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Tunable optical properties of ZnO via doping monovalent (Li+), divalent (Mn2+) and trivalent (Fe3+) cations
Materials Chemistry and Physics xxx (2014) 1e5
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Tunable optical properties of ZnO via doping monovalent (Liþ), divalent (Mn2þ) and trivalent (Fe3þ) cations Ajnur Hoque, Ratan Boruah, Shyamal K. Das* Department of Physics, Tezpur University, Tezpur 784028, Assam, India
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Li, Mn and Fe doped ZnO have been synthesized by simple one step method. Band gap and luminescence of ZnO exhibit strong dependence on dopant type and concentration. Red shift in band gap is observed for all doped ZnO. Highest red shift of 11% is observed for Fe doped ZnO.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 July 2013 Received in revised form 7 April 2014 Accepted 19 April 2014
Implications of Li, Mn and Fe doping on optical properties of ZnO are discussed here. Simple one step sol egel process was utilized to synthesize pristine and doped ZnO. The optical properties of ZnO exhibited a strong function of dopant type and concentration. Incorporation of Li, Mn and Fe resulted in red shift in band gap of ZnO; highest shift being for Fe doped ZnO followed by Mn and Li doped ZnO. Band gap narrowing of 11% is realized for 20% Fe doped ZnO, whereas it is 9% and 6% for Mn and Li doped ZnO. The emission spectra show quenching of UV emission for doped ZnO. Mn and Fe doped ZnO exhibited green luminescence. Ó 2014 Elsevier B.V. All rights reserved.
Keywords: Optical materials Luminescence Semiconductors Optical properties
1. Introduction Zinc oxide (ZnO), a direct wide band gap (3.37 eV) semiconductor at room temperature, has been successfully demonstrated as a promising material for variety of applications such as transparent electronics, ultraviolet light emitters, piezoelectric devices and spin electronics due to its important electrical and optical properties [1,2]. Recently, nanostructuring of ZnO played a pivotal role in optimizing the electro-optical properties [2]. A plethora of ZnO nanoarchitectures exhibited several remarkable
* Corresponding author. Tel.: þ91 3712275586; fax: þ91 3712267005. E-mail address: [email protected] (S.K. Das).
beneficial outcomes [2]. However, materials processing in reduced dimension involve complexity and controlling their dimensions is difficult that limits their overall impact. Another effective approach for optimization of electro-optical properties of ZnO is via doping [1,3e20]. Several reports demonstrated that band gap and luminescence of ZnO can be red or blue shifted via doping different cations with varying degrees [1,3e10]. However, quite ambiguous results on doped ZnO have been reported. For instances, Kawasaki et al. and Voyles et al. reported band gap widening in Mn and Fe doped ZnO respectively [21,22] while Kim et al. and Punnoose et al. observed band gap bowing [23,24]. On the other hand, both red and blue shift in band gap of Mn/Fe doped ZnO were also reported [25,26]. However, different discrete experiments showed divided results. Because of these
http://dx.doi.org/10.1016/j.matchemphys.2014.04.031 0254-0584/Ó 2014 Elsevier B.V. All rights reserved.
Please cite this article in press as: A. Hoque, et al., Tunable optical properties of ZnO via doping monovalent (Liþ), divalent (Mn2þ) and trivalent (Fe3þ) cations, Materials Chemistry and Physics (2014), http://dx.doi.org/10.1016/j.matchemphys.2014.04.031
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A. Hoque et al. / Materials Chemistry and Physics xxx (2014) 1e5
conflicts, it renders utmost importance to investigate the influence of dopant type on ZnO properties while adopting similar materials processing, particle size and shape. Herein, in this context, a systematic study concerning the influence of monovalent (Liþ), divalent (Mn2þ) and trivalent (Fe3þ) cation doping on optical properties of ZnO is reported.
2. Experimental
Fig. 1. (A) X-ray diffraction (XRD) patterns for all ZnO and (B) enlarged view of XRD patterns in 2q ¼ 30e34 range.
