Molecular Layer Deposition of Thiol−Ene Multilayers on

Oct 30, 2009 - ... resulting in depletion of the available functional groups for the next molecular layer and finally arresting film growth after seve...
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Molecular Layer Deposition of Thiol-Ene Multilayers on Semiconductor Surfaces Yun-hui Li,†,‡,§ Dong Wang,‡ and Jillian M. Buriak*,†,‡ †

Department of Chemistry, University of Alberta, Edmonton, AB T6G 2G2, Canada, and ‡the National Institute for Nanotechnology (NINT), National Research Council, 11421 Saskatchewan Drive, Edmonton, AB T6G 2M9, Canada. § Present address: Steacie Institute for Molecular Sciences (SIMS), National Research Council, 100 Sussex Drive, Ottawa, ON K1A 0R6, Canada. Received July 3, 2009

The fabrication of organic thin films with controlled chemical structure in the vertical direction (parallel to surface normal) is important for many practical and technological applications in organic electronics, chemical-resistant films, and biocompatible materials, among others. In order to achieve composition control in the z-direction, molecular layer deposition (MLD: covalent layer-by-layer assembly) of thin, organic films on silicon, silicon oxide, and germanium surfaces was carried out, using the well-established UV-induced thiol-ene reaction. Through successive contact of an interface with dithiol and diene molecules under UV irradiation for short periods (∼30 min, room temperature), welldefined thin films can be obtained. Linear increases in film thickness with respect to layer number were obtained for shorter aliphatic dienes and dithiols (C e 8), but with longer molecules and with aromatic substrates a self-limiting situation sets in whereby both ends of the molecule react with the surface, arresting film growth. The functionalized interfaces were characterized by ellipsometry, X-ray photoelectronic spectroscopy, and atomic force microscopy.

Introduction The controlled assembly of multilayer structures on solid substrates has attracted much interest for fabricating advanced film structures with multifunctional compositions.1 Layerby-layer (LBL) assembly has been clearly demonstrated to be a simple, effective, and versatile method to prepare multilayer films.2 Thanks to innate control of film thickness and composition in the vertical direction (parallel to the surface normal), LBL provides a route toward the formation of robust, multidimensional structures with incorporation of desired elements in a straightforward and efficient manner. A range of different interactions has been utilized to produce multilayer films via a LBL assembly approach, including electrostatic interactions,3 hydrogen *To whom correspondence should be addressed. E-mail: jburiak@ ualberta.ca. (1) (a) Kuhn, H.; Mobius, D. Angew. Chem., Int. Ed. Engl. 1971, 10, 620. (b) Roberts, G. Langmuir-Blodgett Films; Plenum Press: New York, 1990; and references therein.(c) Decher, G. Science 1997, 277, 1232. (d) Talham, D. R. Chem. Rev. 2004, 104, 5479–5502. (e) Cao, G.; Hong, H.-G.; Mallouk, T. E. Acc. Chem. Res. 1992, 25, 420. (f) Multilayer Thin Films; Decher, G., Schlenoff, J., Eds.; Wiley-VCH: Weinheim, 2003.(g) Hammond, P. T. Adv. Mater. 2004, 16, 1271. (2) (a) Decher, G.; Hong, J. D. Ber. Bunsen-Ges. 1991, 95, 1430. (b) Quinn, J. F.; Johnston, A. P. R.; Such, G. K.; Zelikin, A. N.; Caruso, F. Chem. Soc. Rev. 2007, 36, 707. (c) Johnston, A. P. R.; Cortez, C.; Angelatos, A. S.; Caruso, F. Curr. Opin. Colloid Interface Sci. 2006, 11, 203. (d) Zhang, X.; Shen, J. Adv. Mater. 1999, 11, 1139. (3) (a) Decher, G.; Hong, J.-D.; Schmitt, J. Makromol. Chem., Macromol. Symp. 1991, 46, 321. (b) Sukhishvili, S. A. Curr. Opin. Colloid Interface Sci. 2005, 10, 37. (c) Bertrand, P.; Jonas, A.; Laschewsky, A.; Legras, R. Macromol. Rapid Commun. 2000, 21, 319–348. (d) Donath, E.; Sukhorukov, G. B.; Caruso, F.; Davis, S. A.; M€ohwald, H. Angew. Chem., Int. Ed. 1998, 37, 2202. (e) Caruso, F.; Caruso, R. A.; M€ohwald, H. Science 1998, 282, 1111. (f) Sukhorukov, G. B.; Donath, E.; Lichtenfeld, H.; Knippel, E.; Knippel, M.; Budde, A.; M€ohwald, H. Colloids Surf., A 1998, 137, 253. (g) Peyratout, C. S.; D€ahne, L. Angew. Chem., Int. Ed. 2004, 43, 3762. (4) (a) Stockton, W. B.; Rubner, M. F. Macromolecules 1997, 30, 2717. (b) Wang, L.; Wang, Z. Q.; Zhang, X.; Shen, J. C.; Chi, L. F.; Fuchs, H. Macromol. Rapid Commun. 1997, 18, 509. (c) Sukhishvili, S. A.; Granick, S. J. Am. Chem. Soc. 2000, 122, 9550. (d) Quinn, J. F.; Caruso, F. Macromolecules 2005, 38, 3414. (e) Yang, S. Y.; Rubner, M. F. J. Am. Chem. Soc. 2002, 124, 2100. (f) Chen, J.; Cao, W. Chem. Commun. 1999, 1711. (g) Zhang, Y. J.; Guan, Y.; Yang, S. G.; Xu, J.; Han, C. C. Adv. Mater. 2003, 15, 832. (h) Kozlovskaya, V.; Ok, S.; Sousa, A.; Libera, M.; Sukhishvili, S. A. Macromolecules 2003, 36, 8590. (i) Yang, S. Y.; Lee, D.; Cohen, R. E.; Rubner, M. F. Langmuir 2004, 20, 5978. (j) Zelikin, A. N.; Li, Q.; Caruso, F. Angew. Chem., Int. Ed. 2006, 45, 7743.

