Decomposition Processes of an Organic Monolayer Formed on Si

The decomposition processes of an organic monolayer, which was formed on Si(111) via a Si C covalent bond, induced by exposure to UV light irradiation...
0 downloads 0 Views 158KB Size
Download PDF
Langmuir 2004, 20, 1207-1212

1207

Decomposition Processes of an Organic Monolayer Formed on Si(111) via a SiliconsCarbon Bond Induced by Exposure to UV Irradiation or Ozone Kohei Uosaki,* M. Emran Quayum, Satoshi Nihonyanagi, and Toshihiro Kondo† Physical Chemistry Laboratory, Division of Chemistry, Graduate School of Science, Hokkaido University, Sapporo 060-0810, Japan Received May 20, 2003. In Final Form: October 15, 2003 The decomposition processes of an organic monolayer, which was formed on Si(111) via a SisC covalent bond, induced by exposure to UV light irradiation or ozone, were investigated using attenuated total reflectance Fourier transform infrared spectroscopy. Exposure to both ozone and UV light resulted in a reduction of the intensities of the IR peaks corresponding to CH stretching vibration and bending scissors and the appearance of peaks corresponding to CO stretching and COH in-plane bending. The latter peaks initially increased, reached a maximum, and then decreased, indicating that the monolayer was decomposed through the formation of intermediates such as aldehyde and carboxylic acid. The monolayer was also decomposed by exposure only to UV light or ozone but more slowly as the time dependencies of the CH peaks showed. While the peaks corresponding to the CO stretching and the COH in-plane bending behaved similarly under the condition of exposure to ozone, they were not observed during decomposition induced by UV irradiation. These results show that, while the monolayer was decomposed through the formation of oxidized intermediates such as aldehyde and carboxylic acid under the condition of exposure to ozone, the decomposition of the monolayer under the condition of UV irradiation proceeded via cleavage of SisC bonds by photogenerated electrons or holes without such oxidized intermediates. An increase of gauche defects as the decomposition proceeded was demonstrated by sum frequency generation spectroscopy.

Introduction The formation of a well-defined nanosized pattern of organic molecules on a solid surface is one of the most important subjects not only for fundamental science but also for a wide variety of applications in nanotechnology. The formation of a well-defined nanosized pattern of organic molecules requires the construction of a highly ordered organic layer on the solid surface. Self-assembled monolayers (SAMs) of alkanethiols on various metals,1-3 especially gold, and SAMs of alkylsiloxane on metal oxides3,4 have been most widely studied in this field because a well-ordered structure can be very easily prepared without expensive equipment. Thus, SAMs with a wide variety of functionalities have been constructed, and many research groups, including our group, have investigated the formation processes, structures, and decomposition processes of SAMs in detail.5-10 * To whom correspondence should be addressed. Tel.: +81-11706-3812.Fax: +81-11-706-3440.E-mail: [email protected]. † Present address: Department of Chemistry, Faculty of Science, Ochanomizu University, Bunkyo-ku, Tokyo 112-8610, Japan. (1) Nuzzo, R.; Allara, D. J. Am. Chem. Soc. 1983, 105, 4481. (2) Finklea, H. O. In Electroanalytical Chemistry; Bard, A. J., Rubinstein, I., Eds.; Marcel Dekker: New York, 1996; Vol. 19, p 109. (3) Ulman, A. An Introduction to Ultrathin Organic Films from Langmuir-Blodgett to Self-Assembly; Academic Press: New York, 1991. (4) Sagiv, J. J. Am. Chem. Soc. 1980, 102, 92. (5) (a) Uosaki, K.; Sato, Y.; Kita, H. Langmuir 1991, 7, 1510. (b) Uosaki, K.; Sato, Y.; Kita, H. Electrochim. Acta 1991, 36, 1799. (c) Shimazu, K.; Yagi, I.; Sato, Y.; Uosaki, K. Langmuir 1992, 8, 1385. (d) Sato, Y.; Itoigawa, H.; Uosaki, K. Bull. Chem. Soc. Jpn. 1993, 66, 1032. (e) Sato, Y.; Frey, B. L.; Corn, R. M.; Uosaki, K. Bull. Chem. Soc. Jpn. 1994, 67, 21. (f) Ohtsuka, T.; Sato, Y.; Uosaki, K. Langmuir 1994, 10, 3658. (g) Kondo, T.; Takechi, M.; Sato, Y.; Uosaki, K. J. Electroanal. Chem. 1995, 381, 203. (h) Ye, S.; Sato, Y.; Uosaki, K. Langmuir 1997, 13, 3157. (6) (a) Uosaki, K.; Kondo, T.; Zhang, X.-Q.; Yanagida, M. J. Am. Chem. Soc. 1997, 119, 8367. (b) Yanagida, M.; Kanai, T.; Zhang, X.-Q.; Kondo, T.; Uosaki, K. Bull. Chem. Soc. Jpn. 1998, 71, 2555. (c) Kondo, T.; Kanai, T.; Iso-o, K.; Uosaki, K. Z. Phys. Chem. 1999, 212, 23. (d) Kondo, T.; Yanagida, M.; Zhang, X.-Q.; Uosaki, K. Chem. Lett. 2000, 964. (7) Kondo, T.; Horiuchi, S.; Yagi, I.; Ye, S.; Uosaki, K. J. Am. Chem. Soc. 1999, 121, 391.

