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shell nanorod arrays
Materials Letters 86 (2012) 80–83
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Effects of annealing temperature on the properties of ZnO/SnO2 core/shell nanorod arrays Ya-Fang Tu a, Qiu-Ming Fu b, Jian-Ping Sang a,c,n, Zhi-Jie Tan c, Xian-Wu Zou c a
Department of Physics, Institute for Interdisciplinary Research, Jianghan University, Wuhan 430056, China School of Materials Science and Engineering, Wuhan Institute of Technology, Wuhan 430073, China c Department of Physics, Wuhan University, Wuhan 430072, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 May 2012 Accepted 11 July 2012 Available online 20 July 2012
The effects of annealing temperature on the morphologies, structures and optical properties of ZnO/SnO2 core/shell nanorod arrays were investigated. It is found that the annealing of the samples in a muffle furnace at temperatures up to 900 1C transformed the nanorods into large particles and produced the Zn2SnO4 phase. The photoluminescence results showed that when the annealing temperature was increased up to 700 1C, the ratio of visible/ultraviolet emission increased and the center of the visible emission shifted from yellow to green. When the annealing temperature was 900 1C, the green emission made a red-shift, and a new broad peak appeared in the spectra. Possible emission mechanisms were also discussed. & 2012 Elsevier B.V. All rights reserved.
Keywords: Nanocomposites Semiconductors Microstructure
1. Introduction
2. Experimental methods
Recently, there has been much interest in the fabrication of one-dimensional core/shell structured nanomaterials due to their novel properties and potential applications in photoelectronic nanodevices. As a wide variety of core shell nanostructures has been synthesized, modifying the properties of the nanostructures is becoming increasingly important. It has been reported that the photoluminescence (PL) emission intensity of core/shell nanostructures can be significantly increased and the wavelength of the emission can be controlled by choosing a proper coating material or changing the thickness of coating layer [1,2]. ZnO and SnO2, two well-known semiconductors, have received a considerable amount of attention over the past few years due to their excellent optical, electrical and gas sensing properties. Coaxial one-dimensional nanostructures consisting of a core of ZnO and a shell of SnO2 have also been reported by some groups, and excellent physical and chemical properties have been found [3,4]. However, there is no report on the influence of annealing on the PL properties of ZnO/SnO2 core/shell onedimensional nanostructures to the best of our knowledge. In this paper, the effects of annealing temperatures on the morphology, structure and PL properties of ZnO/SnO2 core/shell nanorod arrays were investigated.
As the core, ZnO nanorod arrays were grown by the aqueous chemical growth method [5]. Briefly, a ZnO seed layer was first deposited on a Si substrate by the sol–gel technique. Then the substrate was placed in a mixture of 0.05 M Zn(NO3)2 and 0.05 M C6H12N4 aqueous solution at 90 1C for 6 h. Deposition of SnO2 shell layers on ZnO nanorods was carried out by the liquid phase deposition method [6]. The Si substrate with ZnO nanorod arrays was immersed in a treatment solution of 0.1 M (NH4)2SnF6 and maintained at 40 1C for 5 min. Finally, the as-prepared samples were annealed in a muffle furnace for 1 h at 500, 700 and 900 1C. The morphology and structure of the samples were examined by a field emission scanning electron microscope (FE-SEM, SIRION, FEI Company) equipped with energy dispersive X-ray spectroscopy (EDS), a X-ray diffractometer (XRD, D8 Advance, Bruker Axs) and a transmission electron microscope (TEM, JEM-2010FEF, JEOL) assisted with selected area electron diffraction (SAED). The PL measurements were performed with a He–Cd laser (l ¼325 nm) at room temperature.
n Corresponding author at: Department of Physics, Wuhan University, Wuhan, Hubei 430072, China. Tel.: þ 86 27 88775901; fax: þ 86 27 68752569. E-mail address: [email protected] (J.-P. Sang).
