Surface Functionalization of Alkyl Monolayers by Free-Radical Activation

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Surface Functionalization of Alkyl Monolayers by Free-Radical Activation: Gas-Phase Photochlorination with Cl2 Matthew R. Linford*,† and Christopher E. D. Chidsey‡ Department of Chemistry, Stanford University, Stanford, California 94305-5080, and Department of Chemistry and Biochemistry, Brigham Young University, Provo, Utah 84602 Received January 28, 2002. In Final Form: June 6, 2002 We report the gas-phase photochlorination of methyl-terminated alkyl monolayers on silicon. We argue that this process proceeds by a free-radical mechanism, as is the case for homogeneous photochlorination. This is an example of what should be a family of convenient methods for the incorporation of various functional groups into simple alkyl monolayers by chlorine-radical activation. Monolayers prepared from 1-octadecene on Si(111) were exposed to Cl2 with illumination at 350 nm. We observe that a fraction of the carbon atoms on the surface become singly chlorinated and a smaller fraction become doubly chlorinated, as measured by the chemically shifted components of the C 1s X-ray photoelectron spectrum. The elemental composition of the resulting monolayers, film thickness, and contact angles are reported as a function of exposure. Under conditions that chlorinate only a fraction of the carbon atoms, IR spectroscopy reveals complete chlorination of the methyl groups, demonstrating a strong steric preference for chlorination at the ends of the alkyl chains. In the absence of illumination, a much slower dark reaction is observed.

Introduction Monomolecular organic layers1,2 bonded to a variety of solid surfaces (for example, gold,3-7 silicon,8-23 silicon dioxide,24 and other metals25 and metal oxides26,27) are valuable model systems with which to study electron * To whom correspondence should be addressed. E-mail: [email protected]. † Brigham Young University. ‡ Stanford University. (1) Ulman, A. An Introduction to Ultrathin Organic Films from Langmuir-Blodgett to Self-Assembly; Academic Press: Boston, 1991. (2) Ulman, A. Chem. Rev. 1996, 96, 1533-1554. (3) Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105, 44814483. (4) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321-335. (5) Bain, C. D.; Whitesides, G. M. J. Am. Chem. Soc. 1989, 111, 71647175. (6) Bain, C. D.; Evall, J.; Whitesides, G. M. J. Am. Chem. Soc. 1989, 111, 7155-7164. (7) Schreiber, F. Prog. Surf. Sci. 2000, 65, 151-256. (8) Linford, M. R.; Chidsey, C. E. D. J. Am. Chem. Soc. 1993, 115, 12631-12632. (9) Chidsey, C. E. D.; Linford, M. R. Mechanism for the Chemisorption of Contaminants on Hydrogen-Terminated Silicon Surfaces. Proceedings of the Fourth International Symposium on Cleaning Technology in Semiconductor Device Manufacturing, Oct 8, 1995; Novak, R. E., Ruzyllo, J., Eds.; Electrochemical Society: Pennington, NJ, 1996; pp 455-463. (10) Buriak, J. M. Chem. Commun. 1999, 1051-1060. (11) Sieval, A. B.; Linke, R.; Zuilhof, H.; Sudho¨lter, E. J. R. Adv. Mater. 2000, 12, 1457-1460. (12) Linford, M. R.; Fenter, P.; Eisenberger, P. M.; Chidsey, C. E. D. J. Am. Chem. Soc. 1995, 117, 3145-3155. (13) Sieval, A. B.; Demirel, A. L.; Nissink, J. W. M.; Linford, M. R.; van der Maas, J. H.; de Jeu, W. H.; Zuilhof, H.; Sudho¨lter, E. J. R. Langmuir 1998, 14, 1759-1768. (14) Bateman, J. E.; Eagling, R. D.; Worrall, D. R.; Horrocks, B. R.; Houlton, A. Angew. Chem., Int. Ed. Engl. 1998, 37, 2683-2685. (15) Lopinski, G. P.; Wayner, D. D. M.; Wolkow, R. A. Nature 2000, 406, 48-51. (16) Sieval, A. B.; Vleeming, V.; Zuilhof, H.; Sudho¨lter, E. J. R. Langmuir 1999, 15, 8288-8291. (17) Boukherroub, R.; Morin, S.; Bensebaa, F.; Wayner, D. D. M. Langmuir 1999, 15, 3831-3835. (18) Buriak, J. M.; Stewart, M. P.; Geders, T. W.; Allen, M. J.; Choi, H. C.; Smith, J.; Raftery, D.; Canham, L. T. J. Am. Chem. Soc. 1999, 121, 11491-11502. (19) Cicero, R. L.; Linford, M. R.; Chidsey, C. E. D. Langmuir 2000, 16, 5688-5695.

