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Effect of annealing temperature on the structural and optical properties of Zn1−xMgxO particles prepared by oxalate precursor
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Colloids and Surfaces A: Physicochem. Eng. Aspects 320 (2008) 156–160
Effect of annealing temperature on the structural and optical properties of Zn1−xMgxO particles prepared by oxalate precursor Zhijie Li a , Wenzhong Shen b , Shuwen Xue a , Xiaotao Zu a,∗ a
Department of Applied Physics, University of Electronic Science and Technology of China, Chengdu 610041, PR China b State Key Laboratory of Heavy Oil, China University of Petroleum, Dongying 257061, PR China Received 28 October 2007; received in revised form 21 December 2007; accepted 29 January 2008 Available online 7 February 2008
Abstract Zn1−x Mgx O particles were prepared using zinc and magnesium oxalate precursor by co-precipitated method. The lattice constants of Zn1−x Mgx O proved that the interstitial Mg formed at 500 ◦ C and Mg replaced Zn in ZnO tetrahedral coordination at 800 ◦ C. Compared with the ZnO, the absorbing band edge of the Zn1−x Mgx O displayed blue shifts. The room temperature photoluminescence was similar to ZnO and variation of Mg content did not change the shape or peak position of the emission spectra markedly when it was annealed at 500 ◦ C. However, its blue emission band disappeared, and a relatively strong green light emission at 498 nm appeared after annealed at 800 ◦ C. The photoluminescence intensity ratios I(green) /I(UV) of Zn1−x Mgx O varied with Mg content and the green light emission peak shifted from 498 nm to 472 nm when Mg content increased from 0 to 2.0 at.%. © 2008 Elsevier B.V. All rights reserved. Keywords: ZnO; Zn1−x Mgx O; Photoluminescence; Co-precipitated
1. Introduction As a luminescent material, ZnO has attracted increasing attention because of its wide applications in vacuum fluorescent display (VFD) [1], field emission display (FED), [2] electroluminescent display (ELD) fields [3]. How to realize the band gap engineering by alloying ZnO with other metallic oxides was one important step to design the ZnO-based optoelectronic devices. The energy band gap and luminescence of ZnO could be adjusted by alloying with MgO. The band gap of the ternary alloys Zn1−x Mgx O could be expanded from 3.3 eV to 4.20 eV and a brighter UV luminescence was produced at room temperature. Both the Mg concentration and the annealing temperature affected its UV luminescence [4–6]. Recently, it was also found that the sintered Zn1−x Mgx O pellets emitted bright orange lights induced by UV excitation [7,8]. Therefore, it was important to investigate the effect of annealing temperature on the structural and optical properties of Zn1−x Mgx O system.
∗
Corresponding author. Tel.: +86 28 83201939; fax: +86 28 83201939. E-mail address: [email protected] (X. Zu).
0927-7757/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2008.01.041
At present, many methods had been used to prepare Zn1−x Mgx O alloys, such as pulsed-laser deposition (PLD) [9–11], thermal decomposition method [4,7], and sol–gel method [12–14]. However, in order to form homogeneous mixing of MgO with ZnO, the above-mentioned methods required either relatively high temperature or complicated procedures and expensive equipment. Oxalate precursor method had the advantage of maximum homogeneity, excellent stoichiometry, low trace impurity content, low decomposition temperature, and very fine particle nature. But it had not been used to prepare Zn1−x Mgx O. In this paper, the zinc and magnesium oxalates were adopted to prepare Zn1−x Mgx O particles and the effect of annealing temperature on its structural and optical properties were investigated. 2. Experimental 2.1. Synthesis of samples Zn(CH3 COO)2 and Mg(CH3 COO)2 was selected as the co-precipitated precursors. Firstly, 120 mL Zn(CH3 COO)2 aqueous solutions of 0.3 mol L−1 were prepared, and 0.0571 g Mg(CH3 COO)2 was added. Secondly, 0.3 mol L−1 H2 C2 O4
Z. Li et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 320 (2008) 156–160
Fig. 1. Thermo-gravimetric curve of the oxalate precursor of Zn0.99 Mg0.01 O particles.
