Scanning Electrochemical Microscopy Investigations of Monolayers

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Anal. Chem. 2006, 78, 6019-6025

Scanning Electrochemical Microscopy Investigations of Monolayers Bound to p-Type Silicon Substrates Jalal Ghilane, Fanny Hauquier, Bruno Fabre, and Philippe Hapiot*

Matie` re Condense´ e et Syste` mes Electroactifs, Sciences Chimiques de Rennes, UMR CNRS 6226, Universite´ de Rennes 1, Campus de Beaulieu, F-35042 Rennes, France

p-Si type electrodes modified with different organic monolayers were investigated by reaction with radical anion and cation electrogenerated at a microelectrode operating in the configuration of a scanning electrochemical microscope. The method proves to be a convenient tool for investigating both the quality and the redox properties of the layer as previously demonstrated on metallic electrodes especially when the sample cannot be electrically connected. Approach curves recorded with the different mediators were used to investigate the electron-transfer rates across alkyl monolayers bound to p-type silicon substrates. Preliminary results indicate that the interfacial electron transfer occurs via electron tunneling through the organic layer as generally described for SAMs grafted on gold electrodes. The functionalization of silicon surfaces through the covalent attachment of organic monolayers is the subject of intense attention due to the numerous potential applications of controlled and stable organic/Si interfaces. The scope of these investigations covers large fields in molecular electronics, chemistry, and bioanalytical chemistry, going from the preparation of monolayer surface insulators to the incorporation of a chemical/biochemical functionality at interfaces for use in photovoltaic conversion or the development of new chemical/biological sensing devices.1 Among the various approaches, reactions of hydrogen-terminated silicon surfaces have been demonstrated to be one of the most versatile methods to yield robust and well-organized monolayers showing considerable chemical stability.2-8 Organic films pro* To whom correspondence should be addressed. E-mail: philippe.hapiot@ univ-rennes1.fr. (1) For reviews, see for example: (a) Buriak, J. M. Chem. Rev. 2002, 102, 1271-1308. (b) Wayner, D. D. M.; Wolkow, R. A. J. Chem. Soc., Perkin Trans. 2 2002, 23-34. (c) Bent, S. F. J. Phys. Chem. B 2002, 106, 2830-2842. (2) Cicero, R. L.; Linford, M. R.; Chidsey, C. E. D. Langmuir 2000, 16, 56885695. (3) (a) Haber, J. A.; Lewis, N. S. J. Phys. Chem. B 2002, 106, 3639-3656. (b) Bansal, A.; Li, X.; Lauermann, I.; Lewis, N. S. Yi, S. I.; Weinberg, W. H. J. Am. Chem. Soc. 1996, 118, 7225-7226. (4) (a) Boukherroub, R.; Wayner, D. D. M. J. Am. Chem. Soc. 1999, 121, 11513-11515. (b) Boukherroub, R.; Morin, S.; Bensebaa, F.; Wayner, D. D. M. Langmuir 1999, 15, 3831-3835. (c) Mitchell, S. A.; Ward, T. R.; Wayner, D. D. M.; Lopinski, G. P. J. Phys. Chem. B 2002, 106, 9873-9882. (5) (a) Sieval, A. B.; Demirel, A. L.; Nissink, J. W. M.; Linford, M. R.; van der Maas, J. H.; de Jeu, W. H.; Zuilhof, H.; Sudholter, E. J. R. Langmuir 1998, 14, 1759-1768. (b) Sieval, A. B.; Linke, R.; Heij, G.; Meijer, G. Zuilhof, H.; Sudho ¨fer, E. J. R. Langmuir 2001, 17, 7554-7559. 10.1021/ac060058h CCC: $33.50 Published on Web 07/25/2006

