NANO LETTERS
Periodic ZnO Nanorod Arrays Defined by Polystyrene Microsphere Self-Assembled Monolayers
2006 Vol. 6, No. 10 2375-2378
D. F. Liu,† Y. J. Xiang,† X. C. Wu,‡ Z. X. Zhang,† L. F. Liu,† L. Song,† X. W. Zhao,† S. D. Luo,† W. J. Ma,† J. Shen,† W. Y. Zhou,† G. Wang,† C. Y. Wang,† and S. S. Xie*,† Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Graduate School of the Chinese Academy of Sciences, Beijing 100080, P R China, and National Center for Nanoscience and Nanotechnology, Beijing 100080, P R China Received June 19, 2006; Revised Manuscript Received August 9, 2006
ABSTRACT We demonstrate a low-cost and effective method to fabricate hexagonally patterned, vertically aligned ZnO nanorod arrays. Selective wetetching is used to develop the catalyzing gold particle hexagonal pattern with the aid of a polystyrene microsphere self-assembled monolayer. The gold particles have tunable sizes independent of the polystyrene microsphere’s diameter and are inherently round in shape. Each ZnO rod is grown individually from a catalyzing site via catalyst-initiated epitaxy, and the original hexagonal periodicity is well-preserved. The rods have flat ends, and the diameters of the rods can be controlled well by the amount of source materials. This method provides a promising way to create ZnO one-dimensional nanostructures for applications as two-dimensional photonic crystal, sensor arrays, nanolaser arrays, and optoelectronic devices.
Introduction. ZnO one-dimensional (1D) nanoscale materials, such as nanorods, as important functional oxide nanostructures, have received increasing attention over the past few years because of their exciting potential applications in optoelectronic devices, sensors, highly efficient photonic devices, and near-UV lasers.1-4 However, most of these device applications require a high degree of precision regarding alignment, position, and size of the rods. Traditionally, aligned nanorods are achieved on a lattice matching substrate using a catalyzing metal particle, often gold, to initiate or guide the growth.5,6 It is therefore crucial to be able to control the position of the catalyst particles for the growth of periodic nanorod arrays. There have been efforts to obtain nanoscale-patterned metal catalysts by electronbeam lithography,7 nanoimprint lithography,8 and mask lithography via porous alumina,9,10 self-assembled micro- or nanospheres.11-13 Among them, nanosphere lithography (NSL) has been proven to be a very simple and cost-effective technique for large-scale fabrication of particle arrays with long-range periodicity.14-17 Ren’s and Wang’s groups have recently demonstrated the large-scale preparation of ZnO nanorod arrays templated by NSL.11,12 But the obtained ZnO nanorod arrays are either not patterned individually because of the interconnection of catalyzing sites12 or not truly * Corresponding author. E-mail:
[email protected]. † Beijing National Laboratory for Condensed Matter Physics. ‡ National Center for Nanoscience and Nanotechnology. 10.1021/nl061399d CCC: $33.50 Published on Web 09/23/2006
© 2006 American Chemical Society
vertically aligned possibly because of unoptimized growth conditions.11 Later on, the work of Fan et al. improved the individual patterning of the nanorods via a modified NSL.13 In the conventional NSL, the metal particle patterns are prepared using the interstices between nanospheres by thermal evaporation. Thus, it is inevitable that the periodicity of the obtained metal particle arrays be damaged by the inherent structural defects of the masks, such as dislocations, vacancies, and cracks. And it is also difficult for the conventional NSL to selectively pattern metal particles on a substrate because any regions without nanospheres as mask are accessible for the evaporated metals. Additionally, nanospheres with small diameters are necessary for getting a pattern of metal particles with small size because of the size of the interstices are dependent on the diameter of the used nanospheres. Annealing of the mask can be used to tune the size of the interstices,17 but it will introduce cracks into the mask. In this letter, we develop a method using a polystyrene (PS) microsphere self-assembled monolayers (SAM) to get hexagonally patterned gold particles with inherent round shapes and tunable sizes independent of the PS particle’s diameter via selective wet-etching of Au film unoccupied by PS microspheres. It inherently avoids the presence of undesired Au particles that result from the structural defects of the used SAM and would be applicable for the NSL to fabricate designed metal particle patterns
Figure 1. Schematic illustration of the strategy for fabricating periodic ZnO nanorod arrays: (a) deposition of gold on a sapphire(0001) substrate, (b) modification of the gold surface with ODT SAM, (c) deposition of PS microsphere SAM and slightly melting of the PS microspheres, (d) reduction of the size of PS microspheres and removal of the ODT molecules uncovered by PS microspheres with oxygen plasma, (e) removal of PS microspheres and development of gold particle pattern by selective wet-etching, and (f) growth of ZnO nanorods by vapor transport and condensation technique.
toward device applications. Furthermore, hexagonally individually patterned and vertically aligned ZnO nanorod arrays are achieved using the obtained Au particle patterns. This method provides a promising route to create ZnO 1D nanostructures for applications as two-dimensional photonic crystals, sensor arrays, nanolaser arrays, and optoelectronic devices.
