Annealing Effect of Self-Assembled Monolayers Generated from

These data demonstrate that the CH3-terphenyl-derivatized thiol SAM had the ..... (32) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whiteside...
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Langmuir 2002, 18, 83-92

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Annealing Effect of Self-Assembled Monolayers Generated from Terphenyl Derivatized Thiols on Au(111) Takao Ishida,*,†,‡ Hitoshi Fukushima,§,| Wataru Mizutani,⊥,# Satoru Miyashita,§ Hisato Ogiso,†,⊥ Koichi Ozaki,† 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, Technology Platform Research Center (TPRC), SEIKO-EPSON Corporation, Fujimi Plant 281, Fujimi-machi, Suwa-gun, Nagano 392-8502, Japan, Joint Research Center for Atom Technology (JRCAT), National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki, 305-8562, Japan, and Nanotechnology Research Institute (NRI), National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki, 305-8562, Japan Received December 30, 2000. In Final Form: June 26, 2001 We investigated an annealing effect of self-assembled monolayers (SAMs) generated from derivatives of terphenylmethanethiols and tetradecanethiol (C14) on Au(111) using scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy, and contact angle measurement. STM data revealed that the molecules were partially desorbed by the annealing process at temperatures where the contact angle of the hexadecane began to decrease. The decreases in contact angles of both the CF3- and CH3-terminated terphenyl SAMs were caused by annealing at 180 °C for 1 h, while a similar decrease began at a lower temperature of 150 °C in the C14 SAMs. From the STM data, a large amount of CH3-terminated terphenyl molecules remained after annealing at 180 °C for 3 h, while a small amount of molecules were observed in the other two SAMs. These data demonstrate that the CH3-terphenyl-derivatized thiol SAM had the highest thermal stability of the three SAMs, and the molecular backbone structure and end group were crucial for determining the thermal stability of the SAMs.

Introduction 1

Self-assembled monolayers (SAMs) have been attracting much attention recently. However, from the viewpoint of their future industrial application, unanswered questions still exist. The thermal stability of the SAMs is one of the particularly important issues for future potential SAM applications, for applications such as surface coating, molecular devices, etc. The thermal stability of n-alkanethiol SAMs has been investigated;2,3 e.g., Delamarche et al. reported on the annealing effect of dodecanethiol SAMs at around 100 °C.2 During the annealing process, the depressions which are typically observed in alkanethiol SAMs almost disappeared due to the molecular diffusion, and then, larger single molecular domains without depressions were formed. When the annealing was continued, the dodecanethiolates began to desorb from the Au surface. After 48 h of annealing at the same temperature, these dodecanethiolates were almost desorbed. A similar tendency was confirmed with a radio raveled technique.3 * To whom correspondence should be addressed: e-mail, [email protected]; tel, +81-298-61-7203; fax, +81-298-61-7844. † IMSE-AIST. ‡ PRESTO- JST. § SEIKO-EPSON. | Present address: JRCHMM-JCII, c/o AIST-Tsukuba Central 5, Tsukuba, Ibaraki 305-8565, Japan. ⊥ JRCAT-AIST. # NRI-AIST. (1) Ulman, A., An Introduction to Ultrathin Organic Films From Langmuir-Blodgett to Self-Assembly; Academic Press: New York ,1991. Ulman, A. Chem. Rev. 1996, 96, 1533. (2) Delamarche, E.; Michel, B.; Kang, H.; Gerber, Ch. Langmuir 1994, 10, 4103. (3) Schlenoff, J.; Li, M.; Ly, H. J. Am. Chem. Soc. 1995, 117, 12528.