© 2006 American Chemical Society

duced by this route can simply be terminated by unreactive methyl groups2,3b,4b,5a or by more reactive terminal functions (such as acids or esters,4a,c,5a amines,4a,5b,6 alcohols,4a,5a aromatic rings,7 and redox centers3a,8) that could be subsequently used for the attachment of complex organic or bioorganic structures. In that connection, the characterization of transport phenomena at the interface (diffusion, penetration of organic molecules in the layer, electrontransfer exchange at the interface) are key steps for understanding the processes occurring at the interface. Transient electrochemical methods such as cyclic voltammetry are possible approaches that were previously used for characterizing Si substrates modified by alkyl layers.9 However, these methods have some limitations: the sample must be electrically connected, which is not always possible, it should not be too chemically sensitive because the electrode is polarized (this may be a problem in oxidation), and the techniques should provide only a global view of the redox properties of the entire electrode. Among possible methods, scanning electrochemical microscopy (SECM) appears as a promising tool for analysis of silicon surface modifications. SECM is an in situ scanning probe microscopy (SPM) that has been used for the quantitative investigation of a wide range of processes occurring at interfaces.10 SECM can be used when the sample is not electrically connected and as a major advantage provides a localized examination of the redox properties of the surface down to submicrometer scale.10,11 More specifically for our study, there are only a few publications where electron transfers at semiconductor electrodes have been examined in the dark12 or under illumination by SECM.13 For what concerns the functionalized (6) Hart, B. R.; Letant, S. E.; Kane, S. R., Hadi, M. Z.; Shields, S. J.; Reynolds, J. G. Chem. Commun. 2003, 322-323. (7) Henry de Villeneuve, C.; Pinson, J.; Bernard, M. C.; Allongue, P. J. Phys. Chem. B 1997, 101, 2415-2420. (8) 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. (9) (a) Bansal, A.; Lewis, N. S. J. Phys. Chem. B 1998, 102, 1067-1070. (b) Yu H.-Z.; Boukherroub, R.; Morin S.; Wayner, D. D. M. Electrochem. Commun. 2000, 2, 562-566. (c) Cheng, J.; Robinson, D. B.; Cicero R. L.; Eberspacher, T.; Barrelet, C. J.; Chidsey, C. E. D. J. Phys. Chem. B 2001, 105, 1090010904. (d) Fabre, B.; Hauquier, F. J. Phys. Chem. B 2006, 110, 6848-6855. (10) Scanning Electrochemical Microscopy; Bard, A. J., Mirkin, M. V., Eds.; Marcel Dekker: New York, 2001. (11) Bard, A. J.; Fan, F.-R. F.; Pierce, D. T.; Unwin, P. R.; Wipf, D. O.; Zhou, F. Science 1991, 254, 68-74. (12) (a) Mandler D.; Bard, A. J. Langmuir 1990, 6, 1489-1494. (b) Horrocks B. R.; Mirkins, M. V.; Bard, A. J. J. Phys. Chem. 1994, 98, 9106-9114. (13) Haram, S. K.; Bard, A. J. J. Phys. Chem. B 2001, 105, 8192-8195.

Analytical Chemistry, Vol. 78, No. 17, September 1, 2006 6019

Scheme 1. Principle of the Feedback Mode on a Nonconnected p-Si Electrode As Used in This Study

surfaces, the method has also been successfully used for studying the electron transfer through/at self-assembled monolayers but on a polarized metallic electrode.14 As other relevant examples, SECM was also used to study the adsorption kinetics of nalkanethiols on gold,15 the menadione permeability through SAMs,16 or the rates of electron-transfer reaction through molecular monolayer on metallic electrodes.14 In this paper, we focused on the possibility of the method for studying redox processes on p-Si electrodes derivatized with different alkyl layers. The principle of the SECM is based on the electrochemical interactions of a redox species produced at a probe electrode (tip) and the substrate under investigation.10,11,17 The basics of the method in the conditions of this study are shown in Scheme 1. The most common probe electrode is a planar disk-shaped microelectrode with characteristic dimension in the 0.1-10-µm radius, a. In the simplest mode used here (feedback mode), the substrate electrode (the modified p-Si electrode) is not connected (unbiased configuration) and the probe electrode is a metallic (gold or platinum) disk ultramicroelectrode (UME). For simplicity in the interpretation of the experimental data, the redox couple must be chemically reversible (i.e., oxidized and redox forms are stable) and its electron-transfer kinetics should be intrinsically rapid. The oxidized species (Ox) is electrogenerated at the probe electrode from a solution containing only the reduced species of a reversible redox couple (Red). After diffusion of the oxidized species to the sample, an electrochemical reaction is possible on a localized spot on the surface (the diffusion cone of the UME) where the reduced form of the mediator can be regenerated, resulting in an enhancement of the current at the probe electrode. The process is then analyzed by recording the approach curves; i.e., the normalized current It ) it/iinf versus the normalized distance L) d/a, it is the current at the tip electrode localized at a distance d from the substrate, iinf is the steady-state current when the tip is at an infinite distance from the substrate; iinf ) 4nFDCa, with n the number of electrons transferred per species, F the Faraday constant, D and C the diffusion coefficient and the initial (14) Liu, B.; Bard, A. J.; Mirkin, M. V.; Creager, S. E. J. Am. Chem. Soc. 2004, 126, 1485-1492. (15) Forouzan, F.; Bard, A. J.; Mirkin, M. V. Isr. J. Chem. 1997, 37, 155-163. (16) Cannes, C.; Kanoufi, F.; Bard, A. J. Langmuir 2002, 18, 8134-8141. (17) (a) Fan, F.-R. F.; Bard, A. J. Science 1995, 267, 871. (b) Fan, F.-R. F.; Kwak, J.; Bard, A. J. J. Am. Chem. Soc. 1996, 118, 9669-9675.

