniform distribution and good structural and optical properties. XRD patterns showed formation of polycrystalline ZnO NPs with no evidence of Mg segregation. Only a small change in lattice parameters was evident with the increase in Mg concentration in ZnO nanoparticles from Reitveld refinement of XRD pattern of pristine and Mg-doped ZnO NPs. The energy band gap of the Mg doped ZnO NPs was found to be increasing with increase in dopant concentration, which coincides with the results from the PL measurements of Zn1 xMgxO NPs. The doping of 4% Mg content in ZnO NPs is found to enhance the green–yellow–red emission. As the concentration of Mg increases (>4%), the intensity of green emissions suppresses. The obtained strong green emission of ZnO nanoparticle could be a good alternative display materials for next generation portable smart optoelectronics devices. Acknowledgements S. Sharma thank to IUAC, New Delhi for financial support to carry out this work and also extend thank to Prof. D.C. Kothari, National Center for Nanosciences and Nanotechnology, University of Mumbai, for providing research facilities during this work. References [1] D.M. Bagnall, Y.F. Chen, Z. Zhu, T. Yao, S. Koyama, M.Y. Shen, T. Goto, Appl. Phys. Lett. 70 (1997) 2230.
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