Surface Structure of Binary Self-Assembled Monolayers Formed by

Department of Energy and Hydrocarbon Chemistry, Graduate School of ... SAM of 1-undecanethiol (UDT) and 3-mercaptopropionic acid (MPA) is used as a ...
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Langmuir 1999, 15, 5073-5078

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Surface Structure of Binary Self-Assembled Monolayers Formed by Electrochemical Selective Replacement of Adsorbed Thiols Daisuke Hobara,† Takayuki Sasaki,‡,§ Shin-ichiro Imabayashi,‡,⊥ and Takashi Kakiuchi*,† Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University, Kyoto 606-8501, Japan, and Department of Physical Chemistry, Yokohama National University, Yokohama 240-8501, Japan Received November 23, 1998. In Final Form: April 22, 1999 Nanometer-scale structure of binary thiol self-assembled monolayers (SAMs) prepared by the electrochemical selective replacement of adsorbed thiols in phase-separated binary SAMs has been studied by scanning tunneling microscopy (STM) and cyclic voltammetry. A phase-separated binary SAM of 1-undecanethiol (UDT) and 3-mercaptopropionic acid (MPA) is used as a starting system. MPA, which has a less negative desorption potential, is selectively removed by controlling the electrode potential, and replaced with 1-hexadecanethiol (HDT) or 11-mercaptoundecanoic acid (MUA). STM images of the UDT layer remaining after the selective removal of MPA exhibit two types of domains: domains with striped structure and domains where thiol molecules are densely packed, suggesting that some UDT molecules are reoriented and form low-density regions where the molecules are aligned flat on the surface. STM images of UDT-HDT and UDT-MUA binary SAMs formed by the replacement show surfaces which are phase-separated in nanometer scale. The size of the domains after the replacement is approximately equal to that of the initial phase-separated UDT-MPA SAM, indicating that the domain size is not significantly affected by the replacement.

Introduction We recently reported a new method of forming phaseseparated binary thiol self-assembled monolayers (SAMs) using electrochemical selective replacement of adsorbed thiols.1 The method is based on the potential dependence of the reductive desorption of thiols.2-7 A binary SAM formed by coadsorption of long- and short-chain alkanethiols such as 1-hexadecanethiol (HDT) and 3-mercaptopropionic acid (MPA) exhibits phase separation in nanometer scale.8 Domains consisting of MPA can be selectively reduced by controlling the potential of a substrate in an aqueous alkaline solution, as MPA has a less negative reduction potential than HDT.1,7-9 The reduction of MPA, which is highly soluble in an aqueous * To whom correspondence should be addressed. Telephone: +81-75-753-5528. Fax: +81-75-753-3360. E-mail: kakiuchi@scl. kyoto-u.ac.jp. † Kyoto University. ‡ Yokohama National University. § Present address: aMEITEC Corp., Japan. ⊥ Present address: Department of Chemistry and Biotechnology, Yokohama National University, Yokohama 240-8501, Japan. (1) Imabayashi, S.; Hobara, D.; Kakiuchi, T.; Knoll, W. Langmuir 1997, 13, 4502-4504. (2) Widrig, C. A.; Chung, C.; Porter, M. D. J. Electroanal. Chem. 1991, 310, 335-359. (3) Walczak, M. M.; Popenoe, D. D.; Deinhammer, R. S.; Lamp, B. D.; Chung, C.; Porter, M. D. Langmuir 1991, 7, 2687-2693. (4) Weisshaar, D. E.; Lamp, B. D.; Porter, M. D. J. Am. Chem. Soc. 1992, 114, 5860-5862. (5) Weisshaar, D. E.; Walczak, M. M.; Porter, M. D. Langmuir 1993, 9, 323-329. (6) Lamp, B. D.; Hobara, D.; Porter, M. D.; Niki, K.; Cotton, T. M. Langmuir 1997, 13, 736-741. (7) Imabayashi, S.; Iida, M.; Hobara, D.; Feng, Z. Q.; Niki, K.; Kakiuchi, T. J. Electroanal. Chem. 1997, 428, 33-38. (8) Hobara, D.; Ota, M.; Imabayashi, S.; Niki, K.; Kakiuchi, T. J. Electroanal. Chem. 1998, 444, 113-119. (9) Nishizawa, M.; Sunagawa, T.; Yoneyama, H. J. Electroanal. Chem. 1997, 436, 213-218.