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concentration of the mediator, respectively, and a the radius of the disk ultramicroelectrode. The shape of the approach curves provides data about the processes occurring at the surface generally under the form of an apparent electron-transfer rate constant kel.10 The situation is somewhat more complicated for a monolayermodified Si electrode than for a modified metallic surface. Indeed, the response depends on the layer itself (that is the center of interest) but also on the characteristics of the semiconductor (band gap, nature of the doping, presence of impurities near the surface). Besides the fact that the potential of the electrode is not rigorously defined on an unbiased material, another possible difficulty arises from the charge compensation. Indeed when electron transfer occurs at the substrate surface, the injected charge must be compensated by the inverse redox reaction occurring outside the diffusion cone of tips (see Scheme 1). This reaction is not a problem when considering a large conductive metallic electrode (it is generally neglected) but can become limiting for semiconductors especially when the density of the required charge carriers is low, the reverse reaction forbidden for band-gap considerations (rectification), or the reaction involves some chemical modifications inside the materials.18 Low transport rate inside the Si substrate could also result in oxidative damages of the surface, for example, if the positive charges accumulate on the surface. (Etching has been observed on semiconductor electrodes during SECM experiments.)12a However, these phenomena are expected to be minimized if the injected charges are rapidly dispersed in the sample resulting in extremely low current densities outside the diffusion cone of the UME. For that reason, experiments were always performed with the same p-doped Si wafer for which the charge carrier density is large enough to transport the injected charge far from the diffusion cone (see Experimental Section). In this present work, we have prepared different modified p-doped silicon surfaces, namely, silicon oxide (Si/SiO2), hydrogenterminated silicon (Si-H), and alkyl-monolayer-modified silicon (Si-CnH2n+1), and analyzed the kinetics of the interfacial electron transfer by SECM in dimethylformamide (DMF). All samples were prepared from the same Si wafer as the reproducibility in the preparation of the samples is an important point.19 The layers were investigated both in oxidation (hole injection) and in reduction (electron injection) in the same media. The same sample was investigated by SECM in feedback mode after different treatment stages corresponding to the preparation of the organic layergrafted p-type Si(111). Two sets of mediators were considered, the azobenzene/azobenzene radical anion in reduction (E° ) -1.3 V/SCE) and in oxidation the ferrocene/ferrocenium (E° ) 0.45 V/SCE) and the tetrathiafulvalene (TTF)/tetrathiafulvalene radical cation (E° ) 0.42 V/SCE). The oxidation potential of the ferrocene and TTF mediators is more positive than the flat band potential of the modified silicon surfaces (vide infra). In that situation, the p-doped semiconductor studied here should behave as a quasi(18) (a) An example of such a situation has been analyzed in detail in an experimental system composed of a band electrode and Teflon sample.18b (b) Amatore, C.; Combellas, C.; Kanoufi, F.; Sella, C.; Thie´bault, A.; Thouin, L. Chem. Eur. J. 2000, 6, 820-835. (19) The quality of the silicon interface and the presence of impurities may considerably affect the response and thus the characterization of the processes occurring at the organic layer.

