- Email: [email protected]
Growth of cubic and hexagonal InN nanorods
Materials Letters 61 (2007) 1563 – 1566 www.elsevier.com/locate/matlet
Growth of cubic and hexagonal InN nanorods X.M. Cai, K.Y. Cheung, A.B. Djurišić ⁎, M.H. Xie Department of Physics, The University of Hong Kong, Pokfulam Road, Hong Kong Received 21 February 2006; accepted 22 July 2006 Available online 10 August 2006
Abstract InN nanorods with polyhedral ends were grown by evaporating indium in the flow of ammonia gas on Si substrates coated with Au catalyst. The samples were characterized by scanning electron microscopy, transmission electron microscopy (TEM), and X-ray diffraction (XRD). XRD shows that the samples contain both cubic and hexagonal phases. From the TEM results, it can be found that the cubic nanorods grow along [010] ¯ 0] direction. The growth mechanism of the obtained direction with a lattice constant of 0.497 nm, while the hexagonal nanorods grow along [112 nanostructures is discussed. © 2006 Elsevier B.V. All rights reserved. PACS: 81.07.Vb; 81.05.Ea Keywords: Nanomaterials; Semiconductors
1. Introduction
2. Experiment
One dimensional nanostructures, such as nanowires and nanorods, have attracted much attention due to their novel properties and potential application in nanodevices. Among various semiconductor materials, InN is very promising for optoelectronic and electronic applications. For example, its large drift velocity makes it a better candidate than GaAs and GaN for field effect transistors [1]. Up to date, several works on the synthesis of InN nanostructures were reported [2–11]. Fabrication of InN nanowires from different starting materials was reported [2–5]. Selective growth of InN nanowires on Au patterned Si (100) was also achieved [3]. Other reported morphologies include InN nanorods encapsulated by In2O3 [6], InN nanotubes [7,8], polycrystalline InN fibers [9], and InN whiskers [10]. Self-catalyzed growth was also studied [11]. All these structures consisted of hexagonal InN. Cubic phase [12] or a mixture of hexagonal and cubic phases [13] has only been reported for InN nanocrystals, not 1D nanostructures. In this work, we report the growth and characterization of InN nanorods with polyhedral ends which exhibited a mixture of hexagonal and cubic phases.
For the growth of InN nanorods, Si substrate coated with Au catalyst was used. Before Au coating, Si was sonicated in toluene, acetone, ethanol and DI water. Then, Au (99.99%) was deposited by thermal evaporation in high vacuum (chamber base pressure of ∼ 4.0 × 10− 6 Torr). The thickness of Au layer (20– 150 Å) was monitored by a quartz thickness monitor. Then, a tungsten boat holding both the Au coated Si substrate and In wire (99.999%, 100 mg) was placed inside a quartz tube in a tube furnace, with the substrate placed 3 mm downstream from the In wire. After the quartz tube was evacuated to 2.0 × 10− 1 Torr, the temperature was increased to 550 °C. The temperature of the furnace was then kept at 550 °C for 30 min under the flow of NH3 (99.999%). The flowrate of NH3 was maintained at 150 standard cubic centimeters per minute (sccm), while the pressure in the tube during growth was 3.5 Torr. After the growth, NH3 was shut off and the tube was kept pumped until the furnace cooled down naturally to room temperature. The fabricated samples were characterized with scanning electron microscopy (SEM, Leo 1530), equipped with energy dispersive X-ray (EDX), X-ray powder diffraction (XRD, Rigaku, Ru300) and Bruker D8-Advance XRD, transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM) using JEOL 2010F, also equipped with EDX. Both normal scan and grazing incidence XRD scans were
⁎ Corresponding author. E-mail address: [email protected] (A.B. Djurišić). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.07.079
1564
X.M. Cai et al. / Materials Letters 61 (2007) 1563–1566
performed using Bruker D8-Advance XRD to verify the peak positions initially measured using Rigaku Ru300 XRD. 3. Results and discussion Fig. 1 shows the representative SEM images of the InN nanorods. It can be observed that the nanorods exhibit polyhedral ends. Two types of polyhedrons can be observed, either polyhedra with small dimensions compared to the nanorod length (Fig. 1(a), (c)) or elongated polyhedra (Fig. 1(b), (d)). Occasionally, some nanorods exhibiting both polyhedron at the top and the root can also be found. Both short and
Fig. 2. XRD pattern of the InN nanorods.
