Highly Doped Silicon Electrodes for the ... - ACS Publications

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Highly Doped Silicon Electrodes for the Electrochemical Modification of Self-Assembled Siloxane-Anchored Monolayers: A Feasibility Study Haviv Grisaru, Yair Cohen, Doron Aurbach,* and Chaim N. Sukenik* Department of Chemistry, Bar Ilan University, Ramat-Gan, Israel 52900 Received May 15, 2000. In Final Form: December 13, 2000 The possibility for electrochemical modification of the surface functional groups of a self-assembled siloxane-anchored monolayer was explored. The primary challenge of this work was to minimize the insulating silicon oxide layer while still providing ample hydrophilic oxide for anchoring siloxane-based monolayers. Working on highly doped silicon electrodes, wetting measurements and scanning probe microscopies (both atomic force microscopy and lateral force microscopy) were combined with electrochemical measurements to evaluate various surface preparations in terms of their suitability for combining electrochemical activity with smooth uniform surfaces on which successful monolayer formation can be achieved. The difficulties encountered in achieving this balance suggest that organic electrochemistry on an oxidized silicon surface will be difficult to achieve, in the presence of even a thin oxide layer.

Introduction In view of the importance of silicon as the primary semiconductor material in modern microelectronic devices, efforts to control its electronic properties and to tailor the chemical and physical characteristics of its surface are of major importance. One approach to this surface modification has been through the use of variously functionalized siloxane-anchored self-assembled monolayers (SAMs) attached to the native silicon oxide. For example, by attaching variously functionalized aromatic rings to the silicon surface1,2 it is possible to envision controlled modulation of the work function and band bending of the silicon surface. This variation of surface functionality can be achieved by either synthesizing different silanes and using each of the different silanes for film formation2 or by in situ transformation of a monolayer based on a single film-forming precursor.1 The advantage of the in situ variation route in terms of convenience is often offset by less than quantitative surface modification reactions, and thus there is an ongoing need to discover efficient, new, versatile in situ chemistry for monolayer surfaces. Although electrochemistry is a well-known tool in synthetic organic chemistry, its application has always been limited by the inconvenience of its use for largescale preparations. This disadvantage disappears when dealing with the very small quantities in monolayer films, and, therefore, we wanted to assess the possibility of using preparative electrochemistry for in situ monolayer transformations. The specific challenges begin with the use of the silicon (even if highly doped) as an electrode and the balance between the oxide layer on the silicon surface typically used for siloxane anchoring3-5 and the insulating (1) Cohen, R.; Zenou, N.; Cahen, D.; Yitzchaik, S. Molecular Electronic Tuning of Si Surfaces. Chem. Phys. Lett. 1997, 279, 270-274. (2) Regev, K.; Cahen, D.; Grinstein, M.; Sukenik, C. N. Use of Organic Thin Films to Change the Surface Electronic Properties of Si. Manuscript being prepared for publication. (3) (a) Netzer, L.; Sagiv, J. A New Approach to Construction of Artificial Monolayer Assemblies. J. Am. Chem. Soc. 1983, 105, 674. (b) Wasserman, S. R.; Tao, Y.-T.; Whitesides, G. M. Structure and Reactivity of Alkylsiloxane Monolayers Formed by Reaction of Alkyltrichlorosilanes on Silicon Substrates. Langmuir 1989, 5, 1074-1087. (4) Tillman, N.; Ulman, A.; Penner, T. L., Formation of Multilayers by Self-Assembly. Langmuir 1989, 5, 101-111.

