pubs.acs.org/Langmuir © 2010 American Chemical Society
Conductivity of Ruthenate Nanosheets Prepared via Electrostatic Self-Assembly: Characterization of Isolated Single Nanosheet Crystallite to Mono- and Multilayer Electrodes Jun Sato,† Hisato Kato,† Mutsumi Kimura,†,‡ Katsutoshi Fukuda,*,‡ and Wataru Sugimoto*,†,‡ †
Faculty of Textile Science and Technology and ‡Collaborative Innovation Center for Nanotech Fiber, Shinshu University, 3-15-1 Tokida, Ueda, Nagano 386-8567, Japan Received September 24, 2010. Revised Manuscript Received October 22, 2010
Ultrathin films composed of ruthenate nanosheets (RuO2ns) were fabricated via electrostatic self-assembly of unilamellar RuO2ns crystallites derived by total exfoliation of an ion-exchangeable layered ruthenate. Ultrathin films with submonolayer to monolayer RuO2ns coverage and multilayered RuO2ns thin films were prepared by controlled electrostatic self-assembly and layer-by-layer deposition using a cationic copolymer as the counterion. Electrical properties of a single RuO2ns crystallite were successfully measured by means of scanning probe microscopy. The sheet resistance of an isolated single RuO2ns crystallite was 12 kΩ sq-1. Self-assembled submonolayer films behaved as a continuous conducting film for coverage above 70%, which was discussed based on a two-dimensional percolation model. Low sheet resistance was attained for multilayered films with values less than 1 kΩ sq-1. Interestingly, the grain boundary resistance between nanosheets seems to contribute only slightly to the sheet resistance of self-assembled films.
1. Introduction Transparent conducting films are one of the capital components in optoelectronics applications such as liquid crystal displays, solar cells, and organic light-emitting diodes.1 Currently, the most widely used material for many applications is indium tin oxide (ITO) possessing high transparency and conductivity at room temperature (transparency, 80-90%; sheet resistance, 14-250 Ω sq-1).2,3 From the viewpoint of the limited abundance of indium as well as thermal instability, brittleness and poor corrosion resistance against acidic/basic media, a variety of alternative materials including fluorine-doped tin oxide,4 zinc oxide,5 conductive polymer,6 metal nanowire,7 carbon nanotube,8 and graphene9-12 have also been pursued as indium-free transparent conducting systems. Among them, graphene, which is obtained via physical or chemical exfoliation of single to a few layers of *To whom correspondence should be addressed. Tel: þ81-268-21-5455. Fax: þ81-268-21-5452. E-mail: (W.S.)
[email protected]; (K.F.) kfukuda@ shinshu-u.ac.jp. (1) Ohta, H.; Hosono, H. Mater. Today 2004, 7(6), 42–51. (2) Kulkarni, A. K.; Schulz, K. H.; Lim, T. S.; Khan, M. Thin Solid Films 1999, 345, 273–277. (3) Sandoval-Paz, M. G.; Ramı´ rez-Bon, R. Thin Solid Films 2009, 517, 2596– 2601. (4) (a) Manifacier, J.