Well-Controlled Crystal Growth of Zinc Oxide Films on Plastics at

19096

J. Phys. Chem. C 2009, 113, 19096–19101

Well-Controlled Crystal Growth of Zinc Oxide Films on Plastics at Room Temperature Using 2D Nanosheet Seed Layer Tatsuo Shibata,*,† Tsuyoshi Ohnishi,‡ Isao Sakaguchi,§ Minoru Osada,‡ Kazunori Takada,‡ Toshihiro Kogure,| and Takayoshi Sasaki‡ International Center for Young Scientist & International Center for Materials Nanoarchitectonics, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan, International Center for Materials Nanoarchitectonics, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan, Optronic Materials Center, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan, Department of Earth and Planetary Science, Graduate School of Science, The UniVersity of Tokyo, Bunkyo, 7-3-1 Hongo, Tokyo 113-0033, Japan ReceiVed: August 2, 2009; ReVised Manuscript ReceiVed: September 11, 2009

ZnO crystal films having a nearly perfect (001) orientation were successfully grown at room temperature on plastic and glass substrates by modifying their surface with Cs4W11O36 nanosheets through a solution-based process. To clarify the effect of the monolayer film of Cs4W11O36 nanosheets, the films were characterized by X-ray diffraction (XRD), 2D X-ray diffraction (2D-XRD), and high-resolution transmission electron microscopy (HRTEM). The results showed that the introduction of a typical 2D crystal surface with 2D-hexagonal structure (a ) 0.727 nm) of Cs4W11O36 nanosheet promotes high-quality crystal growth of (001) oriented ZnO film (a ) 0.325 nm). Highly crystalline texture directly formed on top of the substrates has been, to the best of our knowledge, realized for the first time, and it dramatically improves the film property, especially at the small film thickness region of 18 Ωcm) was used throughout. The unilamellar nanosheet of Cs4W11O36 was synthesized from the layered tungstate Cs6+xW11O36 via soft-chemical delamination.31 A polycrystalline sample of Cs6+xW11O36 was prepared by solidstate reaction of a mixture of Cs2CO3 and WO3 (4:11 molar ratio) at 900 °C. The sample was acid-exchanged by 6 M HCl solution to produce a protonic oxide of H2.1Cs3.9W11O36 · 6H2O, which was subsequently treated with 0.0028 M tetrabutylammonium hydroxide (TBA) solution. A resulting colloidal suspension contained monodispersed nanosheets of Cs4W11O36, the average lateral size being 300 nm. A #1737 alkali-free borosilicate glass (Corning Inc.) as a substrate was cleaned by a standard procedure involving immersion in a HCl/CH3OH solution and then concentrated H2SO4 for 30 min each. Commercially available polyethylene terephthalate (PET, 500 µm thick) and polyethylene naphthalate (PEN, 200 µm thick, Teonex, Teijin DuPont Films Co. Ltd.) were used as polymeric substrates. These substrates were ultrasonically cleaned in ethanol prior to use. Assembly of Cs4W11O36 Nanosheets into a Seed Layer. A USI FSD-3-777 double barrier Langmuir trough equipped with a Wilhelmy-type balance for surface pressure measurement was used to deposit the nanosheets on substrates. A diluted colloidal suspension (∼0.032 g dm-3) of Cs4W11O36 nanosheets was used as a subphase. Similar to Ti0.91O2 nanosheet,59,60 Cs4W11O36 nanosheet spontaneously floated at the air/liquid interface by adhering to TBA ions through electrostatic interaction when the suspension was left to stand still for 30 min at 25 °C. Then, the surface was compressed to a pressure of 16 ( 0.5 mN m-1, and the monolayer of neatly packed Cs4W11O36 nanosheets was transferred onto the glass substrate with a vertical lifting rate of 1 mm min-1. In the case of PET and PEN substrates, they were primed with polydiallyldimethylammonium ions by immersing them into its chloride solution (20 g dm-3) for 20 min. This operation was needed to tightly hold the nanosheets on the plastic substrates in the subsequent LB deposition process. Finally, the substrates were dried under N2 gas flow, and the substrates coated with the nanosheets were obtained. Film Deposition of ZnO Crystal. Before the film deposition, substrates were photochemically cleaned by UV light irradiation in ozone for 15 min. Then, ZnO thin films were deposited by PLD using a KrF excimer laser (λ ) 248 nm: Lambda physik COMPex 201). ZnO pellets sintered at 1100 °C were used as PLD targets. During the deposition, substrate temperature was maintained at room temperature, and the PLD chamber was backfilled with oxygen gas. The oxygen partial pressure and typical laser density were maintained at 2.1 Pa and ∼0.4 J cm-2

