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Giant domain formation in Langmuir–Blodgett films based on Alkylammonium–Au(dmit)2 salt
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Thin Solid Films 516 (2008) 2518 – 2521 www.elsevier.com/locate/tsf
Giant domain formation in Langmuir–Blodgett films based on Alkylammonium–Au(dmit)2 salt Y.F. Miura ⁎, K. Hayashi, M. Kitao, M. Takagi, H. Matsui, M. Sugi Graduate School of Engineering, Toin University of Yokohama, 1614 Kurogane-cho, Aoba, Yokohama, 225-8502, Japan Available online 3 May 2007
Abstract The crossed polarized-light microscopy has revealed that the as-deposited LB film of the 2C14–Au(dmit)2 salt consists of giant domains of several hundred microns, which have a uni-axial molecular arrangement. By the electrochemical oxidation, the giant domains disappear and smaller domains of 5–10 μm appear. According to the atomic force microscopy (AFM) observation, the as-deposited film consists of domains of several microns but it appears that the observed domains are fragments formed by cracks generated in much larger domains, as seen by the shapes. It is hypothesized that there exist giant domains at the air/water interface but cracks are made during the transfer process. Thus, the uni-axial molecular arrangement is kept over the macroscopic range. Considering the random potentials set up by grain boundaries and/or defects, fabrication of the crack-free LB film is considered to be indispensable for realizing metallic properties down to much lower temperatures and global superconductivity. © 2007 Elsevier B.V. All rights reserved. Keywords: Langmuir–Blodgett film; Metal(dmit)2 anion; Giant domains; Uni-axial molecular arrangement
1. Introduction The Langmuir–Blodgett (LB) technique is a powerful tool that allows us to assemble organic molecules into tailored twodimensional (2D) molecular sheets with fairly low energy cost [1]. Among various trials for fabricating functional LB films, realization of highly conductive LB systems is one of the important subjects [2]. We have already reported that the LB films based on ditetradecyldimethylammonium–Au(dmit)2 [Fig. 1, 2C14–Au(dmit)2] salt exhibit a high room temperature conductivity of 40 S/cm with a metallic temperature dependence in the range of 230–300 K after electrochemical oxidation [3]. Furthermore, we have also reported that the ac magnetic susceptibility and resistance measurements suggest the existence of a superconducting phase below 4 K [4–6]. In general, at the air/water interface, charge–transfer (CT) complexes and radical salts tend to form domains whose typical sizes are several microns, in spite of the introduction of alkyl chains in the molecular systems. In the earlier stage of studies on the conductive LB films based on dialkyldimethylammonium– Au(dmit)2 salts, we utilized the electrode gap of 0.5 mm for ⁎ Corresponding author. Tel./fax: +81 45 974 5290. E-mail address: [email protected] (Y.F. Miura). 0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2007.04.137
measuring the resistance of the 2C14–Au(dmit)2 LB films, however, the gap distance might be too wide to obtain the intrinsic electrical properties because the film consists of domains of several microns in lateral size as revealed by atomic force microscopy (AFM) [7]. We have already reported that the metallic behavior of resistance of the 2C14–Au(dmit)2 LB films extends down to 58 K by utilizing an inter-digitated electrode gap of 5 μm [7]. We assume that the broad minimum in the resistance vs. temperature plot emerges in a crossover region, where metallic nature inside the domains are competing with the random potentials set up by grain boundaries and/or defects, and then, the metallic behavior extends down to lower temperatures by narrowing the electrode gap. Further morphological characterization is considered to be necessary. In this paper, we report on morphological studies on the 2C14–Au(dmit)2 LB film by means of crossed-polarized light microscopy and atomic force microscopy (AFM). 2. Experimental 2.1. Sample preparation The 2C14–Au(dmit)2 salt (Fig. 1) was synthesized following the procedure of Steimecke et al. [8] and spread on the surface of
Y.F. Miura et al. / Thin Solid Films 516 (2008) 2518–2521
Fig. 1. Molecular structure of the 2C14–Au(dmit)2 salt.
