Epitaxial Growth of ZnO Nanowires on ZnS Nanobelts by Metal Organic Chemical Vapor Deposition Zhiqiang Wang, Xuedong Liu, Jiangfeng Gong, Hongbo Huang, Shulin Gu, and Shaoguang Yang*
CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 11 3911–3913
National Laboratory of Solid State Microstructures, Nanjing UniVersity, Nanjing 210093, China ReceiVed June 12, 2008; ReVised Manuscript ReceiVed August 21, 2008
ABSTRACT: In this manuscript, we report the first successful epitaxial growth of ZnO nanowires on ZnS nanobelts via the metal organic chemical vapor deposition (MOCVD) method. The compositional and structural analyses show that the product consists of wurtzitestructured ZnS/ZnO nanobelt/nanowire heterostructures, which have an epitaxial relationship with identical orientation along the (100) and (002) planes, respectively. Single crystallinity, the same orientation between the ZnS and ZnO, and a short growth time make the new epitaxial growth method with the pre-prepared nanostructures as substrates appropriate for the construction and investigation of onedimensional heterostructures. The development of nanostructures and nanotechnologies in the past few decades supplied us with a new platform to produce materials and devices with a smaller size and higher performance, while lowering the cost and power consumption. Nanowires, one of the most important one-dimensional (1D) nanostructures, have attracted much attention due to their potential use as nanoscale building blocks for future optoelectronic devices and systems.1,2 So far, there are many methods to fabricate nanowires,3 such as vapor-phase synthesis,4-6 vapor-liquid-solid methods,7-9 solution-liquid-solid methods,10,11 solvothermal methods,12,13 solution-phase methods based on capping reagents,14,15 and the self-assembly of nanoparticles.16,17 Among these methods, various templates are very useful for the synthesis of nanowires, such as V-groove substrate,18 cross-sections of multilayer films,19 step edges on the surface of a solid substrate20 and porous membrane (anodic alumina membrane).21 On the basis of the above methods, the studies of the dimensionally confined transport phenomenon and optoelectronic property are limited to the binary nano-system. For the growth and applications of the 1D nanostructures, the synthesis of epitaxial semiconductor heterostructures has been a serious problem. Recently, a new method has been applied for the synthesis of nanowires and nanobelts, which uses pre-prepared 1D nanostructures as the substrates for the epitaxial growth of nanowires and nanobelts.22-24 The novel method not only breaks down the compositional limitation, but also constructs the nanostructures with multiple functionalities such as luminescence, ferromagnetism, ferroelectric or superconducting properties. As important II-VI semiconductors, ZnO and ZnS with a wide bandgap energy (3.37 and 3.7 eV at room temperature, respectively) have a wide range of applications including flat panel displays, sensors, lasers, transducers, and photovoltaic devices. Recently, the study of biocompatibility of ZnO nanowires suggested their potential applications for drug carries, biosensing and biodetection.25 The theoretical calculation of the optical properties of the ZnO/ZnS heterostructures predicted they could be used for potential photovoltaic devices.26 The new experimental results illustrated that ZnS/ZnO biaxial nanobelt heterostructures could be applied for optoelectronic nanoscale devices.27 However, before applying the ZnS/ZnO heterostructures to optoelectronic nanodevices, the synthesis of such heterostructures with high quality becomes one of the main challenges. Here we report the first successful epitaxial growth of ZnO nanowires on pre-prepared ZnS nanobelts by the metal organic chemical vapor deposition (MOCVD) * Author to whom correspondence should be addressed. Phone: +86-2583597483. Fax: +86-25-83595539. E-mail:
[email protected].
