Silicon (100) Electrodes Resistant to Oxidation in Aqueous Solutions

Jan 21, 2009 - Radostina K. Manova , Sidharam P. Pujari , Carel A. G. M. Weijers , Han Zuilhof , and Teris A. van Beek. Langmuir 2012 28 (23), 8651-86...
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Silicon (100) Electrodes Resistant to Oxidation in Aqueous Solutions: An Unexpected Benefit of Surface Acetylene Moieties Simone Ciampi,† Paul K. Eggers,† Guillaume Le Saux,† Michael James,‡ Jason B. Harper,† and J. Justin Gooding*,† School of Chemistry, The UniVersity of New South Wales, Sydney, NSW 2052, Australia, and The Bragg Institute, Australian Nuclear Science and Technology Organisation (ANSTO), Lucas Heights Research Laboratory, Lucas Heights, NSW 2234, Australia ReceiVed NoVember 8, 2008 Here we report on the functionalization of alkyne-terminated alkyl monolayers on highly doped Si(100) using “click” reactions to immobilize ferrocene derivatives. The reaction of hydrogen-terminated silicon surfaces with a diyne species was shown to afford very robust functional surfaces where the oxidation of the underlying substrate was negligible. Detailed characterization using X-ray photoelectron spectroscopy, X-ray reflectometry, and cyclic voltammetry demonstrated that the surface acetylenes had reacted in moderate yield to give surfaces exposing ferrocene moieties. Upon extensive exposure of the redox-active architecture to oxidative environments during preparative and characterization steps, no evidence of SiOx contaminants was shown for derivatized SAMs prepared from singlecomponent 1,8-nonadiyne, fully acetylenylated, monolayers. An analysis of the redox behavior of the prepared Si(100) electrodes based on relevant parameters such as peak splitting and position and shape of the reduction/oxidation waves depicted a well-behaved redox architecture whose spectroscopic and electrochemical properties were not significantly altered even after prolonged cycling in aqueous media between -100 and 800 mV versus Ag|AgCl. The reported strategy represents an experimentally simple approach for the preparation of silicon-based electrodes where, in addition to close-to-ideal redox behavior, remarkable electrode stability can be achieved. Both the presence of a distal alkyne moiety and temperatures of formation above 100 °C were required to achieve this surface stabilization.

1. Introduction The past decade has seen dramatic technical advances in the nanoscale manipulation of silicon substrates toward the preparation of molecular-based devices1-5 not only for microelectronics6-8 but also for memory devices,6,9,10 photovoltaics,11 sensors,12-19 and biologically active surfaces.20-22 Silicon-based architectures offer advantages over organic assemblies on metal surfaces that include (1) providing a well-established technology for the bulk manufacture of semiconductor materials; (2) the ready availability of relatively inexpensive high-purity singlecrystal silicon wafers, as a result of their widespread use in the semiconductor industry; (3) the ability to tailor bulk properties of the silicon substrate by altering the dopant type and/or concentration;23 and (4) the ability to tune the chemical properties of the silicon surface via their modification with very stable self-assembled monolayers.24,25 The challenge is to modify silicon electrodes without the formation of significant SiOx, which * To whom correspondence should be addressed. E-mail: justin.gooding@ unsw.edu.au. † The University of New South Wales. ‡ The Bragg Institute. (1) Lopinski, G. P.; Wayner, D. D. M.; Wolkow, R. A. Nature 2000, 406, 48–51. (2) Richter, C. A.; Hacker, C. A.; Richter, L. J.; Vogel, E. M. Solid-State Electron. 2004, 48, 1747–1752. (3) Rangel, N. L.; Seminario, J. M. J. Phys. Chem. A 2006, 110, 12298–12302. (4) Ashkenasy, G.; Cahen, D.; Cohen, R.; Shanzer, A.; Vilan, A. Acc. Chem. Res. 2002, 35, 121–128. (5) Nishikawa, T.; Mitani, T. Sci. Technol. AdV. Mater. 2003, 4, 81–89. (6) Roth, K. M.; Yasseri, A. A.; Liu, Z.; Dabke, R. B.; Malinovskii, V.; Schweikart, K.-H.; Yu, L.; Tiznado, H.; Zaera, F.; Lindsey, J. S.; Kuhr, W. G.; Bocian, D. F. J. Am. Chem. Soc. 2003, 125, 505–517. (7) Cerofolini, G. F.; Arena, G.; Camalleri, C. M.; Galati, C.; Reina, S.; Renna, L.; Mascolo, D. Nanotechnology 2005, 16, 1040–1047. (8) Cerofolini, G. F.; Mascolo, D. Semicond. Sci. Technol. 2006, 21, 1315– 1325. (9) Li, Q.; Mathur, G.; Gowda, S.; Surthi, S.; Zhao, Q.; Yu, L.; Lindsey, J. S.; Bocian, D. F.; Misra, V. AdV. Mater. 2004, 16, 133–137.

adversely affects the electronic properties of the device.26,27 This has been achieved on Si(111) surfaces27 where the high density of silicon atoms on this crystal face (15.66 × 1014 cm-2)23 allows densely packed organic layers to be generated from linear molecules. In contrast, on the more technologically relevant Si(100) orientation there have yet to be reports on effective passivation strategies such that the silicon substrate can withstand oxidation in aqueous media.27 (10) Li, Q.; Mathur, G.; Homsi, M.; Surthi, S.; Misra, V.; Malinovskii, V.; Schweikart, K.-H.; Yu, L.; Lindsey, J. S.; Liu, Z.; Dabke, R. B.; Yasseri, A.; Bocian, D. F.; Kuhr, W. G. Appl. Phys. Lett. 2002, 81, 1494–1496. (11) Canfield, D.; Morrison, S. R. Electrochemical Storage Cell Based on Polycrystalline Silicon; Lawrence Berkeley Laboratory, Berkeley, CA, 1982. (12) Kilian, K. A.; Bo¨cking, T.; Gaus, K.; Gal, M.; Gooding, J. J. ACS Nano 2007, 1, 355–361. (13) Kilian, K. A.; Bo¨cking, T.; Gaus, K.; King-Lacroix, J.; Gal, M.; Gooding, J. J. Chem. Commun. 2007, 1936–1938. (14) Dancil, K.-P. S.; Greiner, D. P.; Sailor, M. J. J. Am. Chem. Soc. 1999, 121, 7925–7930. (15) George, M.; Parak, W. J.; Gaub, H. E. Sens. Actuators 2000, B69, 266– 275. (16) Sakata, T.; Miyahara, Y. Biosens. Bioelectron. 2005, 21, 827–832. (17) Stewart, M. P.; Buriak, J. M. AdV. Mater. 2000, 12, 859–869. (18) Nikolaides, M. G.; Rauschenbach, S.; Bausch, A. R. J. Appl. Phys. 2004, 95, 3811–3815. (19) Ouyang, H.; Christophersen, M.; Viard, R.; Miller, B. L.; Fauchet, P. M. AdV. Funct. Mater. 2005, 15, 1851–1859. (20) de-Leon, S. B.-T.; Oren, R.; Spira, M. E.; Korbakov, N.; Yitzchaik, S.; Sa’ar, A. Phys. Status Solidi A 2005, 202, 1456–1461. (21) Kilian, K. A.; Bo¨cking, T.; Ilyas, S.; Gaus, K.; Wendy, J.; Gal, M.; Gooding, J. J. AdV. Funct. Mater. 2007, 17, 2884–2890. (22) Kilian, K. A.; Bo¨cking, T.; Gaus, K.; Gal, M.; Gooding, J. J. Biomaterials 2007, 28, 3055–3062. (23) Zhang, G. Electrochemistry of Silicon and its Oxide; Kluwer/Plenum: New York, 2001. (24) Linford, M. R.; Chidsey, C. E. D. J. Am. Chem. Soc. 1993, 115, 12631– 12632. (25) Linford, M. R.; Fenter, P.; Eisenberger, P. M.; Chidsey, C. E. D. J. Am. Chem. Soc. 1995, 117, 3145–3155. (26) Seitz, O.; Bo¨cking, T.; Salomon, A.; Gooding, J. J.; Cahen, D. Langmuir 2006, 22, 6915–6922. (27) Rohde, R. D.; Agnew, H. D.; Yeo, W.-S.; Bailey, R. C.; Heath, J. R. J. Am. Chem. Soc. 2006, 128, 9518–9525.

10.1021/la803710d CCC: $40.75  2009 American Chemical Society Published on Web 01/21/2009

Si Electrodes for Aqueous EnVironments.