For technological applications, however, the construction of ordered molecular layers for nanosized patterning on semiconductor surfaces may be more important. Although several attempts have made to use GaAs11,12 and InP13 as substrates, silicon should be the most important substrate for organic layer formation because of the possible applications for molecular and biomolecular devices in conjunction with advanced silicon technology. There are two main methods for forming an organic layer on silicon. One is via the SisOsSi bond formed by silane coupling reactions on a preoxidized Si surface as described previously, and the other is via a SisC bond.14-29 The (8) Kondo, T.; Kanai, T.; Uosaki, K. Langmuir 2001, 17, 6317. (9) (a) Yamada, R.; Uosaki, K. Langmuir 1997, 13, 5218. (b) Yamada, R.; Uosaki, K. Langmuir 1998, 14, 855. (c) Yamada, R.; Sakai, H.; Uosaki, K. Chem. Lett. 1999, 667. (d) Yamada, R.; Wano, H.; Uosaki, K. Langmuir 2000, 16, 5523. (e) Yamada, R.; Uosaki, K. Langmuir 2001, 17, 4148. (f) Wano, H.; Uosaki, K. Langmuir 2001, 17, 8224. (10) (a) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559. (b) Bain, C. D.; Troughton, E. B.; Tao, Y. T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321. (c) Laibinis, P. E.; Whitesides, G. M.; Allara, D. L.; Tao, Y.; Parikh, A. N.; Nuzzo, R. G. J. Am. Chem. Soc. 1991, 113, 7152. (11) Baum, T.; Ye, S.; Uosaki, K. Langmuir 1999, 15, 8577. (12) Sheen, C. W.; Shi, J.-X.; Ma¨rtensson, J.; Parikh, A. N.; Allara, D. J. Am. Chem. Soc. 1992, 114, 1514. (13) Gu, Y.; Lin, Z.; Butera, R. A.; Smentkowski, V. S.; Waldeck, D. H. Langmuir 1995, 11, 1849. (14) Bozack, M. J.; Taylor, P. A.; Choyke, W. J.; Yates, J. T., Jr. Surf. Sci. 1986, 177, L933. (15) Hamers, R. J.; Hovis, J. S.; Lee, S.; Liu, H.; Shan, J. J. Phys. Chem. B 1997, 101, 1489. (16) Lopinski, G. P.; Wayner, D. D. M.; Wolkow, R. A. Nature 2000, 406, 48. (17) (a) Linford, M. R.; Chidsey, C. E. D. J. Am. Chem. Soc. 1993, 115, 12631. (b) Linford, M. R.; Fenter, P.; Eisenberger, P. M.; Chidsey, C. E. D. J. Am. Chem. Soc. 1995, 117, 3145. (18) (a) Boukherroub, R.; Morin, S.; Bensebaa, F.; Wayner, D. D. M. Langmuir 1999, 15, 3831. (b) Yu, H.-Z.; Morin, S.; Wayner, D. D. M.; Allongue, P.; Henry de Villenueve, C. J. Phys. Chem. B 2000, 104, 11157. (c) Yu, H.-Z.; Boukherroub, R.; Morin, S.; Wayner, D. D. M. Electrochem. Commun. 2000, 2, 562. (19) (a) Yamada, T.; Takano, N.; Yamada, K.; Yoshitomi, S.; Inoue, T.; Osaka, T. Electrochem. Commun. 2001, 3, 67. (b) Ashurst, W. R.; Yau, C.; Carraro, C.; Howe, R. T.; Maboudian, R. Sens. Actuators, A 2001, 91, 239.

