Epitaxial Growth of Horizontally Aligned Zinc Oxide Nanonecklace

J. Phys. Chem. C 2009, 113, 20845–20854

20845

Epitaxial Growth of Horizontally Aligned Zinc Oxide Nanonecklace Arrays on r-Plane Sapphire Jian Shi,† Xin Sun,† Jiaming Zhang,‡ Jie Lian,‡,§ Qingkai Yu,| Mengshi Lin,⊥ and Hao Li*,† Department of Mechanical and Aerospace Engineering, and Food Science Program, DiVision of Food Science System & Bioengineering, UniVersity of Missouri, Columbia, Missouri 65211, Department of Geological Sciences, UniVersity of Michigan, Ann Arbor, Michigan 48109, Department of Mechanical, Aerospace and Nuclear Engineering, Rensselaer Polytechnic Institute, Troy, New York 12180, and Department of Electrical and Computer Engineering, UniVersity of Houston, Houston, Texas 77081 ReceiVed: July 26, 2009; ReVised Manuscript ReceiVed: September 17, 2009

We report the synthesis of faceted single crystalline ZnO nanonecklace (ZnO NN) arrays horizontally aligned on r-plane sapphire using Au nanoparticles catalyzed chemical vapor deposition. High resolution TEM data show that ZnO NNs, without any grain boundary observed, grow along the ZnO [0001] direction horizontally on r-plane sapphire and also reveal the epitaxial relationships between the ZnO NN and r-plane sapphire with ZnO [0001] | sapphire [101j1] and ZnO (1j21j0) | sapphire (011j2). It was found that the Au nanoparticles with diameter of ∼5 nm and lower particle density are critical for the formation of horizontally aligned ZnO NN arrays, while the larger size or the higher density of Au nanoparticles results in other types of ZnO nanostructures, such as vertical ZnO nanoblades and nanowires. Thermodynamic analysis indicates that faster increase of strain energy compared to slower increase of surface free energy and interfacial energy with size increase of one-dimensional ZnO nanostructures might be the critical reason for the size effect that controls the vertical vs horizontal growth of ZnO nanostructures. The smaller lattice mismatch of 1.5% in the growth direction (ZnO [0001] | sapphire [101j1]) compared to the larger lattice mismatch of 18.3% in the other horizontal direction (ZnO [101j0] | sapphire [21j1j0]) that is normal to the growth direction contributes to the one-dimensional growth. The width of ZnO NNs increases with growth time, indicative of continuous postgrowth deposition of ZnO on ZnO NNs. The evolution of necklacelike structures of ZnO might be related to liquid catalyst surface tension, lattice mismatch/strain energy, and surface decoration of ZnO facets with Al and Au atoms. Introduction A wide variety of ZnO nanostructures, such as nanowires, nanocombs, nanorings, nanowalls, nanocages, nanobows, nanobelts, nanodisks, nanopropellers, nanotubes, nanotetropods, nanocantilevers, and nanohelixes, have been explored for various applications.1-13 Among all these structures, the one-dimensional ZnO nanostructures, such as nanowires and nanobelts, are of great importance because they are components of many nanodevices.7,9 In the past 10 years there has been a large amount of literature on epitaxial growth of ZnO nanowires grown vertically or with certain angles on a-plane and c-plane sapphires using different methods. Recently, more efforts have been made to fabricate horizontally aligned one-dimensional nanostructures, including Ge,14 GeSn,15 PtSi,16 silicides,17 In,18 Ga,18 CaF2,19 Bi20 quantum wires with diameters of a few nanometers, ZnO,21 VO2,22 GaN,23 and In2O324 nanowires of diameter of tens of nanometers, and single-walled carbon nanotubes (SWCNTs),25 because such nanostructures are compatible with conventional top-down microfabrication process and * To whom correspondence should be addressed. E-mail: liha@ missouri.edu. Telephone: (01)-573-884-5510. Fax: (01)-573-884-5090. † Department of Mechanical and Aerospace Engineering, University of Missouri. ‡ University of Michigan. § Rensselaer Polytechnic Institute. | University of Houston. ⊥ Division of Food Science System & Bioengineering, University of Missouri.