Formerly, to increase the thermal stability of SAMs on the Au surface, different strategies were employed, e.g., lateral polymerization,4 hydrogen bond network formation derived from amide groups,5 insertion of a underpotentially deposited metal layer between the molecules and surface,6 and double binding molecules.7 Another possible method to increase thermal stability and avoid molecular decomposition is to utilize rigid molecules such as conjugated molecules, e.g., oligophenylenes,8-13 because these oligophenylene SAMs are expected to be rigid due to the presence of phenyl rings. However, the thermal stability of oligophenylene SAMs had not been investigated so far. The wetting property of the SAMs is another important research subject for applications such as a surface coating techniques. In particular, fluorinated alkanethiol or disulfide SAMs have been the most studied examples of functionalized SAMs because these molecules can produce (4) Batchelder, P. N.; Evans, T. H.; Freeman, T.L.; Haussling, L.; Ringsdorf, H. J. Am. Chem. Soc. 1994, 116, 1050. (5) Clegg, R. S.; Reed, S. M.; Hutchison, J. E. J. Am. Chem. Soc. 1996, 118, 2486. (6) Jeannings, G. K.; Laibinis, P. E. Langmuir 1996, 12, 6137. Jeannings, G. K.; Laibinis, P. E. J. Am. Chem. Soc. 1997, 119, 5208. (7) Mizutani, W.; Motomatsu, M.; Tokumoto, H. Thin Solid Films 1996, 273, 70. (8) Sabatani, E.; Cohne-Boulakia, J.; Bruening, M.; Rubinstein, I. Langmuir 1993, 9, 2974. (9) Tao, Y.-T.; Wu, C.-C.; Eu, J.-Y.; Lin, W.-L.; Wu, K.-C.; Chen, C. Langmuir 1997, 13, 4018. (10) Himmel, H.-J.; Terfort, A.; Wo¨ll, Ch. J. Am. Chem. Soc. 1998, 120, 12069. (11) Ishida, T.; Choi, N.; Mizutani, W.; Tokumoto, H.; Kojima, I.; Azehara, H.; Akiba, U.; Hokari, H.; Fujihira, M. Langmuir 1999, 15, 6799. (12) Ishida T.; Mizutani, W.; Akiba, U.; Umemura, K.; Inoue, A.; Choi, N.; Fujihira, M.; Tokumoto, H. J. Phys. Chem. 1999, B103, 1686. (13) Ishida T.; Mizutani, W.; Choi, N.; Akiba, U.; Fujihira, M.; Tokumoto, H. J. Phys. Chem. 2000, B104, 11680.

10.1021/la001816g CCC: $22.00 © 2002 American Chemical Society Published on Web 12/08/2001

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Langmuir, Vol. 18, No. 1, 2002 Chart 1. Molecular Structures of Conjugated Molecules Used in This Study

highly hydrophobic surfaces.14-27 Among these SAMs, the presence of alkyl spacer groups is well-known to influence the molecular assembly of surfactants. Fukushima et al. examined the influence of alkyl spacers on the thermal stability of semifluorinated alkanethiol SAMs and concluded that thermal stability of these SAMs increased as the length of the methylene spacer was increased.24 However, from the viewpoint of nanoscale tribological applications, e.g., coating onto the microchannel of fluid, highly hydrophobic SAMs of shorter molecular length are required, because the thick film itself prevents the smooth transport of fluid as the channel size decreases from microscale to nanoscale. Thus, if the longer alkyl spacer can be replaced by a shorter backbone spacer, like terphenyl, we could expect the formation of thin and robust SAMs with a highly hydrophobic surface. In the present study, we newly synthesized CF3- and CH3-terminated terphenyl derivative thiols ([1,1′:4′,1′′terphenyl]-1-methyl-4-methanethiol (CH3-TP1) and [1,1′:4′,1′′-terphenyl]-4-methanethiol (CF3-TP1)), to investigate the role of the terphenyl group as a spacer instead of methylene groups. We chosen derivatives of terphenylmethanethiols because these conjugated molecules form a well-ordered structure on the Au(111) surface, i.e., (x3 × x3)R30° structures, when the molecule has one methylene group between the sulfur and aromatic rings.9 The molecular structures are shown in Chart 1. For comparison, we also used tetradecanethiol (C14) SAMs, which have a molecular length that is identical to both the TP1 molecules. The annealing effects of these SAMs were examined to understand the thermal stability and annealing mechanism of conjugated molecular SAMs using scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and contact angle techniques. (14) Alves, C. A.; Porter, M. D. Langmuir 1993, 9, 3507. (15) Liu, G.; Fenter, P.; Eberhardt, A.; Chidsey, C. E. D.; Ogletree, D. F.; Eisenberger P.; Salmeron, M. J. Chem. Phys. 1994, 101, 4301. (16) Tsao, M. W.; Hoffman, C. L.; Rabolt, J. F.; Johnson, H. E.; Castner, D. G.; Erdelen, C.; Ringsdorf, H. Langmuir 1997, 13, 4317. (17) Lenk, T. J.; Hallmark, V. M.; Hoffmann, C. L.; Rabolt, J. F.; Castner, D. G.; Erdelen, C.; Ringsdorf, H. Langmuir 1994, 10, 4610. (18) Scho¨nherr, H.; Vansco, G. J. Langmuir 1997, 13, 3769. (19) Scho¨nherr, H.; Ringsdorf, H. Langmuir 1996, 12, 3891. (20) Scho¨nherr, H.; Ringsdorf, H.; Jaschke, M.; Butt, H. J.; Bamberg, E.; Allinson, H.; Evans, S. D. Langmuir 1996, 12, 3898. (21) Ishida, T.; Yamamoto, S.-I.; Mizutani, W.; Motomatsu, M.; Tokumoto, H.; Hokari, H.; Azehara, H.; Fujihira, M. Langmuir 1997, 13, 3261. (22) Tsao, M. W.; Rabolt, J. F.; Scho¨nherr, H.; Castner, D. G. Langmuir 2000, 16, 1734. (23) Tamada, K.; Nagasawa, J.; Nakanishi, F.; Abe, K.; Hara, M.; Knoll, W.; Ishida, T.; Fukushima, H.; Miyashita, S.; Usui, T.; Koini, T.; Lee, T. R. Thin Solid Films 1998, 150, 327. (24) Fukushima, H.; Seki, S.; Nishikawa, T.; Takiguchi, H.; Tamada K.; Abe, K.; Colorado, R., Jr.; Graupe, M.; Shmakova, O. E.; Lee, T. R. J. Phys. Chem. 2000, B104, 7417. (25) Tamada, K.; Ishida, T.; Knoll, W.; Fukushima, H.; Colorado, R., Jr.; Graupe, M.; Shmakova O. E.; Lee, T. R. Langmuir 2001, 17, 1913. (26) Kim, H. I.; Koini, T.; Lee, T. R.; Perry, S. S Langmuir 1997, 13, 7192. (27) Miura, Y. F.; Takenaga, M.; Koini, T.; Graupe, M.; Garg, N.; Graham, R. L., Jr.; Lee, T. R. Langmuir 1998, 14, 5821.

