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Morphology-controlled synthesis, growth mechanism and optical properties of ZnO nanonails
Chemical Physics Letters 401 (2005) 414–419 www.elsevier.com/locate/cplett
Morphology-controlled synthesis, growth mechanism and optical properties of ZnO nanonails Guozhen Shen *, Jung Hee Cho, Cheol Jin Lee
*
Department of Nanotechnology, Hanyang University, 17-Haengdang-dong, Seongdong-gu, Seoul 133-791, Korea Received 6 November 2004; in final form 6 November 2004 Available online 15 December 2004
Abstract High-quality ZnO nanonails with different morphologies were synthesized on silicon substrate through a simple low-temperature thermal evaporation process. The obtained ZnO nanonails exhibit well-defined morphologies, single-crystalline orientations, and clean surface without amorphous contamination. The morphologies of the products can be easily controlled by simply tuning the evaporation temperature of indium powder (TIn). The optical properties of the products were studied by Raman and photoluminescence procedures. The growth of nanonails is based on firstly epitaxial sprouting of small nanorods and then the epitaxial growth of nanonails structures with different microstructures due to different zinc vapor pressures and different deposition sites. Ó 2004 Elsevier B.V. All rights reserved.
1. Introduction With the help of the discovery of carbon nanotubes [1], one-dimensional semiconducting nanomaterials with controlled dimension and morphology have attracted great interest because of their interesting optical, electrical and magnetic properties and the ability to fabricate nanodevices based on their unique properties [2–4]. To make full use of these nanoscale materials, control of their structures and their assembly system is inevitably required because the intrinsic properties of nanomaterials are determined by their structures, such as size, shape, composition, and crystallinity [5,6]. ZnO, an important II–VI semiconductor, has been widely used for its electrical, optoelectronic, and photochemical properties [7,8]. Much effort has been made to synthesize ZnO one-dimensional nanocrystals and then assemble these nanostructures into special nanoarchitectures such
*
Corresponding authors. Fax: +82 2 2290 0768. E-mail addresses: [email protected] (G. Shen), [email protected] (C.J. Lee). 0009-2614/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2004.11.096
as nanorings, nanohelixes, tetrapods and other hierarchical structures. [3,7,9–19]. To get novel complicated ZnO nanostructures, hightemperature carbothermal method is usually applied. The evaporation temperature is always higher than 800 °C to decompose ZnO. Recently, Ren and co-workers [20] reported the high-temperature carbothermal synthesis of novel ZnO nanonails by a vapor transport and condensation process. In their method, ZnO, In2O3, and graphite powders were evaporated at 950–970 °C and quite low pressure of 0.5–1.0 Torr. And the obtained ZnO nanonails are simply composed of a large diameter cap and a small diameter shaft. It is well known that low-temperature and atmospheric pressure technique is necessary especially in nowadays from a technology point of view. In this Letter, we report a very simple low-temperature and atmospheric pressure route to large-scale synthesis of ZnO nanonails with quite complicated structures. By using metallic Zn powders and In powders as source materials, we can not only dramatically lower the growth temperature to 550 °C (Zn vapor pressure can reach up to 10.8 Pa at a temperature down to
G. Shen et al. / Chemical Physics Letters 401 (2005) 414–419
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400 °C), but also we can easily control the morphologies of the obtained ZnO nanonails by simply tuning the temperature of In powders (TIn). We suggested a growth model to explain the possible growth mechanism. The optical properties of these nanonails were also investigated by Raman spectra and photoluminescence measurement at room temperature.
2. Experimental section The ZnO nanonails were fabricated in a two heating zone horizontal tube furnace system. P-type silicon wafer was used as the deposited substrate. In a typical procedure, zinc and indium powders were put into two ceramic boats separately in the two heating zone system (indium powders were put at first heating zone in the furnace upstream, and zinc powders at the second heating zone downstream), Si substrate was placed downstream to collect the products. The vertical distance between the zinc source and the substrate was about 1–2 cm. Prior to heating, the quartz tube was purged with Ar gas (99.9%) for 10 min. The temperature of zinc powders was set to 550 °C and In powders was set to 300–800 °C, respectively. The tube was heated to desired temperatures and kept for 60 min with Ar gas flowing rate of about 500 sccm. After cooling to room temperature, it was found that the substrate was deposited with a layer of wax-like products. The structure of the products was analyzed using XRD (Rigaku DMAX 2500). The morphology of the products was analyzed by scanning electron microscopy (SEM, Hitachi S-4700). The morphology and microstructure analysis were conducted using highresolution transmission electron microscope (TEM, Tecnai F20). The Raman spectra (LABRAM-HR) were produced at room temperature using Ar-ion laser with 514.5 nm emission lines as the excitation laser. Photoluminescence measurement was performed at room temperature using He-Cd laser line of 325 nm as an excitation source.
