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Adsorption Processes of Self-Assembled Monolayers Made from Terphenyl Thiols Takao Ishida,*,†,‡ Wataru Mizutani,§ Hiroaki Azehara,§,| Fuminobu Sato,| Nami Choi,⊥ Uichi Akiba,| Masamichi Fujihira,| and Hiroshi Tokumoto§ Institute for Mechanical Systems Engineering (IMSE), National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8564, Japan, PRESTOsJapan Science and Technology Corporation (JST), 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan, Joint Research Center for Atom Technology (JRCAT), Nanotechnology Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki, 305-8562, Japan, Department of Biomolecular Engineering, Faculty of Bioengineering and Biotechnology, and Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8501, Japan, and JRCAT, Angstrom Technology Partnership (ATP), Tsukuba, Ibaraki, 305-0046, Japan Received March 1, 2001. In Final Form: August 21, 2001 We investigated the adsorption processes of terphenyl (TP) derivatized thiols, [1,1′:4′,1′′-terphenyl]4-thiol (TP0), which form self-assembled monolayers (SAMs) on Au(111). Scanning tunneling microscopy observation revealed that the adsorption process is dependent on the solvent in which the TP0 molecules can dissolve. When methylene chloride was used as a solvent, the TP0 molecules nucleate anisotropically along 〈112〉 directions with a 3-fold symmetry at the initial stage. At 1 min of immersion, a phase-separated image was taken. In the topographically lower region, molecular lattices (a ) 0.65 ( 0.05 nm, b ) 1.3 ( 0.05 nm) appeared, where the TP0 molecules were arranged parallel to the Au surface. After more than 5 min of immersion, the molecular lattices disappeared and larger striped patterns with a spacing of ca. 8 nm were observed. On the other hand, when ethanol was used as a solvent, the adsorption process of the TP0 molecules completely changed, and such larger striped patterns were not observed after 1 day of immersion. Our data demonstrate that ethanol facilitated the formation of the more densely packed TP0 SAMs than methylene chloride solvent.
Introduction Recently, self-assembled monolayers (SAMs)1 of conjugated molecules have been extensively studied.2-27 The conjugated molecules are useful for future molecular * To whom correspondence should be addressed. E-mail:
[email protected]. TEL: +81-298-61-7203. FAX; +81-298-61-7844. † IMSE-AIST. ‡ PRESTOsJST. § JRCAT-AIST. | Tokyo Institute of Technology. ⊥ JRCAT-ATP. (1) Ulman, A. Chem. Rev. 1996, 96, 1533. (2) Tour, J. M.; Jones, II, L.; Pearson, D. L.; Lamba, J. J.; Burgin, T. P.; Whitesides, G. M.; Allara, D. L.; Parkh, A. N.; Atre, S. V. J. Am. Chem. Soc. 1995, 117, 9529. (3) Zehner, R. W.; Parsons, B. F.; Hsung, R. P.; Sita, L. R. Langmuir 1999, 15, 1121. (4) Sabatani, E.; Cohne-Boulakia, J.; Bruening, M.; Rubinstein, I. Langmuir 1993, 9, 2974. (5) Tao, Y.-T.; Wu, C.-C.; Eu, J.-Y.; Lin, W.-L.; Wu, K.-C.; Chen, C. Langmuir 1997, 13, 4018. (6) Himmel, H.-J.; Terfort, A.; Wo¨ll, Ch. J. Am. Chem. Soc. 1998, 120, 12069. (7) Geyer, W.; Stadler, V.; Eck, W.; Zharnikov, M.; Golzhauser, A.; Grunze, M. Appl. Phys. Lett. 1999, 75, 2401. (8) Kang, J. F.; Ulman, A.; Liao, S.; Jordan, R.; Yang, G.; Liu, L.-Y. Langmuir 2001, 17, 95. (9) Nakamura, T.; Kondoh, H.; Matsumoto, M.; Nozoe, H. Langmuir 1996, 12, 5977. (10) Dishner, M. H.; Hemminger, J. C.; Feher, F. J. Langmuir 1996, 12, 6176. (11) (a) Bumm, L. A.; Arnold, J. J.; Cygan, M. T.; Dunbar, T. D.; Jones, L., II; Allara, D. L.; Tour, J. M.; Weiss, P. S. Science 1996, 271, 1705. (b) Cygan, M. T.; Dunbar, T. D.; Arnold, J. J.; Bumm, L. A.; Shedlock, N. F.; Burgin, T. P.; Jones, L., II; Allara, D. L.; Tour, J. M.; Weiss, P. S. J. Am. Chem. Soc. 1998, 120, 2721. (12) Leatherman, G.; Durantini, E. N.; Gust, D.; Moore, T. A.; Moore, A. L.; Stone, S.; Zhou, Z.; Lez, P.; Liu, Y. Z.; Lindsay, S. M. J. Phys. Chem. B 1999, 103, 4006. (13) Kelly, K. F.; Shom, T.-S.; Lee, T. R.; Halas, N. J. J. Phys. Chem. B 1999, 103, 8639.