properties of this oxide. It is also important to make an appropriate choice of reactions and functional groups so as to deal with accessible redox potentials and to deal with the insulating properties of the alkyl chains upon which organic SAMs are typically built. Finally, we note that although methods for the attachment of organic SAMs directly to silicon and not via an oxide layer have been reported6,7 they are still not as convenient, reliable, and well-studied as the silane hydrolysis used for deposition onto the silicon oxide. We have chosen highly N-doped silicon wafers (e0.005 Ω cm) as our electrode material and have studied the removal (etching) and regrowth of its oxide with respect (5) (a) Jin, Z. H.; Vezenov, D. V.; Lee, Y. W.; Zull, J. E.; Sukenik, C. N.; Savinell, R. F. Alternating Current Impedance Characterization of the Structure of Alkylsiloxane Self-Assembled Monolayers on Silicon. Langmuir 1994, 10, 2662-2671. (b) Although there has been a great deal of work addressing the penetrability of thiol on gold monolayers to electron transfer (e.g.: Braach-Maksvytis, V.; Raguse, B.; Highly Impermeable “Soft” Self-Assembled Monolayers. J. Am. Chem. Soc. 2000, 122, 9544-9545 and references therein; also see: Smalley, J. F.; Feldberg, S. W.; Chidsey, C. E. D.; Linford, M. R.; Newton, M. D.; Liu, Y. P. The Kinetics of Electron Transfer Through Ferrocene-Terminated Alkanethiol Monolayers on Gold. J. Phys. Chem. 1995, 99, 1314113149.), relatively little work has been done on the electron permeability of siloxane-anchored systems. An excellent recent treatment of this subject which highlights the difficulty in obtaining siloxane-anchored films which present effective barriers to electron transfer is: Markovich, I.; Mandler, D. The Effect of an Alkyl Silane Monolayer on an Indium Tin Oxide Surface on the Electrochemistry of Hexacyanoferrate. J. Electroanal. Chem. 2000, 484, 194-202 and references therein. (6) Connection via Si-C bond formation has been reported; see, for example: (a) Linford, M. R.; Chidsey, C. E. D. Alkyl Monolayers Covalently Bonded to Silicon Surfaces. J. Am. Chem. Soc. 1993, 115, 12631-12632. (b) Linford, M. R.; Fenter, P.; Eisenberger. P. M.; Chidsey, C. E. D. Alkyl Monolayers on Silicon Prepared from 1-Alkenes and Hydrogen-Terminated Silicon. J. Am. Chem. Soc. 1995, 117, 31453155. (c) Sieval, A. B.; Vleeming, V.; Zuilhof, H.; Sudholter, E. J. R. An Improved Method for the Preparation of Organic Monolayers of 1-Alkenes on Hydrogen-Terminated Silicon Surfaces. Langmuir 1999, 15, 8288-8291. (d) Boukherroub, R.; Morin, S.; Bensebaa, F.; Wayner, D. D. M. New Synthetic Routes to Alkyl Monolayers on the Si(111) Surface. Langmuir 1999, 15, 3831-3835. Elaboration of such films has recently been reported in: Boukherroub, R.; Wayner, D. D. M. Controlled Functionalization and Multistep Chemical Manipulation of Covalently Modified Si(111) Surfaces. J. Am. Chem. Soc. 1999, 121, 11513-11515. (7) An alternative mode of direct connection has also been reported wherein alcohols are attached to fluoride-etched silicon via discrete Si-O-C linkage: Cleland, G.; Horrocks, B. R.; Houlton, A. Direct Functionalization of Silicon via the Self-Assembly of Alcohols. J. Chem. Soc., Faraday Trans. 1995, 91, 4001-4003.

10.1021/la0006768 CCC: $20.00 © 2001 American Chemical Society Published on Web 02/02/2001

Monolayer Attachment and Electrochemistry on Silicon

to wetting and insulating properties. The ω-trichlorosilane of methyl undecanoate3b was used as our SAM-forming material. Polymethylene chain lengths of 16-18 carbons are most commonly used in SAM formation, and they most easily form well-packed films. A chain of 12 carbons is long enough to provide organized monolayer films,5a but, without special precautions, it is likely to give a film with sufficient disorder so as to not present an insurmountable barrier to electron transfer.5b The alkyl ester functionality was chosen as a redox reaction that had been previously applied to monolayer assemblies using conventional reagent chemistry. It has been reported4 that the reduction of a methyl carboxylate to a terminal alcohol (-CH2COOCH3 f -CH2CH2OH) can be achieved using a reducing agent like lithium aluminum hydride. This reaction can be monitored by Fourier transform infrared (FTIR) spectroscopy (i.e., the disappearing carbonyl) and by the change from the relatively hydrophobic ester surface to a relatively hydrophilic alcohol surface. The in situ creation of surface alcohol functionality was a desirable synthetic target for further monolayer elaboration. We also note that electron transfer based reductions of esters are known to be capable of undergoing reductive coupling (classical acyloin reaction), and there are reports of electrochemically induced ester cleavage to acid, by a radical pathway.8 Thus, this system held out the possibility of fine-tuning reaction variables to control the nature of the reduction product. Finally, the electrochemical reduction of esters is not surface specific (surface catalyzed); it has been reported to occur on lead electrodes9 which should not provide specific catalysis. Also, the reduction potential for ester reduction is expected to fall within our operating window in an aqueous acid electrolyte. Reduction of esters in nonaqueous systems can require very low potentials (less than -2 V versus saturated calomel electrode (SCE) as reference),8 although there are also reports on the reduction of esters in a nonaqueous solution at potentials around -1.6 f -1.8 V versus SCE.10 However, when the electrolyte is an acidic aqueous medium, ester reduction may be carried out at potentials greater than -1 V versus SCE.9 The work reported herein combines FTIR spectroscopy, wetting measurements, and atomic force and lateral force microscopies (AFM and LFM) to address the issues that are critical to the use of preparative electrochemical transformations on a siloxane-anchored SAM surface. Experimental Section 1. Synthesis. Octadecyltrichlorosilane, 10-undecenoyl chloride, hydrogen hexachloro-platinate(IV) hydrate, and all organic solvents were obtained from Aldrich Chemical Co. and used as received. Bicyclohexyl (Aldrich, vacuum distilled), pyridine (Aldrich, distilled from CaH2), and trichlorosilane (Aldrich, distilled from quinoline) were distilled immediately before use. Water was deionized and then distilled in an all-glass apparatus. The synthetic procedures were as follows: Preparation of Methyl 10-Undecenoate. In a dry, two-neck 500mL flask equipped with a magnetic stirring bar, a drying tube, and a septum was placed methanol (250 mL, 197.8 g, 6.17 mol). (8) Webster, R. D.; Bond, A. M.; Compton, R. G. Voltammetric and EPR Spectroscopic Studies Associated with the Reduction of Pyridineand Benzene-Substituted n-Alkyl Esters and Thioic S-Esters in Aprotic Solvents. J. Phys. Chem. 1996, 100, 10288-10297. (9) Romulus, A. M.; Savall, A. Electrochemical Reduction of Ethyl2-Picolinate on Lead in a Sulfuric-Acid Medium. Electrochim. Acta 1992, 37, 625-630. (10) Fussing, I.; Hammerich, O.; Hussain, A.; Nielsen, M. F.; Utley, J. H. P. The Influence of Solvent on the Mechanism and Stereochemistry of Electrohydrodimerization. The Reduction of Cinnamic Acid Esters in Methanol. Acta Chem. Scand. 1998, 52, 328-337.