10.1021/la030211s CCC: $27.50 © 2004 American Chemical Society Published on Web 01/23/2004

1208

Langmuir, Vol. 20, No. 4, 2004

latter method is more attractive for the following reasons: (1) a SisC bond is expected to be more stable for hydrolysis than a SisOsSi bond, (2) the monolayer prepared by the latter method is expected to have a higher structural order because an organic molecule directly bonds to the Si atom of the single-crystal surface, and (3) control of the thickness of the oxide is difficult in the former method. Both gas-phase14-16 and liquid-phase17-29 reactions have been proposed as means for forming organic layers via a SisC bond, but the latter is more useful for practical applications because the former requires expensive ultrahigh vacuum equipment to prepare and maintain a clean substrate surface. Recently, we investigated the formation process of an organic monolayer on a hydrogen-terminated Si(111) in a neat alkene liquid, a reactant, at 200 °C, by using attenuated total reflectance Fourier transform infrared (ATR FT-IR) and sum frequency generation (SFG) spectroscopy and found that organic monolayers formed on Si(111) surfaces via SisC bonds show a relatively high conformational order as do alkanethiol SAMs on Au(111).29 Ishibashi et al. also used SFG spectroscopy to investigate the structures of organic monolayers of various chain lengths formed on Si(111) surfaces via SisC bonds and observed only very small amounts of gauche defect.30 They found that the monolayer was formed epitaxially on the Si(111) substrate. To prepare nanosized patterns of organic monolayers on solid substrates, an understanding of and means for controlling the decomposition processes of monolayers are essential. Although much less attention has been paid to decomposition processes than to preparation processes, the decomposition processes of alkanethiol and alkylsiloxane SAMs by UV/ozone treatment have been studied by many groups.31-35 It has been reported that the decomposition of alkanethiol SAMs subjected to UV/ozone treatment proceeds via the attack on thiolate by photo(20) (a) Wagner, P.; Nock, S.; Spudich, J. A.; Volkmuth, W. D.; Chu, S.; Cicero, R. L.; Wade, C. P.; Linford, M. R.; Chidsey, D. E. D. J. Struct. Biol. 1997, 119, 189. (b) Cicero, R. L.; Linford, M. R.; Chidsey, C. E. D. Langmuir 2000, 16, 5688. (c) Barrelet, C. J.; Robinson, D. B.; Cheng, J.; Hunt, T. P.; Quate, C. F.; Chidsey, C. E. D. Langmuir 2001, 17, 3460. (d) Cheng, J.; Robinson, D. B.; Cicero, R. L.; Eberspacher, T.; Barrelet, C. J.; Chidsey, C. E. D. J. Phys. Chem. B 2001, 105, 10900. (21) Wojtyk, J. T. C.; Tomietto, M.; Boukherroub, R.; Wayner, D. D. M. J. Am. Chem. Soc. 2001, 123, 1535. (22) (a) Henry de Villeneuve, C.; Pinson, J.; Bernard, M. C.; Allongue, P. J. Phys. Chem. B 1997, 101, 2415. (b) Allongue, P.; Henry de Villeneuve, C.; Pinson, J.; Ozanam, F.; Chazalviel, J. N.; Wallart, X. Electrochim. Acta 1998, 43, 2791. (c) Fide´lis, A.; Ozanam, F.; Chazalviel, J.-N. Surf. Sci. 2000, 444, L7. (d) Allongue, P.; Henry de Villeneuve, C.; Pinson, J. Electrochim. Acta 2000, 45, 3241. (23) Effenberger, F.; Go¨tz, G.; Bidlingmaier, B.; Wezstein, M. Angew. Chem., Int. Ed. 1998, 37, 2462. (24) (a) Bansal, A.; Li, S.; Lauermann, I.; Lewis, N. S.; Yi, S. I.; Weinberg, W. H. J. Am. Chem. Soc. 1996, 118, 7225. (b) Bansal, A.; Lewis, N. S. J. Phys. Chem. B 1998, 102, 1067. (c) Bansal, A.; Lewis, N. S. J. Phys. Chem. B 1998, 102, 4058. (d) Royea, W. J.; Juang, A.; Lewis, N. S. Appl. Phys. Lett. 2000, 77, 1988. (e) Lewis, N. S. J. Electroanal. Chem. 2001, 508, 1. (f) Bansal, A.; Li, X.; Yi, S. I.; Weinberg, W. H.; Lewis, N. S. J. Phys. Chem. B 2001, 105, 10266. (g) Juang, A.; Scherman, O. A.; Grubbs, R. H.; Lewis, N. S. Langmuir 2001, 17, 1321. (25) (a) He, J.; Patitsas, S. N.; Preston, K. F.; Wolkow, R. A.; Wayner, D. D. M. Chem. Phys. Lett. 1998, 286, 508. (b) Okubo, T.; Tsuchiya, H.; Sadakata, M.; Yasuda, T.; Tanaka, K. Appl. Surf. Sci. 2001, 171, 252. (26) (a) Terry, J.; Linford, M. R.; Wigren, C.; Cao, R.; Pianetta, P.; Chidsey, C. E. D. Appl. Phys. Lett. 1997, 71, 1056. (b) Terry, J.; Linford, M. R.; Wigren, C.; Cao, R.; Pianetta, P.; Chidsey, C. E. D. J. Appl. Phys. 1999, 85, 213. (27) (a) Buriak, J. M. Chem. Commun. 1999, 1051. (b) Robins, E. G.; Stewart, M. P.; Buriak, J. M. Chem. Commun. 1999, 2479. (c) Buriak, J. M. Chem. Rev. 2002, 102, 1271. (28) Sung, M. M.; Kluth, G. J.; Yauw, O. W.; Maboudian, R. Langmuir 1997, 13, 6164. (29) Quayum, M. E.; Kondo, T.; Nihonyanagi, S.; Miyamoto, D.; Uosaki, K. Chem. Lett. 2002, 208. (30) Ishibashi, T.; Ara, M.; Tada, H.; Onishi, H. Chem. Phys. Lett. 2003, 367, 376.