The repertoire of wet chemical strategies for the fabrication of well-defined molecular architectures on silicon is rapidly expanding.28,29 The preparation of silicon-carbon-bound monolayers from nonoxidized, hydrogen-terminated silicon surfaces represents the method of choice when spatially defined ensembles of confined chemical functionalities30,31 with high stability32-34 are required. The formation of a silicon-carbon bond is generally accepted to be the most reliable approach to preventing extensive oxidation of the underlying substrate35-39 while maintaining a sufficiently low surface recombination velocity of charge carriers.40 Detailed studies on the structural and electrical properties of thin organic layers covalently bound to oxidized silicon substrates have been reported,6,41 but these are less well defined and less stable. Furthermore, as a result of a low isoelectric point, adventitious SiOx species are charged at physiological pH,23 and the resulting charged interface might limit the applications and stability of silicon-based sensing devices or increase carrier recombination rates of hybrid electronic devices. The subsequent derivatization of a monolayer in a stepwise fashion is a popular strategy for the fabrication of chemically heterogeneous and complex molecular assemblies onto a surface.42-45 The Cu(I)-catalyzed “click” version of the Huisgen 1,3-dipolar cycloaddition reaction between azides and alkynes46-48 is an emerging strategy for building up molecular assemblies on surfaces in a stepwise manner, and this chemistry has been shown to be a nearly ideal coupling procedure with its high yields and high selectivity.49,50 We and others have recently reported applications of the click cycloaddition of terminal alkynes and substituted azides to yield surface-bound triazole species. This provided a versatile and experimentally simple approach to covalently modified flat27,51-53 and porous silicon surfaces.54 (28) Buriak, J. M. Chem. Commun. 1999, 1051–1060. (29) Buriak, J. M. Chem. ReV. 2002, 102, 1271–1308. (30) Bo¨cking, T.; James, M.; Coster, H. G. L.; Chilcott, T. C.; Barrow, K. D. Langmuir 2004, 20, 9227–9235. (31) Bo¨cking, T.; Kilian, K. A.; Gaus, K.; Gooding, J. J. Langmuir 2006, 22, 3494–3496. (32) Cerofolini, G. F.; Galati, C.; Reina, S.; Renna, L. Appl. Phys. A: Mater. Sci. Process. 2004, 80, 161–166. (33) Terry, J.; Linford, M. R.; Wigren, C.; Cao, R.; Pianetta, P.; Chidsey, C. E. D. J. Appl. Phys. 1999, 85, 213–221. (34) Sung, M. M.; Kluth, G. J.; Yauw, O. W.; Maboudian, R. Langmuir 1997, 13, 6164–6168. (35) Cerofolini, G. F.; Galati, C.; Reina, S.; Renna, L. Mater. Sci. Eng. 2003, C23, 253–257. (36) Bo¨cking, T.; Salomon, A.; Cahen, D.; Gooding, J. J. Langmuir 2007, 23, 3236–3241. (37) Cerofolini, G. F.; Galati, C.; Reina, S.; Renna, L. Trends Semicond. Res. 2005, 1–32. (38) Cerofolini, G. F.; Galati, C.; Reina, S.; Renna, L.; Viscuso, O.; Condorelli, G. G.; Fragala, I. L. Mater. Sci. Eng. 2003, C23, 989–994. (39) Cerofolini, G. F.; Galati, C.; Reina, S.; Renna, L. Semicond. Sci. Technol. 2003, 18, 423–429. (40) Gstrein, F.; Michalak, D. J.; Royea, W. J.; Lewis, N. S. J. Phys. Chem. B 2002, 106, 2950–2961. (41) Boukherroub, R.; Morin, S.; Sharpe, P.; Wayner, D. D. M.; Allongue, P. Langmuir 2000, 16, 7429–7434. (42) Dutta, S.; Perring, M.; Barrett, S.; Mitchell, M.; Kenis, P. J. A.; Bowden, N. B. Langmuir 2006, 22, 2146–2155. (43) Perring, M.; Dutta, S.; Arafat, S.; Mitchell, M.; Kenis, P. J. A.; Bowden, N. B. Langmuir 2005, 21, 10537–10544. (44) Collman, J. P.; Devaraj, N. K.; Chidsey, C. E. D. Langmuir 2004, 20, 1051–1053. (45) Devaraj, N. K.; Collman, J. P. QSAR Comb. Sci 2007, 26, 1253–1260. (46) Huisgen, R. Pure Appl. Chem. 1989, 61, 613–628. (47) Rostovtsev, V. V.; Green, L. G.; Fokin, V. V.; Sharpless, K. B. Angew. Chem., Int. Ed. 2002, 41, 2596–2599. (48) Tornøe, C. V.; Christensen, C.; Meldal, M. J. Org. Chem. 2002, 67, 3057–3064. (49) Kolb, H. C.; Sharpless, K. B. Drug DiscoVery Today 2003, 8, 1128–1137. (50) Bock, V. D.; Hiemstra, H.; van Maarseveen, J. H. Eur. J. Org. Chem. 2006, 51–68. (51) Ciampi, S.; Bo¨cking, T.; Kilian, K. A.; James, M.; Harper, J. B.; Gooding, J. J. Langmuir 2007, 23, 9320–9329. (52) Marrani, A. G.; Dalchiele, E. A.; Zanoni, R.; Decker, F.; Cattaruzza, F.; Bonifazi, D.; Prato, M. Electrochim. Acta 2007.

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In the developed protocol,51,53,54 the foundation of the hybrid organic/silicon structure is prepared via a thermal, noncatalyzed hydrosilylation reaction. Commercially available diacetylene species are used to afford a conveniently presented dipolarophile (the acetylene function). The thus-passivated Si(100) surface generally showed negligible levels of spectroscopically detectable silicon oxide species, even upon exposure to aqueous environments.51,53 We seek to exploit the advantages of silicon listed above in electrochemical applications such as biosensors and molecular electronics as we have done in the past using the gold thiol system.26,55-61 The challenge in using silicon for electrochemical applications in aqueous solution has been the need to passivate the silicon surface against oxidation during potential cycling. We have shown previously that even trace amounts of silicon oxide influence electron transfer between a silicon electrode and the organic monolayer significantly.26 The key to preventing the oxidation of the silicon surface is to prevent the ingress of water to the silicon surface. Hence the application of silicon surfaces in electrochemistry requires effective passivation of the silicon electrode such that the silicon surface is essentially inaccessible to species in the solution in which it is placed. With this requirement, we believe a major application of the silicon electrode in electrochemistry will be redox-active monolayers. That is, a redox-active species attached to the distal end of the monolayer such that electron transfer proceeds through the organic monolayers. Electroactive monolayers on silicon have been previously prepared by grafting either vinylferrocene or ferrocenecarboxyaldehyde62-64 directly onto H-terminated silicon or by further functionalization of chemically passivated Si(100) and Si(111) surfaces with ferrocene.6,27,52,65 Even though such reports describe well-behaved electronic communication between ferrocene and silicon, simultaneous protection against oxidation for the underlying silicon surface is generally either lacking or has not been investigated.62,65 In the vast majority of these systems, especially for those based on Si(100), oxide growth either with the electrode aging or with its exposure to oxidative environments is generally observed.52,66 The purpose of the current work is twofold: first, to show that very robust oxide-free modified silicon surfaces can be produced via the hydrosilylation of a H-terminated Si(100) surface using a diyne species (Scheme 1) and second, to show these surfaces are sufficiently stable to allow nearly ideal electrochemistry over (53) Ciampi, S.; Le Saux, G.; Harper, J. B.; Gooding, J. J. Electroanalysis 2008, 20, 1513–1519. (54) Ciampi, S.; Bo¨cking, T.; Kilian, K. A.; Harper, J. B.; Gooding, J. J. Langmuir 2008, 24, 5888–5892. (55) Beebe, J. M.; Engelkes, V. B.; Liu, J.; Gooding, J. J.; Eggers, P. K.; Jun, Y.; Zhu, X.; Paddon-Row, M. N.; Frisbie, C. D. J. Phys. Chem. B 2005, 109, 5207–5215. (56) Gooding, J. J.; Mearns, F.; Yang, W.; Liu, J. Electroanalysis 2003, 15, 81–96. (57) Liu, G.; Liu, J.; Bo¨cking, T.; Eggers, P. K.; Gooding, J. J. Chem. Phys. 2005, 319, 136–146. (58) Wong, E. L. S.; Gooding, J. J. Anal. Chem. 2006, 78, 2138–2144. (59) Liu, J.; Paddon-Row, M. N.; Gooding, J. J. J. Phys. Chem. B 2004, 108, 8460–8466. (60) Losic, D.; Shapter, J. G.; Gooding, J. J. Langmuir 2001, 17, 3307–3316. (61) Salomon, A.; Bo¨cking, T.; Gooding, J. J.; Cahen, D. Nano Lett. 2006, 6, 2873–2876. (62) Decker, F.; Cattaruzza, F.; Coluzza, C.; Flamini, A.; Marrani, A. G.; Zanoni, R.; Dalchiele, E. A. J. Phys. Chem. B 2006, 110, 7374–7379. (63) Zanoni, R.; Cattaruzza, F.; Coluzza, C.; Dalchiele, E. A.; Decker, F.; Di Santo, G.; Flamini, A.; Funari, L.; Marrani, A. G. Surf. Sci. 2005, 575, 260–272. (64) Zanoni, R.; Aurora, A.; Cattaruzza, F.; Coluzza, C.; Dalchiele, E. A.; Decker, F.; Di Santo, G.; Flamini, A.; Funari, L.; Marrani, A. G. Mater. Sci. Eng. 2006, 26, 840–845. (65) Fabre, B.; Hauquier, F. J. Phys. Chem. B 2006, 110, 6848–6855. (66) Dalchiele, E. A.; Aurora, A.; Bernardini, G.; Cattaruzza, F.; Flamini, A.; Pallavicini, P.; Zanoni, R.; Decker, F. J. Electroanal. Chem. 2005, 579, 133–142.