metallic electrode.12b,20 On the contrary, the azobenzene reduction is expected not to occur on a p-Si electrode in dark.21 EXPERIMENTAL SECTION Reagents. All SECM experiments were performed in dry DMF (puriss, Fluka), stored over molecular sieves. Tetrabutylammonium perchlorate (NBu4ClO4) (electrochemical grade, Fluka) was used at 0.1 mol L-1 as supporting electrolyte. Redox mediators used for SECM experiments were purchased from Aldrich and used as received. 1-Hexene (Acros, 97%), 1-decene (Fluka, >95%), and 1-hexadecene (Alfa-Aesar, 94%) were passed through a neutral, activated alumina column, distilled over sodium under reduced pressure (1-decene and 1-hexadecene) or atmospheric pressure (1-hexene) and stored under argon in the refrigerator. The chemicals used for cleaning and etching of silicon wafer pieces (30% H2O2, 96-97% H2SO4, and 40% NH4F solutions) were of VLSI semiconductor grade (Riedel-de-Hae¨n). Electrodes. The ultramicroelectrodes were prepared in the laboratory following a general published procedure.10 A diskshaped UME of 20-µm diameter was made by sealing platinum or gold wires (Goodfellow) in a soft glass tube that was subsequently ground at one end. The glass edge was conically shaped with an outer diameter of ∼100 µm (5 < RG < 10). Prior to use, the UME was polished using diamond pastes with decreasing grain sizes. Electrode diameter was measured by studying the steady-steady current of the ferrocene oxidation with a defined concentration of mediator. A homemade electrochemical cell was used for SECM experiments similar to the one previously described for EC-AFM experiments.22 The reference electrode was a quasi-reference electrode, made with an Ag wire covered with AgNO3. Its potential was checked versus the ferrocene/ferrocenium couple. A platinum wire (i.d. ) 0.5 mm) was used as the auxiliary electrode. Electrochemical Measurements. Scanning electrochemical microscopy experiments were carried out with a homemade SECM based on a technical description previously published in the literature.10 The microelectrode tip was moved by a horizontal translation stage (speed 2 µm/s) driven by an electrical microstep motor piloted by a computer. The microstep motors were driven by a Pico SPM II (Molecular Imaging) classically used in AFM or STM experiments. The electrode potential tip was controlled using a PAR potentiostat/galvanostat model M273 (EG&G) that also measures and digitalizes the current. The potential applied to the tip electrode was chosen to be at the level of the diffusion plateau of the mediator. The approach curve showing feedback was then recorded, and the coordinate of substrate/solution interface (d ) 0) was determined from the shape change of the tip current that occurred when the tip just touched the substrate surface. The procedure was checked by considering a similar approach in conditions of insulating substrate.10 Diffusion coefficients, D, of the mediators were measured in DMF + 0.1 mol L-1 NBu4ClO4 by cyclic voltammetry on a millimetric electrode. (D ) 8.2 × 10-6 cm2 s-1 for the ferrocene/ferrocenium and for the 7.6 × 10-6 cm2 s-1 TTF/TTF radical cation couples in DMF.) (20) The cyclic voltammetry of ferrocene on a p-Si electrode is reversible: see, for example, ref 9d. (21) Bocarsly, A. B.; Bookbinder, D. C.; Dominey, R. N.; Lewis, N. S.; Wrighton, M. S. J. Am. Chem. Soc. 1980, 102, 3683-3688. (22) Bergamini, J.-F.; Ghilane, J.; Guilloux-Viry, M.; Hapiot, P. Electrochem. Commun. 2004, 6, 188-192.

Alkyl Monolayer Deposition. Electron transfer at an Si electrode is very sensitive to the sample preparation. A defined procedure was used to ensure the maximum of reproducibility in the obtained surface.9b A single-side polished silicon(111) shard (1-5 Ω cm, p-type, boron doped, thickness 525 ( 25 µm, from Siltronix) was sonicated for 10 min successively in acetone, methanol, and ultrapure 18.2 MΩ cm water. It was then cleaned in 3:1 v/v concentrated H2SO4/30% H2O2 at 100 °C for 30 min, followed by copious rinsing with ultrapure water. Caution! The concentrated H2SO4/H2O2 (aq) piranha solution is very dangerous, particularly in contact with organic materials, and should be handled extremely carefully. The surface was etched with argon-deoxygenated ppb grade 40% aqueous NH4F for 15 min to obtain atomically flat Si(111)H.23 It was then dipped in argon-deoxygenated ultrapure water for several seconds and dried under an argon stream. For the covalent attachment of the alkyl monolayer, a Si(111)-H shard was transferred into a Pyrex Schlenk tube containing ∼10 mL of deoxygenated alkene, i.e., 1-hexene, 1-decene, or 1-hexadecene, which had been previously degassed with argon for at least 3 h at 100°C. The decyl- and hexadecyl-modified surfaces were prepared by heating the alkene solution at 150 °C overnight under argon. The hexyl-modified surface was produced by irradiating the solution for 4 h in a Rayonet photochemical reactor (300 nm). The modified surfaces (Si(111)-CnH2n+1) were then rinsed copiously with toluene, dichloromethane, and ethanol and dried under an argon stream. It has been previously demonstrated that alkyl monolayers prepared by either thermal or UV photochemical hydrosilylation routes showed comparable properties, in terms of stability, molecular density, and ordering.5a,24 RESULTS AND DISCUSSION p-Si(111)/SiO2 Substrate. As a first test experiment, p-Si(111)/SiO2 surfaces, previously prepared using piranha solution, were examined (they correspond to the first treatment of the sample) to test the validity of the setup as the SiO2 layer is expected to block completely the interfacial electron transfer to the Si. The theoretical approach curve corresponding to the insulating substrate case is unique (see eq 1)

(

Iins T ) 0.15 +

1.5358 + 0.58e(-1.14/L) + L -1

)

0.0908e(L-6.3/1.017L)

(1)

and depends only on the adimensional distance L. The approach curve on an insulator surface can be used for a precise determination of the distance origin if the radius of the electrode has been measured beforehand. Figure 1 shows the experimental approach curves obtained for two redox couples, namely, the azobenzene/azobenzene radical anion and the ferrocene/ferrocenium (FeCp2). The first one is used for reduction (formation of the azobenzene radical anion) (23) Wade, C. P., Chidsey, C. E. D. Appl. Phys. Lett. 1997, 71, 1679-1682. (24) (a) Linford, M. R.; Fenter, P.; Eisenberger; Chidsey, C. E. D. J. Am. Chem. Soc. 1995, 117, 3145-3155. (b) Cicero, R. L.; Linford, M. R.; Chidsey, C. E. D. Langmuir 2000, 16, 5688-5695. (c) Sun, Q.-Y.; de Smet, L. C. P. M.; van Lagen, B.; Giesbers, M.; Thu ¨ ne, P. C.; van Engelenburg, J.; de Wolf, F. A.; Zuilhof, H.; Sudho ¨lter, E. J. R. J. Am. Chem. Soc. 2005, 127, 25142523.

