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plates on Si substrate grown by low-temperature hydrothermal reaction
Applied Surface Science 256 (2010) 2781–2785
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
ZnO nanorods/plates on Si substrate grown by low-temperature hydrothermal reaction S.Y. Gao, H.D. Li *, J.J. Yuan, Y.A. Li, X.X. Yang, J.W. Liu State Key Laboratory of Superhard materials, Jilin University, Changchun 130012, PR China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 4 October 2009 Received in revised form 4 November 2009 Accepted 6 November 2009 Available online 14 November 2009
The zinc oxide (ZnO) nanorods/plates are obtained via hydrothermal method assisted by etched porous Al film on Si substrate. The products consist of nanorods with average diameter of 100 nm and nanoplates with thickness of 200–300 nm, which are uniformly distributed widely and grown perpendicularly to the substrate. The ZnO nanoplates with thickness of 150–300 nm were grown on Si substrate coated with a thin continuous Al film (without etching) in the same aqueous solution. The growth mechanism and room temperature photoluminescence (PL) properties of ZnO nanorods/plates and nanoplates were investigated. It is found that the introduction of the etched Al film plays a key role in the formation of ZnO nanorods/plates. The annealing process is favorable to enhance the UV PL emissions of the ZnO nanorods/plates. Crown Copyright ß 2009 Published by Elsevier B.V. All rights reserved.
Keywords: Zinc oxide Nanorods/plates Hydrothermal reaction Photoluminescence
1. Introduction Zinc oxide (ZnO) has a direct wide band gap of 3.37 eV and a large exciton binding energy of 60 meV, and has been widely applied [1], such as room-temperature and high-temperature ultraviolet (UV) lasers [2,3], field emission displays [4], surface acoustic wave devices [5], and gas sensors [6]. Recently, nanostructure materials have received growing interests due to their unique properties and various potential applications in the fabrication of the nanodevices. Therefore, fabricating suitable nanostructures become more important, and will offer a base for further building nanodevices. Over the past few years, many ZnO nanostructures with one-dimensional (1D) and two-dimensional (2D) morphologies have been successfully synthesized, such as nanobelts [7], nanowires [8], nanorods [9], nanotubes [10], nanoflowers [11], and nanoplates [12]. Once the 1D and 2D ZnO nanostructures are combined, a novel structure composed of nanorods and nanoplates (nanorods/plates) would lead to highperformance with advantages of both 1D nanorods and 2D nanoplates. However, ZnO nanorods/plates are generally prepared at high-temperature [13–15], which is detrimental to the fabrication and performance of nanodevices. How to fabricate the ZnO nanorods/plates on substrates at low-temperature is still a great challenging issue. Many methods have been carried out on synthesizing ZnO nanostructures, such as chemical vapor deposition (CVD) [16],
* Corresponding author. Tel.: +86 431 85168095; fax: +86 431 85168095. E-mail address: [email protected] (H.D. Li).
physical vapor deposition (PVD) [17], thermal evaporation [18], and electrochemical process [19]. However, the vacuum condition, high-temperature, sophisticated equipment, and other rigid experimental conditions are required in these methods, which may increase the cost and limit the choice of substrates [20]. Comparatively, the solution approach is more attractive for its simplicity, commercial feasibility, and good potential for a largescale production. Hydrothermal synthesis, as an important method of solution synthesis, has been proven to be a versatile approach for ZnO preparation due to its convenience and simplicity in the fabrication. Considering Si is low-cost and potential for integration with Sibased microelectronic devices, it is desirable to grow high-quality ZnO nanostructures on Si substrate. In this work, we present a simple hydrothermal route to the fabrication of ZnO thin films composed of nanorods and nanoplates on Si substrate at a lowtemperature (95 8C), which shows a more promising prospect in the nanodevice fabrication. The structural characteristics, growth mechanism, and photoluminescence (PL) properties of the asprepared nanostructure are investigated. 2. Experimental The synthesis processes are described as follows: First, the pure Al film was deposited on the (1 0 0) oriented ntype Si substrate by the RF (13.56 MHz) magnetron sputtering process. The substrate was ultrasonically cleaned with acetone and ethanol, rinsed in deionized water, and subsequently dried in a flowing nitrogen gas before deposition. Prior to sputtering, the background pressure of sputtering chamber was evacuated below
0169-4332/$ – see front matter . Crown Copyright ß 2009 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2009.11.028
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2 103 Pa with a turbo molecular pump. The argon gas was introduced with the flow rate of 60 sccm (standard cubic centimeter per minute). The Al film deposition was performed at the sputtering pressure of 1.0 Pa and the power of 100 W. Second, the Al-coated Si wafer was anodized in the 0.3 M oxalic solution at 40 V and 0 8C for about 30 min. Then the formed aluminum oxide layer was removed in an etching solution composed of 0.2 M chromic acid and 0.4 M phosphoric acid at 60 8C for about 30 min. Finally, the reaction solution is prepared by adding 1 ml ammonia (25%) into a 30 ml zinc acetate solution (0.05 M) at the room temperature (RT). Those solutions were transferred into Teflon lined stainless steel autoclave of 40 ml in volume. The Si substrate with etched Al film was vertically immersed into the reaction