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Self-assembly growth of ZnO-based axial and radial junctions via a two-step method
Applied Surface Science 264 (2013) 687–691
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Self-assembly growth of ZnO-based axial and radial junctions via a two-step method Yongqin Chang a,∗ , Yingdong Lu a , Mingwen Wang b , Yi Long a , Rongchang Ye a a b
School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China School of Chemistry and Biological Engineering, University of Science and Technology Beijing, Beijing 100083, China
a r t i c l e
i n f o
Article history: Received 18 August 2012 Received in revised form 10 October 2012 Accepted 15 October 2012 Available online 23 October 2012 Keywords: ZnO based junction Chemical vapor deposition Aqueous solution method Photoluminescence Growth mechanism
a b s t r a c t Self-assembly growth of ZnO-based axial and radial junctions via a kind of two-step method was reported. It was first prepared by a chemical vapor deposition (CVD) method, and then followed by an aqueous solution method to obtain ZnO-based junctions. The formation of the axial or radial junction depends on the sample prepared by the first step. If the microrods grown by the CVD method are pure ZnO, axial junction forms when the sample is put into the solution for further growth, otherwise, radial junction forms if the microrods grown by the first step are doped ZnO. It is attributed to the fastest growth direction in the formation of axial junction and radial junction is quite different. The photoluminescence spectra of the samples reveal that the junctions are quite different from the microrod films prepared by the first step. This work provides a useful way to fabricate ZnO-based junctions, which is important for designing nanodevices. © 2012 Elsevier B.V. All rights reserved.
1. Introduction ZnO, with a large band gap (3.37 eV at room temperature) and a high exciton binding energy (60 meV), has great potential applications in short wavelength light emission diodes (LED), laser diodes (LD), and photodetectors. ZnO with a large family of demonstrated 1D nanostructures, including nanowires, nanorods, nanobelts and nanotubes have also been reported [1–4]. Recently, great attention has been focused on the synthesis of complex ZnO hierarchical nanomaterials [5–14]. Among them, nanostructured ZnO-based junctions have been of great interest for the possibility to integrate optical–electrical devices into the high-density functional nanochips, such as low dimensional LED [10–12] and ultraviolet detector [8]. G.P. Wang et al. reported that ZnO p–n homojunctions based on Sb-doped p-type nanowire array and n-type film were grown by combining chemical vapor deposition (for nanowires) with molecular-beam epitaxy (for film) [8]. Kumar et al. reported that ZnO nanowires were fabricated on the ZnO:As films to form nanosized ZnO junction by e-beam and metalorganic chemical vapor deposition [9]. Nanostructured ZnO junctions have also been fabricated by hydrothermal method [10], As+ ion implantation [11], CVD method [12], chemical bath deposition method [13], and pulsed laser deposition combined with hydrothermal growth
∗ Corresponding author. Tel.: +86 10 62334958. E-mail address: [email protected] (Y. Chang). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.10.096
[14]. However, most of such preparation methods need expensive instruments or high preparation temperature, which poses a significant hurdle to their industrial application. In this work, we report a easy-control and low-cost way to fabricate ZnO-based axis and radial junctions, which were grown by a two-step method. The growth mechanism of the ZnO-based axial and radial junctions was also discussed. 2. Experimental details 2.1. Axial junction Axial junction was prepared by growing ZnO microrods and Mg doped ZnO nanotip arrays (or ZnO nanotip arrays) in order. (1) ZnO microrod films were first deposited on a silicon substrate by a simple CVD method, and the typical fabrication process was described as follows. Mixture powders of Zn (700 mg) and C (60 mg) as source materials were put into an alumina boat, and a cleaned silicon substrate was covered on the source materials with vertical distance of 6 mm. The boat was then loaded into a horizontal quartz tube. The furnace was heated to a certain temperature and kept at this temperature for 5 min at atmospheric pressure. The heating temperature is 750 ◦ C and 800 ◦ C for sample 1 (named as S1) and sample 2 (named as S2), respectively. After the furnace was cooled down to room temperature, the sample was pulled out. (2) Mg doped ZnO nanotips (ZnMgO) were then deposited on the S1 sample by an aqueous solution method. The solution was prepared with the
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Fig. 1. (a) SEM image of the ZnO microrods (S2) prepared by CVD method, and (b) ZnO nanotips deposited on the ZnO microrods to form axial junction, the right-up inset shows the enlarged part of the junction.