Uosaki et al.

generated ozone or its decomposition product because UV irradiation under an inert atmosphere cannot degrade them31-33 and both ozone and UV are needed for the degradation of alkylsiloxane SAMs.34,35 On the other hand, the UV/ozone-induced decomposition of organic layers formed on Si(111) via SisC bonds has not yet been studied, although there have been a few studies on the thermal decomposition of a monolayer on Si.28 In the present study, we investigated the decomposition process of an organic monolayer on a Si(111) exposed to UV light irradiation or ozone using ATR FT-IR spectroscopy. Exposure to both ozone and UV light resulted in the reduction of IR peaks corresponding to a CH stretching vibration and bending scissors and the appearance of peaks corresponding to a CO stretching vibration and COH inplane bending. The latter peaks initially increased, reached a maximum, and then decreased. While the peaks corresponding to the carboxylic acid group behaved similarly under the condition of exposure to ozone, they were not observed during the decomposition induced by UV irradiation, indicating that the monolayer was decomposed through the formation of oxidized intermediates such as aldehyde and carboxylic acid under the condition of exposure to ozone but via the cleavage of SisC bonds under the condition of UV irradiation. SFG spectroscopy clearly showed that gauche defects increased as the decomposition process proceeded. Experimental Section Materials. Ultrapure-grade sulfuric acid and reagent-grade hydrofluoric acid, hydrogen peroxide, acetone, diethyl ether, ethanol, dichloromethane, and 1-octadecene from Wako Pure Chemicals, hydrochloric acid (ultrapure grade) from Merck, and special-grade (for semiconductor industries) ammonium fluoride solution (40%) from Morita Chemical were used without further purification except for 1-octadecene, which was purified by vacuum distillation before use. Water was purified using a Milli-Q system (Yamato, WQ-500). Ultrapure argon (99.999%) and oxygen (99.95%) gases were purchased from Air Water. A double-sided polished Si(111) single-crystal wafer of 100 mm in diameter, 0.48 mm in thickness, and resistivity of 5-10 Ω cm (phosphorus-doped, 5.0 × 1014 to 1.0 × 1015 cm-3) was donated by Shin-Etsu Semiconductor. Si(111) wafers were cleaned with acetone and then with Milli-Q water several times in an ultrasonic bath for 5 min each time. The silicon wafers were treated in a sulfuric acid and hydrogen peroxide mixture (H2SO4/H2O2, 2:1 by volume) for about 30 min to grow a thin oxide layer on the surface. Parallelogram ATR prisms (28 × 35 × 0.48 mm3) were prepared by cutting and polishing with 45° bevels.36 Sample Preparation. A Si(111) ATR prism was treated by sequential immersion of the sample in freshly prepared sulfuric (31) (a) Tarlov, M. J.; Burgess, D. R. F., Jr.; Gillen, G. J. Am. Chem. Soc. 1993, 115, 5305. (b) Lewis, M.; Tarlov, M.; Carron, K. J. Am. Chem. Soc. 1995, 117, 9574. (c) Poirier, G. E.; Herne, T. M.; Miller, C. C.; Tarlov, M. J. J. Am. Chem. Soc. 1999, 121, 9703. (32) (a) Huang, J.; Hemminger, J. C. J. Am. Chem. Soc. 1993, 115, 3342. (b) Huang, J.; Dahlgren, D. A.; Hemminger, J. C. Langmuir 1994, 10, 626. (c) Behm, J. M.; Lykke, K. R.; Pellin, M.; Hemminger, J. C. Langmuir 1996, 12, 2121. (33) (a) Schoenfisch, M. H.; Pemberton, J. E. J. Am. Chem. Soc. 1998, 120, 4502. (b) Zhang, Y.; Terrill, R. H.; Tanzer, T. A.; Bohn, P. W. J. Am. Chem. Soc. 1998, 120, 2654. (c) Norrod, K. L.; Rowlen, K. L. J. Am. Chem. Soc. 1998, 120, 2656. (d) Ferris, M. M.; Rowlen, K. L. Appl. Spectrosc. 2000, 54, 664. (34) (a) Paz, Y.; Trakhtenberg, S.; Naaman, R. J. Phys. Chem. 1992, 96, 10964. (b) Paz, Y.; Trakhtenberg, S.; Naaman, R. J. Am. Chem. Soc. 1994, 116, 10344. (c) Paz, Y.; Trakhtenberg, S.; Naaman, R. J. Phys. Chem. 1994, 98, 13517. (35) (a) Moon, D. W.; Kurokawa, A.; Ichimura, S.; Lee, H. W.; Jeon, I. C. J. Vac. Sci. Technol., A 1999, 17, 150. (b) Ye, T.; Wynn, D.; Dudek, R.; Borguet, E. Langmuir 2001, 17, 4497. (36) (a) Ye, S.; Ichihara, T.; Uosaki, K. Appl. Phys. Lett. 1999, 75, 1562. (b) Ye, S.; Ichihara, T.; Uosaki, K. J. Electrochem. Soc. 2001, 148, C421.