pure water (17 °C, Milli-Q SP reagent water system, Millipore Co., Ltd.) using a 1:1 mixture of acetonitrile and benzene as the spreading solvent. After keeping the salts on the water surface for 5 min, they were compressed up to 25 mN/m and then a Teflon plate with rectangular apertures of 13 mm × 38 mm was placed on the water surface. The 2C14–Au(dmit)2 film was divided in the rectangular fractions of 13 mm × 38 mm. By the horizontal lifting method, the films were transferred onto a quartz or Si(100) substrate. The dimensions of the quartz and Si(100) substrates are 1 mm × 13 mm × 38 mm and 0.5 mm × 13 mm × 38 mm, respectively. A NIMA Type 622 trough was used. For the morphological characterization by the crossedpolarized light microscopy and the conductivity measurement, a quartz substrate (Oken-Ohyo Koken Kogyo Co., Ltd.) was used. For the atomic force microscopy (AFM) observation, an Si (100) substrate, cut from a commercially available wafer (n-type, 1–10 Ω cm, Sumitomo Metal Fine Technology Co., Ltd.), was used. The quartz and Si(100) substrates were hydrophobized by keeping them in a container filled with the vapor of 1,1,1,3,3,3hexamethyldisilazane for more than a day.
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Onto the quartz substrate, gold was vacuum-evaporated through a mask to form four electrode strips separated by a gap of 0.5 mm, as shown in Fig. 2(a). Then, the 2C14–Au(dmit)2 films were deposited onto the electrode-coated surface by the horizontal lifting method, as shown in Fig. 2(b). After attaching copper lead wires to the electrodes using a silver paste (DOTITE Type D-550, Fujikura Kasei Co., Ltd.), as shown in Fig. 2(c), the as-deposited film was immersed in an aqueous solution of LiClO4 (0.1 M) and electrochemically oxidized by a constant current of 0.8 μA, as shown in Fig. 2(d). The gold electrode underneath the 2C14–Au(dmit)2 layer and a platinum rod were used as the working and counter electrodes, respectively. Positive potential was applied to the gold electrode. The details of the sample preparation are given in our previous papers [3–7,9]. 2.2. Characterization of the LB film The electrochemical oxidation was intervened every 30 min for the surface observation by crossed polarized-light microscopy and the measurement of conductivity. After the sample was taken out from the electrolyte, the film surface was rinsed by pure water and dried by nitrogen gas. The conductivity was then measured along the film plane using the gold electrode strips and the film surface was observed by crossed polarized-light microscopy (Olympus, Model BH-2 Microscope). The conductivity was measured by a dc two-probe method using a Keithley, Type 2001 digital multimeter at room temperature. The surface of the 2C14–Au(dmit)2 film transferred on the hydrophobized Si(100) surface was observed by a Seiko Instruments, Model SPI3800N atomic force microscope (AFM) using a tapping mode. A Seiko Instruments cantilever Model SI-DF20 was used. 3. Results and discussion Fig. 3 shows a typical change in conductivity as the electrochemical oxidation proceeds. As shown in Fig. 3, the lateral
Fig. 2. Schematic representation of the sample preparation and the method for the conductivity measurement and the optical microscopy observation of the 2C14–Au(dmit)2 LB films.
Fig. 3. The room-temperature conductivity of the 20-layered 2C14–Au(dmit)2 film plotted against the time of the electrochemical oxidation (Tox).