Li, Mn and Fe doped ZnO particles were synthesized by a simple one step method. In a typical synthesis, polyvinyl alcohol (PVA) (0.25 g) is dissolved in distilled water (10 ml) at 70 C. In this transparent solution required amount of Zn(O2CCH3)2$2H2O was dissolved. Required amount of respective dopant precursors (LiCl/ MnCl2$2H2O/FeCl3$6H2O) was added in the above solution followed by addition of 1 M NaOH to obtain solution pHw8. NaOH addition resulted in formation of white, light yellow and light orange colored solid for Li, Mn and Fe doped reactions respectively. The reaction was continuously stirred for 3 h at 70 C. The resultant products were recovered by centrifugation and washed with deionized water and finally dried at 120 C for 24 h. The dried powders were annealed at 450 C for 1 h in air. The final calcined product also assumed powder form (volume material). All structural and optical measurements were performed directly using the ZnO powder. For pure ZnO, no dopant precursor was added in the aqueous solution of Zn(O2CCH3)2$2H2O and PVA which gives white product after addition of NaOH. The crystallographic phase identification of the products was performed using powder X-ray diffraction (XRD; Rigaku; Cu-Ka radiation, l ¼ 1.5418 A). The morphology was observed by scanning electron microscopy (SEM, JEOL JSM 6390LV) and transmission electron microscopy (TEM, FEI Technai T20; accelerating voltage 200 kV). For TEM experiment, small amount of the powder particles were homogeneously dispersed in acetone by sonication. This
Fig. 2. SEM images of (a) pure ZnO, (b) 20% LieZnO, (e) 20% MneZnO, (i) 20% FeeZnO, elemental mapping images of (c, f, j) Zn, (d, g, k) O, (h) Mn and (l) Fe.
Please cite this article in press as: A. Hoque, et al., Tunable optical properties of ZnO via doping monovalent (Liþ), divalent (Mn2þ) and trivalent (Fe3þ) cations, Materials Chemistry and Physics (2014), http://dx.doi.org/10.1016/j.matchemphys.2014.04.031
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Fig. 3. TEM images of (a) pure ZnO and (b) 20% FeeZnO.
acetone solution was dropped in a TEM grid followed by drying at 60 C. Ultravioletevisible (UVeVis) and photoluminescence (PL) spectroscopy techniques were utilized respectively for determining the optical absorption and emission properties of pure and doped ZnO particles. The UVeVis absorption spectra were recorded with Shimadzu UV 2450 spectrometer. The PL spectra were recorded with Perkin Elmer LS55 spectrometer at excitation wavelength of 325 nm. The spectra were recorded using solid state measurement. Barium sulfate powder was used as reference in UVeVis
measurement. All experiments were performed at room temperature (25 C). 3. Results and discussion Fig. 1(A) shows the XRD patterns of pure and Li, Mn, Fe doped ZnO for various doping level. The diffraction peaks can be indexed to hexagonal wurtzite crystal phase (JCPDS No. 36-1451). The strong and sharp diffraction peaks demonstrate that products are
Fig. 4. UVevis absorption spectra for all ZnO. Dopants are (A) Li, (C) Mn and (E) Fe. (ahy)2 vs hy plots are for (B) Li, (D) Mn and (F) Fe.
Please cite this article in press as: A. Hoque, et al., Tunable optical properties of ZnO via doping monovalent (Liþ), divalent (Mn2þ) and trivalent (Fe3þ) cations, Materials Chemistry and Physics (2014), http://dx.doi.org/10.1016/j.matchemphys.2014.04.031
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A. Hoque et al. / Materials Chemistry and Physics xxx (2014) 1e5
Fig. 5. Band gap energy variation with dopant concentration. Inset shows percentage band gap energy decrease with dopant concentration.