0167-577X/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2012.07.037
3. Results and discussion Fig. 1(a) and (b) shows the SEM images in top view of the ZnO nanorod arrays and the as-grown ZnO/SnO2 core/shell nanorod arrays. Core/shell structures can be clearly observed from the magnified view of some broken nanorods (the inset of Fig. 1(b)). Fig. 1(c) and (d) shows the EDS profiles of the sample in (a) and (b), respectively. While the EDS pattern of the ZnO nanorods
Y.-F. Tu et al. / Materials Letters 86 (2012) 80–83
81
Fig. 1. Top view SEM images of (a) ZnO nanorod arrays and (b) ZnO/SnO2 core/shell nanorod arrays. The insets show the magnified images. (c) and (d) are the corresponding EDS patterns of (a) and (b), respectively.
Fig. 2. 451 tilted SEM images of (a) as-grown ZnO/SnO2 core/shell nanorod arrays and annealed at (b) 500 1C, (c) 700 1C and (d) 900 1C.
shows only elemental Zn and O (the Si and Pt signals have originated from the substrate and the evaporated conducting layer for SEM analysis, respectively), additional Sn element is found to be present after SnO2 deposition, which provides powerful evidence for successful deposition of SnO2 on ZnO nanorods. Fig. 2(a)–(d) shows 451 tilted SEM images of the as-grown core/shell nanorod arrays and those annealed at several temperatures. It can be seen that the morphologies of the samples annealed up to 700 1C are similar to that of the unannealed one. When the annealing temperature is 900 1C, very large grains are generally present in the sample, as shown in Fig. 2(d). This indicates that the nanorods can be melted at about 900 1C, which
is much lower than the melting point of the bulk ZnO (1976 1C) and SnO2 (1630 1C) [7]. Similar results have been reported in Refs. [8,9], in which ZnO nanorods melted and changed into particles at 950 1C and 900 1C. Moreover, theoretical investigation has also demonstrated that the melting temperature will decrease with the reduction of the sizes of nanostructures [10]. Morphology and structure analyses were further performed using XRD and TEM (Fig. 3). For the as-grown ZnO/SnO2 core/shell nanorod arrays, a very strong peak corresponding to the (002) plane of hexagonal ZnO is observed without any SnO2-associated peaks. After being annealed at 500 1C, a very weak peak of SnO2 appears at 26.61, which becomes a little stronger when the
82
Y.-F. Tu et al. / Materials Letters 86 (2012) 80–83
Fig. 3. (a) XRD patterns of the as-grown core/shell nanorod arrays and those annealed at different temperatures. (b) TEM image of a core/shell nanorod annealed at 900 1C. (c) The corresponding SAED pattern of the sample in (b).
defects [12]. After being annealed, the visible/UV ratio increases with the increase of the annealing temperature, and the positions of the visible emission are changed. It can be seen that the center of the visible emission shifts from yellow to green after being annealed at temperatures up to 700 1C. The yellow emission band was commonly observed in hydrothermally grown ZnO nanorods [13,14]. The origin of this emission has been attributed to oxygen interstitial defects and the presence of OH groups. In addition, SnO2 may also contribute to the origin of the yellow emission, since it has been reported that SnO2 nanostructures showed a weak broad yellow emission band, which originated from deep levels such as O and Sn vacancies [15]. The green emission has been attributed to the singly ionized oxygen vacancy [12]. As to our sample, the shift from yellow to green in the visible emission by annealing can be attributed to the desorption of hydroxyl group [13,14] and the increase of oxygen vacancies. At a sufficiently high annealing temperature in the relatively oxygen-deficient ambient employed in our experiment, oxygen easily escapes from ZnO [16], thus resulting in the presence of more O vacancies. Therefore the corresponding green emission appears stronger with the increase of the annealing temperature. When the annealing temperature is 900 1C, the green emission shifts from 523 to 537 nm, and a broad peak centered at about 720 nm appears in the spectrum. Since the above XRD results demonstrated the formation of Zn2SnO4 under such a high temperature, some Sn atoms can also diffuse into ZnO. Therefore, the redshift of the green emission can be ascribed to the doping of Sn in ZnO. Although the photoluminescence mechanism for Zn2SnO4 is still unclear, a red emission centered at 740 nm has been reported for Zn2SnO4 microprisms [17]. So the broad peak at around 720 nm is considered to originate from Zn2SnO4.