Decomposition Processes of an Organic Monolayer acid and hydrogen peroxide (2:1 by volume) at 60 °C for 20 min, in 0.5% HF aqueous solution at room temperature (RT) for 5 min, and in freshly prepared RCA solution (H2O/H2O2/HCl, 4:1:1 by volume)36-38 at 80 °C for 20 min. After these treatments, the sample was immersed in 40% deaerated aqueous NH4F solution for 5 min to obtain a monohydride-terminated surface, that is, H-Si(111).36,37 Organic monolayers were prepared by immersing a H-Si(111) ATR prism in neat 1-octadecene at 200 °C for more than 2 h under an Ar atmosphere.29 Before dipping the prism, distilled 1-octadecene was deaerated by a freeze-dry method more than twice, and then the prism was dipped in the deaerated 1-octadecene under an Ar flow. The 1-octadecene liquid was deaerated once more by the same procedure. The liquid containing the prism was then heated to 200 °C for more than 2 h under an Ar atmosphere. The prism was then rinsed with deaerated hexadecane at 200 °C and cooled to RT. The prism was further rinsed with deaerated diethyl ether, deaerated ethanol, and deaerated dichloromethane. Before the IR measurements, the prism was dried with an Ar stream. Decomposition of the Organic Monolayer. Decomposition treatments of the organic monolayer were carried out in the chamber of an UV/ozone cleaner (Nippon Laser & Electronics Lab., NL-UV253), which was equipped with three mercury lamps (254 nm, 2 mW at 3 cm; 185 nm, 20 µW at 3 cm). UV/ozone treatment was carried out in this chamber filled with pure oxygen. Ozone treatment was carried out in the same chamber, but the sample was shaded from UV light by an aluminum shield. UV treatment was carried out in the same chamber filled with ultrapure Ar. ATR FT-IR Measurement.29,36 ATR FT-IR spectra were measured using a Bio-RAD FTS-30 spectrometer equipped with an HgCdTe (mercury cadmium telluride) detector cooled with liquid nitrogen. All of the spectra were measured in p polarization with respect to a spectrum of an oxidized Si(111) surface. All of the spectra were recorded by integrating 256 interferrograms with a resolution of 2 cm-1. SFG Measurement. Detailed description of the SFG system used in the present study has been given elsewhere.39-41 Briefly, a picosecond Nd:YAG laser (PL2143B, EKSPLA) was employed to pump an optical parametric generation/optical parametric amplification/differential frequency generation system, which generates tunable infrared radiation from 2.3 to 8.5 µm. The second harmonic output (532 nm) from the Nd:YAG laser was used as the visible light. The SFG, visible, and IR lights were all p polarized. An azimuthal angle (φ) was defined to be the angle between the (110) direction of the Si(111) substrate and the plane of incidence. The SFG signal was separated from reflected visible and IR light by passing through irises, and a monochromator (Oriel Instruments, MS257) was detected by a photomultiplier tube (Hamamatsu, R3896) and was normalized to the intensities of the infrared and visible beams.