Typical SEM image of the S2 sample grown by CVD method was shown in Fig. 1a. ZnO microrods are aligned well and closely with the average diameter of 3 m. After the S2 sample was put into the solution for further growth, many ZnO nanotips were deposited on the top of the microrods to form ZnO nanotips/ZnO microrods axial junction (Fig. 2b). Fig. 2 shows the XRD pattern of the ZnO nanotips/ZnO microrods axial junction, the diffraction peaks reveal that the products are well-known hexagonal wurtzite ZnO. A strong and sharp peak appeared at 34.2◦ (assigned as (0 0 2)) is highest in intensity as compared to the other peaks in the XRD pattern, which reveals that both ZnO nanotips and ZnO microrods are
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Pure ZnO layer epitaxially grows from Ga doped ZnO microrods to form ZnO/ZnGaO radial junction. (1) Ga doped ZnO microrod arrays (ZnGaO) were firstly grown on a Si wafer using a CVD method. Pure Zn (500 mg) powders and mixture of Ga (30 mg) and C (20 mg) powders as source materials were put into a quartz boat, and a cleaned Si substrate was covered on the sources materials. The boat was then loaded into the center of the tube furnace. The furnace was maintained at 900 ◦ C for 30 min with argon flow of 80 sccm. The sample was carried out when the furnace was cooled down to room temperature. (2) Solution was prepared by zinc nitrate hexahydrate [Zn(NO3 )2 ·6H2 O] and methenamine (C6 H12 N4 ) with the molar ratio of 1:1. The sample prepared by the first step was subsequently put into the solution with the microrod film side facing downwardly. The solution was heated up to 90 ◦ C and maintained at this temperature for 3 h. The sample was then carried out, cleared in ethanol and dried in air, respectively. The morphology and composition of the samples were characterized by a field-emission scanning electron microscopy (FESEM, SUPRA 55) equipped with energy dispersive spectrometry (EDS). Phase structure was carried out by X-ray diffraction (XRD, Rigaku D/max-RB, Cu K␣). Room temperature photoluminescence (PL) measurement was performed using a HITACHI 4500-type visible–ultraviolet spectrophotometer with a Xe lamp as the excitation light source, and the excited wavelength was 325 nm.
3.1. Axial junction
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3. Results and discussion
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molar ratio of 1:1 by zinc nitrate hexahydrate and methenamine, and Mg(NO3 )2 with the content of 0.6 at.% was added. The S1 sample was then put into the solution, and it was heated up to 90 ◦ C and maintained at this temperature for 3 h. The sample was then carried out, and cleaned in ethanol and dried in air, respectively (this sample named as ZnMgO nanotips/ZnO microrods). For comparison, undoped ZnO nanotips were deposited on the S2 sample via the similar growth procedure without adding Mg(NO3 )2 in the solution to form ZnO nanotips/ZnO microrods junction.
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Fig. 3. (a) SEM image of the ZnO microrods (S1) with the mean diameter of several hundred nanometers, (b) SEM image of the ZnMgO nanotips/ZnO microrods axial junction, ZnMgO nanotips grow on the top of the ZnO microrods instead of other places, and (c) initial growth conditions of the ZnMgO nanotips/ZnO microrods axial junction. The right-up inset in (b) and (c) is the corresponding side-view image of the junction.