4. Conclusions
Fig. 4. PL spectra of the as-grown core/shell nanorod arrays and those annealed at different temperatures.
annealing temperature is 700 1C. The appearance of this peak indicates the crystallization of amorphous SnO2. However, when the sample is annealed at 900 1C, the peak of SnO2 is hardly seen, but some new peaks appear, which can be assigned to Zn2SnO4. It has been reported that the solid state chemical reaction between ZnO and SnO2 will happen and produce Zn2SnO4 when the temperature is high [11]. Fig. 3(b) shows the TEM image of a typical ZnO/SnO2 core/shell nanorod annealed at 900 1C. A core and two shell layers along the length direction of the nanorod can be clearly seen. The corresponding SAED pattern (Fig. 3(c)) exhibits a spotted pattern that corresponds to the single-crystalline ZnO, and a set of diffraction rings that are consistent with polycrystalline SnO2, which confirms the core/shell structure and the crystallization of SnO2 after being annealed at 900 1C. Fig. 4 illustrates the PL spectra of the as-grown core/shell nanorods and those annealed at different temperatures. The PL spectrum of the as-grown core/shell nanorods shows a UV emission centered at 380 nm and a broad visible emission spanning the range from green to red. The UV emission can be attributed to the recombination of near-band edge excitons of ZnO, while the visible emission is known to come from deep level
ZnO/SnO2 core/shell nanorod arrays were annealed at different temperatures in a muffle furnace. The increased temperature (lower than 900 1C) led to the increased ratio of the visible/ ultraviolet emission and a band-shift from yellow to green in PL spectra. This was attributed to the desorption of hydroxyl group and the increase of oxygen vacancies. When the annealing temperature is 900 1C, the green emission made a red-shift, and a new broad peak appeared, which were ascribed to the doping of Sn in ZnO and the appearance of Zn2SnO4, respectively.
Acknowledgements This work was supported by Science and Technology Program of Wuhan City (No. 201150699189-19), Scientific Research Foundation of Jianghan University (No. 2009008), Youth Science Foundation of Wuhan Institute of Technology (No. Q201006) and Scientific Research Foundation of Wuhan Institute of Technology (11105021) References [1] Thomas MA, Cui JB. J Vac Sci Technol A 2012;30:01A116. [2] Fabbri F, Rossi F, Attolini G, Salviati G, Dierre B, Sekiguchi T, et al. Mater Lett 2012;71:137–40. [3] Jin C, Kim H, Ryu HY, Kim HW, Lee C. J Phys Chem C 2011;115:8513–8. [4] Hwang IS, Kim SJ, Choi JK, Choi J, Ji H, Kim GT, et al. Sensors Actuators B 2010;148:595–600. [5] Vayssieres L. Adv Mater 2003;15:464–6. [6] Yasuhiro S, Yukinari SI, Minoru M, Shigehito D. J Ceram Soc Jpn 2007;115: 856–60. [7] Zhou YW, Wu FY, Zheng CY. Chin Phys Lett 2011;28:107307.
Y.-F. Tu et al. / Materials Letters 86 (2012) 80–83
[8] Su X, Zhang ZJ, Zhu MM. Appl Phys Lett 2006;88:061913. [9] Yan ZJ, Ma YW, Wang DL, Wang JH, Gao ZS, Wang L, et al. Appl Phys Lett 2008;92:081911. [10] Guisbiers G, Pereira1 S. Nanotechnology 2007;18:435710. [11] Lee JH, Leu IC, Hsu MC, Chung YW, Hon MH. J Phys Chem B 2005;109: 13056–9. [12] Liu J., Xie S.F., Cao Y., Chen Y.L., Zeng H.D., Yang J.H., et al. Mater Lett 2012;70: 80-82.
83
[13] Tam KH, Cheung CK, Leung YH, Djurisˇic AB, Ling CC, Beling CD, et al. J Phys Chem B 2006;110:20865–71. [14] Kwok WM, Djuriˇsic´ AB, Leung YH, Li D, Tam KH, Phillips DL, et al. Appl Phys Lett 2006;89:183112. [15] He JH, Wu TH, Hsin CL, Li MK, Chen LJ, Chueh YL, et al. Small 2006;2:116–20. [16] Yang JH, Gao M, Zhang YJ, Yang LL, Lang JH, Wang DD, et al. Superlattice Microstruct 2008;44:137–42. [17] Zhao JW, Qin LR, Zhang LD. Solid State Commun 2007;141:663–6.
