10496
Langmuir 2002, 18, 10496-10499
Stability of Terphenyl Self-Assembled Monolayers Exposed under UV Irradiation
Chart 1. Molecular Structures of Molecules Used in This Study
Takao Ishida,*,† Miki Sano,† Hitoshi Fukushima,‡ Masaya Ishida,‡ and Shinya Sasaki† Institute for Mechanical Systems Engineering (IMSE), National Institute of Advanced Industrial Science and Technology (AIST), 1-2-1 Namiki, Tsukuba, Ibaraki 305-8564, Japan, Technology Platform Research Center (TPRC), SEIKO-EPSON Corporation, Fujimi Plant 281, Fujimi-machi, Suwa-gun, Nagano 392-8502, Japan Received July 8, 2002. In Final Form: October 25, 2002
Introduction Self-assembled monolayers (SAMs)1 have been attracting much attention recently, from a viewpoint of basic surface science, surface coating, and molecular devices. However, for the future industrial application of SAMs, many unanswered questions still remain. For example, the stability of SAMs is a particularly important unresolved issue.2,3 SAM stability under exposure to light irradiation, especially ultraviolet (UV) light, is quite important. Many studies have been done to investigate the influence of UV irradiation on SAMs as a resist material, for the purpose of nanoscale patterning.4-16 Oxidization of bound sulfur atoms during UV irradiation of typical alkanethiol SAMs has been reported.12,13 Hara et al. reported that alkyl chains were also oxidized during UV irradiation.15 However, these studies were mainly performed using UV light at wavelengths shorter than 200 nm, because the UV light can easily decompose the SAMs from ozone formation. The number of studies using UV light at wavelengths longer than 200 nm was quite few. For example, Zhang et al. found no changes in alkanethiol monolayers upon irradiation in air when the wavelength was longer than 200 nm.16 * To whom correspondence should be addressed. E-mail:
[email protected]. Tel: +81-298-61-7203. Fax: +81-298-61-7844. † AIST. ‡ SEIKO-EPSON. (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. (4) Li, Y.; Huang, J.; McIver, R. T.; Hemminger, J. C. Langmuir 1994, 10, 626. (5) Huang, J.; Hemminger, J. C. J. Am. Chem. Soc. 1993, 115, 3342. (6) Tarlov, M. J.; Burgess, D. R. F.; Gillen, G. J. Am. Chem. Soc. 1993, 115, 2170. (7) Zhang, Y.; Terrill, R. H.; Tanzer, T. A.; Bohn, P. W. J. Am. Chem. Soc. 1998, 120, 2654. (8) Frisbie, C. D.; Wollman, E. W.; Martin, J. R.; Wrighton, M. S. J. Vac. Sci. Technol., A 1993, 11, 2368-72. (9) Wollman, E. W.; Kang, D.; Frisbie, C. D.; Lorkovic, I. M.; Wrighton, M. S. J. Am. Chem. Soc. 1994, 116, 4395. (10) Chan, K. C.; Kim, T.; Schoer, J. K.; Crooks, R. M. J. Am. Chem. Soc. 1995, 117, 5875. (11) Dishner, M. H.; Feher, F. J.; Hemminger, J. C. Chem. Commun. 1996, 1971-1972. (12) Hutt, D. A.; Leggett, G. J. J. Phys. Chem. 1996, 100, 6657. (13) Schoenfisch, M. H.; Pemberton, J. E. J. Am. Chem. Soc. 1998, 120, 4502. (14) Ishida, M.; Kasuga, M.; Kaneko, T.; Shimoda, T. Jpn. J. Appl. Phys. 2000, 39, L227. (15) Hara, M.; Tamada, K.; Hahn, C.; Nishida, N.; Knoll, W. Supramol. Sci. 1996, 3, 103. (16) (a) Zhang, Y.; Terrill, R. H.; Tanzer, T. A.; Bohn, P. W. J. Am. Chem. Soc. 1998, 120, 2654. (b) Zhang, Y.; Terrill, R. H.; Bohn, P. W. Chem. Mater. 1999, 11, 2191.