alkaline solution, leads to selective removal of MPA from the surface.1,9 Other alkanethiols having different chain lengths and terminal groups may be adsorbed in the MPAdesorbed region, yielding phase-separated binary SAMs of various combination of alkanethiols. This method is advantageous in forming phase-separated binary SAMs composed of two thiols which would form homogeneously mixed SAMs if prepared by coadsorption from mixed ethanol solutions. However, the relationship between the structure of a starting SAM and that of a final SAM after the selective replacement has not been examined. In the present paper, we report the nanometer-scale structure of the SAMs prepared by the electrochemical selective replacement of adsorbed thiols. Two different monolayers composed of 1-undecanethiol (UDT) and HDT, and UDT and 11-mercaptoundecanoic acid (MUA), were formed on Au(111) surfaces by the selective replacement of adsorbed MPA in a phase-separated monolayer of UDTMPA. STM measurements and cyclic voltammetry for the reductive desorption of adsorbed thiols revealed the composition and structure of adsorbed thiol molecules at each step of the selective replacement. Experimental Section MPA (Dojindo Laboratories), UDT (Tokyo Kasei Co.), and HDT (Aldrich Chemical Co.) were used without further purification. MUA was synthesized according to the procedure.10 Water was distilled and purified through a Milli-Q system (Millipore Co.). All other chemicals were of reagent grade and used without purification. Au substrates were prepared by vapor deposition of Au (99.99%) on freshly cleaved mica sheets (Nilaco Co.) which were baked at 580 °C prior to the deposition and maintained at 580 °C during the deposition.7,11 Binary SAMs of UDT and MPA (10) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321-335. (11) DeRose, J. A.; Thundat, T.; Nagahara, L. A.; Lindsay, S. M. Surf. Sci. 1991, 256, 102-108.

10.1021/la981631y CCC: $18.00 © 1999 American Chemical Society Published on Web 06/29/1999

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were prepared by immersing the Au substrates for 25 ( 5 h in an ethanol solution of the two thiols where the total thiol concentration was kept at 1 mM. The substrates were then rinsed with ethanol and dried in air. The composition of the binary SAMs was controlled by changing the ratio of the two thiols in a mixed thiol solution. STM measurements were made under ambient conditions using NanoScope E (Digital Instruments) equipped with a low-current amplifier (Model CSTMLC, Digital Instruments) which allows measurements with the lowest practical tunneling current in the picoampere range. All images were obtained in the constant-current mode with mechanically cut Pt|Ir tips. Cyclic voltammetry for the reductive desorption of adsorbed thiols was used to examine a surface composition at each process of the replacement. The voltammograms were recorded in a deaerated 0.5 mol dm-3 KOH aqueous solution at 20 mV s-1. A thiol-adsorbed Au substrate was mounted at the bottom of a cone-shaped cell using an elastic O-ring. An Ag|AgCl (saturated KCl) reference electrode was used. All measurements were made at 20 ( 2 °C.