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Y.F. Miura et al. / Thin Solid Films 516 (2008) 2518–2521
conductivity increases with increasing the oxidation time (Tox) up to Tox = 90 min, but it turns to decrease modestly later. The broad peaks of the conductivity are mostly seen in the range Tox =90–120 min. We assume that the perchlorate anions are incorporated in the film for the compensation of the charge during the electrochemical oxidation and that the possible reaction is as follows: −δ Dþ ½AuðdmitÞ2 − þ ð1−δÞClO−4 →Dþ δ ½AuðdmitÞ2 þ − − þ ð1−δÞD ClO4 þ ð1−δÞe ;
where D + = ditetradecyldimethylammonium and δ = charge transfer ratio (0 b δ b 1) [9]. Since the electronic properties of the bulk metal(dmit)2 (metal = Ni and Pd) salts strongly depend on the values of the charge transfer ratio (δ), there should be a charge transfer ratio (δ) that realizes the maximum conductivity for the present thin-film system as well. We currently assume that the optimal charge transfer state is realized after the
Fig. 5. A typical AFM image of a 30 μm × 30 μm region of the 2C14–Au(dmit)2 film of a single layer transferred on the Si(100) substrate.
Fig. 4. The crossed-polarized light microscopy images of the as-deposited (a) and electrochemically oxidized LB films of 30 min (b) and 90 min (c).
constant current of 0.8 μA is applied for 90–120 min, although the changes in morphology and molecular disorders should be also taken into account. Infrared spectroscopic studies are now in progress for determining the charge transfer ratio (δ) and will be published elsewhere. Fig. 4(a) shows a typical crossed-polarized light microscopy image taken for the as-deposited 2C14–Au(dmit)2 LB film of 20 layers. As shown in Fig. 4(a), giant domains, whose sizes are reaching several hundred microns, are observed. Furthermore, when the specimen is rotated, the giant domains become dark in four positions 90° apart and they become bright in between the four extinction positions. These results indicate that the domains have a uni-axial molecular arrangement extended in the giant domains. The image changes as the electrochemical oxidation proceeds, as shown in Fig. 4(b) (electrochemically oxidized for 30 min) and 4(c) (electrochemically oxidized for 90 min). By the electrochemical oxidation of 30 min, the giant domains disappear and smaller domains of 5–10 μm are seen, as shown in Fig. 4(b). The smaller domains of 5–10 μm also become dark in four positions 90° apart and they become bright in between the four extinction positions, indicating the smaller domains also have a uni-axial molecular arrangement. No significant change is seen on the surface by further oxidation (90 min b Tox), as shown in Fig. 4(c). Fig. 5 shows a typical AFM image of the 2C14–Au(dmit)2 film of a single layer transferred on the Si(100) surface. The film consists of domains of several microns but it appears that the observed domains are fragments due to cracks generated in much larger domains, as seen by the shapes. We hypothesize that there exist giant domains of several hundred microns at the air/water interface but cracks are made during the film-lifting process because of the movement of the solid–liquid–gas contact line.
Y.F. Miura et al. / Thin Solid Films 516 (2008) 2518–2521
2521
Therefore, it is plausible that the uni-axial molecular arrangement is kept over the macroscopic range of several hundred microns despite the film is of fractions of several microns, as seen by AFM observation.
realizing metallic properties down to lower temperatures and global superconductivity.
4. Conclusion
This work was supported in part by Grant-in-Aid for Scientific Research of MEXT (Ministry of Education, Culture, Sports, Science and Technology, Japan) under Grant No. 12750019, University-Industry Joint Research Project for Private University: matching fund subsidy from MEXT, 2002–2007 and KAST (Kanagawa Academy of Science and Technology, Japan) under Grant No. 0012011.