-C.; Szepessy, L.; Bresse, J. F.; Perotin, M.; Stuck, R. Mater. Res. Bull. 1979, 14, 109–119. (b) Gottlieb, B.; Koropecki, R.; Arce, R.; Crisalle, R.; Ferron, J. Thin Solid Films 1991, 199, 13–21. (5) Ott, A. W.; Chang, R. P. H. Mater. Chem. Phys. 1999, 58, 132–138. (6) Martin, B. D.; Nikolov, N.; Pollack, S. K.; Saprigin, A.; Shashidhar, R.; Zhang, F.; Heiney, P. A. Synth. Met. 2004, 142, 187–193. (7) De, S.; Higgins, T. M.; Lyons, P. E.; Doherty, E. M.; Nirmalraj, P. N.; Blau, W. J.; Boland, J. J.; Coleman, J. N. ACS Nano 2009, 3, 1767–1774. (8) Zhang, D.; Ryu, K.; Liu, X.; Polikarpov, E.; Ly, J.; Tompson, M. E.; Zhou, C. Nano Lett. 2006, 6, 1880–1886. (9) Wassei, J. K.; Kaner, R. B. Mater. Today 2010, 13(3), 52–59. (10) (a) Becerril, H. A.; Mao, J.; Liu, Z.; Stoltenberg, R. M.; Bao, Z.; Chen, Y. ACS Nano 2008, 2, 463–470. (b) Eda, G.; Fanchini, G.; Chhowalla, M. Nat. Nanotechnol. 2008, 3, 270–274. (11) Li, X.; Zhang, G.; Bai, X.; Sun, X.; Wang, X.; Wang, E.; Dai, H. J. Nat. Nanotechnol. 2008, 3, 538–542. (12) (a) Kim, K. S.; Zhao, Y.; Jang, H.; Lee, S. Y.; Kim, J. M.; Kim, K. S.; Ahn, J.-H.; Kim, P.; Choi, J.-Y.; Hong, B. H. Nature 2009, 457, 706–710. (b) Li, X.; Zhu, Y.; Cai, W.; Borysiak, M.; Han, B.; Chen, D.; Piner, R. D.; Colombo, L.; Ruoff, R. S. Nano Lett. 2009, 9, 4359–4363.
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carbon from graphite,10,11 chemical vapor deposition on metal surfaces,12 and epitaxial growth on SiC surfaces,13 is an attractive two-dimensional (2D) conducting material in terms of flexibility, transparency, and mobility of charge carrier. By utilizing such two-dimensional nanomaterial, a rational design of transparent conducting films based on a bottom-up approach can be implemented. Another two-dimensional nanomaterial with electronic conductivity is ruthenate nanosheets (RuO2ns). Rutile type RuO2 is known as one of the highest electronic conducting oxides. The layered forms, K0.2RuO2.1 and NaRuO2, also exhibit metallic electrical and magnetic behavior.14,15 Exfoliation of these layered alkali metal ruthenates results in ruthenate nanosheets having thickness in molecular scale and lateral size ranging from submicrometer to micrometers.16-18 We have so far concentrated our research toward the application of RuO2ns to various energyrelated applications, such as electrochemical capacitors and fuel cells.16,18-20 As an example of utilizing the atypical twodimensional nanostructure and excellent physicochemical properties, we have also shown that transparent and/or flexible electrochemical capacitors electrodes can be prepared by electrophoretic deposition on glass or plastic ITO.19 However, the electrophoretic deposition method in principle requires the use of conductive substrate, and thus the use of transparent or flexible conductive (13) Berger, C.; Song, Z.; Li, X.; Wu, X.; Brown, N.; Naud, C.; Mayou, D.; Li, T.; Hass, J.; Marchenkov, A. N.; Conrad, E. H.; First, P. N.; de Heer, W. A. Science 2006, 312, 1191–1196. (14) Sugimoto, W.; Omoto, M.; Yokoshima, K.; Murakami, Y.; Takasu, Y. J. Solid State Chem. 2004, 177, 4542–4545. (15) Shikano, M.; Delmas, C.; Darriet, J. Inorg. Chem. 2004, 43, 1214–1216. (16) Sugimoto, W.; Iwata, H.; Yasunaga, Y.; Murakami, Y.; Takasu, Y. Angew. Chem., Int. Ed. 2003, 42, 4092–4096. (17) Fukuda, K.; Kato, H.; Sato, J.; Sugimoto, W.; Takasu, Y. J. Solid State Chem. 2009, 182, 2997–3002. (18) Fukuda, K.; Saida, T.; Sato, J.; Yonezawa, M.; Takasu, Y.; Sugimoto, W. Inorg. Chem. 2010, 49, 4391–4393. (19) Sugimoto, W.; Yokoshima, K.; Ohuchi, K.; Murakami, Y.; Takasu, Y. J. Electrochem. Soc. 2006, 153, A255–A260. (20) (a) Sugimoto, W.; Saida, T.; Takasu, Y. Electrochem. Commun. 2006, 8, 411–415. (b) Saida, T.; Sugimoto, W.; Takasu, Y. Electrochim. Acta 2010, 55, 857–864.
