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Surface Characterization and Electrochemical Properties of Alkyl, Fluorinated Alkyl, and Alkoxy Monolayers on Silicon Carl J. Barrelet,† David B. Robinson,† Jun Cheng,† Thomas P. Hunt,‡ Calvin F. Quate,‡ and Christopher E. D. Chidsey*,† Department of Chemistry and E. L. Ginzton Laboratory, Stanford University, Stanford, California 94305 Received March 5, 2001 Alkyl and fluorinated alkyl monolayers covalently bonded to the silicon (111) surface have been prepared by UV illumination of the H-Si(111) surface while immersed in a solution of olefin precursor under high vacuum. An alkoxy monolayer covalently bonded to the silicon (111) surface is formed by the reaction of the H-Si(111) surface with a heated solution of primary alcohol precursors under high vacuum. Ellipsometry, X-ray photoelectron spectroscopy, and atomic force microscopy are used to characterize the three monolayers. The alkyl monolayer is measured to have the largest number of organic adsorbates per surface silicon atom. Properties of the monolayers on the silicon surface are probed by cyclic voltammetry. The hydrophobic alkyl monolayer slows the oxidation of the silicon surface by water. The three monolayers also slow the rate of electron transfer across the silicon-electrolyte interface by acting as a tunneling barrier. The alkyl monolayer in tetrahydrofuran exhibits a large and reproducible blocking of the electron transfer.
Introduction Organic monolayers offer a powerful way to modify the chemical and physical properties at silicon surfaces. Kinetically stable covalent bonds can be formed between the silicon surface and organic species in the monolayer.1-12 The nature of the organic species and their packing in the monolayer should determine many of the properties at the surface. A hydrophobic organic monolayer could slow the corrosion of the silicon surface. By analogy with alkylthiols absorbed on gold,13-19 a densely packed mono* To whom correspondence should be addressed. E-mail: chidsey@ stanford.edu. † Department of Chemistry. ‡ E. L. Ginzton Laboratory. (1) Linford, M. R.; Chidsey, C. E. D. J. Am. Chem. Soc. 1993, 115, 12631. (2) Haran, A.; Waldeck, D. H.; Naaman, R.; Moons, E.; Cahen, D. Science 1994, 263, 948. (3) Cleland, G.; Horrocks, B. R.; Houlton, A. J. Chem. Soc., Faraday Trans. 1995, 91, 4001. (4) Linford, M. R.; Fenter, P.; Eisenberger, P. M.; Chidsey, C. E. D. J. Am. Chem. Soc. 1995, 117, 3145. (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) deVilleneuve, C. H.; Pinson, J.; Bernard, M. C.; Allongue, P. J. Phys. Chem. B 1997, 101, 2415. (7) Laibinis, P. E.; Kim, N. Y. Y. Abstr. Pap.sAm. Chem. Soc. 1997, 214, 26. (8) Lewis, N. S.; Bansal, A.; Li, X.; Lieberman, M.; Yi, S. I.; Weinberg, W. H. Abstr. Pap.sAm. Chem. Soc. 1997, 214, 161. (9) Boukherroub, R.; Morin, S.; Bensebaa, F.; Wayner, D. D. M. Langmuir 1999, 15, 3831. (10) Bergerson, W. F.; Mulder, J. A.; Hsung, R. P.; Zhu, X. Y. J. Am. Chem. Soc. 1999, 121, 454. (11) Buriak, J. M. Chem. Commun. 1999, 1051. (12) Cicero, R. L.; Linford, M. R.; Chidsey, C. E. D. Langmuir 2000, 16, 5688. (13) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559. (14) Chidsey, C. E. D. Science 1991, 251, 919. (15) Terrettaz, S.; Becka, A. M.; Traub, M. J.; Fettinger, J. C.; Miller, C. J. J. Phys. Chem. 1995, 99, 11216. (16) Becka, A. M.; Miller, C. J. J. Phys. Chem. 1992, 96, 2657. (17) Finklea, H. O.; Hanshew, D. D. J. Am. Chem. Soc. 1992, 114, 3173. (18) Cheng, J.; Saghiszabo, G.; Tossell, J. A.; Miller, C. J. J. Am. Chem. Soc. 1996, 118, 680.