3. Results and discussion Fig. 1a–c are the XRD patterns of the ZnO products obtained at TIn = 300, 500 and 800 °C, respectively. It can be seen that whatever TIn is, all the strong peaks in the patterns can be indexed to pure ZnO with a wurtzite structure. The strong intensity of diffraction peaks indicates high crystallinity of the ZnO products. No peaks of other impurities, such as In, In2O3, are detected, indicating the relatively high purity of the samples. The strongest (0 0 2) peak of the samples shows that the preferred growth orientation of the deposited ZnO products is along the c-axis.
Fig. 1. XRD patterns of the sample obtained at: (a) TIn = 300 °C, (b) TIn = 500 °C and (c) TIn = 800 °C.
The morphology was analyzed using SEM and the results are shown in Fig. 2a–l. Fig. 2a–d are the SEM images of the ZnO product obtained at TIn = 300 °C. It can be seen that large-scale nail-shaped ZnO nanowires are obtained on the Si substrate. From a highmagnified SEM image shown in Fig. 2b, we can see that typical ZnO nanonail is consisted of a hexagonal (Fig. 2b inset) cap and a small diameter shaft, which is of similar morphology with previous report [20]. The diameters are 200 nm for the cap and 70 nm for the shaft. The lengths of the ZnO nanonails are ca. 1–2 lm. Besides these ZnO nanonails, some ZnO comb-like nanonail arrays were also formed just as shown in Fig. 2c,d. The structure of ZnO nanonails obtained at TIn = 300 °C was clearly indicated in Fig. 2d. In our experiments, it was found that temperature of indium source played an important role in the microstructure of the obtained samples. With the increase of temperature of In powder, the productÕs morphologies changed dramatically. Fig. 2e–h show the SEM images of the ZnO nanonails obtained at TIn = 500 °C. These images clearly indicate that the ZnO nanonail is consisted of a well hexagonal shaped cap, a prismatic shaft and a needle shaped tail. The diameter of the hexagonal cap is about 400 nm. The length of the shaft is ca. 1–2 lm and its diameter is about 200 nm. Fig. 2i–l are the corresponding SEM image of the products obtained at TIn = 800 °C, which also shows a large-scale production of ZnO nanonails on Si substrate. But from a highmagnified SEM image shown in Fig. 2k,l, we can see that they are quite different in shape compared with those ZnO nanonails obtained at TIn = 300 °C and 500 °C. At a higher indium temperature, each ZnO nanonail is consisted of a well hexagonal cap and a ZnO prism connected with a small diameter neck. The hexagonal cap is with similar diameter with that
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Fig. 2. SEM images of ZnO nanonails with different morphologies obtained at: (a)–(d) TIn = 300 °C, (e)–(h) TIn = 500 °C and (i)–(l) TIn = 800 °C.