2532 Langmuir, Vol. 25, No. 4, 2009 Scheme 1. Acetylenyl Monolayers as Molecular Scaffolds toward Ferrocene Groups Irreversibly Confined on Si(100) Electrodesa,b

a Hydride-terminated Si(100) surfaces were reacted with either neat (terminal) diacetylene species 1 or with different molar mixtures of alkynes 1 and 2 (mole fraction of diyne 1 was either 0.5, 0.1, or 0.05) to afford the corresponding hydrosilylation product (surface 1a and surfaces 1b-d, single component and mixed methyl/acetylenyl-terminated surfaces, respectively). Monolayers prepared from heptyne 2 only were prepared to assess their stability toward oxidation (surface 1e). Covalently modified acetylenyl surfaces were further reacted to yield the corresponding redox-active assemblies (surfaces 2a-d) through Cu(I)-catalyzed alkyne-azide “click” coupling reactions with substituted azide 3. b Alternative reaction conditions for this step were 95 °C for 7 h.

many redox cycles with a ferrocene attached to the distal end of the monolayer. The conditions under which this excellent electrochemistry is achieved are also presented. Ferrocene-based monolayers are prepared here using a click reaction to confine azidomethylferrocene 3 onto alkyne-terminated Si(100) substrates (Scheme 1).

2. Experimental Section 2.1. Materials. 2.1.1. Chemicals. All chemicals, unless noted otherwise, were of analytical grade and used as received. Cyclohexane, dichloromethane, diethyl ether, ethanol, ethyl acetate, hexane, and i-propanol for substrate cleaning and chemical reactions were redistilled prior to use. Milli-Q″ water (>18 MΩ cm) was used for preparing solutions, chemical reactions, and surface-cleaning procedures. Hydrogen peroxide (30 wt % in water, Sigma-Aldrich), hydrofluoric acid (Riedel-de Hae¨n, 48 wt % in water), and sulfuric acid (J. T. Baker) were of semiconductor grade. 1,8-Nonadiyne (1; Aldrich, 98%) and 1-heptyne (2; Fluka, 98%) were redistilled from sodium borohydride (Sigma-Aldrich, 99+%) under reduced pressure (79 °C, 8-9 Torr) and atmospheric pressure, respectively, collected over activated molecular sieves (Fluka, 3 Å pore diameter, 10-20 mesh beads, dehydrated with indicator), and stored under a dry argon atmosphere prior to use. Anhydrous solvents used in chemical reactions were purified as follows: (a) diethyl ether was distilled from a benzophenone/sodium still; (b) dichloromethane was distilled from calcium hydride; (c) N,N-dimethylformamide (Fluka, 98+%) was distilled under reduced pressure from calcium hydride; (d) ethylenediamine was distilled under reduced pressure from barium oxide; and (e) ethanol was distilled from sodium. p-Toluensulfonyl chloride (Sigma-Aldrich, 98%) was recrystallized from chloroform (Sigma-Aldrich, HPLC grade)/hexane (redistilled). Sodium azide (Sigma-Aldrich) was crystallized from water by the addition of ethanol. Ferrocenecarboxylic acid was purchased from Lancaster (a; 98%). Sodium ascorbate (98%) and copper(II) sulfate pentahydrate (99%) were obtained from Aldrich. The gallium indium eutectic (99.99+%) was from Aldrich.

Ciampi et al. 2.1.2. Silicon Wafers. Prime-grade double-side-polished silicon wafers, 100-oriented (〈100〉 ( 0.9°), p-type (boron), 500 ( 25 µm thick, 0.007-0.009 Ω cm resistivity, were obtained from Virginia Semiconductors, Inc. 2.2. Purification and Analysis of Synthesized Compounds. Thin-layer chromatography (TLC) was performed on Merck silica gel aluminum sheets (60 F254). Merck silica gel (grade 9385, 230-400 mesh) was used for column chromatography. NMR spectra were recorded on a Bruker Avance 300 spectrometer using the solvent signal (CDCl3 from Aldrich, passed through basic alumina) as an internal reference. Fourier transform infrared (FTIR) spectra were recorded on a Thermo Nicolet Avatar 370 FTIR spectrometer, accumulating a minimum of 32 scans and selecting a resolution of 2 cm-1. Azide 3 was synthesized from ferrocenecarboxylic acid (a) via the corresponding alcohol derivative hydroxymethylferrocene (b) according to literature procedures with minor modifications (Scheme S1, Supporting Information).67 Oct-7-yn-1-thiol 4 was prepared from oct-3-yn-1-ol (c) via a four-step procedure (Scheme S2, Supporting Information). 2.2.1. Hydroxymethylferrocene (b). To a rapidly stirring suspension of LiAlH4 (1.06 g, 28 mmol) in 45 mL of dry diethyl ether a solution of ferrocenecarboxylic acid (a) (0.92 g, 4 mmol) in dry ether (50 mL) was rapidly added at room temperature. Stirring was continued at 32 °C for 10 h under an argon atmosphere. The crude reaction mixture was allowed to cool to room temperature, and a 1:1 ethanol/water mixture (approximately 25 mL) was added to it in portions to destroy excess LiAlH4 before the reaction mixture was transferred to excess ice-cold 2 M NaOH (approximately 200 mL). The aqueous and organic phases were separated, and the aqueous layer was extracted with ether (2 × 50 mL). The combined organic extracts were washed with 1 M NaOH solution (3 × 50 mL) and water (3 × 50 mL) and dried over MgSO4. Filtration through celite and evaporation of the solvent in vacuo afforded crude b as a orange/ brown oil. Column chromatography (ethyl acetate/hexane, 1:2) gave pure product b (0.68 g, 77%) as a yellow solid. 1H NMR (300 MHz, CDCl3) δ: 4.33 (d, 2H, J ) 6.1 Hz), 4.24 (t, 2H, J ) 1.9 Hz), 4.17 (bm, 7H); 13C NMR (75.5 MHz, CDCl3) δ: 68.46, 68.42, 68.0, 60.9; IR (KBr, cm-1): 3422, 3245, 2957, 2930, 1104, 988. 2.2.2. Azidomethylferrocene (3). A solution of hydroxymethylferrocene (b) (60 mg, 0.28 mmol) and sodium azide (108 mg, 1.68 mmol) in glacial acetic acid (3 mL) was stirred at 50 °C for 3 h under an argon atmosphere. The crude reaction mixture was then diluted with dichloromethane (50 mL), and the organic phase was washed with saturated NaHCO3 (3 × 50 mL) and water (1 × 50 mL), dried over Na2SO4, filtered, and dried in vacuo to yield an orange oil. The crude azide was purified by column chromatography (ethyl acetate/ hexane, 1:2) to give title compound 3 as a yellow/orange solid (58 mg, 85%). 1 H NMR (300 MHz, CDCl3) δ: 4.24 (t, 2H, J ) 1.7 Hz), 4.20 (t, 2H, J ) 1.7 Hz), 4.17 (bs, 5H), 4.12 (bs, 2H); 13C NMR (75.5 MHz, CDCl3) δ: 82.25, 68.83, 68.0, 51.03; IR (KBr, cm-1): 3101, 2957, 2936, 2099, 2072. 2.2.3. Oct-7-yn-1-ol (d). To a flask containing dry ethylenediamine (8 mL) was added sodium hydride (60% in mineral oil, 640 mg, 16 mmol), and the resulting suspension, kept under an argon atmosphere, was stirred at room temperature for 40 min and then at 60 °C for 1 h. Upon cooling of the mixture to -35 °C (in a liquid nitrogen/sat. calcium chloride slurry), oct-3-yn-1-ol (c) (504 mg, 4 mmol) was added to the reaction mixture, and stirring was continued at 50 °C for 50 min. Upon cooling to 0 °C in an water/ice bath, diethyl ether (20 mL) and ice (approximately 50 g) were added to the flask. The diluted mixture was then transferred to a separation funnel, and the organic layer was separated. The aqueous layer was then extracted with diethyl ether (2 × 25 mL), and the pooled organic phase was washed with water (3 × 25 mL), dried over MgSO4, filtered, and dried in vacuo to yield an orange oil. The crude product was purified by column chromatography (diethyl ether/hexane, 1:1) to give d as a pale-yellow oil (258 mg, 51%). 1H NMR (300 MHz, CDCl3) δ: (67) Casas-Solvas, J. M.; Vargas-Berenguel, A.; Capitan-Vallvey, L. F.; Santoyo-Gonzalez, F. Org. Lett. 2004, 6, 3687–3690.