Published on Web 11/11/2010
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substrate is inevitable. To be liberated from the drawbacks of present conducting thin films, a more facile film fabrication technique without the necessity of a conductive substrate needs to be applied. A variety of film fabrication techniques using exfoliated nanosheets have been proposed, for example, LangmuirBlodgett method,21 electrostatic self-assembly,22-24 and electrophoretic deposition.19,25 In particular, the electrostatic selfassembly method, which utilizes electrostatic interaction of intrinsically charged nanosheets and oppositely charged cationic polymer for thin film fabrication, is a simple process and is feasible for scale-up. The amount of nanosheet deposition, that is, nanosheet coverage on the substrate, can be controlled by parameters such as reaction time and concentration of the nanosheet suspension; short deposition time in low nanosheet concentration leads to submonolayer coverage while long deposition time and concentrated nanosheet suspensions results in monolayer coverage. Multilayered films can be easily fabricated by repeating the deposition cycle of the cationic polymers and the anionic nanosheets. Thus, this method offers easy control of film thickness in nanoscale precision on the substrate surface, which seems favorable for fabricating transparent conducting films based on the bottomup assembly under ambient conditions. Here, we report the fabrication of ultrathin conductive films on a nonconductive substrate via a self-assembly approach using RuO2ns and cationic polymer. Films with submonolayer to monolayer and multilayer films were prepared and its conductivity is discussed. In addition, the conductivity of the RuO2ns crystallite in molecular entity, in other words, characterization of a single isolated nanosheet crystallite, is of significant importance for understanding conducting system consisting of the nanosheets. In this study, the electrical properties of a single nanosheet crystallite have been characterized by using scanning probe microscopy (SPM), which to the best of our knowledge is the first example of such measurements.
2. Experimental Section 2.1. Material Synthesis. Aqueous ruthenate nanosheet (RuO2ns) colloid was prepared and characterized following previously described procedures.16,17 Briefly, a pelletized mixture of K2CO3 and rutile-type RuO2 (5:8, in molar ratio) was heated at 850 °C for 12 h under Ar flow. The sample was ground with an agate mortar and washed with ultrapure water (>18 MΩ cm), leading to layered potassium ruthenate (formulated as K0.2RuO2.1 3 nH2O). Ion-exchange of the interlayer potassium was conducted with 1 M (M = mol dm-3) HCl for 3 days at 60 °C, resulting in layered protonic ruthenate (formulated as H0.2RuO2.1 3 0.9H2O).16 A sample of 0.1 g of layered protonic ruthenate was added to 25 cm3 of tetrabutylammonium hydroxide (TBAOH) aqueous solution (the molar ratio of TBA ions to the exchangeable protons in ruthenate was TBAþ/Hþ = 10) and vigorously shaken for 10 days to exfoliate the layered ruthenate into elementary RuO2ns. Nonexfoliated impurity was removed by centrifugation at 2000 rpm for 30 min. The final suspension contains (21) (a) Yamaki, T.; Asai, K. Langmuir 2001, 17, 2564–2567. (b) Muramatsu, M.; Akatsuka, K.; Ebina, Y.; Wang, K.; Sasaki, T.; Ishida, T.; Miyake, K.; Haga, M. Langmuir 2005, 21, 6590–6595. (22) (a) Sasaki, T.; Ebina, Y.; Tanaka, T.; Harada, M.; Watanabe, M.; Decher, G. Chem. Mater. 2001, 13, 4661–4667. (b) Sasaki, T.; Ebina, Y.; Fukuda, K.; Tanaka, T.; Harada, M.; Watanabe, M. Chem. Mater. 2002, 14, 3524–3530. (23) Wang, L.; Omomo, Y.; Sakai, N.; Fukuda, K.; Nakai, I.; Ebina, Y.; Takada, K.; Watanabe, M.; Sasaki, T. Chem. Mater. 2003, 15, 2873–2878. (24) Huang, J.; Ma, R.; Ebina, Y.; Fukuda, K.; Takada, K.; Sasaki, T. Chem. Mater. 2010, 22, 2582–2587. (25) Sugimoto, W.; Terabayashi, O.; Murakami, Y.; Takasu, Y. J. Mater. Chem. 2002, 12, 3814–3818.