In fact, from the viewpoint of SAM stability, UV light at longer wavelengths (e.g., 245 nm) is commonly present in the atmosphere and may also decompose the SAMs. For future optical device application of SAMs, the use of optically sensitive molecules such as the azo-benzene moiety has been proposed.17 In such applications, SAM decomposition caused by the UV light irradiation must be avoided. Thus, further investigation and improvements in SAM stability under exposure to UV light at longer wavelengths are important. Oligophenylene SAMs18-31 are typical conjugated molecular SAMs. Such SAMs are believed to be rigid due to the presence of phenyl rings. For example, in our previous study, we demonstrated that terphenyl (TP) SAMs exhibited higher thermal stability than alkanethiol SAMs.29 However, the stability of TP SAMs under UV irradiation has not been investigated as much as that of other molecular SAMs. In the present study, we investigated the stability of [1,1′:4′,1′′-terphenyl]-4-methanethiol (TP1) SAMs under exposure to UV at longer wavelengths. We chose terphenyl methanethiols because these conjugated molecules form a well-ordered structure on Au(111) surfaces, that is, (x3 × x3)R30° structures, when the molecule has one methylene group between the sulfur and aromatic rings (see Chart 1).20,29 Especially, the first TP1 SAM formation was reported by Himmel and co-workers.20 For compari(17) (a) Wolf, H.; Ringsdolf, H.; Delamarche, E.; Takami, T.; Kang, H.; Michel, B.; Gerber, Ch; Jaschcke, M.; Butt, H.-J.; Bamberg, E. J. Phys. Chem. 1995, 99, 7102. (b) Wang, R.; Iyoda, T.; Jiang, L.; Hashimoto, K.; Fujishima, A. Chem. Lett. 1996, 1005. (c) Tamada, K.; Nagasawa, J.; Nakanishi, F.; Abe, K.; Ishida, T.; Hara, M.; Knoll, W. Langmuir 1998, 14, 3264. (18) Tao, Y.-T.; Wu, C.-C.; Eu, J.-Y.; Lin, W.-L.; Wu, K.-C.; Chen, C. Langmuir 1997, 13, 4018. (19) Sabatani, E.; Cohne-Boulakia, J.; Bruening, M.; Rubinstein, I. Langmuir 1993, 9, 2974. (20) Himmel, H.-J.; Terfort, A.; Wo¨ll, Ch. J. Am. Chem. Soc. 1998, 120, 12069. (21) Geyer, W.; Stadler, V.; Eck, W.; Zharnikov, M.; Golzhauser, A.; Grunze, M. Appl. Phys. Lett. 1999, 75, 2401. (22) Kang, J. F.; Ulman, A.; Liao, S.; Jordan, R.; Yang, G.; Liu, L.-Y. Langmuir 2001, 17, 95. (23) Ishida, T.; Mizutani, W.; Azehara, H.; Sato, F.; Choi, N.; Akiba, U.; Fujihira, M.; Tokumoto, H. Langmuir 2001, 17, 7459. (24) Zharnikov, M.; Frey, S.; Rong, H.; Yang, Y.-J.; Heister, K.; Buck, M.; Grunze, M. Phys. Chem. Chem. Phys. 2000, 2, 3359. (25) Frey, S.; Stadler, K.; Heister, K.; Eck, W.; Zharnikov, M.; Grunze, M.; Zeysing, B.; Terfort, A. Langmuir 2001, 17, 2408. (26) Heister, K.; Zharnikov, M.; Grunze, M.; Johanson, L. S. O. J. Phys. Chem. B 2001, 105, 4058. (27) Leung, T. Y. B.; Schwartz, P.; Scoles, G.; Schreiber, F.; Ulman, A. Surf. Sci. 2000, 458, 34. (28) Ishida, T.; Mizutani, W.; Choi, N.; Akiba, U.; Fujihira, M.; Tokumoto, H. J. Phys. Chem. B 2000, B104, 11680. (29) Ishida, T.; Fukushima, H.; Mizutani, W.; Miyashita, S.; Ogiso, H.; Ozaki, K.; Tokumoto, H. Langmuir 2002, 18, 83. (30) Ishida, T.; Mizutani, W.; Azehara, H.; Miyake, K.; Aya, Y.; Sasaki, S.; Tokumoto, H. Surf. Sci. 2002, 514, 187. (31) Ishida, T.; Mizutani, W.; Aya, Y.; Ogiso, H.; Sasaki, S.; Tokumoto, H. J. Phys. Chem. B 2002, 106, 5866.