layer could also act as a barrier to electron tunneling between the silicon surface and another phase. By more fully understanding the reactivity of the surface with organic species and the electronic properties of the silicon surface, we hope to develop a model monolayer system for the study of electron transfer at the semiconductor-liquid interface. In this spirit, we have characterized three representative organic monolayers on silicon surfaces and compared their properties. The hydrogen-terminated silicon (111) surface is an ideal substrate on which to form monolayers because the chemical functionality at the surface is homogeneous and the surface has large atomically flat terraces.20,21 We have treated H-Si(111) with three different monolayer precursors to form an alkyl, a fluorinated alkyl, and an alkoxy monolayer. The three monolayers are characterized by ellipsometry, X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM). The thickness of the monolayers, the surface elemental composition, and the topography of the surface are reported for each monolayer. Cyclic voltammetry is used to probe the properties of the monolayer. The alkyl monolayer in water is observed to slow the corrosion of the silicon surface. Each of the three monolayers also slows the rate of electron transfer to electron acceptors in methanol, acetonitrile, and tetrahydrofuran. The best blocking is observed with the alkyl monolayer in tetrahydrofuran. Though none of these monolayers meets all the criteria one would like for a model system for the study of long-distance electron transfer at the semiconductor-electrolyte interface, they do give insight into the features required of such a model and suggest future strategies for attaining it. Experimental Section Silicon (111) wafers (Silicon Quest International, n-type, 1-4 Ω cm) were used in all of the experiments. For the AFM imaging, (19) Weber, K.; Hockett, L.; Creager, S. J. Phys. Chem. B 1997, 101, 8286. (20) Higashi, G. S.; Chabal, Y. J.; Trucks, G. W.; Raghavachari, K. Appl. Phys. Lett. 1990, 56, 656. (21) Wade, C. P.; Chidsey, C. E. D. Appl. Phys. Lett. 1998, 72, 133.
10.1021/la010333p CCC: $20.00 © 2001 American Chemical Society Published on Web 05/05/2001
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Table 1. Ellipsometric Thickness reacted with H-Si(111) n
CH3(CH2)n-3CHdCH2 (nf ) 1.45) (Å)
5 6 7 8 10
6 7 8 9 11.5
CF3(CF2)n-3CHdCH2 (nf ) 1.33) (Å)
reacted with Au(111) (ref 25) CH3(CH2)n-2CH2OH (nf ) 1.45) (Å)
CH3(CH2)n-1SH (Å)
CF3(CF2)n-3(CH2)2SH (Å)
15.6
17.0
8 11 14
the wafer had a 0° ( 0.25° miscut. Chemicals for the cleaning and etching of the silicon surface were of the highest available commercial grade from Kanto Chemical Company. The HSi(111) surface was prepared following the same procedure for each of the three monolayers. The silicon (111) is first oxidized in 3:1 (v/v) H2SO4:30% (by weight) aqueous H2O2 for 10 min at 100 °C and rinsed with ultrapure (18 MΩ cm) water from a fourbowl Millipore purifier. It is then etched by immersion in a 40% aqueous NH4F solution for 15 min.20 This solution is slightly basic with a pH of about 8. The etching takes place in a quartz cuvette. Before etching, the NH4F solution is sparged with argon for 15 min. The removal of O2 ensures the formation of atomically flat, uniformly terraced H-Si(111) without etch-pits.21 The H-Si(111) surface is hydrophobic and thus emerges dry from the aqueous NH4F into air. The surface is not rinsed with any solvent to avoid oxidation or organic contamination of the surface. The 1-octene (Fluka, g99.8%) is dried by passage through a column of activated neutral alumina. The solution is degassed by three freeze, pump, and thaw cycles. To ensure further removal of water and oxygen, the 1-octene is vacuum transferred onto freshly prepared sodium-potassium (NaK) alloy and stored under argon in a sealed flask. 1H,1H,2H-Perfluoro-1-octene (Oakwood) is first dried by passage through a column of activated neutral alumina and then degassed by three freeze, pump, and thaw cycles. Because NaK alloy cannot be used with 1H,1H,2H-perfluoro-1-octene, dispersed copper (Engelhard CU-0226S, formerly known as Q-5 reactant) was used to remove trace O2. Dispersed copper is activated before use at 100 °C under hydrogen gas for 2 h and stored under argon in a storage flask.22 Anhydrous 1-octanol (>99%) was obtained from Aldrich packaged under nitrogen in Sure/Seal bottles containing less than 0.005% oxygen and water. No further pretreatment was performed on the 1-octanol. 