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Figure 1. Approach curves of a 10-µm-radius disk ultramicroelectrode to the p-Si/SiO2 surface in DMF + 10-1 mol L-1 NBu4ClO4. Mediator: (0) 10-2 mol L-1 FeCp2; (O) 10-2 mol L-1 azobenzene. Solid line: theoretical approach curve for an insulating substrate (eq 1).

Figure 2. Approach curves of UME (a ) 10 µm) to a p-Si(111)-H surface in DMF + 10-1 mol L-1 NBu4ClO4 containing (O) 10-2 mol L-1 FeCp2, (0) 10-2 mol L-1 TTF, or (]) 10-2 mol L-1 azobenzene. (-) Solid thick line: theoretical curve for insulating substrate. Dashed line: theoretical curve for conducting substrate. Inset: Fittings with the theoretical curves corresponding to the finite kinetics case (eq 2) (O) 10-2 mol L-1 FeCp2 (κ ) 1.6); (0) 10-2 mol L-1 TTF (κ ) 3.3); (]) 10-2 mol L-1 azobenzene. (-) Insulating behavior (eq 1).

as the second one works in oxidation, meaning that we can test negative and positive charge injections in the p-Si sample. As seen in Figure 1, the normalized approach curves are similar for both mediators and can be superposed. For L > 4, It is close to 1 as no interaction occurs between the electrogenerated radical anion (or cation) and the substrate for this distance. On the contrary, for L < 4, It rapidly diminishes with the microelectrode/substrate distance demonstrating the absence of the regeneration of mediator at the p-Si(111)/SiO2 surface. This behavior corresponds to a negative feedback and confirms the insulating properties of the layer as it can be expected from a compact and thick SiO2 coating. Furthermore, when comparing the experimental approach curves with theoretical ones, we observed that all experimental curves give a good agreement with the theory. p-Si(111)-H Substrate. After investigating the p-Si(111)/ SiO2 interface, a hydrogen-terminated silicon surface was considered. Atomically flat areas of monohydride ≡Si-H termination are achieved with a commercial silicon wafer with the (111) orientation, treated with degassed 40% aqueous NH4F (see Experimental Section). Hydride-terminated surfaces offer some general advantages especially in terms of chemical homogeneity (>99% H termination).23 However, long-term use of a Si-Hterminated surface is generally precluded due to its propensity to oxidize. SECM should be advantageous as it does not require the prepolarization of the Si electrode and allows testing directly the stability of the interface.25 Our sample (p-Si(111)-H surface) was examined after such treatment using SECM in feedback mode with the same redox mediator in DMF. Different approach curves were performed on the same spot of the surface. No considerable modification of the signal after three to four approaches was observed by comparison with the examination of a “new” area, indicating that the ferrocenium does not chemically react with the Si-H surface contrary to the situation reported for the n-doped electrode.12 Additionally to ferrocene, another redox couple, TTF/TTF+•, was used to illustrate the different behaviors of the silicon interface as a function of the mediator nature. Figure 2 shows the approach

curves obtained for the redox couples at the same concentration. As expected from the theoretical analyses, for all mediators and for L > 4 the current is equal to ilim. When the distance decreases (typically for L < 4), different shapes are observed depending on the nature of the interfacial electron transfer (oxidation or reduction). In the case of the cathodic mediator (azobenzene), a negative feedback is obtained showing the absence of regeneration of the mediator onto the silicon surface. This negative feedback confirms the insulating behavior of p-type silicon at cathodic processes without illumination and is in agreement with previous voltammetric studies over the semiconductor electrode.21 On the contrary, in the case of the anodic mediators, the approach curves show positive feedbacks for both couples and thus the occurrence of electron transfers between the radical cation electrogenerated at the UME and the silicon through the Si-H interface. The electron transfer regenerates the mediator under its reduced form at the surface causing the enhancement of the current. From differential capacitance measurements, the flat band potential of this surface is estimated at ∼0.25 V versus SCE, which is less positive than the redox potential of the two used anodic mediators (see Supporting Information). Consequently, the space-charge region of the semiconductor can be considered in accumulation at the equilibrium. The kinetics of the transfer can be characterized by measuring the apparent electron-transfer rate constant kel. Using the formalism previously introduced by Mirkin and Bard,10,26 when interfacial electron transfer at the substrate/solution is the rate-determining step, the normalized current IT can be described by the following set of equations:

(25) For example, it was reported for the oxidation of ferrocene on an n-Si electrode that ferrocenium reacts chemically with the surface.12b (26) (a) Bard, A. J.; Mirkin, M. V.; Unwin, P. R.; Wipf, D. O. J. Phys. Chem. 1992, 96, 1861-1868. (b) Wei, C.; Bard, A. J.; Mirkin, M. V. J. Phys. Chem. 1995, 99, 16033-16042. (27) (a) Yu, H.-Z.; Morin, S.; Wayner, D. D. M.; Allongue, P.; Henry de Villeneuve, C. J. Phys. Chem. B 2000, 104, 11157-11161. (b) Fabre, B.; Lopinski, G. P.; Wayner, D. D. M. J. Phys. Chem. B 2003, 107, 14326-14335.

where IcT, IkS, and Iins T are respectively the normalized current tip for the diffusion-controlled regeneration, for the kinetically limited electron transfer at the substrate interface, and for an insulating surface (see the preceding eq 1). IkS depends on a single adimensional parameter κ ) kela/D, where a is the electrode tip radius and D the diffusion coefficient

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c ins IT ) IkS(1 - (Iins T /IT)) + IT

(2)

IcT ) 0.68 + (0.78377/L) + 0.3315e(-1.0672/L)

(3)

Figure 3. Approach curves of UME (a ) 10 µm) to a p-Si(111)-H surface for different mediator concentrations in DMF + 10-1 mol L-1 NBu4ClO4: (0) 10-3 mol L-1 FeCp2 (κ ) 1.8); (O) 5 10-3 mol L-1 FeCp2 (κ ) 1.7); (4) 10-2 mol L-1 FeCp2 (κ ) 1.6).

of the mediator. For L < 2 and 0.01 < κ < 1000, IkS can be approximated by the following eq 4:

IkS )

[

]

0.68 + 0.3315e(-1.0672/L) 0.78377 + L(1 + 1/Λ} 1 + F(L,Λ)

(4)

with

F(L,Λ) )

(11 + 7.3Λ) Λ(110 - 40L)

and

Λ ) κL

It is clearly visible on the rough data in Figure 2 that the positive feedback for the TTF couple is higher than for the ferrocene corresponding to a faster kinetics for the first one. Despite the large differences in the data, the derived rate constant for TTF is only twice as high as that measured for the ferrocene. As a matter of fact, the positive feedback is highly sensitive to the value of kel in this range of κ value. To assess whether more complex phenomena interfere within the apparent kinetics, it is interesting to examine the variation of the approach curves for different mediator concentrations ranging from 10-3 to 10-2 mol L-1 (Figure 3). A small decrease of κ (∼10%) was observed for the higher mediator concentration. In the case of a metallic electrode and a global kinetics controlled by an interfacial electron-transfer step, the adimensional parameter κ should be independent of the concentration. The apparent variation (small but reproducible) is thus indicative of the existence of other limiting phenomena, which begin to interfere at the highest transfer rates (possibly the conduction of the charge inside the p-Si). In that sense, the Si-H surface is the worst situation as the interfacial electron-transfer rate is expected to be lower for the p-Si surfaces derivatized with an organic layer (see below). To test the chemical stability of the interface over a long time, these experiments were repeated after 1 day during which the sample was kept in the deoxygenated solution maintained under argon atmosphere. Almost similar responses were obtained showing a surprising stability of the Si-H surface in deoxygenated DMF. p-Si(111)-CnH2n+1 Substrates. After the preliminary studies described above, the alkyl monolayers covalently bound to silicon were examined with SECM in feedback mode. Surface grafting was performed by reaction of a freshly prepared Si(111)-H crystal in the presence of the neat corresponding n-alkene as described

Figure 4. Approach curves of UME (a ) 10 µm) to a p-Si(111)C6H13 surface in DMF + 10-1 mol L-1 NBu4ClO4 solutions containing the following: (O) 2 10-3 mol L-1 FeCp2 (κ ) 1.4); (4) 10-2 mol L-1 FeCp2 (κ ) 1.4); (0) 10-2 mol L-1 TTF (κ ) 1.7); (]) 10-2 mol L-1 azobenzene. (Solid lines): theoretical curve for finite electron-transfer kinetics and an insulating substrate. (Dashed line): theoretical curve for a conducting substrate behavior.