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Figure 1. The electrochemical cell used in this study: (1) a brass cover, (2) a copper foil current collector for the silicon wafer working electrode, (3) the rear side of the silicon wafer UE spread with an amalgam of In/Ga, (4) the conducting (Ndoped) silicon working electrode, (5) a rubber gasket between the UE and the cell body, (6) the cell body (made of Teflon), (7) a saturated calomel reference electrode mounted in the cell body, (8) an O-ring (rubber), (9) glass tubing, (10) a glass frit which isolates the anode compartment, and (11) counter electrode (Pt wire). Table 1. Wetting Properties of Surface Layers on N-Doped Silicon Wafers as a Function of Various Surface Treatments

wafer treatment after cleaning, before etch after fluoride etch (10 min) after etch + 1 min piranha after etch + 2 min piranha after etch + 3 min piranha after etch + 4 min piranha after etch + 5 min piranha after etch + 10 min piranha

surfaces covered after OTS by an oxide layer coating Adv CA/Rec CA Adv CA/Rec CA 56°/44° 75°/59° 35° (adv) 33° (adv) 31° (adv) 28° (adv) ∼0° ∼0°

71°/63° 92°/70° 102°/80° 105°/85° 105°/98° 107°/97°

The flask was cooled in an ice/water bath. To the flask was added (by syringe) 4 mL of dry, freshly distilled pyridine (0.05 mol). The mixture was stirred, and 10-undecenoyl chloride (5.1 g, 0.025 mol) was added dropwise. After this addition was complete, the mixture was stirred at 0 °C for 10 min, the ice bath was removed, and stirring was continued at room temperature for 1 h. Most (≈90%) of the methanol was then evaporated, and the contents of the flask were partitioned between water and diethyl ether. The water layer was extracted with diethyl ether (2 × 150 mL). The combined ether extracts were washed with 0.1 N HCl, saturated aqueous NaHCO3, water, and finally saturated aqueous NaCl. The ether layer was dried over anhydrous MgSO4, filtered, and concentrated by vacuum. The crude product was purified by flash chromatography (silica gel, 3% EtOAc; 97% hexane); yield, 4.75 g (96%). 1H NMR δ (ppm): 5.81 (ddt, J ) 16.8, 10.1, 6.7 Hz, 1H, CHd), 4.99 (ddt, J ) 16.0, 2.1, 1.1 Hz, 1H, CHtrans), 4.92 (ddt, J ) 9.0, 2.2, 1.1 Hz, 1H, CHcis), 3.66 (s, 3H, -OCH3), 2.30 (t, J ) 7.5 Hz, 2H, CH2CO-), 2.03 (b quin, J ) 6.6 Hz, 2H, CH2CHd), 1.7-1.52 (m, 2H, CH2CH2CO-), 1.45-1.2 (m, 10H, (5CH2)). 13C NMR δ (ppm): 174.0 (-CdO); 138.98 (CHd); 113.82 (CH2d); 51.11 (-OCH3); 34.04, 33.73, 29.29, 29.14, 29.07, 29.00, 28.83, 24.88 (8CH2). IR (neat) ν (cm-1): 3077, 2962, 2927, 2855, 1742, 1641, 1462, 1436, 1361, 1241, 1197, 1171, 1117, 994, 910. Preparation of Methyl 11-(Trichlorosilyl)undecanoate (MTU)3b. In a 50-mL pressure tube, containing a magnetic stirring bar and maintained under a nitrogen atmosphere, were placed methyl 10-undecenoate (4.6 g, 23.2 mmol), HSiCl3 (24.7 mL, 245 mmol), and 200 µL of a 4% solution of hydrogen hexachloro-platinate(IV) hydrate in distilled 2-propanol. The tube was sealed and then kept for 20 h at 60 °C. The progress of the reaction was monitored by the disappearance of olefinic protons by 1H NMR. After the reaction was complete, it was transferred (under N2)