Si Electrodes for Aqueous EnVironments. 3.60 (t, 2H, J ) 6.6 Hz), 2.15 (dt, 2H, J ) 2.6 Hz, J ) 6.6 Hz), 2.10 (bs, 1H), 1.91 (t, 1H, J ) 2.6 Hz), 1.52 (bm, 4H), 1.37 (bm, 4H); 13 C NMR (75.5 MHz, CDCl3) δ: 84.64, 68.35, 62.36, 31.83, 28.52, 28.38, 28.26; 18.53; IR (NaCl, cm-1): 3365, 3295, 2930, 2857, 1461. 2.2.4. Oct-7-yn-1-yl p-toluenesulonate (e). To an ice-cold solution of oct-7-yn-1-ol (d) (258 mg, 2 mmol) in pyridine (8 mL) was added p-toluensulfonyl chloride (762 mg, 4 mmol) in one portion. Stirring was continued at 0 °C under an argon atmosphere for 6 h. The reaction mixture was then allowed to warm to room temperature, transferred to a separation funnel, and washed with saturated CuSO4 solution (50 mL), water (2 × 10 mL), and saturated NaCl (25 mL). The organic phase was dried over MgSO4, filtered, and dried in vacuo to yield a yellow oil. The crude product was purified by column chromatography (diethyl ether/hexane, 1:1) to give e as a yellow oil (489 mg, 88%). 1H NMR (300 MHz, CDCl3) δ: 7.79 (d, 2H, J ) 8.1 Hz,), 7.34 (d, 2H, J ) 8.1 Hz), 4.03 (t, 2H, J ) 6.6 Hz), 2.45 (s, 3H), 2.15 (dt, 2H, J ) 2.6 Hz, J ) 6.6 Hz), 1.92 (t, 1H, J ) 2.6 Hz), 1.69 (bm, 2H), 1.47 (bm, 2H), 1.33 (bm, 4H); 13C NMR (75.5 MHz, CDCl3) δ: 144.81, 133.43, 129.96, 128.03, 70.63, 68.47, 28.85, 28.14, 25.03, 21.77, 18.38; IR (NaCl, cm-1): 3291, 2938, 2861, 1598, 1464, 1359, 1189, 1176. 2.2.5. Octyn-7-yl-1-ethanethioate (f). Oct-7-yn-1-yl p-toluenesulonate (e) (320 mg, 1.14 mmol) was added to a stirred solution of potassium thioacetate (188 mg, 1.65 mmol) in dry N,Ndimethylformamide (20 mL). Stirring was continued at room temperature under an argon atmosphere for 20 h. The reaction mixture was then evaporated under reduced pressure (∼65 °C, 20 mbar), and the residue was purified by column chromatography (diethyl ether/ hexane, 2:3) to give title compound f as a yellow oil (155 mg, 74%).1H NMR (300 MHz, CDCl3) δ: 2.86 (t, 2H, J ) 7.2 Hz), 2.32 (s, 3H), 2.18 (dt, 2H, J ) 2.6 Hz, J ) 6.6 Hz), 1.93 (t, 1H, J ) 2.6 Hz), 1.55 (bm, 4H), 1.41 (bm, 4H); 13C NMR (75.5 MHz, CDCl3) δ: 185.97, 79.88, 68.85, 30.76, 29.73, 28.58, 27.46, 27.61; 18.59. 2.2.6. Oct-7-yn-1-thiol (4). To a solution of octyn-7-yl-1ethanethioate (f) (155 mg, 0.84 mmol) in anhydrous, degassed ethanol (15 mL) were added 3 drops of conc. hydrochloric acid. Stirring was continued at 60 °C under an argon atmosphere for 5 h. The solvent was removed under reduced pressure, and the residue was purified by column chromatography (hexane/ethyl acetate, 10:1) to give thiol 4 as a pale-yellow oil (75 mg, 63%). 1H NMR (300 MHz, CDCl3) δ: 2.59 (t, 2H, J ) 7.7 Hz,), 2.18 (dt, 2H, J ) 2.6 Hz, J ) 6.6 Hz), 1.93 (t, 1H, J ) 2.6 Hz), 1.54 (bm, 4H), 1.41 (bm, 4H); 13C NMR (75.5 MHz, CDCl3) δ: 84.72, 68.34, 55.78, 39.18, 31.18, 29.29, 28.90; 28.48, 18.53; IR (NaCl, cm-1): 3465, 3297, 2930, 2856, 1461, 1152. 2.3. Surface Modification. 2.3.1. Single-Component AcetyleneFunctionalized Silicon (100) Surfaces (Surface 1a). The assembly of the acetylenylated Si(100) surface by the covalent attachment of diyne 1 followed a previously reported procedure.51 Silicon samples approximately 20 × 10 mm2 in size were successively rinsed in dichloromethane and ethanol, allowed to dry under a stream of argon, and then cleaned in 1:3 (v/v) 30% by mass aqueous hydrogen peroxide/concentrated sulfuric acid at 100 °C for 30 min. Caution! Concentrated H2O2/H2SO4 solution reacts Violently with organic materials and should be handled with extreme care. Samples were then rinsed with copious amounts of water and etched with 2.5% aqueous hydrogen fluoride solution for 1.5 min to afford a hydrogen-terminated surface.23 Wafers were then immediately transferred to previously degassed (through a minimum of four freeze-pump-thaw cycles) 1,8-nonadiyne 1 (approximately 0.5 mL). The reaction mixture was kept under an argon atmosphere while the reaction vessel was immersed in an oil bath set to 170 °C for 3 h. After being cooled to room temperature, the modified silicon wafers were rinsed several times with dichloromethane, ethanol, and water before being either analyzed or further reacted with adduct 3. Alternative conditions for this step were investigated. A longer reaction time of 7 h and a lower reaction temperature of 95 °C were employed in a control experiment aimed to assess the effect of both parameters on the monolayer quality. 2.3.2. Mixed Methyl/Acetylenyl-Terminated Silicon (100) Surfaces (Surfaces 1b-d). The assembly of a mixed monolayer of 1,8-

Langmuir, Vol. 25, No. 4, 2009 2533 nonadiyne 1 and 1-heptyne 2 proceeded, following a surface cleaning/ etching protocol identical to the one reported above for surface 1a. Hydrogen-terminated samples were immersed in a degassed (through a minimum of four freeze-pump-thaw cycles) mixture of 1,8nonadiyne 1/1-heptyne 2 (either 1:1, 1:10, or 1:20 by mole) prepared by mixing (a) 1 (0.301 mL, 2 mmol) and 2 (0.262 mL, 2 mmol) to give surface 1b; (b) 1 (0.301 mL, 2 mmol) and 2 (2.624 mL, 20 mmol) to give surface 1c; and (c) 1 (0.301 mL, 2 mmol) and 2 (5.248 mL, 40 mmol) to give surface 1d. The reaction chamber was maintained under an argon stream and then transferred to an oil bath set to 95 °C for 7 h. After being cooled to room temperature, the modified silicon wafers were rinsed several times with dichloromethane, ethanol, and water before being further reacted. 2.3.3. Single-Component Methyl-Terminated Silicon (100) Surfaces (Surface 1e). The assembly of a 1-heptyne 2 monolayer on hydrogen-terminated Si(100) by thermal hydrosilylation of the 1-alkyne species followed a protocol similar to the one described in section 2.3.1. In brief, freshly etched hydrogen-terminated samples were immersed in a reaction vessel containing a liquid sample of degassed 1-heptyne 2 kept under argon and then transferred to an oil bath set to 95 °C for 7 h. After being cooled to room temperature, the modified silicon wafers (surface 1e) were rinsed several times with dichloromethane, ethanol, and water before being analyzed. 2.3.4. Immobilization of Ferrocene DeriVatiVes on SingleComponent and Mixed Methyl/Acetylenyl Surfaces. In a typical click procedure, to a reaction vial containing the alkyne-functionalized silicon surface (surfaces 1a-d) were added (i) azide 3 (10 mM, i-propanol/water, 2:1), (ii) copper(II) sulfate pentahydrate (1 mol % relative to the azide compound), and (iii) sodium ascorbate (10 mol % relative to the azide). Reactions were carried out at room temperature without excluding air from the reaction environment and stopped after 24 h by removal of the modified sample from the reaction vessel. The prepared surface-bound [1,2,3]-triazole samples (surfaces 2a-d, prepared respectively from surfaces 1a-d) were rinsed consecutively with copious amounts of hexane, ethyl acetate, ethanol, and water and then stored for a 24 h period in a 0.05% (w/v) ethylenediaminetetraacetic acid (EDTA) solution (pH 7.4). Samples were then rinsed with copious amounts of water before being analyzed. 2.3.5. Single-Component Acetylene-Functionalized Gold Surfaces (Surface 3; Scheme S3, Supporting Information). Commercial polycrystalline gold-wire electrodes (CH Instruments, Inc., part number CHI101) were used for the preparation of gold-based SAMs. Prior to the self-assembly of oct-7-yn-1-thiol 4, the gold electrodes were polished successively with 0.3 and 0.05 µm alumina slurry on microcloth pads (Buehler), sonicated for approximately 1 min in a 1 mM sodium hydroxide solution, and then extensively washed with copious quantities of water and ethanol. SAMs of 4 were prepared by immersing for 24 h the polished gold electrode in ethanol solutions containing thiol 4 at a concentration of 10 mM. Coated electrodes (surface 3) were then rinsed with copious amounts of ethanol and water before being transferred to the click mixture. 2.3.6. Formation of Ferrocene-Functionalized Gold Substrates (Surface 4; Scheme S3, Supporting Information). To catalyze click reactions on Au acetylenyl substrates (surface 3), procedures analogous to that described above for silicon substrates (section 2.3.4) were used. 2.4. Surface Characterization. 2.4.1. Contact Angle Goniometry. Water contact angle measurements were determined on a Rame´Hart 200-F1 goniometer. Samples were prepared in triplicate with at least three separate spots being measured for each sample. The reproducibility of the contact angle measurements was 3-5°. 2.4.2. XPS Measurements. X-ray photoelectron spectroscopy experiments were performed on an ESCALAB 220iXL. Monochromatic Al KR X-rays (1486.6 eV) incident at 58° to the analyzer lens were used to excite electrons from the sample. Emitted photoelectrons at a takeoff angle of 90° from the plane of the sample surface were collected on an hemispherical analyzer with a multichannel detector. The analyzing chamber operated below 10-10 mbar, and the spot size was approximately 1 mm2. The resolution of the spectrometer is ∼0.6 eV as measured from the Ag 3d5/2 signal

