Low-Temperature Growth of Uniform ZnO Particles with Controllable

Apple pectin-mediated green synthesis of hollow double-caged peanut-like ZnO hierarchical superstructures and photocatalytic applications. Ai-Jun Wang...
0 downloads 0 Views 684KB Size
Download PDF
J. Phys. Chem. B 2006, 110, 19147-19153

19147

Low-Temperature Growth of Uniform ZnO Particles with Controllable Ellipsoidal Morphologies and Characteristic Luminescence Patterns Rongguo Xie State Key Laboratory of Silicon Materials, Zhejiang UniVersity, Hangzhou 310027, People’s Republic of China, and Nanomaterials Laboratory, National Institute for Materials Science, Tsukuba 305-0047, Japan

Dongsheng Li, Hui Zhang, and Deren Yang* State Key Laboratory of Silicon Materials, Zhejiang UniVersity, Hangzhou 310027, People’s Republic of China

Minhua Jiang State Key Laboratory of Silicon Materials, Zhejiang UniVersity, Hangzhou 310027, People’s Republic of China, and State Key Laboratory of Crystal Materials, Shandong UniVersity, Jinan 250100, People’s Republic of China

Takashi Sekiguchi Nanomaterials Laboratory, National Institute for Materials Science, Tsukuba 305-0047, Japan

Baodan Liu and Yoshi Bando Graduate School of Pure and Applied Sciences, UniVersity of Tsukuba, Tsukuba, 305-0005, Japan, and AdVanced Materials Laboratory, National Institute for Materials Science, Namiki 1-1, Tsukuba, 305-0047, Japan ReceiVed: January 25, 2006; In Final Form: May 24, 2006

Uniform ellipsoidal ZnO particles have been synthesized in an aqueous solution in the presence of triethonalamine (TEA) mediated by sonication at the temperature below 80 °C. Scanning electron microscopy observations reveal that the ellipsoidal particles are highly uniform with a hexagonal cross-section. The morphologies of the ZnO particles can be systematically controlled from elongated rugby ball-like ellipsoidal to half-ellipsoidal by increasing the TEA concentration. Spatial resolved cathodoluminescence measurements at room temperature show that the ellipsoidal ZnO particles are intrinsically encoded with barcode-like ultraviolet luminescence patterns, which are of either a wide stripe or a narrow stripe perpendicular to the length at the core of the particles depending on the growth temperature. Moreover, the luminescence spectra of the ellipsoidal particles can be tuned by heat treatments at elevated temperatures, while maintaining the luminescence patterns. We believe that the well-defined uniform ellipsoidal ZnO particles embedded with unique luminescence characteristics hold great potential for use in bioengineering and photonics, such as biological labeling and optical probes.

Introduction Luminescent submicro- and microparticles with uniform size and shape have been utilized in an extensive variety of practical applications, including optical devices, biological labeling, drug delivery, and multiplexed bioassays.1 Besides, they are of potential importance as ideally controlled advanced materials for fundamental studies. For example, they might be used as optical probes to study the effect of photonic crystals on spontaneous emission.2 On the other hand, ZnO is a direct gap semiconductor with a large band gap of 3.37 eV and a large exciton binding energy of 60 meV. The strong exciton binding energy can ensure an efficient ultraviolet (UV) emission at room temperature. ZnO is also biosafe and biocompatible and can be directly used for * Corresponding author. Tel: +86-571-87951667; fax: +86-57187932322; e-mail: [email protected].

biomedical applications without coating.3 Up to now, ZnO particles with various interesting morphologies, such as spherical, ellipsoidal, pyramidal, rod-like, doughnut-like, dumbbelllike, flower-like, and so on, have been synthesized with various synthetic methods.4 Among them, the ellipsoidal particles with an adjustable aspect ratio should provide complexity or new types of functionalities and are of great scientific and technological interest.5 However, the reported ellipsoidal ZnO particles are irregular and nonuniform.4b Especially, the morphologies (aspect ratios) of the ellipsoidal particles have not been systematically controlled, and the luminescence properties of the ellipsoidal particles have not been well-studied, thus limiting them to be used for few applications. In this paper, we report a simple low-temperature (less than 80 °C) approach of synthesizing uniform submicrometer-sized ZnO particles with well-defined ellipsoidal morphologies in an aqueous solution. We investigate the role of reaction conditions

10.1021/jp0605449 CCC: $33.50 © 2006 American Chemical Society Published on Web 09/08/2006

19148 J. Phys. Chem. B, Vol. 110, No. 39, 2006

Xie et al.