Results and Discussion Characterization of SAMs of UDT and MPA before and after Reductive Desorption of MPA. A binary SAM composed of UDT and MPA was first prepared as a starting system by the coadsorption of UDT and MPA from a mixed ethanol solution. Figure 1a shows an STM image of a binary SAM of UDT and MPA on Au(111). Islands, whose size ranged from 10 to 20 nm in diameter, appeared on Au(111) terraces in the image. These islands are similar to those previously observed for phaseseparated HDT-MPA SAMs,8 where two types of domains having different heights were distinguished. In the crosssectional height profile shown in Figure 1a the apparent height of the islands is ∼0.4 nm, which is slightly smaller than that of the HDT-MPA SAMs,8 suggesting that the MPA-rich and UDT-rich domains correspond to the darker and brighter domains observed in Figure 1a, respectively.8 Figure 1b shows a cyclic voltammogram for the binary SAM of UDT and MPA. Two distinct cathodic peaks appeared at -1.05 and -0.80 V in the voltammogram. These peaks are attributable to the reductive desorption of UDT-rich and MPA-rich domains, since single-component SAMs of HDT and MPA give reductive desorption peaks at -1.07 and -0.74 V, respectively. The mole fraction of the MPA-rich domains in the binary SAM in Figure 1b was estimated to be 0.68 from the area of voltammetric peaks. The mole fraction estimated from the voltammogram agrees well with that estimated from the STM image in Figure 1a (0.70 for MPA-rich domains). Thus, Figure 1, a and b, confirms that the SAM composed of UDT and MPA is in a phase-separated state like HDTMPA binary SAMs.8 The peak potential for the reductive desorption of UDT is more positive than that of a singlecomponent UDT SAM, while the peak potential of MPA is more negative than that of a single-component MPA SAM. The slight shifts of the peak potentials in Figure 1b with respect to those of the single-component SAMs probably originate from the dissolution of the thiols into the other type of domains. The larger shift of the peak potential for the reductive desorption of the MPA-rich domains indicates the greater extent of the dissolution of UDT into the MPA-rich domains than that of MPA into the UDT-rich domains.8 For the reductive desorption of MPA, the potential of a UDT-MPA coadsorbed substrate was held at -0.85 V for 45 min in a 0.5 M KOH solution. The substrate was then rinsed with water. Figure 2a shows a cyclic voltammogram of reductive desorption recorded for a substrate after this treatment. The peak for the reduction of MPA observed in Figure 1b at -0.8 V disappeared in the

Figure 1. (a) An STM image (400 nm × 400 nm), and (b) a cyclic voltammogram for the reductive desorption, of a binary SAM of UDT and MPA. The mole fraction of MPA in a solution from which the monolayer was formed was 0.91. A crosssectional profile is along the line drawn on the STM image. Bias voltage 1.5 V; setpoint 8 pA. The scan rate for the voltammogram was 20 mV s-1.

voltammogram, while the area and the potential of the peak for UDT domains were almost unaltered. Our previous study using in-situ STM and cyclic voltammetry showed that, after the electrochemical desorption, MPA molecules dissolve away into the aqueous solution phase.12 The disappearance of the peak of MPA confirms the selective removal of the MPA-rich domains from the surface. Figure 2b shows an STM image of the surface after the selective desorption of MPA. The image mainly shows three features; domains observed as the brightest tone, pits in the darkest tone, and striped phases in an (12) Hobara, D.; Miyake, K.; Imabayashi, S.; Niki, K.; Kakiuchi, T. Langmuir 1998, 14, 3590-3596.

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Figure 3. Cyclic voltammograms for the reductive desorption of binary SAMs of UDT and HDT after the selective replacement of MPA with HDT. The mole fractions of MPA in solutions from which starting systems (binary SAMs of UDT and MPA) were formed were (a) 0.82, (b) 0.88, and (c) 0.91. The scan rate was 20 mV s-1.

Figure 2. (a) A cyclic voltammogram for the reductive desorption, and (b) an STM image (120 nm × 120 nm), of adsorbed UDT after the selective desorption of MPA from a binary SAM of UDT and MPA. The mole fraction of MPA in a solution from which the UDT-MPA SAM was formed was 0.91. A cross-sectional profile is along the line drawn on the STM image. Bias voltage 1.5 V; setpoint 8 pA. The scan rate for the voltammogram was 20 mV s-1.

intermediate tone (shown by arrows). A molecularly resolved STM image of the brighter domains showed (x3 × x3)R30° array of thiol molecules (data not shown), indicating that the islands are composed of closely packed UDT molecules. The other regions in Figure 2b, including the pits, are covered with the striped structure. The interrow spacing of the stripes in Figure 2b is 1.7 ( 0.2 nm, which is comparable to the length of a UDT molecule. The cross-sectional height profile along the line drawn on the STM image in Figure 2b shows that the depth of the pits is ∼0.3 nm which is close to the Au(111) single-atom step height (0.24 nm). The height profile also shows that the apparent height of the islands composed of the closely packed thiol molecules with respect to that of the stripe is ∼0.2 nm, which is approximately twice larger than that of HS(CH2)6OH.13 The stripes are aligned with three

