Self-Assembly of High-Quality Covalently Bound Organic Monolayers onto Silicon Luc Scheres, Ahmed Arafat, and Han Zuilhof* Laboratory of Organic Chemistry, Wageningen UniVersity, Dreijenplein 8, 6703 HB Wageningen, The Netherlands ReceiVed May 10, 2007 A very mild method has been developed to obtain covalently attached alkyl monolayers from the attachment of 1-alkynes onto hydrogen-terminated silicon surfaces at room temperature in the dark. Apart from being the mildest method reported so far for the preparation of such monolayers, their quality, as indicated by water contact angles, XPS, and infrared spectroscopy, equals within experimental error that of the best reported alkyl monolayers on silicon. Introduction Over the past decade, the formation of organic monolayers onto hydrogen-terminated silicon (H-Si) has attracted a lot of attention because of their potential application in future electron- ics. 1 Direct covalent linkage to the Si surface via a Si-C bond makes these monolayers chemically and thermally very stable compared to, for example, organosilane monolayers on silicon dioxide and thiols on gold. 2 Currently, several methods are available for producing these monolayers, all of which require a certain type of activation, such as heating, 3 UV light, 4 hydrosilylation catalysts, 5 Lewis acid catalysts, 6 Grignard and lithium reagents, 7 electrochemistry, 8 and chemomechanical scribing. 9 Covalent attachment without external activation (room temperature in the dark) has recently also been reported. 10 However, this reaction required chemically activated alkynes and very long reaction times (up to 40 h), whereas the activating ester moiety itself disturbs the packing of the monolayer, resulting in moderate-quality monolayers. This would limit the applicability thereof because only high-quality organic monolayers proved to possess excellent electrical and passivating properties. 11 In the search for mild and generally applicable attachment methods, our group recently reported a visible-light-initiated modification of H-Si at room temperature. 12 In the present work, we report the first method to obtain high-quality covalently bound organic monolayers on H-Si with unactivated 1-alkynes at room temperature in the dark. Apart from further extending the range of compounds that can be attached in one step onto a Si surface, the quality of the alkyl monolayers onto Si prepared in this manner is at least as good as that obtained via any other method we know of. Experimental Details Pieces of n-Si(111) wafer (single polished, 475-550 μm thick, resistivity 1-5 Ω cm, Addison Engineering, San Jose, CA) were first rinsed several times with acetone (p.a. grade) followed by sonication for 10 min in acetone. Then the samples were cleaned using oxygen plasma (Harrick PDC-002 setup) for 3 min. Subse- quently, the Si(111) substrates were etched in an argon-saturated 40% aqueous NH 4 F solution for 15 min under an argon atmosphere. After being etched, the samples were thoroughly rinsed with deionized water and finally blown dry with a stream of dry nitrogen. A small three-necked flask equipped with a capillary as the argon inlet, a reflux-condenser that was connected to a vacuum pump, and a stopper was charged with 1 g of neat 1-hexadecyne (distilled twice, GC purity >99.9%) followed by positioning the tip of the capillary in the hexadecyne and turning on the argon flow through the capillary. The pressure in the flask was reduced to approximately 10 mbar, and the flask was immersed in an oil bath at the appropriate * Corresponding author. E-mail: han.zuilhof@wur.nl. (1) (a) Sieval, A. B.; Linke, R.; Zuilhof, H.; Sudho ¨lter, E. J. R. AdV. Mater. 