Figure 1. Morphological and structural characterization of ellipsoidal ZnO particles prepared at a TEA concentration of 0.05 M at 80 °C for 2 h. (a) Low-magnification SEM image; (b) high-magnification SEM image; (c) analysis of an ellipsoidal ZnO particle; and (d) XRD pattern.

in determining the morphology of the ZnO particles and demonstrate that the morphologies of ZnO particles can be controlled from elongated rugby ball-like ellipsoidal to halfellipsoidal by increasing the triethanolamine (TEA) concentration. Additionally, we study the close relationship between particle characteristics and optical properties with a high spatial resolution cathodoluminescence (CL) and show that the ellipsoidal particles are intrinsically encoded with characteristic barcode-like UV luminescence patterns, while their luminescence spectra can be tuned by heat treatments at elevated temperatures. Experimental Procedures All chemicals are analytical grade without further purification (purchased from Wako Pure Chemical Industries, Ltd.). Deionized water with a resistivity of 18.0 MΩ cm was used in all experiments. Uniform ellipsoidal ZnO particles were synthesized by the hydrolysis of zinc acetate dihydrate in the presence of TEA in an aqueous solution mediated by sonication. In a typical synthesis process, a 50 mL aqueous solution containing 0.05 M zinc acetate and 0.05 M TEA was ultrasonically irradiated at 80 °C for 2 h. To investigate the role of preparation conditions in determining the particle morphologies, the concentration of TEA and the reaction temperatures were varied. The resulting products were washed, first with water and then ethanol by centrifugation/dispersion cycles 5 times, and then dried at 200 °C in air for 2 h. To tune the luminescence properties of the particles, heat treatments were performed at 400, 600, and 800 °C in air for 2 h. The products were characterized using a Japan Rigaku D/maxga X-ray diffractometer with graphite monochromatized CuKR

Figure 2. (a) TEM image of ellipsoidal ZnO particles prepared at a TEA concentration of 0.05 M at 80 °C for 2 h; (b) corresponding SAED pattern; and (c) representative HRTEM image at the tip of a ellipsoidal ZnO particle.

radiation (λ ) 1.54178 Å). Transmission electron microscopy (TEM) observation was performed with a JEM 3000F microscope operated at 300 kV. Microstructures of the products were obtained with a field-emission scanning electron microscope (FE-SEM, Hitachi S-4300E/N). High-spatial resolution CL measurements were carried out in a thermal field-emission SEM (TFE-SEM, Hitachi S-4200) at 5 kV at room temperature. The CL images were recorded with a photomultiplier (Hamamatsu R3302), and the CL spectra were obtained with a liquid-nitrogen

Low-Temperature Growth of Uniform ZnO Particles

J. Phys. Chem. B, Vol. 110, No. 39, 2006 19149

Figure 3. TEM image of ellipsoidal ZnO particles prepared at a TEA concentration of 0.05 M at 80 °C for 40 min.