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Results STM Observation of the SAMs before and after Annealing. Figure 1 shows STM images of the C14 SAMs. The cross-sectional profiles are shown except for a molecular resolution image. After the immersion of C14 solution for 24 h, we could see (x3 × x3)R30° structures with a molecular distance of ca. 0.5 nm, with many depressions (Figure 1a,b).28,29 The root-mean-square (rms) values were measured to discuss the surface roughness quantitatively, except for the STM images after 180 °C for 3 and 5 h. The rms of the as deposited C14 SAM was 8.6 × 10-2 nm. When the SAM was annealed at 130 °C for 1 h, the depressions were gathered due to Ostwalt ripening (Figure 1c).2,30,31 Even after the annealing, ordered structures were confirmed in the magnified image (Figure 1d). The rms value became 6.5 × 10-2 nm, indicating that the annealed SAM surface roughness became slightly smooth. At domain boundaries, some disordered molecular structures were seen. After the sample was annealed at 150 °C for 1 h (Figure 1e), ordered structures were seen in some regions, whereas molecular desorbed regions appeared in other parts. The rms value increased to 1.2 × 10-1 nm. The depth between the lowest and the highest parts was about 0.2 nm. The height difference was smaller than expected from the C14 SAM thickness (ca. 1.56 nm).32 This phenomenon is explained by the poor electrical conduction of the C14 molecules.33 The C14 SAM surface was completely disordered by the annealing at 180 °C for 1 h (Figure 1f). The rms value increased to 1.6 × 10-1 nm. After the sample was annealed at 180 °C for 3 h, small protrusions with diameters of 5-10 nm and heights of 0.2-0.3 nm were observed (Figure 1g). Note that no striped phases which appear at low coverage33,34 were observed. Finally, the structures of C14 molecules almost disappeared by annealing at 180 °C for 5 h (Figure 1h). Figure 2 shows STM images of the CH3-TP1 SAMs. As the deposited SAMs exhibited well-ordered structures similar to (x3 × x3)R30° with many depressions (panels a and b of Figure 2), the surface morphology was similar to that of C14 SAMs.9 The rms value was 5.2 × 10-2 nm. The depth of the depressions was about 0.2 nm,13 and we confirmed the presence of molecules in these depressions. After the sample was annealed at 130 °C for 1 h, the surface morphology seems to be rough (Figure 2c). However, the rms value was slightly larger than that of as deposited CH3-TP1 SAM. The ordered structures with a spacing of each spot of 0.5 nm were still observed (Figure 2d). However, this is not typical of (x3 × x3)R30° structures and looks like an incommensurate square type lattice. Since after the annealing at a higher temperature we observed typical (x3 × x3)R30° structures (cf. Figure 2), this square type arrangement might be an intermediate phase. Also, this phase might be formed by an increase in the surface roughness. We did not determine the detailed molecular arrangement because this may be due (28) Poirier, G. E.; Tarlov, M. J. Langmuir 1994, 10, 2853. (29) Delamarche, E.; Michel, B.; Gerber, C.; Anselmetti, D.; Guntherodt, H.-J.; Wolf, H.; Ringsdorf, H. Langmuir 1994, 10, 2869. (30) Bumm, L. A.; Arnold, J. J.; Charles, L. S.; Dunbar, T. D.; Allara, D. L.; Weiss, P. S. J. Am. Chem. Soc. 1999, 121, 8017. (31) Yamada, R.; Wano, H.; Uosaki, K. Langmuir 2000, 16, 5523. (32) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321. (33) Poirier, G. E.; Pylant, E. D. Science 1996, 272, 1145. (34) (a) Poirier, G. E. Langmuir 1997, 13, 2019. Poirier, G. E. Langmuir 1999, 15, 1167. Poirier, G. E.; Fitts, W. P.; White, J. M. Langmuir 2001, 17, 1176. (b) Kondoh, H.; Kodama, C.; Nozoye, H. J. Phys. Chem. B 1998, 102, 2310. (c) Yamada, R.; Uosaki, K. Langmuir 1997, 13, 5218. Yamada, R.; Uosaki, K. Langmuir 1998, 14, 855. (d) Kawasaki, M.; Sato, T.; Tanaka, T.; Takao, K. Langmuir 2000, 16, 1719.