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unilamellar RuO2ns with a composition of RuO2.10.2- and was further used for thin film fabrication. 2.2. Film Fabrication. Films of RuO2ns with monolayer coverage were fabricated via electrostatic self-assembly on various substrates such as Si wafer, Pyrex glass, quartz glass, polyethylene terephthalate (PET) film, polyethylene (PE) film, and poly(pphenylene benzobisoxazole) (PBO) fiber. Si wafer, Pyrex glass and quartz glass were washed with a mixture of 12 M HCl and methanol (1:1 by volume), followed by 18 M sulfuric acid and finally ultrapure water prior to the self-assembly process. PET film, PE film, and PBO fiber were washed with ethanol. The precleaned substrate was immersed into an aqueous solution of 1 mass % cationic diblock copolymer composed of ∼14% polyvinylamine and ∼86% polyvinylalcohol (hereafter designated as PVAm-PVA copolymer) for 10 min to form a positively charged layer on the substrate surface. Following mild washing with ultrapure water, the substrate was immersed into an aqueous colloid containing anionic RuO2.10.2- stabilized by TBAþ counter cations (typical concentration is 0.08 g of RuO2 per 1 dm3) for various durations. After dipping for a designated time, the substrate was carefully drawn out and washed with ultrapure water to remove physically adsorbed species. Multilayer films of cationic copolymer/anionic nanosheet pair were prepared by repeating the above-mentioned process up to 10 times. An automatic Multilayer Film Coater (MC-4000, Aiden Co., Ltd.) was used for fabrication of multilayer thin films. 2.3. Film Characterizations. The surface topographic images of ultrathin nanosheet films were obtained by tapping-mode atomic force microscopy (SPA400, Seiko instruments) with Si cantilever (SI-DF20, SII NanoTechnology Inc.; resonant frequency 135 kHz, force constant 15 N m-1). Ultraviolet-visible (UV-vis) absorption spectra were recorded with a Hitachi U-4100 spectrophotometer. Out-of-plane X-ray diffraction (XRD) patterns of multilayer thin films were collected by means of Bragg-Brentano-type diffractometer (Rint-Ultima/S2K, Rigaku Co.) with Cu KR radiation. Synchrotron radiated in-plane XRD patterns were taken with a four axis diffractometer equipped with NaI scintillation counter at the BL-6C of the Photon Factory in the High Energy Accelerator Research Organization. 2.4. Electrical Properties. The electrical properties of continuous mono- and multilayered RuO2ns thin films were characterized using the two-probe method. Current collectors of 3 mm 3 mm Au pads were vacuum-deposited on the RuO2ns films using a mask. The intervals between the current collectors were 0.3, 0.5, 1.0, 2.0, 3.0, 6.0, and 12.0 mm. I-V curves were acquired by contacting two tungsten probes to the Au electrodes. The electrical properties of an isolated single RuO2ns crystallite were measured using a scanning probe microscope (SPM, JSPM5400, JEOL Ltd.) with conducting Pt-coated Si cantilever (OMCL-AC240TM-B2, OLYMPUS Co.; resonant frequency 70 kHz, force constant 2 N m-1 and tip diameter 30 nm). A submonolayer RuO2ns film was prepared on Pyrex glass substrate by electrostatic self-assembly employing short dipping time of 1 to 5 min in the RuO2ns colloid. A continuous Au film was vacuumdeposited on the submonolayer film. A portion of the Au film was mechanically peeled off using adhesive tape. In this way, a section of Au was left to act as a current collector in contact with RuO2ns. Isolated single RuO2ns crystallite in contact with the current collector was first located by measuring topographic images along the current collector edge by means of frequency modulation AFM (FM-AFM). FM-AFM utilizes the resonant frequency shift of the AFM cantilever as a result of the interaction between the tip of the cantilever and the sample in very close range, allowing visualization without contact. The noncontact visualization protects the brittle conducting AFM tip and the RuO2ns from damage. However, the use of a soft cantilever for damageless contact causes low lateral resolution. Thus, after locating the desired position by FM-AFM, the nanosheet sample was visualized by tapping-mode AFM, which gives a higher resolution image than FM-AFM. The conducting tip was then allowed to Langmuir 2010, 26(23), 18049–18054
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Figure 1. The 3 μm 3 μm tapping-mode AFM images of self-assembled RuO2ns films deposited on Si substrate with various dipping time of (a-g) 1, 5, 10, 20, 30, 60, and 90 min in nanosheet colloid. (h) Surface coverage of RuO2ns on the Si substrate as a function of the dipping time.