obtained at TIn = 500 °C and the ZnO prism is about 1–2 lm in length. Fig. 3a is a typical TEM image of the ZnO nanonail obtained at TIn = 300 °C. It clearly shows the nail-shaped nanostructures as analyzed by SEM observation. The ZnO nanonail consists of a cap and a shaft just as previous report [20]. Fig. 3b is the corresponding HRTEM image. The measured plane spacing is about 0.51 nm, corresponding to the (0 0 1) plane of ZnO. The image also shows that the surface of the ZnO nanonail is relatively clean and no amorphous layer is observed. No dislocations or stacking faults are observed in the areas examined. The inset SAED pattern was taken from the ZnO nanonail shown in Fig. 3b with an electron beam along its [1 0 0] zone axis. It, together with the HRTEM image, confirms that the ZnO nanonails grow along the [0 0 1] direction. Fig. 3c is the TEM image of a single ZnO nanonail obtained at TIn = 800 °C, which also consists with the SEM observations. The ZnO nanonail is composed of a hexagonal cap and a small diameter shaft connected with a small diameter neck. The HRTEM image
shown in Fig. 3d also confirms the single-crystal nature and the [0 0 1] growth direction. In our experiments, indium is obvious the key factor to get ZnO nanonails. Without indium, only ZnO nanorod arrays are formed. The use of In powder not only affect the morphology of the products, but also control their microstructures. Studies from SEM and TEM procedures show that the ZnO nanonails formed in the present process are of much complexed structures under different In temperatures. When TIn = 300 °C, the typical ZnO nanonail is composed of a hexagonal cap and a small diameter shaft. At TIn = 500 °C, the diameter of ZnO nanonail increased and the typical ZnO nanonail is composed of a hexagonal cap, a prism shaft and a needle-shaped tail. While at TIn = 800 °C, the typical ZnO nanonail is composed of a hexagonal cap and a prism shaft connected with a small diameter neck. The possible mechanism was list in Fig. 4 and discussed as follows. Indium is a low melting point metal and it is easily vaporized at quite low temperature. With the increase of temperature, the concentration of vaporized
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Fig. 3. TEM and HRTEM images of ZnO nanonails obtained at: (a)–(b) TIn = 300 °C, (c)–(d) TIn = 800 °C.
Fig. 4. Growth mechanism of ZnO nanonails with different morphologies.
In gases increase greatly, which have great influence on the concentration of vaporized Zn gases. It is well known that the concentration of Zn gases has great influence on the morphologies of ZnO products. During the synthesis, Zn powder is firstly heated to generate Zn vapors, which are transferred to the low-temperature region by Ar gas. And then the transferred elements directly react with residual oxygen, resulting in ZnOx vapor. At low temperature region, the ZnOx nucleates on Si substrate and small nanorods sprouts out epitaxially. The incoming ZnOx vapor then continuously deposited on the nanorods and finally ZnO nanonails are formed due to the different deposition ratio of the cap and the shaft [20]. We speculate that in the process, In powder is also vaporized and transferred to the low temperature zone together with Zn vapors by Ar gas to form In or InOx and ZnOx vapors. But the In or InOx
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Fig. 5. Typical Raman spectrum (a) and representative PL spectra (b) of the ZnO products.
vapors are generated very slowly at low In temperature (300 °C), so the ZnOx vapors almost kept unchanging and formed the small diameter shaft. With the increase of In temperature (TIn = 500 °C), the ZnOx vapors also increased and they deposited on the substrate and resulted in the formation of needle-shaped tail due to the cap of the nail has longer time to absorb the ZnOx vapor than the bottom. With the prolonging of the reaction, more In or InOx and ZnOx vapors are generated and finally result in the balance of these vapors, so prism shaft with mean diameter are formed and finally ZnO nanonail with a cap, a prism shaft and a needle-shaped tail is formed. While at high In temperature (TIn = 800 °C), In or InOx vapor is generated very fast and it keeps a balance with ZnOx vapor at the beginning of the reaction so prism shaft with mean diameter was formed firstly. With the prolonging of reaction, ZnOx vapor is exhausted and few ZnOxvapor deposits on the shaft so the small diameter neck is formed between the shaft and the cap. The optical properties of the ZnO nanonails products were investigated using Raman and PL spectra. Fig. 5a is the Raman spectra of a typical product. It is well known that the space group of hexagonal ZnO belongs to C46v . Single-crystalline ZnO has eight sets of optical phonon modes at C point of the Brillouin zone, in which the A1 + E1 + 2E2 modes show the Raman activity. Moreover, the A1 and E1 modes split into LO and TO components. The figure shows a typical Raman spectrum of the deposited sample. It shows only E2 and A1 (LO) modes at 436 and 580 cm 1, respectively. The absence of the TO modes could be attributed to the special angle between the wave vector of photons and the c-axial direction of the wurtzite ZnO crystals [21]. Fig. 5b shows the room temperature PL spectra of the ZnO products. The spectra of all products consist of a sharp, weak emission band located at 380 nm and a strong, broad emission band centered at 500 nm. The
near-UV emission at 380 nm agrees with the band gap of bulk ZnO [22], which comes from the recombination of free excitons [23]. The green emission at 500 nm is related to the singly ionized oxygen vacancy, and this emission results from the recombination of a photo-generated hole with a singly ionized charge state of the specific defect [24]. The intensity of the green emission is comparatively strong for the reason of high density of oxygen vacancies and special surface structures in these ZnO nanonails.
