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Notes Photochemistry and Patterning of Monolayer Films from 11-Phenylundecyltrichlorosilane Andrienne C. Friedli,*,† Rachel D. Roberts,†,‡ Charles S. Dulcey,§ Andro R. Hsu,‡ Stephen W. McElvany,| and Jeffrey M. Calvert§,⊥ Department of Chemistry, Middle Tennessee State University, Murfreesboro, Tennessee 37132, Geo-Centers, Inc., Fort Washington, Maryland 20744, Center for Bio/ Molecular Science and Engineering (Code 6950), Naval Research Laboratory, Washington, D.C. 20375-5348, and Chemistry Division (Code 6500), Naval Research Laboratory, Washington, D.C. 20375-5348 Received September 8, 2003. In Final Form: February 17, 2004
Self-assembled monolayer (SAM) films offer an effective means for tailoring the surface chemical functionality of a substrate and, consequently, properties such as reactivity, wetting, and adhesion.1 Οrganosilanes are a particularly useful class of SAM film precursors because of their ability to modify a wide variety of hydroxylated substrates.2 Patterning of organosiloxane SAMs with radiation has been reported using visible,3 UV,4-6 extreme UV,7 soft8 and synchrotron7 X-ray, and electron9 and ion beam9,10 sources. Phenyl-5 and 4-(chloromethyl)phenyl (i.e., benzyl chloride) terminated11,12 SAM films with short alkyl chains * To whom correspondence may be addressed. Phone/Fax: 615898-2071/5182. E-mail:
[email protected]. † Middle Tennessee State University. ‡ Geo-Centers, Inc. § Center for Bio/Molecular Science and Engineering, Naval Research Laboratory. | Chemistry Division, Naval Research Laboratory. ⊥ Principal author. Present address: Shipley Co., 455 Forest St., Marlborough, MA 01752. (1) Ulman, A. Chem. Rev. 1996, 96, 1533-1554 and references therein. (2) Plueddemann, E. P. Silane Coupling Agents; Plenum Press: New York, 1982. (3) For example: Collins, R. J.; Bae, I. T.; Scherson, D. A.; Sukenik, C. N. Langmuir 1996, 12, 5509-5511. Wrighton, M. S.; Rozsnyai, L. F. Langmuir 1995, 11, 3913-3920. Fodor, S. P. A.; Read, J. L.; Pirrung, M. C.; Stryer, L.; Lu, A. T.; Solas, D. Science 1991, 251, 767-773. (4) Calvert, J. M. In Thin Films; Ulman, A., Ed.; Academic Press: Boston, 1995, Vol. 20, 109-141 and references therein. (5) 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, 16381650. (6) Stenger, D. A.; Georger, J. H.; Dulcey, C. S.; Hickman, J. J.; Rudolph, A. S.; Nielson, T. B.; McCort, S. M.; Calvert, J. M. J. Am. Chem. Soc. 1992, 114, 8435-8442. (7) Yang, X. M.; Peters, R. D.; Kim, T. K.; Nealey, P. F. J. Vac. Sci. Technol., B 1999, 17 (6), 3203-3207. (8) Kim, T. K.; Yang, X. M.; Peters, R. D.; Sohn, B. H.; Nealey, P. F. J. Phys. Chem. B 2000, 104, 7403-7410 and references therein. (9) Perkins, F. K.; Dobisz, E. A.; Brandow, S. L.; Calvert, J. M.; Kosakowski, J. E.; Marrian, C. R. K. Appl. Phys. Lett. 1996, 68, 550552 and references therein. (10) Ada, E. T.; Hanley, L.; Etchin, S.; Melngailis, J.; Dressick, W. J.; Chen, M.-S.; Calvert, J. M. J. Vac. Sci. Technol., B 1995, 13, 21892196. (11) Brandow, S. L.; Chen, M.-S.; Aggarwal, R.; Dulcey, C. S.; Calvert, J. M.; Dressick, W. J. Langmuir 1999, 15, 5429-5432. (12) Dressick, W. J.; Chen, M.-S.; Brandow, S. L. J. Am. Chem. Soc. 2000, 122, 982-983.
(zero to two methylene groups) are efficient low-dose imaging films that can be photopatterned at high resolution using conventional UV lamp and laser exposure sources. The benzyl chloride films readily undergo grafting reactions in irradiated regions, while masked regions are unreactive.11,12 Films with phenyl, benzyl, and phenethyl groups5 photocleave selectively at the Si-C bond to remove the entire organic fragment and subsequently form surface silanols (Si-OH). The photogenerated silanol can then be backfilled, reacted with another organosilane precursor, to modify the surface with a different chemical functionality in the exposed regions. However, fabrication of welldefined coplanar assemblies of SAMs using short-chain precursors has been limited by interpenetration of a second backfilled component into the original masked film.5,6 Films prepared from organosilanes with long alkyl chains, such as octadecyltrichlorosilane, are excellent barriers against penetration by incoming solution reagents but are virtually inert to visible or UV irradiation.13 In this work, we examine the properties and photochemistry of 11-phenylundecyltrichlorosilane (1) SAMs, with the goal of utilizing both the enhanced surface blocking properties observed with long alkyl chains,14 and photochemical sensitivity of the phenyl group.