Figure 2. UV-vis absorption spectra of self-assembled RuO2ns films on both sides of quartz glass obtained with various dipping time. The inset shows absorbance at 360 nm as a function of dipping time. approach the RuO2ns surface close enough to get into contact. To realize both soft and stable contact, the z-displacement of the piezo scanner was moved stepwise by 1 nm. I-V measurement was conducted by applying (20 mV between the tip in contact with the RuO2ns and the Au current collector. Sheet resistance was deduced from the gradient of the I-V curve.
3. Results and Discussion 3.1. Fabrication of Monolayer and Multilayer Films via Electrostatic Self-Assembly. A submonolayer film of RuO2ns was successfully fabricated on various substrates by means of electrostatic self-assembly employing PVAm-PVA copolymer as a countercation. Typical AFM images of the surface of Si wafer after dipping the substrate in RuO2ns colloid for various durations are displayed in Figure 1. The images clearly reveal the presence of nanosheets with thickness of 1.1 ( 0.2 nm (average of ca. 70 nanosheets) and lateral size ranging from several hundred nanometers to a few micrometers on the Si surface. The surface coverage was calculated from the height distribution histograms in the AFM images and is plotted as a function of the dipping time of the substrate in the RuO2ns colloid (Figure 1h). The coverage increased rapidly for the first 30 min, reaching saturation after 90 min. The maximum coverage is approximately 92%, which can be regarded as a near monolayer coverage of the RuO2ns with inevitable overlaps of the nanosheets (approximately 40%). Langmuir 2010, 26(23), 18049–18054
Figure 3. (a) Digital photograph and (b) UV-vis absorption spectra of the 1 to 10 layer films of PVAm-PVA/RuO2ns deposited on both sides of quartz glass. The inset shows absorbance at 360 nm as a function of numbers of deposition cycle.
Folded nanosheets are occasionally observed (indicated with arrows in Figure 1a,b) reflecting the high flexibility in molecular level of the ruthenate nanosheets. UV-vis absorption spectra of self-assembled RuO2ns films on quartz glass are shown in Figure 2 (both sides are coated). The RuO2ns film has a sharp absorption peak at 220 nm and two broad peaks at 360 and 500 nm, which is in agreement with the UV-vis absorption spectra of the RuO2ns colloid.17 The absorbance at 360 nm as a function of dipping time (Figure 2 inset) saturates after 90 min, yielding a film with transmittance of >92% within the visible region. The trend in the change in the absorbance as a function of dipping time is consistent with that of the surface coverage, giving support to relevancy of the coverage estimation. Multilayer RuO2ns films were fabricated via sequential layerby-layer deposition of the cationic copolymer and anionic RuO2ns pair (PVAm-PVA/RuO2ns) on the substrate, yielding auburn-colored films (Figure 3a). The out-of-plane XRD pattern DOI: 10.1021/la103848f
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Sato et al. Table 1. Sheet Resistance and Coverage of RuO2ns on the Si substrate of the Self-Assembled Films Obtained at Various Dipping Times dipping time/min
coverage (%)
sheet resistance/kΩ sq-1
1 5 10 20 30 60 90
32 ( 3 37 ( 2 49 ( 4 72 ( 2 82 ( 2 90 ( 3 92 ( 1
not determined not determined not determined 223 78.0 23.3 21.1
Figure 4. (a) The 3D topographic image of a single RuO2ns in contact with the Au current collector visualized by tapping-mode AFM. (b) Height profile along C-D in (a).