different directions (shown by arrows). The angle between the rows of the stripes is 60°, which probably reflects the symmetry of Au(111) lattice. These features of the observed striped phases are similar to the structure reported at a low coverage of alkanethiol molecules on Au (111),13-17 suggesting that the stripes are composed of thiol molecules lying flat on the surface.13,15 It is thus likely that, after the selective desorption of MPA, the part of UDT molecules at the edges of the domains fall over to the Au surface and spread out to cover the region where MPA molecules were initially adsorbed. SAMs of UDT and HDT Prepared by Electrochemical Replacement and Coadsorption. The substrates with MPA-desorbed UDT-MPA binary SAM were immersed in a 1 mM HDT ethanol solution for 1.5 h to form a binary SAM of UDT and HDT. Within this relatively short immersion time, the exchange of adsorbed UDT with HDT is small.18 Figure 3 shows cyclic voltammograms for the reductive desorption of UDT-HDT SAMs which were prepared by the selective replacement of MPA with HDT in starting systems of UDT-MPA binary SAMs having three different compositions. The composition of the starting systems was varied by changing the ratio of mixed solutions from which the monolayers were formed. All voltammograms exhibit a single peak for the reductive desorption of adsorbed thiol molecules. However, the peak became broader as the ratio of MPA in the starting system was increased. The reoxidation peak at ∼-0.95 V in the (13) Poirier, G. E.; Pylant, E. D. Science 1996, 272, 1145-1148. (14) Poirier, G. E. Langmuir 1997, 13, 2019-2026. (15) Poirier, G. E. Chem. Rev. 1997, 97, 1117-1127. (16) Yamada, R.; Uosaki, K. Langmuir 1997, 13, 5218-5221. (17) Yamada, R.; Uosaki, K. Langmuir 1998, 14, 855-861. (18) Kakiuchi, T.; Sato, K.; Iida, M.; Hobara, D.; Imabayashi, S.; Niki, K. Manuscript in preparation.

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reverse scan also grew with increasing the ratio of MPA in the mixed solution. Since a single-component HDT SAM gives a relatively larger reoxidation peak than a UDT SAM,2,12,19 the voltammograms in Figure 3 indicate that the larger amount of HDT molecules were adsorbed on the surface with increasing the ratio of MPA in the starting system. Two distinct peaks were not resolved in Figure 3, unlike the case of the binary UDT-MUA SAMs prepared by the selective replacement.1 The reduction potentials of singlecomponent SAMs of UDT and HDT are -1.07 and -1.13 V, respectively, at the scan rate of 20 mV s-1. Both of the single-component SAMs give a sharp reduction peak (fwhm ) ∼25 mV). The two peaks would therefore be distinguished if the two thiols are completely phaseseparated into sizable domains. The single peak in the voltammograms shown in Figure 3 suggests that the mixing of the two thiols occurs to some extent.8,20,21 A small peak such as that appearing at -0.9 V in Figure 3c is sometimes observed at a more positive potential than that of the main reduction peak in a voltammogram for the reductive desorption of SAMs prepared by the selective replacement. The peak would reflect the presence of disordered regions due to the insufficiency of the immersion time for HDT molecules to be organized. Figure 4 shows STM images of the UDT-HDT SAMs prepared by the selective replacement of MPA with HDT. The conditions for the replacement are the same as those in voltammograms b and c in Figure 3. The stripes seen in Figure 2b no longer existed. Instead, two types of domains having different heights were observed. The height difference of the two domains is ∼0.4 nm, as can be seen in the cross-sectional height profile of the STM image of Figure 4a. This is slightly smaller than the ∼0.6 nm expected from the size of the thiol molecules extended with a 30° tilt angle.10 The measured height difference depends on the condition of the tip and usually the height from STM images of thiol SAMs tends to be smaller than the actual value.13,22 The area of the brighter domains increased as the ratio of MPA increased in the starting system (Figure 4, a and b), suggesting that the brighter domains mainly consist of HDT molecules. The STM image of Figure 4c shows (x3 × x3)R30° structure of thiol molecules on the brighter domains (shown by arrows). The image indicates that the domains consist of closely packed molecules. On the other hand, darker domains of the STM image of Figure 4c show irregular heights where a molecularly resolved image was hardly obtained. It is, therefore, most probable that a certain number of HDT molecules are accommodated in darker UDT domains, while the brighter domains mainly consist of HDT molecules. Such incorporation of HDT molecules into UDTrich domains is caused either by surface diffusion of HDT molecules or by the exchange reaction of adsorbed UDT molecules with HDT in the ethanol solution of HDT during the immersion of the substrate for the adsorption of HDT.23-25 The size of the HDT domains ranges from 10 to 20 nm in diameter. The domain size is similar to that (19) Zhong, C.-J.; Porter, M. D. J. Electroanal. Chem. 1997, 425, 147-153. (20) Arnold, S.; Feng, Z. Q.; Kakiuchi, T.; Knoll, W.; Niki, K. J. Electroanal. Chem. 1997, 438, 91-97. (21) Kakiuchi, T.; Iida, M.; Gon, N.; Hobara, D.; Imabayashi, S.; Niki, K. Manuscript in preparation. (22) Han, T.; Thomas P. Beebe, J. Langmuir 1994, 10, 2705-2709. (23) Bain, C. D.; Evall, J.; Whitesides, G. M. J. Am. Chem. Soc. 1989, 111, 7155-7164. (24) Chidsey, C. E. D.; Bertozzi, C. R.; Putvinski, T. M.; Mujsce, A. M. J. Am. Chem. Soc. 1990, 112, 4301-4306. (25) Biebuyck, H. A.; Whitesides, G. M. Langmuir 1993, 9, 17661770.