Compound 115 was synthesized in an overall yield of 26% from chlorobenzene and 11-bromoundecene via a Kumada coupling reaction, followed by hydrosilylation. Acid-cleaned substrates16 were immersed for 150 min in 0.1% solutions of 1 in anhydrous toluene, followed by (13) Wasserman, S. R.; Tao, Y.-T.; Whitesides, G. M. Langmuir 1989, 5, 1074-1087. (14) Friedli, A. C.; Dulcey, C. S.; Hsu, A. R.; Roberts, R. D.; Calvert, J. M. Book of Abstracts; 209th National American Chemical Society Meeting, Anaheim, CA, April 2-5, 1995. Abstract COL 010. Friedli, A. C.; Dulcey, C. S.; Hsu, A. R.; Roberts, R. D.; Calvert, J. M. Abstracts, Materials Research Society Meeting; Boston, MA, Nov. 27-Dec. 1, 1995, Abstract O157. (15) Compound 1: bp (Kugelrohr) 120-132 °C (6 mT); 1H NMR δ 7.32-7.20 (m, max at 7.27, 2H), 7.19-7.14 (m, max at 7.17, 3H), 2.59 (t, J ) 7.7 Hz, 2H), 1.60-1.54 (m, center at 1.57, 4H), 1.39-1.22 (bm, max at 1.26, 16H); 13C NMR δ 142.8, 128.4, 128.2, 125.5, 36.0, 31.8, 31.5, 29.5 (3 × C), 29.3 (2 × C), 29.0, 24.3, 22.3; IR 3026, 2926, 2860, 1609, 1458, 1029, 751, 696, 594, 567 cm-1; UV λmax (log ) 191, 261, 268; EIMS, m/z 368(M + 4, 4), 366(M + 2, 8), 364(M+, 8), 133(19), 105(17), 104(11), 92(90), 91(100). Anal. Calcd for C17H27Cl3Si: C, 55.81; H, 7.44; Si, 7.68. Found: C, 56.17; H, 7.69; Si, 8.21. (16) Cleaning of silicon (quartered 2 in. Si 100 wafers with native oxide, Wafernet, Inc.), Si with a thermal oxide (380 Å controlled oxidation at NRL) and 1 cm2 fused silica (Dell Optics), and aluminum-coated microscope slide (NRL) substrates included immersion in 1:1 HCl/ methanol for 30 min, rinsing with ultrapure water (Barnstead Nanopure filtration system, 18 MΩ), immersion in 18 M H2SO4 for 30 min, rinsing three times with ultrapure water (the last rinse in boiling water), and blowing dry with nitrogen. Pt surfaces (prepared by evaporation of Pt onto Si wafers at NRL) were cleaned/oxidized in piranha solution (30: 70 35% H2O2 in H2SO4).
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several rinses in toluene and baking 5 min on a 120° hotplate to form monolayer films. Good quality films are formed reproducibly, in contrast to phenoxyalkylsiloxane17 and midlength (three to six methylene) ω-phenylalkylsiloxane18 films. Surface characteristics consistent with a monolayer film include an absorbance at λmax ) 191 nm of 0.060 ( 0.005, an ellipsometric thickness of 17 ( 2 Å, and a static water contact angle (θw) of 86 ( 4°.19 Contact angles for 1 films were independent of substrate and were essentially unchanged after storage under ambient conditions for >1 year; boiling in deionized water resulted in only slight changes in wettability ((4° of initial value), indicating excellent film stability.20 The atomic force microscopy (AFM) root-mean-square roughness values of 1.4-1.8 Å21 were comparable to values measured for films from the short-chain analogue, phenyltrichlorosilane 2. The degree of interpenetration of a silane backfilled into an existing film was probed by treatment of a 1 SAM with a 1% solution of (tridecafluoro(1,1,2,2-tetrahydro)octyl)dimethylchlorosilane (3) in toluene for 30 min according to a literature procedure.6 The resulting film exhibited an Si/F ratio measured by X-ray photoelectron spectroscopy (XPS) corresponding to 6% of a monolayer of 3. This value represents a ∼7-fold improvement in surface coverage when compared to a film of 2, in which backfilling of 44% of a monolayer of 3 was observed under the same conditions. The physical and chemical properties described above present a picture which is consistent with monolayer coverage of fully extended 1 molecules and enhanced barrier properties compared to short-chain silanes. The deep UV photochemistry of films of 1 was investigated using 193 nm excimer laser exposure in ambient air, with pulse energies between 0.5 and 1.5 mJ/cm2. The UV signature of 1 was reduced to baseline noise after irradiation with