4. Conclusion Large-scale ZnO nanonails were successfully synthesized on silicon substrate via a low-temperature thermal evaporation process under atmosphere pressure. The obtained ZnO nanonails exhibit well-defined morphologies, single-crystalline orientations, and clean surface without amorphous contamination. The temperature of In powder plays a key role to control the microstructures of ZnO nanonails. Such a low temperature growth of ZnO nanonails may provide a possibility to fabricate nanodevices onto various low temperature endurance substrates. Acknowledgements This work was supported by Center for Nanotubes and Nanostructured Composites at SKKU, the National R&D Project for Nano Science and Technology of MOST. References [1] S. Iijima, Nature 354 (1991) 56. [2] X.F. Duan, Y. Huang, J.F. Wang, C.M. Lieber, Nature 409 (2001) 66.
G. Shen et al. / Chemical Physics Letters 401 (2005) 414–419 [3] Y. Wu, H. Yan, P. Yang, Chem. Eur. J. 8 (2002) 1261. [4] C.N.R. Rao, G. Gundiah, F.L. Deepak, A. Govindaraj, A.K. Cheetham, J. Mater. Chem. 14 (2004) 440. [5] Y. Cui, C.M. Lieber, Science 291 (2001) 851. [6] L. Manna, D.J. Milliron, A. Meisel, E.C. Scher, A.P. Alivisatos, Nat. Mater. 2 (2003) 382. [7] L. Vayssieres, K. Keis, A. Hagfeldt, S.E. Lindquist, Chem. Mater. 13 (2001) 4395. [8] Z.K. Tang, G.K.L. Wong, P. Yu, M. Kawasaki, A. Ohtomo, H. Koinuma, Y. Segawa, Appl. Phys. Lett. 72 (1998) 3270. [9] C.J. Lee, T.J. Lee, S.C. Lyu, Y. Zhang, H. Ruh, H.J. Lee, Appl. Phys. Lett. 81 (2002) 3648. [10] Z.W. Pan, Z.R. Dai, Z.L. Wang, Science 291 (2001) 1947. [11] J.H. Park, H.J. Choi, S. Sohn, J.H. Park, J. Mater. Chem. 14 (2004) 35. [12] J.Q. Hu, Q. Li, N.B. Wong, C.S. Lee, S.T. Lee, Chem. Mater. 14 (2002) 1216.
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[13] Y. Sun, G.M. Fuge, M.N.R. Ashfold, Chem. Phys. Lett. 396 (2004) 21. [14] X.Y. Kong, Y. Ding, R. Yang, Z.L. Wang, Science 303 (2004) 1348. [15] X.Y. Kong, Z.L. Wang, Nano Lett. 3 (2003) 1625. [16] A.B. Djurisic, Y.H. Leung, W.C.H. Choy, K.W. Cheah, W.K. Chan, Appl. Phys. Lett. 84 (2004) 2635. [17] P.X. Gao, Z.L. Wang, Appl. Phys. Lett. 84 (2004) 2883. [18] J.Y. Lao, J.G. Wen, Z.F. Ren, Nano Lett. 2 (2002) 1287. [19] S.C. Lyu, Y. Zhang, C.J. Lee, Chem. Mater. 15 (2003) 3294. [20] J.Y. Lao, J.Y. Huang, D.Z. Wang, Z.F. Ren, Nano Lett. 2 (2003) 235. [21] 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. [22] E.M. Wong, P.C. Searson, Appl. Phys. Lett. 74 (1999) 2939. [23] V. Stikant, D.R. Clarke, J. Appl. Phys. 83 (1998) 5447. [24] Y. Li, G.S. Cheng, L.D. Zhang, J. Mater. Res. 15 (2000) 2305.
Morphology-controlled synthesis, growth mechanism and optical properties of ZnO nanonails Guozhen Shen *, Jung Hee Cho, Cheol Jin Lee
*
Department of Nanotechnology, Hanyang University, 17-Haengdang-dong, Seongdong-gu, Seoul 133-791, Korea Received 6 November 2004; in final form 6 November 2004 Available online 15 December 2004
Abstract High-quality ZnO nanonails with different morphologies were synthesized on silicon substrate through a simple low-temperature thermal evaporation process. The obtained ZnO nanonails exhibit well-defined morphologies, single-crystalline orientations, and clean surface without amorphous contamination. The morphologies of the products can be easily controlled by simply tuning the evaporation temperature of indium powder (TIn). The optical properties of the products were studied by Raman and photoluminescence procedures. The growth of nanonails is based on firstly epitaxial sprouting of small nanorods and then the epitaxial growth of nanonails structures with different microstructures due to different zinc vapor pressures and different deposition sites. Ó 2004 Elsevier B.V. All rights reserved.