Fig. 1. (a), (b) The typical low magnification SEM images of different types of InN nanorods; (c), (d) higher magnification images of polyhedron ends.
elongated polyhedra typically show four facets at the top, as can be observed in high magnification SEM images (Fig. 1(a), (d)). The XRD pattern of the sample is shown in Fig. 2. The diffraction peaks can be indexed to hexagonal and cubic phase InN [14,15]. Letters h and c in Fig. 2 denote peaks corresponding to hexagonal and cubic phases, respectively, while the numbers in the brackets are the corresponding Miller indexes. The c(002) peak [14,15] at 36.04° implies that the cubic phase has a lattice constant of ac = 4.98 Å which is close to previously reported values [16]. Since lattice constants of cubic InN reported in the literature range from 4.532 Å to 5.09 Å [12], the obtained value in our work is in good agreement with the literature. ¯ 0) show that the hexagonal InN has a lattice The h(0002) and h(112 parameter of ah = 3.54 Å and ch = 5.72 Å. ah agrees with that in JCPDS (02-1450), while ch is slightly larger than the corresponding data in JCPDS (02-1450), possibly due to the overlapping [14] of peaks corresponding to h(0002) and c(111). The cubic lattice constant ac calculated from the peak at c(111) is 4.95 Å which is also slightly smaller than that calculated from the c(002) peak. It should be noted that there are large differences in the reported lattice parameters of InN in the literature. One possible reason for this is oxygen incorporation, which results in broadening and small shift of the XRD peaks [16]. It is well known that there is a close proximity in lattice spacing of In2O3 and InN [6], which results in difficulties in conclusive identification of the possible phases present in the sample. However, the lattice parameter of In2O3 is almost exactly double that of cubic InN, but its lattice is bcc rather than fcc, which allows distinction between the two using HRTEM and SAED [12]. However, it should be noted that the presence of oxygen in our nanorods cannot be entirely excluded, especially since InN is known to incorporate oxygen after storage in air at room temperature [16]. Also, it is expected that oxygen incorporation in nanorods would occur after shorter storage time than in the case of films due to their larger surface to volume ratio, which introduces additional difficulties in the study of their physical properties. Fig. 3 shows the TEM and HRTEM images of the InN nanorods. Fig. 3(e) and its inset show the corresponding HRTEM images and selective area electron diffraction (SAED) patterns of the nanorods in Fig. 3(a) and (b). The SAED pattern corresponding to the cubic InN can be indexed to the diffraction along [100]. The lattice constants measured along [001] and [010] are both 0.497 nm as shown in Fig. 3(e), which is similar to previously reported values [17]. From the obtained results, it can be concluded that the nanorods grow along [010] direction. Fig. 3(f) and its inset correspond to the HRTEM and SAED pattern of the hexagonal nanorods shown in Fig. 3(c) and (d). The c
X.M. Cai et al. / Materials Letters 61 (2007) 1563–1566
1565
Au could successfully catalyze the growth of InN nanorods, as shown in Fig. 4, and different morphology is obtained in the case of Ag catalyst. Au is a commonly used catalyst for the growth of InN nanostructures. It has been proposed that the presence of residual oxygen in the tube plays a significant role in the growth of InN using Au catalyst, due to the low miscibility of N and Au [6]. Based on the proposed model, nitrogen from the ammonia dissolves in the Au–InOx nanoclusters, with possible formation of In–O–N as an intermediate state, from which InN nanorods grow. Although we observed no clear In2O3 clusters in the HRTEM images unlike work reported in Ref. [6], similar mechanism likely plays a role in the growth of InN nanorods fabricated here. This is supported by the fact that obtained InN nanorods are brown and show no luminescence in the infrared spectral region, while oxygen-free nanorods are expected to be black and show PL in the range 0.7–0.8 eV [6]. We also found that the morphology of the obtained nanorods was dependent not only on the catalyst type, but also on the catalyst thickness. The InN nanorods grown on the substrate with 2 nm thick Au usually have much smaller polyhedrons at the end than those grown on the substrate with 15 nm thick Au, which suggests that Au plays an important role in the formation of the polyhedrons. EDX analyses conducted during TEM observations showed that the polyhedrons contain a very low content of Au, while the part outside the polyhedrons does not contain any Au. Therefore, it is likely that the original form of the polyhedrons is the catalytic droplet containing Au, In and N. The catalytic Au droplets form by aggregation of the deposited Au upon annealing in the tube furnace prior to nanostructure growth. A thicker Au layer will likely give rise to droplets of larger sizes and so the dimensions of the polyhedrons are also larger. Once the droplets become supersaturated, InN will precipitate forming InN nanorods. In most cases, the droplets are seen to be suspended from the substrate surface (see Fig. 1), suggesting the vapor–liquid–solid (VLS) mechanism [18]. When the growth is stopped and the sample is cooled down, the catalytic droplets solidify and form crystalline facets, similar to that observed during GaN nanowire growth by using Fe catalyst [18].