1-Octene and 1H,1H,2H-perfluoro-1-octene precursors are both photoreacted with the H-Si(111) surface using the same procedure.12 A vacuum system, composed of a quartz cuvette and a Schlenk line connected to a liquid-nitrogen-trapped diffusion pump, minimizes oxidation of the H-Si(111) surface by trace O2 and H2O. Prior to each monolayer synthesis, a forerun is distilled from the olefin storage flask. The H-Si(111) sample emersed from the aqueous NH4F solution is directly inserted into the quartz cuvette. The cuvette is twice evacuated to 2 × 10-5 Torr and backfilled with argon. Then, it is evacuated to 5 × 10-6 Torr, at which point the quartz cuvette is isolated from the pump and cooled in a dry ice/acetone bath. The olefin is then vacuum transferred onto the H-Si(111) surface. Once the H-Si(111) wafer is immersed in olefin, the cuvette is allowed to warm and is backfilled with argon. The H-Si(111) surface is then illuminated with a 254-nm pen lamp placed 1.5 cm from the surface while blowing air across the cuvette to prevent heating. The sample is illuminated for at least 2 h for maximum monolayer coverage. The quartz cuvette is opened to air, and the olefin is decanted. The silicon surface is rinsed and then sonicated twice in CH2Cl2 for 5 min. The alkoxy monolayer preparation requires a similar vacuum apparatus to avoid the presence of O2 and H2O. A freshly prepared H-Si(111) surface sample is inserted into a glass cuvette immediately after being emersed from the NH4F solution. The cuvette containing the H-Si(111) is twice evacuated to approximately 5 × 10-5 Torr and backfilled with argon. The 1-octanol is cannula transferred under argon directly from the (22) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.; Timmers, F. J. Organometallics 1996, 15, 1518.
10
Sure/Seal bottle to fully cover the silicon surface. The cuvette is isolated under an argon atmosphere and heated to 80 °C for 30 min. The cuvette is then opened to air, the 1-octanol is decanted, and the coated silicon surface is rinsed and twice sonicated for 5 min with CH2Cl2. Ellipsometry and XPS were performed as previously described.4 The XPS takeoff angle was 35° from the surface. The AFM image was obtained using the Digital Instruments 3000 AFM in tapping mode with a silicon cantilever at a resonance frequency of 360 kHz.23 All the AFM imaging experiments were performed in air with an ambient humidity of about 35%. The electrochemical characterizations were performed in a deoxygenated glass electrochemical cell sealed to the silicon by a Chemraz (ACE Glass) perfluoroelastomer O-ring with an inside area of 0.58 cm2. The silicon surface was used as a working electrode. An ohmic contact was obtained using a layer of Ga/In eutectic (Alfa) painted on the back of the silicon sample which is then pressed onto a copper plate. The counter and reference electrodes were platinum. A PAR 273 potentiostat was employed for cyclic voltammetry. Cyclic voltammetry was performed in four solvents: water, tetrahydrofuran, methanol, and acetonitrile. Cyclic voltammetry was conducted in the dark, under an argon atmosphere at ambient temperature for the nonaqueous electrolytes. For the aqueous electrochemistry, a freshly prepared solution of 3 mM potassium ferricyanide, 3 mM potassium ferrocyanide, and 1 M potassium chloride was used. For the nonaqueous electrochemistry, stock solutions of 1 mM decamethylferricenium hexafluorophosphate,24 1 mM decamethylferrocene, and 1 M lithium perchlorate in each solvent were prepared. These solutes were dried overnight at 110 °C before use. The solvents (Aldrich Sure/Seal) were cannula transferred into the storage flask containing the salts. The solutions were subject to three freeze, pump, and thaw cycles before being stored under argon. For each measurement, a few milliliters of the solution was transferred from the storage flask into the deoxygenated electrochemical cell under argon.