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Figure 1. STM images for C14 SAMs before and after annealing: (a) C14 SAM formed by 24 h of immersion in C14 solution; (b) magnified image of (a); (c) after annealing at 130 °C for 1 h; (d) magnified image of (c); (e) after annealing at 150 °C for 1 h; (f) after annealing at 180 °C for 1 h; (g) after annealing at 180 °C for 3 h; (h) after annealing at 180 °C for 5 h. Cross-sectional profiles are shown except for the magnified images. All the images were taken by a constant current mode at a bias of -1.8 V and a current of 20 pA.

Figure 2. STM images of the CH3-TP1 SAMs before and after annealing: (a) CH3-TP1 SAM formed by a 24 h immersion; (b) magnified image of (a); (c) after annealing at 130 °C for 1 h; (d) magnified image of (c); (e) after annealing at 150 °C for 1 h; (f) after annealing at 180 °C for 1 h; (g) after annealing at 180 °C for 3 h; (h) magnified image of (g) on the molecular remained area. Cross-sectional profiles are shown except for the magnified images. All the images were taken by a constant current mode at a bias of -0.5 V and a current of 20 pA.

to an increase in the Au roughness. After the sample was annealed at 150 °C for 1 h, the surface morphology became similar to that seen after annealing at 130 °C (Figure 2e). Although it looks like a rough surface, the height difference between the lowest and the highest parts was less than 0.2 nm. In fact, the rms value became 4.5 × 10-2 nm. Furthermore, after the sample was annealed at 180 °C

for 1 h, the surface morphology became rougher (Figure 2f) and the rms value increased to 2.9 × 10-1 nm. We consider that the lower regions correspond to the molecular desorbed area, because the height difference between the highest and lowest regions was estimated to be 0.7 nm, a value that is larger than the depressions. Furthermore, when this SAM was characterized by contact mode atomic

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Figure 3. STM images of the CF3-TP1 SAMs before and after annealing: (a) CF3-TP1 SAM formed by a 24 h immersion; (b) magnified image of (a); (c) after annealing at 130 °C for 1 h; (d) after annealing at 150 °C for 1 h; (e) after annealing at 180 °C for 1 h; (f) after annealing at 180 °C for 3 h. The cross-sectional profiles are shown except for the magnified images. All the images were taken by a constant current mode at a bias of -0.5 V and a current of 20 pA.