Figure 4. STM images of binary SAMs of UDT and HDT after the selective replacement of MPA with HDT. The mole fractions of MPA in solutions from which starting systems (binary SAMs of UDT and MPA) were formed were (a) and (b) 0.88, and (c) 0.91. A cross-sectional profile is along the line drawn on the STM image. Scan area: 170 nm × 170 nm for (a) and (b), 25 nm × 25 nm for (c); bias voltage 1.5 V; setpoint 9 pA for (a) and (c), and 13 pA for (b).

of the starting system, indicating that the domain size is maintained after the replacement. It is noteworthy that

Surface Structure of Self-Assembled Monolayers

Figure 5. (a) An STM image (170 nm × 170 nm), and (b) a cyclic voltammogram for the reductive desorption, of a UDTHDT binary SAM formed by coadsorption of UDT and HDT from a mixed solution. The mole fraction of UDT in a solution from which the SAM was formed was 0.8. Bias voltage 2.0 V; setpoint 14 pA. The scan rate for the voltammogram was 20 mV s-1.

pits are hardly seen on the HDT domains generated by the replacement. The phase separation of binary SAMs of UDT and HDT in a similar scale as observed in Figure 4 cannot be attained with a SAM prepared by coadsorption of these thiols. Figure 5a shows an STM image of a binary UDT-HDT SAM formed by the coadsorption from a mixed solution of UDT and HDT. The image is different from the SAM prepared by the selective replacement shown in Figure 4 and shows no distinctive domains as in Figure 1a, indicating a higher degree of mixing of the two thiols. Small brighter spots (less than 4 nm in diameter) appearing in Figure 5a probably correspond to the HDTrich regions. Figure 5b shows a cyclic voltammogram obtained for the same substrate as used in Figure 5a. The peak at -1.1 V for the reductive desorption of the SAM formed by the coadsorption is sharper than that for the SAM prepared by the replacement (Figure 3b), indicating that the state of the thiol molecules is more homogeneous in the UDT-HDT SAM formed by the coadsorption from an ethanol solution. The difference in the shape of the

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Figure 6. (a) A cyclic voltammogram for the reductive desorption, and (b) an STM image (170 nm × 170 nm), of a binary SAM of UDT and MUA after the selective replacement of MPA with MUA. The mole fraction of MPA in a solution from which a starting system (a binary SAM of UDT and MPA) was formed was 0.91. A cross-sectional profile is along the line drawn on the STM image. Bias voltage 1.5 V; setpoint 8 pA. The scan rate for the voltammogram was 20 mV s-1.

voltammograms in Figures 3b and 5b thus reflects the difference in the mixing state of the thiols as shown in the STM images in Figures 4a and 5a. SAMs of UDT and MUA Prepared by Electrochemical Replacement. The electrochemical selective replacement was also performed to yield a phase-separated binary SAM of UDT and MUA, which would form a homogeneously mixed binary SAM if prepared by the coadsorption.21 Figure 6a shows a cyclic voltammogram for the reductive desorption of a SAM of UDT and MUA formed by the selective replacement of MPA with MUA. The SAM was prepared by the same procedure as the SAMs in Figure 3, i.e., holding the electrode potential at -0.85 V for 45 min and immersing the substrate in 1 mM MUA ethanol solution for 1.5 h. Unlike the case of the SAM of UDT and HDT, two distinct peaks appeared at -1.05 and -0.95 V, which are attributable to the reduction