in the Experimental Section. The method results in the formation of alkyl monolayers covalently bound to silicon via Si-C interfacial bonds.1 Different organic chains with increasing lengths were grafted onto p-Si. As a result of the weak polarization of the Si-C bond, the flat band potential of the alkyl-modified surfaces is found to be not significantly different from that of Si(111)-H, i.e., 0.20 ( 0.04 V versus SCE. Therefore, as this is the case for the hydrogen-terminated surface, the silicon underlying the alkyl chains is in accumulation when contacted with the anodic mediators. Figure 4 shows the approach curves recorded with the p-Si(111) functionalized with the shortest chain, p-Si(111)-C6H13. We first check that the response in reduction was not affected by the presence of the layer. Indeed, a decrease of the faradic current with the distance L is observed for the cathodic mediator. The curve recorded with azobenzene as the mediator shows the insulating behavior of the surface and provides a good fit with the theoretical curve. In opposition, the two anodic mediators exhibit an important positive feedback. The experimental approach curves were fitted with the theoretical curves for a process governed by finite substrate heterogeneous kinetics to extract the apparent electron-transfer rate constant κ (or kel). We found a κ of ∼1.4 for ferrocene and one only slightly higher for TTF (1.7). Noticeably, these values are just a little smaller than the rates obtained when no organic layer is grafted to the surface, showing evidence for very low blocking effects. This can be ascribed to the short tunneling distance between the redox mediator and the silicon surface, but also to the weakly densely packed character of the hexyl monolayer, as proved by the large value of its dielectric constant m ) 4.5 (see Supporting Information). It is also interesting to remark that negligible concentration effects are now visible with this interface (compare the curves obtained for the two ferrocene concentrations). Similar experiments were repeated with p-Si surfaces covered by longer organic chains. Figure 5 shows the behavior obtained for the p-Si(111)-C10H21 surface. A negative feedback is now observed, but comparison with the insulating behavior proves that interfacial electron transfer between the mediator and the surface is still possible because the curves remain higher than those predicted for an insulating material. As observed before, the apparent electron-transfer Analytical Chemistry, Vol. 78, No. 17, September 1, 2006

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Table 1. Apparent Interfacial Electron Transfer Rate Constants on Modified p-Si(111) Electrodes

Figure 5. Approach curves of UME (a ) 10 µm) to a p-Si(111)C10H21 surface in a DMF +10-1 mol L-1 NBu4ClO4 solution containing the following: (O)10-2 mol L-1 FeCp2; (0) 10-2 mol L-1 TTF. The line (-) is the theoretical behavior for a finite electron-transfer kinetic for κ ) 0.12. The dashed lines are the two limiting cases for conductive and insulating substrates.

Figure 6. Approach curves of UME (a ) 10 µm) on a p-Si-C16H33 surface in DMF + 10-1 mol L-1 NBu4ClO4 containing the following: (O) 10-2 mol L-1 FeCp2 and (0) 10-2 mol L-1 TTF.

kinetics seems a bit slower for ferrocene than for TTF. A reasonable agreement with the theoretical behavior for a finite electron-transfer kinetics difference can be obtained for L > 0.8. However, some discrepancies appear for shorter distances, and the experimental data slightly deviate from the theoretical behavior. Following the fitting, an average value of κ of ∼0.12 was extracted. The approach curves obtained for the longest chain p-Si(111)C16H33 are displayed in Figure 6. A negative feedback was also obtained, but the currents remain higher than the curve calculated for the insulating behavior. As before, the two mediators show some differences and the curve for ferrocene remains below that for TTF, indicating that the electron transfer with ferrocene is always slower than for TTF. However, the experimental data deviate from the theoretical behavior for L < 1 suggesting a change in the kinetics regime for the highest diffusion rate (shortest electrode-substrate distance). From the adjustments for L > 1, the adimensional constants κ were extracted and led to kel rate constants smaller than those determined for the previous surfaces. This diminution in interfacial electron transfer evidences a blocking character more efficient than that observed for the decyl monolayer. This is believed to be directly related to the organic chain lengthening 6024 Analytical Chemistry, Vol. 78, No. 17, September 1, 2006

surface

κ (TTF)

kel x 100 (cm s-1)

κ (FeCp2)

kel x 100 (cm s-1)