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Figure 2. (a) An AFM image of a typical Si surface after cleaning with organic solvents, before etching, 1 × 1 µm. A height scale appears along with the following data: AH ) average height, SD ) standard deviation from the average height, and RF ) roughness factor. (b) A typical profile of the area scanned in image 2a. (c) A lateral force image of the surface scanned in image 2a. to a 50-mL round-bottom flask, excess HSiCl3 was evaporated, and the product was isolated by Kugelrohr distillation at 160170 °C and 0.007 mmHg; yield, 4.5 g (58%). 1H NMR δ (ppm): 3.66 (s, 3H, -OCH3), 2.30 (t, J ) 7.51 Hz, 2H, CH2CO-), 1.60 (m, 2H, CH2CH2CO-), 1.42-1.25 (m, 16H, CH2 × 8). 13C NMR δ (ppm): 174.02 (-CdO); 51.22 (-OCH3); 33.91, 31.66, 29.22, 29.13, 29.07, 28.98, 28.84, 24.79, 24.16, 22.12 (10 × CH2). IR (neat) ν (cm-1): 2937, 2928, 2921, 2855, 1736, 1466, 1440, 1209, 1177, 920. HRMS (DCI CH4) m/z: 333 (100%, MH+), 297 (36%,

MH+-HCl), 283 (8%, MH+-CH3-Cl•). m/e: 333.0611 (calcd), 333.0615 (found). 2. Procedures for Wafer Cleaning and Etching and Oxide Regrowth. N-doped (As) silicon (100) wafers, 525 ( 25 µm thick, resistivity < 0.005 Ω cm, and polished on one side, were obtained from Silicon Sense Inc. (U.S.). Their surfaces were cleaned by rinsing in analytical grade chloroform, acetone, and ethanol, followed by drying in a stream of filtered nitrogen. The wafers were then etched in aqueous HF (53 wt %) for 10 min at room


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Figure 3. (a-c) Same as Figure 2 for a silicon surface after etching by 53% HF solution for 2 min. temperature (in a Teflon vessel), followed by washing in triply distilled water and drying in a stream of filtered nitrogen. This treatment removes both organic contaminants and the SiO2 surface layer.11,12 To obtain an oxide layer on which self-assembly can take place (OH surface groups), the etched wafers were oxidized by a mixture of concentrated sulfuric acid (98%) and 30% (by weight) aqueous H2O2 solution 7:3 v/v (“piranha” solution) at 80 °C for times between 1 and 10 min, followed by washing with water and drying (11) D’Aragona, F. S. J. Electrochem. Soc. 1972, 119, 948-951.

(filtered N2 stream). The suitability of the treated wafers for SAM coating was ascertained from their very low water contact angles (i.e., high hydrophilicity). Silane coating of these wafers was done within 2 h of their cleaning/oxidation. 3. Procedure for SAM Coating of the N-Doped Silicon Surfaces by MTU. Cleaned silicon wafers were immersed for (12) (a) Jakob, P.; Chabal, Y. J. J. Chem. Phys. 1991, 95, 2897. (b) Higashi, G. S.; Chabal, Y. J. In Handbook of Semiconductor Wafer Cleaning Technology: Science, Technology and Applications; Kern, W., Ed.; Noyes Publications: Park Ridge, NJ, 1993; pp 433-496. (c) Dumas, P.; Chabal, Y. J.; Jakob. P. Surf. Sci. 1992, 269, 867.