well crystallized. There is no signature of presence of any additional impurity phases of Li, Mn and Fe in the XRD patterns. It is observed that peak positions of Mn doped ZnO shift consistently to the lower angle side while it shifts to higher angle side for Fe doped ZnO (Fig. 1(B)). It can be attributed to expansion and contraction of ZnO lattice due to substitution of Mn and Fe since ionic radius of Zn2þ (0.74 A at coordination number 6) is smaller than that of Mn2þ (0.83 A at coordination number 6) and larger than Fe3þ (0.64 A at coordination number 6) [22,23]. Shifting to lower and higher angles respectively for Mn and Fe doped ZnO signify substitution of Mn2þ and Fe3þ ions in Zn2þ sites; not Mn3þ and Fe2þ ions. It is because ionic radius of Zn2þ is larger than that of Mn3þ (0.65 A at coordination number 6) and smaller than Fe2þ (0.77 A at coordination number 6) [27,28]. No peak position change is observed for Li (ionic
radius ¼ 0.76 A at coordination number 6) doped ZnO. Moreover, it is also observed that peak intensity decreases with increase in impurity content. This might be due to lattice disorder and strain induced by the dopants on ZnO lattice. From full width at half maximum (FWHM) for the (100) peak (2q ¼ 31.64 ), the crystallite size using Scherrer equation is estimated to be w22e24 nm for all synthesized materials. However, SEM images (Fig. 2) show random shaped aggregates of micronsized particles in all samples. It is attributed to uncontrolled nanoparticle agglomeration. This fact is clearly evident from the TEM images shown in Fig. 3. It shows agglomerated clusters of nanoparticles of primary sizes in the range of 20e25 nm. Elemental mapping shows homogeneous distribution of Zn, O, Mn and Fe in doped ZnO. It is evident that the number of spots corresponding to Zn and O (Fig. 2(c, d, f, g, j, k)) are higher in density compared to Mn (Fig. 2(h)) and Fe (Fig. 2(l)). The smaller number of spots for Mn and Fe are due to less dopant amount. It was not possible to detect the distribution of Li due to instrument limitation. Thus, the XRD patterns and elemental mapping confirmed substitution of Li, Mn and Fe in ZnO. Fig. 4(A, C, E) shows the UVeVis absorption spectra of pure and Li, Mn, Fe doped ZnO. Excitonic feature of pure ZnO appears at 355 nm. A red shift as compared to pure ZnO in absorption spectra is observed in all Li, Mn and Fe doped ZnO. The red shift (from 355 nm to 390 nm) is highest for Fe (20%) doped ZnO followed by Mn (20%) and Li (20%) doped ZnO. The optical band gap energy of all ZnO materials was derived from the Tauc equation [29]. According to Tauc equation, for a given direct transition the photon energy (hy) can be related to band gap energy (E) by the following relationship: ahy ¼ Aðhy EÞ1=2 ; where a, E and A are absorption coefficient, band gap and a constant respectively. Band gap were estimated by extrapolating the linear portion near the onset on the plot of (ahy)2 versus hy (Fig. 4(B, D, F)). The estimated band gap for pure ZnO is 3.36 eV which is equivalent to bulk band gap of ZnO (3.37 eV). It signifies no size dependent quantum confinement effect [25]. Therefore, it is necessary to note here that any change in optical properties of doped ZnO is only due to dopants and not due
Fig. 6. The PL spectra of all ZnO. Dopants are (A) Li, (B) Mn and (C) Fe.
Please cite this article in press as: A. Hoque, et al., Tunable optical properties of ZnO via doping monovalent (Liþ), divalent (Mn2þ) and trivalent (Fe3þ) cations, Materials Chemistry and Physics (2014), http://dx.doi.org/10.1016/j.matchemphys.2014.04.031
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to size effects (all synthesized materials possesses similar particle size w22 nm). It is clearly observed from Fig. 5 that band gap decreases with increasing dopant (Li, Mn, Fe) concentration. However, decrease is significant for Fe doped ZnO compared to Mn, Li doped ZnO. A maximum of 11% red shift is realized for 20% Fe doped ZnO, whereas it is 9% and 6% for Mn and Li doped ZnO respectively (inset of Fig. 5). It signifies the dependence of band gap on dopant type. It is attributed to the strong spin exchange interactions prevail between the d electrons of Mn/Fe and the s and p electrons of the host [25]. In Fig. 4(C and E), additional features in the form of a broad peak in 400e600 nm range can also be observed in the absorption spectra for 20% Mn and Fe doped ZnO which is attributed to de d transitions of Mn/Fe ions [3,27]. No such feature is noticeable in Li-doped ZnO; again due to absence of any ded transitions in Li ion. The photoluminescence (PL) spectra of pure and doped ZnO at an excitation wavelength 325 nm are shown in Fig. 6. Pure ZnO shows UV emission maxima at 389 nm along with violet-blue (423 nm) and blue (453 nm, 490 nm) luminescence. The PL spectra of doped ZnO are complex with emissions at different wavelengths. Quenching in emission maxima at 389 nm is observed for all doped samples with increasing dopants content. However, quenching is lowest for Li doped ZnO. Violet (415e 438 nm) and blue (454e475 nm) luminescence peaks with varying intensities are observed for all doped samples. Additional emergence of green (522e526 nm) luminescence is prominently observed in Mn and Fe doped ZnO (Fig. 5(B, C)); not observed in pure ZnO. The doped cations may induce structural defects resulting in emission at different wavelengths [30].