The lateral conductivity of the 2C14–Au(dmit)2 film increases with increasing the oxidation time (Tox) up to Tox = 90 min, but it turns to decrease modestly later. The crossed polarized-light microscopy has revealed that as-deposited 2C14–Au(dmit)2 LB film consists of giant domains of several hundred microns and the domains have a uni-axial molecular arrangement. By the electrochemical oxidation, the giant domains disappear and smaller domains of 5–10 μm are seen by the crossed polarizedlight microscopy. The AFM observation has revealed that the asdeposited film consists of domains of several microns but it appears that the observed domains are fragments due to cracks generated in much larger domains, as seen by the shapes. Therefore, it is hypothesized that there exist giant domains at the air/water interface but cracks are made during the transfer process and the uni-axial molecular arrangement is kept over the macroscopic range of several hundred microns. We have already reported that the metallic behavior of resistance of the 2C14–Au(dmit)2 LB film extends down to the lower temperatures by narrowing the electrode gap from 0.5 mm down to 5 μm; for instance, by the gap of 0. 5 mm, metallic behavior is typically seen from room temperature down to 230 K, while the metallic behavior is seen from room temperature down to 58 K by the gap of 5 μm [7]. Considering the random potentials set up by grain boundaries and/or defects, fabrication of the crack-free LB film is considered to be indispensable for
Acknowledgment
References [1] A. Ulman, An Introduction to Ultrathin Organic Films: From Langmuir– Blodgett to Self-Assembly, Academic Press, San Diego CA, 1991. [2] T. Nakamura, in: H.S. Nalwa (Ed.), Handbook of organic conductive molecules and polymers, Electrically Conductive Langmuir–Blodgett Films, vol. 1, Wiley, New York, 1997, p. 728, Chap. 14. [3] Y.F. Miura, Y. Okuma, H. Ohnishi, T. Kawasaki, M. Sugi, Jpn. J. Appl. Phys. 37 (1998) L1481. [4] Y.F. Miura, M. Horikiri, S.-H. Saito, M. Sugi, Solid State Comm. 113 (2000) 603. [5] Y.F. Miura, M. Horikiri, S.-H. Saito, M. Sugi, Synth. Met. 120 (2001) 727. [6] Y.F. Miura, M. Horikiri, S. Tajima, T. Wakaita, S.-H. Saito, M. Sugi, Synth. Met. 133–134 (2003) 663. [7] M. Kitao, H. Matsui, S. Morita, Y.F. Miura, M. Sugi, M. Hedo, Y. Uwatoko, Trans. Mater. Res. Soc. Jpn. 31 (2006) 621. [8] G. Steimecke, H. J. Sieler, P. Kirmse, E. Hoyer, Phosphorus Sulfur 7 (1979) 49. [9] R. Watanabe, S. Morita, Y.F. Miura, M. Sugi, Trans. Mater. Res. Soc. Jpn. 30 (2005) 135.
Thin Solid Films 516 (2008) 2518 – 2521 www.elsevier.com/locate/tsf
Giant domain formation in Langmuir–Blodgett films based on Alkylammonium–Au(dmit)2 salt Y.F. Miura ⁎, K. Hayashi, M. Kitao, M. Takagi, H. Matsui, M. Sugi Graduate School of Engineering, Toin University of Yokohama, 1614 Kurogane-cho, Aoba, Yokohama, 225-8502, Japan Available online 3 May 2007
Abstract The crossed polarized-light microscopy has revealed that the as-deposited LB film of the 2C14–Au(dmit)2 salt consists of giant domains of several hundred microns, which have a uni-axial molecular arrangement. By the electrochemical oxidation, the giant domains disappear and smaller domains of 5–10 μm appear. According to the atomic force microscopy (AFM) observation, the as-deposited film consists of domains of several microns but it appears that the observed domains are fragments formed by cracks generated in much larger domains, as seen by the shapes. It is hypothesized that there exist giant domains at the air/water interface but cracks are made during the transfer process. Thus, the uni-axial molecular arrangement is kept over the macroscopic range. Considering the random potentials set up by grain boundaries and/or defects, fabrication of the crack-free LB film is considered to be indispensable for realizing metallic properties down to much lower temperatures and global superconductivity. © 2007 Elsevier B.V. All rights reserved. Keywords: Langmuir–Blodgett film; Metal(dmit)2 anion; Giant domains; Uni-axial molecular arrangement
