© Copyright 2002 American Chemical Society
JANUARY 22, 2002 VOLUME 18, NUMBER 2
Letters Olefin Additions on H-Si(111): Evidence for a Surface Chain Reaction Initiated at Isolated Dangling Bonds Ronald L. Cicero† and Christopher E. D. Chidsey* Department of Chemistry, Stanford University, Stanford, California 94305-5080
Gregory P. Lopinski,* Danial D. M. Wayner, and Robert A. Wolkow Steacie Institute for Molecular Sciences, National Research Council, Ottawa, Ontario, Canada Received June 4, 2001. In Final Form: October 1, 2001 We report the surface chain reaction of styrene with hydrogen-terminated Si(111), H-Si(111) initiated at isolated surface dangling bonds. Electrons from a scanning tunneling microscope in ultrahigh vacuum are used to create surface isolated dangling bonds on H-Si(111) prepared by aqueous ammonium fluoride etch. Exposure of dangling bonds on an otherwise hydrogen terminated surface to as little as 5 langmuirs of styrene leads to the formation of compact islands containing multiple styrene adsorbates bonded to the surface through individual C-Si bonds. From this observation we conclude that these adsorbate islands are the result of a surface chain reaction of styrene with H-Si(111). This result suggests that the key radical-chain propagation step of hydrogen atom abstraction from the silicon surface for the proposed mechanism of reaction of liquid phase terminally unsaturated organic molecules with hydrogen-terminated silicon under free-radical conditions readily occurs.
The modification of silicon surfaces by covalent attachment of unsaturated organic molecules has been the subject of much interest in recent years. The structural stability of the films combined with the chemical versatility afforded through simple synthetic organic chemistry creates the potential for these structures to have utility in applications as far reaching as microelectronics and biological assays.1,2 Methods for preparing these films on a variety of silicon surfaces involving both wet chem* To whom correspondence should be addressed. † Present address: Zyomyx Inc., 26101 Research Rd., Hayward, CA 94545. (1) Linford, M. R.; Fenter, P.; Eisenberger, P. M.; Chidsey, C. E. D. J. Am. Chem. Soc. 1995, 117, 3145. (2) Wolkow, R. A. Annu. Rev. Phys. Chem. 1999, 50, 413. (3) Linford, M. R.; Fenter, P.; Eisenberger, P. M.; Chidsey, C. E. D. J. Am. Chem. Soc. 1995, 117, 3145. (4) Cicero, R. L.; Linford, M. R.; Chidsey, C. E. D. Langmuir 2000, 16, 5688. (5) Bansal, A.; Li, X. L.; Lauermann, I.; Lewis, N. S.; Yi, S. I.; Weinberg, W. H. J. Am. Chem. Soc. 1996, 118, 7225. (6) Boukherroub, R.; Morin, S.; Bensebaa, F.; Wayner, D. D. M. Langmuir 1999, 15, 3831.
ical,3-14 and ultrahigh vacuum (UHV)15-20 approaches have been explored. To date, most mechanistic studies have centered on reactions involving olefin addition to (7) Boukherroub, R.; Wayner, D. D. M. J. Am. Chem. Soc. 1999, 121, 11513. (8) Bergerson, W. F.; Mulder, J. A.; Hsung, R. P.; Zhu, X. Y. J. Am. Chem. Soc. 1999, 121, 454. (9) deVilleneuve, C. H.; Pinson, J.; Bernard, M. C.; Allongue, P. J. Phys. Chem. B 1997, 101, 2415. (10) Allongue, P.; deVilleneuve, C. H.; Pinson, J.; Ozanam, F.; Chazalviel, J. N.; Wallart, X. Electrochim. Acta 1998, 43, 2791. (11) Buriak, J. M. Chem. Commun. 1999, 1051. (12) Cleland, G.; Horrocks, B. R.; Houlton, A. J. Chem. Soc., Faraday Trans. 1995, 91, 4001. (13) Kim, N. Y.; Laibinis, P. E. J. Am. Chem. Soc. 1998, 120, 4516. (14) Song, J. H.; Sailor, M. J. J. Am. Chem. Soc. 1998, 120, 2376. (15) Hamers, R. J.; Hovis, J. S.; Lee, S.; Liu, H. B.; Shan, J. J. Phys. Chem. B 1997, 101, 1489. (16) Hovis, J. S.; Hamers, R. J. J. Phys. Chem. B 1997, 101, 9581. (17) Teplyakov, A. V.; Kong, M. J.; Bent, S. F. J. Am. Chem. Soc. 1997, 119, 11100. (18) Lopinski, G. P.; Fortier, T. M.; Moffatt, D. J.; Wolkow, R. A. J. Vac. Sci. Technol., A 1998, 16, 1037. (19) Lopinski, G. P.; Moffatt, D. J.; Wayner, D. D.; Wolkow, R. A. Nature 1998, 392, 909.