2534 Langmuir, Vol. 25, No. 4, 2009 (fwhm). All energies are reported as binding energies in electron volts and are referenced to the Si 2p1/2 signal (corrected to 99.9 eV). Survey scans were carried out by selecting a 100 ms dwell time and an analyzer pass energy of 100 eV. High-resolution scans were run with a 0.1 eV step size, a dwell time of 100 ms, and the analyzer pass energy set to 20 eV. After background subtraction using the Shirley routine, spectra were fitted with a convolution of Lorentzian and Gaussian profiles as described previously.51 The ratios of the integrated areas for the C 1s and N 1s emissions, each normalized for their elemental sensitivity68 and scanning time (number of scans accumulated), afforded an estimate of the conversion of the acetylenyl surface (surface 1a) to the ferrocene-functionalized surface (surface 2a). The monolayer coverage of oxidized silicon was calculated directly from the oxidized/bulk Si 2p peak area ratio according to the method described by Webb and co-workers for very thin oxide overlayers.69,70 Under our XPS experimental conditions, the SiOx detection limit was ∼0.07 monolayer. 2.4.3. X-ray Reflectometry Measurements. X-ray reflectivity spectra were measured on a Panalytical Ltd. X’Pert Pro Reflectometer using Cu KR X-ray radiation (λ ) 1.54056 Å). The X-ray beam was focused using a Go¨bel mirror and collimated with 0.2 mm pre- and postsample slits. A De-Wolf beam knife was mounted above the sample and was used to reduce the sample footprint at very low angles. Reflectivity data were collected over the angular range of 0.05° e θ e 4.00°, with a step size of 0.010° and counting times of 15 s per step. Samples had an average size of 10 × 30 mm2. Structural parameters of the prepared organic thin layers were refined using the MOTOFIT71 reflectivity analysis software with reflectivity data as a function of momentum-transfer vector Qz () 4π(sinθ)/λ). In the fitting routines, the Levenberg-Marquardt method was selected to minimize χ2 values. Single-layer models were proposed when no significant improvement in the fitting quality was observed upon the introduction of additional layer in the refined model. All X-ray reflectivity curves were acquired in air for samples stored under an argon atmosphere prior to analysis. 2.4.4. Electrochemical Characterizations. All electrochemical measurements were performed using a BAS 100B electrochemical analyzer (Bioanalytical Systems, Inc., W. Lafayette, IN). A conventional three-electrode cell was used with either modified silicon or a gold surface as the working electrode, a platinum mesh as the counter electrode, and silver/silver chloride in 3 M sodium chloride as the reference electrode. All cyclic voltammograms were acquired in 1.0 M aqueous perchloric acid at room temperature without the exclusion of air from the cell and with potentials reported versus the reference electrode. Silicon-based electrodes had a geometric area of 400 mm2 with only a designated portion of it (200 mm2) immersed in the electrolyte solution during measurements. Ohmic contact between the silicon substrate and a copper wire was ensured by rapidly rubbing a gallium indium eutectic onto a close series of marks (diamond scribe) aimed to expose the bulk of the silicon electrodes. The area of the uncoated polished gold electrode was determined from the charge passed during the stripping of the gold oxide during voltammetric cleaning in 0.5 M sulfuric acid. The formalism developed by Weber and co-workers to obtain kinetic information,72 namely, the apparent electron-transfer rate constant, kapp, for the electron-transfer process between tethered redox groups and the conducting substrate, has been here used to determine the rate constants for all of the redox assemblies examined in this article. A calculation routine written in MATLAB used the difference in cyclic voltammetric peak potentials at different scan rates in a method similar to that described by Weber et al.72 and Tender et al.73 to calculate the apparent rate constant. The modification of the (68) XPS atomic sensitivities were 10.82 for Fe 2p3/2, 5.6 for Fe 2p1/2, 0.82 for Si 2p, 1.00 for C 1s, and 1.80 for N 1s. (69) Webb, L. J.; Lewis, N. S. J. Phys. Chem. B 2003, 107, 5404–5412. (70) Haber, J. A.; Lewis, N. S. J. Phys. Chem. B 2002, 106, 3639–3656. (71) Nelson, A. J. Appl. Crystallogr. 2006, 39, 273–276. (72) Weber, K.; Creager, S. E. Anal. Chem. 1994, 66, 3164–3172. (73) Tender, L.; Carter, M. T.; Murray, R. W. Anal. Chem. 1994, 66, 3173– 3181.

Ciampi et al.

Figure 1. High-resolution Si 2p XPS narrow scans for the Si 2p region of acetylenyl- and methyl-terminated Si(100) surfaces prepared from acetylene species. (a) Single-component monolayer prepared by the thermal hydrosilylation of 1,8-nonadyine 1 at 170 °C (3 h) (surface 1a) showing no evidence of SiOx species (inset). (b) Alkyl-passivated surfaces prepared by the thermal grafting of alkyne 2 (only) at 95 °C (7 h) and characterized by poor resistance toward the oxidation of the silicon substrate (surface 1e). (c) Spectrograph for an acetylenyl SAM (surface 1a) prepared from diyne 1 at a lower hydrosilylation temperature (95 °C, 7 h) showing detectable levels of oxidized silicon species (inset).

previously described techniques was that both the oxidation and reduction peak potentials were used in the fitting procedure. With the reorganization energy held constant, as suggested by Weber et al., this reduced the number of variables in the fit to one, which increased the confidence in the averaged calculated rate constant.

3. Results and Discussion 3.1. Assembly of Single-Component 1,8-Nonadiyne Monolayers on Hydrogen-Terminated Si(100) Surfaces. As depicted in Scheme 1, the immersion of freshly etched hydrogen-terminated Si(100) surfaces in 1,8-nonadiyne 1 at 170 °C for 3 h afforded the acetylene-functionalized surface (surface 1a).74 An advancing water contact angle for acetylene-terminated surface 1a of 85 ( 5° was consistent with the presence of a hydrophobic monolayer but considerably smaller (by ∼25°) than those reported for analogous methyl-terminated monolayers prepared on Si(100) from 1-alkenes and 1-alkynes.75,76 We have attributed the more hydrophilic character of surface 1a to an increased polarizability of the acetylene function compared with that of the methylterminated substrates.51 XPS spectra acquired on acetylenyl surface 1a are shown in Figure S1 (Supporting Information) and in Figure 1. Figure S1a shows a representative survey spectrum that indicates the presence of Si, C, and O only.77 High-resolution narrow scans were collected for the C 1s and Si 2p regions to gain information on bonding configurations and on the presence (74) An extensive discussion of X-ray reflectivity and XPS data for analagous Si(100) monolayers prepared from diyne 1 has been previously reported.51 (75) Sieval, A. B.; Linke, R.; Zuilhof, H.; Sudholter, E. J. R. AdV. Mater. 2000, 12, 1457–1460. (76) Sieval, A. B.; Opitz, R.; Maas, H. P. A.; Schoeman, M. G.; Meijer, G.; Vergeldt, F. J.; Zuilhof, H.; Sudhoelter, E. J. R. Langmuir 2000, 16, 10359– 10368. (77) The oxygen 1s emission at ∼532 eV is ascribed to adventitiously adsorbed oxygen.

Si Electrodes for Aqueous EnVironments.