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bonding,4 metal-ligand interactions,5 and others. The versatility of the LBL technique is apparent, as films produced via this approach have incorporated a huge variety of host materials, including polymers, nanoparticles, biological molecules, and others.1-5 A recent development within the LBL family, self-limited sequential reactions that result in covalent linkages have been explored to fabricate covalently bonded multilayer assemblies.6-10 Because of the similarity to atomic layer deposition (ALD) with respect to building precisely controlled multilayer film structures of defined thicknesses and makeup, the term molecular layer deposition (MLD) is now used to describe covalently linked LBL films.6 Through usage of bifunctional R,ω-terminated molecules as building blocks, and sequential dosing of these molecules onto a surface, multilayer films with covalently bonded, precisely controlled composition can be obtained. Representative reactions explored for MLD include condensation reactions such as the formation of polyurethane, polyurea, polyimide, polyamide, polyimide-amide, and other polar covalent bonding configurations.11 In order to further expand the scope of reactions (5) Hatzor, A.; Moav, T.; Doron-Mor, I.; Cohen, H.; Matlis, S.; Libman, J.; Vaskevich, A.; Shanzer, A.; Rubinstein, I. J. Am. Chem. Soc. 1998, 120, 13469– 13477. (6) George, S. M.; Yoon, B.; Dameron, A. A. Acc. Chem. Res. 2009, 42, 498–508. (7) Kim, Y.-G.; Crooks, R. M. Langmuir 2005, 21, 11262. (8) Jiao, J.; Anariba, F.; Tiznado, H.; Schmidt, I.; Lindsey, J. S.; Zaera, F.; Bocian, D. F. J. Am. Chem. Soc. 2006, 128, 6965. (9) Kohli, P.; Blanchard, G. J. Langmuir 2000, 16, 4655. (10) Lee, D.-C.; Morales, G. M.; Lee, Y.; Yu, L. Chem. Commun. 2006, 100. (11) (a) Yoshimura, T.; Tatsuura, S.; Sotoyama, W. Appl. Phys. Lett. 1991, 59, 482. (b) Kubono, A.; Yuasa, N.; Shao, H. L.; Umemoto, S.; Okui, N. Thin Solid Films 1996, 289, 107. (c) Nagai, A.; Shao, H. L.; Umemoto, S.; Kikutani, T.; Okui, N. Polymer 2001, 13, S169. (d) Shao, H. I.; Umemoto, S.; Kikutani, T.; Okui, N. Polymer 1997, 38, 459. (e) Yoshimura, T.; Tatsuura, S.; Sotoyama, W.; Matsuura, A.; Hayano, T. Appl. Phys. Lett. 1992, 60, 268. (f) Bitzer, T.; Richardson, N. V. Appl. Phys. Lett. 1997, 71, 662. (g) Haq, S.; Richardson, N. V. J. Phys. Chem. B 1999, 103, 5256. (h) Putkonen, M.; Harjuoja, J.; Sajavaara, T.; Niinisto, L. J. Mater. Chem. 2007, 17, 664. (i) Miyamae, T.; Tsukagoshi, K.; Matsuoka, O.; Yamamoto, S.; Nozoye, H. Jpn. J. Appl. Phys., Part 1 2002, 41, 746. (j) Kim, A.; Filler, M. A.; Kim, S.; Bent, S. F. J. Am. Chem. Soc. 2005, 127, 6123. (k) Lee, J. S.; Lee, Y. J.; Tae, E. L.; Park, Y. S.; Yoon, K. B. Science 2003, 301, 818.