can be further processed for device applications. Particularly, Nikoobakht et al. for the first time reported the horizontally aligned ZnO nanowire arrays grown along one crystallographic orientation on a-plane sapphire, and Zhu et al. reported the horizontal ZnO nanowires grown along three crystallographic orientations on c-plane sapphire.21,26 However, there is no report on synthesis of horizontally grown ZnO nanowires on r-plane sapphire epitaxially even though a few groups reported the epitaxial growth of ZnO thin films on r-plane sapphire.27-29 One-dimensional nanowire growth differs very much from two-dimensional thin film growth due to different growth mechanisms. The preferential growth direction of nanowires could be influenced by many factors through different mechanisms, such as relatively high surface free energy of the growth surface, reduction of strain energy, catalysts-assisted growth process, relatively high anisotropic growth rate, etc. Tersoff et al. simulated the shape transition of Ag nanoislands to Ag nanowires on Si and found strain energy was relaxed by elongating the Ag island to an asymmetrical quantum wire with the aspect ratio as big as 1:50.30 Chen et al. summarized the growth behavior of silicides quantum wires on silicon wafer and found that asymmetrical one-dimensional growth phenomena happened when small lattice mismatch ( γ(10-11) > γ(11-20) > γ(10-10), which is typically the reason why in many cases ZnO grows in the c-axis [0001] with facets (101j0) and (112j0).4 In addition, ZnO (0001) and ZnO (101j1) planes are typical polar surface while ZnO (112j0) and ZnO (101j0) are nonpolar.67 In some ZnO NNs in Figure 2, some ZnO facets, possibly (101j1), are present rather than ordinary ZnO (112j0) and ZnO (101j0) in vertically grown ZnO nanowires.11 It is likely the facets that are not parallel to the c-axis become more energetically favorable than ZnO (112j0) plane and (10-10) plane during the ZnO crystal growth due to surface modification and decoration of the facets of the ZnO.65,66 Zhou et al.67 showed that the ionic liquid changed the surface energy of the (101j1) plane of ZnO and resulted in pyramid structure of ZnO; Fan et al.68 found that indium could induce the a-axis direction growth instead of c-axis direction growth of ZnO nanostructures, which also indicated that the indium atoms should have changed the surface composition and energy of the ZnO facet. It has been found that Au has different effects on the surface free energy of different facets of ZnO crystals.69,70 As Au NPs were used during the growth of ZnO NNs, Au atoms may decorate on the surface of ZnO NNs modifying the facet surface free energy. If the metal elements decoration is the only reason or mechanism for the formation of necklace structures, we should have observed similar ZnO NN structures in c-plane, a-plane, and m-plane sapphire. However, ZnO NN was only observed on r-plane sapphire, which suggests that the substrate indeed plays an important role in the formation of necklacelike structure. Al diffusivity is different on the different planes of sapphire, which may also contribute to the necklace-like structure. Gorla et al.71 reported that spinel ZnAl2O4 film formed on r-plane sapphire through Al diffusion from the substrate, suggesting a high mobility of Al atoms at high temperatures. It is very likely that Al atoms may also decorate the ZnO facets. The dissociation energy of Al and O atoms in sapphire is 2210 J/mol at absolute zero temperature, which is much bigger than 385 J/mol for the dissociation energy of Zn and O atoms in ZnO.72 Since the ZnO polar plane could provide the surplus oxygen ions, such planes/facets may be favorable for formation of the strong Al-O bond with reduced surface energy.