For all of the experiments, (0001)-oriented sapphire wafers were cut into pieces of 5 × 5 mm2, cleaned in H2SO4/H2O2 (7:3) solution at 80 °C for 20 min, and dried in a 100 °C oven. Before usage, the substrates were cleaned again by oxygen plasma for 2 min in order to get rid of some contaminants from the air. The whole fabrication process is illustrated schematically in Figure 1: (a) Au films with thickness of 15-100 Å were ion-sputtered onto the clean substrates. (b) The Au-coated substrates were immersed in an ethanol solution of octadecanethiol (ODT, 10 mM) at 50 °C for 1 h and at room temperature for another 24 h to form a densely packed ODT SAM on the gold surface. The modified substrates were rinsed with ethanol to clear out the superfluous ODT and successively rinsed with deionized water to clean out the ethanol, and finally were stored in deionized water until next usage. (c) For the preparation of PS microsphere SAM, the self-assembly technique on a water surface was used (details of this method in the work of Kosiorek et al.).16 In this experiment, 120 µL of monodispersed PS microsphere suspension (Ø1390 nm, purchased from Microparticles Gmbh Berlin, 10 wt % aqueous dispersion, used as received) was diluted by mixing with equal volume of ethanol and slowly applied to the surface of deionized water in a Ø15 cm Petri dish using a glass pipet. Before distributing the PS microspheres on the surface of water, the ODT-modified Au-coated substrates had been placed on the bottom of the Petri dish. The single-crystal domains of PS microsphere SAM could be promoted up to about 25 cm2 by slow and careful vessel tilting. The PS microsphere SAM was deposited on the substrates by slow water drainage. After the drying process, the substrates covered with PS microsphere SAM were heated on a 110 °C hot plate for 2 min to make the PS microspheres melt slightly and stick to the substrates (it provides a good
Figure 2. SEM images of (a) a PS microsphere SAM deposited on the ODT-modified gold-coated sapphire-(0001) substrate, (b) PS microsphere SAM after oxygen plasma etching, (c) the obtained gold particle pattern, and (d) a ZnO nanorod array grown from the patterned gold particles. Few gold clusters are missing (see circle), and most of the rods have a bigger base, but a few have a uniform size along the axial direction (marked by arrows). The inset in d is an enlarged view, which clearly shows the hexagonal flat end and the big base of the nanorod. 2376
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protection for the ODT SAM under the PS microspheres in the subsequent oxygen plasma etching; glass transition temperature for polystyrene, 95-105 °C). (d) Oxygen plasma etching was used to reduce the size of the PS microspheres and to burn off the ODT molecules uncovered by the PS microspheres. (e) Upon removal of the PS microspheres by ultrasonication in ethanol, the gold particle pattern was developed by wet etching with an aqueous Fe3+/thiourea (20 mM/30 mM) solution because the ODT SAM on Au provides enough etch resistance to this etch bath and prevent etchants from dissolving the Au below.18 (f) Finally, a periodic ZnO nanorod array was grown on the patterned substrate by a vapor transport process that we had used to grow ZnO hexagonal nanoprisms with well-defined shapes.19 For the growth of ZnO nanorods, a mixture of ZnO and activated carbon powder (1:1) was held in a quartz boat and placed at the center of a furnace tube (made of quartz, Ø45 mm), and the substrates prepatterned with Au particles were positioned at the upstream 4-10 cm away from the quartz boat. Before loading the source materials and substrates into the furnace tube, it was heated to the desired temperature of 910 °C. During the growth process, the furnace tube was maintained at 910 °C and ∼150 Torr with a flux of 150 sccm (standard cubic centimeters per minute) helium and 0.7 sccm oxygen. The duration of growth was 30 min. After the growth, the substrates turned colorful. Figure 2a shows a typical SEM image of a PS microsphere SAM on the substrate, indicating that a defectless single-crystal region can be achieved at least on the scale of 100 × 100 µm2. After etching with oxygen plasma, the morphology of the PS microsphere SAM has turned into a non-closepacked pattern from the close-packed one (Figure 2b). Thus, it assures us that the ODT molecules on the regions uncovered by PS microspheres have been completely burned off. Figure 2c is an SEM image of the obtained Au particle patterns. The average size of the Au particles is about 250 nm. (The size can be controlled by either the time of heating on the hot plate or the time of oxygen plasma etching, or both.) Some of the Au particles are missing (see circle). Three factors account for this phenomenon: (i) point defects in the original PS microsphere SAM; (ii) some PS microspheres aren’t tightly adhered to the substrate after heating and can’t prevent oxygen plasma from burning off the ODT SAM underneath; and (iii) few undesired PS microspheres