10.1021/la010823h CCC: $22.00 © 2002 American Chemical Society Published on Web 12/20/2001
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Figure 1. Proposed chain reaction mechanism for the addition of olefins to the H-terminated Si(111) surface. Initial reaction at a dangling bond site leads to formation of a carbon-centered radical which can abstract hydrogen atom from a nearest neighbor site, creating a new reactive dangling bond.
the clean Si(100)-(2 × 1) surface prepared in UHV.21-26 While olefin reactions with the hydrogen-terminated Si(111) surface under wet chemical conditions have been thoroughly investigated, the mechanisms of these reactions are not yet fully established. In this work we use scanning tunneling microscopy (STM) under UHV conditions to investigate the reaction of styrene with isolated single dangling bonds on an otherwise H-terminated surface prepared by etching of the surface in aqueous ammonium fluoride. In this controlled environment, we observe that dangling bonds formed by cleavage of H-Si bonds by electrons from the STM tip act as initiating points of chain reactions, leading to the growth of islands of adsorbed styrene molecules. Some of us have also recently observed the adsorption on the UHV-prepared H-Si(100) (2 × 1) surface of multiple styrene molecules initiated at dangling bonds.27 These observations support the radicalinitiated chain-reaction mechanism of olefin addition proposed by Chidsey and co-workers3,4 for various terminally unsaturated organics. H-Si(111) prepared by treating Si(111) with 40% aqueous NH4F28 is a remarkably stable surface that has been shown to react with terminal olefins and acetylenes under free-radical conditions to form densely packed monolayers of adsorbate attached through C-Si bonds. These reactions are initiated by thermal activation, pyrolysis of organic peroxides in the presence of the adsorbate, or illumination with UV light, which are proposed to break H-Si bonds to create surface dangling bonds that add to olefins or acetylenes to form carboncentered radicals beta to the surface. It has been further proposed that these radicals abstract hydrogen from neighboring H-Si bonds regenerating surface dangling bonds which can then propagate the reaction as depicted (20) Lopinski, G. P.; Moffatt, D. J.; Wayner, D. D. M.; Zgierski, M. Z.; Wolkow, R. A. J. Am. Chem. Soc. 1999, 121, 4532. (21) Bozack, M. J.; Taylor, P. A.; Choyke, W. J.; Yates, J. T. Surf. Sci. 1986, 177, L933. (22) Yoshinobu, J.; Tsuda, H.; Onchi, M.; Nishijima, M. J. Chem. Phys. 1987, 87, 7332. (23) Mayne, A. J.; Cataldi, T. R. I.; Knall, J.; Avery, A. R.; Jones, T. S.; Pinheiro, L.; Hill, H. A. O.; Briggs, G. A. D.; Pethica, J. B.; Weinberg, W. H. Faraday Discuss. 1992, 199. (24) Kiskinova, M.; Yates, J. T. Surf. Sci. 1995, 325, 1. (25) Liu, H. B.; Hamers, R. J. J. Am. Chem. Soc. 1997, 119, 7593. (26) Lopinski, G. P.; Moffatt, D. J.; Wayner, D. D. M.; Wolkow, R. A. J. Am. Chem. Soc. 2000, 122, 3548. (27) Lopinski, G. P.; Wayner, D. D. M.; Wolkow, R. A. Nature 2000, 406, 48. (28) Higashi, G. S.; Chabal, Y. J.; Trucks, G. W.; Raghavachari, K. Appl. Phys. Lett. 1990, 56, 656.