of any oxidation of the silicon substrate.78 The narrow scan of the C 1s region (Figure S1b, Supporting Information) shows a broad signal (1.59 eV, fwhm)79 with a mean binding energy of 285.0 eV for the fitted function (not shown) and agrees well with previous data.51 The high-resolution Si 2p scan reveals important information about the monolayer quality and its ability to prevent appreciable oxidation of the underlying Si substrate.75 Most importantly, in Si 2p narrow scans obtained for surface 1a, as presented in Figure 1a, no silicon oxide was detected in the 102-104 eV region on the high-binding-energy side of the Si 2p1/2-Si 2p3/2 spin-orbitsplit doublet (99.9 and 99.3 eV, respectively).26,80 To gain structural information on the monolayers prepared from diyne 1, X-ray reflectivity spectra were recorded (Figure S2). The refinement of a structural model suggested a monolayer thickness (d) of 10.4(1) Å.81 The electron density (Fel) of the organic layer was also derived from the X-ray reflectivity data and was found to be ∼0.34 e-/Å3, consistent with previous reports of alkane-based SAMs, and supports the formation of a densely packed organic layer with very limited defects.25,30 On the basis of both the refined values of monolayer thickness and electron density, an estimate of the molecular coverage of the silicon surface of 20(3) Å2 per grafted molecule of diyne 1 is proposed. Both the refined silicon-organic (σsilicon) and monolayer/air (σmonolayer) interfacial roughnesses were found to be 4(1) Å, which reflects a good-quality monolayer film.25,30 Given our interest in the development of a robust passivating strategy, we then turned our attention to changes in the surface quality upon exposure of surface 1a to aqueous environments. The high-quality monolayer prepared from diyne 1 was, as a proof-of-concept, extensively82 analyzed by means of cyclic voltammetry between -100 and 800 mV versus Ag/AgCl in aqueous 1.0 M perchloric acid (Figure S3a, Supporting Information) and 0.1 M NaClO4 (pH 7.4, Figure S3b, Supporting Information) before being loaded into the XPS analysis chamber to assess the oxidation level of the silicon substrate. XPS Si 2p narrow scans contained no clear evidence of the formation of silicon oxides (Si+-Si4+), which would appear as a broad peak between 102 and 104 eV (Figure S4, Supporting Information).36,80 Furthermore, acetylenyl surfaces stored for 2 months (in a sealed vial, under an argon atmosphere, in the dark at 4 °C) showed similar XPS profiles to freshly prepared surfaces (Figure S5, Supporting Information), with no SiOx species observed by XPS (Figure S5c, Supporting Information). These stability studies illustrate that acetylenyl-terminated surfaces are exceedingly robust and therefore can be applied to electrochemical studies in aqueous media without degradation of the passivating structure. As a consequence, we next modified the acetyleneterminated surfaces with azidomethylferrocene via click chemistry for electron-transfer studies and as precursor molecular constructs for the fabrication of molecular devices such as sensors. 3.2. Coupling of Azidomethylferrocene onto SingleComponent Acetylenyl Surfaces 3.2.1. Spectroscopic and Electrochemical Characterization of the Ferrocene Assemblies on Si (100) Surfaces. The formation of surface 2a, as produced through click reactions of azide species 3 with surface acetylenes, (78) An FTIR investigation of analogous surfaces prepared on porous silicon rugate filters probed the presence of a surfacial sililated olefin and a terminal acetylene functionality for 1,8-nonadiyne 1-based SAMs on this mid-IR permittivity, high-surface area substrate.54 (79) The observed large dispersion value for the C 1s signal is consistent with a signal arising from contributions of carbon-, hydrogen- and silicon-bonded carbon atoms being either sp, sp2, or sp3 hybridized. Given the chemical composition of the monolayer assembled from diyne 1, a contribution ascribed to carbon atoms in a carbide carbon-silicon configuration was expected around ∼284.1 eV 39 but could not be assigned unambiguously. (80) Himpsel, F. J.; McFeely, F. R.; Taleb-Ibrahimi, A.; Yarmoff, J. A.; Hollinger, G. Phys. ReV. B 1988, 38, 6084–6096.

Langmuir, Vol. 25, No. 4, 2009 2535

Figure 2. XPS analysis of Si(100) electrodes (surface 2a) before (top spectra) and upon (bottom spectra) cyclic voltammetry analysis. Spectra are offset for clarity. (a) XPS survey spectrum. (b) Narrow scan of the Fe 2p region comprising two spin-orbit split components 3/2 and 1/2 found at ∼708.6 and ∼721.3 eV, respectively.

was supported by an ∼16° decrease in the advancing contact angle value (Θa ) 69 ( 3°) relative to surface 1a. It was also in good agreement with the findings of our previous report on the click conditions for the preparation of the same redox surface.53 Increased hydrophilicity for the derivatized acetylenyl silicon substrate was consistent with an interface bearing ferrocenyl derivatives. The contact angles recorded were close to values reported by Fabre et al.65 for organic layers prepared from acidterminated Si(111) SAMs onto which 2-aminoethylferroceneylmethylether was immobilized. From XPS survey spectra, a set of satellite peaks at ∼715 eV, attributed to the Fe 2p emission from immobilized ferrocene units together with silicon-, carbon-, nitrogen-, and oxygenrelated signals, suggested the successful outcome of the cycloaddition reaction to produce surface 2a (Figure 2a). Peaks from the Fe 2p, N 1s, and C 1s core levels of the grafted molecules and Si 2p from the silicon substrate were investigated in detail, and the obtained data were found to be in excellent agreement with those that we previously reported in an optimization study on the preparation of analogous electrodes.53 Curve fitting of the experimental curves for the Fe 2p region (Figure 2b) showed two well-resolved Fe 2p3/2 and Fe 2p1/2 spin-orbit split components at 708.6 and 721.3 eV (fwhm’s of ∼1.1 and ∼1.6 eV, respectively), suggesting a predominant Fe(II) population with a lack of detectable high-binding-energy satellite signals at ∼711 and ∼724 eV, generally observed for Fe(III)-related compounds.64,83,84 This observation is consistent with a predominantly ferrocene population and indicates that no appreciable oxidation of ferrocene to ferricinium occurred under the coupling conditions. Narrow N 1s scans, presented in Figure S6a, showed that a broad signal centered at ∼401 eV was best fit to two functions held 1.3 eV apart (400.4 and 401.7 eV, 2:1 ratio of the integrated areas), which are associated with photoelectrons emitted from nitrogen atoms of the immobilized triazole moiety.27,51,85 Both the observed binding energies and fwhm (∼1.7 eV for both of the fitted functions) compared well with what was observed for other 1,4-disubstituted [1,2,3]-triazoles on Si(100) surfaces.51 The lack of signal at approximately 405 eV, resulting from electron-deficient nitrogen atoms in azido groups, was diagnostic of the absence of any nonspecifically adsorbed 3 on the surface. (81) Estimated standard deviations (esd’s) are given in parentheses. (82) The silicon electrode was scanned at a sweep rate of 100 mV s-1 between -100 and 800 mV for ∼2.5 h (∼1000 segments). (83) Woodbridge, C. M.; Pugmire, D. L.; Johnson, R. C.; Boag, N. M.; Langell, M. A. J. Phys. Chem. B 2000, 104, 3085–3093. (84) Tajimi, N.; Sano, H.; Murase, K.; Lee, K.-H.; Sugimura, H. Langmuir 2007, 23, 3193–3198. (85) Lee Jungkyu, K.; Chi Young, S.; Choi Insung, S. Langmuir 2004, 20, 3844–3847.

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Figure 3. High-resolution Si 2p XPS spectra for ferrocene-modified Si(100) surfaces (surface 2a) prior and upon the electrochemical investigation of their redox behavior in aqueous environments. (a) Ferrocene-modified acetylenyl substrates showing no detectable levels of SiOx species (102-104 eV, inset) for freshly prepared electrode surfaces. (b) Lack of detectable silicon oxidation (inset) for an extensively analyzed (through a minimum of ∼200 cyclic voltammetry cycles) electrode.

High-resolution scans from the C 1s region (Figure S6b) were decomposed into two functions with mean binding energies of 286.4 and 284.9 eV. The unresolved low-binding-energy signal at 284.9 eV (fwhm ) 1.3 eV) was attributed to a contribution of carbon atoms from the cyclopentadienyl rings83 and the aliphatic carbon-bonded (C-C) carbon atoms. The function at 286.4 eV (fwhm ) 1.4 eV) is characteristic of nitrogen-bonded (C-N) carbons and further supports the formation of a triazole ring.30,51,65 On the basis of a previously reported deconvolution procedure,51 XPS data acquired for the N 1s and C 1s regions aided the assessment of the click yields. The click reaction conversion of the acetylene layer (surface 1a) to the corresponding ferrocene-based surface (surface 2a) was estimated to be approximately 40%. Importantly, despite the chemical derivatization of the acetylene surface in aqueous environments, the successive prolonged exposure to aqueous EDTA solutions (surface 2a), and the extensive handling of the sample in air during washing steps, no silicon oxide-related signals were observed in the 102-104 eV region of the Si 2p XPS emission (Figure 3a). X-ray reflectivity measurements performed on surface 2a were fitted to a bilayer structural model (Figure S2b, Supporting Information), with the refined structural parameters summarized in Table S1 (Supporting Information). The thickness of the inner layer was 10.0(1) Å, which is in good agreement with the refined values obtained from surface 1a. The thickness of the outer layer was found to be 6.4(1) Å. The refined total thickness of the organic layer (∼16 Å) is consistent with a significant conversion of the acetylenyl surface to the corresponding triazole product. XRR data suggest an area per molecule for grafted ferrocene compound 3 of 67(15) Å2, indicating a yield of the process close to 25-35%, which correlates well with the X-ray photoelectron spectroscopy data. The profile of CVs obtained for the ferrocene-derivatized Si(100) surface (surface 2a) showed symmetrical (relative to the potential axis) ferrocene oxidation and ferricinium reduction waves at low scan rates (Figure 4a and Figure S7, Supporting Information), with a separation between the anodic (Ea) and cathodic (Ec) peak potentials of 18 mV (ν ) 20 mV s-1). From voltammograms at low scan rates, the half-wave potential, E1/2, was 371 ( 9 and very similar to the value of 370 mV versus Ag|AgCl|3 M NaCl recently reported by Tajimi and co-workers84 for densely packed vinylferrocene assemblies on Si(111). The linear dependence of the anodic peak current on the scan rate provided further evidence that the redox species were surfacebound (Figure S8, Supporting Information).

Figure 4. Representative cyclic voltammograms of azidomethylferrocene 3-functionalized Si(100) and gold electrodes in 1.0 M HClO4. The scan rates are (a-d) 20 mV s-1 (showing data for surfaces 2a-d, respectively) and (e) 20 V s-1 (surface 4). A schematic depiction of the putative electrode surface, showing the mole fraction (χsln) of diyne 1 used in the corresponding hydrosilylation, or SAM deposition step, is inserted on the side of each set of oxidation/reduction waves.