Results and Discussion Figure 1 shows ATR FT-IR spectra of the C18 monolayer formed on the Si(111) surface after exposure to both UV light and ozone for (a) 0, (b) 1, (c) 5, (d) 10, (e) 30, (f) 60, and (g) 90 min. The spectrum in Figure 1h was measured after exposure to both UV light and ozone for 90 min followed by washing with pure water. Before the exposure (Figure 1a), four prominent peaks were observed at 2960 cm-1 (I), 2920 cm-1 (II), 2950 cm-1 (III), and 1468 cm-1 (V), corresponding to the asymmetric CH3 stretching, asymmetric CH2 stretching, symmetric CH2 stretching, and bending scissors, respectively. The spectrum was (37) (a) Kern, W.; Puotinen, D. RCA Rev. 1970, 31, 187. (b) Kern, W. RCA Rev. 1978, 39, 278. (38) Higashi, G. S.; Chabal, Y. J. In Handbook of Semiconductor Wafer Cleaning Technology; Kern, W., Ed.; Noyes: Park Ridge, 1994; p 433. (39) Ye, S.; Nihonyanagi, S.; Uosaki, K. Chem. Lett. 2000, 734. (40) Ye, S.; Saito, T.; Nihonyanagi, S.; Uosaki, K.; Miranda, P. B.; Kim, D.; Shen, Y. R. Surf. Sci. 2001, 476, 121. (41) Ye, S.; Nihonyanagi, S.; Uosaki, K. Nonlinear Opt. 2000, 24, 93.

Langmuir, Vol. 20, No. 4, 2004 1209

Figure 1. ATR FT-IR spectra of the C18 monolayer after exposure to both ozone and UV irradiation for (a) 0, (b) 1, (c) 5, (d) 10, (e) 30, (f) 60, and (g) 90 min. (h) ATR FT-IR spectrum of the C18 monolayer after being washed with pure water following exposure to both ozone and UV irradiation for 90 min (g).

essentially the same as the one previously reported, indicating the formation of the C18 monolayer on the Si(111) surface in the present study. The intensities of these peaks decreased with an increase in the exposure time, indicating that the organic monolayer on the Si(111) surface was degraded by the UV/ozone treatment. A new peak around 1710 cm-1 (IV) appeared after only 1 min of exposure to UV light and ozone, and another peak around 1420 cm-1 (VI) appeared after exposure for 10 min. The former peak can be assigned to the CdO stretching vibration and the latter to the Cs OsH in-plane bending mode. The exposure time dependencies of (a) the integrated peak intensities of the CH2 and CH3 stretching modes, (b) the integrated peak intensity of the CdO stretching mode, and (c) the peak position of the CH2 asymmetric mode are summarized in Figure 2. The integrated peak intensities of the CH2 and CH3 stretching modes monotonically decreased with an increase in the exposure time. The integrated peak intensity of the CdO stretching increased, reached a maximum, and then gradually decreased with an increase in the exposure time. These results indicate that oxidized intermediates were formed during the decomposition process. Because the CdO stretching peak appeared before the CsOsH in-plane bending mode, aldehyde and carboxylic acid seem to be the most likely intermediates, although the formation of alcohol and ketone cannot be denied. The fact that a peak corresponding to the OH stretching vibration of the carboxylic acid group was not observed around 3000 cm-1 in any spectra suggests that the terminal carboxylic groups in the remaining monolayer were not hydrogen-bonded to each other.42,43 The peak position of the CH2 asymmetric mode shifted to higher frequency with an increase in the decomposition time, indicating that the monolayer became less crystalline as the decomposition proceeded.3,5e,29,44 No peaks were observed after 90 min of exposure followed by washing with pure water (Figure 1h), indicating that the monolayer (42) Schubert, C. C.; Pease, R. N. J. Am. Chem. Soc. 1956, 78, 5553. (43) Razumovskii, S. D.; Zaikov, G. E. Ozone and Its Reactions with Organic Compounds; Elsevier: Amsterdam, 1984; p 237.

1210

Langmuir, Vol. 20, No. 4, 2004

Uosaki et al.

Figure 3. SFG spectra in the CH stretching vibration region of the C18 monolayer recorded after exposure to both UV light and ozone for (a) 0, (b) 0.5, (c) 1.5, (d) 2.5, (e) 4.5, (f) 10, (g) 20, and (h) 40 min.

