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Langmuir 2007, 23, 8876-8881
UV Photooxidation and Photopatterning of Alkanethiolate Self-Assembled Monolayers (SAMs) on GaAs (001) Chuanzhen Zhou and Amy V. Walker* Department of Chemistry and Center for Materials InnoVation, Washington UniVersity in St. Louis, Campus Box 1134, One Brooking DriVe, St. Louis, Missouri 63130 ReceiVed March 16, 2007. In Final Form: May 13, 2007 We have investigated the photooxidation of alkanethiolate self-assembled monoalyers (SAMs) adsorbed on GaAs (001) using time-of-flight secondary ion mass spectrometry. Both -CH3- and -COOH-terminated SAMs undergo photoreaction to form sulfonated species upon exposure to UV light from a 500 W Hg arc lamp (λ ) 280-440 nm) in the presence of oxygen. In contrast to SAMs adsorbed on metals, the photooxidation of octadecanethiol adsorbed on GaAs can be fit to two first-order reactions: a fast initial reaction followed by a second slower reaction (∼6 times slower). For SAMs with shorter alkyl chain lengths, the photooxidation process is can be fit to a single first-order reaction. Using the optimal photooxidation time, we also demonstrate that SAMs can be successfully UV photopatterned on GaAs substrates producing sharp, well-defined patterns.
1. Introduction Organic monolayers on semiconductors are important both scientifically and technologically.1-12 Organic monolayers, such as self-assembled monnolayers (SAMs), are able to modify both chemical13-16 and electrical17,18 properties of semiconductors. Applications of SAM/semiconductor constructs include biosensing,10 corrosion inhibition,13 and molecular/organic electronics.17-23 Gallium arsenide is the most widely used III-V semiconductor. There have been a number of reports of organized SAM formation on GaAs using alkanethiols2,3,9,17,18,24-30 and aromatic thiols.8,31,32 * To whom correspondence should be addressed. E-mail: walker@ wustl.edu. Phone: 314 935 8496. Fax: 314 935 4481. (1) Lunt, S. R.; Ryba, G. N.; Santangelo, P. G.; Lewis, N. S. J. Appl. Phys. 1991, 70, 7449-7465. (2) Nakagawa, O. S.; Ashok, S.; Sheen, C. W.; Martensson, J.; Allara, D. L. Jpn. J. Appl. Phys. 1991, 30, 3759-3762. (3) Sheen, C. W.; Shi, J.-X.; Martensson, J.; Parikh, A. N.; Allara, D. L. J. Am. Chem. Soc. 1992, 114, 1514-1515. (4) Yang, G. H.; Zhang, Y.; Kang, E. T.; Neoh, K. G.; Huang, W.; Teng, J. H. J. Phys. Chem. B 2003, 107, 8592-8598. (5) Camillone, N.; Khan, K. A.; Osgood, R. M. Surf. Sci. 2000, 453, 83-102. (6) Donev, S.; Brack, N.; Paris, N. J.; Pigram, P. J.; Singh, N. K.; Usher, B. F. Langmuir 2005, 21, 1866-1874. (7) Cho, Y.; Ivanisevic, A. J. Phys. Chem. B 2005, 109, 12731-12737. (8) Adlkofer, K.; Eck, W.; Grunze, M.; Tanaka, M. J. Phys. Chem. B 2003, 107, 587-591. (9) Ye, S.; Li, G.; Noda, H.; Uosaki, K.; Osawa, M. Surf. Sci. 2003, 529, 163-170. (10) Abdelghani, A. Mater. Lett. 2001, 50, 73-77. (11) Shaporenko, A.; Adlkofer, K.; Johansson, L. S. O.; Ulman, A.; Grunze, M.; Tanaka, M.; Zharnikov, M. J. Phys. Chem. B 2004, 108, 17964-17972. (12) Shaporenko, A.; Adlkofer, K.; Johansson, L. S. O.; Tanaka, M.; Zharnikov, M. Langmuir 2003, 19, 4992-4998. (13) Kirchner, C.; George, M.; Stein, B.; Parak, W. J.; Gaub, H. E.; Seitz, M. AdV. Funct. Mater. 2002, 12, 266-276. (14) Tiberio, R. C.; Craighead, H. G.; Lercel, M.; Lau, T.; Sheen, C. W.; Allara, D. L. Appl. Phys. Lett. 1993, 62, 476-478. (15) Lercel, M. J.; Redinbo, G. F.; Pardo, F. D.; Rooks, M.; Tiberio, R. C.; Simpson, P.; Craighead, H. G.; Sheen, C. W.; Parikh, A. N.; Allara, D. L. J. Vac. Sci. Technol. B 1994, 12, 3663-3667. (16) Lercel, M. J.; Rooks, M.; Tiberio, R. C.; Craighead, H. G.; Sheen, C. W.; Parikh, A. N.; Allara, D. L. J. Vac. Sci. Technol. B 1995, 13, 1139-1143. (17) Nesher, G.; Vilan, A.; Cohen, H.; Cahen, D.; Amy, F.; Chan, C.; Hwang, J.; Kahn, A. J. Phys. Chem. B 2006, 110, 14363-14371. (18) Ramashan, K.; Bhat, K. N. Thin Solid Films 1999, 342, 20-29. (19) Janes, D. B.; Lee, T.; Liu, J.; Batisuta, M.; Chen, N.-P.; Walsh, B. L.; Andres, R. P.; Chen, E.-H.; Melloch, M. R.; Woodall, J. M.; Reifenberger, R. J. Electron. Mater. 2000, 29, 565-569. (20) Hsu, J. W. P.; Loo, Y. L.; Lang, D. V.; Rogers, J. A. J. Vac. Sci. Technol. B 2003, 21, 1928-1935.
However, a large variation in the structure of these SAMs was observed. For example, the thickness of octadecanthiolate (ODT) SAMs adsorbed on GaAs (001) ranges from 12 to 22 Å.9,18,26-28 Recently, McGuinness et al.28 reported reproducible, well-ordered SAM formation for ODT adsorbed on GaAs(001) under carefully controlled experimental conditions. We note, however, that the nature of the bonding between the thiol and GaAs remains controversial. It has been reported the thiol binds via Ga-S,6 As-S,7,9,12,27 and both Ga-S and As-S bonding.4,30 A number of methods have been employed to pattern SAMs including microcontact printing, nanoimprinting, and scanningprobe-based nanolithographies (see refs 33 and 34 and references therein). To date, there have been few studies of the patterning of SAMs adsorbed on semiconductors. One of the advantages of SAMs adsorbed on GaAs is that they act as corrosion inhibitors.13 Thus, SAMs have been employed as etch resists after nanoscale patterning using electron beam lithography, which requires high-coverage, thin (