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Fig. 3. XRD patterns of Zn0.99 Mg0.01 O particles annealed at 500 ◦ C and 800 ◦ C.
3. Results and discussion aqueous solution was added under magnetic stirring. After stirred for 30 min, the precursor was separated by filtration and dried at 80 ◦ C to obtain the oxalate precursor. Finally, the precursor was annealed at 500 ◦ C or 800 ◦ C for 1 h in air and Zn1−x Mgx O particles were obtained. 2.2. Characterization of samples The concentration of Mg in samples was measured by inductively coupled plasma optical emission spectrometer (ICP, AtomScan 16). The thermal gravimetry (TG) analysis was carried out in PerkinElmer TGA-2 thermal gravimeter in air at a heating rate of 10 K/min. The crystalline phase of Zn1−x Mgx O was determined by X-ray diffraction (XRD, Cu K␣, 40 kV, 60 mA, Rigaku D/max-2400). The morphology of particles was observed by transmission electron microscope (TEM, Hitachi-600-2). Diffuse reflectance spectra (DRS) recorded by a Shimadzu UV-2101 apparatus, equipped with an integrating sphere, using BaSO4 as reference. The photoluminescence (PL) spectrum was recorded by RF-5301PC.
3.1. The thermal gravimetry analysis Fig. 1 shows the thermo-gravimetric curve of the oxalate precursor of Zn0.99 Mg0.01 O particles obtained using co-precipitated method. There were two steps weight losses. One occurred at 136 ◦ C, which was the conversion of oxalate precursor dihydrate to anhydrous zinc oxalate. The other took place around 376 ◦ C, which was the conversion of anhydrous oxalate precursor to Zn0.99 Mg0.01 O. So the oxalate precursor should completely convert to Zn0.99 Mg0.01 O at 500 ◦ C. 3.2. Structural analyses The TEM images of Zn0.99 Mg0.01 O annealed at 500 ◦ C and 800 ◦ C showed in Fig. 2(A) and (B), respectively. It could be seen that the particles annealed at 800 ◦ C were round and the average particle sizes were about 500 nm. It was larger than the particles annealed at 500 ◦ C. Fig. 3 shows XRD patterns of Zn0.99 Mg0.01 O particles that were annealed at 500 ◦ C and 800 ◦ C for 1 h, respectively. The XRD pattern obtained at 500 ◦ C was
Fig. 2. The TEM images of Zn0.99 Mg0.01 O (A) annealed at 500 ◦ C; (B) annealed at 800 ◦ C.