1. Introduction With the help of the discovery of carbon nanotubes [1], one-dimensional semiconducting nanomaterials with controlled dimension and morphology have attracted great interest because of their interesting optical, electrical and magnetic properties and the ability to fabricate nanodevices based on their unique properties [2–4]. To make full use of these nanoscale materials, control of their structures and their assembly system is inevitably required because the intrinsic properties of nanomaterials are determined by their structures, such as size, shape, composition, and crystallinity [5,6]. ZnO, an important II–VI semiconductor, has been widely used for its electrical, optoelectronic, and photochemical properties [7,8]. Much effort has been made to synthesize ZnO one-dimensional nanocrystals and then assemble these nanostructures into special nanoarchitectures such
*
Corresponding authors. Fax: +82 2 2290 0768. E-mail addresses: [email protected] (G. Shen), [email protected] (C.J. Lee). 0009-2614/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2004.11.096
as nanorings, nanohelixes, tetrapods and other hierarchical structures. [3,7,9–19]. To get novel complicated ZnO nanostructures, hightemperature carbothermal method is usually applied. The evaporation temperature is always higher than 800 °C to decompose ZnO. Recently, Ren and co-workers [20] reported the high-temperature carbothermal synthesis of novel ZnO nanonails by a vapor transport and condensation process. In their method, ZnO, In2O3, and graphite powders were evaporated at 950–970 °C and quite low pressure of 0.5–1.0 Torr. And the obtained ZnO nanonails are simply composed of a large diameter cap and a small diameter shaft. It is well known that low-temperature and atmospheric pressure technique is necessary especially in nowadays from a technology point of view. In this Letter, we report a very simple low-temperature and atmospheric pressure route to large-scale synthesis of ZnO nanonails with quite complicated structures. By using metallic Zn powders and In powders as source materials, we can not only dramatically lower the growth temperature to 550 °C (Zn vapor pressure can reach up to 10.8 Pa at a temperature down to
G. Shen et al. / Chemical Physics Letters 401 (2005) 414–419
415
400 °C), but also we can easily control the morphologies of the obtained ZnO nanonails by simply tuning the temperature of In powders (TIn). We suggested a growth model to explain the possible growth mechanism. The optical properties of these nanonails were also investigated by Raman spectra and photoluminescence measurement at room temperature.
2. Experimental section The ZnO nanonails were fabricated in a two heating zone horizontal tube furnace system. P-type silicon wafer was used as the deposited substrate. In a typical procedure, zinc and indium powders were put into two ceramic boats separately in the two heating zone system (indium powders were put at first heating zone in the furnace upstream, and zinc powders at the second heating zone downstream), Si substrate was placed downstream to collect the products. The vertical distance between the zinc source and the substrate was about 1–2 cm. Prior to heating, the quartz tube was purged with Ar gas (99.9%) for 10 min. The temperature of zinc powders was set to 550 °C and In powders was set to 300–800 °C, respectively. The tube was heated to desired temperatures and kept for 60 min with Ar gas flowing rate of about 500 sccm. After cooling to room temperature, it was found that the substrate was deposited with a layer of wax-like products. The structure of the products was analyzed using XRD (Rigaku DMAX 2500). The morphology of the products was analyzed by scanning electron microscopy (SEM, Hitachi S-4700). The morphology and microstructure analysis were conducted using highresolution transmission electron microscope (TEM, Tecnai F20). The Raman spectra (LABRAM-HR) were produced at room temperature using Ar-ion laser with 514.5 nm emission lines as the excitation laser. Photoluminescence measurement was performed at room temperature using He-Cd laser line of 325 nm as an excitation source.
3. Results and discussion Fig. 1a–c are the XRD patterns of the ZnO products obtained at TIn = 300, 500 and 800 °C, respectively. It can be seen that whatever TIn is, all the strong peaks in the patterns can be indexed to pure ZnO with a wurtzite structure. The strong intensity of diffraction peaks indicates high crystallinity of the ZnO products. No peaks of other impurities, such as In, In2O3, are detected, indicating the relatively high purity of the samples. The strongest (0 0 2) peak of the samples shows that the preferred growth orientation of the deposited ZnO products is along the c-axis.