1. Introduction The Langmuir–Blodgett (LB) technique is a powerful tool that allows us to assemble organic molecules into tailored twodimensional (2D) molecular sheets with fairly low energy cost [1]. Among various trials for fabricating functional LB films, realization of highly conductive LB systems is one of the important subjects [2]. We have already reported that the LB films based on ditetradecyldimethylammonium–Au(dmit)2 [Fig. 1, 2C14–Au(dmit)2] salt exhibit a high room temperature conductivity of 40 S/cm with a metallic temperature dependence in the range of 230–300 K after electrochemical oxidation [3]. Furthermore, we have also reported that the ac magnetic susceptibility and resistance measurements suggest the existence of a superconducting phase below 4 K [4–6]. In general, at the air/water interface, charge–transfer (CT) complexes and radical salts tend to form domains whose typical sizes are several microns, in spite of the introduction of alkyl chains in the molecular systems. In the earlier stage of studies on the conductive LB films based on dialkyldimethylammonium– Au(dmit)2 salts, we utilized the electrode gap of 0.5 mm for ⁎ Corresponding author. Tel./fax: +81 45 974 5290. E-mail address: [email protected] (Y.F. Miura). 0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2007.04.137
measuring the resistance of the 2C14–Au(dmit)2 LB films, however, the gap distance might be too wide to obtain the intrinsic electrical properties because the film consists of domains of several microns in lateral size as revealed by atomic force microscopy (AFM) [7]. We have already reported that the metallic behavior of resistance of the 2C14–Au(dmit)2 LB films extends down to 58 K by utilizing an inter-digitated electrode gap of 5 μm [7]. We assume that the broad minimum in the resistance vs. temperature plot emerges in a crossover region, where metallic nature inside the domains are competing with the random potentials set up by grain boundaries and/or defects, and then, the metallic behavior extends down to lower temperatures by narrowing the electrode gap. Further morphological characterization is considered to be necessary. In this paper, we report on morphological studies on the 2C14–Au(dmit)2 LB film by means of crossed-polarized light microscopy and atomic force microscopy (AFM). 2. Experimental 2.1. Sample preparation The 2C14–Au(dmit)2 salt (Fig. 1) was synthesized following the procedure of Steimecke et al. [8] and spread on the surface of
Y.F. Miura et al. / Thin Solid Films 516 (2008) 2518–2521
Fig. 1. Molecular structure of the 2C14–Au(dmit)2 salt.
pure water (17 °C, Milli-Q SP reagent water system, Millipore Co., Ltd.) using a 1:1 mixture of acetonitrile and benzene as the spreading solvent. After keeping the salts on the water surface for 5 min, they were compressed up to 25 mN/m and then a Teflon plate with rectangular apertures of 13 mm × 38 mm was placed on the water surface. The 2C14–Au(dmit)2 film was divided in the rectangular fractions of 13 mm × 38 mm. By the horizontal lifting method, the films were transferred onto a quartz or Si(100) substrate. The dimensions of the quartz and Si(100) substrates are 1 mm × 13 mm × 38 mm and 0.5 mm × 13 mm × 38 mm, respectively. A NIMA Type 622 trough was used. For the morphological characterization by the crossedpolarized light microscopy and the conductivity measurement, a quartz substrate (Oken-Ohyo Koken Kogyo Co., Ltd.) was used. For the atomic force microscopy (AFM) observation, an Si (100) substrate, cut from a commercially available wafer (n-type, 1–10 Ω cm, Sumitomo Metal Fine Technology Co., Ltd.), was used. The quartz and Si(100) substrates were hydrophobized by keeping them in a container filled with the vapor of 1,1,1,3,3,3hexamethyldisilazane for more than a day.