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available, we are investigating the application of chemistries that result in covalent, nonpolar linkages that could be useful for electronic applications, such as molecular electronics, construction of controlled dielectrics, and others. Thiol-ene photoinduced coupling has a long history in organic and macromolecular literature, and its chemistry has been extensively developed.12,13 As outlined in Figure 1, thiol-ene coupling occurs via a free-radical chain mechanism.14 Upon UV irradiation at 365 nm with a photoinitiator (PI) or at 254 nm in the absence of a PI, heterolytic cleavage of the thiol group affords a thiyl radical. Addition of the thiyl radical to the double bond of an ene functionality (propagation step 1) and subsequent hydrogen abstraction from a thiol group by a carbon-centered radical produces a thiyl radical (propagation step 2) that sustains the chain reaction. Termination occurs through radical-radical coupling. The rate-determining step in the thiol-ene photoinduced coupling reaction is the chain-transfer hydrogen-abstraction process. Thiol-ene photoinduced coupling has several characteristics that are attractive with regards to MLD: (i) Thiol-ene coupling chemistry can be performed under mild, ambient conditions and is not significantly inhibited by oxygen;15,16 (ii) thiol-ene systems are capable of initiator-free initiation,16,17 thus minimizing a potential source of impurities and other problems associated with light attenuation caused by initiator adsorption; (iii) it has demonstrated tolerance of a range of different structures, including internal alkenes as well as aromatic and aliphatic thiols;14,15,18 (iv) formation of disulfides is not problematic, as they undergo S-S cleavage under 254 nm irradiation (reversibility of this step).19,20 The thiol-ene photocoupling reaction has been utilized previously on surfaces to form micrometer-thick polymer films (under non-self-limiting conditions)21 and to modify the surface wettability of thiol-terminated self-assembled monolayers on silica substrates.22 In this work, we demonstrate the application of thiol-ene chemistry toward MLD of homobifunctional R,ωdithiol and R,ω-diene molecules; the two molecules were alternately photografted onto the surfaces one layer at a time, as outlined schematically in Figure 2a. MLD of thiol-ene multilayers was carried out on three different sets of interfaces, including hydride-terminated silicon, silicon oxide, and germanium, and characterized through atomic force microscopy (AFM), X-ray photoelectronic spectroscopy (XPS), and other techniques.

Results and Discussion The general procedure for construction of covalent thiol-ene multilayer structures on silicon, silicon oxide, and germanium is (12) (a) Posner, T. Chem. Ber. 1905, 38, 646. (b) Griesbaum, K. Angew. Chem., Int. Ed. Engl. 1970, 9, 273–287. (c) Zard, S. Z. Radical Reactions in Organic Synthesis; Oxford University Press: Oxford, 2003. (13) Dondoni, A. Angew. Chem., Int. Ed. 2008, 47, 8995–8997. (14) Hoyle, C. E.; Lee, T. Y.; Roper, T. J. Polym. Sci., Part A: Polym. Chem. 2004, 42, 5301. (15) Kharasch, M. S.; Nudenberg, W.; Mantell, G. J. J. Org. Chem. 1951, 16, 524–532. (16) Cramer, N. B.; Scott, J. P.; Bowman, C. N. Macromolecules 2002, 35, 5361. (17) Cramer, N. B.; Reddy, S. K.; Cole, M.; Hoyle, C.; Bowman, C. N. J. Polym. Sci., Part A: Polym. Chem. 2004, 42, 5817. (18) Roper, T. M.; Guymon, C. A.; Jonsson, E. S.; Hoyle, C. E. J. Polym. Sci., Part A: Polym. Chem. 2004, 42, 6283. (19) Okay, O.; Bowman, C. N. Macromol. Theory Simul. 2005, 14, 267. (20) Sayamol, K.; Knight, A. R. Can. J. Chem. 1968, 46, 999. (21) (a) Khire, V. S.; Benoit, D. S.; Anseth, K. S.; Bowman, C. N. J. Polym. Sci., Part A: Polym. Chem. 2006, 44, 7027. (b) Harant, A. W.; Khire, V. S.; Thibodaux, M. S.; Bowman, C. N. Macromolecules 2006, 39, 1461. (c) Khire, V. S.; Harant, A. W.; Watkins, A. W.; Anseth, K. S.; Bowman, C. N. Macromolecules 2006, 39, 5081. (d) Hagberg, E. C.; Malkoch, M.; Ling, Y.; Hawker, C. J.; Carter, K. R. Nano Lett. 2007, 7, 233. (e) Khire, V. S.; Lee, T. Y.; Bowman, C. N. Macromolecules 2007, 40, 5669. (22) Besson, E.; Gue, A.-M.; Sudor, J.; Korri-Youssoufi, H.; Jaffrezic, N.; Tardy, J. Langmuir 2006, 22, 8346.

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Figure 1. Reaction mechanism of UV-induced thiol-ene chemistry.

Figure 2. (a) Schematic diagram of covalent MLD assembly of a thiol-ene multilayer. (b) Chemical structure of the multilayer assembly. (c) Dienes and dithiols used in this work.