Results To provide a sense for the nature of the films, Table 1 reports the effective film thicknesses as measured by optical ellipsometry assuming a refractive index of 1.45 for the hydrogenated monolayers and 1.33 for the fluorinated monolayers.25 For each extra CH2 or CF2 in the adsorbate chain, the film thickness increases accordingly. From these measurements, we conclude that each film is a monolayer of adsorbate chains oriented away from the surface. As shown in the table, the measured thickness of each film on silicon is somewhat smaller than the previously observed thickness for the same chain length alkylthiol adsorbates on gold (111). At least some of this difference must be due to the absence of sulfur in adsorbates on silicon. The rest may be due to somewhat lower areal densities of the adsorbates on silicon compared with those on gold. (23) Binning, G.; Quate, C. F.; Gerber, C. Phys. Rev. Lett. 1986, 56, 930. (24) Nesmeyanov, A. N.; Materikova, R. B.; Lyatifov, I. R.; Kurbanov, T. K.; Kochetkova, N. S. J. Organomet. Chem. 1978, 145, 241. (25) Chidsey, C. E. D.; Loiacono, D. N. Langmuir 1990, 6, 682.
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Figure 1. X-ray photoelectron survey spectra of (a) H-Si(111) after illumination with 254 nm light for 2 h while immersed in dry deoxygenated 1-octene, (b) H-Si(111) after illumination with 254 nm light for 2 h while immersed in dry deoxygenated 1H,1H,2H-perfluoro-1-octene, and (c) H-Si(111) after immersion in 1-octanol heated to 80 °C for 30 min.
XPS survey scans in Figure 1 identify the elemental composition of each surface. A strong carbon (1s) peak indicates the presence of an organic compound on the silicon surface for all three surface treatments. For the 1H,1H,2H-perfluoro-1-octene-treated surface (Figure 1b), the strong fluorine (1s) peak confirms the presence of a fluorocarbon. For the 1-octanol-treated surface (Figure 1c), the strong oxygen (1s) peak reflects the reaction of the silicon surface with an alcohol precursor. Some oxygen is also observed for the other two surfaces, presumably because of reaction with trace O2 or H2O during the reaction or with atmospheric O2 or H2O after the reaction. Figure 2 displays the XPS carbon (1s) narrow scans. The most obvious feature is the difference between the fluorinated monolayer (Figure 2b) and the hydrogenated monolayers (Figure 2a,c). For the fluorinated monolayer, three distinct carbon (1s) peaks are assigned respectively to CH2, CF2, and CF3. In contrast, all carbons in the hydrogenated monolayers have similar enough chemical environments that, for the spectrometer settings used, a single carbon (1s) feature results. To compare quantitatively the amount of carbon, oxygen, and silicon, we used the integrated peak areas of the carbon (1s), oxygen (1s), and silicon (2p) XPS narrow scans and accounted for the differences in atomic sensitivity, the depth distribution of each element, and the XPS takeoff angle; the oxygen was assumed to be at the organic/silicon interface. We used a calculation that has been previously used for thin organic films on silicon.12 Table 2 reports the ratio of organic adsorbates per surface silicon atom. The 1-octene-treated surface has a larger
Barrelet et al.