Figure 2. (a) Integrated peak intensity of the CH2 and CH3 stretching modes, (b) integrated peak intensity of the CdO stretching mode, and (c) peak position of the CH2 asymmetric mode as a function of exposure time to ozone and UV irradiation.

was totally decomposed and the surface was covered with the oxide. The hydrogen-terminated surface was recovered by the treatment described in the Experimental Section. Figure 3 shows SFG spectra in the CH stretching vibration region of the C18 monolayer formed on the Si(111) surface that were recorded after exposure to both UV light and ozone for (a) 0, (b) 0.5, (c) 1.5, (d) 2.5, (e) 4.5, (f) 10, (g) 20, and (h) 40 min. The peaks observed at 2878, 2945, and 2962 cm-1 can be assigned to CH symmetric (r+), Fermi resonance between r+ and the CH bending overtone, and the CH asymmetric (r-), respectively, of the terminal methyl (CH3) group. The two peaks attributed to CsH symmetric stretching (d+) and asymmetric stretching (d-) of the methylene (CH2) group are at 2850 and 2917 cm-1, respectively. Before the UV/ozone exposure (Figure 3a), only peaks corresponding to the methyl group were observed in the SFG spectrum as reported previously,29 confirming the formation of the C18 monolayer of essentially an all-trans conformation. All of the SFG spectra were fitted to eq 1 with the five peak components mentioned previously.

ISFG ) |

∑n ω

2

An - ωn + iΓ

i 2 + |χ(2) NR|e |

(1)

Here, ω2, χNR, , An, and Γn are the frequency of the incident beam, second-order nonlinear susceptibility corresponding to the nonresonant component, phase angle between the resonant and the nonresonant contributions, strength, and damping constant of the surface vibration or rotation

Figure 4. Amplitude of the SFG peak corresponding to CH asymmetric stretching vibration of the terminal methyl group, r-, normalized to the nonresonant component, χNR (white circles), and the ratio between the SFG peaks corresponding to the CH asymmetric stretching vibration of the terminal methyl (r-) and methylene (d-) groups (black circles) as a function of the UV/ozone exposure time.

mode with frequency ωn, respectively. The results are summarized in Figure 4, in which the amplitude of the SFG peak corresponding to the CH asymmetric stretching vibration of the terminal methyl group, r-, normalized to the nonresonant component, χNR (white circles), and the ratio between the SFG peaks corresponding to the CH asymmetric stretching vibration of the terminal methyl (r-) and methylene, d-, groups (black circles) are plotted as a function of the UV/ozone exposure time. It is clear that the amplitude of the SFG peak corresponding to the CH asymmetric stretching vibration of the terminal methyl group decreased with time, supporting the results of ATR FT-IR measurement that the organic monolayer on the Si(111) surface was degraded by the UV/ozone treatment. Furthermore, the relative intensity of the CH asymmetric stretching peak of the methylene group to that of the terminal methyl group increased with time, indicating that gauche defects increased as the decomposition proceeded. This result is in good agreement with the ATR FT-IR observation that the monolayer became less crystalline as the decomposition proceeded. To clarify the roles of UV and ozone separately, the monolayer-covered surface was exposed only to UV

Decomposition Processes of an Organic Monolayer

Langmuir, Vol. 20, No. 4, 2004 1211

Figure 5. ATR FT-IR spectra of the C18 monolayer after exposure only to (i) UV irradiation and (ii) ozone for (a) 0, (b) 10, (c) 30, (d) 60, (e) 120, (f) 180, and (g) 240 min. (h) ATR FT-IR spectra of the C18 monolayer after (g) and piranha cleaning for 20 min.

irradiation or to ozone. Figure 5 shows ATR FT-IR spectra of the C18 monolayer formed on Si(111) after (i) treatment with only UV irradiation and (ii) treatment with only ozone. In both cases, the originally observed peaks, I, II, III, and V, decreased with the increase in the exposure time, indicating that the organic monolayer can be decomposed by exposure to only UV irradiation or to only ozone. The exposure time dependencies of the integrated peak intensity of the CH2 and CH3 stretching modes shown in Figure 6a clearly indicate that the rates of decomposition under these conditions were much slower than the rate of decomposition under the condition of exposure to both UV irradiation and ozone. No IR peaks were observed after 240 min of exposure to UV irradiation or ozone followed by piranha cleaning for 20 min, and the total removal of the monolayers from the surface was confirmed. Peaks IV and VI corresponding to the carboxylic acid/ aldehyde group were also observed when the monolayer was exposed to only ozone but were absent in the spectra obtained during the decomposition induced only by UV irradiation (Figure 6b). These results clearly show that intermediates with a carboxylic acid/aldehyde group were formed by exposure to ozone. It is well-known that an alkane produces a carboxylate group through the formation of an alcohol and aldehyde induced by ozone treatment.43 The peak position of the CH2 asymmetric mode shifted to a higher frequency in both cases but more rapidly during ozone exposure, although the decomposition rates did not seem to be greatly different, as indicated by the decrease in the integrated peak intensity of the CH2 and CH3 stretching modes. The decomposition processes by UV and ozone are schematically shown in Schemes 1 and 2, respectively. In the case of decomposition under the condition of exposure to ozone, the photogenerated ozone mainly attacks the terminal methyl group of the organic monolayer and converts it to a carboxylic acid group, which is further decomposed via decarboxylation (Scheme 1). On the other hand, no intermediate with an IR-active functional group