preferentially grown along the c-axis direction, it is in accordance with the SEM results (Fig. 1b). Fig. 3a shows the SEM image of the S1 sample, ZnO microrods with the mean diameter of 330 nm were deposited on the Si substrate, and the ZnO microrods are aligned loosely. The typical morphology of the ZnMgO nanotips/ZnO microrods axial junction was shown in Fig. 3b, it clearly shows that ZnMgO nanotips only deposit on the top of the ZnO microrods instead of other places, although there are some spaces along the ZnO microrods. Fig. 3c shows the initial growth conditions of the ZnMgO nanotips, the nuclei only form on the top of the ZnO microrods (c-axis of the ZnO). EDS results show that the content of Mg is around 3 at.%. XRD results of the ZnMgO nanotips/ZnO microrods axial junction indicate that all the diffraction peaks match well with the diffraction pattern of ZnO powders. No characteristic peaks of any impurities were detected in the diffraction pattern within the XRD detection limitation (the results were not shown here). Room temperature PL spectra were measured for the ZnO microrods (S1) and the ZnMgO nanotips/ZnO microrods junction, and the results were shown in Fig. 4. Only an obvious band-gap emission peak (centered at 380.8 nm) was detected for the ZnO microrods, while, two emission peaks were observed for the ZnMgO nanotips/ZnO microrods junction. The weak ultraviolet (UV) emission centered at 386.0 nm ascribes to the band-gap emission, and the 1600 ZnO microrods (S1) ZnMgO/ZnO junction
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high emission intensity located at about 500 nm is green emission. The band-gap emission of the ZnMgO nanotips/ZnO microrods junction shows an obvious red-shift compared to that of the ZnO microrods. Y.S. Chang et al. reported that the band-gap emission of Mg doped ZnO shift to long wavelength because of the doping of Mg [15]. In Fig. 4, the band-gap emission for the ZnMgO nanotips/ZnO microrods junction composes of the emission of the ZnO microrods and that of the ZnMgO nanotips, the contribution of Mg doping leads a red-shift of the UV emission position for the ZnMgO nanotips/ZnO microrods junction. The green emission of the ZnMgO nanotips/ZnO microrods junction may be originated from two reasons. One is the large ratio of surface to volume for the ZnMgO nanotips, another reason is the different ionic radii of Mg (0.66 nm) and Zn (0.74 nm), which introduces some defects into the junction because of Mg doping. Fig. 5 shows the schematic formation for the axial junction. ZnO microrods were firstly deposited on the Si substrate, pure or doped ZnO nanotips were grown on the top of the ZnO microrods to from axial junction. The nanotips only grow along the c-axis direction instead of the other directions even there are some spaces along the ZnO microrods. 3.2. Radial junction Fig. 6 shows the typical SEM image of the radial junction. ZnGaO microrods with the diameter of 250–700 nm were synthesized via CVD method at first. After these microrods were put into the solution, ZnO layer with thickness of 50–270 nm epitaxially grows around the ZnGaO microrods. EDS results indicate that the content of Ga is around 5 at.%. The different color in Fig. 6 might ascribe to the different conductivity of the ZnO layer and the ZnGaO microrods. This kind of structure is named as ZnO/ZnGaO radial junction. Room temperature PL spectra of the ZnGaO microrod films and the ZnO/ZnGaO junction were shown in Fig. 7. It clearly reveals
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Wavelength (nm) Fig. 4. PL spectra of the ZnO microrods (S1) and the ZnMgO nanotips/ZnO microrods junction.