With the potential being scanned at 20 mV s-1, the fwhm for both curves was found to be ∼115 mV, which was slightly higher than the entropically determined value of 90.6 mV for ideal Nernstian behavior.86 The departure from ideality of the experimental fwhm is generally attributed to either one or a combination of the following: (i) the presence of an ensemble of ferrocene moieties exposed to different local environments,87 (ii) non-negligible interactions between neighboring ferrocene sites,88 and (iii) variation of the interfacial charged double layer during the experiment.52,62 From an analysis of the area under the CV’s cathodic or anodic peaks, the ferrocene surface coverage, Γ, for surface 2a was also estimated and was found to be 3.2 ( 0.1 × 10-10 mol cm-2. The obtained Γ was found to be very similar to that observed by Fabre et al.65 for densely packed ferrocene films prepared on acid-terminated Si(111) SAMs ((3.3-3.5) × 10-10 mol cm-2). Considering a surface density of silicon top sites of 6.78 × 1014 atoms cm-2,23 the electrochemically observed density of redox sites in the film yields a silicon atom/ferrocene molecule ratio of ∼3.5-3.6.65 From the perspective that surface acetylene groups have a footprint of 20(3) Å2 (refinement of XRR structural model for surface 1a, section 3.1), a yield between 35-43% for the click step was determined. The value was very close to the XPSderived ∼40% conversion of surface acetylenes to immobilized triazoles (section 3.2).53 (86) Bard, A. J.; Faulkner, L. R. Electrohemical Methods; John Wiley & Sons: New York, 1980. (87) Calvente, J. J.; Andreu, R.; Molero, M.; Lopez-Perez, G.; Dominguez, M. J. Phys. Chem. B 2001, 105, 9557–9568. (88) Eagling, R. D.; Bateman, J. E.; Goodwin, N. J.; Henderson, W.; Horrocks, B. R. J. Chem. Soc., Dalton Trans. 1998, 1273–1276.

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Table 1. Electrochemical Properties of Ferrocene-Modified Electrodes Prepared from Acetylenyl Si(100) Surfacesa entry

monolayer structure (acetylenyl surface/χslnb)

E1/2 (mV)c

∆Ep (mV)d

fwhm (mV),d,e

Γae (mol cm-2)

Kapp (s-1)

1 2 3 4

surface 2a (surface 1a/1.0) surface 2b (surface 1b/0.5) surface 2c (surface 1c/0.1) surface 2d (surface 1d/0.05)

+371 ( 9 +361 ( 6 +368 ( 5 +397 ( 17

18 ( 4 12 ( 3 14 ( 3 122 ( 8

115 ( 4 116 ( 3 123 ( 2 210 ( 35

3.2 ( 0.1 × 10-10 3.4 ( 0.1 × 10-10 5.4 ( 0.9 × 10-11 2.7 ( 1.1 × 10-11

5.3 ( 1.5 5.1 ( 1.1 2.9 ( 0.9

a All entries were obtained from a minimum of three independently prepared and analyzed electrodes. b 1,8-Nonadiyne 1 mole fraction in the hydrosilylation mixture. c E1/2 was taken as the average of the anodic and cathodic peak potentials from voltammograms taken at low scan rates. d Data voltammograms taken at a scan rate of 20 mV s-1. e Data for the ferrocene oxidation waves.

As evident from the data presented in Figure S7 (Supporting Information), with an increase in the sweep rate (ν was varied from 20 to 1500 mV s-1), the separation of Ea and Ec increased. The divergence of peak potentials with the scan rate was used to calculate values of kapp, the apparent electron-transfer rates constant. The value of kapp was found to be 5.3 ( 1.5 s-1. The obtained kapp was lower than the 30 s-1 value calculated for ferrocene assemblies on p-type Si(100) (∼10 Ω cm), where the cyclopentadienyl rings (2-aminoethylferroceneylmethylether) were located further from the surface when immobilized on acidterminated SAMs65 and were lower than the 10 and 130 s-1 reported by Dalchiele et al.66 for monolayers prepared by the direct grafting of ferrocenecarboxyaldehyde or vinylferrocene directly onto hydrogen-terminated p-Si(100) (0.02 Ω cm). We then investigated how the exposure to aqueous environments and the applied potentials during voltammetric investigations altered the electrochemical and spectroscopic properties of the silicon electrode. The electrochemical properties of surface 2a were not significantly affected by ∼200 CV cycles in aqueous environments, as judged by voltammograms taken for extensively analyzed electrodes (Figure S9). Changes in the observed voltammetric-based parameters can be summarized as follows: (1) E1/2 shifted to 385 mV, which is ∼14 mV more anodic than that observed for freshly prepared electrodes (371 ( 10 mV), (2) anodic-to-cathodic peak separations were increased only slightly, with an Ea/Ec difference of 25 mV comparable to the 18 mV initially observed at 20 mV s-1, (3) the fwhm (at 20 mV s-1) increased from to 114 to 126 mV, and (4) the calculated electron-transfer rate constant remained unchanged within the experimental uncertainty (5.5 s-1). The increase in the peak separation suggested a minor departure from reversibility for the confined redox event, whereas the increase in the observed fwhm was interpreted as a minor deterioration of the film quality. Nevertheless, XPS spectra of the used Si(100) electrodes (surface 2a) continue to show negligible SiOx growth for the aged silicon electrodes (Figures 3b), indicating that any degradation of the surface was very minor. Furthermore, the Fe(II) signal remained unchanged (Figure 2b), further supporting the conclusion of the good passivating properties of the acetylenyl SAMs and the chemical robustness of the [1,2,3]-triazole linker. Detailed analysis of the N 1s and C 1s spectral regions (Figure S6a and S6b, respectively) afforded information on binding energies, relative intensities, and the fwhm of fitted functions by all means comparable, within the experimental uncertainty, to what is reported in section 2.3.1 for the freshly prepared electrodes. The estimated surface coverage after 200 CV cycles using XPS (from the nitrogen-bound carbon atoms and N 1s data) was estimated to be ∼40%, again supporting the supposition that any degradation of the surface after 200 cycles in aqueous solution is exceedingly minor. 3.2.2. Ferrocene-Functionalized Gold Electrodes. CVs from ferrocene-coated Au electrodes where the redox moiety was confined on the surface in a similar manner (surface 4, Scheme S3, Supporting Information; sections 2.3.5 and 2.3.6) were

acquired so as to allow a comparison of the result obtained on the silicon substrates with a more conventional metallic surface (Figure 4e). Despite the numerous encouraging reports on the application of click reactions to functionalized SAMs on gold,44,85,89 our results suggested only a limited conversion of the single-component acetylenyl gold surface to the intended redox film, with a calculated Γ of 5.3 × 10-11 mol cm-2. In contrast, Chidsey et al.44 have reported ferrocene coverages of ∼1.3 × 10-10 mol cm-2 for the immobilization of ethynylferrocene on azide-terminated SAMs on gold (where the azideterminated species are diluted to χsln ) 0.3) and proposed quantitative yields for the click reaction. From voltammograms acquired for surface 4, the formal redox potential E1/2 was 387, slightly higher than the value of 371 mV observed for densely packed ferrocenyl assemblies on Si(100) (surface 2a). However, it is comparable, within the experimental uncertainty, to that found for silicon-based electrodes characterized by very modest ferrocene coverages (surface 2d). Attempts to calculate rates of electron transfer for the goldbased assemblies (surface 4) by means of cyclic voltammetry suggested a kapp value higher than 1000 s-1, according to the very limited effect on the observed positions of Ea and Ec with progressively increased scan rates (Figure S10).89 This is not the first time that gold has been shown to exhibit significantly faster electron-transfer rates than nonmetal substrates. It has been shown that glassy carbon produced an electron-transfer rate of 17 ( 10 s-1 whereas the same construct on gold yielded 257 ( 41 s-1.57 There are two possible effects that could account for the decrease in the rate constant in silicon. The first is the plausible influence of the electrode material on the electronic coupling, with changes in the substrate influencing the extent of wave function mixing between grafted organic molecules and the substrates.57 The orbitals of the alkyl chain may mix to a greater degree with gold than they do with silicon. The second possibility relates to the band structure of silicon.23 In gold, being a metal, there is a continuum of available sites for electron transfer up to the Fermi level. However, in silicon there is a band gap, and thus the number of sites available for electron transfer will not be a continuum up to the Fermi level. This means that the number of sites available will depend on the applied energy and will be related to both the instantaneous and maximum rate constants. 3.3. Redox Assemblies Prepared from Multiple-Component Acetylenyl Si(100) Surfaces. Given the very high density of redox sites for surface 2a (section 3.2.1),53 the number of sites to which the ferrocene could attach was reduced to determine if even more ideal electrochemistry could be observed as expected for independent and isolated sites.86 SAMs composed of a mixture of precursors 1 and 2 were prepared with progressive dilutions of diyne 1 (surfaces 1b-d, section 2.3.2). Figure 4b-d shows representative cyclic voltammograms (CVs) for the four different ferrocene-modified surfaces (surface 2b-d). The relevant electrochemical data are summarized in Table 1. (89) Devaraj, N. K.; Decreau, R. A.; Ebina, W.; Collman, J. P.; Chidsey, C. E. D. J. Phys. Chem. B 2006, 110, 15955–15962.