electronics applications because they are considered to be electrically conductive. The electronic properties of these SAMs are influenced by the molecular arrangement and surface morphology of the conjugated molecules. It is, therefore, important to control the molecular ordering and arrangement during SAM formation. The adsorption process of n-alkanethiol SAMs onto an Au(111) surface have been studied extensively by means of contact angle,28 X-ray photoelectron spectroscopy (XPS),29,30 second har(14) Kergueris, C.; Bourgoin, J. P.; Palacin, S. Nanotechnology 1999, 10, 8. (15) Reed, M. A.; Zhou, C.; Muller, C. J.; Burgin, T. P., Tour, J. M. Science 1997, 278, 252. (16) Zhou, C.; Deshpande, M. R.; Reed, M. A.; Jones II, L.; Tour J. M. Appl. Phys. Lett. 1997, 71, 611. (17) Chen, J.; Reed, M. A.; Rawlett, A. M.; Tour, J. M. Science 1999, 286, 1550. (18) Ishida T.; Mizutani, W.; Akiba, U.; Umemura, K.; Inoue, A.; Choi, N.; Fujihira, M.; Tokumoto, H. J. Phys. Chem. B 1999, 103, 1686. (19) Mizutani, W., Ishida, T.; Tokumoto, H. Jpn. J. Appl. Phys. 1999, 38, 3892. (20) Ishida, T.; Mizutani, W.; Tokumoto, H.; Choi, N.; Akiba, U.; Fujihira, M. J. Vac. Sci. Technol., A 2000, 18, 1437. (21) Ishida, T.; Mizutani, W.; Choi, N.; Akiba, U.; Fujihira, M.; Tokumoto, H. J. Phys. Chem. B 2000, 104, 11860. (22) Zharnikov, M.; Frey, S.; Rong, H.; Yang, Y.-J.; Heister, K.; Buck, M.; Grunze, M. Phys. Chem. Chem. Phys. 2000, 2, 3359 (23) Frey, S.; Stadler, K.; Heister, K.; Eck, W.; Zharnikov, M.; Grunze, M.; Zeysing, B.; Terfort, A. Langmuir 2001, 17, 2408. (24) Heister, K.; Zharnikov, M.; Grunze, M.; Johanson, L. S. O. J. Phys. Chem. B 2001, 105, 4058. (25) Leung, T. Y. B.; Schwartz, P.; Scoles, G.; Schreiber, F.; Ulman, A. Surf. Sci. 2000, 458, 34. (26) Dhirani, Al-A.; Zehner, R. W.; Hsung, R. P.; Guyot-Sionnest, P.; Sita, L. R. J. Am. Chem. Soc. 1996, 118, 3319. (27) Yang, G.; Qian, Y.; Engtrakul, C.; Sita, L. R.; Liu, G.-Y. J. Phys. Chem. B 2000, 104, 9059. (28) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321. (29) Buck, M.; Eisert, F.; Fischer, J.; Grunze, M.; Tra¨ger, F. J. Appl. Phys. A 1991, 53, 552.