10.1021/la0262001 CCC: $22.00 © 2002 American Chemical Society Published on Web 12/17/2002
Notes
Langmuir, Vol. 18, No. 26, 2002 10497
Figure 1. S(2p) XPS spectra of the SAMs: C10 SAMs (a) formed by 24 h of immersion; (b) after UV irradiation for 3 h; (c) after UV irradiation for 6 h. BM SAMs (d) formed by 24 h of immersion; (e) after UV irradiation for 3 h; (f) after UV irradiation for 6 h. TP1 SAMs (g) formed by 24 h of immersion; (h) after UV irradiation for 6 h; (i) after UV irradiation for 24 h.
son, we also used decanethiol (C10) and benzyl mercaptane (BM) SAMs. The irradiation effects of these SAMs were examined to understand the stability and desorption mechanism of conjugated molecular SAMs using X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM). Experimental Section The synthesis method of TP1 was described elsewhere.32 BM and C10 were purchased from Aldrich and Kanto Kagaku, respectively. An atomically flat Au(111) surface was epitaxially grown on mica by vacuum deposition. The detailed procedure has been reported elsewhere.28-31 For the BM and C10 SAMs, the Au substrates were immersed into 1 mM ethanol solution for 24 h. For the TP1 molecules, we used dichloromethane as the solvent in which the TP1 molecules were dissolved and immersed the Au substrate into a 0.1 mM solution of dichloromethane for more than 24 h. After it was taken out of the solution, the Au substrate was rinsed with pure solvent to remove the physisorbed multilayer. The UV light source was the Iuchi SLUV-4 at 245 nm. The UV irradiation process was carefully carried out in air for 1-24 h. The intensity of the UV light was 1250 µW cm-2. For the TP1 SAMs, we also utilized the Sen HLR100T which had a higher intensity than the Iuchi SLUV-4 (4700 µW cm-2 at 245 nm). STM images were obtained with a Seiko Instruments SPA340 unit operating in air. A tunneling current of 20 pA and tip biases of 0.5-1.0 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.
Results and Discussion Figure 1 shows the series of S(2p) XPS spectra of the C10, BM, and TP1 SAMs. It has been reported that the S(2p) spectra of alkanethiol SAMs exhibit two strong peaks at around 162 eV (S(2p3/2)) and 163.4 eV (S(2p1/2)).33 Since the spectrum consists of (2p3/2) and (2p1/2) peaks with an (32) Liphardt, B.; Luettke, W. Liebigs Ann. Chem. 1981, 1118.
intensity ratio of 2:1, which has been theoretically determined by the spin-orbit splitting effect,33 the two peaks should be assigned to one species, that is, thiolate (bound sulfur). Besides the bound sulfur, two other sulfur species peaks were seen.34,35 However, in the present study, we did not resolve these peaks in Figure 1 because changes in the sulfur species between 161 and 164 eV were not the focus of our present study. Before UV irradiation, the S(2p) spectra of all the SAMs exhibited these two combined peaks at 162 ( 0.2 eV and 163.4 ( 0.2 eV with a full width at half-maximum (fwhm) of 1.2 ( 0.05 eV, which is assigned to be bound sulfur (Figure 1a,d,g).33 After 245 nm UV irradiation for 3 h, for the C10 and BM SAMs, a peak appeared at around 167 eV which was assigned to be oxidized sulfur.13,15 Both the height and area of the oxidized sulfur peak increased with the UV irradiation time, while the peak intensity of the bound sulfur between 161 and 164 eV decreased (cf. Figure 1b,c,e,g). However, for the TP1 SAM, we did not confirm such a strong change in the S(2p) spectra even after 6 and 24 h of irradiation (cf. Figure 1h,j). Table 1 shows the ratios of 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 ratio of oxidized sulfur in the total sulfur atoms on the Au surface was also noted as Sox (%). The calculation errors of the C(1s)/Au(4f) and S(2p)/Au(4f) ratios were (0.4 and (0.005, respectively. For the BM SAMs, the C(1s)/Au(4f) ratio decreased with longer irradiation time, while the S(2p)/Au(4f) ratio increased. This may be attributed to the molecular decomposition caused by the UV irradiation, that is, only the aromatic ring portion might be desorbed, and then atomic and oxidized sulfurs without an aromatic ring remained on (33) Castner, D. G.; Hinds, K.; Grainger, D. W. Langmuir 1996, 12, 5083. (34) Ishida, T.; Hara, M.; Kojima, I.; Tsuneda, S.; Nishida, N.; Sasabe, H.; Knoll, W. Langmuir 1998, 14, 2092. (35) Ishida, T.; Choi, N.; Mizutani, W.; Tokumoto, H.; Kojima, I.; Azehara, H.; Akiba, U.; Hokari, H.; Fujihira, M. Langmuir 1999, 15, 6799.