Figure 2. AFM images of the monolayer film of nanosheets fabricated on (a) glass and (b) PEN substrates. Note that (b) shows a phase image to confirm the neat tiling of nanosheets.

(pulsed repetition: 5 Hz), respectively. The film thickness was controlled in the range of 20 to 250 nm by the deposition time. Instrumentation. A powder X-ray diffractometer (Rint 2200, Rigaku Corporation) with monochromatized Cu KR radiation was employed to record normal XRD patterns. 2D-XRD measurements were performed with a Bruker AXS General Area Detector Diffraction System (GADDS) equipped with a HISTAR detector. The scattered X-ray photons were detected by a 2D multiwire proportional counter. Atomic force microscopy (AFM) observations were performed using an SPA400 (Seiko Instruments Inc.) in tapping mode with a silicon tip cantilever (force constant: 20 N m-1). HRTEM images were recorded using a JEM-2010 (JEOL Ltd.) operated at 200 kV. The crosssectional TEM specimen was prepared using Ar ion milling. The thickness of ZnO films was measured using a surface profilometer (Dektak 3D, Sloan Tech. Corporation). The electrical resistivity of the films was measured with a standard fourpoint probe technique. The data was obtained by averaging three separate measurements. Results and Discussion Cs4W11O36 Nanosheet Seed Layer. Figure 1 shows a schematic illustration of a Cs4W11O36 nanosheet, which was employed as a seed material to grow ZnO films. The in-plane hexagonal lattice parameter of 0.727 nm is close to double the hexagonal a parameter (0.325 nm) of ZnO with the wurtzite structure. This structural similarity is expected to assist oriented growth of ZnO films on the nanosheet film. Figure 2a depicts a typical AFM image of a monolayer film of the nanosheet deposited via the Langmuir-Blodgett procedure on a glass substrate. Individual nanosheets of several hundred nanometers in lateral size are resolved, confirming the substantial monolayer coverage of neatly packed nanosheets. Although gaps and overlaps are found in a limited area, the surface was fairly flat with the root-mean-square roughness (Ra) of 0.5 nm. On the other hand, it was difficult to obtain a clear topographic image in the case of PEN substrate possibly due to its rougher surface. The phase image, however, provided

19098

J. Phys. Chem. C, Vol. 113, No. 44, 2009

Figure 3. XRD patterns for ZnO thin films deposited on (a) glass, (b) PEN, and (c) PET substrates at room temperature. Symbols (2) and (•) show the peaks assignable to substrates.