outlined in Figure 2. To generate the densest possible multilayers on the surfaces via this chemistry, the hydrosilylation and most thiol-ene reactions were performed using neat reagents.22 Starting with the example of organic monolayers on silicon surfaces with no intervening oxide layer, a hydride-terminated silicon interface was prepared through immersion of a native oxidecapped Si(100) shard in dilute hydrofluoric acid [1% HF (aq)].23 While there are many approaches to induce hydrosilylation of alkenes on a hydride-terminated silicon surface,24-27 photochemistry was utilized simply to be consistent.25 The hydride-terminated silicon sample (∼0.8  0.8 cm2) was coated with ∼40 μL of (23) Chabal, Y. J.; Higashi, G. S.; Raghavachari, K.; Burrows, V. A. J. Vac. Sci. Technol., A 1989, 7, 2104. (24) Buriak, J. M. Chem. Rev. 2002, 102, 1271. (25) (a) Terry, J.; Linford, M. R.; Wigren, C.; Cao, R.; Pianetta, P.; Chidsey, C. E. D. Appl. Phys. Lett. 1997, 71, 1056. (b) Cicero, R.; Linford, M. R.; Chidsey, C. E. D. Langmuir 2000, 16, 5688. (26) (a) Sieval, A. B.; Vleeming, V.; Zuilhof, H.; Sudh€olter, E. J. R. Langmuir 1999, 15, 8288. (b) Linford, M. R.; Fenter, P.; Eisenberger, P. M.; Chidsey, C. E. D. J. Am. Chem. Soc. 1995, 117, 3145. (27) (a) Stewart, M. P.; Buriak, J. M. J. Am. Chem. Soc. 2001, 123, 7821. (b) Sun, Q.-Y.; de Smet, L. C. P. M.; van Lagen, B.; Giesbers, M.; Thune, P. C.; van Engelenburg, J.; de Wolf, F. A.; Zuilhof, H.; Sudholter, E. J. R. J. Am. Chem. Soc. 2005, 127, 2514.

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Figure 3. AFM images and mean line profiles for the three-layer C8-DE-C4-DT film (a) and the nine-layer C8-DE-C4-DT film (b).

the desired R,ω-diene and exposed to UV irradiation from small UV pen lamp source for 3 h, leading to silicon-carbon bond formation via hydrosilylation (Figure 2b). The silicon surface was, at this point, terminated with organic groups with exposed alkenes at the monolayer interface, available for subsequent chemistry. As shown in Figure 2b, the next step involves treatment with a neat R,ω-dithiol, also via irradiation with the mercury pen lamp, for 30 min. The resulting thiol-terminated silicon substrate can then be reacted with a diene, followed by a dithiol, and so on, until the desired number of layers is reached. Figure 2c summarizes the different R,ω-dienes and -dithiols screened in this study; the aliphatic molecules are denoted according to the length of aliphatic chain (i.e., C8 for octane) followed by DE for diene or DT for dithiol, whereas the aromatic dithiols are listed with a unique abbreviation. Thicknesses of MLD Multilayer Films. Film thicknesses were obtained by ellipsometry, but for verification of this measurement tool a number of film values were cross-referenced against thickness data obtained by the commonly used AFM “scratching” technique;28 the data were found to be comparable within experiment error (Figure 3 and Supporting Information section). Ellipsometric data for multilayers on silicon surfaces derived from aliphatic dienes and dithiols are plotted in Figure 4a-d. The black spots represent the calculated thicknesses of the multilayers, based upon the assumption that the aliphatic chains adopt an all-trans, perfectly vertical (parallel to surface normal) configuration on the surface, as shown schematically in Figure 5. The red points correspond to the measured ellipsometric results for the organic layers (each point is the average of a total of 20 measurements, taken on four separately prepared films). The multilayer films with a shorter aliphatic chain length (C e 8, Figure 4a, b) show regular film growth, yielding slopes of 1.18 nm/bilayer for the C8-DE-C2-DT multilayer and 1.54 nm/bilayer for the C8DE-C4-DT multilayer. The observation of a shallower slope compared to the calculated “ideal” black line with increasing layer thickness is most likely due to a combination of film tilt and aliphatic chain disorder.29 (28) (a) Anariba, F.; DuVall, S. H.; McCreery, R. L. Anal. Chem. 2003, 75, 3837. (b) Nishikawa, T.; Nishida, J.; Ookura, R.; Nishimura, S.-I.; Scheumann, V.; Zizlsperger, M.; Lawall, R.; Knoll, W.; Shimomura, M. Langmuir 2000, 16, 1337. (c) Wang, D.; Buriak, J. M. Surf. Sci. 2005, 590, 154. (d) Li, Y.-H.; Buriak, J. M. Inorg. Chem. 2006, 45, 1096. (29) (a) Legay, G.; Markey, L.; Meunier-Prest, R.; Finot, E. Ultramicroscopy 2007, 107, 1111. (b) Gray, D. E.; Case-Green, S. C.; Fell, T. S.; Dobson, P. J.; Southern, E. M. Langmuir 1997, 13, 2833.

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Figure 4. Plots of film thickness versus multilayer number: (a) C8DT-C2-DE on a Si-Hx surface, (b) C8-DE-C4-DT on a Si-Hx surface, (c) C12-DE-C10-DT on a Si-Hx surface, (d) C8DE-BDMT on a Si-Hx surface, (e) C8-DE-C4-DT on a Ge-Hx surface, and (f) C8-DE-C4-DT on a SiOx/Si surface. Black spots are theoretical calculated thicknesses based on the assumption that all the molecules in the film adopt an all-trans upright configuration. Red spots are measured film thickness data by ellipsometry except in (e) which was determined by the AFM scratching method. The ellipsometric thickness measurements were carried out at five different locations on each of four separate samples (each spot is therefore an average of 20 measurements). For the AFM thicknesses, thickness measurements represent the average of three separate samples.