force microscopy (AFM), the friction of the lower region was larger than that of higher region, supporting the above assumption. After the sample was annealed at 180 °C for 3 h, a larger molecular domain (ca. 100 nm size) still remained (Figure 2g). No striped phases which appeared at low coverage33,34 were seen in the case of CH3-TP1 SAMs on the topographic lower parts, as well. On the topographically higher region, we observed a molecular lattice similar to the (x3 × x3)R30° structure with a spacing of 0.5 nm (Figure 2h). Finally, the STM image became similar to that of Figure 1h by annealing at 180 °C for 5 h, indicating that the CH3-TP1 molecules were almost desorbed (data not shown). Figure 3 shows STM images of the CF3-TP1 SAMs. The surface morphology is different from those of the CH3TP1 SAMs (Figure 3a). The topographically lower part, which looks like domain boundaries, may be depressions. The rms was 8.1 × 10-2 nm, whose value was larger than those of other as deposited SAMs. We also observed (x3 × x3)R30° like structures in the magnified image (Figure 3b). Compared with the CH3-TP1 SAMs, it became difficult to obtain molecular resolution images. This may be due to the electronic effect of the CF3 end group. Even after the sample was annealed at 130 °C for 1 h, the surface morphology was hardly changed (Figure 3c); however, some depressions appeared. The rms became 9.5 × 10-2 nm. The depressions did not disappear after annealing at 150 °C for 1 h (Figure 3d). The rms value decreased to 3.2 × 10-2 nm, indicating that the annealed SAM surface became more smooth. The height difference between the lowest and highest regions was less than 0.1 nm. After the sample was annealed at 180 °C for 1 h (Figure 3e), we confirmed molecular desorption and the desorbed area was slightly larger than that of the CH3-TP1 SAMs. The rms value was 2.5 × 10-1 nm. After the sample was annealed at 180 °C for 3 h, only one large molecular domain

remained (Figure 3f). The height difference was more than 0.7 nm. The STM image became similar to Figure 1h by annealing at 180 °C for 5 h, indicating that the CF3-TP1 molecules were also almost desorbed (data not shown) by this annealing condition. We further characterized the influence of annealing on the clean Au surface using both STM and AFM, because small defects present on the Au surface might induce the morphological change of the SAMs. However, we observed an atomically flat Au surface after annealing at conditions of 130, 150, and 180 °C, similar to that shown in Figure 1h. The height difference between the highest and the lowest positions was less than 0.05 nm. A few small defects were observed by AFM on one terrace of about 200 × 200 nm2. The size and density of the larger defects (up to 100 nm) were also not changed by any of the annealing conditions. XPS Measurements. Figure 4 shows the series of S(2p) XPS spectra of the C14, CH3-TP1, and CF3-TP1 SAMs. It has been reported that the S(2p) spectra of alkanethiol or dialkyl disulfide SAMs exhibit two strong peaks at around 162 eV (S(2p3/2)) and 163.4 eV (S(2p1/2)).35,36 Since the spectrum consists of (2p3/2) and (2p1/2) peaks with an intensity ratio of 2:1, which has been theoretically determined by the spin-orbit splitting effect,35,36 the two peaks should be assigned to one species, i.e., thiolate (bound sulfur). In addition, two other sulfur-combined peaks have been confirmed at around 161 eV (isolated or atomic sulfur)11,36 and 163-164 eV (unbound sulfur).35 Before the sample was annealed, the S(2p) spectra of C14 and CH3-TP1 SAMs exhibited two combined peaks (35) Castner D. G.; Hinds, K.; Grainger, D. W. Langmuir 1996, 12, 5083. (36) Ishida, T.; Hara, M.; Kojima, I.; Tsuneda, S.; Nishida, N.; Sasabe, H.; Knoll, W. Langmuir 1998, 14, 2092.