better visibility of the nanosheets, revealing the comparable monolayer coverage as shown in Figure 2b. It is worth pointing out that the highly flexible nature of nanosheets assists adherence to a rough surface. The nanosheet film on a PET substrate showed similar morphology (not shown). In-plane XRD measurements detected sharp diffraction peaks attributable to the nanosheet structure (Figure S1 of the Supporting Information). These results explicitly indicate the successful introduction of a highly crystalline 2D lattice onto the surface of glass and plastic substrates. The domain size with continuous 2D lattice of the nanosheet is in several hundred nanometers, and the macroscopic in-plane orientation of the seed layer is random because the nanosheets were tiled randomly on the substrates. Film Characterization. Figure 3 compares XRD patterns of ZnO films with a thickness of ca. 100 nm grown by PLD method on glass and plastic (PEN and PET) substrates with and without nanosheets. The deposition was carried out at room temperature. The films deposited on the bare substrates showed one weak peak attributable to a 002 reflection. This feature can be understood by the well-known growth habit of ZnO crystal along the c axis, as discussed in detail below. The films on the substrates covered with the nanosheet, on the other hand, also showed the 002 peak, but it was much more intense than that for the films on the bare substrates. In addition, a 004 peak was detectable. These results clearly indicate that the nanosheet at the substrate surface indeed promoted the growth of ZnO crystal on glass and plastic substrates at room temperature and the obtained film had high crystallinity and enhanced crystallographic orientation along the c axis. Cross-sectional TEM observations provided minute information on the film texture at a nanoscopic resolution. The film deposited on the glass substrate covered with the nanosheet had a uniform thickness of ca. 100 nm across a wide area (Figure 4a). An electron diffraction pattern taken from the whole area of the film was composed of sharp spots, indicating the fairly high crystallinity of the film and its strong [001] orientation with respect to the substrate. A magnified image at the film/ substrate interface could resolve a ca. 1.8 nm thick seed layer of unilamellar Cs4W11O36 nanosheets. Because the exfoliated nanosheet is comparable to the cleaved surface of layered materials such as mica, the interface was well-defined and atomically sharp without a noticeable interfacial layer. The

Shibata et al. lattice fringes of ZnO appeared directly on top of the nanosheet, maintaining an epitaxial relationship to the nanosheet. Note that a structural integrity like this has not been attained for ZnO films grown at room temperature. The film deposited on PEN substrate covered with the nanosheet also showed an electron diffraction pattern similar to that on glass substrate, indicating prominent orientation along the c axis (Figure 4b). The somewhat broader diffraction spots may have been caused by a lower flatness of the PEN substrate in comparison with glass, resulting in the relatively larger degree of c axis fluctuation. Still, the film texture was found to be very similar at a nanoscopic scale to that on the glass substrate: the sharp appearance of well-crystallized and oriented ZnO film at the film/substrate interface. In contrast to these well-oriented textures, the film deposited onto the bare PEN substrate exhibited a rather poorly crystalline nature (Figure 4c). Electron diffraction data revealed ringlike features besides spotty components, indicating the presence of poorly or randomly oriented domains in the film. In the TEM image, a conelike texture indicated by the arrow was frequently discerned for the ZnO film. Close observation of the domain near the interface revealed poorly crystalline or even amorphouslike regions for the most part. Even in crystalline regions, there was clearly a large deviation of the c axis from the substrate normal. It is well established that, according to the Van der Drift model for polycrystalline growth, intrinsic anisotropy in the growth rate leads predominantly to overgrowth of (001) preferred oriented ZnO crystals.61 This growth habit often produces the typical cone texture, as actually observed in the film on the bare PEN. This means that the orientation distribution and crystallinity of the film significantly deteriorate at an early stage of deposition for ZnO films as reported so far. Accordingly, this tendency is more pronounced and becomes apparent when the film thickness typically becomes smaller than 100 nm. In contrast, the nanosheet seed layer could promote high-quality crystal growth even at ambient temperature from the initial stage of the film deposition regardless of the underlying substrates. In Figure 4, the typical textures in the central region of the films (at about 50 nm above the interface) are also compared. The (001) lattice fringe was apparently tilted for the film on the bare PEN, in contrast to the flat and parallel lattice plane for the film grown on the nanosheet seed. The results unambiguously confirm that the growth of ZnO crystal can be critically controlled by the topmost surface structure defined by the nanosheet. The obtained films were characterized by 2D-XRD analyses to gain more quantitative information on their degree of crystallographic orientation (Figure 5). The diffraction features at a scattering angle of 2θ ) 34.3° and 72.8° are indexable to 002 and 004 reflections, respectively. Absence of general hkl reflections is consistent with the θ-2θ scan data, confirming preferred orientation along the [001] direction. The 002 and 004 reflections exhibited a noticeable spread along the χ circle. This spread indicates that the crystal films involve some degree of misalignment, and the spread was, in fact, dependent on the films. The profile along the arclike feature produces the χ rocking curve. The ZnO films deposited on the substrates with the nanosheet seed layer displayed rather sharp rocking curves. However, the full width at half-maximum (fwhm) of 3.58° and 4.92° for the films on glass and PEN substrates with the nanosheet, respectively, are still not small enough, when compared with that for films grown on single-crystal substrates at elevated temperature. The lattice mismatch between the ZnO (001) plane and 2D architecture of Cs4W11O36 nanosheet is ca.