4. Conclusion In summary, Li, Mn, Fe doped ZnO were synthesized by a simple solegel procedure. It is demonstrated that dopant nature and concentration critically influence the optical properties of ZnO. Red shift in band gap is more pronounced in Fe doped ZnO compared to Li and Mn doped ZnO. A band gap narrowing of 11% is observed for 20% Fe doped ZnO, whereas it is 9% and 6% for Mn and Li doped ZnO respectively. The photoluminescence studies clearly show emission at different wavelengths (for violet and blue) for Li, Mn, Fe doped ZnO with concurrent quenching of UV emission. Additionally, Mn/ Fe doped ZnO emit at green wavelengths. We believe the present results will be beneficial in optimizing the optical properties of ZnO based optoelectronic devices.
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Acknowledgment SKD thanks Tezpur University for financial support. References an, V. Avrutin, [1] Ü. Özgür, Y.I. Alivov, C. Liu, A. Teke, M.A. Reshchikov, S. Dog S.J. Cho, H. Morkoç, J. Appl. Phys. 98 (2005) 041301. [2] A.B. Djurisic, Y.H. Leung, Small 2 (2006) 944. S. Dutta, S. Chattopadhyay, A. Sarkar, M. Chakrabarti, D. Sanyal, D. Jana, Prog. Mater. Sci. 54 (2009) 89. [3] Y.S. Wang, P.J. Thomas, P. O’Brien, J. Phys. Chem. B 110 (2006) 21412. [4] B.E. Sernelius, K.F. Berggren, Z.C. Jin, I. Hamberg, C.G. Granqvist, Phys. Rev. B 37 (1988) 10244. [5] L. Cao, J. Jiang, L. Zhu, Mater. Lett. 100 (2013) 201. [6] S. Kim, M.S. Kim, G. Nam, H. Park, H. Yoon, J.Y. Leem, J. Nanosci. Nanotechnol. 13 (2013) 6183. [7] W.B. Ko, J.S. Lee, S.H. Lee, S.N. Cha, J.I. Sohn, J.M. Kim, Y.J. Park, H.J. Kim, J.P. Hong, J. Nanosci. Nanotechnol. 13 (2013) 6231. [8] Y.J. Lee, J.H. Kim, B.Y. Oh, K.Y. Kim, J. Nanosci. Nanotechnol. 13 (2013) 6296. [9] S.G. Cho, D.U. Lee, E.K. Kim, J. Nanosci. Nanotechnol. 13 (2013) 6443. [10] Y. Liu, W. Luo, R. Li, X. Chen, J. Nanosci. Nanotechnol. 10 (2010) 1871. [11] R. Vettumperumal, S. Kalyanaraman, R. Thangavel, Mater. Chem. Phys. 145 (2014) 237. [12] S. Kim, H. Yoon, D.Y. Kim, S. Kim, J.-Y. Leem, Opt. Mater. 35 (2013) 2418. [13] J. Sivasankari, S. Sankar, S. Selvakumar, L. Vimaladevi, R. Krithiga, Mater. Chem. Phys. 143 (2014) 1528. [14] R. Bhattacharjee, I.-M. Hung, Mater. Chem. Phys. 143 (2014) 693. [15] S.B. Woodley, A.A. Sokol, C. Richard, A. Catlow, A.A. Al-Sunaidi, S.M. Woodley, J. Phys. Chem. C 117 (2013) 27127. [16] J. Kim, G.V. Naik, A.V. Gavrilenko, K. Dondapati, V.I. Gavrilenko, S.M. Prokes, O.J. Glembocki, V.M. Shalaev, A. Boltasseva, Phys. Rev. X 3 (2013) 041037. [17] F.C. Romeiro, J.Z. Marinho, A.C.A. Silva, N.F. Cano, N.O. Dantas, R.C. Lima, J. Phys. Chem. C 117 (2013) 26222. [18] J. Xu, S. Shi, L. Li, X. Zhang, Y. Wang, Q. Shi, S. Li, H. Wang, J. Electron. Mater. 