method. The product has essential features of single crystallinity and identical orientation between the ZnS and ZnO, which make the novel epitaxial growth method appropriate for the construction and investigation of complex system. The ZnS nanobelts used in this study were synthesized in a traditional horizontal furnace via a simple vapor-phase transport method. ZnS powder was placed in the center of a quartz tube, and then a piece of Si wafer coated with a layer of Au was placed 7 cm away from the ZnS powder. Prior to heating, the system was flushed with high-purity N2 for 10 min to eliminate O2 from the quartz tube. After that, N2 of 100 sccm was continuously blown from the ZnS side into the quartz tube as the protection and carrier gas. When the temperature of the furnace was raised to 900 °C, the quartz tube was quickly inserted into the furnace. After a reaction of 20 min, the furnace was naturally cooled down to room temperature. A white woollike product was obtained on the Si wafer. ZnO nanowires were deposited on the ZnS nanobelts to yield ZnS/ZnO nanobelt/nanowire heterostructures by the MOCVD method. Diethylzinc (DEZn) was used as zinc source which was carried with high purity argon at the flow rate of 16.6 sccm. N2O was employed as the oxygen reactant at a flow rate of 100 sccm. The ZnS nanobelts grown on the Si wafer were used as substrates for the growth of ZnO nanowires. To minimize the gas phase reaction, the metal organic (MO) source and N2O were introduced into the reactor separately and mixed just above the ZnS nanobelts surface. The pressure, reaction temperature, and time were 145 Pa, 500 °C, and 10 min, respectively. The chamber was naturally cooled down to room temperature after the reaction. The crystal structures and morphologies of the products were characterized by X-ray diffraction (XRD; PANational x’pert) with Cu KR radiation, scanning electron microscopy (SEM; JEOL JSM6700F), transmission electron microscopy (TEM; JEOL JEM-100S), and high-resolution transmission electron microscopy (HRTEM; Tecnai F20). The SEM image of the synthesized ZnS nanobelts is shown in Figure 1a. The product consists of large quantities of nanobelts with a width of hundreds of nanometers and a length ranging from tens to hundreds of micrometers. The prepared nanobelts have a smooth surface. The XRD pattern (red pattern in Figure 1c) illustrates the nanobelts are wurtzite-structured ZnS with lattice constants a ) 0.3822 and c ) 0.6260 nm (JCPDS 79-2204). The TEM image of the ZnS nanobelt is shown in Figure 2a, which has a width of 270 nm and a very smooth surface. In the HRTEM image of the ZnS nanobelt (Figure 2b), the lattice spacings are 0.33 and 0.31 nm, corresponding to (100) and (002) planes of the hexagonal ZnS, which suggests that the ZnS nanobelt grows along the 〈100〉 direction. The corresponding selected area electron diffraction
10.1021/cg800588q CCC: $40.75 2008 American Chemical Society Published on Web 10/11/2008
3912 Crystal Growth & Design, Vol. 8, No. 11, 2008
Communications
Figure 1. (a, b) SEM images of the ZnS nanobelts (a) before and (b) after the deposition of ZnO via MOCVD. (c) XRD patterns of the ZnS nanobelts and the ZnS/ZnO nanobelt/nanowire heterostructures. (d) Schematic illustration of procedures that generate the ZnO nanowire on the ZnS nanobelt by MOCVD.
Figure 2. (a) TEM and (b) HRTEM images of the ZnS nanobelt. (c) TEM and (d) HRTEM images of the ZnS/ZnO nanobelt/nanowire heterostructures. The insets of (c) and (d) are the corresponding SAED, respectively.
(SAED, the inset of Figure 2b) further confirms the structure of the nanobelt. Smooth surface and single crystallinity make the ZnS nanobelts excellent substrates for the epitaxial growth of other nanostructures with suited crystal symmetry to construct the heterostructures. Figure 1b shows the SEM image of the ZnS nanobelts after the deposition of ZnO. After the MOCVD process, the morphology of the nanobelt was unchanged and each nanobelt still retained the smooth surface. The average width of the nanobelts became larger than that of the ZnS nanobelts before the deposition of ZnO, which may be caused by the growth of ZnO on ZnS nanobelt. As shown in Figure 1d, we propose the ZnO nanowire was grown on one side of the ZnS nanobelts in the MOCVD process, which resulted