Zinc Oxide Films on Plastics

J. Phys. Chem. C, Vol. 113, No. 44, 2009 19099

Figure 4. Cross-sectional TEM images of ZnO thin films deposited on glass (a) with the nanosheet seed layer, on PEN substrate (b) with, and (c) without the seed layer. Blue and red arrows in the HRTEM of sample (a) help guide the eye to confirm the epitaxial relationship between the nanosheet and ZnO film.

Figure 5. 2D Debye diffraction rings for ZnO thin films deposited on glass substrate (a) with and (b) without the nanosheet seed layer, and those on PEN substrate (c) with and (d) without the nanosheets. Symbol (2) shows the diffraction rings from PEN substrate. (e) and (f) show X-ray rocking curves of ZnO 002 diffraction for the films on glass and PEN substrates, respectively.

12%. This relatively large mismatch as well as room-temperature deposition is responsible for some degree of orientational distribution.3 Improved crystallographic orientation may be expected through the selection of a better-matched nanosheet. The larger fwhm on PEN can be ascribed to its rougher surface. It should be emphasized, however, that the rocking curve for the films fabricated without the assistance of the nanosheet was much broader, highlighting the effective role of the nanosheet as the seed layer. The fwhm was 13.17° on the glass and 20.79° on PEN, indicating poor orientation of the films deposited on the substrates without 2D seed lattice of the nanosheets. Electrical property of ZnO Films. Figure 6a shows the resistivity of ZnO films on the glass substrate with and without nanosheet seed layer as a function of film thickness in a range from 25 to 250 nm. The resistivity stayed constant at ca. 2.4 × 10-2 Ωcm for a thickness of >100 nm, being independent of the films with or without the seed layer. In contrast, the resistivity of the film on the bare glass rapidly increased to over 4 Ωcm as the film thickness decreased to 20 nm. This striking

trend has been commonly reported for ZnO films deposited at low temperature and can be accounted for by the deteriorated film quality involving misalignment and lowered crystallinity, particularly at a very small film thickness.20–22 As described above, the inherent growth habit of ZnO crystal spontaneously yields an oriented texture when the film becomes thick enough, as a rule, for a thickness of >100 nm. Thus, the resistivity in the plateau region is governed by the oriented crystal domain. In contrast, the films on the nanosheet seed layer did not show an appreciable rise in resistivity at the small film thickness. The resistivity was suppressed to 0.1 Ωcm at 20 nm, which was 1/40 of the other case. The highly ordered film texture immediately formed on top of the substrate explains this behavior well. The changes of the film conductivity on PEN substrates (Figure 6b) are substantially the same as those on glass ones. The resistivity on the nanosheet seed was 0.21 Ωcm at the smallest thickness of 20 nm, whereas significant degradation was observed for the comparable film without the seed. These results obviously indicate the high potential of the

19100

J. Phys. Chem. C, Vol. 113, No. 44, 2009

Figure 6. Dependence of electrical resistivity on film thickness. ZnO films (O with and • without seed layer) deposited on (a) glass and (b) PEN substrates, respectively.