Fig. 5. Schematic formation for the ZnO-based axial junction.
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Fig. 6. (a) SEM image of the ZnO/ZnGaO radial junction, the inside is ZnGaO microrods, and the outside is ZnO layer, and (b) enlarged part of (a).
that both of them exhibit two emission peaks, one is the UV emission and another one is the green emission. The UV emission peak of the ZnO/ZnGaO junction (centered at 389.4 nm) shows a little red-shift compared with that of the ZnGaO microrods (centered at 387.6 nm). The intensity of the green emission of the ZnGaO microrod obviously decreases by coating with ZnO layer, which confines that the out layer is quite different from the inside for the ZnO/ZnGaO radial junction. In Fig. 7, the UV emission of the ZnO/ZnGaO junction is superimposed by the emission of the ZnGaO microrods and the out-layer ZnO. Previous research results reported that the UV emission of ZnO shifts to short wavelength because of Ga doping [16]. When the surface of the ZnGaO microrods was overlapped by the layer of ZnO, it will lead the UV emission of the ZnO/ZnGaO junction showing a red-shift compared with that of the ZnGaO microrods. The intensity ratio of the band-edge emission (IUV ) to the deep level emission (IG ) is always used to define the crystal quality. The results in Fig. 7 show that the high density of defects in the ZnGaO microrods gives rise to high IG /IUV ratio of 2.1, while the value of IG /IUV ratio is 0.9 for the ZnO/ZnGaO junction. The high intensity of the green emission in the ZnGaO microrods might relate to the defects at/near surfaces and the introduced defects due to the difference of the ionic radii of Ga (0.62 nm) and Zn
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(0.74 nm). When the ZnGaO microrods was coated by the ZnO layer, the defects at/near surface of the ZnGaO microrods are suppressed, which makes the ratio of IGreen /IUV decrease in some degree. Fig. 8 shows the schematic formation for the ZnO/ZnGaO junction. The inside is the ZnGaO microrods synthesized by the CVD method and the outside is the ZnO layer grown by the solution method. 3.3. Growth mechanism Why do some junctions grow along the axial direction, while other junctions grow along the radial junction? The possible mechanism suggested is that the formation of the different direction junctions is attributed to the difference in the growth rate of the various crystal facets. It is well known that the crystal structure defines the main and secondary growth directions of the materials. For ZnO with wurtzite crystallographic lattice structure, [0 0 0 1] is the fastest growth direction in the formation of nanostructure [17–19]. The structures will grow along the c-axis direction spontaneously if the deposition sustains it. It can easily envelope other lattice planes and determine the preferable orientation to be c-axis, that is the reason why the microrods always grow along the c-axis direction, as shown in Fig. 2. When these ZnO microrods are put into the solution for further growth, the nuclei are also easily formed at the top of the ZnO microrods, which is also the fast growth direction for ZnO, and only in this condition, the growth system has the lowest energy to keep stable. The microrods growth proceeds by a repeated nucleation and growth of epitaxial hexagonal on the c-face, (0 0 0 1) of the microrods, and forms the axis junction (Figs. 1 and 3). When the microrods grown by the CVD method are doped ZnO, many defects are introduced into the microrods
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Wavelength (nm) Fig. 7. PL spectra of the Ga doped ZnO microrods and the ZnO/ZnGaO radial junction.
