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J. Phys. Chem. C 2008, 112, 13922–13925
Grating Network of ZnO Nanostructure Chunxiang Xu,*,† Guangping Zhu,† Yi Yang,‡ Zhili Dong,§ Xiaowei Sun,‡ and Yiping Cui† AdVanced Photonics Center, School of Electronic Science and Engineering, Southeast UniVersity, Nanjing 210096, People’s Republic of China, School of Electrical and Electronic Engineering, Nanyang Technological UniVersity, Nanyang AVenue, 639798, Singapore, and School of Material Science and Engineering, Nanyang Technological UniVersity, Nanyang AVenue, 639798, Singapore ReceiVed: April 11, 2008; ReVised Manuscript ReceiVed: June 2, 2008
Through a vapor-phase transport method, a three-dimensional network nanostructure of ZnO, which is similar to the microstructures of a butterfly wing, was fabricated by using zinc powders as the source. The analysis of X-ray diffraction and energy dispersive spectra demonstrate the ZnO network covered on the surface of a Zn microball. The observation of scanning electron microscopy, high-resolution transmission microscopy, and selected area electron diffraction revealed that the ZnO backbone grew along [0001], 〈101j0〉, 〈11j01〉, and 〈12j11〉 directions to form a three-dimensional network. The growth process was discussed in detail. Introduction In recent years, ZnO nanostructures have been paid considerable attention because of their wide direct bandgap, strong exciton binding energy, rich morphologies, and potential applications as nanolasers, field emitters, nanotransistors, sensors, and nanogenerators.1–5 Up to now, various nanostructures of ZnO, which include quasi-one-dimensional (1D) nanowires, nanorods, and nanobelts, 2D nanocombs, nanodisks, hexagrams, and 3D nanorings and hierarchical whiskers, have been fabricated by control of the growth conditions based on the polarity and surface energy of ZnO.6–12 Some ZnO nanostructures exhibit tangled ridges or contacted junctions and they were claimed as network.13–16 It is possible to extend the optoelectronic function if the ZnO network can be arranged into a regular configuration. It is well-known that some butterfly wings with regular microstructures act as natural photonic crystals and present charming colors due to pigment absorption, diffraction, and interferences.17 Learning from nature, fabricated artificial photonic crystals have been carefully designed by complicated laser technology and chemical self-assembly.18–21 Simplified fabrication and improved performance of photonic crystals are significant in developing photonic devices. In this paper, we shall present the simple fabrication technique and the growth mechanism of a ZnO nanostructure with a regular 3D network similar to that of a butterfly wing. Experimental Section The ZnO nanostructure was produced in a tube furnace with two constant temperature zones. High-purity Zn powders held in a small Al2O3 boat as the source material were placed into a slender one-end-sealed quartz tube near the closed end. A strip of silicon wafer with (100) plane was put into the quartz tube as the substrate. The source temperature was kept at 750 °C, * Author to whom any correspondence should be addressed. E-mail:
[email protected]. Tel: 86-25-83601769. † Advanced Photonics Center, School of Electronic Science and Engineering, Southeast University. ‡ School of Electrical and Electronic Engineering, Nanyang Technological University. § School of Material Science and Engineering, Nanyang Technological University.
Figure 1. XRD pattern with the EDX spectrum of the ZnO grating network nanostructure as an insert.
and the growth temperature was kept at about 450 °C. The sample was taken out of the furnace after 30 min for growth in air. The crystal structure of the sample was characterized by X-ray diffraction (XRD), using a Seimens D5005 XRD diffractometer with the Cu KR1 line under an accelerating voltage of 40 kV. The morphology of the product was examined by JEOL JSM5910LV scanning electron microscopy (SEM). The SEM was also employed to conduct the energy dispersive X-ray (EDX) spectroscopy under the accelerating voltage of 15 kV. A JEM2010 high-resolution transmission electron microscope (HRTEM) operated at 200 kV was employed to detect the lattice structure. Results and Discussion Figure 1 presents the XRD patterns of the sample. As indexed in the figure, the main diffraction peaks match with the wurtzite structural ZnO with lattice constants of a ) 3.250 Å and c ) 5.207 Å. The stronger diffraction peaks appear at 31.8°, 34.3°,
10.1021/jp8031463 CCC: $40.75 2008 American Chemical Society Published on Web 08/14/2008
Grating Network of ZnO Nanostructure
Figure 2. SEM images of the microballs with low magnification (a) and enlarged microballs covered with ZnO network surface (b and c).