Fig. 3. (a)–(d) TEM images of the nanorods; (e) the HRTEM image of the nanorods shown in (a) and (b) (inset: the corresponding SAED pattern); (f) the HRTEM image of the nanorods shown in (c) and (d) (inset: the corresponding SAED pattern).
lattice constant is determined to be 0.574 nm. These hexagonal ¯ 0] direction, in agreement with previously nanorods grow along [112 reported results [3,4]. In order to obtain more information about the growth mechanism, Si substrates without any catalysts and with different catalysts (Al, Cu, Zn, Ni, Ag and Au, with 2 nm thickness) were tested. On Si without any catalysts, no nanorod growth was observed. Differences in the experimental setup and conditions resulting in different vapor pressure of reactants are likely responsible for differences between our results and those previously reported [11], where self-catalyzed growth of InN was achieved. On Si substrates with those metallic layers, only Ag and
Fig. 4. (a) and (b) InN nanorods with 2 nm Au catalyst. (c) and (d) InN nanorods with 2 nm Ag catalyst.
1566
X.M. Cai et al. / Materials Letters 61 (2007) 1563–1566
4. Conclusion InN nanorods with polyhedral ends were grown with Au as the catalyst. A mixture of cubic and hexagonal phases was obtained, as revealed by XRD and TEM results. HRTEM shows that the cubic nanorods grow along [010] direction, while the hexagonal nanorods grow along [112¯ 0] direction. The morphology of the obtained nanorods was dependent on the metal catalyst type (Au and Ag) and thickness. Acknowledgements The authors would like to thank Amy Wong and Wing Song Lee (Electron Microscopy Unit, HKU) for SEM measurements. This work is financially supported by grants from the Research Grant Council of the Hong Kong Special Administrative Region, China (Project Nos. HKU 7035/03P and HKU7019/04P) and the University of Hong Kong University Development Fund Grant. References [1] S.K. O'Leary, B.E. Foutz, M.S. Shur, U.V. Bhapkar, J. Appl. Phys 83 (1998) 826. [2] J. Zhang, L. Zhang, X. Peng, X. Wang, J. Mater. Chem. 12 (2002) 802.