42 (2013) 3438. [19] X. Zhang, X. Pu, Y. Chen, X. Gu, D. Xu, S. Zhang, Mater. Lett. 112 (2013) 129. [20] C.G. Jin, Y. Yang, Z.F. Wu, L.J. Zhuge, Q. Han, X.M. Wu, Y.Y. Li, Z.C. Feng, J. Mater. Chem. C 2 (2014) 2992. [21] T. Fukumura, Z. Jin, A. Ohtomo, H. Koinuma, M. Kawasaki, Appl. Phys. Lett. 75 (1999) 3366. [22] A.P. Palomino, O.P. Perez, R. Singhal, M. Tomar, J. Hwang, P.M. Voyles, J. Appl. Phys. 103 (2008) 07D121. [23] K.K. Kim, Y.R. Park, J. Appl. Phys. 96 (2004) 4150. [24] J. Anghel, A. Thurber, D.A. Tenne, C.B. Hanna, A. Punnoose, J. Appl. Phys. 107 (2010) 09E314. [25] R. Viswanatha, S. Sapra, S.S. Gupta, B. Satpati, P.V. Satyam, B.N. Dev, D.D. Sarma, J. Phys. Chem. B 108 (2004) 6303. [26] S. Kumar, S. Mukherjee, R. Singh, S. Chatterjee, A.K. Ghosh, J. Appl. Phys. 110 (2011) 103508. [27] D.Y. Inamdar, A.K. Pathak, I. Dubenko, N. Ali, S. Mahamuni, J. Phys. Chem. C 115 (2011) 23671. [28] J. Luo, J.K. Liang, O.L. Liu, F.S. Liu, Y. Zhang, B.J. Sun, G.H. Rao, J. Appl. Phys. 97 (2005) 086106. [29] J. Tauc, R. Grigorvici, Y. Yanca, Phys. Stat. Sol. 15 (1966) 627. [30] K. Vanheusden, W.L. Warren, C.H. Seager, D.R. Tallant, J.A. Voigt, B.E. Gnade, J. Appl. Phys. 79 (1996) 7983.
Please cite this article in press as: A. Hoque, et al., Tunable optical properties of ZnO via doping monovalent (Liþ), divalent (Mn2þ) and trivalent (Fe3þ) cations, Materials Chemistry and Physics (2014), http://dx.doi.org/10.1016/j.matchemphys.2014.04.031
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Tunable optical properties of ZnO via doping monovalent (Liþ), divalent (Mn2þ) and trivalent (Fe3þ) cations Ajnur Hoque, Ratan Boruah, Shyamal K. Das* Department of Physics, Tezpur University, Tezpur 784028, Assam, India
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Li, Mn and Fe doped ZnO have been synthesized by simple one step method. Band gap and luminescence of ZnO exhibit strong dependence on dopant type and concentration. Red shift in band gap is observed for all doped ZnO. Highest red shift of 11% is observed for Fe doped ZnO.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 July 2013 Received in revised form 7 April 2014 Accepted 19 April 2014
Implications of Li, Mn and Fe doping on optical properties of ZnO are discussed here. Simple one step sol egel process was utilized to synthesize pristine and doped ZnO. The optical properties of ZnO exhibited a strong function of dopant type and concentration. Incorporation of Li, Mn and Fe resulted in red shift in band gap of ZnO; highest shift being for Fe doped ZnO followed by Mn and Li doped ZnO. Band gap narrowing of 11% is realized for 20% Fe doped ZnO, whereas it is 9% and 6% for Mn and Li doped ZnO. The emission spectra show quenching of UV emission for doped ZnO. Mn and Fe doped ZnO exhibited green luminescence. Ó 2014 Elsevier B.V. All rights reserved.