Figure 2. Carbon (1s) narrow XPS scan for the same surfaces as in Figure 1. Table 2. XPS Surface Elemental Quantification
CH3(CH2)5CHdCH2 CF3(CF2)5CHdCH2 CH3(CH2)6CH2OH
adsorbate/ surface silicon
adsorbate areal density/maximum areal density
oxygen/ surface silicon
0.43 0.27 0.21
0.62 0.58 0.30
0.12 0.47 1.29
coverage of adsorbates than the other two surfaces. To provide a sense of the areal density of the adsorbates, we compare with the maximum areal density these adsorbates have in bulk crystals. For the hydrogenated chains, we take the maximum areal density to be that of the chains in crystalline polyethylene (18.3 Å2)-1. For the fluorinated chains, we take the maximum areal density to be that of the chains in crystalline poly(tetrafluoroethylene) (27.2 Å2)-1.26 The areal density of surface silicon atoms is (12.7 Å2)-1. In the second column of Table 2, we report the ratio of the areal density of the adsorbate to this maximum areal density. The third column of Table 2 reports the ratio of oxygen atoms per surface silicon atom. A significant amount of oxygen per surface silicon atom is observed for each surface treatment. These impurities reflect the limitation in the current state of the art for these monolayer preparations. In particular, the ratio of oxygen atoms per surface silicon atom on the 1-octanol-treated surface is much larger than expected. Figure 3 is a tapping-mode AFM image of the 1-octanoltreated H-Si(111). The surface is composed of atomically flat terraces separated by step heights of 0.3 nm. Similar images have already been observed with the 1-octene(26) Wunderlich, B. Macromolecular Physics; Academic Press: New York, 1973; Vol. 1.
Alkyl, Fluorinated Alkyl, and Alkoxy Monolayers
Figure 3. Tapping-mode AFM images of 1-octanol-treated H-Si(111).
Figure 4. Three successive cyclic voltammograms of (a) the H-Si(111) electrode and (b) the 1-octene-treated H-Si(111) electrode in 3 mM K3Fe(CN)6/3 mM K4Fe(CN)6/1 M KCl/H2O (scan rate, 100 mV/s).
treated H-Si(111).27 In both cases, the functionalized silicon surface retains the nanotopography of the original bare H-Si(111) before the reaction.21 To gain an understanding of the reactivity of these silicon surfaces in electrochemical settings, we start with some measurements in an aqueous electrolyte. Figure 4 shows three successive cyclic voltammograms of the bare H-Si(111) surface and of a 1-octene-treated H-Si(111) surface in water with 3 mM potassium ferricyanide, 3 mM potassium ferrocyanide, and 1 M potassium chloride. A platinum electrode in this aqueous ferri/ferrocyanide (27) Wagner, P.; Nock, S.; Spudich, J. A.; Volkmuth, W. D.; Chu, S.; Cicero, R. L.; Wade, C. P.; Linford, M. R.; Chidsey, C. E. D. J. Struct. Biol. 1997, 119, 189.