2519
Onto the quartz substrate, gold was vacuum-evaporated through a mask to form four electrode strips separated by a gap of 0.5 mm, as shown in Fig. 2(a). Then, the 2C14–Au(dmit)2 films were deposited onto the electrode-coated surface by the horizontal lifting method, as shown in Fig. 2(b). After attaching copper lead wires to the electrodes using a silver paste (DOTITE Type D-550, Fujikura Kasei Co., Ltd.), as shown in Fig. 2(c), the as-deposited film was immersed in an aqueous solution of LiClO4 (0.1 M) and electrochemically oxidized by a constant current of 0.8 μA, as shown in Fig. 2(d). The gold electrode underneath the 2C14–Au(dmit)2 layer and a platinum rod were used as the working and counter electrodes, respectively. Positive potential was applied to the gold electrode. The details of the sample preparation are given in our previous papers [3–7,9]. 2.2. Characterization of the LB film The electrochemical oxidation was intervened every 30 min for the surface observation by crossed polarized-light microscopy and the measurement of conductivity. After the sample was taken out from the electrolyte, the film surface was rinsed by pure water and dried by nitrogen gas. The conductivity was then measured along the film plane using the gold electrode strips and the film surface was observed by crossed polarized-light microscopy (Olympus, Model BH-2 Microscope). The conductivity was measured by a dc two-probe method using a Keithley, Type 2001 digital multimeter at room temperature. The surface of the 2C14–Au(dmit)2 film transferred on the hydrophobized Si(100) surface was observed by a Seiko Instruments, Model SPI3800N atomic force microscope (AFM) using a tapping mode. A Seiko Instruments cantilever Model SI-DF20 was used. 3. Results and discussion Fig. 3 shows a typical change in conductivity as the electrochemical oxidation proceeds. As shown in Fig. 3, the lateral
Fig. 2. Schematic representation of the sample preparation and the method for the conductivity measurement and the optical microscopy observation of the 2C14–Au(dmit)2 LB films.
Fig. 3. The room-temperature conductivity of the 20-layered 2C14–Au(dmit)2 film plotted against the time of the electrochemical oxidation (Tox).
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Y.F. Miura et al. / Thin Solid Films 516 (2008) 2518–2521
conductivity increases with increasing the oxidation time (Tox) up to Tox = 90 min, but it turns to decrease modestly later. The broad peaks of the conductivity are mostly seen in the range Tox =90–120 min. We assume that the perchlorate anions are incorporated in the film for the compensation of the charge during the electrochemical oxidation and that the possible reaction is as follows: −δ Dþ ½AuðdmitÞ2 − þ ð1−δÞClO−4 →Dþ δ ½AuðdmitÞ2 þ − − þ ð1−δÞD ClO4 þ ð1−δÞe ;
where D + = ditetradecyldimethylammonium and δ = charge transfer ratio (0 b δ b 1) [9]. Since the electronic properties of the bulk metal(dmit)2 (metal = Ni and Pd) salts strongly depend on the values of the charge transfer ratio (δ), there should be a charge transfer ratio (δ) that realizes the maximum conductivity for the present thin-film system as well. We currently assume that the optimal charge transfer state is realized after the
Fig. 5. A typical AFM image of a 30 μm × 30 μm region of the 2C14–Au(dmit)2 film of a single layer transferred on the Si(100) substrate.
Fig. 4. The crossed-polarized light microscopy images of the as-deposited (a) and electrochemically oxidized LB films of 30 min (b) and 90 min (c).
constant current of 0.8 μA is applied for 90–120 min, although the changes in morphology and molecular disorders should be also taken into account. Infrared spectroscopic studies are now in progress for determining the charge transfer ratio (δ) and will be published elsewhere. Fig. 4(a) shows a typical crossed-polarized light microscopy image taken for the as-deposited 2C14–Au(dmit)2 LB film of 20 layers. As shown in Fig. 4(a), giant domains, whose sizes are reaching several hundred microns, are observed. Furthermore, when the specimen is rotated, the giant domains become dark in four positions 90° apart and they become bright in between the four extinction positions. These results indicate that the domains have a uni-axial molecular arrangement extended in the giant domains. The image changes as the electrochemical oxidation proceeds, as shown in Fig. 4(b) (electrochemically oxidized for 30 min) and 4(c) (electrochemically oxidized for 90 min). By the electrochemical oxidation of 30 min, the giant domains disappear and smaller domains of 5–10 μm are seen, as shown in Fig. 4(b). The smaller domains of 5–10 μm also become dark in four positions 90° apart and they become bright in between the four extinction positions, indicating the smaller domains also have a uni-axial molecular arrangement. No significant change is seen on the surface by further oxidation (90 min b Tox), as shown in Fig. 4(c). Fig. 5 shows a typical AFM image of the 2C14–Au(dmit)2 film of a single layer transferred on the Si(100) surface. The film consists of domains of several microns but it appears that the observed domains are fragments due to cracks generated in much larger domains, as seen by the shapes. We hypothesize that there exist giant domains of several hundred microns at the air/water interface but cracks are made during the film-lifting process because of the movement of the solid–liquid–gas contact line.