Figure 6. Sheet resistance of the self-assembled RuO2ns multilayer films as a function of layer numbers.
Figure 5. (a) I-V characteristic of an isolated single RuO2ns crystallite. (b) Resistance measured at different points from the current collector of an isolated single RuO2ns crystallite.
of the substrate after 10 deposition cycles (see Supporting Information, Figure S1) exhibited a series of basal reflection peaks corresponding to interlayer space of 1.38 nm. These peaks can be attributed to PVAm-PVA/RuO2ns superlattice, suggesting that uniform layer-by-layer growth of the copolymer/nanosheet pair was achieved. UV-vis absorption spectra of the multilayered RuO2ns film reveals that the absorbance increased monotonously with the number of deposition cycles (Figure 3b). The absorbance at 360 nm increases linearly as a function of the dipping cycles (Figure 3b inset), indicating that a constant amount of RuO2ns were adsorbed on the substrate in each deposition cycle. 3.2. Electrical Properties of an Isolated, Single Ruthenate Nanosheet Crystallite. Figure 4a shows a 3D topographic image of the isolated single nanosheet sample used for I-V measurement. The single RuO2ns (located in center of the image) is in contact with the Au current collector (left-hand side of the image). RuO2ns has a uniform thickness of ∼1 nm, and does not overlap with other nanosheets and thus can be treated as an ideal sample for characterization of the electrical properties. The edge 18052 DOI: 10.1021/la103848f
of the Au current collector is sharp and perpendicular to the RuO2ns. This was made possible by mechanically stripping away a portion of the vacuum-deposited Au film26 by adhesive tape. The sharp Au edge is necessary to distinguish the boundary between the current collector and the nanosheet. Using a mask led to an ill-defined contact between the current collector and the nanosheet, unsuitable for accurate electrical conductivity measurements. I-V measurements were conducted by carefully lowering the cantilever 1 nm at a time by changing the z-displacement of the piezo scanner to allow soft contact of the cantilever and RuO2ns surface. Once the cantilever was in contact with RuO2ns, the z-displacement of the piezo scanner was further lowered in order to obtain good contact with a steady-state resistance (see Supporting Information, Figure S3). Figure 5a shows a typical I-V curve of the single nanosheet crystallite measured at the points indicated by the leftmost arrow in Figure 4a, revealing stable ohmic contact. The deduced resistance from the I-V curve was 47.2 kΩ and should be the sum of the intrinsic resistance of RuO2ns, the contact resistance between the cantilever and RuO2ns, and the system resistance of the apparatus, the current collector and the conductive cantilever. The resistance of the system measured by contacting the cantilever to the current collector was 0.5 to 1 kΩ, negligible compared to the overall resistance. The sheet resistance of the single RuO2ns crystallite was evaluated using the resistance data obtained at the points represented by arrows in Figure 4a. The resistance data was plotted against the distance between the measurement points and the edge of the current collector (Figure 5b). The gradient of Figure 5b gives the resistance per unit distance, which gives 0.166 kΩ nm-1. The C-D line profile along the width of the nanosheet shown in Figure 4b is about 70 nm, taking into account the diameter of the AFM tip as 30 nm.27 Multiplying the gradient by the width of (26) To evaluate the possibility of thermal damage of nanosheets due to the vapor-deposition process, the two-dimensional structure of the ruthenate nanosheet film was studied by means of synchrotron radiated in-plane X-ray diffraction before and after heating at 200 °C under vacuum. No difference was observed in the diffraction patterns before and after heating (see Supporting Information, Figure S2), which indicates that the two-dimensional crystallinity of ruthenate nanosheet is not affected by the Au vapor-deposition process.