J. Phys. Chem. C, Vol. 112, No. 36, 2008 13923 and 36.5°, corresponding to (101j0), (0002), and (101j1) planes of wurtzite ZnO, respectively. Besides the diffraction peaks from ZnO, three peaks at 43.4°, 54.5°, and 70.4° are clearly observed in the XRD patterns. These peaks originate from the diffraction of crystal planes of (101j0), (101j1), and (112j0) of zinc, respectively. However, the EDX spectrum inserted in Figure 1 demonstrates that the atomic ratio of Zn to O is closed to 1:1. It is reasonable to recognize that the EDX signal is mainly from the surface of the sample and the XRD illustrates more information from the bulk crystal because the accelerating voltage is 15 kV for EDX measurement and 40 kV for XRD. The results of XRD and EDX patterns indicate that a layer of ZnO covers the surface of Zn microspheres. Figure 2 shows the top view SEM images of the product with small and medium magnifications. It presents the sphere morphology of about several tens of micrometers in diameter. It is interesting to note, from the enlarged SEM images in Figure 2b,c, that the surface of the microball is covered with regular network structures. The further enlarged SEM images reveal that the ridges of the network backbone present a homogeneous thickness of about 40 nm, and include two kinds of patterns as shown in Figure 3a,b. The typical morphology for both patterns is composed of three ordered structures. The first one is the long stems as shown by highlighted red lines in Figure 3a,b, which are parallel to each other for a period of about several micrometers. The second one is a grating-like structure as highlighted in green in Figure 3a,b, which grows among the first-ordered structures and parallel with a spacing of about 400 nm. From the enlarged second order structures in Figure 3c, the third order is clearly seen. It is also parallel gratings among the second ones with a space of about 200 nm. It is noted that these three ordered structures have fixed orientations. The first order crosses the second one with an angle of 60° as labeled in Figure 3a,b, while the second and the third orders are perpendicular to each other. It is worth mentioning that the grating network morphology of the ZnO belongs to a bionic structure. For comparison, Figure 3d presents a SEM image of a butterfly wing. The similar geometrical shapes of the butterfly wings have also been reported previously.17 The main long stems and the linked short subordinates are analogous to the second and third
Figure 3. The enlarged SEM images of the ZnO network nanostructures with three ordered configurations (a, b, and c) and a similar microstructure of a butterfly wing (d). The first, second, and third order structures are highlighted in red, green, and blue, respectively.
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Figure 4. The optical diffraction pattern from the ZnO grating network.
order structures of the ZnO network nanostructures. It is wellknown that some butterfly wings are natural photonic crystals and have special color effects caused by absorption, interference, or diffraction of their spatial regular structures. Similar to the optical function of the butterfly wings, the ZnO grating network also presents some diffraction patterns when it is irradiated by a laser at the proper incident angle. Under irradiation from a He-Ne laser, a typical optical diffraction pattern from the grating network of ZnO was observed, as shown in Figure 4. The diffraction effect and the pattern variation have been separately discussed in detailed in our previous report.22 To understand the growth process of the nanostructural network of ZnO, the microballs are scraped into fractions for
Xu et al. TEM analysis. Figure 5 shows the typical three kinds of the TEM images with low and high magnifications and the corresponding selected area electron diffraction (SAED) patterns from fractions a, b, and c, respectively. The HRTEM image of fraction a in Figure 5a-2 clearly shows the lattice fringes with the d-space of 0.26 nm, which matches that of (0002) planes of the wurtzite structural ZnO. By combining this with the corresponding SAED pattern in Figure 5a-3, we can determine that the fraction grows along [0001] and [101j0] directions. The HRTEM and the SAED pattern taken from fraction b are exhibited in Figure 5b-2,b-3. It is clearly seen that the lattices arrange into a hexagonal geometry. In the three directions perpendicular to the adjacent ridges of the hexagon, as labeled in Figure 5b-2, the d-spacing are 0.28, 0.24, and 0.24 nm, which correspond to those of (101j0), (11j01), and (01j11) planes. The corresponding SAED also presents a hexagonal pattern in Figure 5b-3. It is noted that the hexagon is not equilateral, whose diagonals exhibit two typical cross-angles of 64° and 52°. By careful calculation, the representative diffraction spots are labeled as 101j0, 11j01, and 01j11 in Figure 5b-3. These indices of the SAED pattern are consistent with the HRTEM parameters in Figure 5b-2. Both HRTEM and SAED results demonstrate that fraction b grows along [101j0] and [11j01], or their equivalent directions. Similarly, the rectangle SAED pattern and the HRTEM from fraction c determine the two growth directions along [101j0] and [12j11]. The projected directions of [11j01] and [12j11] on the (0001) plane are along the [11j00] and [12j10] directions. It is noted that the vector [101j0] crosses [11j00] at 60o and perpendicular to [12j10]. The orientations of these three vectors are consistent with the angle relationship among the three ordered structures of the ZnO network configurations in
Figure 5. TEM images with low (a-1, b-1, and c-1) and high (a-2, b-2, and c-2) resolution and the corresponding SAED patterns (a-3, b-3, and c-3) taken from the three kinds of fractions of the ZnO network surface.