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Z. Li et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 320 (2008) 156–160
Table 1 The structure parameters of Zn0.99 Mg0.01 O ˚ Cell parameter (A)
Samples
(500 ◦ C)
Zn0.99 Mg0.01 O Zn0.99 Mg0.01 O (800 ◦ C) Standard parameters (ZnO)
a or b
c
3.254 3.252 3.249
5.213 5.204 5.205
well-defined. It indicated that Zn0.99 Mg0.01 O particles possessed polycrystalline hexagonal wurtzite structure. The intensity of peaks remarkably increased when it was annealed at 800 ◦ C, suggesting that the particle annealed at 800 ◦ C was better crystallized than 500 ◦ C. Based on the XRD spectra, Rietveld refinement was carried out to determine the crystal structure of Zn1−x Mgx O. The ionic radius of Mg2+ (0.057 nm) is smaller than Zn2+ (0.060 nm), so replacement of Zn by Mg in tetrahedral coordination should not cause a significant change in lattice constants. By doping suitable Mg into ZnO, it should be possible to obtain a ternary Zn1−x Mgx O alloy, which still had a lattice constant similar to that of pure ZnO. The lattice parameters calculated by the Rietveld refinement were listed in Table 1. Here, lattice constants of Zn0.99 Mg0.01 O annealed at 800 ◦ C (resulting values a = 3.251 and c = 5.204) were similar to that of pure ZnO (a = 3.249 and c = 5.205). It meant that Mg replaced Zn in ZnO tetrahedral coordination. But for Zn0.99 Mg0.01 O annealed at 500 ◦ C, the lattice constants (resulting values a = 3.254 and c = 5.213) was slightly larger than those of pure ZnO. The expansion of the lattice constants of Zn0.99 Mg0.01 O indicated that Mg did not really replace Zn in ZnO tetrahedral coordination, but it formed the interstitial Mg. 3.3. The diffuse reflectance spectra and the photoluminescence spectra analysis The diffuse reflectance spectra and the photoluminescence spectra of Zn1−x Mgx O were measured at room temperature to investigate the optical properties. The DRS spectra of Zn1−x Mgx O were shown in Figs. 4 and 5. For samples annealed at 500 ◦ C, the absorbing band edge showed a blue shift with the increase of Mg concentrate. It was 382 nm, 381.5 nm, 381 nm, 380 nm, 379 nm and 378 nm for 0, 0.1%, 0.5%, 1.0%, 1.5% and 2.0% Mg-doped ZnO, respectively. It proved that the absorption edge clearly depended on the Mg content. The increase of the optical band gap energy with doping of Mg concentration was attributed to the so-called Moss–Burstein effect caused by electrons generated by oxygen vacancies [15]. It meant that the lifting of the Fermi level into the conduction band of the degenerate semiconductor due to the increase in the carrier density leaded to the energy band broadening effect. In the case, the Moss–Burstein effect should cause by electrons generated by oxygen vacancies. In the Zn1−x Mgx O materials, oxygen vacancies were the most dominant defect centers and the creation of an oxygen vacancy was associated with the generation of free
Fig. 4. The DRS spectra of Zn1−x Mgx O with difference of Mg annealed at 500 ◦ C.
charge carriers according to the following process [16]: O0 → V0 ++ + 2e− + 21 O2 where O0 represented an oxygen ion at its normal site, V0 ++ the doubly charged oxygen vacancy and e− was the free electronic charge generated through the vacancy formation. The blue shift suggested that the oxygen vacancies in Zn1−x Mgx O annealed at 500 ◦ C increased with Mg content. When it was annealed at 800 ◦ C, Zn would replace by Mg in tetrahedral coordination, and in the process, the vacancies were gradually increased with Mg contents. So the shift of the Eg was more remarkable, it was from 387 nm to 378 nm when Mg content varied from 0 to 2.0 at.%, respectively. The shift of the direct transition energy Eg to higher values due to Moss–Burstein effect had been found in Al-doped ZnO [15], Sn-doped In2 O3 [17], CdIn2 O4 films [18] and so on. Thus, Zn1−x Mgx O could be considered not only as an ultraviolet light emitting material, but also as ultraviolet absorption materials, which absorption energy could be adjust by the Mg content. Fig. 6 shows the room temperature photoluminescence using 350 nm excitation wavelength of Zn1−x Mgx O annealed at 500 ◦ C. The emission consisted of two bands: an intensive
Fig. 5. The DRS spectra of Zn1−x Mgx O with difference of Mg annealed at 800 ◦ C.