[3] C.H. Liang, L.C. Chen, J.S. Hwang, K.H. Chen, Y.T. Hung, Y.F. Chen, Appl. Phys. Lett. 81 (2002) 22. [4] T. Tang, S. Han, W. Jin, X. Liu, C. Li, D. Zhang, C. Zhou, B. Chen, J. Han, M. Meyyapan, J. Mater. Res. 19 (2004) 423. [5] M.C. Johnson, C.J. Lee, E.D. Bourret-Courchesne, S.L. Konsek, S. Aloni, W.Q. Han, A. Zettl, Appl. Phys. Lett. 85 (2004) 5670. [6] Z.H. Lan, W.M. Wang, C.L. Sun, S.C. Shi, C.W. Hsu, T.T. Chen, K.H. Chen, C.C. Chen, Y.F. Chen, L.C. Chen, J. Cryst. Growth 269 (2004) 87. [7] L.W. Yin, Y. Bando, D. Golberg, M.S. Li, Adv. Mater. 16 (2004) 1833. [8] K. Sardar, F.L. Deepak, A. Govindaraj, M.M. Seikh, C.N.R. Rao, Small 1 (2005) 91. [9] S.D. Dingman, N.P. Rath, P.D. Markowitz, P.C. Gibbons, W.E. Buhro, Angew. Chem., Int. Ed. Engl. 39 (2000) 1470. [10] H. Parala, A. Devi, F. Hipler, E. Maile, A. Birkner, H.W. Becker, R.A. Fischer, J. Cryst. Growth 231 (2001) 68. [11] S. Vaddiraju, A. Mohite, A. Chin, M. Meyyappan, G. Sumanasekera, B.W. Alphenaar, M.K. Sunkara, Nano Lett. 5 (2005) 1625. [12] P.S. Schofield, W. Zhou, P. Wood, I.D.W. Samuel, D.J. Cole-Hamilton, J. Mater. Chem. 14 (2004) 3124. [13] Y.J. Bai, Z.G. Liu, X.G. Xu, D.L. Cui, X.P. Hao, X. Feng, Q.L. Wang, J. Cryst. Growth 241 (2002) 189. [14] V. Cimalla, J. Pezoldt, G. Ecke, R. Kosiba, O. Ambacher, L. Spieβ, G. Teichert, H. Lu, W.J. Schaff, Appl. Phys. Lett. 83 (2003) 3468. [15] K. Nishida, Y. Kitamura, Y. Hijikata, H. Yaguchi, S. Yoshida, Phys. Status Solidi, B Basic Res. 241 (2004) 2839. [16] E. Kurimoto, M. Hangyo, H. Harima, M. Yoshimoto, T. Yamaguchi, T. Araki, Y. Nanishi, K. Kisoda, Appl. Phys. Lett. 84 (2004) 212. [17] D. Bagayoko, L. Franklin, G.L. Zhao, J. Appl. Phys. 96 (2004) 4297. [18] X. Duan, C.M. Lieber, J. Am. Chem. Soc. 122 (2000) 188.
Growth of cubic and hexagonal InN nanorods X.M. Cai, K.Y. Cheung, A.B. Djurišić ⁎, M.H. Xie Department of Physics, The University of Hong Kong, Pokfulam Road, Hong Kong Received 21 February 2006; accepted 22 July 2006 Available online 10 August 2006
Abstract InN nanorods with polyhedral ends were grown by evaporating indium in the flow of ammonia gas on Si substrates coated with Au catalyst. The samples were characterized by scanning electron microscopy, transmission electron microscopy (TEM), and X-ray diffraction (XRD). XRD shows that the samples contain both cubic and hexagonal phases. From the TEM results, it can be found that the cubic nanorods grow along [010] ¯ 0] direction. The growth mechanism of the obtained direction with a lattice constant of 0.497 nm, while the hexagonal nanorods grow along [112 nanostructures is discussed. © 2006 Elsevier B.V. All rights reserved. PACS: 81.07.Vb; 81.05.Ea Keywords: Nanomaterials; Semiconductors
1. Introduction
2. Experiment
One dimensional nanostructures, such as nanowires and nanorods, have attracted much attention due to their novel properties and potential application in nanodevices. Among various semiconductor materials, InN is very promising for optoelectronic and electronic applications. For example, its large drift velocity makes it a better candidate than GaAs and GaN for field effect transistors [1]. Up to date, several works on the synthesis of InN nanostructures were reported [2–11]. Fabrication of InN nanowires from different starting materials was reported [2–5]. Selective growth of InN nanowires on Au patterned Si (100) was also achieved [3]. Other reported morphologies include InN nanorods encapsulated by In2O3 [6], InN nanotubes [7,8], polycrystalline InN fibers [9], and InN whiskers [10]. Self-catalyzed growth was also studied [11]. All these structures consisted of hexagonal InN. Cubic phase [12] or a mixture of hexagonal and cubic phases [13] has only been reported for InN nanocrystals, not 1D nanostructures. In this work, we report the growth and characterization of InN nanorods with polyhedral ends which exhibited a mixture of hexagonal and cubic phases.