Fig. 8. Schematic formation for the ZnO-based radial junction.
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microrod films prepared by the first step are doped ZnO. It is ascribed to the fastest growth direction in the formation of axial junction and radial junction is quite different. The PL spectra of the samples show that the junctions are quite different from the microrod films prepared by the first step. This method provides a useful and effective way to fabricate ZnO-based junctions, which have great potential applications in semiconductor devices. Acknowledgments
Fig. 9. Growth direction of the crystal facets in ZnO structure.
because of the different ions diameter of Ga3+ (0.62 nm) and Zn2+ (0.74 nm), and the fastest growing crystallographic plane of (0 0 0 1) may be disturbed. The experiment results show that the relatives velocities of crystal growth in [0 0 0 1] direction is restrained, and the main growth direction turns to be (Fig. 9). In this case, the ZnO nuclei are inclined to form surrounding the microrods, because is the fast growth direction. ZnO layer then epitaxially grows from the doped ZnO microrods and forms radial junction (Fig. 6). In a word, axial junction forms when the microrods prepared by the first step are pure ZnO, otherwise, radial junction forms as the microrods synthesized by the first step are doped ZnO, because the fastest growth direction in the formation of axial junction and radial junction is quite different, which also indicates that the growth direction of the junctions can be controlled by doping in the ZnO structures. 4. Conclusions In conclusion, ZnO-based axial and radial junctions were successfully synthesized by a two-step method. Pure or doped ZnO microrods were firstly synthesized by CVD method, these microrods were then put into the aqueous solution for further growth. Axial junction forms as the microrod films synthesized by CVD method are pure ZnO. Otherwise, radial junction forms when the
This project was financially supported by the National Natural Science Foundation of China (No. 11175014), Beijing Natural Science Foundation (No. 1092014). One of the authors (Y.Q. Chang) is supported by Program for New Century Excellent Talents in University (No. NCET-07-0065). References [1] K.S. Leschkies, R. Divakar, J. Basu, E. Enache-Pommer, J.E. Boercker, C.B. Carter, U.R. Kortshagen, D.J. Norris, E.S. Aydil, Nano Lett. 7 (2007) 1793. [2] W.I. Park, D.H. Kim, S.W. Jung, Y. Gyu-Chul, Appl. Phys. Lett. 80 (2002) 4232. [3] W.Z. Wang, B.Q. Zeng, J. Yang, B. Poudel, J.Y. Huang, M.J. Naughton, Z.F. Ren, Adv. Mater. 18 (2006) 3275. [4] F. Xu, J. Chen, L.Y. Guo, S.Y. Lei, Y.R. Ni, Appl. Surf. Sci. 258 (2012) 8160. [5] L.F. Xu, Q.W. Chen, D.S. Xu, J. Phys. Chem. C 111 (2007) 11560. [6] S.T. Hung, C.J. Chang, M.H. Hsu, J. Hazard. Mater. 198 (2011) 307. [7] W.W. Guo, T.M. Liu, H.J. Zhang, R. Sun, Y. Chen, W. Zeng, Z.C. Wang, Sens. Actuators B 166–167 (2012) 492. [8] G.P. Wang, S. Chu, N. Zhan, Y.Q. Lin, L. Chernyak, J.L. Liu, Appl. Phys. Lett. 98 (2011) 041107. [9] M. Kumar, J.P. Kar, I.-S. Kim, S.-Y. Choi, J.-M. Myoung, Appl. Phys. A 97 (2009) 689. [10] X. Fang, J.H. Li, D.X. Zhao, D.Z. Shen, B.H. Li, X.H. Wang, J. Phys. Chem. C 113 (2009) 21208. [11] Y. Yang, X.W. Sun, B.K. Tay, G.F. You, S.T. Tan, K.L. Teo, Appl. Phys. Lett. 93 (2008) 253107. [12] M.T. Chen, M.P. Lu, Y.J. Wu, J.H. Song, C.Y. Lee, M.Y. Lu, Y.C. Chang, L.J. Chou, Z.L. Wang, L.J. Chen, Nano Lett. 10 (2010) 4387. [13] S. Yılmaz, I. Polata, S. Altındalc, E. Bacaksız, Mater. Sci. Eng. B 177 (2012) 588. [14] Y. Sun, N.A. Fox, G.M. Fuge, M.N.R. Ashfold, J. Phys. Chem. C 114 (2010) 21338. [15] Y.S. Chang, C.T. Chien, C.W. Chen, J. Appl. Phys. 101 (2007) 033502. [16] A. Escobedo-Morales, U. Pal, Appl. Phys. Lett. 93 (2008) 193120. [17] H.Z. Zhang, X.C. Sun, R.M. Wang, D.P. Yu, J. Cryst. Growth 269 (2004) 464. [18] H.Q. Le, S.J. Chua, Y.W. Koh, K.P. Loh, Z. Chen, C.V. Thompson, E.A. Fitzgerald, Appl. Phys. Lett. 87 (2005) 101908. [19] S. Mandal, K. Sambasivarao, A. Dhar, S.K. Ray, J. Appl. Phys. 106 (2009) 024103.
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Self-assembly growth of ZnO-based axial and radial junctions via a two-step method Yongqin Chang a,∗ , Yingdong Lu a , Mingwen Wang b , Yi Long a , Rongchang Ye a a b
School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China School of Chemistry and Biological Engineering, University of Science and Technology Beijing, Beijing 100083, China
a r t i c l e
i n f o
Article history: Received 18 August 2012 Received in revised form 10 October 2012 Accepted 15 October 2012 Available online 23 October 2012 Keywords: ZnO based junction Chemical vapor deposition Aqueous solution method Photoluminescence Growth mechanism
a b s t r a c t Self-assembly growth of ZnO-based axial and radial junctions via a kind of two-step method was reported. It was first prepared by a chemical vapor deposition (CVD) method, and then followed by an aqueous solution method to obtain ZnO-based junctions. The formation of the axial or radial junction depends on the sample prepared by the first step. If the microrods grown by the CVD method are pure ZnO, axial junction forms when the sample is put into the solution for further growth, otherwise, radial junction forms if the microrods grown by the first step are doped ZnO. It is attributed to the fastest growth direction in the formation of axial junction and radial junction is quite different. The photoluminescence spectra of the samples reveal that the junctions are quite different from the microrod films prepared by the first step. This work provides a useful way to fabricate ZnO-based junctions, which is important for designing nanodevices. © 2012 Elsevier B.V. All rights reserved.