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Figure 7. Digital photograph of RuO2ns deposited on both sides of (a) PET and (b) PE film. The values 1 L, 3 L, 6 L, and 10 L indicate layer numbers of PVAm-PVA/RuO2ns pair. (c) Photograph of RuO2ns-coated PBO fibers. Inset shows the resistance of the tied fiber (indicated with an arrow) has a resistance of ∼3 MΩ.
the nanosheet gives the sheet resistance of the single RuO2ns crystallite. The average of three samples gave a sheet resistance of 12 kΩ sq-1 (see Supporting Information, Figure S5). 3.3. Electrical Properties of Self-Assembled Monolayer Films of Ruthenate Nanosheets. To understand the electrical properties of the two-dimensional conducting nanoscale network of RuO2ns with submonolayer to monolayer coverage, the sheet resistance of RuO2ns films with various coverage were studied by means of conventional two-probe method. Ohmic response was observed at coverage higher than 72% (see Supporting Information, Figure S6). The sheet resistances deduced from the I-V curves are summarized in Table 1. The sheet resistance of the monolayer film with 92% coverage was 21 kΩ sq-1, which is only slightly higher than that of the single RuO2ns crystallite. The relatively small amount of contact between the nanosheets in the monolayer film due to the high anisotropic structure of the nanosheet may be the cause of the low sheet resistance. Naturally, the sheet resistance of self-assembled films strongly depended on the coverage of the nanosheets on the substrate. For films with coverage below 50%, no current could be detected within the limit of the apparatus used. At coverage above 70%, the sheet resistance decreased rapidly with the increase in coverage. This insulator-conductor transition indicates that a continuous twodimensional network of the conductive RuO2ns was formed at coverage above 70%, which is in good agreement with the AFM images shown in Figure 1. The electrical properties may be understood based on a 2D percolation theory, which describes the behavior of connection between permeable components randomly distributed in a 2D plane. According to this theory, the components form a continuous, long-range network (cluster) by connecting with each other. The size of the cluster will gradually grow with increase in population of the conducting component. At a percolation threshold, the clusters are connected with each other to form a continuous network (infinity size cluster), forming a permeable path. Of the many models proposed, a recent report regarding the continuum frontier-walk calculation on the percolation thresholds of disks with different radii28 seems to appropriately represent the present nanosheet film of which the permeable nanosheets randomly occupy the surface. According to the model proposed by Quintanilla and Ziff, the percolation threshold for disk-shaped components is 67.6%, which closely matches our experimental data of ∼70%. 3.4. Electrical Properties of Layer-by-Layer Assembled Multilayer Films of Ruthenate Nanosheets. The sheet resistance of self-assembled layer-by-layer multilayer films is plotted (27) The diameter of the cantilever was confirmed by intentionally making indents on the ruthenate nanosheet surface with the AFM tip, which was formed by repeatedly contacting the cantilever to the ruthenate nanosheet surface at the same position tens of times (see Supporting Information, Figure S4). (28) Quintanilla, J. A.; Ziff, R. M. Phys. Rev. E 2007, 76, 051115.
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as a function of the number of the (PVAm-PVA/RuO2ns) layers in Figure 6. The sheet resistance decreased depending on the number of layers, finally reaching a value of 0.36 kΩ sq-1 for the ten-layered film. Practical transparent conducting oxide films for optoelectronic applications generally require optical transmittance of more than 80% in the visible range. In the case of the RuO2ns film, this value was satisfied for layers less than three (see Supporting Information, Figure S7a).The three-layered film with transmittance of 85.3% at 550 nm exhibited sheet resistance of 2.3 kΩ sq-1, which is better than that of similar three-layered film of graphene obtained by the Langmuir-Blodgett method.11 The sheet resistance, however, is higher than a typical ITO film with thickness of 150-500 nm.3 The ratio of DC conductivity and optical conductivity, σDC/σOP, which can be used as figure of merit for transparent conductors29 was estimated and compared with that of the other materials. σDC/σOP was estimated based on following equation,29 T ¼
1þ
Z0 σOP 2Rs σ DC
- 2
where T is transmittance, Rs is sheet resistance, and Z0 is the impedance of free space. From the plot of T-1/2 - 1 versus Z0/2Rs, the slope of the plot is equal to σOP/σDC (see Supporting Information, Figure S7b).The σDC/σOP of self-assembled RuO2ns film was 1.6. This value is lower than that of ITO (several hundred) and carbon nanotube (maximum value = 35) but is higher than that of graphene films obtained by solution processing.29 Nonetheless, the RuO2ns film can be used in severely corrosive conditions where ITO would normally corrode such as in acids and bases. Furthermore, the room-temperature solution process allows formation of thin RuO2ns films on various substrates such as PET film, flexible PE films, as well as PBO, a well-known highmodulus and high-strength fibers (Figure 7). Even when such films and fibers are bended, folded, crumbled, or even tied, the electrical conduction is retained due to the high flexibility of RuO2ns.