transfer, adhesion, sensing, and catalysis and should find application in many other situations that require chemical modification of a surface. For such monolayers to be of significant utility, they often require chemical functionality other than the methyl groups that terminate the most easily prepared monolayers of alkyl chains. The formation of terminally functionalized monolayers of alkyl chains has been studied by many groups.5,13,18,28-40 Unfortunately, few useful R,ω-derivatized long-chain hydrocarbons or (20) Sieval, A. B.; Opitz, R.; Maas, H. P. A.; Schoeman, M. G.; Meijer, G.; Vergeldt, F. J.; Zuilhof, H.; Sudho¨lter, E. J. R. Langmuir 2000, 16, 10359-10368. (21) Boukherroub, R.; Morin, S.; Sharpe, P.; Wayner, D. D. M.; Allonge, P. Langmuir 2000, 16, 7429-7434. (22) Bansal, A.; Li, X.; Lauermann, I.; Lewis, N. S. J. Am. Chem. Soc. 1996, 118, 7225-7226. (23) Terry, J.; Linford, M. R.; Wigren, C.; Cao, R.-Y.; Pianetta, P.; Chidsey, C. E. D. Appl. Phys. Lett. 1997, 71, 1056-1058. (24) Maoz, R.; Sagiv, J. J.Colloid Interface Sci. 1984, 100, 465-496. (25) Laibinis, P. E.; Whitesides, G. M.; Allara, D. L.; Tao, Y.-T.; Parikh, A. N.; Nuzzo, R. G. J. Am. Chem. Soc. 1991, 113, 7152-7167. (26) Allara, D. L.; Nuzzo, R. G. Langmuir 1985, 1, 45-52. (27) Allara, D. L.; Nuzzo, R. G. Langmuir 1985, 1, 52-66. (28) Chechik, V.; Crooks, R. M.; Stirling, C. J. M. Adv. Mater. 2000, 12, 1161-1171. (29) Cicero, R. L.; Wagner, P.; Linford, M. R.; Hawker, C. J.; Waymouth, R. M.; Chidsey, C. E. D. Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.) 1997, 38, 904-905. (30) Wagner, P.; Nock, S.; Spudich, J. A.; Volkmuth, W. D.; Chu, S.; Cicero, R. L.; Wade, C. P.; Linford, M. R.; Chidsey, C. E. D. J. Struct. Biol. 1997, 119, 189-201. (31) Bain, C. D.; Whitesides, G. M. J. Am. Chem. Soc. 1988, 110, 6560-6561. (32) Balachander, N.; Sukenik, C. N. Langmuir 1990, 6, 1621-1627. (33) Fragneto, G.; Lu, J. R.; McDermott, D. C.; Thomas, R. K.; Rennie, A. R.; Gallagher, P. D.; Satija, S. K. Langmuir 1996, 12, 477-486. (34) Heid, S.; Effenberger, F.; Bierbaum, K.; Grunze, M. Langmuir 1996, 12, 2118-2120. (35) Maoz, R.; Sagiv, J. Langmuir 1987, 3, 1045-1051. (36) Maoz, R.; Sagiv, J. Langmuir 1987, 3, 1034-1044. (37) Tillman, N.; Ulman, A.; Schildkraut, J. S.; Penner, T. L. J. Am. Chem. Soc. 1988, 110, 6136-6144. (38) Wagner, P.; Zaugg, F.; Kernen, P.; Hegner, M.; Semenza, G. J. Vac. Sci. Technol., B 1996, 14, 1466-1471. (39) Yan, L.; Marzolin, C.; Terfort, A.; Whitesides, G. M. Langmuir 1997, 13, 6704-6712. (40) Buriak, J. M.; Allen, M. J. J. Am. Chem. Soc. 1998, 120, 13391340.