For longer aliphatic chains (C > 8), however, there is significant deviation from linearity. As shown in the example in Figure 4c, the experimental thickness trend for C12-DE-C10-DT MLD multilayer is not linear with increasing layer number. Upon increasing the layer number from 1 to 3, the film thickness of the C12-DE-C10-DT multilayer increases from 1.3 to 3.7 nm as expected, but with a further increase of the layer number the Langmuir 2010, 26(2), 1232–1238

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Figure 5. Schematic diagram shows the calculated “ideal” height of various molecular fragments in the multilayer assembly.

thickness of the C12-DE-C10-DT multilayer ultimately levels off. A similar nonlinear growth pattern was also observed in the C12-DE-C4-DT and C8-DE-C10-DT multilayers (Figure S1, Supporting Information), in which only one component of the diene/dithiol combination is greater than eight carbons long. Therefore, as long as at least one of the dithiols or dienes is greater than eight carbons long, deviation from linearity with respect to thickness is observed. The discrepancy of expected thickness and experiment results for the films composed of a longer dithiol or diene (alphatic chain length > 8) building blocks indicates that the longer molecules are undergoing a self-restricting event, most likely a looped conformation in which both thiol or ene groups within a single molecule are bonded to the underlying monolayer. Taking a self-assembled monolayer of alkane dithiols on gold surfaces as an analogous case, alkane dithiol molecules have been reported to take on one of the following orientations: (i) an upright orientation, in which one thiol group is bonded to the surface; (ii) a horizontal orientation (relative to the surface plane), in which both thiols are bonded to the surfaces and the molecule lies flat; or (iii) a looped conformation, in which both thiol goups are bonded to surfaces and the alkyl chain forms a loop (Figure 6).30 The structures of alkane dithiol monolayers depend on both molecular coverage and molecular chain length.31-33 In the MLD assembly of a C12-DE-C10-DT multilayer film, the long chain C12-DE and C10-DT molecules are assumed to form a looped conformation on the surface. Compared to weak Au-S bonds, the covalent C-S bonds are essentially irreversible and will not break, under the conditions of these studies. Once a looped conformation of a diene or dithiol molecule is formed on the surfaces, a subsequent transition to vertically oriented bound configuration is not possible since the structures are covalently trapped.34 Therefore, during the formation of each layer, a portion of these longer molecules takes on a looped conformation, which diminishes the number of available functional groups for anchoring the next molecular layer; the surface coverage of molecules therefore drops with an increasing layer number. After two to three layers, buildup is effectively arrested as no vinyl or thiol groups are available for the formation of the next molecular (30) Schreiber, F. Prog. Surf. Sci. 2000, 65, 151. (31) Kobayashi, K.; Horiuchi, T.; Yamada, H.; Matsushige, K. Thin Solid Films 1998, 331, 210. (32) Rieley, H.; Kendall, G. K.; Zemicael, F. W.; Smith, T. L.; Yang, S. Langmuir 1998, 14, 5147. (33) Xu, S.; Cruchon-Dupeyrat, S. J. N.; Garno, J. C.; Liu, G.-Y.; Jennings, G. K.; Yong, T.-H.; Laibinis, P. E. J. . Chem. Phys 1998, 108, 5002. (34) Cicero, R. L.; Chidsey, C. E. D.; Lopinski, G. P.; Wayner, D. D. M.; Wolkow, R. A. Langmuir 2002, 18, 305.

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Figure 6. Schematic representation of three distinct orientations of alkane dithiols on the metal surface corresponding to (a) an upright orientation, (b) a horizontal orientation, and (c) a looped conformation.

layer. Consequently, growth ceases. Attempts were made to modify the reaction conditions through addition of a photoinitiator (2,2-dimethoxy-2-phenylacetophenone, DMPA), or using thermally induced conditions with 1,10 -azobis(cyclohexane)carbonitrile (thermal initiator) for the C12-DE-C10-DT multilayer films. Both of these sets of conditions resulted in a similar pattern of self-restricting behavior, with a plateau of multilayer thickness (Supporting Information, Figure S2). The aromatic dithiols, 1,4-benzenedithiol (BDT) and 1,4benzenedimethanethiol (BDMT), also show a self-restricted MLD growth trend, like those described for longer chain aliphatic molecules (C > 8). Figure 4d shows the thickness of a C8-DE-BDMT multilayer film. Again, making the analogy to the known orientations of BDMT molecules on metallic surfaces, BDMT has two configurations on gold and silver substrates: (i) parallel to the surface on Ag and (ii) upright on Au.35 The comparison of the experimental thicknesses to the theoretical calculated thicknesses suggests that a large proportion of BDMT molecules are lying flat by layer number four, and hence, further thiol-ene reactions are self-restricted. The aromatic BDT molecules show a similar thickness growth trend (Supporting Information, Figure S3). XPS Analysis of MLD Multilayers on Silicon. Figure 7a-c shows the C 1s narrow scan of the surfaces following multilayer construction. After the first C8-DE monolayer was grafted on a Si(100) substrate (Figure 7a), a single-component C 1s peak at 285.0 eV was observed. On the other hand, both the five-layer (Figure 7b) and the nine-layer (Figure 7c) films can be deconvoluted into two peaks and assigned to the C-C links in the carbon-based alkyl chain (285.2 eV) and the C-S-linked carbons (286.3 eV), respectively.36 The peak area ratios between the two peaks for the five-layer film and nine-layer film are approximately 2.85:1 and 2.53:1, which are in reasonable agreement with the expected ratios of 3:1 and 2.5:1, respectively. Figure 7d presents a high resolution S 2s spectrum from a ninelayer film. The S 2s peak (∼228 eV) was acquired instead of the (35) Murty, K. V. G. K.; Venkataramanan, M.; Pradeep, T. Langmuir 1998, 14, 5446. (36) B€ocking, T.; Salomon, A.; Cahen, D.; Gooding, J. J. Langmuir 2007, 23, 3236.