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Effects of annealing temperature on the properties of ZnO/SnO2 core/shell nanorod arrays Ya-Fang Tu a, Qiu-Ming Fu b, Jian-Ping Sang a,c,n, Zhi-Jie Tan c, Xian-Wu Zou c a
Department of Physics, Institute for Interdisciplinary Research, Jianghan University, Wuhan 430056, China School of Materials Science and Engineering, Wuhan Institute of Technology, Wuhan 430073, China c Department of Physics, Wuhan University, Wuhan 430072, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 May 2012 Accepted 11 July 2012 Available online 20 July 2012
The effects of annealing temperature on the morphologies, structures and optical properties of ZnO/SnO2 core/shell nanorod arrays were investigated. It is found that the annealing of the samples in a muffle furnace at temperatures up to 900 1C transformed the nanorods into large particles and produced the Zn2SnO4 phase. The photoluminescence results showed that when the annealing temperature was increased up to 700 1C, the ratio of visible/ultraviolet emission increased and the center of the visible emission shifted from yellow to green. When the annealing temperature was 900 1C, the green emission made a red-shift, and a new broad peak appeared in the spectra. Possible emission mechanisms were also discussed. & 2012 Elsevier B.V. All rights reserved.
Keywords: Nanocomposites Semiconductors Microstructure
1. Introduction
2. Experimental methods
Recently, there has been much interest in the fabrication of one-dimensional core/shell structured nanomaterials due to their novel properties and potential applications in photoelectronic nanodevices. As a wide variety of core shell nanostructures has been synthesized, modifying the properties of the nanostructures is becoming increasingly important. It has been reported that the photoluminescence (PL) emission intensity of core/shell nanostructures can be significantly increased and the wavelength of the emission can be controlled by choosing a proper coating material or changing the thickness of coating layer [1,2]. ZnO and SnO2, two well-known semiconductors, have received a considerable amount of attention over the past few years due to their excellent optical, electrical and gas sensing properties. Coaxial one-dimensional nanostructures consisting of a core of ZnO and a shell of SnO2 have also been reported by some groups, and excellent physical and chemical properties have been found [3,4]. However, there is no report on the influence of annealing on the PL properties of ZnO/SnO2 core/shell onedimensional nanostructures to the best of our knowledge. In this paper, the effects of annealing temperatures on the morphology, structure and PL properties of ZnO/SnO2 core/shell nanorod arrays were investigated.
As the core, ZnO nanorod arrays were grown by the aqueous chemical growth method [5]. Briefly, a ZnO seed layer was first deposited on a Si substrate by the sol–gel technique. Then the substrate was placed in a mixture of 0.05 M Zn(NO3)2 and 0.05 M C6H12N4 aqueous solution at 90 1C for 6 h. Deposition of SnO2 shell layers on ZnO nanorods was carried out by the liquid phase deposition method [6]. The Si substrate with ZnO nanorod arrays was immersed in a treatment solution of 0.1 M (NH4)2SnF6 and maintained at 40 1C for 5 min. Finally, the as-prepared samples were annealed in a muffle furnace for 1 h at 500, 700 and 900 1C. The morphology and structure of the samples were examined by a field emission scanning electron microscope (FE-SEM, SIRION, FEI Company) equipped with energy dispersive X-ray spectroscopy (EDS), a X-ray diffractometer (XRD, D8 Advance, Bruker Axs) and a transmission electron microscope (TEM, JEM-2010FEF, JEOL) assisted with selected area electron diffraction (SAED). The PL measurements were performed with a He–Cd laser (l ¼325 nm) at room temperature.
n Corresponding author at: Department of Physics, Wuhan University, Wuhan, Hubei 430072, China. Tel.: þ 86 27 88775901; fax: þ 86 27 68752569. E-mail address: [email protected] (J.-P. Sang).