Keywords: Optical materials Luminescence Semiconductors Optical properties
1. Introduction Zinc oxide (ZnO), a direct wide band gap (3.37 eV) semiconductor at room temperature, has been successfully demonstrated as a promising material for variety of applications such as transparent electronics, ultraviolet light emitters, piezoelectric devices and spin electronics due to its important electrical and optical properties [1,2]. Recently, nanostructuring of ZnO played a pivotal role in optimizing the electro-optical properties [2]. A plethora of ZnO nanoarchitectures exhibited several remarkable
* Corresponding author. Tel.: þ91 3712275586; fax: þ91 3712267005. E-mail address: [email protected] (S.K. Das).
beneficial outcomes [2]. However, materials processing in reduced dimension involve complexity and controlling their dimensions is difficult that limits their overall impact. Another effective approach for optimization of electro-optical properties of ZnO is via doping [1,3e20]. Several reports demonstrated that band gap and luminescence of ZnO can be red or blue shifted via doping different cations with varying degrees [1,3e10]. However, quite ambiguous results on doped ZnO have been reported. For instances, Kawasaki et al. and Voyles et al. reported band gap widening in Mn and Fe doped ZnO respectively [21,22] while Kim et al. and Punnoose et al. observed band gap bowing [23,24]. On the other hand, both red and blue shift in band gap of Mn/Fe doped ZnO were also reported [25,26]. However, different discrete experiments showed divided results. Because of these
http://dx.doi.org/10.1016/j.matchemphys.2014.04.031 0254-0584/Ó 2014 Elsevier B.V. All rights reserved.
Please cite this article in press as: A. Hoque, et al., Tunable optical properties of ZnO via doping monovalent (Liþ), divalent (Mn2þ) and trivalent (Fe3þ) cations, Materials Chemistry and Physics (2014), http://dx.doi.org/10.1016/j.matchemphys.2014.04.031
2
A. Hoque et al. / Materials Chemistry and Physics xxx (2014) 1e5
conflicts, it renders utmost importance to investigate the influence of dopant type on ZnO properties while adopting similar materials processing, particle size and shape. Herein, in this context, a systematic study concerning the influence of monovalent (Liþ), divalent (Mn2þ) and trivalent (Fe3þ) cation doping on optical properties of ZnO is reported.
2. Experimental
Fig. 1. (A) X-ray diffraction (XRD) patterns for all ZnO and (B) enlarged view of XRD patterns in 2q ¼ 30e34 range.
Li, Mn and Fe doped ZnO particles were synthesized by a simple one step method. In a typical synthesis, polyvinyl alcohol (PVA) (0.25 g) is dissolved in distilled water (10 ml) at 70 C. In this transparent solution required amount of Zn(O2CCH3)2$2H2O was dissolved. Required amount of respective dopant precursors (LiCl/ MnCl2$2H2O/FeCl3$6H2O) was added in the above solution followed by addition of 1 M NaOH to obtain solution pHw8. NaOH addition resulted in formation of white, light yellow and light orange colored solid for Li, Mn and Fe doped reactions respectively. The reaction was continuously stirred for 3 h at 70 C. The resultant products were recovered by centrifugation and washed with deionized water and finally dried at 120 C for 24 h. The dried powders were annealed at 450 C for 1 h in air. The final calcined product also assumed powder form (volume material). All structural and optical measurements were performed directly using the ZnO powder. For pure ZnO, no dopant precursor was added in the aqueous solution of Zn(O2CCH3)2$2H2O and PVA which gives white product after addition of NaOH. The crystallographic phase identification of the products was performed using powder X-ray diffraction (XRD; Rigaku; Cu-Ka radiation, l ¼ 1.5418 A). The morphology was observed by scanning electron microscopy (SEM, JEOL JSM 6390LV) and transmission electron microscopy (TEM, FEI Technai T20; accelerating voltage 200 kV). For TEM experiment, small amount of the powder particles were homogeneously dispersed in acetone by sonication. This
Fig. 2. SEM images of (a) pure ZnO, (b) 20% LieZnO, (e) 20% MneZnO, (i) 20% FeeZnO, elemental mapping images of (c, f, j) Zn, (d, g, k) O, (h) Mn and (l) Fe.