solution. The autoclave was sealed and heated to a constant temperature of 95 8C for 2 h. Subsequently, the autoclave was allowed to cool down naturally. The obtained product on the Si substrate was thoroughly washed with distilled water to remove the residual salts and dried naturally in air for further characterization. For comparison, the Si substrate with Al film without etching was also applied for the same hydrothermal process. The crystal structure and quality of the products were examined by X-ray diffraction (XRD) by a Rigaku D/max-RA diffractometer using Cu Ka radiation of 1.54056 A˚. The scanning electron microscopic (SEM) morphologies of the products were observed with a JEOL JXA-8200 electron probe micro-analyzer. Transmission electron microscopy (TEM) and selected area electron-diffraction (SAED) studies were performed using HITACHI TEM H-8100IV operated at 200 kV. To prepare the TEM samples, the products were first scraped from the substrates, dispersed in ethanol and diluted, followed by placing a droplet of the solution onto a amorphous holey carbon film covered copper grid. RT PL spectra were observed by using HR800 LabRam Infinity spectrometer with a 325 nm Hd–Cd laser as the excitation source. 3. Results and discussion 3.1. Structure and morphology Fig. 1 shows the XRD pattern of ZnO nanorods/plates deposited on the Si substrate with etched Al film. Evidently, the diffraction peaks can be indexed to a hexagonal wurtzite phase of ZnO and the (0 0 2) peak dominates the spectrum, which suggests that the samples are well crystallized and preferentially oriented in the caxis direction. Expect to the ZnO-related diffraction peaks, the peaks at 2u = 38.838 and 45.018 are well assigned to Al (1 1 1) and
Al (2 0 0), respectively. Furthermore, Si (4 0 0) peak appears at 69.458. No other uncertain diffraction peaks are represented, suggesting that high-purity ZnO products have been synthesized. A typical surface morphology of the Si substrate coated with the etched Al film is shown in Fig. 2a. It shows that the Al film has almost the same arrangement of pits with an average diameter about 60 nm and an interspace about 120 nm (inset of Fig. 2a). After hydrothermal reaction, the very thin homogenous film coats on the surface of Si substrate coated with etched Al film. The SEM images of the asprepared products are shown in Fig. 2b and c. It is found that a large number of sheet-like nanocrystals are uniformly grown on the substrate. Observed from the enlarged image of Fig. 2c, one can see the homogenous film consist of nanorods/plates. The nanoplates have smooth surface and the thickness ranges from 200 to 300 nm. The average diameter of the nanorods is about 100 nm. TEM and SAED were employed to investigate the morphology and the structure of the as-prepared products. The representative TEM image of ZnO nanorod is shown in Fig. 3a. The inset of Fig. 3a is the corresponding SAED pattern, which confirms the single crystal nature of the nanorods and growth along the [0 0 0 1] direction [21]. Fig. 3b is the result from ZnO nanoplates, observed from the image contrast, the thickness of the ZnO nanoplate are nearly homogeneous. The SAED pattern taken along the [0 0 0 1] zone axis demonstrates that the crystalline nature of the ZnO nanoplates is dominated by (0 0 0 1) facets. The doping is believed to be related to the growth process, where the Al(OH)4 species play a significant role in the formation of the (0 0 0 1) surface dominated nanoplates [22,23]. As comparison, Fig. 4a and b show the morphology of ZnO nanostructures grown on Si substrate with Al film without etching. An overview SEM image of the films is presented in Fig. 4a, from which one can see that a large amount of flake-like ZnO nanocrystals are produced on the whole substrate. High magnification observation (Fig. 4b) shows that these ZnO nanostructures consist of smooth nanoplates (with 150–300 nm in thickness) and no nanorods appear, which is in contrast to the observations in Fig. 1 mentioned above. This is similar to the previously reported results [22,23], where ZnO nanoplates are synthesized on the Al substrate. Fig. 4c shows the XRD patterns of the corresponding ZnO nanoplates. Evidently, the diffraction pattern can be indexed as hexagonal wurtzite ZnO. Interestingly, the intensity of the (1 0 1) peak is stronger than the (0 0 2) peak, significantly different from that of the ZnO nanorods/plates arrays (Fig. 1). Other peaks can be well assigned to (1 1 1), (2 0 0), (2 2 0), (2 2 2) peaks of Al and (4 0 0) of Si, as labeled in Fig. 4c. 3.2. Growth process and mechanism The major reactions involved in the formation of ZnO nanorods/ plates and nanoplates can be summarized as the following [24]:
Fig. 1. XRD pattern of the nanorods/plates grown on Si substrate with etched Al film.
NH3 H2 O $ NH4 þ þ OH
(1)
Zn2þ þ 4NH3 H2 O $ ZnðNH3 Þ4 2þ þ 4H2 O
(2)
ZnðNH3 Þ4 2þ þ 2OH $ ZnO þ 4NH3 þ H2 O
(3)
In the synthesis systems, ammonia serves as a alkaline buffer to release OH (Eq. (1)). The complexion Zn(NH3)42+ is formed by mixing ammonia and zinc acetate solution (Eq. (2)), and finally the ZnO is formed by the reaction between Zn(NH3)42+ and OH under thermal treatment conditions (Eq. (3)). Based on the above experimental results, a possible formation route of the present ZnO nanostructures can be schematically summarized in Fig. 5. As known, Al is an amphoteric metal, which can be dissolved into the solution under alkaline conditions in the
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Fig. 2. (a) Typical SEM images of the surface morphology of etched Al film on Si substrate and high magnification image (inset); (b) low and (c) high magnification SEM image of the nanorods/plates grown on the Si substrate with etched Al film.