in increased average width of the ZnS nanobelts, while the smooth surfaces were maintained. To confirm the above hypothesis, compositional and structural analysis was carried out on the product after the deposition of ZnO. There are two sets of diffraction peaks in the XRD pattern (blue pattern in Figure 1c) of the ZnS nanobelts after ZnO coating. The diffraction peaks marked with hollow circles are the same with those of the above pattern (red), which come from the ZnS nanobelts. The peak marked with pentagram comes from Au used as the catalyst for the growth of ZnS nanobelts. Those marked with triangles can be indexed to wurtzite-structured ZnO with lattice constants a ) 0.3253 and c ) 0.5209 nm (JCPDS 80-0075). There is no other phase presenting in the product. The XRD result illustrates the formation of single crystal ZnO in the final product. In order to make sure that the ZnO was grown on one side of the ZnS nanobelt, the structure and morphology of the product were further characterized by TEM. A typical single nanobelt/nanowire with clear bright/dark contrast variation running side by side along the growth direction of the nanostructures is shown in Figure 2c. The nanobelt with bright contrast has a width of 130 nm and the nanowire with dark contrast has a width of 20 nm, which illustrates that the nanowire grows on one side of the nanobelt. Figure 2d shows the HRTEM image of the interface between the nanobelt and the nanowire. The lattice spacing of 0.33 and 0.31 nm corresponds to (100) and (002) planes of hexagonal ZnS, and that of 0.28 and 0.26 nm corresponds to (100) and (002) planes of hexagonal ZnO. The HRTEM result further confirms that the nanobelt and nanowire are ZnS and ZnO, respectively, and the hypothesis that the ZnO nanowire was grown on one side of the ZnS nanobelt in the MOCVD process. Furthermore, the HRTEM image shows that the (100) and (002) planes of ZnS are parallel to that of ZnO, respectively. Both ZnS nanobelt and ZnO nanowire in the nanobelt/nanowire structures grow along the 〈100〉 direction. In the corresponding SAED pattern of the nanobelt/nanowire structures (the inset of Figure 2d), there are two sets of diffraction spots corresponding to (100) and (001) planes of ZnS nanobelt and (100) and (001) planes of ZnO nanowire, respectively. The zone axis of the ZnS nanobelt and ZnO nanowire are both 〈010〉. From both HRTEM and SAED images, the ZnS and ZnO in the nanobelt/ nanowire structures have an epitaxial relationship with identical
Communications
Crystal Growth & Design, Vol. 8, No. 11, 2008 3913
References
Figure 3. The model of crystallographic ZnS (ZnO) (a) and the ZnS/ ZnO nanobelt/nanowire heterostructures (b).
orientation along the (100) and (002) planes, respectively. The EDX spectrum of the nanobelt/nanowire heterostructures (not shown here) indicates that the heterostructures are comprised of Zn, S, and O element, which accords well with the result of XRD and illustrates the formation of ZnS and ZnO. In the vapor-phase synthesis method, the 1D nanostructures are normally grown via the vapor-solid (VS) or vapor-liquid-solid (VLS) mechanism. Here, we regard that the growth of ZnO nanowires on the substrates of ZnS nanobelts is dominated by the self-catalyzed growth mechanism.28,29 As shown in Figure 3a, wurtzite ZnS has a character similar to wurtzite ZnO, positively charged Zn terminated (001) surface is chemically active, while the negatively charged S or O terminated (001j) surface is chemically inert.30 In our study, the ZnS nanobelts which grew along the 〈100〉 direction with polar ((001) facets as small side surfaces were preprepared by controlling the synthesis conditions.28,30-32 The Zn terminated (001) surface of the ZnS nanobelt had a role as the “seed” for the growth of ZnO. During the MOCVD process, the ZnO nanowire grew epitaxially from the Zn site on the (001) plane of ZnS nanobelt (Figures 1d and 3b), which resulted in the formation of ZnS/ZnO nanobelt/nanowire heterostructures. In conclusion, we illustrate the synthesis of ZnO nanowires on ZnS nanobelts to form the ZnS/ZnO nanobelt/nanowire heterostructures by the MOCVD process. The method not only successfully achieves the growth of new nanostructures on pre-prepared nanostructures, but also can be used to construct a multicompositional system with multiple functions in nanoscale. The multi-compositional nanostructures prepared by this method have the potential application for the study of complex systems.
Acknowledgment. This work was supported by NSFC (60577002), JSPNSF (BK2006111), NCET (07-0430), and 973 Program (2007CB936300). Supporting Information Available: The introduction of MOCVD system, experimental details, and low magnification TEM images of the ZnS/ZnO nanobelt/nanowire heterostructures. This information is available free of charge via the Internet at http://pubs.acs.org.