Z. Li et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 320 (2008) 156–160
Fig. 6. The room temperature photoluminescence (PL) using 350 nm excitation wavelength of Zn1−x Mgx O with difference of Mg annealed at 500 ◦ C.
ultraviolet emission peak at 387 nm and a relatively weak and broad blue light emission peak at 460 nm. The emission spectra of Zn1−x Mgx O were similar to that of ZnO and the Mg content did not markedly change the shape or peak position of the emission spectra. It was well known that the UV emission peak usually originated from a near-band-edge (NBE) transition of the wide band gap due to the annihilation of excitons [8]. In addition, the sharp and strong UV emission gave powerful evidence that the Zn1−x Mgx O have a hexagonal wurtzite structure. It also indicated that oxalate precursor had completely transferred Zn1−x Mgx O at 500 ◦ C. The weak blue emission possibly resulted from the deep-level (DL) defect. It implied that there were few defects in the Zn1−x Mgx O. For comparison, the room temperature photoluminescence using 350 nm excitation wavelength of Zn1−x Mgx O annealed at 800 ◦ C was also displayed in Fig. 7. For pure ZnO, the emission band was invariability. But for Zn1−x Mgx O, the intensity of the UV emission dramatically decreased without changing its position. At the same time, the blue emission band disappeared. However, a relatively strong and broad green light emission at around 498 nm appeared. The origin of green light emission had been a subject of much debate. The impurities and structural defects, such as oxygen vacancies and so forth, were responsible for the deep level or trap-state emission in the visible range.
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In this case, it was believed that the green emission originated from the recombination of a photogenerated hole with an electron occupying the oxygen vacancy in the Zn1−x Mgx O lattice. The origin of oxygen vacancies in Zn1−x Mgx O was the evaporation of O at high temperature. The broad widths of the observed bands in the green region suggested that it was reasonable for a deep trap level. The ratio of green intensity to UV intensity was an effective measure of Zn1−x Mgx O crystal quality. The calculated PL intensity ratios I(green) /I(UV) of Zn1−x Mgx O were 0.34, 2.32, 2.54, 3.57, 1.07 and 0.62 according to the with different Mg contents of 0, 0.1%, 0.5%, 1.0%, 1.5% and 2.0%, respectively. The change in the ratio of emission intensities was determined by the increase of green emission and the decrease of UV emission. The Zn0.98 Mg0.02 O displayed bright green emission, but it still had high crystal quality. Furthermore, the green PL peak remarkably shifted to the higher energy side with Mg content. It shifted from 498 nm to 472 nm when Mg contents varied from 0 to 2.0 at.%. It indicated that new defects formed in Zn1−x Mgx O, such as Zn interstitials could be regenerated in the crystal at higher annealing temperatures, leading to a blue shift of green emissions. 4. Conclusion Zn1−x Mgx O particles could be prepared using it oxalate precursor by co-precipitated method. The lattice constants indicated that Mg really replaced Zn in ZnO tetrahedral coordination at 800 ◦ C. Due to Moss–Burstein effect, the absorption edges of the Zn1−x Mgx O blue shifted with Mg content and the shift was more remarkable after annealed at 800 ◦ C. After annealed at 500 ◦ C, the room temperature photoluminescence was similar to that of ZnO and Mg content did not markedly change the shape or peak position of the emission spectra. But after annealed at 800 ◦ C, its blue emission band disappeared, and a relatively strong and broad green light emission appeared at 498 nm. The PL intensity ratios I(green) /I(UV) of Zn1−x Mgx O varied with Mg content and the green PL peak shifted from 498 nm to 472 nm when Mg contents increased from 0 to 2.0 at.%. References
Fig. 7. The room temperature photoluminescence (PL) using an excitation wavelength of 350 nm of Zn1−x Mgx O with difference of Mg annealed at 800 ◦ C.
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[11] M. Nakayama, H. Tanaka, K. Masuko, T. Fukushima, A. Ashida, N. Fujimura, Appl. Phys. Lett. 88 (2006) 241908. [12] S. Fujihara, C. Sasaki, T. Kimura, J. Eur. Ceram. Soc. 21 (2001) 2109. [13] G. Ning, X.g. Zhao, J. Li, Opt. Mater. 27 (2004) 1. [14] S. Das, S. Chakrabarti, S. Chaudhuri, J. Phys. D: Appl. Phys. 38 (2005) 4021.