presence of amine. When the Si substrate with Al films without etching is immersed into the reaction solution, the aqueous solution exhibits Al(OH)4 ions due to the reaction between OH and Al films on the Si substrate. The formed Al(OH)4 ions could presumably bind to the Zn terminated (0 0 0 1) surface of ZnO more strongly than to other nonpolar surfaces, which can effectively block the growth along the [0 0 0 1] direction [22]. This allows the lateral growth to occur. Thus, there are ZnO nanoplates obtained shown in Fig. 5a. When the Si substrate coated with etched Al film is immersed into the reaction solution (Fig. 5b), due to a number of pits are produced on the Al film surface, the ZnO nuclei can be formed preferentially in the pits [25]. The ZnO nuclei on the substrate act as the growing points for the next process, and play a key role in the formation of c-axis oriented ZnO nanorods. It is well known that
ZnO grows preferentially along the [0 0 0 1] direction due to the faster growth rate in this direction than in other directions. In this case, large-scale oriented ZnO nanorods are formed. With the increase in the formation of ZnO nanorods, the surface of the etched Al film is protected, which would prevent a mass of Al ions into the solution under alkaline condition. However, a small quantity of Al(OH)4 ions are still exhibited in the aqueous solution and the ZnO nanoplates are consequently formed, as discussed above. As a result, the ZnO nanorods/plates can be formed on the Si substrate with etched Al film, as shown in Fig. 2b and c. 3.3. Photoluminescence Fig. 6 shows the RT PL spectra of ZnO nanorods/plates and nanoplates. The ZnO nanorods/plates exhibit a strong UV emission
Fig. 3. TEM image of ZnO nanorods and the corresponding SAED pattern (inset) (a) and TEM image and SAED pattern of ZnO nanoplates (inset) (b).
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Fig. 4. Low (a) and high (b) magnification SEM image of the nanoplates grown on Si substrate with Al film without etching, (c) the corresponding XRD spectrum.
peak at 378 nm and a relatively weak and broad green emission between 500 and 700 nm. This result indicates a rather high-UV luminescence efficiency of the ZnO nanorods/plates. It is accepted that for ZnO, the near band edge UV emission at 378 nm originates from the recombination of free excitons [3,26]. The origin of the visible emission is generally attributed to deep level defects such as vacancies and interstitials of oxygen and zinc. The broad green emission band is considered resulting from the radiative recombination of a photongenerated hole with an electron occupying the oxygen vacancy [27]. Note that the UV emission of ZnO nanoplates is almost quenched and defect-related emission in visible region is significantly enhanced, with respect to the case of nanorods/plates. It is speculated that more oxygen vacancies are existed in the ZnO self-assembled nanoplates, which might be resulted from the
strong combination between Al and O [28]. The intensity ratio of the UV and the visible emission of the ZnO nanorods/plates and nanoplates are 4.36 and 0.16. Sun et al. reported that the intensity ratio of the UV emission and the visible emission of ZnO nanorods was 0.32 [29]. Compared to the optical results of Ref. [29] and the ZnO nanoplates prepared in this work, it is demonstrated that the ZnO nanorods/plates are of high-quality having strong UV emission. Fig. 7 shows the RT PL spectra of the ZnO nanorods/plates before (a) and after annealing at 500 8C (b) and 600 8C (c) in air for 1 h. The inset of Fig. 7 presents the morphology of the ZnO nanorods/plates annealed at 500 8C. Compared with the unannealed ZnO nanorods/plates, the UV emission intensity of the annealed ZnO nanorods/plates increased drastically by a factor of
Fig. 5. Schematic illustration of the process for fabricating ZnO nanoplates (route a) and nanorods/plates (route b).
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temperature for further enhancing the intensity of UV emission and suppressing the green emission of ZnO nanorods/plates. It is very beneficial to the UV optoelectronic devices development. Further work is now undergoing. 4. Conclusion In summary, we have developed a facile low-temperature hydrothermal route for the synthesis of large-scale uniform ZnO nanorods/plates on Si substrate. It is demonstrated that the etched Al film with pits on the substrate plays a key role in the growth of ZnO nanorods/plates. Additionally, PL measurements show a higher luminescence efficiency of the ZnO nanorods/plates than the ZnO nanoplates. We believe that this route is universal and can be potentially extended to the development of novel nanostructures for practical applications, such as UV sensors, UV lasers, gas sensors and nanodevices.
Fig. 6. RT PL spectra of ZnO nanorods/plates (a) and nanoplates (b).
Acknowledgments This work was financially supported by the Program for New Century Excellent Talents in University (no. 06-0303) and the National Basic Research Program of China under grant no. 2001CB711201). References
Fig. 7. RT PL spectra of ZnO nanorods/plates (a) the as-prepared samples; annealed at 500 8C (b) and 600 8C (c) in air for 1 h. The inset is the SEM image of the ZnO nanorods/plates annealed at 500 8C.