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other such multifunctional precursors are commercially available and any others must be synthesized,4,41 a barrier for those not experienced in organic synthesis. Moreover, in many important cases, the functionality desired is not compatible with the formation of high-quality monolayers. For example, adsorbates containing both the silane functionality required for bonding to oxide surfaces, for example, -SiCl3 or -Si(OR)3 groups, and also the very desirable protic functionalities, for example, -OH, -COOH, and -NH2 groups, either are not stable or else form poorly organized multilayer films. In such cases, a terminal functional group that is compatible with monolayer formation must be chosen and then converted to the desired functional group after monolayer formation. This approach has also been taken to prepare protic-functionalized monolayers from hydrogen-terminated silicon.42-44 We believe that a simpler procedure for preparing functionalized monolayers in many cases would be to first prepare methyl-terminated monolayers from commercially available monofunctional compounds and to then chemically substitute hydrogen at the surface of these monolayers by a useful functional group. Free-radical processes are one method for doing such substitutions. In particular, the chlorine radical can activate the substitution of hydrogen by a wide variety of groups. Many molecules form chlorine radicals on illumination or activation with other radicals. These can lead to chlorination and/or other substitutions of hydrocarbons depending on conditions. Reactions of hydrocarbons with molecules of the form XCl have been shown to form C-X bonds in several cases, where X is Cl,45 Br,46 SO2Cl,47 COCl,48 CN,49,50 SCl,51,52 or SCN.51,53 In addition, reactions of hydrocarbons with Cl2 in the presence of a molecule, Y, often lead to C-Y, C-YCl, or C(Cl)Y functionality: C-CO-Cl groups are formed from CO,48 C-SO2-Cl groups are formed from SO2,54 and C-NO and C(Cl)-NO groups are formed from NO.55 A significant advantage of these reagents is that the chlorine radical is a potent hydrogen abstractor with relatively low selectivity for secondary over primary hydrogens.45 (Another potentially useful gas-phase free-radical method of monolayer functionalization may be I2 activated with 1849 Å light.47 Like the chlorine atom, this reagent effectively abstracts hydrogen from methyl groups. Although this iodination is rarely used in organic chemistry because the HI produced reduces the alkyl iodide,45 the minute quantities of HI produced during monolayer functionalization should make this undesirable side reaction inconsequential.) (41) Effenberger, F.; Heid, S. Synthesis 1995, 9, 1126-1130. (42) Sieval, A. B.; Linke, R.; Heij, G.; Meijer, G.; Zuilhof, H.; Sudho¨lter, E. J. R. Langmuir 2001, 17, 7554-7559. (43) Strother, T.; Hamers, R. J.; Smith, L. M. Nucleic Acids Res. 2000, 28, 3535-3541. (44) Strother, T.; Cai, W.; Zhao, X. S.; Hamers, R. J.; Smith, L. M. J. Am. Chem. Soc. 2000, 122, 1205-1209. (45) March, J. Advanced Organic Chemistry; John Wiley & Sons: New York, 1992. (46) Speier, J. L. J. Am. Chem. Soc. 1951, 73, 826-827. (47) Gover, T. A.; Willard, J. E. J. Am. Chem. Soc. 1960, 82, 38163821. (48) Landauer, F.; Beermann, C. Carboxychlorinated polymers of unsaturated aliphatic or cycloaliphatic hydrocarbons and process for preparing same. Farbwerk Hoechst AG, Frankfurt am Main, Germany. U.S. Patent 3,536,685, 1970. (49) Mu¨ller, E.; Huber, H. Chem. Ber. 1963, 96, 2319-2326. (50) Mu¨ller, E.; Huber, H. Chem. Ber. 1963, 96, 670-682. (51) Mu¨ller, E.; Schmidt, E. W. Chem. Ber. 1963, 96, 3050-3061. (52) Mu¨ller, E.; Schmidt, E. W. Chem. Ber. 1964, 97, 2614-2621. (53) Bacon, R. G. R.; Guy, R. G. J. Chem. Soc. 1961, 2428-2436. (54) Brewer, N. J.; Rawsterne, R. E.; Kothari, S.; Leggett, G. J. J. Am. Chem. Soc. 2001, 123, 4089-4090. (55) Mu¨ller, E.; Metzger, H.; Fries, D.; Heuschkel, U.; Witte, K.; Waidelich, E.; Schmid, G. Angew. Chem. 1959, 71, 229-243.