[15] B.E. Sernelius, K.F. Berggren, Z.C. Jin, I. Hamberg, C.G. Granqvist, Phys. Rev. B 37 (1988) 10244. [16] S.C. Roy, G.L. Sharma, M.C. Bhatnagar, Solid State Commun. 141 (2007) 243. [17] T.J. Vink, M.H.F. Overwijk, W. Walrave, J. Appl. Phys. 80 (1996) 3734. [18] W.L. Wang, J.Y. Gao, K.J. Lao, D.L. Peng, S.R. Jiang, Mater. Lett. 13 (1992) 342.
Colloids and Surfaces A: Physicochem. Eng. Aspects 320 (2008) 156–160
Effect of annealing temperature on the structural and optical properties of Zn1−xMgxO particles prepared by oxalate precursor Zhijie Li a , Wenzhong Shen b , Shuwen Xue a , Xiaotao Zu a,∗ a
Department of Applied Physics, University of Electronic Science and Technology of China, Chengdu 610041, PR China b State Key Laboratory of Heavy Oil, China University of Petroleum, Dongying 257061, PR China Received 28 October 2007; received in revised form 21 December 2007; accepted 29 January 2008 Available online 7 February 2008
Abstract Zn1−x Mgx O particles were prepared using zinc and magnesium oxalate precursor by co-precipitated method. The lattice constants of Zn1−x Mgx O proved that the interstitial Mg formed at 500 ◦ C and Mg replaced Zn in ZnO tetrahedral coordination at 800 ◦ C. Compared with the ZnO, the absorbing band edge of the Zn1−x Mgx O displayed blue shifts. The room temperature photoluminescence was similar to ZnO and variation of Mg content did not change the shape or peak position of the emission spectra markedly when it was annealed at 500 ◦ C. However, its blue emission band disappeared, and a relatively strong green light emission at 498 nm appeared after annealed at 800 ◦ C. The photoluminescence intensity ratios I(green) /I(UV) of Zn1−x Mgx O varied with Mg content and the green light emission peak shifted from 498 nm to 472 nm when Mg content increased from 0 to 2.0 at.%. © 2008 Elsevier B.V. All rights reserved. Keywords: ZnO; Zn1−x Mgx O; Photoluminescence; Co-precipitated
1. Introduction As a luminescent material, ZnO has attracted increasing attention because of its wide applications in vacuum fluorescent display (VFD) [1], field emission display (FED), [2] electroluminescent display (ELD) fields [3]. How to realize the band gap engineering by alloying ZnO with other metallic oxides was one important step to design the ZnO-based optoelectronic devices. The energy band gap and luminescence of ZnO could be adjusted by alloying with MgO. The band gap of the ternary alloys Zn1−x Mgx O could be expanded from 3.3 eV to 4.20 eV and a brighter UV luminescence was produced at room temperature. Both the Mg concentration and the annealing temperature affected its UV luminescence [4–6]. Recently, it was also found that the sintered Zn1−x Mgx O pellets emitted bright orange lights induced by UV excitation [7,8]. Therefore, it was important to investigate the effect of annealing temperature on the structural and optical properties of Zn1−x Mgx O system.
∗
Corresponding author. Tel.: +86 28 83201939; fax: +86 28 83201939. E-mail address: [email protected] (X. Zu).