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solution serves as the reference electrode. The cyclic voltammograms for H-Si(111) show a diffusion-limited peak which reaches its maximum current at an overpotential of about -0.7 V (Figure 4a). Diffusion-limited currents at such large overpotentials are typical of redox electrochemistry at semiconducting electrodes. The cyclic voltammograms for the 1-octene-treated H-Si(111) surface (Figure 4b) show significant blocking of the reduction current. It is only at a much greater overpotential (outside of the potential range shown in Figure 4) that a diffusionlimited peak of comparable amplitude to that of the bare H-Si(111) occurs. For the bare H-Si(111) surface in the aqueous electrolyte, the current decreases between successive cyclic voltammograms (Figure 4a). The decrease in the reduction current over time is assigned to formation of SiO2 at the silicon surface because of corrosion. For the 1-octenetreated surface in water, the cyclic voltammogram changes much less over the same time period (Figure 4b). The monolayer appears to retard significantly the formation of SiO2 at the silicon surface. All further experiments are done in dry deoxygenated methanol, acetonitrile, or tetrahydrofuran solutions. The redox species dissolved in these electrolyte solutions are 1 mM decamethylferrocenium hexafluorophosphate and 1 mM decamethylferrocene with 0.1 M lithium perchlorate. The solvents have been previously deoxygenated to reduce oxidation of the silicon surface. The reference electrode is a bare platinum electrode in contact with each of these decamethylferrocene/decamethylferrocenium solutions. To avoid diffusion limitations, the range of the potential sweep is limited such that the currents during the scans to negative and to positive potentials are essentially identical; only the scans to negative potentials are plotted in subsequent figures. To accommodate the large range of currents observed, the absolute value of current is plotted on a logarithmic scale. Figure 5 shows reproducible current versus potential curves of a bare H-Si(111) surface in the three different solvents (dotted lines). In methanol, the overpotential required to achieve zero current is approximately -190 mV, which is similar to the initial open-circuit potential on contact of the electrode with the electrolyte. The fact that the open-circuit potential occurs at a significantly negative overpotential suggests that a corrosion reaction occurs when a bare H-Si(111) electrode is in contact with methanol; the oxidation of the silicon surface appears to cause electrons to accumulate on the electrode and shift the potential of the silicon electrode with respect to the platinum reference electrode in the redox electrolyte. In contrast, the overpotentials at zero current and the initial open-circuit potentials for acetonitrile and tetrahydrofuran are very close to 0 V. Figure 5 also shows current versus potential curves of silicon electrodes with an alkyl monolayer in the three different solvents (solid lines). Current versus potential curves are reproducible in tetrahydrofuran, whereas in methanol and acetonitrile, currents half an order of magnitude higher and lower have been observed on sequential runs with different monolayer preparations. Current magnitudes as a function of potential are significantly different for each solvent. For bare HSi(111) surfaces, the current density is lower in methanol than in tetrahydrofuran or acetonitrile. For silicon electrodes with an alkyl monolayer, the current density is lower in tetrahydrofuran than in methanol or acetonitrile. Figure 6 shows the current versus potential curves of three silicon electrodes in tetrahydrofuran, each with a different surface treatment. Tetrahydrofuran was chosen because
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where the current is approximately 2 orders of magnitude smaller than the bare H-Si(111). The current for a 1-octanol-treated surface is decreased by approximately 1 order of magnitude from that of bare H-Si(111). The 1H,1H,2H-perfluoro-1-octene-treated surface and the 1-octanol-treated surface have less reproducible current versus potential curves with the measured currents varying within half an order of magnitude between samples. Discussion
Figure 5. Semilog plots of current versus overpotential of the H-Si(111) electrode (dotted line) and the 1-octene-treated H-Si(111) electrode (solid line) in (a) methanol, (b) acetonitrile, and (c) tetrahydrofuran electrolyte solutions composed of 1 mM Me10FcPF6/1 mM Me10Fc/1 M LiClO4 (scan rate, 10 mV/s).
Figure 6. Semilog plots of current versus overpotential of (a) the 1-octene-treated H-Si(111) electrode, (b) the 1H,1H,2Hperfluoro-1-octene-treated H-Si(111) electrode, and (c) the 1-octanol-treated H-Si(111) electrode in 1 mM Me10FcPF6/1 mM Me10Fc/1 M LiClO4 in tetrahydrofuran (scan rate, 10 mV/ s). The dotted curves are semilog plots of current versus overpotential for the untreated H-Si(111) electrode.