Fig. 1. XRD patterns of the sample obtained at: (a) TIn = 300 °C, (b) TIn = 500 °C and (c) TIn = 800 °C.
The morphology was analyzed using SEM and the results are shown in Fig. 2a–l. Fig. 2a–d are the SEM images of the ZnO product obtained at TIn = 300 °C. It can be seen that large-scale nail-shaped ZnO nanowires are obtained on the Si substrate. From a highmagnified SEM image shown in Fig. 2b, we can see that typical ZnO nanonail is consisted of a hexagonal (Fig. 2b inset) cap and a small diameter shaft, which is of similar morphology with previous report [20]. The diameters are 200 nm for the cap and 70 nm for the shaft. The lengths of the ZnO nanonails are ca. 1–2 lm. Besides these ZnO nanonails, some ZnO comb-like nanonail arrays were also formed just as shown in Fig. 2c,d. The structure of ZnO nanonails obtained at TIn = 300 °C was clearly indicated in Fig. 2d. In our experiments, it was found that temperature of indium source played an important role in the microstructure of the obtained samples. With the increase of temperature of In powder, the productÕs morphologies changed dramatically. Fig. 2e–h show the SEM images of the ZnO nanonails obtained at TIn = 500 °C. These images clearly indicate that the ZnO nanonail is consisted of a well hexagonal shaped cap, a prismatic shaft and a needle shaped tail. The diameter of the hexagonal cap is about 400 nm. The length of the shaft is ca. 1–2 lm and its diameter is about 200 nm. Fig. 2i–l are the corresponding SEM image of the products obtained at TIn = 800 °C, which also shows a large-scale production of ZnO nanonails on Si substrate. But from a highmagnified SEM image shown in Fig. 2k,l, we can see that they are quite different in shape compared with those ZnO nanonails obtained at TIn = 300 °C and 500 °C. At a higher indium temperature, each ZnO nanonail is consisted of a well hexagonal cap and a ZnO prism connected with a small diameter neck. The hexagonal cap is with similar diameter with that
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Fig. 2. SEM images of ZnO nanonails with different morphologies obtained at: (a)–(d) TIn = 300 °C, (e)–(h) TIn = 500 °C and (i)–(l) TIn = 800 °C.
obtained at TIn = 500 °C and the ZnO prism is about 1–2 lm in length. Fig. 3a is a typical TEM image of the ZnO nanonail obtained at TIn = 300 °C. It clearly shows the nail-shaped nanostructures as analyzed by SEM observation. The ZnO nanonail consists of a cap and a shaft just as previous report [20]. Fig. 3b is the corresponding HRTEM image. The measured plane spacing is about 0.51 nm, corresponding to the (0 0 1) plane of ZnO. The image also shows that the surface of the ZnO nanonail is relatively clean and no amorphous layer is observed. No dislocations or stacking faults are observed in the areas examined. The inset SAED pattern was taken from the ZnO nanonail shown in Fig. 3b with an electron beam along its [1 0 0] zone axis. It, together with the HRTEM image, confirms that the ZnO nanonails grow along the [0 0 1] direction. Fig. 3c is the TEM image of a single ZnO nanonail obtained at TIn = 800 °C, which also consists with the SEM observations. The ZnO nanonail is composed of a hexagonal cap and a small diameter shaft connected with a small diameter neck. The HRTEM image
shown in Fig. 3d also confirms the single-crystal nature and the [0 0 1] growth direction. In our experiments, indium is obvious the key factor to get ZnO nanonails. Without indium, only ZnO nanorod arrays are formed. The use of In powder not only affect the morphology of the products, but also control their microstructures. Studies from SEM and TEM procedures show that the ZnO nanonails formed in the present process are of much complexed structures under different In temperatures. When TIn = 300 °C, the typical ZnO nanonail is composed of a hexagonal cap and a small diameter shaft. At TIn = 500 °C, the diameter of ZnO nanonail increased and the typical ZnO nanonail is composed of a hexagonal cap, a prism shaft and a needle-shaped tail. While at TIn = 800 °C, the typical ZnO nanonail is composed of a hexagonal cap and a prism shaft connected with a small diameter neck. The possible mechanism was list in Fig. 4 and discussed as follows. Indium is a low melting point metal and it is easily vaporized at quite low temperature. With the increase of temperature, the concentration of vaporized
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Fig. 3. TEM and HRTEM images of ZnO nanonails obtained at: (a)–(b) TIn = 300 °C, (c)–(d) TIn = 800 °C.