Figure 4. Typical SEM images of ellipsoidal ZnO particles prepared at different TEA concetrations at 80 °C for 2 h. (a and b) 0.1 M TEA and (c and d) 0.15 M TEA.

cooled charge-coupled device (CCD) detector (Jobin Yvon, Spectrum One). Results and Discussion The morphology and the size of the products were examined with FE-SEM. Figure 1a shows the products obtained at 0.05 M TEA (pH ) 7.5) at 80 °C, which consist of highly uniform elongated rugby ball-like ellipsoidal particles. Analysis of a number of the rugby ball-like ZnO particles shows that they have an average length of about 620 nm and a mean diameter of about 400 nm (the aspect ratio is about 1.55). The clearer structure of the rugby ball-like ZnO particles is revealed by the magnified SEM image shown in Figure 1b. The well-resolved hexagonal edges and corners can be observed, indicating good crystalline quality of the particles. Furthermore, from Figure

Figure 5. Dependence of the aspect ratios (length to diameter) as a function of TEA concentrations. Inset: schematic illustration of morphology evolution of ellipsoidal ZnO particles with increasing TEA concentration. The arrows represent TEA molecules.

1b,c, one can see that there is a joint boundary perpendicular to the length of the particles, which divides the particles into two sides. Examining a rugby ball-like ellipsoidal particle paralleled to the substrate (see Figure 1c), it is found that each side of the rugby ball-like ellipsoidal ZnO particle is of a different size (the length ratio between the bigger side and the smaller one is calculated to be about 1.6). Therefore, the word ellipsoidal is not strictly pertinent to describe the symmetric ellipsoid, which is just a phenomenological description. The asymmetry of the ellipsoidal particles implies that the nucleation and growth of each half of the particles are not identical. Figure 1d shows the X-ray diffraction (XRD) pattern of the rugby balllike ZnO particles. All the reflection peaks of the products can be indexed as pure hexagonal ZnO with cell parameters a ) 3.249 Å and c ) 5.206 Å, which are in good agreement with the literature values (JCPDS card number 36-1451). The relative

19150 J. Phys. Chem. B, Vol. 110, No. 39, 2006

Xie et al.

Figure 6. SEM images of ellipsoidal ZnO particles prepared at different conditions for 2 h. (a) 0.05 M TEA, 80 °C; (b) 0.05 M TEA, 65 °C; (c) 0.10 M TEA, 80 °C; and (d) 0.10 M TEA, 65 °C.

Figure 7. CL spectra of ellipsoidal ZnO particles prepared at 0.05 M TEA at different temperatures for 2 h. (a) at 80 °C and (b) at 65 °C.

sharp peaks in the XRD pattern confirm that the rugby balllike ZnO particles are well-crystallized. Figure 2a displays the TEM image of the rugby ball-like ZnO particles. Again, the highly uniform elongated ellipsoidal ZnO particles can be observed. The SAED pattern in Figure 2b indicates the single crystalline nature of the sample and its hexagonal phase, which is consistent with the results of the XRD characterization shown in Figure 1d. A typical HRTEM image on the tip of a rugby ball-like ellipsoidal ZnO particle is shown in Figure 2c. The spacing of 0.52 nm between adjacent lattice planes corresponding to the distance of the (001) planes can be observed, indicating that [0001] is the growth direction of the ellipsoidal ZnO particles. To understand how the rugby ball-like ZnO particles were formed, we studied the morphology of the ZnO particles in the early stages of crystal growth. As shown in Figure 3, after 40 min of the reaction, the rugby ball-like ZnO particles were not obtained. Instead, half-ellipsoidal ZnO particles had already been formed, and the second half of the ZnO particles was initiated from their flat bases. From these results, it is believed that the