22 (13) for the ZnO nanorods/plates annealed at 500 8C (600 8C). This result can be attributed to the improved crystallinity of the ZnO nanorods/plates after annealing and the decrease of the nonradiative defects [30]. In addition, the intensity of green emission for ZnO nanorods/plates increased simultaneously after annealing at 600 8C. It could be explained as follows: in the case of the annealing of ZnO in air, the reaction in ZnO can be expressed as follows [31]: Vo þ ð1=2ÞO2 $ Oo
(4)
ZnO $ Vo þ Zni þ ð1=2ÞO2
(5)
where VO, OO and Zni are the oxygen vacancy, oxygen on the lattice place, and interstitial zinc, respectively. When the ZnO nanorods/ plates annealed in air at the high-temperature (600 8C), it is speculated that the desorption rate of oxygen is faster with respect to the adsorption rate due to the large kinetic energy of the oxygen atoms on the ZnO lattice. Consequently, there is an increase in the amount of the oxygen vacancies in the high-temperature-annealed ZnO nanorods/plates and the intensity of the green emission enhances. Therefore, there should be an optimal annealing
¨ zgu¨r, Y.I. Alivov, C. Liu, A. Teke, M.A. Reshchikov, S. Dog˘an, V. Avrutin, S.-J. Cho, ¨.O [1] U H. Morkoc, J. Appl. Phys. 98 (2005) 041301. [2] A. Mitra, R.K. Thareja, V. Ganesan, A. Gupta, P.K. Sahoo, V.N. Kulkarni, Appl. Surf. Sci. 174 (2001) 232. [3] H.D. Li, S.F. Yu, S.P. Lau, E.S.P. Leong, H.Y. Yang, T.P. Chen, A.P. Abiyasa, C.Y. Ng, Adv. Mater. 18 (2006) 771. [4] C.C. Lin, W.H. Lin, Y.Y. Li, J. Phys. D: Appl. Phys. 41 (2008) 225411. [5] N.W. Emanetoglu, J. Zhu, Y. Chen, J. Zhong, Y.M. Chen, Y.C. Lu, Appl. Phys. Lett. 85 (2004) 3702. [6] C.M. Ghimbeu, J. Schoonman, M. Lumbreras, M. Siadat, Appl. Surf. Sci. 253 (2007) 7483. [7] Z.W. Pan, Z.R. Dai, Z.L. Wang, Science 291 (2001) 1947. [8] M.H. Huang, S. Mao, H. Feick, H.Q. Yan, Y.Y. Wu, H. Kind, E. Weber, R. Russo, P.D. Yang, Science 292 (2001) 1897. [9] L.C. Tien, D.P. Norton, S.J. Pearton, H.T. Wang, F. Ren, Appl. Surf. Sci. 253 (2007) 4620. [10] J.J. Wu, S.C. Liu, C.T. Wu, K.H. Chen, L.C. Chen, Appl. Phys. Lett. 81 (2002) 1312. [11] A. Umar, S. Lee, Y.H. Im, Y.B. Hahn, Nanotechnology 16 (2005) 2462. [12] X.X. Liu, Z.G. Jin, Z.F. Liu, K. Yu, S.J. Bu, Appl. Surf. Sci. 252 (2006) 8668. [13] M. Ma¨der, J.W. Gerlach, T. Ho¨che, C. Czekalla, M. Lorenz, M. Grundmann, B. Rauschenbach, Phys. Status Solidi (RRL) 2 (2008) 200. [14] J. Grabowska, A. Meaney, K.K. Nanda, J.-P. Mosnier, M.O. Henry, J.-R. Ducle`re, E. McGlynn, Phys. Rev. B 71 (2005) 115439. [15] C. Li, G.J. Fang, F.H. Su, G.H. Li, X.G. Wu, X.Z. Zhao, Nanotechnology 17 (2006) 3740. [16] Y.J. Zeng, Z.Z. Ye, W.Z. Xu, L.P. Zhu, B.H. Zhao, Appl. Surf. Sci. 250 (2005) 280. [17] Y. Zhang, H.B. Jia, R.M. Wang, C.P. Chen, X.H. Luo, D.P. Yu, C.J. Lee, Appl. Phys. Lett. 83 (2003) 4631. [18] M.X. Qiu, Z.Z. Ye, J.G. Lu, H.P. He, J.Y. Huang, L.P. Zhu, B.H. Zhao, Appl. Surf. Sci. 255 (2009) 3972. [19] G.R. Li, X.H. Lu, W.X. Zhao, C.Y. Su, Y.X. Tong, Cryst. Growth Des. 8 (2008) 1276. [20] W.Q. Peng, S.C. Qu, G.W. Cong, Z.G. Wang, Cryst. Growth Des. 6 (2006) 1518. [21] B.Q. Cao, W.P. Cai, J. Phys. Chem. C 112 (2008) 680. [22] C.H. Ye, Y. Bando, G.Z. Shen, D. Golberg, J. Phys. Chem. B 110 (2006) 15146. [23] J.P. Cheng, X.B. Zhang, Z.Q. Luo, Surf. Coat. Technol. 202 (2008) 4681. [24] Z. Wang, X.F. Qian, J. Yin, Z.K. Zhu, Langmuir 20 (2004) 3441. [25] B.B. Wang, W.L. Wang, K.J. Liao, J.L. Xiao, Phys. Rev. B 63 (2001) 085412. [26] X.Q. Gu, L.P. Zhu, Z.Z. Ye, H.P. He, Y.Z. Zhang, F. Huang, M.X. Qiu, Y.J. Zeng, F. Liu, W. Jaeger, Appl. Phys. Lett. 91 (2007) 022103. [27] K. Vanheusden, W.L. Warren, C.H. Seager, D.R. Tallant, J.A. Voigt, B.E. Genade, J. Appl. Phys. 79 (1996) 7983. [28] J.P. Cheng, Z.M. Liao, D. Shi, F. Liu, X.B. Zhang, J. Alloys Compd. 480 (2009) 741. [29] Y. Sun, G.M. Fuge, N.A. Fox, D.J. Riley, M.N.R. Ashfold, Adv. Mater. 17 (2005) 2477. [30] X.M. Teng, H.T. Fan, S.S. Pan, C. Ye, G.H. Li, J. Appl. Phys. 100 (2006) 053507. [31] K. Ogata, K. Sakurai, Sz. Fujita, S. Fujita, K. Matsushige, J. Cryst. Growth 214/215 (2000) 312.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
ZnO nanorods/plates on Si substrate grown by low-temperature hydrothermal reaction S.Y. Gao, H.D. Li *, J.J. Yuan, Y.A. Li, X.X. Yang, J.W. Liu State Key Laboratory of Superhard materials, Jilin University, Changchun 130012, PR China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 4 October 2009 Received in revised form 4 November 2009 Accepted 6 November 2009 Available online 14 November 2009
The zinc oxide (ZnO) nanorods/plates are obtained via hydrothermal method assisted by etched porous Al film on Si substrate. The products consist of nanorods with average diameter of 100 nm and nanoplates with thickness of 200–300 nm, which are uniformly distributed widely and grown perpendicularly to the substrate. The ZnO nanoplates with thickness of 150–300 nm were grown on Si substrate coated with a thin continuous Al film (without etching) in the same aqueous solution. The growth mechanism and room temperature photoluminescence (PL) properties of ZnO nanorods/plates and nanoplates were investigated. It is found that the introduction of the etched Al film plays a key role in the formation of ZnO nanorods/plates. The annealing process is favorable to enhance the UV PL emissions of the ZnO nanorods/plates. Crown Copyright ß 2009 Published by Elsevier B.V. All rights reserved.