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of UDT and MUA, respectively.21 The mole fraction of the MUA domains in the binary SAM in Figure 6a was estimated to be 0.65 from the area of voltammetric peaks. The mole fraction agrees well with that of the MPA domains in the starting system of a UDT-MPA binary SAM. The peak potential of UDT is 0.02 V more positive than that for a single-component UDT SAM, while the peak potential of MUA is 0.05 V more negative than that for a single-component MUA SAM. The shifts of the peak potentials suggest that UDT is dissolved into the domains of MUA to some extent, and vice versa, though phaseseparated. The larger shift of the peak potential of MUA suggests the greater extent of the dissolution of UDT into the domain mainly composed of MUA. In spite of the presence of the mutual solubility, each domain gives distinct voltammetric peaks due to the different properties of the terminal functional groups of UDT and MUA. The STM image of the SAM shown in Figure 6b shows two types of domains with a different contrast, indicating that the two thiols forms a phase-separated monolayer as in the case of the UDT-HDT SAMs. The different contrast of a UDT-rich domain and an MUA-rich domain probably arises from the difference in the tunneling mechanism. It is likely that water molecules adsorbed on the hydrophilic MUA-rich domains play an important roll in making the different contrast of the domains.26 In Figure 6b, there exist domains whose size is more than 30 nm in diameter. The domains are larger than those for UDT-HDT SAMs prepared by the replacement (Figure 4). This would also be the reason for the distinct voltammetric peaks appeared in Figure 6a. Conclusions STM images of remained UDT molecules after the selective desorption of MPA suggest that the UDT molecules diffuse to the thiol-free region and cover all the surface with two different orientations. Such self-repairing (26) Poirier, G. E.; Pylant, E. D.; White, J. M. J. Chem. Phys. 1996, 105, 2089-2092.

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of surface coating would be important for corrosion prevention using adsorbed molecules.27-32 STM images of the SAMs after the selective replacement show that the domains composed of the replaced molecules are several tens of nanometers in diameter, indicating that the domain size of the SAM formed by the replacement reflects the initial phase-separated SAM. A desired two-dimensional distribution of phase-separated domains can be obtained by controlling the domain size of a starting SAM system which functions as a template. SAMs prepared by the present method can be used for fundamental studies of surface properties of SAMs, e.g., wetting properties in terms of microscopic structure,33 and surface diffusion of adsorbed thiol molecules using artificially phase-separated SAMs.34 Acknowledgment. This work was partially supported by Research Fellowships of the Japan Society for the Promotion of Science for Young Scientists (D.H.), the Saneyoshi Scholarship Foundation (S.I.), the Ministry of Education, Science, Sports and Culture, Japan (T.K., Grant-in-Aid for exploratory research, no. 08640769 and 09875208,Grant-in-AidforPriorityResearch,no.11118235), and Casio Science Promotion Foundation (T.K.). LA981631Y (27) Laibinis, P. E.; Whitesides, G. M. J. Am. Chem. Soc. 1992, 114, 9022-9028. (28) Volmer-Uebing, M.; Stratmann, M. Appl. Surf. Sci. 1992, 55, 19-35. (29) Li, Y.-Q.; Chailapakul, O.; Crooks, R. M. J. Vac. Sci. Technol. B 1995, 13, 1300-1306. (30) Nozawa, K.; Nishihara, H.; Aramaki, K. Corros. Sci. 1997, 39, 1625-1639. (31) Zamborini, F. P.; Crooks, R. M. Langmuir 1997, 13, 122-126. (32) Feng, Y.; Teo, W.-K.; Siow, K.-S.; Gao, Z.; Tan, K.-L.; Hsieh, A.-K. J. Electrochem. Soc. 1997, 144, 55-64. (33) Imabayashi, S.; Gon, N.; Sasaki, T.; Hobara, D.; Kakiuchi, T. Langmuir 1998, 14, 2348-2351. (34) Imabayashi, S.; Hobara, D.; Kakiuchi, T. Manuscript in preparation.