formation of the rugby ball-like ZnO particles results from the first growth of a half-ellipsoidal particle, followed by the germination and growth of a second half at its base. To clarify the role of TEA in determining the morphologies of ZnO particles, controlled experiments were performed by varying the TEA concentration while maintaining the other parameters. At a very low TEA concentration (0.01 M, pH ) 7.3), long double-sided rod-like ZnO particles with the cusp end were formed, and the aspect ratio was 4.56 (see Supporting Information). When the TEA concentration increased to 0.10 M (pH ) 8.0), the ZnO particles (Figure 4a,b) became more asymmetric, and the aspect ratio decreased to 1.22, having a length of about 423 nm and a diameter of about 340 nm. From Figure 4a, it is obvious that one side of the particles is much smaller than the opposite side. By further increasing the TEA concentration to 0.15 M (pH ) 8.5), the morphology of the obtained ZnO particles was changed to half-ellipsoidal with an aspect ratio of 0.93 (see Figure 4c,d). The average diameter of the half-ellipsoidal particles is about 328 nm. Furthermore, from the SEM images of the tilted ZnO particles (Figure 4b,d), the quite flat hexagonal plane at the base of the bigger side can be observed, indicating that the particles still remain in an identical crystalline orientation. In addition, it is clear that the smaller sides consist of an assembly of 30-40 nm diameter nanoparticles covered on the base plane of the bigger side, which further confirmed that the double-sided ellipsoidal ZnO particles were formed from a half-ellipsoidal particle followed by the germination and growth of another side at its base. We have further demonstrated that much higher TEA concentrations (0.30 M, pH ) 8.7) can produce plate-like ZnO particles rather than ellipsoidal particles (see Supporting Information). Figure 5 shows the dependence of the aspect ratios (length to diameter) as a function of TEA concentrations. As

Low-Temperature Growth of Uniform ZnO Particles

J. Phys. Chem. B, Vol. 110, No. 39, 2006 19151

Figure 8. Monochromatic CL (at 385 nm) images of ellipsoidal ZnO particles prepared at 0.05 M TEA for 2 h. (a) At 80 °C and (b) at 65 °C. (c) SEM; (d) monochromatic CL (at 385 nm); and (e) their overlapping images of two ellipsoidal particles prepared at 65 °C for 2 h.

can be seen, the aspect ratio of the ellipsoidal particles decreased with increasing the TEA concentration. This systematic correlation suggests that TEA slows down crystal growth along the c-axis and therefore provides a simple approach to controlling the aspect ratio of the ellipsoidal ZnO particles. The transition from double-sided rods with the cusp end to rugby ball-like ellipsoidal particles, and then to half-ellipsoidal particles with the increase in TEA concentration, is illustrated in the inset of Figure 5. As is well-known, ZnO is a polar crystal of Wurtzite structure, whose positive polar plane is rich in Zn and whose negative polar plane is rich in O. In aqueous solution, ZnO generally prefers to grow along the c-axis to form microparticles with the cusp end due to the different growth rate of the crystalline plane.4e,4f,6 TEA is an important biological ligand. As in many surfactant-assisted or ligand-mediated syntheses of shape-controlled materials,7 TEA may serve as a surface modifier, presumably bound to Zn2+ on the polar (0001) planes. Because of the absorption of the capping agents, ions may bond to the Zn2+ (0001) surfaces. This surface interaction can inhibit ZnO crystals elongated perpendicular to these planes. At low TEA concentrations, the inherent anisotropic growth of ZnO along the c-axis is still favorable; thus, the elongated rugby balllike ZnO particles were formed. At high TEA concentrations, the growth along the c-axis was substantially retarded. As a consequence, the aspect ratio was significantly reduced and the growth of the second half was suppressed; thus, the morphology of the ellipsoidal particles was changed to half-ellipsoidal. Uniform particles with ellipsoidal and half-ellipsoidal morphologies could also be obtained at the lower temperatures (Figure 6b,d). As shown in the SEM images, the particles obtained at 65 °C have similar morphologies to those obtained at 80 °C under the otherwise identical conditions (Figure 6a,c), implying that the growth of uniform ellipsoidal ZnO particles is highly reproducible. However, the particles obtained at the lower temperature have a much rougher surface, which was clearly composed of 30-40 nm subunit nanoparticles. It seemed