Please cite this article in press as: A. Hoque, et al., Tunable optical properties of ZnO via doping monovalent (Liþ), divalent (Mn2þ) and trivalent (Fe3þ) cations, Materials Chemistry and Physics (2014), http://dx.doi.org/10.1016/j.matchemphys.2014.04.031
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3
Fig. 3. TEM images of (a) pure ZnO and (b) 20% FeeZnO.
acetone solution was dropped in a TEM grid followed by drying at 60 C. Ultravioletevisible (UVeVis) and photoluminescence (PL) spectroscopy techniques were utilized respectively for determining the optical absorption and emission properties of pure and doped ZnO particles. The UVeVis absorption spectra were recorded with Shimadzu UV 2450 spectrometer. The PL spectra were recorded with Perkin Elmer LS55 spectrometer at excitation wavelength of 325 nm. The spectra were recorded using solid state measurement. Barium sulfate powder was used as reference in UVeVis
measurement. All experiments were performed at room temperature (25 C). 3. Results and discussion Fig. 1(A) shows the XRD patterns of pure and Li, Mn, Fe doped ZnO for various doping level. The diffraction peaks can be indexed to hexagonal wurtzite crystal phase (JCPDS No. 36-1451). The strong and sharp diffraction peaks demonstrate that products are
Fig. 4. UVevis absorption spectra for all ZnO. Dopants are (A) Li, (C) Mn and (E) Fe. (ahy)2 vs hy plots are for (B) Li, (D) Mn and (F) Fe.
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Fig. 5. Band gap energy variation with dopant concentration. Inset shows percentage band gap energy decrease with dopant concentration.
well crystallized. There is no signature of presence of any additional impurity phases of Li, Mn and Fe in the XRD patterns. It is observed that peak positions of Mn doped ZnO shift consistently to the lower angle side while it shifts to higher angle side for Fe doped ZnO (Fig. 1(B)). It can be attributed to expansion and contraction of ZnO lattice due to substitution of Mn and Fe since ionic radius of Zn2þ (0.74 A at coordination number 6) is smaller than that of Mn2þ (0.83 A at coordination number 6) and larger than Fe3þ (0.64 A at coordination number 6) [22,23]. Shifting to lower and higher angles respectively for Mn and Fe doped ZnO signify substitution of Mn2þ and Fe3þ ions in Zn2þ sites; not Mn3þ and Fe2þ ions. It is because ionic radius of Zn2þ is larger than that of Mn3þ (0.65 A at coordination number 6) and smaller than Fe2þ (0.77 A at coordination number 6) [27,28]. No peak position change is observed for Li (ionic
radius ¼ 0.76 A at coordination number 6) doped ZnO. Moreover, it is also observed that peak intensity decreases with increase in impurity content. This might be due to lattice disorder and strain induced by the dopants on ZnO lattice. From full width at half maximum (FWHM) for the (100) peak (2q ¼ 31.64 ), the crystallite size using Scherrer equation is estimated to be w22e24 nm for all synthesized materials. However, SEM images (Fig. 2) show random shaped aggregates of micronsized particles in all samples. It is attributed to uncontrolled nanoparticle agglomeration. This fact is clearly evident from the TEM images shown in Fig. 3. It shows agglomerated clusters of nanoparticles of primary sizes in the range of 20e25 nm. Elemental mapping shows homogeneous distribution of Zn, O, Mn and Fe in doped ZnO. It is evident that the number of spots corresponding to Zn and O (Fig. 2(c, d, f, g, j, k)) are higher in density compared to Mn (Fig. 2(h)) and Fe (Fig. 2(l)). The smaller number of spots for Mn and Fe are due to less dopant amount. It was not possible to detect the distribution of Li due to instrument limitation. Thus, the XRD patterns and elemental mapping confirmed substitution of Li, Mn and Fe in ZnO. Fig. 4(A, C, E) shows the UVeVis absorption spectra of pure and Li, Mn, Fe doped ZnO. Excitonic feature of pure ZnO appears at 355 nm. A red shift as compared to pure ZnO in absorption spectra is observed in all Li, Mn and Fe doped ZnO. The red shift (from 355 nm to 390 nm) is highest for Fe (20%) doped ZnO followed by Mn (20%) and Li (20%) doped ZnO. The optical band gap energy of all ZnO materials was derived from the Tauc equation [29]. According to Tauc equation, for a given direct transition the photon energy (hy) can be related to band gap energy (E) by the following relationship: ahy ¼ Aðhy EÞ1=2 ; where a, E and A are absorption coefficient, band gap and a constant respectively. Band gap were estimated by extrapolating the linear portion near the onset on the plot of (ahy)2 versus hy (Fig. 4(B, D, F)). The estimated band gap for pure ZnO is 3.36 eV which is equivalent to bulk band gap of ZnO (3.37 eV). It signifies no size dependent quantum confinement effect [25]. Therefore, it is necessary to note here that any change in optical properties of doped ZnO is only due to dopants and not due
Fig. 6. The PL spectra of all ZnO. Dopants are (A) Li, (B) Mn and (C) Fe.