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Figure 4. S(2p) XPS spectra of the SAMs: C14 SAMs (a) formed by 24 h of immersion; (b) after annealing at 130 °C for 1 h; (c) after annealing at 180 °C for 1 h. CH3-TP1 SAMs: (d) formed by a 24 h immersion; (e) after annealing at 130 °C for 1 h; (f) after annealing at 180 °C for 1 h. CF3-TP1 SAMs: (g) formed by 24 h of immersion; (h) after annealing at 130 °C for 1 h; (i) after annealing at 180 °C for 1 h; (j) after annealing at 180 °C for 5 h. In (a), (b), (c), (f), and (g), the C species peaks were not resolved because of weakness of the peak. In (a) and (b), the B species peaks were not mentioned for the same reason. In (j), we did not resolved any peaks, because the purpose of showing this spectrum was to discuss the possibility of oxidation during the annealing process.

at 162 ( 0.2 eV and 163.4 ( 0.2 eV with full width at half-maximum (fwhm) of 1.2 ( 0.05 eV (denoted A), which is assigned to bound sulfur (Figure 4a-c).11,35,36 We also detected the unbound sulfur peaks (denoted B) at higher binding energy at around 163.8 eV (S(2p3/2) peak) with fwhm of 1.8 ( 0.05 eV. After the sample was annealed above 130 °C, all the S(2p) peaks became broader. The peak intensity at around 163-164 eV increased after the annealing process. In the case of as deposited CF3-TP1 SAM, other combined peaks (denoted C) at around 161 and 162.5 eV were observed, in addition to the A and B peaks (Figure 4g). The increase in C peaks was also confirmed for both TP1 SAMs (spectra e, f, h, and i of

Figure 4). We resolved these peaks into 161.0 ( 0.2 eV (S(2p3/2) peak) and 162.5 ( 0.2 eV (S(2p1/2) peak) with fwhm of 1.0 ( 0.05 eV. In some SAMs, we did not resolve the B or C species peaks, because of the weakness of the peaks. The influence of oxygen on the sulfur should be taken into account here.37 It is likely that the sulfurs easily were oxidized by annealing in air. However, it should be noted that no oxidized sulfur peak at around 168-169 eV was observed in the S(2p) region even after annealing at 180 (37) Schoenfisch, M. H.; Pemberton, J. E. J. Am. Chem. Soc. 1998, 120, 4502.

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Figure 5. C(1s) XPS spectra of the SAMs: C14 SAMs (a) formed by a 24 h immersion; (b) after annealing at 130 °C for 1 h; (c) after annealing at 180 °C for 1 h. CH3-TP1 SAMs: (d) formed by a 1 day immersion; (e) after annealing at 180 °C for 1 h. CF3-TP1 SAMs: (f) formed by a 24 h immersion; (g) after annealing at 180 °C for 5 h. Table 1. F(1s)/Au(4f), C(1s)/Au(4f), and S(2p)/Au(4f) Ratios of the SAMs, Estimated from XPS Peak Areasa CH3-TP1

CF3-TP1 as deposited annealed at 130 °C for 1 h annealed at 150 °C for 1 h annealed at 180 °C for 1 h annealed at 180 °C for 3 h annealed at 180 °C for 5 h a

C14

F(1s)/Au(4f)

C(1s)/Au(4f)

S(2p)/Au(4f)

C(1s)/Au(4f)

S(2p)/Au(4f)

C(1s)/Au(4f)

S(2p)/Au(4f)

0.880 0.773 0.725 0.316 0.175 0.083

3.00 2.88 2.99 2.87 1.41 1.97

0.084 0.091 0.093 0.033 0.031 0.032

3.20 2.83 3.22 2.48 1.92 1.88

0.083 0.089 0.085 0.075 0.052 0.040

2.41 2.04 1.90 1.79 1.31 1.28

0.091 0.067 0.084 0.073 0.063 0.073

The errors of F(1s)/Au(4f), C(1s)/Au(4f), and S(2p)/Au(4f) ratios are ( 0.15, ( 0.3, and ( 0.003, respectively.