10498
Langmuir, Vol. 18, No. 26, 2002
Notes
Table 1. C(1s)/Au(4f) and S(2p)/Au(4f) Ratios of the SAMs, Estimated from XPS Peak Areasa BM as deposited UV 3 h UV 6 h UV 24 h
C10
TP1
C(1s)/Au(4f)
S(2p)/Au(4f)
Sox (%)
C(1s)/Au(4f)
S(2p)/Au(4f)
Sox (%)
C(1s)/Au(4f)
S(2p)/Au(4f)
Sox (%)
1.324 0.721 0.882
0.0885 0.133 0.121
0 31.0 60.1
1.31 1.24 1.35
0.0817 0.0887 0.0464
0 56.7 100
2.53 2.64 2.73 2.54
0.1088 0.1149 0.0816 0.0956
0 0 0 0
a Calculation errors of the C(1s)/Au(4f) and S(2p)/Au(4f) ratios were (0.4 and (0.005, respectively. The ratios of oxidized sulfur were also noted as Sox (%).
the Au surface. Even after 6 h of irradiation, all the sulfur atoms remaining on the Au surface were not necessarily oxidized. On the other hand, for the C10 SAMs, the C(1s)/ Au(4f) ratio did not change much after irradiation, while the S(2p)/Au (4f) ratio decreased after 6 h of irradiation. After 6 h of irradiation, the Sox value became 100%, indicating that all the sulfur atoms remaining on the Au surface were oxidized. Zhang et al. found no changes in alkanethiol monolayers upon irradiation in air when the wavelength was larger than 200 nm,16 although our C10 SAMs easily decomposed by the UV irradiation. We consider that this discrepancy might be owing to the light intensity, because our used UV light intensity of 1250 µW cm-2 was much stronger than that used in ref 16 (50 µW cm-2).16 In addition, the humidity difference between Japan and the United States may be also taken into account. On the other hand, for TP1 SAMs, both the C(1s)/Au(4f) and S(2p)/Au(4f) ratios did not change much after the UV irradiation even after 24 h of irradiation. Slight decreases in the S(2p)/Au(4f) ratios were confirmed after 6 and 24 h of irradiation. However, we could not detect any oxidized sulfur even after 24 h of irradiation. Thus, we concluded that small amounts of TP1 molecules were desorbed from the Au surface during UV irradiation. We checked the surface morphology using STM before and after UV irradiation. Figure 2 shows STM images of the BM and C10 SAMs. The BM SAM exhibited the structures with a spacing of each spot of 0.5 nm, similar to (x3 × x3)R30° with many depressions (Figure 2a,b), and the surface morphology was similar to that of C10 SAMs.30 The depth of the depressions was about 0.2 nm,35 and we confirmed the presence of molecules in these depressions. After UV irradiation for 3 h, the structures with a spacing of each spot of 0.5 nm had almost disappeared (Figure 2c,d). Since the BM molecules should be present in this condition from the XPS data, the BM molecules were adsorbed as a disordered structure. In Figure 2d, the striped structures with a spacing of each spot of about 0.3 nm were seen. This may be a lattice structure of the bare Au(111) surface after molecular desorption. For the C10 SAMs, after immersion for 24 h, we could see domain structures with many depressions (Figure 2e), which is a typical surface of alkanethiol SAMs.35,36 When the UV light irradiated the C10 SAM for 3 h (Figure 2f), in some regions such molecular domains disappeared. Note that the striped phases that appear at low coverage37-40 were not seen in the desorbed area. In any case, the STM data confirmed that BM and C10 SAMs were decomposed by UV irradiation. (36) Poirier, G. E.; Tarlov, M. J. Langmuir 1994, 10, 2853. (37) Delamarche, E.; Michel, B.; Gerber, C.; Anselmetti, D.; Guntherodt, H.-J.; Wolf, H.; Ringsdorf, H. Langmuir 1994, 10, 2869. (38) Poirier, G. E.; Pylant, E. D. Science 1996, 272, 1145. (39) Kondoh, H.; Kodama, C.; Nozoye, H. J. Phys. Chem. B 1998, 102, 2310. (40) Yamada, R.; Uosaki, K. Langmuir 1998, 14, 855.
Figure 2. STM images of BM and C10 SAMs before and after UV irradiation: (a) BM SAM formed by 24 h of immersion in BM solution; (b) magnified image of (a); (c) after UV irradiation for 3 h; (d) magnified image of (c); (e) C10 SAM formed by 24 h of immersion; (f) after UV irradiation for 3 h.