Please cite this article in press as: A. Hoque, et al., Tunable optical properties of ZnO via doping monovalent (Liþ), divalent (Mn2þ) and trivalent (Fe3þ) cations, Materials Chemistry and Physics (2014), http://dx.doi.org/10.1016/j.matchemphys.2014.04.031
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to size effects (all synthesized materials possesses similar particle size w22 nm). It is clearly observed from Fig. 5 that band gap decreases with increasing dopant (Li, Mn, Fe) concentration. However, decrease is significant for Fe doped ZnO compared to Mn, Li doped ZnO. A maximum of 11% red shift is realized for 20% Fe doped ZnO, whereas it is 9% and 6% for Mn and Li doped ZnO respectively (inset of Fig. 5). It signifies the dependence of band gap on dopant type. It is attributed to the strong spin exchange interactions prevail between the d electrons of Mn/Fe and the s and p electrons of the host [25]. In Fig. 4(C and E), additional features in the form of a broad peak in 400e600 nm range can also be observed in the absorption spectra for 20% Mn and Fe doped ZnO which is attributed to de d transitions of Mn/Fe ions [3,27]. No such feature is noticeable in Li-doped ZnO; again due to absence of any ded transitions in Li ion. The photoluminescence (PL) spectra of pure and doped ZnO at an excitation wavelength 325 nm are shown in Fig. 6. Pure ZnO shows UV emission maxima at 389 nm along with violet-blue (423 nm) and blue (453 nm, 490 nm) luminescence. The PL spectra of doped ZnO are complex with emissions at different wavelengths. Quenching in emission maxima at 389 nm is observed for all doped samples with increasing dopants content. However, quenching is lowest for Li doped ZnO. Violet (415e 438 nm) and blue (454e475 nm) luminescence peaks with varying intensities are observed for all doped samples. Additional emergence of green (522e526 nm) luminescence is prominently observed in Mn and Fe doped ZnO (Fig. 5(B, C)); not observed in pure ZnO. The doped cations may induce structural defects resulting in emission at different wavelengths [30].
4. Conclusion In summary, Li, Mn, Fe doped ZnO were synthesized by a simple solegel procedure. It is demonstrated that dopant nature and concentration critically influence the optical properties of ZnO. Red shift in band gap is more pronounced in Fe doped ZnO compared to Li and Mn doped ZnO. A band gap narrowing of 11% is observed for 20% Fe doped ZnO, whereas it is 9% and 6% for Mn and Li doped ZnO respectively. The photoluminescence studies clearly show emission at different wavelengths (for violet and blue) for Li, Mn, Fe doped ZnO with concurrent quenching of UV emission. Additionally, Mn/ Fe doped ZnO emit at green wavelengths. We believe the present results will be beneficial in optimizing the optical properties of ZnO based optoelectronic devices.
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Please cite this article in press as: A. Hoque, et al., Tunable optical properties of ZnO via doping monovalent (Liþ), divalent (Mn2þ) and trivalent (Fe3þ) cations, Materials Chemistry and Physics (2014), http://dx.doi.org/10.1016/j.matchemphys.2014.04.031