schematically in Figure 1. Preliminary DFT calculations (B3LYP/6-31G) of the energetics of this reaction scheme, based on a cluster model of the Si(111) surface, suggest that for styrene, the first step is exothermic by 0.8 eV with a further 0.1 eV stabilization gained by hydrogen atom abstraction. The STM studies were carried out in a UHV chamber with a base pressure below 5 × 10-11 Torr. Hydrogenterminated Si(111) surfaces were prepared by etching samples cut from Si(111) wafers (Virginia Semiconductor, n-type, 1.0 Ω cm) in 40% NH4F as previously described.29 Upon withdrawal from the etching solution, the samples were quickly mounted into a sample holder and transferred into the UHV chamber within 5-10 min. Surfaces prepared in this way exhibited large terraces of order 500 Å with some triangular-shaped etch pits. Atomic resolved STM images showed the expected (1 × 1) hexagonal array with a spacing between features of ∼3.8 Å and an average corrugation of ∼0.12 Å. The STM tip was used to locally desorb hydrogen by using conditions well established on the H-terminated Si(100) surface,30,31 employing a sample bias of +6.5 V and currents below 100 pA. Styrene (Aldrich 99%) was degassed by performing several freeze-pumpthaw cycles. The molecules were controllably introduced into the UHV chamber via a variable leak valve. Figure 2a shows an occupied state STM image of a region of H-Si(111) with a low concentration of isolated silicon dangling bonds. A single, monatomic step runs through the center of the image. The dangling bonds appear as ∼0.5 Å high protrusions. A faint “halo” of reduced density of occupied states surrounds the dangling bond, as previously observed around isolated dangling bonds in unoccupied state images of H-Si(100).32 In addition to these dangling bond features there are small depressions which are attributed to defects, possibly due to single and multiple atom vacancies in the surface. There are also a few bright protrusions of undetermined origin that do not change in appearance or location upon dosing with styrene. Figure 2b contains an image of the identical region shown in Figure 2a after the surface was exposed to 12 langmuirs (1 langmuir ) 1 × 10-6 Torr‚s) at a partial pressure of 1 × 10-7 Torr of styrene. The appearance of (29) Wade, C. P.; Chidsey, C. E. D. Appl. Phys. Lett. 1997, 71, 1679. (30) Shen, T. C.; Wang, C.; Abeln, G. C.; Tucker, J. R.; Lyding, J. W.; Avouris, P.; Walkup, R. E. Science 1995, 268, 1590. (31) Avouris, P.; Walkup, R. E.; Rossi, A. R.; Akpati, H. C.; Nordlander, P.; Shen, T. C.; Abeln, G. C.; Lyding, J. W. Surf. Sci. 1996, 363, 368. (32) Hashizume, T.; Heike, S.; Lutwyche, M. I.; Watanabe, S.; Wada, Y. Surf. Sci. 1997, 386, 161.
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Figure 2. Occupied state STM images (215 Å × 130 Å, -2.1 V, 44 pA) of an H-terminated Si(111) surface with isolated dangling bonds created by desorption activated with the STM tip (a) before dosing with styrene and (b) after exposure to 12 langmuirs of styrene. The black dots in (b) mark the positions of the initial dangling bonds, showing that these sites serve to nucleate the growth of styrene islands. The gray scale ranges in the two images are different; the maximum height range in (a) is ∼5 Å and ∼10 Å in (b).