above the PS microsphere SAM draw away the below microsphere from its original site during the oxygen plasma etching. Figure 2d presents a tilted SEM image of the obtained ZnO nanorod arrays. It is obvious that the nanorods possess a hexagonal lattice, similar to the initial PS microspheres. The hexagonal ends of the rods indicate that their main axis direction is [0001]. Most of the rods have bigger bases, but a few have a uniform size along the axial direction (marked by arrows), indicating that the bases can be eliminated by adjusting the size of the Au particles and/or the growth conditions. All of the rods have flat ends, indicating that the growth belongs to the catalyst-initiated epitaxy.19 The length of the rod can be regulated easily by growth time. In contrast, it was found that it was not easy to control Nano Lett., Vol. 6, No. 10, 2006
Figure 3. Control of the size of ZnO nanorods by adjusting the amount of source materials while keeping the other growth parameters unchanged. SEM images of the ZnO nanorod arrays grown with (a) 5.0 g, (b) 4.0 g, and (c) 2.0 g of source materials. The average diameter of the rods is about 160, 230, and 500 nm in a-c, respectively. The inset in a is an enlarged view, with the electron beam focused on the base of the rod, which shows the polyhedral morphology of the base.
the diameter of the rod by the size of Au particles or their thickness. When the size of the Au particles was more than 200 nm, frequently, more than one rod grew out from one original catalyzing site because of the splitting of the Au pad into several tiny particles at the growth temperature. When the thickness of the Au particles was thicker than 5 nm, the grown rods were hardly kept in a good alignment, not normal to the substrate or branched at the bottom end (not shown here) because the lattice matching between the ZnO nanorod and the sapphire substrate was broken by the excess gold. So we realized the regulation of the diameter of the rod by adjusting the amount of source materials. Figure 3 exhibits the SEM images of the nanorod arrays grown under different amount of the source materials while keeping the other growth parameters unchanged (the size of Au particles was about 150 nm and their thickness was about 2377
1.5 nm). It can be found that the average diameter of the rods is about 160, 230, and 500 nm for the source materials of 5.0, 4.0, and 2.0 g, respectively, and the difference between the sizes of the rod and its base becomes smaller when the diameter of the rod is bigger. In the process of catalyst-initiated homoepitaxy, as discussed in detail in our previous report,19 the catalyst facilitates the formation of ZnO nanoparticles. Then the ZnO nanopaticles grow into bigger polyhedral particles or several nanoparticles merge into one large polyhedral particle through a crystal growth process. Subsequently, the ZnO rods grow up on the c-plane of the polyhedral particle via the homoepitaxy. Therefore, the diameter of the rod is determined by the size of the c-plane of the polyhedral particle. However, during crystal growing, the shape of the polyhedral particle is regulated by the Bravais-Friedel law,20 which states that high-index crystal planes with small interplanar spacings grow faster than lowindex planes and thus are not seen in the final shape of the crystal. Whereupon, when the amount of source materials are relatively large, as long as the c-plane of the polyhedral particle has grown larger than the critical size for nucleation to the homoepitaxy, a rod begins to grow on the c-plane at a relatively faster rate so that there is not enough time for some high-index crystal planes of the polyhedral particle to grow themselves out of existence, thus leaving a large base to the rod. But when the amount of source materials become less, the growth rate of the rod slows down and there is enough time for the high-index crystal planes of the polyhedral particle to grow themselves smaller or out of existence, the rod grows on a larger c-plane and gets a larger diameter. Furthermore, it can be safely inferred that there would be no bases for the rods if they were grown from very small catalyst particles under a relatively small amount of source material. In summary, we have developed a strategy for fabricating hexagonally patterned Au particle arrays to grow periodic ZnO nanorod arrays with the aid of PS microsphere SAM. This method is inexpensive and effective. The key factors for the successful growth of individually patterned and vertically aligned ZnO nanorod arrays are formation of very thin gold particles with well-separated distances and small sizes and controlled-growth with a relatively small amount of source material. The ZnO rods have flat ends, and the diameter can be controlled well by the amount of the source materials. The uniformity of size along the axial direction of the rod can be achieved by using small catalyst particles (less than 150 nm) to initiate the growth of the ZnO rod under a small amount of source material.