nanosheet seed layer method to produce high-quality ZnO films on glass and plastic substrates. Conclusions In this study, we have demonstrated successful fabrication of well crystallized ZnO (001) films without any interfacial layer onto glass and plastic substrates at room temperature with the assistance of a Cs4W11O36 nanosheet as a seed layer. Needless to say, not only plastics but also various kinds of substrates such as metals and ceramics could be used because nanosheets are readily deposited on them via the colloid process. Moreover, this method is not limited to ZnO film deposition, but is, in principle, applicable to a range of crystals by selecting nanosheets with appropriate 2D structures. Therefore, we expect that this nanosheet seed layer technique will pave the way to the fabrication of various functional crystal films on plastic films at low temperature and will thus contribute to the development of various flexible electronic devices such as displays, touch panels, light emitting diodes, and MEMS. Acknowledgment. We thank Teijin DuPont Films Co. Ltd., Japan, for providing Teonex (200 µm thick PEN film). This work was supported by CREST of the Japan Science and Technology Agency (JST) and World Premier International Research Center (WPI) Initiative on Materials Nanoarchitectonics, MEXT, Japan. Supporting Information Available: In-plane XRD pattern of the monolayer film of Cs4W11O36 nanosheet seed layer. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Funakubo, H.; Mizutani, N.; Yonetsu, M.; Saiki, A.; Shinozaki, K. J. Elecroceram. 1999, 4, 25. (2) Sasaki, A.; Hara, W.; Matsuda, A.; Tateda, N.; Otaka, S.; Akiba, S.; Saito, K.; Yodo, T.; Yoshimoto, M. Appl. Phys. Lett. 2005, 86, 2319111. (3) Kuppusami, P.; Vollweiler, G.; Rafaja, D.; Ellmer, K. Appl. Phys. A: Mater. Sci. Process. 2005, 80, 183. (4) Ohta, J.; Fujioka, H.; Oshima, M. Appl. Phys. Lett. 2003, 83, 3060.