Keywords: Zinc oxide Nanorods/plates Hydrothermal reaction Photoluminescence
1. Introduction Zinc oxide (ZnO) has a direct wide band gap of 3.37 eV and a large exciton binding energy of 60 meV, and has been widely applied [1], such as room-temperature and high-temperature ultraviolet (UV) lasers [2,3], field emission displays [4], surface acoustic wave devices [5], and gas sensors [6]. Recently, nanostructure materials have received growing interests due to their unique properties and various potential applications in the fabrication of the nanodevices. Therefore, fabricating suitable nanostructures become more important, and will offer a base for further building nanodevices. Over the past few years, many ZnO nanostructures with one-dimensional (1D) and two-dimensional (2D) morphologies have been successfully synthesized, such as nanobelts [7], nanowires [8], nanorods [9], nanotubes [10], nanoflowers [11], and nanoplates [12]. Once the 1D and 2D ZnO nanostructures are combined, a novel structure composed of nanorods and nanoplates (nanorods/plates) would lead to highperformance with advantages of both 1D nanorods and 2D nanoplates. However, ZnO nanorods/plates are generally prepared at high-temperature [13–15], which is detrimental to the fabrication and performance of nanodevices. How to fabricate the ZnO nanorods/plates on substrates at low-temperature is still a great challenging issue. Many methods have been carried out on synthesizing ZnO nanostructures, such as chemical vapor deposition (CVD) [16],
* Corresponding author. Tel.: +86 431 85168095; fax: +86 431 85168095. E-mail address: [email protected] (H.D. Li).
physical vapor deposition (PVD) [17], thermal evaporation [18], and electrochemical process [19]. However, the vacuum condition, high-temperature, sophisticated equipment, and other rigid experimental conditions are required in these methods, which may increase the cost and limit the choice of substrates [20]. Comparatively, the solution approach is more attractive for its simplicity, commercial feasibility, and good potential for a largescale production. Hydrothermal synthesis, as an important method of solution synthesis, has been proven to be a versatile approach for ZnO preparation due to its convenience and simplicity in the fabrication. Considering Si is low-cost and potential for integration with Sibased microelectronic devices, it is desirable to grow high-quality ZnO nanostructures on Si substrate. In this work, we present a simple hydrothermal route to the fabrication of ZnO thin films composed of nanorods and nanoplates on Si substrate at a lowtemperature (95 8C), which shows a more promising prospect in the nanodevice fabrication. The structural characteristics, growth mechanism, and photoluminescence (PL) properties of the asprepared nanostructure are investigated. 2. Experimental The synthesis processes are described as follows: First, the pure Al film was deposited on the (1 0 0) oriented ntype Si substrate by the RF (13.56 MHz) magnetron sputtering process. The substrate was ultrasonically cleaned with acetone and ethanol, rinsed in deionized water, and subsequently dried in a flowing nitrogen gas before deposition. Prior to sputtering, the background pressure of sputtering chamber was evacuated below
0169-4332/$ – see front matter . Crown Copyright ß 2009 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2009.11.028
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2 103 Pa with a turbo molecular pump. The argon gas was introduced with the flow rate of 60 sccm (standard cubic centimeter per minute). The Al film deposition was performed at the sputtering pressure of 1.0 Pa and the power of 100 W. Second, the Al-coated Si wafer was anodized in the 0.3 M oxalic solution at 40 V and 0 8C for about 30 min. Then the formed aluminum oxide layer was removed in an etching solution composed of 0.2 M chromic acid and 0.4 M phosphoric acid at 60 8C for about 30 min. Finally, the reaction solution is prepared by adding 1 ml ammonia (25%) into a 30 ml zinc acetate solution (0.05 M) at the room temperature (RT). Those solutions were transferred into Teflon lined stainless steel autoclave of 40 ml in volume. The Si substrate with etched Al film was vertically immersed into the reaction solution. The autoclave was sealed and heated to a constant temperature of 95 8C for 2 h. Subsequently, the autoclave was allowed to cool down naturally. The obtained product on the Si substrate was thoroughly washed with distilled water to remove the residual salts and dried naturally in air for further characterization. For comparison, the Si substrate with Al film without etching was also applied for the same hydrothermal process. The crystal structure and quality of the products were examined by X-ray diffraction (XRD) by a Rigaku D/max-RA diffractometer using Cu Ka radiation of 1.54056 A˚. The scanning electron microscopic (SEM) morphologies of the products were observed with a JEOL JXA-8200 electron probe micro-analyzer. Transmission electron microscopy (TEM) and selected area electron-diffraction (SAED) studies were performed using HITACHI TEM H-8100IV operated at 200 kV. To prepare the TEM samples, the products were first scraped from the substrates, dispersed in ethanol and diluted, followed by placing a droplet of the solution onto a amorphous holey carbon film covered copper grid. RT PL spectra were observed by using HR800 LabRam Infinity spectrometer with a 325 nm Hd–Cd laser as the excitation source. 