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There are a number of trends in this data that warrant further discussion. First, the electrochemistry of the surface prepared from mixtures containing diyne 1 at χsln ) 0.05 was vastly inferior with much broader peaks and a very large separation between the anodic and cathodic peaks. Both of these deviations from ideality indicate that the ferrocene moieties are in a range of environments. As a result, a reliable fit to the Marcus equation could not be obtained for the kinetic data, and hence no rate constant for electron transfer could be calculated. The reason for this decay in the quality of the electrochemistry will be discussed in section 3.4. We will not discuss the electrochemistry of these surfaces further. Second, there were also minor differences in the electrochemistry for the surfaces prepared from mixtures containing diyne 1 at χsln ) 1.0, 0.5, and 0.1 that warrant discussion. The dilution of the coupling points from χsln ) 1.0-0.5 surfaces made almost no difference, with very similar surface coverage and rate constants for electron transfer being derived. The estimated surface coverage approximates that of a close-packed layer.90 The similarity in coverage clearly indicates that in surface 2a (χsln)1.0) the click coupling yield of 40% is limited by the footprint of the ferrocene molecule.91 Under the assumption of comparable reactivities between acetylene species 1 and 2 at the silanic sites during the hydrosilylation step, the measured ferrocene coverage on surface 2b (χsln ) 0.5 mixture) would thus suggest an ∼80% yield for the click step.92 In contrast, the surface coverage is almost an order of magnitude lower for surface 2c (χsln ) 0.1 mixture) because the distance between the surface acetylenes, assuming a homogeneous distribution of the species resulting from the coupling of alkynes 1 and 2 on the surface, is greater than the footprint of the ferrocene species. The coverage of ferrocene species determined on surface 2c suggests a close to quantitative yield of ferrocene species on the surface, which is consistent with the results of an FTIR93 and electrochemical study reported by Chidsey and co-workers suggesting a quantitative yield for clicking ethynylferrocene to an azide-terminated SAM on gold. 44 Note that in the case of surface 2c the apparent rate constant for electron transfer, kapp, decreased slightly to 2.9 ( 0.9 s-1. A lower rate of electron transfer is consistent with previous reports by Zanoni et al.64 in which interactions of Fe(III) species with deprotonated silanol termini94 (surface SiO- species, section 3.4) might account for a reduced ion-pairing capability of solution perchlorate counterions with the confined ferricinium species. Thus, it will negatively impact the electron-transfer event kinetics. Furthermore, because the growth of SiOx species is likely to have occurred because of the poorer passivation of the substrate (section 3.4),95 the inferior electrical properties observed for surface 2c may have resulted from detrimentally affected (90) From the obtained Γ value, an average individual area for the immobilized ferrocene units can be approximated to 49 and 52 Å2 (surfaces 2a and 2b, respectively), a value close to the modeled footprint of a ferrocene molecule if approximated as a sphere having a ∼34 Å2 cross section.91 (91) Seo, K.; Jeon, I. C.; Yoo, D. J. Langmuir 2004, 20, 4147–4154. (92) As discussed in section 3.1, from the analysis of XPS C 1s signals for surfaces 1a and 1e, individual contributions of carbon-, hydrogen-, and siliconbonded carbon atoms being either sp, sp2, or sp3 hybridized could not be resolved. As a direct consequence of this experimental limitation in assessing for surface 1e ratios of grafted individual alkyne 1 and 2 species, “click” yields for surfaces 2b, 2c, and 2d are approximated only by electrochemical measurements. (93) In the cited work,44 the disappearance of a strong azide absorption at 2101 cm-1 was correlated with the immobilization of ethynylferrocene on the azideterminated gold surface. (94) Hu, K.; Fan, F.-R. F.; Bard, A. J.; Hillier, A. C. J. Phys. Chem. B 1997, 101, 8298–8303. (95) An evident XPS SiOx signal at 102 eV (Figure 1b) was always seen for SAMs prepared from 1-heptyne only.

Ciampi et al.

carrier recombination rates69 as a result of increased densities of electrically active trap sites.96 3.4. Assembly of Single-Component 1-Heptyne Monolayers on Hydrogen-Terminated Si(100) Surfaces. The reaction of 1-alkynes at hydrogen-terminated Si(100) surfaces has been studied by several authors and is generally believed to yield interfaces with small but detectable levels of SiOx.32,35,97 Prompted by the unexpected degradation of the redox behavior that we observed for surface 2d, where 1-heptyne 2 was used to separate distal acetylene moieties of grafted 1,8-nonadiyne 1 molecules, we prepared passivated silicon surfaces via the hydrosilylation of 2 only (surface 1e, section 2.3.3). An advancing water contact angle of 106 ( 5° was observed for this surface, which compares well with other methyl-terminated surfaces prepared by the thermal reaction of 1-alkenes on hydrogen-terminated Si(100) surfaces.75 To characterize surface 1e further, XPS spectra were obtained. Figure S13a (Supporting Information) shows a survey spectrum with only Si, C, and O species evident. A narrow scan of the C 1s region (Figure S13b, Supporting Information) showed an emission peak at ∼284.9 eV similar to that generally observed for monolayers prepared from 1, with the only difference being a comparatively small dispersion value (1.43 eV vs 1.59 eV for surface 1a, fwhm).98 The quality of 1-heptyne 2 monolayer was judged to be unsatisfactory with regard to its ability to prevent or limit the formation of oxide because a significant fraction of surface silicon (Figure 1b) was found to be in an oxidized state with the XPS SiOx/Si 2p peak area ratio found to be ∼0.07 (equivalent to ∼0.8 monolayer of SiOx).99 The question that arises from the observation of a significant increase in the number of SiOx species with 1-heptyne 2 compared to the number with 1,8-nonadiyne 1 is whether this can be attributed to either the lower monolayer formation temperature or the different distal moieties. As reported in section 2.3.3, the temperature of the reaction chamber during the hydrosilylation of 1-heptyne 2 onto the hydrogen-terminated Si(100) substrate was kept at a temperature of 95 °C, which is significantly lower than the 170 °C used for the corresponding grafting procedure of 1,8-nonadiyne 1. The lower reaction temperature was required by the relatively low boiling point of 1-alkyne species 2 (99-100 °C for 2 versus 171-172 °C for species 1, both values were measured at 760 Torr). To shed some light on this question, thermal grafting of diyne compound 1 was also performed at 95 °C, under identical experimental conditions to those described for surface 1e. Figure 1c shows the XPS Si 2p spectra for surface 1a formed at 95 °C (section 2.3.1). The level of silicon oxide species, as determined from the peak between 102 and 104 eV, was significantly lower than with 1-heptyne 2 (SiOx/Si 2p peak area ratio found to be ∼0.01 and thus equivalent to ∼0.1 monolayer of SiOx69,70) but greater than that described above for SAMs prepared from the same diyne species (1) at 170 °C.100 These results suggest that the presence of a distal acetylene is exceedingly important for achieving the excellent-quality, oxidefree monolayers that we obtain on Si(100) surfaces with diyne (96) Royea, W. J.; Juang, A.; Lewis, N. S. Appl. Phys. Lett. 2000, 77, 1988– 1990. (97) Cerofolini, G. F.; Galati, C.; Reina, S.; Renna, L. Surf. Interface Anal. 2006, 38, 126–138. (98) The lack of distal acetylene moieties in surface 1e and the corresponding photoemissions might partially account for the reduced fwhm when compared to acetylenyl surface 1a. (99) The SiOx/Si 2p ratio was used to compute the monolayer equivalents of SiO2 present on the surface according to the method described by Webb and co-workers.69,70 The surface density of Si atoms was set to 6.78 × 1014 atoms cm-2 for Si(100), and the escape depth was equal to 1.6 nm. (100) No appreciable differences were seen between monolayers of different quality by means of water contact angle measurements.

Si Electrodes for Aqueous EnVironments.

1 but that higher temperatures of formation also assist in achieving good-quality surfaces.

4. Conclusions Here we show a route to producing organic-monolayermodified silicon surfaces that enable stable, robust, and nearly ideal surface electrochemistry to be performed in aqueous media. The excellent electrochemistry is achieved by modifying a Si(100) surface with a dialkyne species, 1,8-nonadiyne 1. Clicking azidomethylferrocene 3 to the surface by copper(I)-catalyzed alkyne-azide cycloaddition reactions afforded electrochemically active surface-bound [1,2,3]-triazole species with nearly ideal surface electrochemistry. The surface showed no evidence of silicon oxide species (below an XPS detection limit of approximately 0.07 SiOx monolayers) even after prolonged cycling between -100 and 800 mV (Vs. Ag|AgCl) in aqueous electrolytes. We conclude that π binding between the distal alkynes provides a surface that is resistant to silicon oxidation. Evidence for this conclusion comes from a deviation from ideality of the surface electrochemistry as the dialkyne is diluted with 1-heptyne such

Langmuir, Vol. 25, No. 4, 2009 2539

that the distal alkynes are diluted. This approach opens the door to the application of modified silicon (100) electrodes in a wide variety of solvents and is hence applicable to bioapplications and photovoltaics and provides a route to oxide-free silicon surfaces for other areas such as molecular electronics and nonelectrochemical applications. Acknowledgment. This research was supported by the Australian Research Council’s Discovery Projects funding scheme (project number DP0772356). S.C. was supported by an International Postgraduate Research Scholarship from the Australian Government and by a Research Postgraduate Award from the Australian Institute of Nuclear Science and Engineering (AINSE). Supporting Information Available: Additional XPS spectrographs, X-ray reflectivity data, cyclic voltammograms, and synthesis schemes. This material is available free of charge via the Internet at http://pubs.acs.org. LA803710D