0927-7757/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2008.01.041
At present, many methods had been used to prepare Zn1−x Mgx O alloys, such as pulsed-laser deposition (PLD) [9–11], thermal decomposition method [4,7], and sol–gel method [12–14]. However, in order to form homogeneous mixing of MgO with ZnO, the above-mentioned methods required either relatively high temperature or complicated procedures and expensive equipment. Oxalate precursor method had the advantage of maximum homogeneity, excellent stoichiometry, low trace impurity content, low decomposition temperature, and very fine particle nature. But it had not been used to prepare Zn1−x Mgx O. In this paper, the zinc and magnesium oxalates were adopted to prepare Zn1−x Mgx O particles and the effect of annealing temperature on its structural and optical properties were investigated. 2. Experimental 2.1. Synthesis of samples Zn(CH3 COO)2 and Mg(CH3 COO)2 was selected as the co-precipitated precursors. Firstly, 120 mL Zn(CH3 COO)2 aqueous solutions of 0.3 mol L−1 were prepared, and 0.0571 g Mg(CH3 COO)2 was added. Secondly, 0.3 mol L−1 H2 C2 O4
Z. Li et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 320 (2008) 156–160
Fig. 1. Thermo-gravimetric curve of the oxalate precursor of Zn0.99 Mg0.01 O particles.
157
Fig. 3. XRD patterns of Zn0.99 Mg0.01 O particles annealed at 500 ◦ C and 800 ◦ C.
3. Results and discussion aqueous solution was added under magnetic stirring. After stirred for 30 min, the precursor was separated by filtration and dried at 80 ◦ C to obtain the oxalate precursor. Finally, the precursor was annealed at 500 ◦ C or 800 ◦ C for 1 h in air and Zn1−x Mgx O particles were obtained. 2.2. Characterization of samples The concentration of Mg in samples was measured by inductively coupled plasma optical emission spectrometer (ICP, AtomScan 16). The thermal gravimetry (TG) analysis was carried out in PerkinElmer TGA-2 thermal gravimeter in air at a heating rate of 10 K/min. The crystalline phase of Zn1−x Mgx O was determined by X-ray diffraction (XRD, Cu K␣, 40 kV, 60 mA, Rigaku D/max-2400). The morphology of particles was observed by transmission electron microscope (TEM, Hitachi-600-2). Diffuse reflectance spectra (DRS) recorded by a Shimadzu UV-2101 apparatus, equipped with an integrating sphere, using BaSO4 as reference. The photoluminescence (PL) spectrum was recorded by RF-5301PC.
3.1. The thermal gravimetry analysis Fig. 1 shows the thermo-gravimetric curve of the oxalate precursor of Zn0.99 Mg0.01 O particles obtained using co-precipitated method. There were two steps weight losses. One occurred at 136 ◦ C, which was the conversion of oxalate precursor dihydrate to anhydrous zinc oxalate. The other took place around 376 ◦ C, which was the conversion of anhydrous oxalate precursor to Zn0.99 Mg0.01 O. So the oxalate precursor should completely convert to Zn0.99 Mg0.01 O at 500 ◦ C. 3.2. Structural analyses The TEM images of Zn0.99 Mg0.01 O annealed at 500 ◦ C and 800 ◦ C showed in Fig. 2(A) and (B), respectively. It could be seen that the particles annealed at 800 ◦ C were round and the average particle sizes were about 500 nm. It was larger than the particles annealed at 500 ◦ C. Fig. 3 shows XRD patterns of Zn0.99 Mg0.01 O particles that were annealed at 500 ◦ C and 800 ◦ C for 1 h, respectively. The XRD pattern obtained at 500 ◦ C was
Fig. 2. The TEM images of Zn0.99 Mg0.01 O (A) annealed at 500 ◦ C; (B) annealed at 800 ◦ C.