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Figure 4. (a-c) Same as Figure 3 after etching the Si surface for 10 min. 2.5 h in a 1% v/v solution of MTU in dicyclohexyl (DCH, distilled) under a N2 atmosphere in a glovebox. Coated wafers were extensively rinsed with chloroform and ethanol. The same procedure was used for coating octadecyltrichlorosilane (OTS). 4. Wetting Measurements. Contact angles were measured using a Rame-Hart model 100 contact angle goniometer. Advancing contact angles (Adv CA) were measured by placing a measured drop of H2O (by syringe) on the solid substrate, advancing the periphery of the drop by adding more liquid, and measuring the contact angle within no more than 30 s. The receding contact (Rec CA) angles were measured after first

withdrawing part of the liquid from the drop. Reported values are an average of five measurements taken at different points on the surface, at 22 ( 2 °C. 5. FTIR Spectroscopy. FTIR spectroscopy of the silicon surfaces used a Nicolet model 860 spectrometer equipped with a Spectratech FT-80 accessory (external reflectance mode, 80° grazing angle). The spectrometer is in a H2O- and CO2-free atmosphere in a glovebox. One thousand interferometer scans (4000-650 cm-1) were sufficient to obtain acceptable spectra at 4 cm-1 resolution. Reference spectra were obtained using uncoated (cleaned/oxidized) silicon wafers.


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Figure 5. Same as Figures 2-4. An AFM image, surface profile, and LFM image (a-c, respectively) of a silicon surface after etching with HF solution for 10 min, followed by regrowing of a thin oxide film by mild oxidation with a piranha solution (80 °C, 5 min). 6. AFM Measurements. AFM and LFM used the Discoverer model TMX 2010 AFM system from Topometrix Inc. in a vibration-protected, argon atmosphere glovebox.13 Tips from Park Scientific Instruments (Si3N4, spring constant ) 0.02 N/m, torsional force constant ) 0.008 N/rad) were used in contact mode. Each sample was sonicated for 15 min in chloroform before being introduced into the glovebox of the AFM system. It was confirmed that the AFM measurements in contact mode are not destructive to the surfaces studied.

7. Electrochemical Measurements. Electrochemical measurements used an EG&G model 273A potentiostat and model 270 software. The electrochemical cell (Figure 1) involved a SCE: Pt/Hg, Hg2Cl2/KCl salt reference electrode and a platinum-wire counter electrode. The electrical contact to the silicon wafer working electrode was obtained by scratching the unpolished side of the wafer with a quartz crystal and spreading an indium/ gallium (24/76 by weight) mixture on the scratched surface. A copper foil contact adhered well to the In/Ga covered surface and

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maintained a stable electrical contact to the silicon wafer. No IR compensation was applied. The electrolyte solution was 1 M aqueous H2SO4. After electrochemistry, wafers were rinsed twice in triply distilled water and ethanol, sonicated at 60 °C in CHCl3 (30 min), and then dried with a stream of nitrogen and characterized.

Results 1. Cleaning, Control, and Characterization of Oxide on Silicon Wafer. To create silicon wafers with an oxide thickness suitable for siloxane monolayer coating yet thin enough to enable electron transfer from the N-doped silicon to the organic monolayer surface, we first cleaned the wafer and removed the native oxide by a fluoride etch. We proceeded to regrow the oxide by treatment with piranha solution and monitored the wetting properties of the new oxide. These data are shown in Table 1. The suitability of a wafer for SAM deposition was ascertained by depositing a film of OTS. This material readily gives a very hydrophobic film with low hysteresis, if the underlying oxide surface is suitably cleaned and wetted. The results of the OTS coatings on the variously treated wafers are also shown in Table 1. Hence, we concluded that 5 min treatment with the piranha solution was the mildest oxidation (i.e., that formed the thinnest oxide layer) that provided a reasonable coverage of the silicon by a uniform hydrophilic oxide layer (on which MTU could be self-assembled). Figure 2a presents a typical AFM image of the silicon wafer area after cleaning with organic solvents, before etching. The average height (AH), the standard deviation from the AH (SD), and the roughness factor (RF) of the surface imaged are also provided (listed on the left side of the image). A typical profile of the surface appears in Figure 2b, and the lateral force imaging (which reflects friction forces in addition to topography14) is presented in Figure 2c. Similar AFM and LFM data of the silicon surfaces after etching with the HF solution for 2 min, after etching for 10 min, and after regrowing a thin oxide film by treating the Si wafers with the piranha solution (80 °C, 5 min) are presented in Figures 3-5, respectively. As clearly seen from the images in Figures 2-5, the pristine Si surfaces, before etching, contain morphological features (Figure 2), probably organic contaminants, which are removed by etching (Figures 2 and 3). It is evident from these images that etching for 10 min has a pronounced impact on the surface morphology (compare the images in Figure 4 with those in Figures 2 and 3). This correlates well with the literature data,11,12 according to which treating Si surfaces with a concentrated HF solution for 10 min completely removes the pristine oxide layer. Further regrowing of the surface oxide layer by a mild oxidation (piranha solution, 80 °C, 5 min, Figure 5) seems to make the surface smoother and more uniform. This is seen both visually and by comparing the standard deviation from the average height, which is smaller for the mildly oxidized surfaces. The profiles presented in Figures 4b and 5b also reflect the smoothing effect of regrowing the oxide. The lateral force imaging, Figures 4c and 5c, also reflects the formation of chemically uniform surfaces in both the etching and the oxidation processes. 2. Coating and Characterization of Methyl Ester Terminated SAM. Mildly oxidized silicon surfaces were coated by MTU, followed by wettability, FTIR spectroscopy, and AFM/LFM measurements. The advancing (13) Cohen, Y.; Aurbach, D. Rev. Sci. Instrum. 1999, 70, 4668. (14) ) (a) Marti, O.; Amrein, M. STM and SFM in Biology; Academic Press Inc.: San Diego, CA, 1993; pp 76-78, 82-83. (b) Marti, O.; Colehero, J.; Mlynek, J. Nanotechnology 1990, 1, 141.