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monic generation (SHG),29 and scanning probe microscopy (SPM).31-35 However, the adsorption processes of conjugated molecules onto a gold surface has not been studied systematically. Terphenyl (TP) or biphenyl (BP) derivatized thiol SAMs are representative types of the conjugated molecular SAMs.4-8,18-25 Sabatani et al. first assembled BP and TP onto a gold surface.4 They predicted, using molecular mechanics simulations, that the adsorbed terphenylthiols may contain large regions of (x3×x3)R30° overlayers. Tao et al.5 studied the effect of the methylene spacer of the conjugated molecules on the molecular arrangement on the Au surface. We have also confirmed the influence of the methylene spacers on the molecular arrangement as well as on monolayer electrical conduction.21 Our SPM images clearly indicated that the TP derivatives exhibited (x3×x3)R30° structures when the methylene group was present. On the other hand, for the terphenylthiol without a methylene group, the molecules adsorbed on the Au surface exhibited specific larger striped patterns whose average spacing between the strips was about 8 nm (the image of this larger striped pattern is shown in Figure 1), and thus molecularly resolved SPM images could not be obtained.21 On the contrary, Kang et al. observed molecularly resolved SPM images of 4-chlorobiphenylthiol SAM without a methylene group on the Au(111) surface.8 Recently, Zharnikov and co-workers investigated the structures of BP derivatized thiol SAMs on Au and Ag substrates using X-ray adsorption fine structure spectroscopy (NEXAFS)22,23 and high-resolution XPS.22-24 They concluded that highly oriented and densely packed SAMs formed on both the Au and Ag substrates. They also found an even-odd effect in the number of methylene groups on the molecular orientation.22 Thus, the molecular arrangement and ordering of TP and BP derivaized thiol SAMs became controversial, and further investigations are now necessary. In addition, recent data seem to be contrary to our previous SPM observation of the TP0 SAMs. However, from the viewpoint of future molecular electronics application, there is a distinct advantage of using conjugated molecules without a methylene group, because a direct connection between the conjugated rings and the Au surface is expected to facilitate smooth electrical transport at the molecular/ metal interface. We found that the solvents in which the TP molecules were dissolved were different and, as such, the solvent is likely to be one factor causing the above discrepancy. For example, the research groups that obtain densely packed SAMs made from conjugated molecules without a methylene group used ethanol as the solvent. However, we previously used methylene chloride, because it is difficult to dissolve conjugated molecules into ethanol. Tao et al. used a mixed tetrahydrofuran and ethanol solution for the same reason.5 On the other hand, since it was reported that the highly ordered SAMs were successfully obtained in the case of oligo(phenylethynyl)(30) (a) Ishida, T.; Nishida, N.; Tsuneda, S.; Hara, M.; Sasabe H.; Knoll, W. Jpn. J. Appl. Phys. 1996, 35, L1710. (b) Ishida, T.; Hara, M.; Kojima, I.; Tsuneda, S.; Nishida, N.; Sasabe H.; Knoll, W. Langmuir 1998, 14, 2092. (31) (a) Poirier, G. E.; Pylant, E. D. Science 1996, 272, 1145. (b) Poirier, G. E. Langmuir 1997, 13, 2019. (c) Poirier, G. E. Langmuir 1999, 15, 1167. (32) Kondoh, H.; Kodama, C.; Nozoye, H. J. Phys. Chem. B 1998, 102, 2310. (33) Tamada, K.; Hara, M.; Sasabe H.; Knoll, W. Langmuir 1997, 13, 1558. (34) (a) Yamada, R.; Uosaki, K. Langmuir 1997, 13, 5218. (b)Yamada, R.; Uosaki, K. Langmuir 1998, 14, 855. (35) Kawasaki, M.; Sato, T.; Tanaka, T.; Takao, K. Langmuir 2000, 16, 1719.
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arenethiol without a methylene group SAMs,26,27 when the methylene chloride was used as a solvent, such a solvent dependence may be only seen in the terphenyl or biphenyl thiol SAMs. In the present study, we investigated the adsorption processes of terphenylthiols with scanning tunneling microscopy (STM) to clarify the influence of solvents on the TP SAM morphology. We found that an anisotropic nucleation formed larger striped patterns when methylene chloride solvent was used, but these larger striped patterns did not appear among the terphenylthiol SAMs even after 1 day of immersion in ethanol solvent. Experimental Section Here we used the following molecules: [1,1′:4′,1′′-terphenyl]4-thiol (TP0) which was synthesized by the same way as Sabatani et al.4
The molecules were dissolved in methylene chloride or ethanol. To prepare the SAMs, we used atomically flat Au(111) surface deposited on mica. The Au(111) surface was epitaxially grown on mica by vacuum deposition under a base pressure of about 4 × 10-8 Torr. The mica was preheated at 440 °C for 4 h before the deposition. The deposition rate of Au was kept at 0.2 nm/s. After the deposition, the substrate was annealed at 480 °C for 30-60 min to obtain a large terrace on the Au surface. The flatness of the terrace on the Au surface was checked using scanning tunneling microscopy (STM) and was found to be atomically flat over 200 nm. The Au substrate was immersed in a 0.1 mM solution for 10 s to 24 h. The adsorption process of SAMs has been recently studied at very low concentration solution (ca. 10-2-10-4 mM) and revealed that the adsorption process is dependent on the concentration.33-35 However, we used the 0.1 mM concentration in order to reveal the adsorption of SAMs under the same condition as before. After the Au substrates were taken out of the solution, they were rinsed with pure methylene chloride or ethanol to remove the physisorbed multilayer. Then the surface structure was observed by STM (Seiko Instruments SPA340 unit in air) with a typical tunneling current of 20-100 pA and a tip bias of 0.5 V.