Grating Network of ZnO Nanostructure
Figure 6. Diagrams of the growth directions in the top view of the 3D (a) and the projected 2D (b and c) ZnO grating network nanostructures. The first, second, and third orders are represented in red, green, and blue, respectively.
SEM images in Figure 5. These results demonstrate that the TEM images and the SAED patterns shown in Figure 5, parts a, b, and c are correspond to the first, second, and third order structures in the ZnO network, respectively. On the other word, the backbone of the network vertically grows along the [0001] direction on the microball surface. In the lateral directions, the first, second, and third ordered structures grow along [101j0], [11j01] and [12j11] or their equivalent directions. In the top view, the first and second orders present a 60° cross-angle while the second and third orders exhibit 90°, as highlighted in Figure 3. The growth mechanism of the present spherical profiles is similar to that of the reported microcages and microballs of ZnO.23–25 Zn powders were quickly evaporated into Zn vapor in the high temperature region, and condensed to form liquid clusters of Zn in the low temperature region. The liquid Zn droplets deposited on the substrate and fast solidified into Zn spheres. The surface oxidation of the Zn spheres produced textured ZnO nanostructures. The backbone of the ZnO network is similar to the growth process of nanobelts and hierachical structures.6,26 In the initial growth stage, the Zn vapor is high due to the quick evaporation of Zn powders. In this case, the ZnO crystal grew along the low-energy surfaces of {101j0}, {11j00}, and {12j10} because the general favored growing direction of [0001] was suppressed by the high pressure. The [11j00] and [12j10] directions are just the projected orientations of the observed [11j01] and [12j11] on the (0002) surface. The Zn source for further epitaxy of ZnO along the [0001] direction is provided slowly and stably by evaporation of the residual Zn powders and sublimation of the inner Zn spheres. The ideal 3D grating network and the spatial relationship among the three ordered structures in the top view are clearly illustrated in Figure 6, which shows the [101j0], [11j00], and [12j10] directions. Conclusion In summary, the grating network nanostructure of ZnO similar to the microstructure of a butterfly wing was fabricated in air by the vapor-phase transport method, using Zn powder as the source. The Zn vapor condensed and solidified on the substrate to form microballs. The microball surfaces were oxidized and self-organized into the network backbone of wurtzite ZnO with a homogeneous ridge thickness of about 40 nm. The top view