Figure 6. An FTIR spectrum (external reflectance) obtained from a doped silicon surface coated by a SAM of MTU.

contact angle measured for the coated Si surfaces was around 75°, compared with 73° previously reported3b and compared with ∼70° for Adv CA measured for Si surfaces coated by similar ester monomers (based on chains of 1722 carbons).4,15 These measurements reflect the relatively hydrophobic nature of the surface after the coating, because of the formation of a layer of methyl ester edge groups (compared with Adv CA of 0° for the pristine, mildly oxidized surface). Figure 6 shows a typical FTIR spectrum of the coated surface (external reflectance mode). In addition to the IR bands between 1500 and 800 cm-1 which relate to the silicon surface and the oxide layer on it, the ester carbonyl peak (νCdO) appears at 1738 cm-1 and methyl and methylene νC-H peaks appear at 2961 and 2854 cm-1, respectively. The pronounced νC-H peak at 2924 cm-1 belongs to both groups.16 Figure 7 shows the AFM image, a typical profile, and the LFM image of the MTUcoated Si surface (a-c, respectively). The morphological picture obtained from the AFM measurements suggests a relatively uniform coating despite the low degree of crystallinity indicated by the IR data (i.e., a crystalline, well-ordered film would have signals at lower values than the 2924 and 2854 cm-1 reported herein). The LFM image of the coated surface (Figure 7c) shows dark spots suggesting the presence of relatively hydrophobic areas. These may result from local disorder in the MTU coating wherein there is a larger presence of the more hydrophobic methylene chains on the surface. 3. Effect of the Electrochemical Treatment on the Coated Surfaces. Figure 8 shows a typical CV obtained when the coated silicon wafers were used as working electrodes in the cell described in Figure 1. A typical voltammogram of a doped (cleaned) Si electrode before coating by the SAM, measured at the same conditions, is also presented in the inset of Figure 8. The voltammograms of the uncoated doped Si electrodes are characterized by nearly zero currents at potentials above -1.3 V (SCE). Below -1.3 V, cathodic currents are measured. They are attributed to the reduction of protons to hydrogen. The voltammograms of the coated electrodes are characterized by nearly zero current at a potential above -0.6 V (vs (15) Pomerantz, M.; Segmuller, A.; Netzer, L.; Sagiv, J. J. Thin Solid Films 1985, 132, 153-162. (16) Silverstein, R. M.; Bassler, C. G.; Morril, T. C. Spectrometric Identification of Organic Compounds, 5th ed.; J. Wiley & Sons: New York, 1991; Chapter 3. (17) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications; J. Wiley & Sons: New York, 1980; Chapter 12.


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Figure 7. Same as Figures 2-5. An AFM image, surface profile, and LFM image (a-c, respectively) of a silicon surface coated by a SAM of MTU.

SCE). Two corresponding cathodic and anodic peaks appear at potentials around -1 V. These peaks, which are absent in the CV of the uncoated electrodes, very much resemble the CV of adsorption processes.17 The charge involved in each of these processes is around 30 µC/cm2. At potentials below -1.1 V (vs SCE), a cathodic current is measured. It is very significant that the cathodic currents measured with the coated electrodes at potentials

below -1.2 V (vs SCE) are 1 order of magnitude higher than those measured at the same potential with the coated electrodes (Figure 8). This means that in the absence of a coating our doped silicon electrodes are repassivated by thin oxide films in the acidic aqueous solutions. In contrast, the coated silicon electrodes show much more pronounced electrochemical activity. Hence, we conclude that the coating stabilizes the thin oxide layer (that was grown after cleaning with HF) and prevents its further growth.

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Figure 8. A typical cyclic voltammogram of a silicon surface coated by MTU in 1 M H2SO4 solution, in the cell described in Figure 1. The potential scan rate was 40 mV/s. A typical voltammogram of an uncoated doped (and cleaned) Si electrode is also presented as an inset in the figure.