that the low temperature could not provide enough energy required for the growth of well-crystallized ellipsoidal particles. To evaluate the luminescence qualities of the obtained products, we further carried out CL measurements at room temperature. As shown in Figure 7, the products obtained at both 80 and 65 °C show the UV emission band centered at 385 nm with negligible visible luminescence. It is worth mentioning the other products also show similar CL spectrum features, except the absolute intensities (not shown here). The close relationship of the luminescence properties and particle characteristics was examined by the high-spatial resolution CL imaging. Figure 8a,b shows the monochromatic CL images (at 385 nm) of the two kinds of rugby ball-like ellipsoidal particles obtained at 80 and 65 °C, respectively. An interesting feature of the rugby ball-like ZnO particles is that the UV luminescence is not homogeneously distributed in the particles. The core region in the ellipsoidal particles has the strongest UV emission, whereas the tip region has the weakest. Figure 8c-e reveals the closer relationship between luminescence properties and particle characteristics. As can be seen, the most luminescent region is located at the base of the bigger side. The inhomogeneous distribution of the UV luminescence in ellipsoidal ZnO particles might be explained in terms of some nonirradiative defects, which were introduced due to nonequilibrium crystal growth. As suggested previously, the ellipsoidal ZnO particles were formed from a half-ellipsoidal particle followed by the germination of a second half at its base. The ellipsoidal ZnO particles may have grown under a steady state at the initial stage of crystal growth, so that the UV emission was strongest at the base of the bigger side. Moreover, the ellipsoidal ZnO particles obtained at the different temperatures show the different barcode-like UV luminescence patterns. As illustrated in Figure 9a,b, the ellipsoidal ZnO particles obtained at 80 °C exhibit a wide striping pattern, whereas those obtained at 65 °C show a narrow line-like striping pattern perpendicular to the length. The luminescent region that was more closely confined at the base of the bigger side of the ellipsoidal particles obtained at 65 °C

19152 J. Phys. Chem. B, Vol. 110, No. 39, 2006

Xie et al. the hydrogen evolved from the ZnO. Therefore, it is reasonable that the intensity of the UV emission decreased dramatically and that the visible emission increased. The significant increase of the UV emission and the visible emission at 800 °C possibly results from the reduction of nonirradiative centers owing to the annealing effect. It is worth mentioning that UV luminescence patterns embedded in the ellipsoidal beads remained nearly unchanged except for the brightness. The exact origin of the variation of the luminescence properties of the ZnO particles on the heat treatments demands much in-depth work.

Figure 9. Line profiles of CL intensities (at 385 nm) along the length of ellipsoidal ZnO particles prepared at 0.05 M TEA for 2 h. (a) at 80 °C and (b) at 65 °C.

Figure 10. (a) CL spectra of ellipsoidal ZnO particles prepared at 0.05 M TEA at 80 °C for 2 h after heat treatments at 300, 400, 600, and 800 °C.

might be due to more nonirradiative centers introduced in the particles during the growth at the lower temperatures. The barcode-like UV luminescence patterns, which were intrinsically encoded in the ellipsoidal particles, should inspirit the exploration of more practical applications to ZnO particles.8 The luminescence properties of the ZnO particles can be further tuned by heat treatments. Figure 10 shows the CL spectra of the ZnO ellispsoidal particles grown at 80 °C after heat treatments at 300, 400, 600, and 800 °C, respectively. The ZnO particles still show a strong UV emission band after heat treatment at 300 °C. With an increase of temperature to 400 °C, the intensity of the UV emission decreased slightly, whereas that of the visible emission increased. When the temperature reached 600 °C, the intensity of the UV emission decreased dramatically, and the visible emission kept on increasing. The 800 °C heat treatment led to a great increase in the intensity of both the UV and the visible emissions. At room temperature, ZnO typically exhibits UV band edge emission and broad visible emissions at the green and yellow bands. The UV band edge emission is generally attributed to free excitonic emission. For the green and yellow emissions, it has been believed that these visible emissions are due to transitions in defect states, in particular, oxygen vacancies.9 Generally, for the luminescence properties of ZnO prepared by wet chemical methods, the roles of the adsorbed species, such as surface hydroxyl groups and hydrogen, should be taken into account. Our previous thermal desorption studies on ZnO prepared in aqueous solution have demonstrated that most of the adsorbed water and hydroxyl groups are released from ZnO at temperatures around 200 °C, while a large amount of hydrogen was retained in ZnO until the temperature reached 450 °C.10 It is well-known that hydrogen doping can strongly enhance UV emission efficiency while suppressing the visible emission of ZnO.11 Thus, only the strong UV emission that was observed from all ZnO samples annealed below 400 °C might be ascribed to the inherent hydrogen passivation. When the ZnO was annealed at 600 °C,