°C for 1-3 h. We only observed oxidized sulfur peaks on the CF3-TP1 surface after annealing at 180 °C for 5 h (Figure 4j). This indicated that the oxidation of the sulfur might not have occurred as quickly. The oxidation mechanism will be discussed later. Figure 5 shows the series of C(1s) XPS spectra of C14, CH3-TP1, and CF3-TP1 SAMs. In the case of the asdeposited C14 SAM, a single strong peak was observed at around 284.8 eV with a fwhm of 1.2 ( 0.05 eV. The peak position changed gradually to lower binding energy. In a previous study, it was reported that the C(1s) peak position of alkanethiol SAMs was dependent on the annealing condition.36 The fully covered C14 SAM acts as a good insulator while the insulating property of the low coverage C14 SAM was poor. In the case of insulating films, the binding energy tends to be at more positive position, because of charge up phenomena (Figure 5a-c).36 Therefore, this peak shift may be due to the breaking of the insulating property of C14 SAM.25,36 A similar phenomenon was also reported recently.38a These XPS data indicated that the alkyl chains were more tilted together with the contaminant from air, after the annealing. Another explanation is the formation of the CdC bond during the annealing. However, from the XPS data, we could not judge which factor was dominant. The C(1s) peak of asdeposited CH3-TP1 SAM was 1.0 eV lower than that of C14 SAMs, because the TP1 molecule is considered to be more conductive than C14 (spectra d and e of Figure 5).38a For the as-deposited CF3-TP1 SAM, another weak peak assigned to CF3 group appeared at 293 eV (Figure 5e). No specific oxidized carbon species peaks were detected before and after the annealing, suggesting that the influence of

oxygen on the carbon was also small. On the other hand, we observed an additional peak at 288.4 eV in the CF3TP1 SAM annealed at 180 for 5 h (Figure 5h). We consider that this peak was formed by a bond cleavage at the CF3 group. Similar peak formation during long time electron beam irradiation has been reported.39 Table 1 shows the ratios of F(1s)/Au(4f), C(1s)/Au(4f), and S(2p)/Au(4f) for all the SAMs. All the signal intensities were normalized by the photoionization cross sections of each element. The F(1s)/Au(4f) ratio is shown only for the CF3-TP1 SAMs. The experimental errors of the F(1s)/Au(4f), C(1s)/Au(4f), and S(2p)/Au(4f) ratios were (0.15, (0.3, and (0.003, respectively. For the C14 SAMs, the C(1s)/ Au(4f) ratio decreased with higher annealing temperatures. On the other hand, the S(2p)/Au(4f) ratios did not decrease by annealing at 130 and 150 °C. This can be explained by the following two factors: (1) an increase in the photoelectron intensity due to the decrease in the alkyl chain density as a result of the molecular desorption; (2) C-S bond cleavage40,41 during annealing. We will discuss which factor was dominant later. On the other hand, for both TP1 SAMs, the S(2p)/Au(4f) ratios did not decrease by annealing below 150 °C but did decrease by annealing (38) In the following paper, it was reported that the C(1s) peak position of alkanethiol SAMs depended on the thickness of the SAM: (a) Heister, K.; Zharnikov, M.; Grunze, M.; Johanson, L. S. O. J. Phys. Chem. B 2001, 105, 4058. (b) Ishida, T.; Nishida, N.; Tsuneda, S.; Hara, M.; Sasabe H.; Knoll, W. Jpn. J. Appl. Phys. 1996, 35, L1710. (39) Frey, S.; Heister, K.; Zharnikov, M.; Grunze, M. Phys. Chem. Chem. Phys. 2000, 1979. (40) Zhong, C.-J.; Porter, M. D. J. Am. Chem. Soc. 1994, 116, 11616. (41) Zhong, C.-J.; Brush, R. C.; Anderegg, J.; Porter, M. D. Langmuir 1999, 15, 518.

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Table 2. Contact Angles (deg) of C14 SAMs before and after the Annealing Processa as deposited annealed at 130 °C for 1 h annealed at 150 °C for 1 h annealed at 180 °C for 1 h annealed at 180 °C for 3 h annealed at 180 °C for 5 h