Results and Discussion Figure 1 shows a series of STM images of the TP0 SAMs formed in a methylene chloride solution. The 23×x3 reconstruction, which appeared on the clean Au(111) surface (Figure 1a),36,37 quickly disappeared after 10 s of immersion in the solvent, and elongated domains (hereafter R phase) are clearly seen on the Au surface (Figure 1b). These R domains are aligned to three of the 〈112〉 directions of the Au(111) surface. After the surfaces were dipped in the TP0 solution for 1 min (Figure 1c), about 70-80% of the Au surface was covered with TP0 molecules. There exist two kinds of molecular domains (denoted β and χ in Figure 1d), i.e., phase separation. Many small χ protrusions are visible between the two larger long β domains. The difference in height between the lower β domains and χ protrusions was measured to be 0.5-0.7 nm. In domain β, we observed molecular lattice structures (Figure 1d). In the small χ protrusions, we could not see any molecularly resolved images. Figure 1e shows a molecularly resolved STM image of the β phase. We also displayed the cross-sectional profiles of lines A and B. Along line A, the spacing of each spot (which correspond (36) Wo¨ll, Ch.; Chiang, S.; Wilson, R. J.; Lippel, P. H. Phys. Rev. B 1989, 39, 7988. (37) Mizutani, W.; Ohi, A.; Motomatsu, M.; Tokumoto, H. Jpn. J. Appl. Phys. 1995, 34, L1151.
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Figure 1. STM images for the bare Au surface and TP0 SAMs formed in methylene chloride solvent: (a) bare Au(111) surface before SAM formation; (b) TP0 SAM formed by 10 s of immersion. We denoted R in (b); (c) TP0 SAM formed by 1 min of immersion; (d) magnified image of (c). We denoted the β and χ phases in (d); (e) magnified image of β phase. Lines A and B in the image e express the cross-sectional profiles of the β phase; (f) TP0 SAM formed by 5 min of immersion. We denoted δ in (f); (g) TP0 SAM formed by 1 day of immersion. We also denoted δ. All images were taken in constant current mode at a bias of 0.50 V and a current of 20-100 pA.
to intermolecular spacing) and molecular level corrugation were 0.65 ( 0.05 nm and ca. 0.05 nm, respectively. For line B, the spacing of each spot was 1.35 ( 0.05 nm. The angle between lines A and B was 50 ( 5°. Since the
difference in height between the β and χ phase was 0.50.7 nm, the TP0 molecules in the β phase domains were considered to be arranged almost parallel to the Au surface. The detailed molecular arrangement will be described
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Figure 2. STM images for the TP0 SAMs formed in ethanol solvent: (a) TP0 SAM formed by 10 s of immersion; (b) TP0 SAM formed by 1 min of immersion; (c) magnified image of (b) We denoted in (c); (d) TP0 SAM formed by 1 h of immersion. We denoted φ in (d); (e) magnified image of (d). Lines C and D in the image e express the cross-sectional profiles of the φ phase; (f) TP0 SAM formed by 1 day of immersion We denoted γ and η in (d); (g) magnified image of (f) Line E in the image f indicates the cross-sectional profiles of the γ phase. All images were taken in constant current mode at a bias of 0.50 V and a current of 20 pA.
later. After 5 min of immersion, the surface was fully covered with the TP0 molecules with larger striped patterns (hereafter δ phase) (Figure 1f). The δ phase structure may be identical to that of the χ phase. After more than 5 min of immersion, we could not observe molecular ordered β structures even in the magnified image (data are not shown). A similar STM image was obtained after 1 day of immersion (Figure 1g). The average spacing between the strips of the δ phase was about 8 nm. Figure 2 shows STM images of the TP0 SAMs formed in ethanol solution. At 10 s of immersion, the Au surface would be fully covered with TP0 molecules so we could not observe elongated domains such as the R phase as shown Figure 1a. The surface morphology did not changed much after 1 min of immersion (Figure 2b). The average
domain size was less than 10 nm. In the magnified image (Figure 2c), the molecular lattice was confirmed (hereafter phase). For the phase, the spacings of each spot were 0.65 ( 0.05 nm and 1.25 ( 0.04 nm, respectively. Since these spacings were almost identical to those of the β phase, we considered that the phase correspond to the β phase of the TP0 SAMs prepared in the methylene chloride solvent. Thus, even when ethanol solvent was used, at the initial stage of SAM growth, the lying down phases appeared.31-35 The SAM surface after 1 h of immersion was likely to be disordered (Figure 2d). However, in the case of ethanol solvent, the larger striped patterns such as the δ phase were not seen even after 1 h of immersion, while clear square molecular lattices were observed in the magnified image (Figure 2e, hereafter φ
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Figure 3. Proposed molecular lattices of (a) β or phase and (b) φ phase.