Linford and Chidsey

To study the feasibility of using chlorine radical based methods to functionalize monolayers, we focus here on the simplest case, which is the reaction of Cl2 with alkyl monolayers. We have examined this reaction with gasphase Cl2, using a procedure that requires only a simple vacuum line. The basic strategy that we are proposing to make modified monolayer surfaces with has previously been used to modify bulk polymers. For instance, chlorinated polyethylene has been prepared from inexpensive polyethylene and chlorine gas.47 This paper joins a family of investigations on the gasphase and/or radical modification of alkyl monolayers on inorganic substrates. For example, Maoz,56 working with Sagiv, reported the gas-phase chlorination of siloxane monolayers on silicon oxide in her thesis. Chidsey and co-workers29,30 demonstrated photochemical chlorosulfonation (the Reed reaction57) of alkyl monolayers using mixtures of Cl2 and SO2 for the attachment of primary amines. Baker and Watling58 brominated monolayers with solution-phase bromine radicals, which probably do not effectively functionalize terminal methyl groups in alkyl monolayers because of bromine’s higher selectivity for secondary over primary hydrogen atoms.45 Robinson and co-workers59 studied the reaction of self-assembled monolayers (SAMs) of thiols on gold with atomic fluorine, and Naaman and co-workers60-62 examined the reactivity of atomic oxygen with alkyl monolayers. Cooks and coworkers63-66 have used low-energy ion beams to chemically modify and study monolayers. Czanderna and co-workers evaporated thin metal overlayers onto SAMs with different terminal functional groups.67-69 Maboudian and co-workers70 investigated the interaction of H(D) atoms with octadecylsiloxane SAMs, and Rowntree and Olsen71 have studied the bond-selective dissociation of thiol on gold monolayers with low-energy electrons. Admittedly, radical functionalization suffers from a general lack of selectivity. For this reason, free-radical reactions, such as photochlorination with Cl2, are not often employed in organic synthesis, except where the possible products are easily separated and their number is limited by the precursor structure, for example, methane, ethane, or cyclohexane. In this work, we find evidence that the (56) Maoz, R. Organic Reactions of Organized Monomolecular Systems Adsorbed on Solid Substrates. Thesis (in Hebrew), Weizmann Institute of Science, Rehovot, Israel, 1985. (57) Gilbert, E. E. Sulfonation and Related Reactions; John Wiley & Sons: New York, 1965; pp 126-131. (58) Baker, M. V.; Watling, J. D. Tetrahedron Lett. 1995, 36, 46234624. (59) Robinson, G. N.; Freedman, A.; Graham, R. L. Langmuir 1995, 11, 2600-2608. (60) Paz, Y.; Trakhtenberg, S.; Naaman, R. J. Phys. Chem. 1992, 96, 10964-10967. (61) Paz, Y.; Trakhtenberg, S.; Naaman, R. J. Phys. Chem. 1993, 97, 9075-9077. (62) Paz, Y.; Trakhtenberg, S.; Naaman, R. J. Phys. Chem. 1994, 98, 13517-13523. (63) Winger, B. E.; Julian, R. K., Jr.; Cooks, R. G.; Chidsey, C. E. D. J. Am. Chem. Soc. 1991, 113, 8967-8969. (64) Riederer, D. E., Jr.; Cooks, R. G.; Linford, M. R. J. Mass Spectrom. 1995, 30, 246. (65) Luo, H.; Miller, S. A.; Cooks, R. G.; Pachuta, S. J. Int. J. Mass Spectrom. Ion Processes 1998, 174, 193-217. (66) Pradeep, T.; Riederer, D. E., Jr.; Hoke, S. E.; Ast, T.; Cooks, R. G.; Linford, M. R. J. Am. Chem. Soc. 1994, 116, 8658-8665. (67) Czanderna, A. W.; King, D. E.; Spaulding, D. J. Vac. Sci. Technol., A 1991, 9, 2607-2613. (68) Jung, D. R.; King, D. E.; Czanderna, A. W. J. Vac. Sci. Technol., A 1993, 11, 2382-2386. (69) Jung, D. R.; Czanderna, A. W. J. Vac. Sci. Technol., A 1994, 12, 2402-2409. (70) Kluth, G. J.; Sung, M. M.; Maboudian, R. Langmuir 1997, 13, 6491-6496. (71) Olsen, C.; Rowntree, P. A. J. Chem. Phys. 1998, 108, 37503764.