1. Introduction ZnO, with a large band gap (3.37 eV at room temperature) and a high exciton binding energy (60 meV), has great potential applications in short wavelength light emission diodes (LED), laser diodes (LD), and photodetectors. ZnO with a large family of demonstrated 1D nanostructures, including nanowires, nanorods, nanobelts and nanotubes have also been reported [1–4]. Recently, great attention has been focused on the synthesis of complex ZnO hierarchical nanomaterials [5–14]. Among them, nanostructured ZnO-based junctions have been of great interest for the possibility to integrate optical–electrical devices into the high-density functional nanochips, such as low dimensional LED [10–12] and ultraviolet detector [8]. G.P. Wang et al. reported that ZnO p–n homojunctions based on Sb-doped p-type nanowire array and n-type film were grown by combining chemical vapor deposition (for nanowires) with molecular-beam epitaxy (for film) [8]. Kumar et al. reported that ZnO nanowires were fabricated on the ZnO:As films to form nanosized ZnO junction by e-beam and metalorganic chemical vapor deposition [9]. Nanostructured ZnO junctions have also been fabricated by hydrothermal method [10], As+ ion implantation [11], CVD method [12], chemical bath deposition method [13], and pulsed laser deposition combined with hydrothermal growth
∗ Corresponding author. Tel.: +86 10 62334958. E-mail address: [email protected] (Y. Chang). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.10.096
[14]. However, most of such preparation methods need expensive instruments or high preparation temperature, which poses a significant hurdle to their industrial application. In this work, we report a easy-control and low-cost way to fabricate ZnO-based axis and radial junctions, which were grown by a two-step method. The growth mechanism of the ZnO-based axial and radial junctions was also discussed. 2. Experimental details 2.1. Axial junction Axial junction was prepared by growing ZnO microrods and Mg doped ZnO nanotip arrays (or ZnO nanotip arrays) in order. (1) ZnO microrod films were first deposited on a silicon substrate by a simple CVD method, and the typical fabrication process was described as follows. Mixture powders of Zn (700 mg) and C (60 mg) as source materials were put into an alumina boat, and a cleaned silicon substrate was covered on the source materials with vertical distance of 6 mm. The boat was then loaded into a horizontal quartz tube. The furnace was heated to a certain temperature and kept at this temperature for 5 min at atmospheric pressure. The heating temperature is 750 ◦ C and 800 ◦ C for sample 1 (named as S1) and sample 2 (named as S2), respectively. After the furnace was cooled down to room temperature, the sample was pulled out. (2) Mg doped ZnO nanotips (ZnMgO) were then deposited on the S1 sample by an aqueous solution method. The solution was prepared with the
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Fig. 1. (a) SEM image of the ZnO microrods (S2) prepared by CVD method, and (b) ZnO nanotips deposited on the ZnO microrods to form axial junction, the right-up inset shows the enlarged part of the junction.
Typical SEM image of the S2 sample grown by CVD method was shown in Fig. 1a. ZnO microrods are aligned well and closely with the average diameter of 3 m. After the S2 sample was put into the solution for further growth, many ZnO nanotips were deposited on the top of the microrods to form ZnO nanotips/ZnO microrods axial junction (Fig. 2b). Fig. 2 shows the XRD pattern of the ZnO nanotips/ZnO microrods axial junction, the diffraction peaks reveal that the products are well-known hexagonal wurtzite ZnO. A strong and sharp peak appeared at 34.2◦ (assigned as (0 0 2)) is highest in intensity as compared to the other peaks in the XRD pattern, which reveals that both ZnO nanotips and ZnO microrods are
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Pure ZnO layer epitaxially grows from Ga doped ZnO microrods to form ZnO/ZnGaO radial junction. (1) Ga doped ZnO microrod arrays (ZnGaO) were firstly grown on a Si wafer using a CVD method. Pure Zn (500 mg) powders and mixture of Ga (30 mg) and C (20 mg) powders as source materials were put into a quartz boat, and a cleaned Si substrate was covered on the sources materials. The boat was then loaded into the center of the tube furnace. The furnace was maintained at 900 ◦ C for 30 min with argon flow of 80 sccm. The sample was carried out when the furnace was cooled down to room temperature. (2) Solution was prepared by zinc nitrate hexahydrate [Zn(NO3 )2 ·6H2 O] and methenamine (C6 H12 N4 ) with the molar ratio of 1:1. The sample prepared by the first step was subsequently put into the solution with the microrod film side facing downwardly. The solution was heated up to 90 ◦ C and maintained at this temperature for 3 h. The sample was then carried out, cleared in ethanol and dried in air, respectively. The morphology and composition of the samples were characterized by a field-emission scanning electron microscopy (FESEM, SUPRA 55) equipped with energy dispersive spectrometry (EDS). Phase structure was carried out by X-ray diffraction (XRD, Rigaku D/max-RB, Cu K␣). Room temperature photoluminescence (PL) measurement was performed using a HITACHI 4500-type visible–ultraviolet spectrophotometer with a Xe lamp as the excitation light source, and the excited wavelength was 325 nm.
3.1. Axial junction
(100)
2.2. Radial junction
3. Results and discussion
Intensity (a.u.)
molar ratio of 1:1 by zinc nitrate hexahydrate and methenamine, and Mg(NO3 )2 with the content of 0.6 at.% was added. The S1 sample was then put into the solution, and it was heated up to 90 ◦ C and maintained at this temperature for 3 h. The sample was then carried out, and cleaned in ethanol and dried in air, respectively (this sample named as ZnMgO nanotips/ZnO microrods). For comparison, undoped ZnO nanotips were deposited on the S2 sample via the similar growth procedure without adding Mg(NO3 )2 in the solution to form ZnO nanotips/ZnO microrods junction.