phase). For the φ phase, the spacing of each spot along both lines C and D was 0.75 ( 0.05 nm. This detailed molecular arrangement will also be described later. After 1 day of immersion, the SAM surface morphology became rougher (Figure 2f). In the magnified image (Figure 2g), we observed hexagonal-like molecular lattices (hereafter γ phase). We could not confirm clear domain boundaries in the STM images. The average spacing of each spot of γ phase along line E was about 0.5 nm, and this value was similar to that of well-ordered (x3×x3)R30° structures.38,39 Thus, for the γ phase, the TP0 molecules may arrange with (x3×x3)R30° structure whose intermolecular spacing is expected to be ca. 0.5 nm, which is seen in typical alkanethiol SAMs38,39 and TP SAMs with a methylene group.18,21 However, since a clearer image than Figure 2g was not obtained, we could not conclude that the γ phase correspond to (x3×x3)R30° structure at the moment. In Figure 2f, small protrusions were observed (hereafter η phase). Hexagonal-like molecular lattices were also seen at the η phase (data are not shown). Figure 3 shows the proposed models of molecular arrangements observed in the TP0 SAMs. In all the cases, we assume that the sulfur atoms locate at the 3-fold hollow sites.40 For the β or phase, the TP0 molecules may favor the sp3 configuration,5 because the 1.3 nm of spacing for each spot is consistent with the expected intermolecular (38) Poirier, G. E.; Tarlov, M. J. Langmuir 1994, 10, 2853. (39) Delamarche, E.; Michel, B.; Gerber, C.; Anselmetti, D.; Guntherodt, H.-J.; Wolf, H.; Ringsdorf, H. Langmuir 1994, 10, 2869. (40) Sellers, H.; Ulman, A.; Shindman Y.; Eilers, J. E. J. Am. Chem. Soc. 1993, 115 9389.
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spacing value when the TP0 molecules are arranged with the sp3 configuration. Thus, we propose the following TP0 molecular arrangement of the β or phase as shown in Figure 3a. These β and phases should correspond to the lying down phase which is often observed at the initial stage of alkanethiol SAM growth in low concentration solutions33,34 or in vapor phase deposition.31,32 In addition, in the case of BP SAMs, the presence of a lying down phase was confirmed.25 For the φ phase, we propose a square-type lattice model as shown in Figure 3b. The intermolecular spacing is expected to be ca. 0.75 nm. Since the molecular packing of the TP0 molecules gradually increased with the immersion time in ethanol solvent, our data showed that ethanol solvent facilitated the formation of the more densely packed SAMs of TP0 SAMs than methylene chloride solvent. However, in the case of oligo(phenylethynyl)arenethiol without a methylene group,26,27 highly ordered SAMs were successfully obtained when the methylene chloride was used as a solvent. Thus, such a solvent dependence can only be seen in the terphenyl or biphenyl thiol SAMs. Even in the case of ethanol solvent, it has been reported that there are difficulties in the preparation of the commensurate domains of TP or BP SAMs without a methylene group.24,25 Thus, other factors should exist to determine the molecular density, e.g., strength of intermolecular interaction, molecular length, dihedral angle of TP group, etc. In any case, further study concerning to find the controlling in the molecular packing and the density on the SAMs of conjugated molecules is required. Conclusions We investigated the adsorption processes of SAMs of terphenyl-derivatized thiols using STM. The adsorption process was strongly affected by the organic solvents in which the molecules were dissolved. In methylene chloride solvent, an anisotropic nucleation along the 〈112〉 direction occurred at the initial stage of the TP0 SAM growth. At 1 min of immersion, they were phase-separated. After more than 5 min of immersion, the ordered β phases disappeared and changed into the δ domain via χ phase. We could not observe molecular lattice of the TP0 SAMs after immersion for more than 5 min. On the other hand, we found that the δ phase domains did not form when we used ethanol as the solvent. In ethanol, the molecular packing of the TP0 molecules increased with the immersing time, while the domain size was small. Our data demonstrate that ethanol facilitated the formation of the more densely packed TP0 SAMs than methylene chloride solvent. Acknowledgment. This work was partly supported by the New Energy and Industrial Technology Development Organization (NEDO) of Japan. We thank Dr. S. Sasaki and Dr. K. Miyake (AIST) for their helpful suggestions. LA010322W