it gives the most reproducible electrochemical measurement. The 1-octene-treated surface gives reproducible current versus potential curves where the current at -0.1 V is more than 3 orders of magnitude smaller than for H-Si(111). The 1H,1H,2H-perfluoro-1-octene-treated surface gives a characteristic current versus potential curve
We start by considering the differences between the hydrogenated and the fluorinated alkyl monolayers. 1-Olefins are thought to react by a radical-chain mechanism initiated by the photodissociation of the surface H-Si by UV light.12 The alkyl monolayer has a significantly larger number of adsorbates per surface silicon atom than the fluorinated alkyl monolayer (Table 2, column 1). Perhaps surprisingly, Table 1 shows that the ellipsometric thicknesses are larger for the fluorinated than for the hydrogenated chains and the adsorbate per surface silicon atom is lower for the fluorinated than for the hydrogenated chains. This discrepancy can be explained by the different cross-sectional areas of hydrogenated and fluorinated alkyl chains as emphasized by the similarities of the ratios of the adsorbate areal density to their maximum areal density (Table 2, column 2). On the other hand, this ratio of areal densities is in fact distinct for the alkoxy monolayer compared with the alkyl and fluorinated alkyl monolayers, presumably because the alkoxy monolayers form by a different mechanism. We suspect that primary alcohols react with the HSi(111) surface by a nucleophilic substitution of hydrogen. Despite the difference in formation mechanism, the fact that the terraced nanotopography typical of the bare HSi(111) surface and of the olefin-modified surface is maintained (Figure 3) tells us that the reaction is confined to the topmost layers of the silicon crystal. We also see that the alkoxy monolayer has the smallest coverage of all three (Table 2). A lower monolayer coverage would involve a greater tilt angle and the presence of a larger free volume per adsorbate, resulting in a decreased ellipsometric thickness. However, the measured ellipsometric thickness is similar to those of the alkyl monolayer even though the alkoxy coverage is about half that of the alkyl. For the alkoxy monolayer, one oxygen is expected to accompany each adsorbate. This leaves a coverage of 1.08 oxygen per surface silicon atom (Table 2) that presumably arises from reaction with O2 or H2O. We suggest that this oxygen may have migrated into approximately a third of the topmost Si-Si bonds, converting them to Si-O-Si bonds. No pretreatment other than the use of Sure/Seal is done to deoxygenate the 1-octanol. Deliberate deoxygenation of alcohols immediately prior to use to form monolayers might decrease the extra oxygen coverage. Organic monolayers on silicon allow us to learn about the electron transfer across the semiconductor-electrolyte interface. Figure 4 illustrates the role of the monolayer as a tunneling barrier to electron transfer to a redox species in an aqueous solution. For the bare H-Si surface (111), the electron transfer is fast and diffusion becomes limiting at moderated overpotential as shown by the diffusionlimited peak. For the alkyl-coated silicon electrode, the absence of a diffusion-limited peak shows that electron transfer is much slower. That is, the monolayer acts as a tunneling barrier that slows the electron transfer from the silicon electrode to the redox molecule in solution as illustrated schematically in Figure 7.
Alkyl, Fluorinated Alkyl, and Alkoxy Monolayers
Figure 7. Schematic figure to illustrate electron tunneling from a silicon (111) electrode across a monolayer to a decamethylferrocenium cation in solution.