Figure 3 shows STM images of the TP1 SAMs. The surface morphology is similar to that of typical TP1 SAMs as we reported elsewhere28 (Figure 3a). We also observed (x3 × x3)R30°-like structures in the magnified images28 (Figure 3b). After irradiation for 6 h, the surface morphology was hardly changed (Figure 3c). Moreover, the surface did not change after UV irradiation for 24 h (Figure 3d), confirming that the TP1 SAM did not decompose even after 24 h of irradiation. We further checked the irradiation effect on the TP1 SAMs using stronger UV light which can decompose typical alkanethiol SAMs.15 Even for the octadecanethiol SAMs with a carbon number of 18, all the sulfur atoms were oxidized by 100 min of irradiation using this light source.15 Table 2 shows the ratios of C(1s)/Au(4f) and S(2p)/ Au(4f) for the TP1 SAMs before and after stronger UV irradiation. The S(2p) XPS spectrum and STM images are shown in Figure 4. The C(1s)/Au(4f) ratios were unchanged, and the S(2p)/Au(4f) ratios decreased slightly. These data indicated that, surprisingly, strong decomposition did not occur even under stronger UV irradiation. Here, we shall discuss the desorption mechanism during UV irradiation. It has been believed that sulfur atoms are oxidized first, and oxygen or water is likely to be the
Notes
Langmuir, Vol. 18, No. 26, 2002 10499
Figure 3. STM images of TP1 SAMs before and after UV irradiation: (a) TP1 SAM formed by 24 h of immersion; (b) magnified image of (a); (c) after UV irradiation for 3 h; (d) after UV irradiation for 24 h. All images were taken by a constant current mode at a bias of -0.5 V and a current of 100 pA. Table 2. C(1s)/Au(4f) and S(2p)/Au(4f) Ratios of the Strong UV Irradiated TP1 SAMs, Estimated from XPS Peak Areasa TP1 as deposited UV 3 h UV 6 h UV 24 h
C(1s)/Au(4f)
S(2p)/Au(4f)
Sox (%)
2.53 2.71 2.67 2.74
0.1088 0.1000 0.0918 0.0908
0 0 0 0
a The errors of C(1s)/Au(4f) and S(2p)/Au(4f) ratios are (0.3 and (0.005, respectively.
oxidant. For the BM and C10 SAMs, such an oxidant can easily attack the sulfur present in the molecular/Au interface, because of the loose packing of the molecules. For the BM SAMs, even if we could obtain structures with a spacing of each spot of 0.5 nm, similar to (x3 × x3)R30°, the BM molecules might not be as dense as expected. From the STM images, the monolayer decomposition of the BM SAMs was faster than that of C10 SAMs. The reason for the fast decomposition of the BM SAMs might be owing to not only the loose packing but also the reactivity of the benzyl group. The principles of photochemistry suggest that the benzyl group should cleave more easily by photoirradiation.41 Thus, for the above two reasons (less dense packing and reactivity of the benzyl group), the BM SAM has the lowest stability against the UV irradiation. On the other hand, the TP1 SAMs did not decompose even after 24 h of irradiation under both the UV sources. We confirmed only small amounts of desorption after more than 6 h of irradiation from the XPS data. Why was such a discrepancy observed between the TP and the other SAMs? Typically, both the C10 SAMs and TP1 SAMs were arranged with (x3 × x3)R30° structures.1,28 However, since the molecular diameter of terphenyl is larger than that of the C10 molecules, this suggests a higher atomic density. In addition, the BM layer might not be as dense as expected even if we could observe lattice images as described before. Thus, we suppose that the higher atomic (41) See for example: Gazith, M.; Noyes, R. M. J. Am. Chem. Soc. 1955, 77, 6091.
Figure 4. S(2p) XPS spectrum (a) and STM image (b) of the TP1 SAMs after stronger UV irradiation for 24 h. The STM image was taken by a constant current mode at a bias of -0.5 V and a current of 100 pA.
density of the TP1 SAMs at the Au surface prevented the penetration of the oxidants into the molecular/Au interface. Therefore, we concluded that TP1 SAMs have a higher stability under UV irradiation. In addition, UV irradiation (also electron irradiation) may cause double bonds to cleave and then form the strong molecular lateral network.21 Such a network formation might further increase the stability of the TP1 SAMs under exposure to UV irradiation. In conclusion, we investigated the effect of 245 nm UV light on SAMs on Au(111) generated from derivatives of TP1, BM, and C10. XPS and STM data revealed that the BM and C10 molecules were partially desorbed by irradiation for 3 h, while such desorption was not observed in the TP1 SAMs. Furthermore, even when using stronger UV light which could decompose alkanethiol SAMs more efficiently, XPS and STM data confirmed no decomposition after irradiation for 24 h. Our data demonstrate that TP1 SAMs had higher stability under UV irradiation. Acknowledgment. We acknowledge Dr. W. Mizutani, Dr. H. Azehara, Dr. K. Miyake, and Dr. H. Tokumoto (AIST) for many useful suggestions and their experimental support. This work was supported by the Science and Technology Research Grant Program for Young Researchers with a Term from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan. LA0262001