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Z. Li et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 320 (2008) 156–160
Table 1 The structure parameters of Zn0.99 Mg0.01 O ˚ Cell parameter (A)
Samples
(500 ◦ C)
Zn0.99 Mg0.01 O Zn0.99 Mg0.01 O (800 ◦ C) Standard parameters (ZnO)
a or b
c
3.254 3.252 3.249
5.213 5.204 5.205
well-defined. It indicated that Zn0.99 Mg0.01 O particles possessed polycrystalline hexagonal wurtzite structure. The intensity of peaks remarkably increased when it was annealed at 800 ◦ C, suggesting that the particle annealed at 800 ◦ C was better crystallized than 500 ◦ C. Based on the XRD spectra, Rietveld refinement was carried out to determine the crystal structure of Zn1−x Mgx O. The ionic radius of Mg2+ (0.057 nm) is smaller than Zn2+ (0.060 nm), so replacement of Zn by Mg in tetrahedral coordination should not cause a significant change in lattice constants. By doping suitable Mg into ZnO, it should be possible to obtain a ternary Zn1−x Mgx O alloy, which still had a lattice constant similar to that of pure ZnO. The lattice parameters calculated by the Rietveld refinement were listed in Table 1. Here, lattice constants of Zn0.99 Mg0.01 O annealed at 800 ◦ C (resulting values a = 3.251 and c = 5.204) were similar to that of pure ZnO (a = 3.249 and c = 5.205). It meant that Mg replaced Zn in ZnO tetrahedral coordination. But for Zn0.99 Mg0.01 O annealed at 500 ◦ C, the lattice constants (resulting values a = 3.254 and c = 5.213) was slightly larger than those of pure ZnO. The expansion of the lattice constants of Zn0.99 Mg0.01 O indicated that Mg did not really replace Zn in ZnO tetrahedral coordination, but it formed the interstitial Mg. 3.3. The diffuse reflectance spectra and the photoluminescence spectra analysis The diffuse reflectance spectra and the photoluminescence spectra of Zn1−x Mgx O were measured at room temperature to investigate the optical properties. The DRS spectra of Zn1−x Mgx O were shown in Figs. 4 and 5. For samples annealed at 500 ◦ C, the absorbing band edge showed a blue shift with the increase of Mg concentrate. It was 382 nm, 381.5 nm, 381 nm, 380 nm, 379 nm and 378 nm for 0, 0.1%, 0.5%, 1.0%, 1.5% and 2.0% Mg-doped ZnO, respectively. It proved that the absorption edge clearly depended on the Mg content. The increase of the optical band gap energy with doping of Mg concentration was attributed to the so-called Moss–Burstein effect caused by electrons generated by oxygen vacancies [15]. It meant that the lifting of the Fermi level into the conduction band of the degenerate semiconductor due to the increase in the carrier density leaded to the energy band broadening effect. In the case, the Moss–Burstein effect should cause by electrons generated by oxygen vacancies. In the Zn1−x Mgx O materials, oxygen vacancies were the most dominant defect centers and the creation of an oxygen vacancy was associated with the generation of free
Fig. 4. The DRS spectra of Zn1−x Mgx O with difference of Mg annealed at 500 ◦ C.
charge carriers according to the following process [16]: O0 → V0 ++ + 2e− + 21 O2 where O0 represented an oxygen ion at its normal site, V0 ++ the doubly charged oxygen vacancy and e− was the free electronic charge generated through the vacancy formation. The blue shift suggested that the oxygen vacancies in Zn1−x Mgx O annealed at 500 ◦ C increased with Mg content. When it was annealed at 800 ◦ C, Zn would replace by Mg in tetrahedral coordination, and in the process, the vacancies were gradually increased with Mg contents. So the shift of the Eg was more remarkable, it was from 387 nm to 378 nm when Mg content varied from 0 to 2.0 at.%, respectively. The shift of the direct transition energy Eg to higher values due to Moss–Burstein effect had been found in Al-doped ZnO [15], Sn-doped In2 O3 [17], CdIn2 O4 films [18] and so on. Thus, Zn1−x Mgx O could be considered not only as an ultraviolet light emitting material, but also as ultraviolet absorption materials, which absorption energy could be adjust by the Mg content. Fig. 6 shows the room temperature photoluminescence using 350 nm excitation wavelength of Zn1−x Mgx O annealed at 500 ◦ C. The emission consisted of two bands: an intensive
Fig. 5. The DRS spectra of Zn1−x Mgx O with difference of Mg annealed at 800 ◦ C.