Shibata et al. (5) Tsukazaki, A.; Ohtomo, A.; Onuma, T.; Ohtani, M.; Makino, T.; Sumiya, M.; Ohtani, K.; Chichibu, S. F.; Fuke, S.; Segawa, Y.; Ohno, H.; Koinuma, H.; Kawasaki, M. Nat. Mater. 2005, 4, 42. (6) Pan, F.; Song, C.; Liu, X. J.; Yang, Y. C.; Zeng, F. Mater. Sci. Eng., R. 2008, 62, 1. ¨ zgu¨r, U ¨ .; Alivov, Ya. I.; Liu, C.; Teke, A.; Reshchikov, M. A.; (7) O Dog˘an, S.; Avrutin, V.; Cho, S.-J.; Morkoc¸, H. J. Appl. Phys. 2005, 98, 041301. (8) Klingshirn, C. ChemPhysChem 2007, 8, 782. (9) Tsukazaki, A.; Ohtomo, A.; Kita, T.; Ohno, Y.; Ohno, H.; Kawasaki, M. Science 2007, 315, 1388. (10) Chichibu, S. F.; Onuma, T.; Kubota, M.; Uedono, A.; Sota, T.; Tsukazaki, A.; Ohtomo, A.; Kawasaki, M. J. Appl. Phys. 2006, 99, 093505. (11) Masuda, S.; Kitamura, K.; Okumura, Y.; Miyatake, S.; Tabata, H.; Kawai, T. J. Appl. Phys. 2003, 93, 1624. (12) Carcia, P. F.; McLean, R. S.; Reilly, M. H.; Nunes, G., Jr. Appl. Phys. Lett. 2003, 82, 1117. (13) Fortunato, E.; Barquinha, P.; Pimentel, A.; Gonc¸alves, A.; Marques, A.; Pereira, L.; Martins, R. Thin Solid Films 2005, 487, 205. (14) Fortunato, E.; Barquinha, P.; Pimentel, A.; Gonc¸alves, A.; Marques, A.; Pereira, L.; Martins, R. AdV. Mater. 2005, 17, 590. (15) Fortunato, E.; Gonc¸alves, A.; Pimentel, A.; Barquinha, P.; Gonc¸alves, G.; Pereira, L.; Ferreira, I.; Martins, R. Appl. Phys. A: Mater. Sci. Process. 2009, 96, 197. (16) Guziewicz, E.; Godlewski, M.; Krajewski, T.; Wachnicki, Ł.; Szczepanik, A.; Kopalko, K.; Woˇjcik-Głodowska, A.; Przez´dziecka, E.; Paszkowicz, W.; Łusakowska, E.; Kruszewski, P.; Huby, N.; Tallarida, G.; Ferrari, S. J. Appl. Phys. 2009, 105, 122413. (17) Minami, T.; Sato, H.; Nanto, H.; Takata, S. Jpn. J. Appl. Phys. 1985, 24, L781. (18) Agura, H.; Suzuki, A.; Matsushita, T.; Aoki, T.; Okuda, M. Thin Solid Films 2003, 445, 263. (19) Kim, J.-P.; Lee, S.-A; Bae, J. S.; Park, S.-K.; Choi, U.-C.; Cho, C.-R. Thin Solid Films 2008, 516, 5223. ´ guas, (20) Fortunato, E.; Gonc¸alves, A.; Assunc¸a˜o, V.; Marques, A.; A H.; Pereira, L.; Ferreira, I.; Martins, R. Thin Solid Films 2003, 442, 121. (21) Yu, X.; Ma, J.; Ji, F.; Wang, Y.; Cheng, C.; Ma, H. Appl. Surf. Sci. 2005, 245, 310. (22) Zhu, B. L.; Sun, X. H.; Guo, S. S.; Zhao, X. Z.; Wu, J.; Wu, R.; Liu, J. Jpn. J. Appl. Phys. 2006, 45, 7860. (23) Nakagawara, O.; Kishimoto, Y.; Seto, H.; Koshido, Y.; Yoshino, Y.; Makino, T. Appl. Phys. Lett. 2006, 89, 091904. (24) Yamada, T.; Miyake, A.; Kishimoto, S.; Makino, H.; Yamamoto, N.; Yamamoto, T. Appl. Phys. Lett. 2007, 91, 051915. (25) Lee, J.; Lee, D.; Lim, D.; Yang, K. Thin Solid Films 2007, 515, 6094. (26) Miyake, A.; Yamada, T.; Makino, H.; Yamamoto, N.; Yamamoto, T. Thin Solid Films 2009, 517, 3130. (27) Ferna´ndez, S.; Martı´nez-Steele, A.; Gandı´a, J. J.; Naranjo, F. B. Thin Solid Films 2009, 517, 3152. (28) Koidis, C.; Logothetidis, S.; Georgiou, D.; Laskarakis, A.; Lousinian, S.; Tsiaoussis, I.; Frangis, N. Phys. Status Solidi A 2008, 205, 1988. (29) Koidis, C.; Logothetidis, S.; Laskarakis, A.; Tsiaoussis, I.; Frangis, N. Micron 2009, 40, 130. (30) Shibata, T.; Fukuda, K.; Ebina, Y.; Kogure, T.; Sasaki, T. AdV. Mater. 2008, 20, 231. (31) Fukuda, K.; Akatsuka, K.; Ebina, Y.; Ma, R.; Takada, K.; Nakai, I.; Sasaki, T. ACS Nano 2008, 2, 1689. (32) Nadeau, P. H.; Wilson, M. J.; McHardy, W. J.; Tait, J. M. Science 1984, 225, 923. (33) Nakato, T.; Furumi, Y.; Terao, N.; Okuhara, T. J. Mater. Chem. 2000, 10, 737. (34) Treacy, M. M. J.; Rice, S. B.; Jacobson, A. J.; Lewandowski, J. T. Chem. Mater. 1990, 2, 279. (35) Fang, M.; Kim, C. H.; Saupe, G. B.; Kim, H.-N.; Waraksa, C. C.; Miwa, T.; Fujishima, A.; Mallouk, T. E. Chem. Mater. 1999, 11, 1526. (36) Schaak, R. E.; Mallouk, T. E. Chem. Mater. 2000, 12, 2513. (37) Schaak, R. E.; Mallouk, T. E. Chem. Mater. 2000, 12, 3427. (38) Ebina, Y.; Sasaki, T.; Watanabe, M. Solid State Ionics 2002, 151, 177. (39) Ozawa, T. C.; Fukuda, K.; Akatsuka, K.; Ebina, Y.; Sasaki, T. Chem. Mater. 2007, 19, 6575. (40) Ozawa, T. C.; Fukuda, K.; Akatsuka, K.; Ebina, Y.; Sasaki, T.; Kurashima, K.; Kosuda, K. J. Phys. Chem. C 2008, 112, 17115. (41) Ida, S.; Ogata, C.; Eguchi, M.; Youngblood, W. J.; Mallouk, T. E.; Matsumoto, Y. J. Am. Chem. Soc. 2008, 130, 7052. (42) Miyamoto, N.; Yamamoto, H.; Kaito, R.; Kuroda, K. Chem. Commun. 2002, 2378. (43) Takagaki, A.; Lu, D.; Kondo, J. N.; Hara, M.; Hayashi, S.; Domen, K. Chem. Mater. 2005, 17, 2487. (44) Takagaki, A.; Yoshida, T.; Lu, D.; Kondo, J. N.; Hara, M.; Domen, K.; Hayashi, S. J. Phys. Chem. B 2004, 108, 11549.