Fig. 4. Growth mechanism of ZnO nanonails with different morphologies.
In gases increase greatly, which have great influence on the concentration of vaporized Zn gases. It is well known that the concentration of Zn gases has great influence on the morphologies of ZnO products. During the synthesis, Zn powder is firstly heated to generate Zn vapors, which are transferred to the low-temperature region by Ar gas. And then the transferred elements directly react with residual oxygen, resulting in ZnOx vapor. At low temperature region, the ZnOx nucleates on Si substrate and small nanorods sprouts out epitaxially. The incoming ZnOx vapor then continuously deposited on the nanorods and finally ZnO nanonails are formed due to the different deposition ratio of the cap and the shaft [20]. We speculate that in the process, In powder is also vaporized and transferred to the low temperature zone together with Zn vapors by Ar gas to form In or InOx and ZnOx vapors. But the In or InOx
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Fig. 5. Typical Raman spectrum (a) and representative PL spectra (b) of the ZnO products.
vapors are generated very slowly at low In temperature (300 °C), so the ZnOx vapors almost kept unchanging and formed the small diameter shaft. With the increase of In temperature (TIn = 500 °C), the ZnOx vapors also increased and they deposited on the substrate and resulted in the formation of needle-shaped tail due to the cap of the nail has longer time to absorb the ZnOx vapor than the bottom. With the prolonging of the reaction, more In or InOx and ZnOx vapors are generated and finally result in the balance of these vapors, so prism shaft with mean diameter are formed and finally ZnO nanonail with a cap, a prism shaft and a needle-shaped tail is formed. While at high In temperature (TIn = 800 °C), In or InOx vapor is generated very fast and it keeps a balance with ZnOx vapor at the beginning of the reaction so prism shaft with mean diameter was formed firstly. With the prolonging of reaction, ZnOx vapor is exhausted and few ZnOxvapor deposits on the shaft so the small diameter neck is formed between the shaft and the cap. The optical properties of the ZnO nanonails products were investigated using Raman and PL spectra. Fig. 5a is the Raman spectra of a typical product. It is well known that the space group of hexagonal ZnO belongs to C46v . Single-crystalline ZnO has eight sets of optical phonon modes at C point of the Brillouin zone, in which the A1 + E1 + 2E2 modes show the Raman activity. Moreover, the A1 and E1 modes split into LO and TO components. The figure shows a typical Raman spectrum of the deposited sample. It shows only E2 and A1 (LO) modes at 436 and 580 cm 1, respectively. The absence of the TO modes could be attributed to the special angle between the wave vector of photons and the c-axial direction of the wurtzite ZnO crystals [21]. Fig. 5b shows the room temperature PL spectra of the ZnO products. The spectra of all products consist of a sharp, weak emission band located at 380 nm and a strong, broad emission band centered at 500 nm. The
near-UV emission at 380 nm agrees with the band gap of bulk ZnO [22], which comes from the recombination of free excitons [23]. The green emission at 500 nm is related to the singly ionized oxygen vacancy, and this emission results from the recombination of a photo-generated hole with a singly ionized charge state of the specific defect [24]. The intensity of the green emission is comparatively strong for the reason of high density of oxygen vacancies and special surface structures in these ZnO nanonails.
4. Conclusion Large-scale ZnO nanonails were successfully synthesized on silicon substrate via a low-temperature thermal evaporation process under atmosphere pressure. The obtained ZnO nanonails exhibit well-defined morphologies, single-crystalline orientations, and clean surface without amorphous contamination. The temperature of In powder plays a key role to control the microstructures of ZnO nanonails. Such a low temperature growth of ZnO nanonails may provide a possibility to fabricate nanodevices onto various low temperature endurance substrates. Acknowledgements This work was supported by Center for Nanotubes and Nanostructured Composites at SKKU, the National R&D Project for Nano Science and Technology of MOST. References [1] S. Iijima, Nature 354 (1991) 56. [2] X.F. Duan, Y. Huang, J.F. Wang, C.M. Lieber, Nature 409 (2001) 66.
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