3. Results and discussion 3.1. Structure and morphology Fig. 1 shows the XRD pattern of ZnO nanorods/plates deposited on the Si substrate with etched Al film. Evidently, the diffraction peaks can be indexed to a hexagonal wurtzite phase of ZnO and the (0 0 2) peak dominates the spectrum, which suggests that the samples are well crystallized and preferentially oriented in the caxis direction. Expect to the ZnO-related diffraction peaks, the peaks at 2u = 38.838 and 45.018 are well assigned to Al (1 1 1) and
Al (2 0 0), respectively. Furthermore, Si (4 0 0) peak appears at 69.458. No other uncertain diffraction peaks are represented, suggesting that high-purity ZnO products have been synthesized. A typical surface morphology of the Si substrate coated with the etched Al film is shown in Fig. 2a. It shows that the Al film has almost the same arrangement of pits with an average diameter about 60 nm and an interspace about 120 nm (inset of Fig. 2a). After hydrothermal reaction, the very thin homogenous film coats on the surface of Si substrate coated with etched Al film. The SEM images of the asprepared products are shown in Fig. 2b and c. It is found that a large number of sheet-like nanocrystals are uniformly grown on the substrate. Observed from the enlarged image of Fig. 2c, one can see the homogenous film consist of nanorods/plates. The nanoplates have smooth surface and the thickness ranges from 200 to 300 nm. The average diameter of the nanorods is about 100 nm. TEM and SAED were employed to investigate the morphology and the structure of the as-prepared products. The representative TEM image of ZnO nanorod is shown in Fig. 3a. The inset of Fig. 3a is the corresponding SAED pattern, which confirms the single crystal nature of the nanorods and growth along the [0 0 0 1] direction [21]. Fig. 3b is the result from ZnO nanoplates, observed from the image contrast, the thickness of the ZnO nanoplate are nearly homogeneous. The SAED pattern taken along the [0 0 0 1] zone axis demonstrates that the crystalline nature of the ZnO nanoplates is dominated by (0 0 0 1) facets. The doping is believed to be related to the growth process, where the Al(OH)4 species play a significant role in the formation of the (0 0 0 1) surface dominated nanoplates [22,23]. As comparison, Fig. 4a and b show the morphology of ZnO nanostructures grown on Si substrate with Al film without etching. An overview SEM image of the films is presented in Fig. 4a, from which one can see that a large amount of flake-like ZnO nanocrystals are produced on the whole substrate. High magnification observation (Fig. 4b) shows that these ZnO nanostructures consist of smooth nanoplates (with 150–300 nm in thickness) and no nanorods appear, which is in contrast to the observations in Fig. 1 mentioned above. This is similar to the previously reported results [22,23], where ZnO nanoplates are synthesized on the Al substrate. Fig. 4c shows the XRD patterns of the corresponding ZnO nanoplates. Evidently, the diffraction pattern can be indexed as hexagonal wurtzite ZnO. Interestingly, the intensity of the (1 0 1) peak is stronger than the (0 0 2) peak, significantly different from that of the ZnO nanorods/plates arrays (Fig. 1). Other peaks can be well assigned to (1 1 1), (2 0 0), (2 2 0), (2 2 2) peaks of Al and (4 0 0) of Si, as labeled in Fig. 4c. 3.2. Growth process and mechanism The major reactions involved in the formation of ZnO nanorods/ plates and nanoplates can be summarized as the following [24]:
Fig. 1. XRD pattern of the nanorods/plates grown on Si substrate with etched Al film.
NH3 H2 O $ NH4 þ þ OH
(1)
Zn2þ þ 4NH3 H2 O $ ZnðNH3 Þ4 2þ þ 4H2 O
(2)
ZnðNH3 Þ4 2þ þ 2OH $ ZnO þ 4NH3 þ H2 O
(3)
In the synthesis systems, ammonia serves as a alkaline buffer to release OH (Eq. (1)). The complexion Zn(NH3)42+ is formed by mixing ammonia and zinc acetate solution (Eq. (2)), and finally the ZnO is formed by the reaction between Zn(NH3)42+ and OH under thermal treatment conditions (Eq. (3)). Based on the above experimental results, a possible formation route of the present ZnO nanostructures can be schematically summarized in Fig. 5. As known, Al is an amphoteric metal, which can be dissolved into the solution under alkaline conditions in the
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Fig. 2. (a) Typical SEM images of the surface morphology of etched Al film on Si substrate and high magnification image (inset); (b) low and (c) high magnification SEM image of the nanorods/plates grown on the Si substrate with etched Al film.