Y.F. Miura et al. / Thin Solid Films 516 (2008) 2518–2521
2521
Therefore, it is plausible that the uni-axial molecular arrangement is kept over the macroscopic range of several hundred microns despite the film is of fractions of several microns, as seen by AFM observation.
realizing metallic properties down to lower temperatures and global superconductivity.
4. Conclusion
This work was supported in part by Grant-in-Aid for Scientific Research of MEXT (Ministry of Education, Culture, Sports, Science and Technology, Japan) under Grant No. 12750019, University-Industry Joint Research Project for Private University: matching fund subsidy from MEXT, 2002–2007 and KAST (Kanagawa Academy of Science and Technology, Japan) under Grant No. 0012011.
The lateral conductivity of the 2C14–Au(dmit)2 film increases with increasing the oxidation time (Tox) up to Tox = 90 min, but it turns to decrease modestly later. The crossed polarized-light microscopy has revealed that as-deposited 2C14–Au(dmit)2 LB film consists of giant domains of several hundred microns and the domains have a uni-axial molecular arrangement. By the electrochemical oxidation, the giant domains disappear and smaller domains of 5–10 μm are seen by the crossed polarizedlight microscopy. The AFM observation has revealed that the asdeposited film consists of domains of several microns but it appears that the observed domains are fragments due to cracks generated in much larger domains, as seen by the shapes. Therefore, it is hypothesized that there exist giant domains at the air/water interface but cracks are made during the transfer process and the uni-axial molecular arrangement is kept over the macroscopic range of several hundred microns. We have already reported that the metallic behavior of resistance of the 2C14–Au(dmit)2 LB film extends down to the lower temperatures by narrowing the electrode gap from 0.5 mm down to 5 μm; for instance, by the gap of 0. 5 mm, metallic behavior is typically seen from room temperature down to 230 K, while the metallic behavior is seen from room temperature down to 58 K by the gap of 5 μm [7]. Considering the random potentials set up by grain boundaries and/or defects, fabrication of the crack-free LB film is considered to be indispensable for
Acknowledgment
References [1] A. Ulman, An Introduction to Ultrathin Organic Films: From Langmuir– Blodgett to Self-Assembly, Academic Press, San Diego CA, 1991. [2] T. Nakamura, in: H.S. Nalwa (Ed.), Handbook of organic conductive molecules and polymers, Electrically Conductive Langmuir–Blodgett Films, vol. 1, Wiley, New York, 1997, p. 728, Chap. 14. [3] Y.F. Miura, Y. Okuma, H. Ohnishi, T. Kawasaki, M. Sugi, Jpn. J. Appl. Phys. 37 (1998) L1481. [4] Y.F. Miura, M. Horikiri, S.-H. Saito, M. Sugi, Solid State Comm. 113 (2000) 603. [5] Y.F. Miura, M. Horikiri, S.-H. Saito, M. Sugi, Synth. Met. 120 (2001) 727. [6] Y.F. Miura, M. Horikiri, S. Tajima, T. Wakaita, S.-H. Saito, M. Sugi, Synth. Met. 133–134 (2003) 663. [7] M. Kitao, H. Matsui, S. Morita, Y.F. Miura, M. Sugi, M. Hedo, Y. Uwatoko, Trans. Mater. Res. Soc. Jpn. 31 (2006) 621. [8] G. Steimecke, H. J. Sieler, P. Kirmse, E. Hoyer, Phosphorus Sulfur 7 (1979) 49. [9] R. Watanabe, S. Morita, Y.F. Miura, M. Sugi, Trans. Mater. Res. Soc. Jpn. 30 (2005) 135.