ZnO Nanonecklace Arrays on r-Plane Sapphire Lastly, it is worthy of pointing out that our results suggesting the initially grown ZnO NNs have only a size of 20-30 nm upon VLS growth and postgrowth deposition have been confirmed by our observation of variation of ZnO NN width and by our experiments with different growth times as shown in Figure 3a. The postgrowth deposition, while ZnO keeps the single crystalline structure, will also favor formation of facets with lower surface free energy and lower strain energy. So it is very likely that lattice mismatch/strain energy and metal elements decoration also play a role in the postgrowth deposition process, which eventually leads to the final faceted ZnO NN structures. In summary, we have demonstrated a single crystalline ZnO nanonecklace structure on r-plane sapphire using chemical vapor deposition at 900 °C and using Au nanoparticles of 5 nm diameter as the catalyst. The epitaxial relationships between ZnO and sapphire were found to be ZnO [0001] | sapphire [101j1] and ZnO (1j21j0) | sapphire (011j2). Our results indicate that catalysts size and the competition between surface free energy, interfacial energy between ZnO and sapphire, and strain energy are the major factors driving ZnO NN growth along the ZnO c-axis and sapphire [101j1] direction on r-plane sapphire. We also speculate the Au and Al atoms decoration and strain energy might contribute to the formation of necklacelike structures. The fact that horizontal ZnO NN could only be formed on r-plane sapphire, not on other types of sapphire, indicates the epitaxial relationship and lattice mismatch in different directions play an important role in necklace-like structure formation. More detailed studies are required for better understanding of evolution of such horizontal single crystalline nanonecklace structures, and more efforts are also needed to explore their electrical and optical applications. Acknowledgment. This work was supported by the National Science Foundation (CMMI-0620906). References and Notes (1) Wang, Z. L. Appl. Phys. A 2007, 88, 7–15. (2) Lieber, C. M.; Wang, Z. L. MRS Bull. 2007, 32, 99–104. (3) Ding, Y.; Wang, Z. L. J. Phys. Chem. B 2004, 108, 12280–12291. (4) Fu, Z.; Wang, Z.; Yang, B.; Yang, Y.; Yan, H.; Xia, L. Mater. Lett. 2007, 61, 4832–4835. (5) Gao, P.; Wang, Z. L. J. Phys. Chem. B 2002, 106, 12653–12658. (6) Gao, P. X.; Wang, Z. L. Appl. Phys. Lett. 2004, 84, 2883–2885. (7) Greene, L. E.; Yuhas, B. D.; Law, M.; Zitoun, D.; Yang, P. Inorg. Chem. 2006, 45, 7535–7543. (8) Kong, X. Y.; Ding, Y.; Wang, Z. L. J. Phys. Chem. B 2004, 108, 570–574. (9) Wang, X.; Song, J.; Liu, J.; Wang, Z. L. Science 2007, 316, 102– 105. (10) Wang, X.; Summers, C. J.; Wang, Z. L. AdV. Mater. 2004, 16, 1215–1218. (11) Wang, Z. L. Mater. Today 2004, 7, 26–33. (12) Wang, Z. L.; Kong, X. Y.; Zuo, J. M. Phys. ReV. Lett. 2003, 91, 185502.1-185502.4. (13) Yang, P.; Yan, H.; Mao, S.; Russo, R.; Johnson, J.; Saykally, R.; Morris, N.; Pham, J.; He, R.; Choi, H.-J. AdV. Funct. Mater. 2002, 12, 323–331. (14) Sunamura, H.; Usami, N.; Shiraki, Y.; Fukatsub, S. Appl. Phys. Lett. 1996, 68, 1847–1849. (15) Deng, X.; Yang, B.-K.; Hackney, S. A.; Williams, D. R. M.; Krishnamurthy, M. Phys. ReV. Lett. 1998, 80, 1022–1025. (16) Kavanagh, K. L.; Reuter, M. C.; Tromp, R. M. J. Cryst. Growth 1997, 173, 393–401. (17) Chen, Y.; Ohlberg, D. A. A.; Williams, R. S. Appl. Phys. Lett. 2002, 91, 3213–3218. (18) Evans, M. M. R.; Nogami, J. Phys. ReV. B 1999, 59, 7644–7648. (19) Viernow, J.; Petrovykh, D. Y.; Men, F. K.; Kirakosian, A.; Lin, J.-L.; Himpsel, F. J. Appl. Phys. Lett. 1999, 74, 2125–2127. (20) Miki, K.; Bowler, D. R.; Owen, J. H. G.; Briggs, G. A. D.; Sakamoto, K. Phys. ReV. B 1999, 59, 14868–14871.

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