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densely packed array of alkyl chains in a well-formed monolayer sterically limits functionalization to the nearsurface region, although not exclusively to the terminal carbon of the alkyl chain as can be obtained in monolayers prepared from specifically synthesized ω-functionalized compounds. In many applications, the functionalized monolayers produced by the free-radical methods would be adequate, thus making the extra work required to synthesize an ω-functionalized compound unnecessary. Finally, we note the possibility of exchanging chlorine in monolayers for iodine, a better leaving group, using the Finkelstein reaction (halogen exchange using sodium iodide in acetone).72 Experimental Section Methyl-Terminated Monolayer Preparation. The methylterminated monolayers used in this study were made by the following procedures: (i) pyrolysis of a mixture of 90% 1-octadecene and 10% [CH3(CH2)16COO]2 in the presence of hydrogenterminated silicon (111)12 and (ii) UV irradiation of hydrogenterminated silicon (111) in the presence of neat, oxygen-free 1-octadecene.9,19 Monolayer Chlorination. Methyl-terminated monolayers on silicon and oxidized silicon were placed in quartz cuvettes, evacuated to less than 10 mTorr, and backfilled with Ar. This process was repeated five times, after which the entire system was pumped to less than 6 × 10-6 Torr. An appropriate pressure of Cl2 (Matheson, 99.999%) gas, as measured by an uncalibrated Convectron gauge (Granville Phillips), was admitted to the sample, and the system was either held in the dark at this pressure or illuminated with a broadband 350-nm UV lamp. The lamp (Spectronics, MB-100, 100 W) has an average intensity of 6 mW/cm2 at a distance of 6 in. over a 7 in. diameter. Its emission is centered at 352 nm with a full-width at half-height of about 35 nm. Characterization. The characterization73 and the errors associated with physical measurements of methyl-terminated and functionalized monolayers that were used in this work have been described earlier.12 Fourier transform infrared (FTIR) spectra were acquired in the attenuated total internal reflection (ATR) mode using 50 × 20 × 1 mm trapezoidal silicon (111) crystals. X-ray photoelectron spectroscopy (XPS) peak fitting was performed with the built-in software of the XPS spectrometer (Surface Science model 150 XPS spectrometer). An adequate fit for the components of the C 1s regions is obtained with a functional form available in the software that is the sum of a Gaussian and a Lorentzian. The specific function used was 70% Gaussian and 30% Lorentzian, both with full width at half-height of 1.75 eV. The atomic ratio is calculated using atomic sensitivity factors for the XPS instrument that are tabulated in the instrument software and neglects differential attenuation of Cl 2s and C 1s photoelectrons. It is an upper limit because the C 1s photoelectrons, being distributed over a greater depth, should be more attenuated. To within the accuracy of the measurement, similar atomic ratios are calculated using tabulated atomic sensitivity factors.74