Figure 6. Effects of the exposure time to UV (open circles) and ozone (closed circles) on the (a) integrated peak intensity of the CH2 and CH3 stretching, (b) integrated peak intensity of the CdO stretching, and (c) peak position of the CH2 asymmetric stretching.

1212

Langmuir, Vol. 20, No. 4, 2004

Uosaki et al.

Scheme 1. Schematic Representation of the Decomposition Process of a C18 Monolayer Bonded to a Si(111) Surface under the Condition of Exposure to Ozone

Scheme 2. Schematic Representation of the Decomposition Process of a C18 Monolayer Bonded to a Si(111) Surface under the Condition of Exposure to UV Irradiation.

seems to be formed during the decomposition induced by only UV irradiation because no additional IR peaks were observed. It has been reported that alkanethiol SAMs on Au or Ag cannot be decomposed by UV irradiation in an oxygen-free atmosphere.31-33 This means that neither the CsH nor the CsC bond can be cleaved by UV irradiation. Photocleavage of SisC bonds of organic silane films by deep UV was reported by Calvert and colleagues,45,46 but the dose required for the photocleavage of a monolayer without a π bond is much higher, for example, 13-15 J/cm2 for the monolayer formed from N-(2-aminoethyl)-3-aminopropyltrimethoxy silane,46 than the dose in the present case, which was about 0.072 J/cm2 for the 1-hr exposure to 185-nm light. Thus, SisC bond cleavage by UV irradiation is also not feasible. These considerations and the fact that both ozone and UV are needed for the degradation of the alkylsiloxane SAMs on SiO234,35 strongly suggest that the Si substrate plays a critical role in the decomposition of the monolayer induced by UV irradiation. Because Si is a semiconductor with an energy gap of 1.06 eV, UV photons are absorbed by Si near the Si/organic monolayer interface and generate electron-hole pairs. Electrons or holes are transferred to the SisC bond, leading to the cleavage of this bond and the decomposition of the monolayer (Scheme 2). It seems reasonable to assume that the order of the monolayer is maintained until a certain amount of the monolayer has been (44) Anderson, M. R.; Gatin, M. Langmuir 1994, 10, 1638. (45) Stenger, D. S.; Georger, J. H.; Dulcey, C. S.; Hickman, J. J.; Rudolph, A. S.; Nielsen, T. B.; McCort, S. M.; Calvert, J. M. J. Am. Chem. Soc. 1992, 114, 8435. (46) Dulcey, C. S.; Georger, J. H., Jr.; Chen, M.-S.; McElvany, S. W.; O’Ferrall, C. E.; Benezra, V. I.; Calvert, J. M. Langmuir 1996, 12, 1638.

degraded, resulting in the relatively slow shift of the peak position of the CH2 asymmetric mode to higher frequencies, as shown in Figure 6c. To clarify the decomposition mechanisms more quantitatively, the irradiation wavelength dependence is now under investigation. Conclusions ATR FT-IR spectroscopy showed that an organic monolayer formed on Si(111) via a SisC covalent bond was decomposed by exposure to UV light irradiation or ozone. In all cases, the IR peaks corresponding to the CH stretching vibration and bending scissors decreased with time, but the rates of decomposition were lower during exposure only to UV light or ozone. When the monolayer was exposed to ozone, peaks corresponding to CO stretching and COH in-plane bending initially increased, reached a maximum, and then decreased, indicating that the monolayer was decomposed through the formation of intermediates with carboxylic acid/aldehyde groups. Peaks corresponding to intermediates were not observed during the decomposition induced by UV irradiation. SFG measurement confirmed an increase of gauche defects as the decomposition proceeded. Mechanisms for the monolayer decomposition induced by UV irradiation and ozone exposure are proposed. Acknowledgment. This work was partially supported by Grants-in-Aid for Scientific Research from the Ministry of Education, Science and Culture of Japan (13304047, 13554026, and 13874085). LA030211S