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Acknowledgment. This work is supported by National Natural Science Foundation of China (Grant No. 90406022) and “973” National Key Basic Research Program of China (Grant No. 2005CB623602). Supporting Information Available: Figures showing the process of oxygen plasma etching on the PS microsphere monolayer and the tunable size of the Au particles independent of the PS microsphere diameter, and a group of SEM images of the Au particle arrays (1.5 nm thick and 300-150 nm diameter) before and after annealing at 910 °C. This material is available free of charge via the Internet at http:// pubs.acs.org. References (1) 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. (2) Johnson, J. C.; Knutsen, K. P.; Yan, H. Q.; Law, M.; Zhang, Y. F.; Yang, P. D.; Saykally, R. J. Nano Lett. 2004, 4, 197. (3) Liu, C. H.; Zapien, J. A.; Yao, Y.; Meng, X. M.; Lee, C. S.; Fan, S. S.; Lifshitz, Y.; Lee, S. T. AdV. Mater. 2003,15, 838. (4) Choy, J. H.; Jang, E. S.; Won, J. H.; Chung, J. H.; Jang, D. J.; Kim, Y. W. AdV. Mater. 2003, 15, 1911. (5) Huang, M. H.; Mao, S.; Feick, H.; Yan, H.; Wu, Y.; Kind, H.; Weber, E.; Russo, R.; Yang, P. D. Science 2001, 292, 1897. (6) Ng, H. T.; Chen, B.; Li, J.; Han, J.; Meyyappan, M.; Wu, J.; Li, X.; Haller, E. E. Appl. Phys. Lett. 2003, 82, 2023. (7) Ng, H. T.; Han, J.; Yamada, T.; Nguyen, P.; Chen, Y. P.; Meyyappan, M. Nano Lett. 2004, 4, 1247. (8) Ma˚rtensson, T.; Carlberg, P.; Borgstro¨m, M.; Montelius, L.; Seifert, W.; Samuelson, L. Nano Lett. 2004, 4, 699. (9) Chik, H.; Liang, J.; Cloutier, S. G.; Kouklin, N.; Xu, J. M. Appl. Phys. Lett. 2004, 84, 3376. (10) Fan, H. J.; Lee, W.; Scholz, R.; Dadgar, A.; Krost, A.; Nielsch, K.; Zacharias, M. Nanotechnology 2005, 16, 913. (11) Rybczynski, J.; Banerjee, D.; Kosiorek, A.; Giersig, M.; Ren, Z. F. Nano Lett. 2004, 4, 2037. (12) Wang, X. D.; Summers, C. J.; Wang, Z. L. Nano Lett. 2004, 4, 423. (13) Fan, H. J.; Fuhrmann, B.; Scholz, R.; Syrowatka, F.; Dadgar, A.; Krost, A.; Zacharias, M. J. Cryst. Growth 2006, 287, 34. (14) Rybczynski, J.; Ebels, U.; Giersig, M. Colloids Surf., A 2003, 219, 1. (15) Kempa, K.; Kimball, B.; Rybczynski, J.; Huang, Z. P.; Wu, P. F.; Steeves, D.; Sennett, M.; Giersig, M.; Rao, D. V. G. L. N.; Carnahan, D. L.; Wang, D. Z.; Lao, J. Y.; Li, W. Z.; Ren, Z. F. Nano Lett. 2003, 3, 13. (16) Kosiorek, A.; Kandulski, W.; Chudzinski, P.; Kempa, K.; Giersig, M. Nano Lett. 2004, 4, 1359. (17) Kosiorek, A.; Kandulski, W.; Glaczynska, H.; Giersig, M. Small 2005, 4, 439. (18) Mclellan, J. M.; Geissier, M.; Xia, Y. N. J. Am. Chem. Soc. 2004, 126, 10830. (19) Liu, D. F.; Xiang, Y. J.; Zhang, Z. X.; Wang, J. X.; Gao, Y.; Song, L.; Liu, L. F.; Dou, X. Y.; Zhao, X. W.; Luo, S. D.; Wang, C. Y.; Zhou, W. Y.; Wang, G.; Xie, S. S. Nanotechnology 2005, 16, 2665. (20) Donnay, J. D. H.; Harker, D. Am. Mineral. 1937, 22, 446.
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