0167-577X/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2012.07.037
3. Results and discussion Fig. 1(a) and (b) shows the SEM images in top view of the ZnO nanorod arrays and the as-grown ZnO/SnO2 core/shell nanorod arrays. Core/shell structures can be clearly observed from the magnified view of some broken nanorods (the inset of Fig. 1(b)). Fig. 1(c) and (d) shows the EDS profiles of the sample in (a) and (b), respectively. While the EDS pattern of the ZnO nanorods
Y.-F. Tu et al. / Materials Letters 86 (2012) 80–83
81
Fig. 1. Top view SEM images of (a) ZnO nanorod arrays and (b) ZnO/SnO2 core/shell nanorod arrays. The insets show the magnified images. (c) and (d) are the corresponding EDS patterns of (a) and (b), respectively.
Fig. 2. 451 tilted SEM images of (a) as-grown ZnO/SnO2 core/shell nanorod arrays and annealed at (b) 500 1C, (c) 700 1C and (d) 900 1C.
shows only elemental Zn and O (the Si and Pt signals have originated from the substrate and the evaporated conducting layer for SEM analysis, respectively), additional Sn element is found to be present after SnO2 deposition, which provides powerful evidence for successful deposition of SnO2 on ZnO nanorods. Fig. 2(a)–(d) shows 451 tilted SEM images of the as-grown core/shell nanorod arrays and those annealed at several temperatures. It can be seen that the morphologies of the samples annealed up to 700 1C are similar to that of the unannealed one. When the annealing temperature is 900 1C, very large grains are generally present in the sample, as shown in Fig. 2(d). This indicates that the nanorods can be melted at about 900 1C, which
is much lower than the melting point of the bulk ZnO (1976 1C) and SnO2 (1630 1C) [7]. Similar results have been reported in Refs. [8,9], in which ZnO nanorods melted and changed into particles at 950 1C and 900 1C. Moreover, theoretical investigation has also demonstrated that the melting temperature will decrease with the reduction of the sizes of nanostructures [10]. Morphology and structure analyses were further performed using XRD and TEM (Fig. 3). For the as-grown ZnO/SnO2 core/shell nanorod arrays, a very strong peak corresponding to the (002) plane of hexagonal ZnO is observed without any SnO2-associated peaks. After being annealed at 500 1C, a very weak peak of SnO2 appears at 26.61, which becomes a little stronger when the
82
Y.-F. Tu et al. / Materials Letters 86 (2012) 80–83
Fig. 3. (a) XRD patterns of the as-grown core/shell nanorod arrays and those annealed at different temperatures. (b) TEM image of a core/shell nanorod annealed at 900 1C. (c) The corresponding SAED pattern of the sample in (b).
defects [12]. After being annealed, the visible/UV ratio increases with the increase of the annealing temperature, and the positions of the visible emission are changed. It can be seen that the center of the visible emission shifts from yellow to green after being annealed at temperatures up to 700 1C. The yellow emission band was commonly observed in hydrothermally grown ZnO nanorods [13,14]. The origin of this emission has been attributed to oxygen interstitial defects and the presence of OH groups. In addition, SnO2 may also contribute to the origin of the yellow emission, since it has been reported that SnO2 nanostructures showed a weak broad yellow emission band, which originated from deep levels such as O and Sn vacancies [15]. The green emission has been attributed to the singly ionized oxygen vacancy [12]. As to our sample, the shift from yellow to green in the visible emission by annealing can be attributed to the desorption of hydroxyl group [13,14] and the increase of oxygen vacancies. At a sufficiently high annealing temperature in the relatively oxygen-deficient ambient employed in our experiment, oxygen easily escapes from ZnO [16], thus resulting in the presence of more O vacancies. Therefore the corresponding green emission appears stronger with the increase of the annealing temperature. When the annealing temperature is 900 1C, the green emission shifts from 523 to 537 nm, and a broad peak centered at about 720 nm appears in the spectrum. Since the above XRD results demonstrated the formation of Zn2SnO4 under such a high temperature, some Sn atoms can also diffuse into ZnO. Therefore, the redshift of the green emission can be ascribed to the doping of Sn in ZnO. Although the photoluminescence mechanism for Zn2SnO4 is still unclear, a red emission centered at 740 nm has been reported for Zn2SnO4 microprisms [17]. So the broad peak at around 720 nm is considered to originate from Zn2SnO4.