This passivation inhibition by the MTU coating seems to make the coated electrodes much more electroactive than the uncoated ones. Because ester electroreduction in an aqueous acid medium should not be a reversible process, we suggest that the reversible processes seen in the voltammogram of Figure 8 around -1 V are proton reduction to adsorbed hydrogen and the corresponding reoxidation of the adsorbed hydrogen back to protons (during the anodic scan). This process may be influenced by the MTU coating. The negative currents measured below -1.2 V with the coated electrodes may relate both to some reduction of the MTU layer and to reduction of protons to hydrogen. Below -1.5 V, there is a pronounced increase in the cathodic current due to H+ reduction to H2 (hydrogen evolution was observed). Hence, we could not work at potentials below -1.5 V versus SCE because of interference from vigorous hydrogen formation. However, in light of reports9 that esters can be reduced in acidic solution at potentials greater than -1 V versus SCE we assume that polarization of our doped silicon electrodes to -1.5 V should be adequate to reduce the ester groups anchored to the SAM surface. The electrochemical processes, after which the electrodes were characterized by contact angle measurements, FTIR spectroscopy, and AFM, included polarization of the electrode by linear potential scanning (40 mV/s) from OCV (≈ 0 V vs SCE) to -1.5 V. When the potential applied reached this limit, the electrode was disconnected from the cell, washed, and dried. The contact angle measurements after the electrochemical process reflect some changes. The surface becomes less hydrophobic, depending on the solvents with which the surfaces were washed after the electrochemical process. Washing by polar solvents (H2O, ethanol) led to CA Adv around 50°, and using chloroform as the washing solvent led to CA Adv around 65°, compared with CA Adv around 75° for the pristine coated surface. Figure 9 shows a typical FTIR spectrum measured from an electrochemically treated MTU-coated silicon surface. Except for a low, broad peak centered around 3330 cm-1, which can represent surface hydroxyl groups, the spectra thus obtained are similar to those measured for the pristine coated surface (compare Figures 9 and 6 above). This suggests that if any reduction of the coating took place it was, at best, partial. By use of the silicon peaks as internal standards (e.g., the pronounced peak around 1107 cm-1) and calculation of the relative intensities of the νC-H and νCdO peaks compared with the Si peaks before and after the electrochemical treatment, it appears that the intensities of the peaks around 1738

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Figure 9. An FTIR spectrum (external reflectance mode) obtained from a doped silicon surface coated by a SAM of MTU after a single electrochemical polarization to -1.5 V (vs SCE). The potential was scanned from OCV (0 V vs SCE) to -1.5 V at 40 mV/s. When this low potential was reached, the electrode was disconnected from the cell, washed, and dried.

cm-1 (νCdO), 2853 cm-1 (νCH2), 2924 cm-1 (νCH2, νCH3), and 2960 cm-1 (νCH3) decreased by ∼10, 20, 6, and 18%, respectively, because of the electrochemical treatment. From the FTIR data, it is not clear whether possible hydrophilic edge groups such as -OH (as evident from both the IR peak around 3330 cm-1 and the LFM image) are due to reduction of the ester (COOR) to an alcohol (CH2OH) or possible electrochemically induced ester cleavage to form acid (COOH) groups. There is also a pronounced decrease in the intensity of the methylene νC-H peaks, so it is possible that the electrochemical treatment removed coating molecules, thus leaving SiOH edge groups on the surface. Figure 10 provides the AFM image, the typical profile, and the LFM image of coated Si surfaces after electrochemical treatment (Figure 10a-c, respectively). Both the AFM image and a typical profile of the surface (Figure 10a,b) reflect a uniform morphology, which is similar to that of the pristine MTU-coated surface (see Figure 7 above). However, the LFM images of these surfaces show a pronounced change in the chemical composition of the surface as a result of the electrochemical treatment (i.e., compare Figure 10c with Figure 7c). Dark and bright spots appear on the LFM picture, reflecting a compositionally nonuniform surface. The bright spots, which indicate local high friction forces between surface groups and the AFM tips, may belong to surface species that contain OH groups. These should interact attractively with the AFM tip. The dark spots in the LFM image, which indicate lower local friction forces, show that in parallel to a possible partial reduction process of the MTU coating that increased the hydrophilicity of the surface, there were also changes that led to a local increase in the surface hydrophobicity on a submicroscopic scale (e.g., changes in chain conformation and/or packing order). In control experiments, MTU-coated wafers were immersed for different periods (from 30 min to 24 h) in a 1 M H2SO4 solution, followed by AFM and LFM imaging, to verify that the changes discussed above (Figure 10c) are indeed due to the electrochemical process and not to exposure to the acidic solution. Figure 11 presents AFM and LFM images (parts a and b, respectively) of MTUcoated Si surfaces after storage in 1 M H2SO4 solution for 24 h. The LFM image of this surface does not show any of the features that characterize LFM images of electro-


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Figure 10. Same as Figures 2-5 and 7. An AFM image, surface profile, and an LFM image (a-c, respectively) of a Si surface coated with MTU after electrochemical treatment.

chemically treated MTU-coated silicon surfaces (compare Figures 10c and 11b). Comparing parts a and b of Figure 11 confirms that the few differences seen in the image in Figure 11b relate to larger scale topography (e.g., occasional scratches) and not to differences in surface chemistry (as reflected in Figure 10). Hence, we conclude that the LFM imaging described above (Figure 10c) reflects changes in surface chemistry of the coating resulting from the electrochemical treatment.