presence of amine. When the Si substrate with Al films without etching is immersed into the reaction solution, the aqueous solution exhibits Al(OH)4 ions due to the reaction between OH and Al films on the Si substrate. The formed Al(OH)4 ions could presumably bind to the Zn terminated (0 0 0 1) surface of ZnO more strongly than to other nonpolar surfaces, which can effectively block the growth along the [0 0 0 1] direction [22]. This allows the lateral growth to occur. Thus, there are ZnO nanoplates obtained shown in Fig. 5a. When the Si substrate coated with etched Al film is immersed into the reaction solution (Fig. 5b), due to a number of pits are produced on the Al film surface, the ZnO nuclei can be formed preferentially in the pits [25]. The ZnO nuclei on the substrate act as the growing points for the next process, and play a key role in the formation of c-axis oriented ZnO nanorods. It is well known that
ZnO grows preferentially along the [0 0 0 1] direction due to the faster growth rate in this direction than in other directions. In this case, large-scale oriented ZnO nanorods are formed. With the increase in the formation of ZnO nanorods, the surface of the etched Al film is protected, which would prevent a mass of Al ions into the solution under alkaline condition. However, a small quantity of Al(OH)4 ions are still exhibited in the aqueous solution and the ZnO nanoplates are consequently formed, as discussed above. As a result, the ZnO nanorods/plates can be formed on the Si substrate with etched Al film, as shown in Fig. 2b and c. 3.3. Photoluminescence Fig. 6 shows the RT PL spectra of ZnO nanorods/plates and nanoplates. The ZnO nanorods/plates exhibit a strong UV emission
Fig. 3. TEM image of ZnO nanorods and the corresponding SAED pattern (inset) (a) and TEM image and SAED pattern of ZnO nanoplates (inset) (b).
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Fig. 4. Low (a) and high (b) magnification SEM image of the nanoplates grown on Si substrate with Al film without etching, (c) the corresponding XRD spectrum.
peak at 378 nm and a relatively weak and broad green emission between 500 and 700 nm. This result indicates a rather high-UV luminescence efficiency of the ZnO nanorods/plates. It is accepted that for ZnO, the near band edge UV emission at 378 nm originates from the recombination of free excitons [3,26]. The origin of the visible emission is generally attributed to deep level defects such as vacancies and interstitials of oxygen and zinc. The broad green emission band is considered resulting from the radiative recombination of a photongenerated hole with an electron occupying the oxygen vacancy [27]. Note that the UV emission of ZnO nanoplates is almost quenched and defect-related emission in visible region is significantly enhanced, with respect to the case of nanorods/plates. It is speculated that more oxygen vacancies are existed in the ZnO self-assembled nanoplates, which might be resulted from the
strong combination between Al and O [28]. The intensity ratio of the UV and the visible emission of the ZnO nanorods/plates and nanoplates are 4.36 and 0.16. Sun et al. reported that the intensity ratio of the UV emission and the visible emission of ZnO nanorods was 0.32 [29]. Compared to the optical results of Ref. [29] and the ZnO nanoplates prepared in this work, it is demonstrated that the ZnO nanorods/plates are of high-quality having strong UV emission. Fig. 7 shows the RT PL spectra of the ZnO nanorods/plates before (a) and after annealing at 500 8C (b) and 600 8C (c) in air for 1 h. The inset of Fig. 7 presents the morphology of the ZnO nanorods/plates annealed at 500 8C. Compared with the unannealed ZnO nanorods/plates, the UV emission intensity of the annealed ZnO nanorods/plates increased drastically by a factor of
Fig. 5. Schematic illustration of the process for fabricating ZnO nanoplates (route a) and nanorods/plates (route b).
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temperature for further enhancing the intensity of UV emission and suppressing the green emission of ZnO nanorods/plates. It is very beneficial to the UV optoelectronic devices development. Further work is now undergoing. 4. Conclusion In summary, we have developed a facile low-temperature hydrothermal route for the synthesis of large-scale uniform ZnO nanorods/plates on Si substrate. It is demonstrated that the etched Al film with pits on the substrate plays a key role in the growth of ZnO nanorods/plates. Additionally, PL measurements show a higher luminescence efficiency of the ZnO nanorods/plates than the ZnO nanoplates. We believe that this route is universal and can be potentially extended to the development of novel nanostructures for practical applications, such as UV sensors, UV lasers, gas sensors and nanodevices.
Fig. 6. RT PL spectra of ZnO nanorods/plates (a) and nanoplates (b).
Acknowledgments This work was financially supported by the Program for New Century Excellent Talents in University (no. 06-0303) and the National Basic Research Program of China under grant no. 2001CB711201). References
Fig. 7. RT PL spectra of ZnO nanorods/plates (a) the as-prepared samples; annealed at 500 8C (b) and 600 8C (c) in air for 1 h. The inset is the SEM image of the ZnO nanorods/plates annealed at 500 8C.