For the growth of InN nanorods, Si substrate coated with Au catalyst was used. Before Au coating, Si was sonicated in toluene, acetone, ethanol and DI water. Then, Au (99.99%) was deposited by thermal evaporation in high vacuum (chamber base pressure of ∼ 4.0 × 10− 6 Torr). The thickness of Au layer (20– 150 Å) was monitored by a quartz thickness monitor. Then, a tungsten boat holding both the Au coated Si substrate and In wire (99.999%, 100 mg) was placed inside a quartz tube in a tube furnace, with the substrate placed 3 mm downstream from the In wire. After the quartz tube was evacuated to 2.0 × 10− 1 Torr, the temperature was increased to 550 °C. The temperature of the furnace was then kept at 550 °C for 30 min under the flow of NH3 (99.999%). The flowrate of NH3 was maintained at 150 standard cubic centimeters per minute (sccm), while the pressure in the tube during growth was 3.5 Torr. After the growth, NH3 was shut off and the tube was kept pumped until the furnace cooled down naturally to room temperature. The fabricated samples were characterized with scanning electron microscopy (SEM, Leo 1530), equipped with energy dispersive X-ray (EDX), X-ray powder diffraction (XRD, Rigaku, Ru300) and Bruker D8-Advance XRD, transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM) using JEOL 2010F, also equipped with EDX. Both normal scan and grazing incidence XRD scans were
⁎ Corresponding author. E-mail address: [email protected] (A.B. Djurišić). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.07.079
1564
X.M. Cai et al. / Materials Letters 61 (2007) 1563–1566
performed using Bruker D8-Advance XRD to verify the peak positions initially measured using Rigaku Ru300 XRD. 3. Results and discussion Fig. 1 shows the representative SEM images of the InN nanorods. It can be observed that the nanorods exhibit polyhedral ends. Two types of polyhedrons can be observed, either polyhedra with small dimensions compared to the nanorod length (Fig. 1(a), (c)) or elongated polyhedra (Fig. 1(b), (d)). Occasionally, some nanorods exhibiting both polyhedron at the top and the root can also be found. Both short and
Fig. 2. XRD pattern of the InN nanorods.
Fig. 1. (a), (b) The typical low magnification SEM images of different types of InN nanorods; (c), (d) higher magnification images of polyhedron ends.
elongated polyhedra typically show four facets at the top, as can be observed in high magnification SEM images (Fig. 1(a), (d)). The XRD pattern of the sample is shown in Fig. 2. The diffraction peaks can be indexed to hexagonal and cubic phase InN [14,15]. Letters h and c in Fig. 2 denote peaks corresponding to hexagonal and cubic phases, respectively, while the numbers in the brackets are the corresponding Miller indexes. The c(002) peak [14,15] at 36.04° implies that the cubic phase has a lattice constant of ac = 4.98 Å which is close to previously reported values [16]. Since lattice constants of cubic InN reported in the literature range from 4.532 Å to 5.09 Å [12], the obtained value in our work is in good agreement with the literature. ¯ 0) show that the hexagonal InN has a lattice The h(0002) and h(112 parameter of ah = 3.54 Å and ch = 5.72 Å. ah agrees with that in JCPDS (02-1450), while ch is slightly larger than the corresponding data in JCPDS (02-1450), possibly due to the overlapping [14] of peaks corresponding to h(0002) and c(111). The cubic lattice constant ac calculated from the peak at c(111) is 4.95 Å which is also slightly smaller than that calculated from the c(002) peak. It should be noted that there are large differences in the reported lattice parameters of InN in the literature. One possible reason for this is oxygen incorporation, which results in broadening and small shift of the XRD peaks [16]. It is well known that there is a close proximity in lattice spacing of In2O3 and InN [6], which results in difficulties in conclusive identification of the possible phases present in the sample. However, the lattice parameter of In2O3 is almost exactly double that of cubic InN, but its lattice is bcc rather than fcc, which allows distinction between the two using HRTEM and SAED [12]. However, it should be noted that the presence of oxygen in our nanorods cannot be entirely excluded, especially since InN is known to incorporate oxygen after storage in air at room temperature [16]. Also, it is