2000, 12, 1457-1460. (b) Wayner, D. D. M.; Wolkow, R. A. J. Chem. Soc., Perkin Trans. 2 2002, 23-34. (c) Buriak, J. M. Chem. ReV. 2002, 102, 1271- 1308. (d) Boukherroub, R. Curr. Opin. Solid State Mater. Sci. 2005, 9, 66-72. (e) Shirahata, N.; Hozumi, A.; Yonezawa, T. Chem. Rec. 2005, 5, 145-159. (2) Sung, M. M.; Kluth, G. J.; Yauw, O. W.; Maboudian, R. Langmuir 1997, 13, 6164-6168. (3) (a) Linford, M. R.; Fenter, P.; Eisenberger, P. M.; Chidsey, C. E. D. J. Am. Chem. Soc. 1995, 117, 3145-3155. (b) 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. (4) (a) Cicero, R. L.; Linford, M. R.; Chidsey, C. E. D. Langmuir 2000, 16, 5688-5695. (b) Terry, J.; Linford, M. R.; Wigren, C.; Cao, R. Y.; Pianetta, P.; Chidsey, C. E. D. Appl. Phys. Lett. 1997, 71, 1056-1058. (5) (a) 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. (b) Boukherroub, R.; Morin, S.; Bensebaa, F.; Wayner, D. D. M. Langmuir 1999, 15, 3831-3835. (6) (a) Buriak, J. M.; Allen, M. J. J. Am. Chem. Soc. 1998, 120, 1339-1340. (b) Buriak, J. M.; Allen, M. J. J. Lumin. 1998, 80, 29-35. (c) Holland, J. M.; Stewart, M. P.; Allen, M. J.; Buriak, J. M. J. Solid State Chem. 1999, 147, 251- 258. (7) (a) Hurley, P. T.; Nemanick, E. J.; Brunschwig, B. S.; Lewis, N. S. J. Am. Chem. Soc. 2006, 128, 9990-9991. (b) Juang, A.; Scherman, O. A.; Grubbs, R. H.; Lewis, N. S. Langmuir 2001, 17, 1321-1323. (c) Royea, W. J.; Juang, A.; Lewis, N. S. Appl. Phys. Lett. 2000, 77, 1988-1990. (8) Vieillard, C.; Warntjes, M.; Ozanam, F.; Chazalviel, J.-N. Proc. Electrochem. Soc. 1996, 95, 250. (9) (a) Niederhauser, T. L.; Jiang, G. L.; Lua, Y. Y.; Dorff, M. J.; Woolley, A. T.; Asplund, M. C.; Berges, D. A.; Linford, M. R. Langmuir 2001, 17, 5889- 5900. (b) Niederhauser, T. L.; Lua, Y. Y.; Jiang, G. L.; Davis, S. D.; Matheson, R.; Hess, D. A.; Mowat, I. A.; Linford, M. R. Angew. Chem., Int. Ed. 2002, 41, 2353-2356. (c) Lee, M. V.; Guo, D.; Linford, M. R.; Zuilhof, H. Langmuir 2004, 20, 9108-9113. (d) Yang, L.; Lua, Y.-Y.; Lee, M. V.; Linford, M. R. Acc. Chem. Res. 2005, 38, 933-942. (10) Liu, Y.; Yamazaki, S.; Yamabe, S.; Nakato, Y. J. Mater. Chem. 2005, 15, 4906-4913. (11) (a) Liu, Y.-J.; Yu, H.-Z. Chem. Phys. Chem. 2002, 3, 799-802. (b) Webb, L. J.; Lewis, N. S. J. Phys. Chem. B 2003, 107, 5404-5412. (c) Faber, E. J.; de Smet, L. C. P. M.; Olthuis, W.; Zuilhof, H.; Sudho ¨ lter, E. J. R.; Bergveld, P.; van den Berg, A. Chem. Phys. Chem. 2005, 6, 2153-2166. (d) Seitz, O.; Bo ¨cking, T.; Salomon, A.; Gooding, J. J.; Cahen, D. Langmuir 2006, 22, 6915-6922. (e) Faber, E. J.; Sparreboom, W.; Groeneveld, W.; de Smet, L. C. P. M.; Olthuis, W.; Zuilhof, H.; Sudho ¨ lter, E. J. R.; Bergveld, P.; Van den Berg, A. Chem. Phys. Chem. 2007, 8, 101-112. (12) (a) Sun, Q. Y.; de Smet, L.; van Lagen, B.; Giesbers, M.; Thu ¨ne, P. C.; van Engelenburg, J.; de Wolf, F. A.; Zuilhof, H.; Sudho ¨ lter, E. J. R. J. Am. Chem. Soc. 2005, 127, 2514-2523. (b) Sun, Q. Y.; de Smet, L.; van Lagen, B.; Wright, A.; Zuilhof, H.; Sudho ¨ lter, E. J. R. Angew. Chem., Int. Ed. 2004, 43, 1352-1355. (c) de Smet, L. C. P. M.; Stork, G. A.; Hurenkamp, G. H. F.; Sun, Q.-Y.; Topal, H.; Vronen, P. J. E.; Sieval, A. B.; Wright, A.; Visser, G. M.; Zuilhof, H.; Sudho ¨lter, E. J. R. J. Am. Chem. Soc. 2003, 125, 13916-13917. 8343 Langmuir 2007, 23, 8343-8346 10.1021/la701359k CCC: $37.00 © 2007 American Chemical Society Published on Web 06/22/2007