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Figure 7. XPS data of C8-DE-C4-DT multilayer films on a Si-Hx surface: (a) C 1s spectrum of a one-layer film, (b) C 1s spectrum of a five-layer film, (c) C 1s spectrum of a nine-layer film, (d) S 2s spectrum of a nine-layer film, (e) Si 2p spectrum of a nine-layer film, and (f) plot of concentration ratios between sulfur and carbon versus layer numbers in the C8-DE-C4-DT multilayer films (each point is an average of three separate samples).

more typical S 2p peak (∼164 eV) to avoid the silicon electron energy loss peak at ∼167 eV.36 The low binding energy of the S 2s feature at ∼228 eV indicates that all of the surface sulfurs exist as either thiols or disulfides.37,38 No oxidized sulfur signals are observed in the high resolution XPS spectra, presumably because the thiol-ene reaction occurs in a reducing environment with excess dithiol molecules. As shown in Figure 4e, the Si 2p narrow scan of the nine-layer film shows the Si 2p3/2- 2p1/2 features (99.5 and 100.1 eV) arising from the silicon substrate. The lack of features at 100-104 eV reveals little or no formation of silicon oxides under these conditions.39 The atomic concentrations of S and C and thus their atomic ratio in the C8-DE-C4-DT multilayer films were estimated from XPS spectra. To calculate atomic concentrations, the raw XPS peak intensities were divided by the relative sensitivity factors (RSFs) and then normalized over all the elements detected. The zigzag shape evolution of the calculated S/C ratios with the change of layer numbers, as shown in Figure 7f, provides direct evidence for the formation of the multilayer films. The first layer, where only 1,7-octadiene molecules are grafted onto the silicon surfaces, does not contain any sulfur. C4-DT molecules are subsequently deposited on the silicon surfaces from the second layer; the sulfur signal then appears and is used to compare with the carbon signal. Predictably, the intensity of the top layer is stronger than that of the underlying layer, and therefore, the (37) Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. D. Handbook of X-ray Photoelectron Spectroscopy; Perkin-Elmer Corporation: Eden Prairie, MN, 1992. (38) Lenigk, R.; Carles, M.; Ip, N. Y.; Sucher, N. J. Langmuir 2001, 17, 2497. (39) (a) Hurley, P. T.; Nemanick, E. J.; Brunschwig, B. S.; Lewis, N. S. J. Am. Chem. Soc. 2006, 128, 9990. (b) Stewart, M. P.; Maya, F.; Kosynkin, D. V.; Dirk, S. M.; Stapleton, J. J.; McGuiness, C. L.; Allara, D. L.; Tour, J. M. J. Am. Chem. Soc. 2004, 126, 370.

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intensity of sulfur is higher when the top layer is composed of C4-DT molecules. The intensity of sulfur is lower when the top layer is composed of C8-DE molecules, since the sulfur element is buried by the carbon-terminated layer. Since C8-DE and C4-DT were deposited onto the surfaces alternately, the atomic ratios between sulfur and carbon versus layer numbers show an oscillating pattern. Surface Morphology of Multilayer Films. The morphology of the silicon multilayer films after modification by MLD was studied by tapping mode AFM. Figure 8 shows the representative AFM images of the multilayer films on Si(100) and an AFM image of a silicon hydride precursor surface for comparison. The root-mean-square (rms) surface roughness values of a five-layer film of a C12-DE-C10-DT (looped/self-restricted) multilayer, a seven-layer film of a C8-DE-C2-DT multilayer, and a nine-layer film of a C8-DE-C4-DT multilayer are less than 1 nm: 0.3, 0.2, and 0.5 nm, respectively. Multilayer film roughness therefore is not significantly higher than that of the silicon hydride surface, whose rms is 0.2 nm (Figure 8d). Successive AFM images of onelayer, three-layer, and sever-layer C8-DE-C2-DT multilayers with increasing film thicknesses (Supporting Information, Figure S7) indicate that the rms roughness values stay approximately constant at about 0.2 nm as the film grows. No significant topographic change during the layer-by-layer assembly process is observed. The formation of a smooth film indicates that patchy growth appears to be avoided,40 suggesting consistent growth over the entire surface. MLD Multilayer Formation on Different Interfaces. The thiol-ene multilayer assembly protocol can be carried out on a variety of surfaces, and as examples C8-DE-C4-DT multilayer data are shown for both Ge(111) and silicon oxide surfaces. In the case of Ge(111), the first layer utilized the known Ge-S bondformation chemistry of the Maboudian group, based upon selfassembly of C4-DT on a hydride-terminated Ge(111) surface.41 MLD assembly of the C4-DT-C8-DE multilayer film was then built up via the standard UV illumination approach. The thickness trend of C4-DT-C8-DE multilayers was obtained by the AFM “scratching” method as shown in Figure 4e and Figure S5 (Supporting Information). XPS spectra of the multilayer-coated samples (Supporting Information, Figure S6) show the expected signature peaks for C, S, and Ge. MLD assembly of a C8-DE-C4-DT multilayer was also carried out on a silicon oxide surface functionalized with octenyltrichlorosilane. The thicknesses obtained from ellipsometry (red spots in Figure 4f) match well with theoretical thicknesses (black line in Figure 4f).