several adsorbate islands is clearly observed. These islands are ∼3 Å in height with a characteristic lateral size of ∼20 Å, significantly larger than a single styrene adsorbate. We conclude that the islands are each composed of many styrene adsorbates. Each of the islands is correlated with one of the pre-existing dangling bonds and each of the dangling bonds has spawned the growth of an island. To make this clear, the black dots in Figure 2b mark the locations of the dangling bonds in Figure 2a. Further exposures of this surface to styrene do not result in further growth of the adsorbate islands. In fact we find that dangling bonds exposed to up to 100 langmuirs of styrene do not form adsorbate islands significantly bigger than those pictured in Figure 2b. From several such experiments we can conclude that the growth of the adsorbate islands is complete after exposures of only ∼5 langmuirs. Although one island in Figure 2b (near the right edge of the image) has a more extended appearance, most of the islands have a compact shape. Internal structure of the islands is apparent in this as well as in smaller area scans. The features within the islands are somewhat too large to be attributed to individual adsorbates. Interactions between the phenyl rings on adjacent styrenes may lead to aggregation of phenyl rings within an island which can account for the observed internal structure of the adsorbate clusters. Figure 2 clearly shows that many styrenes adsorb for each isolated silicon dangling bond. Two obvious explanations for this observation are that styrene reacts with a surface dangling bond via the radical chain mechanism depicted in Figure 1 to give islands of styrene adsorbed
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to the surface through individual C-Si bonds or that styrene polymerizes from a surface dangling bond to give a single oligostyrene molecule bonded to the surface through a single C-Si bond. Which of these occurs is determined by competition between hydrogen-atom abstraction and carbon-carbon bond formation. In the UVinitiated reaction of liquid styrene with H-Si(111),4 there is strong evidence that carbon-carbon bond formation does not occur. Because in that case the reaction is inferred to also go through a carbon-centered radical and the local concentration of styrene is vastly greater, we conclude that, for the reaction under UHV conditions, polymerization does not occur. An additional argument against polymerization is that the analogous homogeneous phase radical-initiated reaction of molecular hydrosilanes with olefins, including styrene, gives the hydrosilylated product in high yield.33 The fact that growth stops after exposures of only 5 langmuirs implies that the chain reaction is self-limited. We propose the reaction terminates when there is no available neighboring hydrogen for the carbon-centered radical to abstract. We have previously modeled this process as a self-avoiding random walk on a triangular lattice using a Monte Carlo simulation and found that, on average, 76 steps (adsorption events) occurred before the chain reaction stopped due to the absence of a nearestneighbor hydrogen atom.4 This model did not account for steric crowding effects, which might favor abstraction of the least hindered neighboring hydrogen resulting in the reaction propagating in one direction along the surface. Such unidirectional propagation is observed on the H-Si(100)(2 × 1) surface where the reconstruction provides a unique propagation direction.27 It is clear from Figure 2b that the chain reaction does not normally propagate in a single direction on H-Si(111) and furthermore that the reaction takes far fewer steps than those predicted before growth is terminated. On the basis of the size of the observed islands, we can estimate that on average the chain reaction is stopped after only ∼20 steps. The rather compact shape of most islands suggests that, contrary to the expectation based on steric hindrance arguments, the chain reaction prefers to turn back on itself. This preference suggests an attractive interaction between the phenyl rings in a growing island. We note that this attractive interaction is also consistent with the reaction terminating in fewer steps than expected on the basis of the simulation with no adsorbate-adsorbate interactions. However, this attractive interaction may be diminished when the reaction is carried out with liquid styrene allowing for greater coverage per H-Si bond broken. The current results show that multiple styrenes adsorb per surface dangling bond, from which we have inferred that the key radical-chain propagation step of hydrogen atom abstraction from the silicon surface readily occurs. From this we conclude that the same step must be possible in the case of the radical reaction of liquid styrene with H-Si(111), although other processes, such as differing chain termination and possible chain transfer reactions may modify, among other factors, the size and shape of adsorbate islands. Acknowledgment. This work was partially supported by the Center for Environmentally Benign Semiconductor Manufacturing sponsored by the National Science Foundation and Semiconductor Research Corporation. LA010823H (33) Kopping, B.; Chatgilialoglu, C.; Zehnder, M.; Giese, B. J. Org. Chem. 1992, 57, 3994.