Results and Discussion Chlorination of Alkyl Monolayers on Silicon. Figure 1a shows the XPS survey spectrum and C 1s narrow scan of an alkyl monolayer on silicon prepared by method i (see Experimental Section). The survey scan shows peaks due to silicon, carbon, and a small amount of oxygen that are characteristic of this preparation of alkyl monolayers on Si(111). The methyl-terminated monolayer was then exposed to 0.2 Torr Cl2 and illuminated with a 350-nm (72) Finkelstein, H. Ber. Dtsch. Chem. Ges. 1910, 43, 1528-1532. (73) Ulman, A. Characterization of Organic Thin Films; ButterworthHeinemann: Boston, 1994. (74) Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. D. Handbook of X-ray Photoelectron Spectroscopy; Physical Electronics: Eden Prarie, MN, 1995.

Figure 1. X-ray photoelectron spectra of variously treated alkyl monolayers on Si(111): (a) as-prepared, methyl-terminated monolayer, (b) methyl-terminated monolayer after exposure to 0.2 Torr Cl2 with simultaneous illumination with a broadband 350 nm lamp for 15 s, and (c) methyl-terminated monolayer after exposure to 0.2 Torr Cl2 in the dark for 1 h. The insets show narrow scan data of the C 1s region as well as the components and the residuals of fits to these C 1s spectra (see Experimental Section).

broadband UV lamp for 15 s. Figure 1b shows the XPS survey and C 1s narrow scan spectra of the resulting surface. The survey scan shows peaks due to Cl 2p and Cl 2s photoemission74 in addition to the peaks seen in Figure 1a. The ratio of the Cl 2s to the C 1s peak areas is 0.55 ( 0.15, giving a Cl to C atomic ratio of 0.33 ( 0.09. The narrow scan of the C 1s region in the inset to Figure 1b can be fit with four components at 283.7 eV (2%), 284.8 eV (65%), 286.4 eV (27%), and 287.8 eV (6%). We assign the largest of these components to carbon bonded only to hydrogen and other carbon atoms. We assign the two components that are shifted by +1.6 and +3.0 eV from the largest peak to mono- and dichlorinated carbons. This assignment is based on the reported chemical shift difference of the two C 1s signals in poly(vinyl chloride) ((CH2CHCl)n) (1.5 eV).75 The very small component at 283.7 eV in Figure 1b is assigned to carbon bonded to silicon, which has suffered significant attenuation during escape through the monolayer.23 Neglecting differential attenuation of the photoelectrons, these assignments of the C 1s peaks imply a Cl to C atomic ratio of 0.39, in reasonable agreement with the atomic ratio of 0.33 obtained above from the ratio of Cl 2s peak area to the total C 1s peak area. Finally, because of the strength of the Si-Cl bond, it is important to consider a potential reaction between the silicon substrate beneath the mono(75) Pireaux, J. J.; Riga, J.; Caudano, R.; Verbist, J. J.; Delhalle, J.; Delhalle, S.; Andre´, J. M.; Gobillon, Y. Phys. Scr. 1977, 16, 329-338.

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Figure 2. Plots of several properties of methyl-terminated alkyl monolayers on Si(111) after exposure to Cl2 gas as a function of the product of the exposure pressure and exposure time both during UV illumination (open symbols) and in the dark (closed symbols): (a) ratio of the area of the Cl 2s to C 1s peaks, (b) final minus initial ellipsometric thickness, and (c) advancing (square) and receding (circle) water contact angles and advancing (upright triangle) and receding (inverted triangle) hexadecane contact angles.