Conclusion In summary, a new class of well-defined uniform ellipsoidal ZnO particles has been synthesized in an aqueous solution at low temperatures mediated by sonication. Aside from the high uniformity in morphology and size, the ellipsoidal particles synthesized here have several unique characteristics. First, the morphology of the ellipsoidal ZnO particles can be systematically controlled from the elongated rugby ball-like ellipsoidal to half-ellipsoidal in a controlled manner by increasing the TEA concentrations. Second, spatial resolved CL measurements show that the UV luminescence is remarkably strong at the base of the bigger side of the ellipsoidal particles, and the UV luminescence patterns are of either a wide stripe or a narrow stripe at the core of the ellipsoidal particles, which depends on the growth temperatures. Third, the luminescence spectra of the ellipsoidal particles can be tuned by the heat treatments while maintaining the luminescence patterns. With these unique characteristics embedded in the ellipsoidal ZnO particles, we believe that the ellipsoidal particles presented here may find a wide variety of practical applications, such as biological labeling, multiplexed bioassays, and optical probe inside photonic crystals.1,2,8 Acknowledgment. The authors from China thank the Natural Science Foundation of China (60225010) and the Key Project of Chinese Ministry of Education for financial support. Supporting Information Available: SEM images of ellipsoidal ZnO particles prepared at very low and very high TEA concentrations at 80 °C for 2 h. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Sugimoto, T. Monodispersed Particles; Elsevier: Amsterdam, 2001. (b) Wang, D.; Rogach, A. L.; Caruso, F. Chem. Mater. 2003, 15, 2724. (c) Lin, Y.; Zhang, J.; Sargent, E. H.; Kumacheva, E. Appl. Phys. Lett. 2002, 81, 3134. (d) Gao, X.; Chan, W.; Nie, S. J. Biomed. Opt. 2002, 7, 532. (e) Han, M.; Gao, X.; Su, J. Z.; Nie, S. Nat. Biotechnol. 2001, 19, 631. (f) Chan, Y.; Zimmer, J. P.; Stroh, M.; Steckel, J. S.; Jain, R. K.; Bawendi, M. G. AdV. Mater. 2004, 16, 2092. (2) (a) Megens, M.; Wijnhoven, J. E. G. J.; Lagendijk, A.; Vos, W. L. J. Opt. Soc. Am. B 1999, 16, 1403. (b) Vos, W. L.; Polman, A. MRS Bull. 2001, 26, 642. (c) Lin, Y.; Zhang, J.; Sargent, E. H.; Kumacheva, E. Appl. Phys. Lett. 2002, 81, 3134. (3) Yi, G. C.; Wang, C. R.; Park, W. I. Semicond. Sci. Technol. 2005, 20, S22. (4) (a) Jezequel, D.; Guenot, J.; Jouini, N.; Fievet, F. Mater. Sci. Forum 1994, 152-153, 339. (b) Zhong, Q. P.; Matijevic, E. J. Mater. Chem. 1996, 6, 443. (c) Zhang, J.; Sun, L. D.; Yin, J. L.; Su, H. L.; Liao, C. S.; Yan, C. H. Chem. Mater. 2002, 14, 4172. (d) Oliveira, A. P. A.; Hochepied, J. F.; Franc¸ osi, G.; Berger, M. H. Chem. Mater. 2003, 15, 3202. (e) Zhang, H.; Yang, D.; Ji, Y.; Ma, X.; Xu, J.; Que, D. J. Phys. Chem. B 2004, 108, 3955. (f) Liang, J. B.; Liu, J. W.; Xie, Q.; Bai, S.; Yu, W. C.; Qian, Y. J. Phys. Chem. B 2005, 109, 9463. (g) Zhang, H.; Yang, D.; Li, D.; Ma, X.; Li, S.; Que, D. Cryst. Growth Design 2005, 5, 547. (h) Choi, S. H.; Kim, E. G.; Park, J.; An, K.; Lee, N.; Kim, S. C.; Hyeon, T. J. Phys. Chem. B 2005, 109, 14792. (i) Pal, U.; Santiago, P. J. Phys. Chem. B 2005, 109, 15317. (j) Andelman, T.; Gong, Y.; Polking, M.; Yin, M.; Kuskovsky, I.; Neumark, G.; O’Brien, S. J. Phys. Chem. B 2005, 109, 14314.