J. Phys. Chem. C, Vol. 112, No. 36, 2008 13925 morphology presents three ordered parallel stripes: the first one is long stripes with an inter-space of several micrometers, the second ones grew among the first orders with an inter-space of about 400 nm and is linked the adjacent first ones, and the third ones are short gratings linking the adjacent second ones and has an inter-space of about 200 nm. The analysis on the microstructure of the bionic geometry demonstrated that the ZnO grating network grew along [0001] in the vertical direction to the sphere surfaces, and [101j0], [11j01], and [12j11] or their equivalent lateral directions. For the top view observation, the first and second orders cross each other at 60°, and the third order is perpendicular to the second one. The grating network not only cultivates a new member for the ZnO family with abundant aesthetic morphologies but also presents the optical diffraction function, which is expected to apply in optoelectronic, photonic areas as novel nanodevices, such as microoptical microelectronic mechanical systems, optic splitters, and sensors. Acknowledgment. This work is support by NSFC (60725413, 60576008, and 10674023), 863 Program (2006AA03Z313), 973 program (2007CB936300), and RFDP (20050286004). The authors also appreciate Dr. X. H. Ji, who provided the SEM image of a butterfly wing. References and Notes (1) Huang, M.; Mao, H.; Feick, H.; Yan, H.; Wu, Y.; Kind, H.; Weber, E.; Russo, R.; Yang, P. Science 2001, 292, 1897. (2) Xu, C. X.; Sun, X. W.; Chen, B. J. Appl. Phys. Lett. 2004, 84, 1540. (3) Arnold, M. S.; Avouris, P.; Pan, Z. W.; Wang, Z. L. J. Phys. Chem, B 2003, 107, 659. (4) Law, M.; Sirbuly, D. J.; Johnson, J. C.; Goldberger, J.; Saykally, R. J.; Yang, P. Science 2004, 305, 1269. (5) Wang, M.; Song, J. H. Science 2006, 312, 242. (6) Pan, Z. W.; Dai, Z. R.; Wang, Z. L. Science 2001, 291, 1947. (7) Xu, C. X.; Sun, X. W.; Dong, Z. L.; Yu, M. B. J. Cryst. Growth 2004, 270, 498. (8) Xu, C. X.; Sun, X. W.; Dong, Z. L.; Yu, M. B. Appl. Phys. Lett. 2004, 85, 3878. (9) Xu, C. X.; Sun, X. W.; Dong, Z. L.; Zhu, G. P.; Cui, Y. P. Appl. Phys. Lett. 2006, 88, 093101. (10) Kong, X. Y.; Ding, Y.; Yang, R. S.; Wang, Z. L. Science 2004, 303, 1348. (11) Gao, P. X.; Ding, Y.; Mai, W.; Hughes, W. L.; Lao, C. S.; Wang, Z. L. Science 2005, 309, 1700. (12) Xu, C. X.; Sun, X. W.; Chen, B. J.; Dong, Z. L. Appl. Phys. Lett. 2005, 86, 011118. (13) Gao, P. X.; Lao, G. S.; Hughes, W. L.; Wang, Z. L. Chem. Phys. Lett. 2005, 408, 174. (14) Zhang, B. P.; Wakatsuki, K.; Binh, N. T.; Segawa, Y.; Usami, N. J. Appl. Phys. 2004, 96, 340. (15) Kim, S. W.; Fujita, S.; Yi, M. S.; Yoon, D. H. Appl. Phys. Lett. 2006, 88, 253114. (16) Xu, W.; Ye, Z.; Zhu, L.; Zeng, Y.; Jiang, L.; Zhao, B. J. Cryst. Growth. 2005, 277, 490. (17) Vukusic, P.; Sambles, J. R. Nature 2003, 424, 852. (18) Birner, A.; Wehrspohn, R. B.; Go¨sele, U. M.; Busch, K. AdV. Mater. 2001, 13, 377. (19) van Blaadere, A. Science 1998, 282, 887. (20) Miguez, H.; Te´treault, N.; Yang, S. M.; Kitaev, V.; Ozin, G. A. AdV. Mater. 2003, 15, 597> (21) Breen, M. L.; Dinsmore, A. D.; Pink, R. H.; Qadri, S. B.; Ratna, B. R. Langmuir 2001, 17, 903. (22) Xu, C. X.; Zhu, G. P.; Liu, Y. J.; Sun, X. W.; Li, X.; Liu, J. P.; Cui, Y. P. New J. Phys. 2007, 9, 381. (23) Gao, P. X.; Wang, Z. L. J. Am. Chem. Soc. 2003, 125, 11299. (24) Shen, G.; Bando, Y.; Lee, C. J. J. Phys. Chem. B. 2005, 109, 10578. (25) Xia, X.; Zhu, L.; Ye, Z.; Yuan, G.; Zhao, B.; Qian, Q. J. Cryst. Growth 2005, 282, 506. (26) Lao, J. Y.; Wen, J. G.; Ren, Z. F. Nano Lett. 2002, 2, 1287.
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