layer and atomic or molecular chlorine. If such a reaction were occurring, one would expect to see chemically shifted silicon by XPS. A comparison of the Si 2p narrow scans from the samples used to obtain the spectra shown in Figure 1a,b reveals that they are superimposable. We conclude that the monolayer shields the silicon substrate from reaction with chlorine. To determine the role of UV light in the above reaction, we conducted a dark control experiment by exposing another methyl-terminated alkyl monolayer on silicon to a 10-fold greater pressure of Cl2 gas (2 Torr) and greater than a 10-fold increase in the total exposure time (1 h). Figure 1c shows the XPS survey and C 1s narrow scan spectra of this monolayer. The ratio of the areas of the Cl 2s and C 1s photoemission is 0.27 ( 0.15, giving a Cl to C atomic ratio of 0.16 ( 0.08. The shoulder in the inset of Figure 1c indicates that there is some carbon that is singly bonded to chlorine. Despite the significantly greater pressure and exposure time, the chlorine uptake is only about 1/2 of that seen following exposure to UV light (Figure 1b). This result demonstrates that the process observed in Figure 1b is largely a photoreaction, although some dark reaction does occur. Chlorine gas and UV light are standard conditions for chlorination of hydrocarbons, which has been extensively studied.76,77 The particular (76) Chiltz, G.; Goldfinger, P.; Huybrechts, G.; Martens, G.; Verbeke, G. Chem. Rev. 1963, 63, 355-372.

Linford and Chidsey

Figure 3. Fourier transform infrared spectroscopy of the C-H stretching region of a methyl-terminated alkyl monolayer on Si(111) before and after exposure to 0.2 Torr Cl2 with simultaneous illumination with a broadband 350 nm lamp for 15 s.

UV lamp used in these studies overlaps the broad, photodissociative Cl2 absorption maximum between 300 and 350 nm78 and thus forms chlorine radicals, Cl•, which probably are essential to extract hydrogen from the monolayer and initiate the chlorination process. The dark reaction is probably due to adventitious formation of chlorine radicals. Figure 2 shows various physical properties of monolayers prepared by a photoreaction and a dark reaction as a function of the product of pressure and exposure time. Figure 2a shows that the ratio of the Cl 2s to C 1s peaks increases at low exposures in the case of the photoreaction but requires more than 4 orders of magnitude greater exposure to reach similar levels in the case of the dark reaction. Similarly, Figure 2b shows an increase in thickness at low photoexposure but only small increases after larger exposures in the dark. Finally, Figure 2c shows the water (circles and squares) and hexadecane (triangles) advancing and receding contact angles. Note in particular that the hexadecane contact angles drop abruptly to zero at low photoexposures but decay much more slowly in the dark. Figure 3 shows the C-H stretching region of the infrared absorption spectra of the same monolayer surfaces shown in Figure 1 before and after exposure to chlorine gas and UV light. Before exposure, we see two strong peaks at 2850.4 cm-1 (full width at half-maximum (fwhm), 10.1 (77) Poutsma, M. L. In Halogenation; Poutsma, M. L., Ed.; John Wiley: New York, 1973; pp 159-229. (78) Noyes, W. A., Jr.; Leighton, P. A. The Photochemistry of Gases; Reinhold Publishing: New York, 1941; pp 263-265.