4. Conclusions
Fig. 4. PL spectra of the as-grown core/shell nanorod arrays and those annealed at different temperatures.
annealing temperature is 700 1C. The appearance of this peak indicates the crystallization of amorphous SnO2. However, when the sample is annealed at 900 1C, the peak of SnO2 is hardly seen, but some new peaks appear, which can be assigned to Zn2SnO4. It has been reported that the solid state chemical reaction between ZnO and SnO2 will happen and produce Zn2SnO4 when the temperature is high [11]. Fig. 3(b) shows the TEM image of a typical ZnO/SnO2 core/shell nanorod annealed at 900 1C. A core and two shell layers along the length direction of the nanorod can be clearly seen. The corresponding SAED pattern (Fig. 3(c)) exhibits a spotted pattern that corresponds to the single-crystalline ZnO, and a set of diffraction rings that are consistent with polycrystalline SnO2, which confirms the core/shell structure and the crystallization of SnO2 after being annealed at 900 1C. Fig. 4 illustrates the PL spectra of the as-grown core/shell nanorods and those annealed at different temperatures. The PL spectrum of the as-grown core/shell nanorods shows a UV emission centered at 380 nm and a broad visible emission spanning the range from green to red. The UV emission can be attributed to the recombination of near-band edge excitons of ZnO, while the visible emission is known to come from deep level
ZnO/SnO2 core/shell nanorod arrays were annealed at different temperatures in a muffle furnace. The increased temperature (lower than 900 1C) led to the increased ratio of the visible/ ultraviolet emission and a band-shift from yellow to green in PL spectra. This was attributed to the desorption of hydroxyl group and the increase of oxygen vacancies. When the annealing temperature is 900 1C, the green emission made a red-shift, and a new broad peak appeared, which were ascribed to the doping of Sn in ZnO and the appearance of Zn2SnO4, respectively.
Acknowledgements This work was supported by Science and Technology Program of Wuhan City (No. 201150699189-19), Scientific Research Foundation of Jianghan University (No. 2009008), Youth Science Foundation of Wuhan Institute of Technology (No. Q201006) and Scientific Research Foundation of Wuhan Institute of Technology (11105021) References [1] Thomas MA, Cui JB. J Vac Sci Technol A 2012;30:01A116. [2] Fabbri F, Rossi F, Attolini G, Salviati G, Dierre B, Sekiguchi T, et al. Mater Lett 2012;71:137–40. [3] Jin C, Kim H, Ryu HY, Kim HW, Lee C. J Phys Chem C 2011;115:8513–8. [4] Hwang IS, Kim SJ, Choi JK, Choi J, Ji H, Kim GT, et al. Sensors Actuators B 2010;148:595–600. [5] Vayssieres L. Adv Mater 2003;15:464–6. [6] Yasuhiro S, Yukinari SI, Minoru M, Shigehito D. J Ceram Soc Jpn 2007;115: 856–60. [7] Zhou YW, Wu FY, Zheng CY. Chin Phys Lett 2011;28:107307.
Y.-F. Tu et al. / Materials Letters 86 (2012) 80–83
[8] Su X, Zhang ZJ, Zhu MM. Appl Phys Lett 2006;88:061913. [9] Yan ZJ, Ma YW, Wang DL, Wang JH, Gao ZS, Wang L, et al. Appl Phys Lett 2008;92:081911. [10] Guisbiers G, Pereira1 S. Nanotechnology 2007;18:435710. [11] Lee JH, Leu IC, Hsu MC, Chung YW, Hon MH. J Phys Chem B 2005;109: 13056–9. [12] Liu J., Xie S.F., Cao Y., Chen Y.L., Zeng H.D., Yang J.H., et al. Mater Lett 2012;70: 80-82.
83
[13] Tam KH, Cheung CK, Leung YH, Djurisˇic AB, Ling CC, Beling CD, et al. J Phys Chem B 2006;110:20865–71. [14] Kwok WM, Djuriˇsic´ AB, Leung YH, Li D, Tam KH, Phillips DL, et al. Appl Phys Lett 2006;89:183112. [15] He JH, Wu TH, Hsin CL, Li MK, Chen LJ, Chueh YL, et al. Small 2006;2:116–20. [16] Yang JH, Gao M, Zhang YJ, Yang LL, Lang JH, Wang DD, et al. Superlattice Microstruct 2008;44:137–42. [17] Zhao JW, Qin LR, Zhang LD. Solid State Commun 2007;141:663–6.