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Fig. 3. (a) SEM image of the ZnO microrods (S1) with the mean diameter of several hundred nanometers, (b) SEM image of the ZnMgO nanotips/ZnO microrods axial junction, ZnMgO nanotips grow on the top of the ZnO microrods instead of other places, and (c) initial growth conditions of the ZnMgO nanotips/ZnO microrods axial junction. The right-up inset in (b) and (c) is the corresponding side-view image of the junction.
preferentially grown along the c-axis direction, it is in accordance with the SEM results (Fig. 1b). Fig. 3a shows the SEM image of the S1 sample, ZnO microrods with the mean diameter of 330 nm were deposited on the Si substrate, and the ZnO microrods are aligned loosely. The typical morphology of the ZnMgO nanotips/ZnO microrods axial junction was shown in Fig. 3b, it clearly shows that ZnMgO nanotips only deposit on the top of the ZnO microrods instead of other places, although there are some spaces along the ZnO microrods. Fig. 3c shows the initial growth conditions of the ZnMgO nanotips, the nuclei only form on the top of the ZnO microrods (c-axis of the ZnO). EDS results show that the content of Mg is around 3 at.%. XRD results of the ZnMgO nanotips/ZnO microrods axial junction indicate that all the diffraction peaks match well with the diffraction pattern of ZnO powders. No characteristic peaks of any impurities were detected in the diffraction pattern within the XRD detection limitation (the results were not shown here). Room temperature PL spectra were measured for the ZnO microrods (S1) and the ZnMgO nanotips/ZnO microrods junction, and the results were shown in Fig. 4. Only an obvious band-gap emission peak (centered at 380.8 nm) was detected for the ZnO microrods, while, two emission peaks were observed for the ZnMgO nanotips/ZnO microrods junction. The weak ultraviolet (UV) emission centered at 386.0 nm ascribes to the band-gap emission, and the 1600 ZnO microrods (S1) ZnMgO/ZnO junction
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high emission intensity located at about 500 nm is green emission. The band-gap emission of the ZnMgO nanotips/ZnO microrods junction shows an obvious red-shift compared to that of the ZnO microrods. Y.S. Chang et al. reported that the band-gap emission of Mg doped ZnO shift to long wavelength because of the doping of Mg [15]. In Fig. 4, the band-gap emission for the ZnMgO nanotips/ZnO microrods junction composes of the emission of the ZnO microrods and that of the ZnMgO nanotips, the contribution of Mg doping leads a red-shift of the UV emission position for the ZnMgO nanotips/ZnO microrods junction. The green emission of the ZnMgO nanotips/ZnO microrods junction may be originated from two reasons. One is the large ratio of surface to volume for the ZnMgO nanotips, another reason is the different ionic radii of Mg (0.66 nm) and Zn (0.74 nm), which introduces some defects into the junction because of Mg doping. Fig. 5 shows the schematic formation for the axial junction. ZnO microrods were firstly deposited on the Si substrate, pure or doped ZnO nanotips were grown on the top of the ZnO microrods to from axial junction. The nanotips only grow along the c-axis direction instead of the other directions even there are some spaces along the ZnO microrods. 3.2. Radial junction Fig. 6 shows the typical SEM image of the radial junction. ZnGaO microrods with the diameter of 250–700 nm were synthesized via CVD method at first. After these microrods were put into the solution, ZnO layer with thickness of 50–270 nm epitaxially grows around the ZnGaO microrods. EDS results indicate that the content of Ga is around 5 at.%. The different color in Fig. 6 might ascribe to the different conductivity of the ZnO layer and the ZnGaO microrods. This kind of structure is named as ZnO/ZnGaO radial junction. Room temperature PL spectra of the ZnGaO microrod films and the ZnO/ZnGaO junction were shown in Fig. 7. It clearly reveals
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Fig. 5. Schematic formation for the ZnO-based axial junction.