Low-Temperature Growth of Uniform ZnO Particles (5) (a) Noeckel, J. U.; Stone, A. D. Nature 1997, 385, 45. (b) Snoeks, E.; van Blaaderen, A.; Dillen, T. V.; Kats, C. M. V.; Brongersma, M. L.; Polman, A. AdV. Mater. 2000, 12, 1511. (c) Lu, Y.; Yin, Y. D.; Xia, Y. N. AdV. Mater. 2001, 13, 415. (6) Li, W. J.; Shi, E. W.; Zhong, W. Z.; Yin, Z. W. J. Cryst. Crowth 1999, 203, 186. (7) See examples: (a) Peng, X. G. AdV. Mater. 2003, 15, 459. (b) Liu, Z. P.; Li, S.; Yang, Y.; Peng, S.; Hu, Z. K.; Qian, Y. T. AdV. Mater. 2003, 15, 1946. (c) Sun, Y.; Yin, Y.; Mayer, B.; Herricks T.; Xia, Y. N. Chem. Mater. 2002, 14, 4736. (d) Tian, Z. R.; Voigt, J. A.; Liu, J.; Mckenzie, B.; Mcdermott, M. J.; Rodriguez, M. A.; Konishi, H.; Xu, H. F. Nat. Mater. 2003, 2, 821. (8) Nicewarner-Pena, S. R.; Freeman, R. G.; Reiss, B. D.; He, L. Science 2001, 294, 137. (9) (a) Damen, T. C.; Porto, S. P. S.; Tell, B. Phys. ReV. 1966, 142,

J. Phys. Chem. B, Vol. 110, No. 39, 2006 19153 570. (b) Arguello, C. A.; Rousseau, D. L.; Porto, S. P. S. Phys. ReV. 1969, 181, 1351. (10) (a) Xie, R. G.; Sekiguchi, T.; Ishigaki, T.; Ohashi, N.; Li, D. S.; Yang, D. R.; Liu, B. D.; Bando, Y. Appl. Phys. Lett. 2005, 88, 134103. (b) Xie, R. G.; Li, D. S.; Yang, D. R.; Sekiguchi, T.; Jiang, M. H. Nanotechnology 2006, 17, 2789. (11) (a) Sekiguchi, T.; Ohashi, N.; Terada, Y. Jpn. J. Appl. Phys. 1997, 36, L289. (b) Ohashi, N.; Ishigaki, T.; Okada, N.; Sekiguchi, T.; Sakaguchi, I.; Haneda, H. Appl. Phys. Lett. 2002, 80, 2869. (c) Ohashi, N.; Ishigaki, T.; Okada, N.; Taguchi, H.; Sakaguchi, I.; Hishita, S.; Sekiguchi, T.; Haneda, H. J. App. Phys. 2003, 93, 6386. (d) Lee, J. M.; Kim, K. K.; Parka, S. J.; Choi, W. K. Appl. Phys. Lett. 2001, 78, 3842. (e) Chen, Z. Q.; Kawasuso, A.; Xu, Y.; Naramoto, H.; Yuan, X. L.; Sekiguchi, T.; Suzuki, R.; Ohdaira, T. Phys. ReV. B 2005, 71, 115213.