4. Conclusions We have demonstrated the successful fabrication of flexible and transparent thin films consisting of ruthenate nanosheet coatings via self-assembly reaction of the anionic ruthenate nanosheets and a cationic polyvinylamine-polyvinylalcohol block copolymer. Nanoscale two-probe method based on AFM revealed that single isolated ruthenate nanosheet exhibits high conductivity with a sheet resistance of 12 kΩ sq-1. Conductivity measurements of self-assembled films with submonolayer to monolayer coverage revealed that the percolation threshold for conductivity was approximately 70% coverage, which is in good (29) De, S.; Coleman, J. N. ACS Nano 2010, 4, 2713–2720.
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agreement to calculations for two-dimensional percolated networks. The sheet resistance decreased with increasing layer numbers from 21 kΩ sq-1 for the monolayer film to 0.36 kΩ sq-1 for the 10-layered film. A three-layered film gave transmittance of 85.3% at 550 nm with a sheet resistance of 2.3 kΩ sq-1, which is superior to values reported for similar films composed of graphene. The findings in the present work should aid in understanding the mechanism of electron flow in nanosheet systems including graphene-related materials, which in turn may allow facile tuning of electronic or optoelectronic devices utilizing exfoliated nanosheets.30 Acknowledgment. This work was supported in part by a “Creation of Innovation Centers for Advanced Interdisciplinary Research Areas” Project in Special Coordination Funds for (30) (a) Yui, T.; Tsuchino, T.; Akatsuka, K.; Yamauchi, A.; Kobayashi, Y.; Hattori, T.; Haga, M.; Takagi, K. Bull. Chem. Soc. Jpn. 2006, 79, 386–396. (b) Akatsuka, K.; Ebina, Y.; Muramatsu, M.; Sato, T.; Hester, H.; Kumaresan, D.; Schmehl, R. H.; Sasaki, T.; Haga, M. Langmuir 2007, 23, 6730–6736. (c) Sakai, N.; Fukuda, K.; Omomo, Y.; Ebina, Y.; Takada, K.; Sasaki, T. J. Phys. Chem. C 2008, 112, 5197–5202.
18054 DOI: 10.1021/la103848f
Sato et al.
Promoting Science and Technology of the Ministry of Education, Culture, Sports, Science, and Technology, Japan and CREST of the Japan Science and Technology Agency (JST). Synchrotron radiated in-plane diffraction experiments were performed with the approval of the Photon Factory Program Advisory Committee (2009G546). The authors wish to thank Aiden Co., Ltd. for development of Automatic Multilayer Film Coater (MC-4000) and JEOL Ltd. for introduction of fine control system of piezo scanner into SPM (JSPM-5400). Supporting Information Available: Out-of-plane XRD pattern of multilayer film, in-plane XRD pattern of monolayer film before and after heating at 200 °C under vacuum, force curve and resistance data measured on contacting process with conducting cantilever to isolated single nanosheet, indentations produced by contacting with conducting cantilever, I-V measurement data of isolated single nanosheet, I-V characteristic of self-assembled film, electrical and optical properties of multilayered films. This material is available free of charge via the Internet at http://pubs.acs.org.
Langmuir 2010, 26(23), 18049–18054