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Fig. 6. (a) SEM image of the ZnO/ZnGaO radial junction, the inside is ZnGaO microrods, and the outside is ZnO layer, and (b) enlarged part of (a).
that both of them exhibit two emission peaks, one is the UV emission and another one is the green emission. The UV emission peak of the ZnO/ZnGaO junction (centered at 389.4 nm) shows a little red-shift compared with that of the ZnGaO microrods (centered at 387.6 nm). The intensity of the green emission of the ZnGaO microrod obviously decreases by coating with ZnO layer, which confines that the out layer is quite different from the inside for the ZnO/ZnGaO radial junction. In Fig. 7, the UV emission of the ZnO/ZnGaO junction is superimposed by the emission of the ZnGaO microrods and the out-layer ZnO. Previous research results reported that the UV emission of ZnO shifts to short wavelength because of Ga doping [16]. When the surface of the ZnGaO microrods was overlapped by the layer of ZnO, it will lead the UV emission of the ZnO/ZnGaO junction showing a red-shift compared with that of the ZnGaO microrods. The intensity ratio of the band-edge emission (IUV ) to the deep level emission (IG ) is always used to define the crystal quality. The results in Fig. 7 show that the high density of defects in the ZnGaO microrods gives rise to high IG /IUV ratio of 2.1, while the value of IG /IUV ratio is 0.9 for the ZnO/ZnGaO junction. The high intensity of the green emission in the ZnGaO microrods might relate to the defects at/near surfaces and the introduced defects due to the difference of the ionic radii of Ga (0.62 nm) and Zn
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(0.74 nm). When the ZnGaO microrods was coated by the ZnO layer, the defects at/near surface of the ZnGaO microrods are suppressed, which makes the ratio of IGreen /IUV decrease in some degree. Fig. 8 shows the schematic formation for the ZnO/ZnGaO junction. The inside is the ZnGaO microrods synthesized by the CVD method and the outside is the ZnO layer grown by the solution method. 3.3. Growth mechanism Why do some junctions grow along the axial direction, while other junctions grow along the radial junction? The possible mechanism suggested is that the formation of the different direction junctions is attributed to the difference in the growth rate of the various crystal facets. It is well known that the crystal structure defines the main and secondary growth directions of the materials. For ZnO with wurtzite crystallographic lattice structure, [0 0 0 1] is the fastest growth direction in the formation of nanostructure [17–19]. The structures will grow along the c-axis direction spontaneously if the deposition sustains it. It can easily envelope other lattice planes and determine the preferable orientation to be c-axis, that is the reason why the microrods always grow along the c-axis direction, as shown in Fig. 2. When these ZnO microrods are put into the solution for further growth, the nuclei are also easily formed at the top of the ZnO microrods, which is also the fast growth direction for ZnO, and only in this condition, the growth system has the lowest energy to keep stable. The microrods growth proceeds by a repeated nucleation and growth of epitaxial hexagonal on the c-face, (0 0 0 1) of the microrods, and forms the axis junction (Figs. 1 and 3). When the microrods grown by the CVD method are doped ZnO, many defects are introduced into the microrods
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Fig. 8. Schematic formation for the ZnO-based radial junction.
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microrod films prepared by the first step are doped ZnO. It is ascribed to the fastest growth direction in the formation of axial junction and radial junction is quite different. The PL spectra of the samples show that the junctions are quite different from the microrod films prepared by the first step. This method provides a useful and effective way to fabricate ZnO-based junctions, which have great potential applications in semiconductor devices. Acknowledgments
Fig. 9. Growth direction of the crystal facets in ZnO structure.
because of the different ions diameter of Ga3+ (0.62 nm) and Zn2+ (0.74 nm), and the fastest growing crystallographic plane of (0 0 0 1) may be disturbed. The experiment results show that the relatives velocities of crystal growth in [0 0 0 1] direction is restrained, and the main growth direction turns to be (Fig. 9). In this case, the ZnO nuclei are inclined to form surrounding the microrods, because is the fast growth direction. ZnO layer then epitaxially grows from the doped ZnO microrods and forms radial junction (Fig. 6). In a word, axial junction forms when the microrods prepared by the first step are pure ZnO, otherwise, radial junction forms as the microrods synthesized by the first step are doped ZnO, because the fastest growth direction in the formation of axial junction and radial junction is quite different, which also indicates that the growth direction of the junctions can be controlled by doping in the ZnO structures. 4. Conclusions In conclusion, ZnO-based axial and radial junctions were successfully synthesized by a two-step method. Pure or doped ZnO microrods were firstly synthesized by CVD method, these microrods were then put into the aqueous solution for further growth. Axial junction forms as the microrod films synthesized by CVD method are pure ZnO. Otherwise, radial junction forms when the
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