Surface Functionalization of Alkyl Monolayers

cm-1) and 2918.6 cm-1 (fwhm, 16.8 cm-1), that correspond to the symmetric and asymmetric methylene stretches of the alkyl chains in the monolayer, as well as the asymmetric in-plane methyl mode at 2962 cm-1.79 After chlorination, this methyl peak is no longer discernible and the methylene stretches are decreased to about half of their original height, broadened, and shifted to 2850.9 cm-1 (fwhm, 14.0 cm-1) and 2920.0 cm-1 (fwhm, 26.5 cm-1), respectively. The areas (fwhm times height) of these CH2 stretches decrease by 31%, which is about the same as the combined area (33%) of the peaks assigned to carbon bonded to one and two chlorine atoms in the inset to Figure 1b. The shift to higher wavenumbers and broadening of the CH2 peaks suggest that more gauche defects have been introduced into the alkyl chains of the monolayer.79,80 The complete disappearance of the infrared methyl stretches shown in Figure 3 provides strong evidence for chlorination primarily at the chain ends. Moreover, since it is known that methyl groups photochlorinate somewhat more slowly than methylene groups,45 the complete disappearance of methyl groups under the fractional chlorination conditions of Figures 1 and 3 proves the presence of a substantial steric effect directing the chlorination to the near-surface carbon atoms. Because the XPS signal from the less highly chlorinated carbon atoms deep in the monolayer will be attenuated, the areas of the chemically shifted peaks in the carbon 1s narrow scans compared with the total area of the carbon 1s peaks (inset to Figure 1b) place an upper limit on the number of singly and doubly chlorinated carbons in the alkyl chains. Hence, each alkyl chain in the 18-carbon monolayer shown in Figure 1b has no more than 4.9 singly and 1.1 doubly chlorinated carbon atoms. That is, less than 1/3 of the total carbons have some form of chlorine attached to them after photochlorination under these conditions. In the case of the dark reaction, even after 7200 Torr s of exposure, less than 3.2 carbons per alkyl chain (1/6 of the total) in this monolayer are singly chlorinated, with no measurable double chlorination. In fact, the dark reaction actually provides a convenient method to achieve low chlorine incorporation in the monolayer without careful control of small exposures as would be required for the photoreaction. Free-Radical Mechanism of Photochlorination. The UV-light dependence of the chlorination observed here is solid evidence for a free-radical mechanism, which may even be the same free-radical chain mechanism that is known for the UV-initiated reaction of Cl2 with hydrocarbons in homogeneous situations.45 That mechanism is (79) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559-3568. (80) Snyder, R. G.; Strauss, H. L.; Elliger, C. A. J. Phys. Chem. 1982, 86, 5145-5150.

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initiated by the photodissociation of Cl2 to generate two chlorine atoms:

Cl2 + hν f 2Cl•

(1)

The chain propagation steps are

R1R2R3C-H + Cl• f R1R2R3C• + HCl

(2)

R1R2R3C• + Cl2 f R1R2R3C-Cl + Cl•

(3)

where R1, R2, and R3 can be H, alkyl, or Cl. Termination can occur by recombination of any two radicals. Note that the Cl2 and Cl• species freely diffuse in the gas phase while the alkyl species (R1R2R3C-H, R1R2R3C•, and R1R2R3CCl) are immobilized on the surface. Also, chain reactions of this sort at surfaces may require that the mean free path of the freely diffusing chain-carrying species in the gas phase (Cl• in this case) be small compared with the dimensions of the container. If not, the chain-carrying species may either escape or react at the wall of the container. In that case, the reaction would have to be stoichiometric rather than catalytic in the initiating photons. For this reason, it is unlikely that reactions of the type used here will be of utility in single-collision dosing experiments under ultrahigh vacuum conditions. On the other hand, this class of reactions is quite appropriate for functionalizing surfaces under viscous-flow gas-phase conditions (as used here) or under liquid-phase conditions. Conclusion We have demonstrated that free-radical activation is a viable method to functionalize methyl-terminated alkyl monolayers. We have illustrated the general strategy with the simple case of the UV-initiated, gas-phase chlorination of alkyl monolayers on silicon. There is a significant steric effect that favors chlorination at the chain ends. Other free-radical reactions are possible and promise a range of convenient ways to functionalize simple methyl-terminated alkyl monolayers. Acknowledgment. This work was supported by Stanford University and the National Science Foundation (CHE-9412720) and has benefitted from the facilities and equipment made available to Stanford University by the NSF-MRSEC program through the Center for Materials Research at Stanford University. M.R.L. thanks the Fannie and John Hertz Foundation for their kind support. C.E.D.C. is a Camille and Henry Dreyfus TeacherScholar. We acknowledge Ron Cicero for assisting in XPS data analysis and thank Brittany Peterson and Rachel Parkinson for their help in preparing this manuscript. LA020095D