Si-H Si-C6H13 Si-C10H21 Si-C16H33

3.3 1.7 0.14 0.1

2.5 1.3 0.11 0.076

1.6 1.4 0.10 0.05

1.3 1.15 0.082 0.041

rather than differences in the quality of the deposited monolayers. Indeed, the dielectric constant values determined from capacitance measurements are 3.9 and 3.3 for the decyl and hexadecyl monolayers, respectively. These values are in excellent agreement with that determined for alkyl monolayers on n-type silicon (3.3 ( 0.6).27 Considering the large experimental uncertainty in the determination of m, it can be concluded that the decyl and hexadecyl monolayers studied here are similarly packed and are less defective than the hexyl one. Three majors pathways are classically envisaged in the literature for the interfacial electron transfer, namely: (i) electron tunneling through the monolayer, (ii) permeation of the electroactive species into the monolayer followed by a transfer near the electrode surface, and (iii) diffusion of the electroactive species through pinhole defects. When the electron transfer occurs by tunneling across an organic layer, the electron-transfer rate decreases exponentially with the layer thickness with a slope -β, where β is the electron-tunneling constant related to the barrier height.28,29 In that relation for characterizing the blocking effect of the chain, it is suitable to examine the variation of the logarithm of κ (or kel) with the number of carbon atoms in the alkyl chain (see the data gathered in Table 1). Comparable trends are obtained for both mediators, which after linear regression analysis gives very similar slopes, β ) 0.92 and β ) 0.93 for ferrocene and TTF, respectively (Figure 7). These constants are similar to the β values determined experimentally with closely packed SAMs on gold (β ≈ 0.9-1.1/CH2)28,29 and thus suggest that the main pathway for the interfacial electron transfer at the p-Si substrate is the electron tunneling through the organic monolayer. These results show the good blocking properties of long-chain alkyl monolayers grafted to Si(111), which act as effective electrontransfer barriers. Our experiments can also be directly compared with previous investigations performed on polarized and modified n-Si(111) electrodes. In a first investigation, the heterogeneous standard rate constants of the anthraquinone/anthraquinone radical anion couple were measured by cyclic voltammetry.9b Contrary to our results, a small β constant was determined, ∼0.05/CH2, suggesting the occurrence of different electrontransfer pathways such as the penetration of the mediator inside the layer. In a more recent work, the variation of the electrochemical current of the ferrocenium reduction measured at a fixed potential was investigated as a function of the alkyl chain length.9c (28) Smalley, J. F.; Feldberg, S. W., Chidsey, C. E. D.; Linford, M. R.; Newton, M. D.; Liu, Y.-P. J. Phys. Chem. 1995, 99, 13141-13149. (29) (a) Becka, A. M.; Miller, C. J. J. Phys. Chem. 1992, 96, 2657-2668. (b) Weber, K.; Hockett, L.; Creager, S. J. Phys. Chem. B 1997, 101, 82868291. (c) Yu, H. Z.; Shao, H. B.; Luo, Y.; Zhang, H. L.; Liu Z. F. Langmuir 1997, 13, 5774-5778.

in feedback mode reflect the global process, namely, the interfacial electron transfer between the oxidized form of the mediator and p-Si, but also the reverse reaction occurring outside the diffusion cone of the UME and the intrinsic conduction inside the semiconductor. However, classical test as the variation of the apparent kel with the mediator concentration seems to reject such a complication in the analysis of the SECM data.

Figure 7. Variation of the adimensional electron-transfer rate constant κ for (O) ferrocene and (0) TTF vs the number of carbons in the alkyl chain grafted to the p-Si(111) surface. The line is the linear regression (slope -0.92) calculated for the TTF/TTF+. couple.

A value of β equal to 1.0 closely matching our β value (0.93 on p-Si substrate) for the ferrocene/ferrocenium couple was obtained, showing good agreement with our SECM results when the same redox mediator is considered.9c At this stage of our work, the origin of the discrepancy observed with the anthraquinone/ anthraquinone radical anion couple is not clear. Similar “softer” dependency of the interfacial electron transfer with distance was described on n-InP electrodes (a factor of 2 softer by comparison with the same organic layer on a gold electrode) and was attributed to different reorganization energies at semiconductor interface than on a metal/liquid junction.30 Another possibility is that the penetration of the mediator depends on its chemical nature. However, no considerable variation was detected between ferrocene and TTF. Finally, we should also remind that our electrode is not connected. It results that SECM measurements (30) Gu, Y.; Waldeck, D. H. J. Phys. Chem. 1996, 100, 9573-9576.

CONCLUSION In this work, we have examined the possibility of SECM in feedback mode to examine the properties of organic layers attached to an unbiased silicon electrode. The method has proved to be a sensible tool to probe the interfacial electron transfer through the organic layer at such a semiconductor interface. Several interfaces were investigated: Si/SiO2, Si-H, and SiCnH2n+1. The approach curves demonstrate the insulating character of p-Si (in the dark) for cathodic processes and, on the contrary, large positive feedback with two anodic mediators. The derived apparent rate constants for the interfacial electron transfer show a large dependence with the chain length, which strongly supports that electron transfer occurs through electron tunneling across the organic layer. More generally, this study shows the capacity of SECM for analyzing different nonconnected silicon surfaces covered by organic monolayers. SUPPORTING INFORMATION AVAILABLE Impedances and differential capacitance measurements performed with the alkyl-modified surfaces in acetonitrile. This material is available free of charge via the Internet at http:// pubs.acs.org. Received for review January 10, 2006. Accepted June 18, 2006. AC060058H

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