Conclusions In this work, a simple and effective procedure for preparing assemblies of covalent multilayer films based upon thiol-ene chemistry has been developed. Aliphatic dienes and dithiols of various chain lengths, and aromatic dithiols were used to build molecular layer deposited assemblies on Si, Ge, and Si/SiOx surfaces. Film thickness data indicate that short chain molecules C8-DE, C2-DT, and C4-DT can be used to build up multilayer films in a predictable manner and can sustain continuous growth of the organic layer. On the other hand, long chain molecules such as C12-DE and C10-DT adopt looped configurations in the multilayer assembly, resulting in depletion of the available (40) (a) Meyer zu Heringdorf, F.-J.; Reuter, M. C.; Tromp, R. M. Nature 2001, 412, 517. (b) Goring, G. L. G.; Brown, R. S.; Horton, J. H. J. Mater. Chem. 2001, 11, 2282. (41) Han, S. M.; Ashurst, W. R.; Carraro, C.; Maboudian, R. J. Am. Chem. Soc. 2001, 123, 2422.

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Figure 8. AFM images (2  2 μm2, z range = 20 nm) of (a) a seven-layer C8-DE-C2-DT multilayer film on SiHx, (b) a nine-layer C8-DE-C4-DT multilayer film on SiHx, (c) a five-layer C12-DE-C10-DT multilayer film on SiHx, and (d) a freshly prepared silicon-hydride surface.

functional groups for the next molecular layer and finally arresting film growth after several layers. Similar results were observed when aromatic BDMT and BDT molecules were used. To summarize, MLD based upon thiol-ene multilayers is a viable approach toward controlled film growth of precise thickness and composition.

Experimental Section Generalities. All procedures were carried out in a nitrogenfilled Vacuum Atmospheres glovebox or with standard Schlenk techniques under argon. Water was obtained from a Millipore system (resistivity > 18 MΩ). Si(100) (n-type, P-doped, F = 1-5 Ω 3 cm) wafers were purchased from SQI. Sb-doped Ge(111) wafers were purchased from MTI Corporation. Toluene, dichloromethane (DCM) and tetrahydrofuran (THF) were purified by an Innovative Technologies solvent purification system. 1,11Dodecadiene C12-DE [The Chemical Institute (TCI)] was sparged with argon, and 1,7-octadiene C8-DE (Aldrich) was distilled under vacuum, followed by passing through dry, neutral alumina before use. 1,2-Ethanedithiol C2-DT (TCI), 1,4-butanedithiol C4-DT (TCI), 1,10-decanedithiol C10-DT (TCI), 1,4benzenedimethanethiol (BDMT) (TCI), 1,4-benzenedithiol (BDT) (TCI), 1,4-diethynylbenzene (TCI), octadecyltrichlorosilane (OTS) (Aldrich), octenyltrichlorosilane (Aldrich), trichlorododecylsilane (Aldrich), the UV photoinitiator, 2,2-dimethoxy-2phenylacetophenone (DMPA) (Aldrich), and the thermal initiator, 1,10 -azobis(cyclohexane)carbonitrile (Aldrich), were used as received. Ellipsometry data were collected using a Gaertner multiangle ellipsometer. XPS (Kratos Axis 165) was performed using the monochromatic Al KR line with a photon energy of 1486.6 eV. The atomic force microscope used in this study was a Nanoscope IV (Digital Instruments/Veeco), and commercially available Si cantilevers were used. Substrate Preparation. Si(100) wafers were cut to size (∼1 cm2) and then cleaned via the standard RCA procedure.42 The clean Si(100) shards were dipped in 1% HF (aq) to produce a (42) Kern, W.; Puotinen, D. RCA Rev. 1970, 31, 187.