expected that oxygen incorporation in nanorods would occur after shorter storage time than in the case of films due to their larger surface to volume ratio, which introduces additional difficulties in the study of their physical properties. Fig. 3 shows the TEM and HRTEM images of the InN nanorods. Fig. 3(e) and its inset show the corresponding HRTEM images and selective area electron diffraction (SAED) patterns of the nanorods in Fig. 3(a) and (b). The SAED pattern corresponding to the cubic InN can be indexed to the diffraction along [100]. The lattice constants measured along [001] and [010] are both 0.497 nm as shown in Fig. 3(e), which is similar to previously reported values [17]. From the obtained results, it can be concluded that the nanorods grow along [010] direction. Fig. 3(f) and its inset correspond to the HRTEM and SAED pattern of the hexagonal nanorods shown in Fig. 3(c) and (d). The c
X.M. Cai et al. / Materials Letters 61 (2007) 1563–1566
1565
Au could successfully catalyze the growth of InN nanorods, as shown in Fig. 4, and different morphology is obtained in the case of Ag catalyst. Au is a commonly used catalyst for the growth of InN nanostructures. It has been proposed that the presence of residual oxygen in the tube plays a significant role in the growth of InN using Au catalyst, due to the low miscibility of N and Au [6]. Based on the proposed model, nitrogen from the ammonia dissolves in the Au–InOx nanoclusters, with possible formation of In–O–N as an intermediate state, from which InN nanorods grow. Although we observed no clear In2O3 clusters in the HRTEM images unlike work reported in Ref. [6], similar mechanism likely plays a role in the growth of InN nanorods fabricated here. This is supported by the fact that obtained InN nanorods are brown and show no luminescence in the infrared spectral region, while oxygen-free nanorods are expected to be black and show PL in the range 0.7–0.8 eV [6]. We also found that the morphology of the obtained nanorods was dependent not only on the catalyst type, but also on the catalyst thickness. The InN nanorods grown on the substrate with 2 nm thick Au usually have much smaller polyhedrons at the end than those grown on the substrate with 15 nm thick Au, which suggests that Au plays an important role in the formation of the polyhedrons. EDX analyses conducted during TEM observations showed that the polyhedrons contain a very low content of Au, while the part outside the polyhedrons does not contain any Au. Therefore, it is likely that the original form of the polyhedrons is the catalytic droplet containing Au, In and N. The catalytic Au droplets form by aggregation of the deposited Au upon annealing in the tube furnace prior to nanostructure growth. A thicker Au layer will likely give rise to droplets of larger sizes and so the dimensions of the polyhedrons are also larger. Once the droplets become supersaturated, InN will precipitate forming InN nanorods. In most cases, the droplets are seen to be suspended from the substrate surface (see Fig. 1), suggesting the vapor–liquid–solid (VLS) mechanism [18]. When the growth is stopped and the sample is cooled down, the catalytic droplets solidify and form crystalline facets, similar to that observed during GaN nanowire growth by using Fe catalyst [18].
Fig. 3. (a)–(d) TEM images of the nanorods; (e) the HRTEM image of the nanorods shown in (a) and (b) (inset: the corresponding SAED pattern); (f) the HRTEM image of the nanorods shown in (c) and (d) (inset: the corresponding SAED pattern).
lattice constant is determined to be 0.574 nm. These hexagonal ¯ 0] direction, in agreement with previously nanorods grow along [112 reported results [3,4]. In order to obtain more information about the growth mechanism, Si substrates without any catalysts and with different catalysts (Al, Cu, Zn, Ni, Ag and Au, with 2 nm thickness) were tested. On Si without any catalysts, no nanorod growth was observed. Differences in the experimental setup and conditions resulting in different vapor pressure of reactants are likely responsible for differences between our results and those previously reported [11], where self-catalyzed growth of InN was achieved. On Si substrates with those metallic layers, only Ag and
Fig. 4. (a) and (b) InN nanorods with 2 nm Au catalyst. (c) and (d) InN nanorods with 2 nm Ag catalyst.