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HD

CH2I2

110 112 108 102 99 90

54 53 44 34 24 18

69 72 45 43 30 28

a Before SAM formation 80°, 300 °C (dec). 1H NMR (CDCl3): 4.57 (s, 2H, ArCH2Br), 7.50 (d, 8.7 Hz, 2H, H-3′′, H-5′′), 7.63 (d, 8.7 Hz, 2H, H-2′′, H-6′′), 7.69 (s, 4H, H-2′ to H-6′), 7.73 (m, 4H, H-2 to H-6). Thioacetic Acid S-(4-Trifluoromethyl-[1,1′;4′,1′′]terphenyl-4′′-ylmethyl) Ester (7). Potassium thioacetate (1.8 g, 16 mmol) was dissolved in 20 mL of DMF and placed in a flask with a nitrogen atmosphere. 6 (3.1 g, 7.9 mmol) dissolved in 45 mL of DMF was dropped into the above solution and stirred at room temperature under nitrogen for 3 h. Subsequently, the mixture was refluxed for a half hour. The reaction mixture was filtered to remove KBr, and the filtrate was evaporated in a vacuum to remove DMF. The residue was mixed with water and extracted by dichloromethane. The organic layer was washed with water several times and dried with MgSO4. After dichloromethane was removed, a crude brown precipitate appeared and was purified using silica gel chromatography (dichloromethane/hexane ) 1/1). The pale yellow crystal was washed with ether and hexane (yield 1.41 g, 50%), mp 190-192 °C. 1H NMR (CDCl3): 2.38 (s, 3H, COCH3), 4.18 (s, 2H, ArCH2SCO), 7.40 (d, 9 Hz, 2H, H-3′′, H-5′′), 7.59 (d, 9 Hz, 2H, H-2′′, H-6′′), 7.68 (s, 4H, H-2′ to H-6′), 7.727.73 (m, 4H, H-2′ to H-6′, H-2 to H-6). (4-Trifluoromethyl[1,1′;4′,1′′]terphenyl-4′′-yl)methanethiol (CF3-TP1) (8). 7 (0.6 g, 1.6 mmol) was suspended in 100 mL of ethanol and 0.5 g (8.9 mmol) of KOH pellets was added to the suspension. The mixture was stirred at 90 °C for 5 h. The mixture is cooled, and 15 mL of HCl was added to the solution. Subsequently, 40 mL of water is added and stirred for another half hour at room temperature. The emerging precipitate was filtered off and washed with water. The crude precipitate was purified using silica gel chromatography (dichloromethane/ hexane ) 1/1). The colored crystal was recrystallized by a mixed solvent (dichloromethane/hexane ) 1/2). A thin-yellow crystal was obtained (yield 0.36 g, 65%), mp 220-222 °C. 1H NMR

Ishida et al. (CDCl3): 1.82 (t, 6.7 Hz, 1H, SH), 3.81 (d, 6.7 Hz, 2H, ArCH2S), 7.43 (d, 8.7 Hz, 2H, H-3′′, H-5′′), 7.61 (d, 8.7 Hz, 2H, H-2′′, H-6′′), 7.69 (s, 4H, H-2′ to H-6′), 7.72-7.73 (m, 4H, H-2 to H-6). SAM Formation and Annealing Process. An atomically flat Au(111) surface was epitaxially grown on mica by vacuum deposition. The detailed procedure has been reported elsewhere.7,11-13 For the C14 SAMs, the Au substrates were immersed into a 1 mM ethanol solution for 24 h. In the case of conjugated molecules, we used dichloromethane as the solvent in which the conjugated molecules were dissolved and immersed the Au substrate into a 0.1 mM solution of dichloromethane for more than 24 h. After the solvent was removed from the solution, the Au substrate was rinsed with pure solvent to remove the physisorbed multilayer. The annealing process was carefully carried out in air at 130-180 °C for 1-5 h, inside a temperaturecontrolled oven system. The temperature at the Au/mica surface was checked by a thermocouple directly, and the deviation of surface temperature was less than (2 °C STM, XPS, and Contact Angle Measurements. STM images were obtained with a Seiko Instruments SPA340 unit operating in air. A tunneling current of 20 pA and tip biases of 0.5-2.0 V were used. XPS spectra were recorded using a Surface Science Inc. SSX-100 system with a monochromatic Al KR X-ray source (1486.6 eV). The binding energy was calibrated using the Au (4f7/2) peak energy (84.0 eV) as an energy standard. The X-ray power, pass energy of the analyzer, and takeoff angle of the photoelectrons were set at 200 W, 58 eV, and 35°, respectively. XPS peaks were fitted using the spectra processing program in the XPS system. To prevent CF3 group bond cleavage during the XPS measurements,39 except for the S(2p) region, we measured C(1s), Au(4f), and F(1s) within 5 min. Contact angles were measured by the free drop standing method. The solvents used here were water, hexadecane (HD), and CH2I2.

Acknowledgment. We gratefully acknowledge Drs. K. Abe and H. Azehara (AIST) for their helpful suggestions regarding the contact angle measurements.We thank I. Yamane (Hodogaya Contract Lab.) for synthesizing for CH3-TP1 molecule. LA001816G