Although it can be useful for qualitative electrochemical studies, water is not an ideal solvent in which to study electron transfer because water is known to corrode the silicon surface. Methanol also corrodes the silicon surface, as shown by the large overpotential at zero current in Figure 5. On the other hand, tetrahydrofuran does not corrode the silicon surface. Tetrahydrofuran is also the solvent in which the alkyl monolayer blocks electron transfer from the silicon electrode to the redox acceptor most effectively and most reproducibly (Figure 6a). The alkyl monolayer appears to hold the decamethylferrocenium (the electron acceptor) at a fixed distance from the silicon surface (the electron donor) as illustrated schematically in Figure 7. The length of the alkyl chain determines the distance between the electron donor and the electron acceptor. In a separate body of work,28 we have taken advantage of this more reproducible blocking behavior to study the distance dependence of electron transfer. By varying the number of CH2 units in the alkyl chains, a chain-length dependence of electron transfer through monolayers with chain lengths from 5 to 8 carbons was measured in tetrahydrofuran with the expected negative logarithmic slope, β, of about 1.0 per CH2. We have also examined the electrochemistry with other monolayers on silicon. For a similar chain length, the fluorinated alkyl and alkoxy monolayers are not capable of blocking the current to the same degree as the alkyl monolayer (Figure 6b,c). This result correlates with the fact that the 1-octene treatment gives a significantly higher surface coverage and a lower free volume per chain than the 1H,1H,2H-perfluoro-1-octene and the 1-octanol treatments. We postulate that the lower monolayer coverage and larger free volumes per chain for the 1H,1H,2H-perfluoro1-octene- and the 1-octanol-treated surfaces cause monolayer defects which account for the larger currents. Additionally, the variation in the current magnitudes between samples is characteristic of the presence of defects in the monolayers. These defect sites can behave as an ensemble of microelectrodes that quickly become diffusionlimited. Variation in the distribution of this ensemble may cause the observed variability in the currents.29 (28) Cheng, J.; Robinson, D. B.; Cicero, R. L.; Eberspacher, T.; Barrelet, C. J.; Chidsey, C. E. D. Langmuir, to be submitted.
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Chazalviel and others have observed methoxylation of H-Si(111) in methanol vapor and that this modification of the surface protects against oxidation.30,31 Lewis et al. have suggested that methanol passivates surface states on the silicon surface by methoxylation.32 In tetrahydrofuran and acetonitrile, surface states are probably not similarly passivated. Indeed, Figure 5 shows that the current densities on bare H-Si(111) at highly negative potentials are larger in tetrahydrofuran and acetonitrile than in methanol, as expected for semiconductor electrodes with high surface state densities. For the silicon electrode with an alkyl monolayer, current versus potential curves are reproducible in tetrahydrofuran, whereas the same silicon electrode in acetonitrile or in methanol shows less reproducible current versus potential curves. Additionally, tetrahydrofuran is the solvent which gives the lowest current density. Tetrahydrofuran solvent molecules are more bulky than acetonitrile and methanol, which could minimize permeation of the monolayer and lead to better barrier properties. Another possibility, which may additionally account for the greater reproducibility of the current densities in tetrahydrofuran, is that the density of electronic surface states is sensitive to the degree of oxidation of the silicon surface and that the least polar solvent, tetrahydrofuran, leads to the least amount of oxidation in the presence of a monolayer. These and other possibilities will be explored in future work. Conclusions Three types of monolayers (alkyl, fluorinated alkyl, and alkoxy) on silicon (111) have been characterized by ellipsometry and XPS, with the alkyl monolayer showing the highest coverage on the surface. Cyclic voltammetry shows that these monolayers can act as corrosion and electron-tunneling barriers. The blocking of electron transfer is greatest with an alkyl monolayer in tetrahydrofuran. Because it minimizes the effects of corrosion, diffusion, and oxidation, this system is likely to be valuable for the study of electron transfer at the silicon-electolyte interface. However, the properties of even the best monolayer-coated silicon surfaces reported here could be substantially improved. Methods to reduce adventitious oxidation are being explored, as well as other ways to reduce the density of defects and surface states on monolayer-coated silicon electrodes. Acknowledgment. This work was supported by the National Science Foundation through Grant CHE9412720. We also thank the Center for Materials Research at Stanford University for the use of the XPS made possible through funds from the NSF-MRSEC program. D.B.R. acknowledges a National Science Foundation graduate fellowship. LA010333P (29) Finklea, H. O.; Snider, D. A.; Fedyk, J.; Sabatani, E.; Gafni, Y.; Rubinstein, I. Langmuir 1993, 9, 3660. (30) Chazalviel, J.-N. J. Electroanal. Chem. 1987, 233, 37. (31) Bateman, J. E.; Horrocks, B. R.; Houlton, A. J. Chem. Soc., Faraday Trans. 1997, 93, 2427. (32) Lewis, N. S.; Haber, J. A. Abstr. Pap.sAm. Chem. Soc. 2000, 205, 51.