1566
X.M. Cai et al. / Materials Letters 61 (2007) 1563–1566
4. Conclusion InN nanorods with polyhedral ends were grown with Au as the catalyst. A mixture of cubic and hexagonal phases was obtained, as revealed by XRD and TEM results. HRTEM shows that the cubic nanorods grow along [010] direction, while the hexagonal nanorods grow along [112¯ 0] direction. The morphology of the obtained nanorods was dependent on the metal catalyst type (Au and Ag) and thickness. Acknowledgements The authors would like to thank Amy Wong and Wing Song Lee (Electron Microscopy Unit, HKU) for SEM measurements. This work is financially supported by grants from the Research Grant Council of the Hong Kong Special Administrative Region, China (Project Nos. HKU 7035/03P and HKU7019/04P) and the University of Hong Kong University Development Fund Grant. References [1] S.K. O'Leary, B.E. Foutz, M.S. Shur, U.V. Bhapkar, J. Appl. Phys 83 (1998) 826. [2] J. Zhang, L. Zhang, X. Peng, X. Wang, J. Mater. Chem. 12 (2002) 802.
[3] C.H. Liang, L.C. Chen, J.S. Hwang, K.H. Chen, Y.T. Hung, Y.F. Chen, Appl. Phys. Lett. 81 (2002) 22. [4] T. Tang, S. Han, W. Jin, X. Liu, C. Li, D. Zhang, C. Zhou, B. Chen, J. Han, M. Meyyapan, J. Mater. Res. 19 (2004) 423. [5] M.C. Johnson, C.J. Lee, E.D. Bourret-Courchesne, S.L. Konsek, S. Aloni, W.Q. Han, A. Zettl, Appl. Phys. Lett. 85 (2004) 5670. [6] Z.H. Lan, W.M. Wang, C.L. Sun, S.C. Shi, C.W. Hsu, T.T. Chen, K.H. Chen, C.C. Chen, Y.F. Chen, L.C. Chen, J. Cryst. Growth 269 (2004) 87. [7] L.W. Yin, Y. Bando, D. Golberg, M.S. Li, Adv. Mater. 16 (2004) 1833. [8] K. Sardar, F.L. Deepak, A. Govindaraj, M.M. Seikh, C.N.R. Rao, Small 1 (2005) 91. [9] S.D. Dingman, N.P. Rath, P.D. Markowitz, P.C. Gibbons, W.E. Buhro, Angew. Chem., Int. Ed. Engl. 39 (2000) 1470. [10] H. Parala, A. Devi, F. Hipler, E. Maile, A. Birkner, H.W. Becker, R.A. Fischer, J. Cryst. Growth 231 (2001) 68. [11] S. Vaddiraju, A. Mohite, A. Chin, M. Meyyappan, G. Sumanasekera, B.W. Alphenaar, M.K. Sunkara, Nano Lett. 5 (2005) 1625. [12] P.S. Schofield, W. Zhou, P. Wood, I.D.W. Samuel, D.J. Cole-Hamilton, J. Mater. Chem. 14 (2004) 3124. [13] Y.J. Bai, Z.G. Liu, X.G. Xu, D.L. Cui, X.P. Hao, X. Feng, Q.L. Wang, J. Cryst. Growth 241 (2002) 189. [14] V. Cimalla, J. Pezoldt, G. Ecke, R. Kosiba, O. Ambacher, L. Spieβ, G. Teichert, H. Lu, W.J. Schaff, Appl. Phys. Lett. 83 (2003) 3468. [15] K. Nishida, Y. Kitamura, Y. Hijikata, H. Yaguchi, S. Yoshida, Phys. Status Solidi, B Basic Res. 241 (2004) 2839. [16] E. Kurimoto, M. Hangyo, H. Harima, M. Yoshimoto, T. Yamaguchi, T. Araki, Y. Nanishi, K. Kisoda, Appl. Phys. Lett. 84 (2004) 212. [17] D. Bagayoko, L. Franklin, G.L. Zhao, J. Appl. Phys. 96 (2004) 4297. [18] X. Duan, C.M. Lieber, J. Am. Chem. Soc. 122 (2000) 188.