(1) Hu, J. T.; Odom, T. W.; Lieber, C. M. Acc. Chem. Res. 1999, 32, 435–445. (2) Huang, M. H.; Mao, S.; Feick, H.; Yan, H. Q.; Wu, Y. Y.; Kind, H.; Weber, E.; Russo, R.; Yang, P. D. Science 2001, 292, 1897–1899. (3) Xia, Y. N.; Yang, P. D.; Sun, Y. G.; Wu, Y. Y.; Mayers, B.; Gates, B.; Yin, Y. D.; Kim, F.; Yan, H. Q. AdV. Mater. 2003, 15, 353–389. (4) Yang, P. D.; Lieber, C. M. Science 1996, 273, 1836–1840. (5) Shi, W. S.; Zheng, Y. F.; Wang, N.; Lee, C. S.; Lee, S. T. AdV. Mater. 2001, 13, 591–594. (6) Zhang, Y. J.; Wang, N. L.; Gao, S. P.; He, R. R.; Miao, S.; Liu, J.; Zhu, J.; Zhang, X. Chem. Mater. 2002, 14, 3564–3568. (7) Morales, A. M.; Lieber, C. M. Science 1998, 279, 208–211. (8) Duan, X. F.; Lieber, C. M. AdV. Mater. 2000, 12, 298–302. (9) Huang, M. H.; Wu, Y. Y.; Feick, H.; Tran, N.; Weber, E.; Yang, P. D. AdV. Mater. 2001, 13, 113–116. (10) Trentler, T. J.; Hickman, K. M.; Geol, S. C.; Viano, A. M.; Gibbons, P. C.; Buhro, W. E. Science 1995, 270, 1791–1794. (11) Trentler, T. J.; Goel, S. C.; Hickman, K. M.; Viano, A. M.; Chiang, M. Y.; Beatty, A. M.; Gibbons, P. C.; Buhro, W. E. J. Am. Chem. Soc. 1997, 119, 2172–2181. (12) Heath, J. R.; Legoues, F. K. Chem. Phys. Lett. 1993, 208, 263–268. (13) Wang, X.; Li, Y. D. J. Am. Chem. Soc. 2002, 124, 2880–2881. (14) Peng, X. G.; Manna, L.; Yang, W. D.; Wickham, J.; Scher, E.; Kadavanich, A.; Alivisatos, A. P. Nature 2000, 404, 59–61. (15) Peng, Z. A.; Peng, X. G. J. Am. Chem. Soc. 2001, 123, 1389–1395. (16) (a) Mirkin, C. A.; Letsinger, R. L.; Mucic, R. C.; Storhoff, J. J. Nature 1996, 382, 607–609. (b) Alibisatos, A. P.; Johnsson, K. P.; Peng, X. G.; Wilson, T. E.; Loweth, C. J.; Bruchez, M. P., Jr.; Schultz, P. G. Nature 1996, 382, 609–611. (17) Yin, Y. D.; Lu, Y.; Gates, B.; Xia, Y. N. J. Am. Chem. Soc. 2001, 123, 8718–8729. (18) Jorritsma, J.; Gijs, M. A. M.; Kerkhof, J. M.; Stienen, J. G. H. Nanotechnology 1996, 7, 263–265. (19) Fasol, G. Science 1998, 280, 545–546. (20) Zach, M. P.; Ng, K. H.; Penner, R. M. Science 2000, 290, 2120– 2123. (21) Martin, C. R. Science 1994, 266, 1961–1966. (22) He, R. R.; Law, M.; Fan, R.; Kim, F.; Yang, P. D. Nano. Lett. 2002, 2, 1109–1112. (23) Hu, J. Q.; Bando, Y.; Liu, Z. W.; Sekiguchi, T.; Golberg, D.; Zhan, J. H. J. Am. Chem. Soc. 2003, 125, 11306–11313. (24) Law, M.; Zhang, X. F.; Yu, R.; Kuykendall, T.; Yang, P. D. Small 2005, 1, 858–865. (25) Zhou, J.; Xu, N. S.; Wang, Z. L. AdV. Mater. 2006, 18, 2432–2435. (26) Schrier, J.; Demchenko, D. O.; Wang, L. W.; Alivisatos, A. P. Nano. Lett. 2007, 7, 2377–2382. (27) Yan, J.; Fang, X. S.; Zhang, L. D.; Bando, Y.; Gautam, U. K.; Dierre, B.; Sekiguchi, T.; Golberg, D. Nano. Lett. 2008, 8, 2794–2799. (28) Wang, Z. L.; Kong, X. Y.; Zuo, J. M. Phys. ReV. Lett. 2003, 91, 185502. (29) Huang, H. B.; Yang, S. G.; Gong, J. F.; Liu, H. W.; Duan, J. H.; Zhao, X. N.; Zhang, R.; Liu, Y. L.; Liu, Y. C. J. Phys. Chem. B 2005, 109, 20746–20750. (30) Moore, D.; Wang, Z. L. J. Mater. Chem. 2006, 16, 3898–3905. (31) Moore, D.; Ronning, C.; Ma, C.; Wang, Z. L. Chem. Phys. Lett. 2004, 385, 8–11. (32) Fang, X. S.; Ye, C. H.; Zhang, L. D.; Wang, Y. H.; Wu, Y. C. AdV. Funct. Mater. 2005, 15, 63–68.
CG800588Q