Z. Li et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 320 (2008) 156–160
Fig. 6. The room temperature photoluminescence (PL) using 350 nm excitation wavelength of Zn1−x Mgx O with difference of Mg annealed at 500 ◦ C.
ultraviolet emission peak at 387 nm and a relatively weak and broad blue light emission peak at 460 nm. The emission spectra of Zn1−x Mgx O were similar to that of ZnO and the Mg content did not markedly change the shape or peak position of the emission spectra. It was well known that the UV emission peak usually originated from a near-band-edge (NBE) transition of the wide band gap due to the annihilation of excitons [8]. In addition, the sharp and strong UV emission gave powerful evidence that the Zn1−x Mgx O have a hexagonal wurtzite structure. It also indicated that oxalate precursor had completely transferred Zn1−x Mgx O at 500 ◦ C. The weak blue emission possibly resulted from the deep-level (DL) defect. It implied that there were few defects in the Zn1−x Mgx O. For comparison, the room temperature photoluminescence using 350 nm excitation wavelength of Zn1−x Mgx O annealed at 800 ◦ C was also displayed in Fig. 7. For pure ZnO, the emission band was invariability. But for Zn1−x Mgx O, the intensity of the UV emission dramatically decreased without changing its position. At the same time, the blue emission band disappeared. However, a relatively strong and broad green light emission at around 498 nm appeared. The origin of green light emission had been a subject of much debate. The impurities and structural defects, such as oxygen vacancies and so forth, were responsible for the deep level or trap-state emission in the visible range.
159
In this case, it was believed that the green emission originated from the recombination of a photogenerated hole with an electron occupying the oxygen vacancy in the Zn1−x Mgx O lattice. The origin of oxygen vacancies in Zn1−x Mgx O was the evaporation of O at high temperature. The broad widths of the observed bands in the green region suggested that it was reasonable for a deep trap level. The ratio of green intensity to UV intensity was an effective measure of Zn1−x Mgx O crystal quality. The calculated PL intensity ratios I(green) /I(UV) of Zn1−x Mgx O were 0.34, 2.32, 2.54, 3.57, 1.07 and 0.62 according to the with different Mg contents of 0, 0.1%, 0.5%, 1.0%, 1.5% and 2.0%, respectively. The change in the ratio of emission intensities was determined by the increase of green emission and the decrease of UV emission. The Zn0.98 Mg0.02 O displayed bright green emission, but it still had high crystal quality. Furthermore, the green PL peak remarkably shifted to the higher energy side with Mg content. It shifted from 498 nm to 472 nm when Mg contents varied from 0 to 2.0 at.%. It indicated that new defects formed in Zn1−x Mgx O, such as Zn interstitials could be regenerated in the crystal at higher annealing temperatures, leading to a blue shift of green emissions. 4. Conclusion Zn1−x Mgx O particles could be prepared using it oxalate precursor by co-precipitated method. The lattice constants indicated that Mg really replaced Zn in ZnO tetrahedral coordination at 800 ◦ C. Due to Moss–Burstein effect, the absorption edges of the Zn1−x Mgx O blue shifted with Mg content and the shift was more remarkable after annealed at 800 ◦ C. After annealed at 500 ◦ C, the room temperature photoluminescence was similar to that of ZnO and Mg content did not markedly change the shape or peak position of the emission spectra. But after annealed at 800 ◦ C, its blue emission band disappeared, and a relatively strong and broad green light emission appeared at 498 nm. The PL intensity ratios I(green) /I(UV) of Zn1−x Mgx O varied with Mg content and the green PL peak shifted from 498 nm to 472 nm when Mg contents increased from 0 to 2.0 at.%. References
Fig. 7. The room temperature photoluminescence (PL) using an excitation wavelength of 350 nm of Zn1−x Mgx O with difference of Mg annealed at 800 ◦ C.
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