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hydride-terminated silicon surface. The hydride-terminated Ge(111) surface was produced by previously reported methods.43 Briefly, the Ge(111) wafers were sonicated in acetone for 5 min to clean organic contaminants, blow-dried with N2, and then rinsed in deionized water to dissolve GeO2. Finally, the surface was treated with a concentrated HF solution (49 wt % in H2O) for 10 min to remove the residual oxide and produce an H-terminated surface. Surface Chemistry Reaction Vessel. All photoinduced surface chemistry was carried out in a simple homemade reactor vessel. In a block of Teflon, four shallow square 1.0  1.0 cm2 holes were drilled. All chemistry was carried out on ∼0.8  0.8 cm2-sized silicon or germanium shards placed within the square holes in the Teflon block. The smallest volume of reagents, ∼40 μL, was used to coat each wafer shard, and evaporation was limited with a quartz microscope slide over top of each hole. Hydrosilylation on Hydride-Terminated Si(100). UV light (254 nm) was produced from an ozone-free Hg pen lamp (Pen-ray lamp, UVP). The hydrosilylation procedure was carried out following reported methods.25 Briefly, a freshly prepared hydride-terminated Si(100) substrate was coated with neat C8DE, or neat C12-DE, in the Teflon cell and covered by a quartz slide. The Hg pen lamp was placed 1.0 cm above the surface, and irradiation was carried out for 3 h. After the reaction, the substrate modified by 1,7-octadiene was thoroughly rinsed with toluene and CH2Cl2.

Alkanethiol Modification on Hydrogen-Terminated Ge(111). A hydrogen-terminated Ge(111) substrate was immersed in a 1,4-butanedithiol solution (10-3 M in 2-propanol) for 1 day to obtain high-quality self-assembled films.41 The Ge substrate was then sonicated in neat 2-propanol for 1 min and then rinsed with 2-propanol and CH2Cl2. Self-Assembled Monolayers on SiO2. A Si(100) substrate cleaned via the RCA procedure was placed in an octenyltrichlorosilane (43) (a) Prabhakaran, K.; Ogino, T. Surf. Sci. 1995, 325, 263. (b) Deegan, T.; Hughes, G. Appl. Surf. Sci. 1998, 123/124, 66. (c) Prabhakarana, K.; Ogino, T.; Hull, R.; Bean, J. C.; Peticolas, L. J. Surf. Sci. 1994, 316, L1041.

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solution (50 μL) in toluene (10 mL, 1.0% w/w) for 24 h. The samples were then washed with toluene, CH2Cl2, and ethanol.

DMPA, 2,2-dimethoxy-2-phenylacetophenone, was added to all grafting solutions.

C8-DE-C2-DT Multilayer and C8-DE-C4-DT Multilayer on Functionalized Si. The alkene-terminated silicon sub-

C12-DE-C10-DT Multilayer on Functionalized Si(100) via Thermal Initiation. The initiator, 1,10 -azobis(cyclo-

strate [Si(100) or SiO2] was coated with neat C4-DT or C2-DT in the Teflon cell and covered by a quartz slide. After 30 min of UV irradiation (254 nm), the substrate was rinsed with THF, toluene and CH2Cl2. The substrate was then irradiated in neat C8-DE for 30 min, followed by rinsing with toluene and CH2Cl2. The C8-DE-C2-DT multilayer or C8-DE-C4-DT multilayer was formed on the substrate through repeated iteration of these processes.

hexane)carbonitrile (2 wt % relative to the C10-DT), was dissolved in mesitylene and introduced into a 5 mL round-bottom flask containing C10-DT. A freshly prepared alkene-terminated C12-DE Si(100) substrate was then placed in the solution. The flask was evacuated, flooded with argon, vented to a bubbler, and heated to 120 °C for 1 h. The sample was then removed and rinsed with toluene, sonicated in toluene for 2 min, and finally rinsed with toluene and CH2Cl2. The sample then was placed in the 5 mL round-bottom flask containing C12-DE and thermal initiator and heated to 120 °C for another 1 h, followed by sonication and rinsing as above. By repeating this procedure, a C12-DEC10-DT multilayer was formed on the Si(100) surface.

C8-DE-C4-DT Multilayer on Functionalized Ge(111). The formation of a C8-DE-C4-DT multilayer on thiolterminated Ge(111) was completed by the same procedure as above except that the UV irradiation step in neat diene C8-DE was first executed, followed by the reaction in dithiol C4-DT.

C12-DE-C10-DT, C12-DE-C4-DT, and C8-DE-C10DT Multilayers on Functionalized Si(100). (i) Without photoinitiator: MLD was carried out in the same manner as the above description with one exception. First, after deposition of each layer of C12-DE and C10-DT molecules, the substrate was rinsed with toluene, sonicated in toluene for 2 min, and finally rinsed again with toluene and CH2Cl2. (ii) With photoinitiator: MLD was carried out as above, except that 5% mol of the photoinitiator

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Supporting Information Available: Plots and tables showing film thicknesses obtained via ellipsometry and AFM scratching methods; AFM images and mean line profiles for C8-DE-BMDT film and C8-DE-C4-DT multilayer film; XPS spectra of 1.7-octadiene and 1,4-butanedithiol multilayers; AFM images of 1,7-octadiene modified monolayer, three-layer C8-DE-C2-DT multilayer film, and seven-layer C8-DE-C2DT multilayer film. This material is available free of charge via the Internet at http://pubs.acs.org.

Langmuir 2010, 26(2), 1232–1238