Zinc Oxide Films on Plastics (45) Fukuda, K.; Nakai, I.; Ebina, Y.; Ma, R.; Sasaki, T. Inorg. Chem. 2007, 46, 4787. (46) Sasaki, T.; Watanabe, M.; Hashizume, H.; Yamada, H.; Nakazawa, H. J. Am. Chem. Soc. 1996, 118, 8329. (47) Sasaki, T.; Watanabe, M. J. Am. Chem. Soc. 1998, 120, 4682. (48) Sugimoto, W.; Terabayashi, O.; Murakami, Y.; Takasu, Y. J. Mater. Chem. 2002, 12, 3814. (49) Miyamoto, N.; Kuroda, K.; Ogawa, M. J. Mater. Chem. 2004, 14, 165. (50) Liu, Z. H.; Ooi, K.; Kanoh, H.; Tang, W. P.; Tomida, T. Langmuir 2000, 16, 4154. (51) Omomo, Y.; Sasaki, T.; Wang, L. Z.; Watanabe, M. J. Am. Chem. Soc. 2003, 125, 3568. (52) Liu, Z.; Ma, R.; Ebina, Y.; Takada, K.; Sasaki, T. Chem. Mater. 2007, 19, 6504. (53) Shaack, R. E.; Mallouk, T. E. Chem. Commun. 2002, 706.

J. Phys. Chem. C, Vol. 113, No. 44, 2009 19101 (54) Sugimoto, W.; Iwata, H.; Yasunaga, Y.; Murakami, Y.; Takasu, Y. Angew. Chem., Int. Ed. 2003, 42, 4092. (55) Adachi-Pagano, M.; Forano, C.; Besse, J.-P. Chem. Commun. 2000, 91. (56) Hibino, T.; Jones, W. J. Mater. Chem. 2001, 11, 1321. (57) Li, L.; Ma, R.; Ebina, Y.; Iyi, N.; Sasaki, T. Chem. Mater. 2005, 17, 4386. (58) Liu, Z.; Ma, R.; Osada, M.; Iyi, N.; Ebina, Y.; Takada, K.; Sasaki, T. J. Am. Chem. Soc. 2006, 128, 4872. (59) Muramatsu, M.; Akatsuka, K.; Ebina, Y.; Wang, K.; Sasaki, T.; Ishida, T.; Miyake, K.; Haga, M. Langmuir 2005, 21, 6590. (60) Akatsuka, K.; Haga, M.; Ebina, Y.; Osada, M.; Fukuda, K.; Sasaki, T. ACS Nano 2009, 3, 1097. (61) Van der Drift, A. Philips Res. Rep. 1967, 22, 267.

JP9074288