22 (13) for the ZnO nanorods/plates annealed at 500 8C (600 8C). This result can be attributed to the improved crystallinity of the ZnO nanorods/plates after annealing and the decrease of the nonradiative defects [30]. In addition, the intensity of green emission for ZnO nanorods/plates increased simultaneously after annealing at 600 8C. It could be explained as follows: in the case of the annealing of ZnO in air, the reaction in ZnO can be expressed as follows [31]: Vo þ ð1=2ÞO2 $ Oo
(4)
ZnO $ Vo þ Zni þ ð1=2ÞO2
(5)
where VO, OO and Zni are the oxygen vacancy, oxygen on the lattice place, and interstitial zinc, respectively. When the ZnO nanorods/ plates annealed in air at the high-temperature (600 8C), it is speculated that the desorption rate of oxygen is faster with respect to the adsorption rate due to the large kinetic energy of the oxygen atoms on the ZnO lattice. Consequently, there is an increase in the amount of the oxygen vacancies in the high-temperature-annealed ZnO nanorods/plates and the intensity of the green emission enhances. Therefore, there should be an optimal annealing
¨ zgu¨r, Y.I. Alivov, C. Liu, A. Teke, M.A. Reshchikov, S. Dog˘an, V. Avrutin, S.-J. Cho, ¨.O [1] U H. Morkoc, J. Appl. Phys. 98 (2005) 041301. [2] A. Mitra, R.K. Thareja, V. Ganesan, A. Gupta, P.K. Sahoo, V.N. Kulkarni, Appl. Surf. Sci. 174 (2001) 232. [3] H.D. Li, S.F. Yu, S.P. Lau, E.S.P. Leong, H.Y. Yang, T.P. Chen, A.P. Abiyasa, C.Y. Ng, Adv. Mater. 18 (2006) 771. [4] C.C. Lin, W.H. Lin, Y.Y. Li, J. Phys. D: Appl. Phys. 41 (2008) 225411. [5] N.W. Emanetoglu, J. Zhu, Y. Chen, J. Zhong, Y.M. Chen, Y.C. Lu, Appl. Phys. Lett. 85 (2004) 3702. [6] C.M. Ghimbeu, J. Schoonman, M. Lumbreras, M. Siadat, Appl. Surf. Sci. 253 (2007) 7483. [7] Z.W. Pan, Z.R. Dai, Z.L. Wang, Science 291 (2001) 1947. [8] M.H. Huang, S. Mao, H. Feick, H.Q. Yan, Y.Y. Wu, H. Kind, E. Weber, R. Russo, P.D. Yang, Science 292 (2001) 1897. [9] L.C. Tien, D.P. Norton, S.J. Pearton, H.T. Wang, F. Ren, Appl. Surf. Sci. 253 (2007) 4620. [10] J.J. Wu, S.C. Liu, C.T. Wu, K.H. Chen, L.C. Chen, Appl. Phys. Lett. 81 (2002) 1312. [11] A. Umar, S. Lee, Y.H. Im, Y.B. Hahn, Nanotechnology 16 (2005) 2462. [12] X.X. Liu, Z.G. Jin, Z.F. Liu, K. Yu, S.J. Bu, Appl. Surf. Sci. 252 (2006) 8668. [13] M. Ma¨der, J.W. Gerlach, T. Ho¨che, C. Czekalla, M. Lorenz, M. Grundmann, B. Rauschenbach, Phys. Status Solidi (RRL) 2 (2008) 200. [14] J. Grabowska, A. Meaney, K.K. Nanda, J.-P. Mosnier, M.O. Henry, J.-R. Ducle`re, E. McGlynn, Phys. Rev. B 71 (2005) 115439. [15] C. Li, G.J. Fang, F.H. Su, G.H. Li, X.G. Wu, X.Z. Zhao, Nanotechnology 17 (2006) 3740. [16] Y.J. Zeng, Z.Z. Ye, W.Z. Xu, L.P. Zhu, B.H. Zhao, Appl. Surf. Sci. 250 (2005) 280. [17] Y. Zhang, H.B. Jia, R.M. Wang, C.P. Chen, X.H. Luo, D.P. Yu, C.J. Lee, Appl. Phys. Lett. 83 (2003) 4631. [18] M.X. Qiu, Z.Z. Ye, J.G. Lu, H.P. He, J.Y. Huang, L.P. Zhu, B.H. Zhao, Appl. Surf. Sci. 255 (2009) 3972. [19] G.R. Li, X.H. Lu, W.X. Zhao, C.Y. Su, Y.X. Tong, Cryst. Growth Des. 8 (2008) 1276. [20] W.Q. Peng, S.C. Qu, G.W. Cong, Z.G. Wang, Cryst. Growth Des. 6 (2006) 1518. [21] B.Q. Cao, W.P. Cai, J. Phys. Chem. C 112 (2008) 680. [22] C.H. Ye, Y. Bando, G.Z. Shen, D. Golberg, J. Phys. Chem. B 110 (2006) 15146. [23] J.P. Cheng, X.B. Zhang, Z.Q. Luo, Surf. Coat. Technol. 202 (2008) 4681. [24] Z. Wang, X.F. Qian, J. Yin, Z.K. Zhu, Langmuir 20 (2004) 3441. [25] B.B. Wang, W.L. Wang, K.J. Liao, J.L. Xiao, Phys. Rev. B 63 (2001) 085412. [26] X.Q. Gu, L.P. Zhu, Z.Z. Ye, H.P. He, Y.Z. Zhang, F. Huang, M.X. Qiu, Y.J. Zeng, F. Liu, W. Jaeger, Appl. Phys. Lett. 91 (2007) 022103. [27] K. Vanheusden, W.L. Warren, C.H. Seager, D.R. Tallant, J.A. Voigt, B.E. Genade, J. Appl. Phys. 79 (1996) 7983. [28] J.P. Cheng, Z.M. Liao, D. Shi, F. Liu, X.B. Zhang, J. Alloys Compd. 480 (2009) 741. [29] Y. Sun, G.M. Fuge, N.A. Fox, D.J. Riley, M.N.R. Ashfold, Adv. Mater. 17 (2005) 2477. [30] X.M. Teng, H.T. Fan, S.S. Pan, C. Ye, G.H. Li, J. Appl. Phys. 100 (2006) 053507. [31] K. Ogata, K. Sakurai, Sz. Fujita, S. Fujita, K. Matsushige, J. Cryst. Growth 214/215 (2000) 312.