Discussion and Conclusions The objective of this study was to examine the possibility of electrochemical reduction of ester functional groups of siloxane-anchored self-assembled monolayers on conducting silicon. We attempted to achieve electrochemical reduction in a system that had been successfully reduced on a monolayer surface4,15 using either LiAlH4 or BH3, standard chemical reducing agents. It appears that even

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Figure 11. AFM and LFM images (a and b, respectively) of a MTU-coated Si surface, after being immersed for 24 h in a 1 M H2SO4 solution.

the relatively high electrical conductivity of heavily N-doped silicon wafers is not sufficient to allow their use as electrodes in the face of a silicon oxide overlayer. To obtain any electrochemical activity on the silicon wafers, a balance had to be achieved. The oxide film on the silicon is electrically insulating, but its complete removal eliminated the possibility of self-assembly by siloxane-anchoring chemistry. Hence, a compromise was sought between minimizing the oxide in order to allow electrochemical activity and achieving an ample oxide to obtain SAM formation. The net result seems to be minimal electrochemical activity. A priori, the remaining oxide layer (necessary for the self-assembly process) and the 12-carbon chain of the MTU may each inhibit electrochemical processes at the electrode-solution interface (i.e., at the functional group of the coating). However, the electrochemical response of these electrodes seen in Figure 8 shows that the MTU coating seems to inhibit passivation of the Si electrodes and to allow electrochemical processes of the coated

electrodes (above). The reversible processes around -1 V seen in Figure 8 can be attributed to proton reduction to adsorbed hydrogen and its reoxidation, suggesting that the MTU coating does not preclude electron transfer. We polarized N-doped silicon electrodes in 1 M H2SO4/H2O solutions to potentials as low as -1.5 V versus SCE, conditions that should provide the ester monolayer with an appropriate reducing environment. It seems that the monolayer is only partially reduced because of intrinsic problems of the oxide-covered Si electrode. From the contact angle measurements and the scanning probe microscopy (SPM) imaging, it is clear that both removing the oxide and regrowing the thin oxide layer result in highly uniform surfaces. It was impossible to isolate a specific reduction process of the monolayer in the electrochemical measurements. The wetting properties, FTIR spectroscopy, and the SPM measurements all indicate that a partial change in the monolayer occurred because of cathodic polarization of the coated silicon wafers. Both the CA measurements and the LFM images


showed that the surface become more hydrophilic as a result of the electrochemical process. The residual carbonyl peak in the IR argues against extensive reduction to alcohol, and the residual methyl peaks in the C-H region of the IR argue against extensive ester hydrolysis. The regions of high lateral force (bright spots in Figure 10c) indicating increased attraction between the hydrophilic tip and the surface groups, the decrease in the advancing contact angle after reduction, and the broad peak around 3330 cm-1 in the FTIR spectrum of the electrochemically treated Si surface (Figure 9) suggest that cathodic polarization of the coated Si water in the H2SO4 solution indeed led to some new surface hydroxyl groups. However, the process is not efficient and occurred only in scattered spots on the surface. We assume that the oxide layer on the silicon, although thin, is still a pronounced barrier to efficient electron transfer. Hence, the electrochemical reduction depends on electron tunneling through the oxide layer at spots in which the surface resistance is sufficiently low. The AFM images and the morphological profiles of the Si surfaces covered by the regrown oxide (Figure 5) show that despite the overall uniformity on the microscopic level there are fluctuations from the average surface height, indicating the existence of local

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spots where the oxide layer is particularly thin (and hence may afford sites for electron tunneling and, thus, ester reduction). We conclude that the use of conducting silicon wafers as electrodes on which siloxane-anchored self-assembled monolayers can be electrochemically modified is problematic. It seems that an oxide film on the silicon surface which is sufficient for the formation of uniformly bound siloxane-anchored monolayers provides excessively high surface resistance for adequate charge transfer. Hence, electrochemistry seems not to be an appropriate tool for the modification of the surface functionality of a siloxaneanchored SAM. Attaching SAMs directly to the reduced silicon6,7 is more likely to lead to workable in situ electrochemistry. Acknowledgment. This work was partially supported by the German Ministry of Science (BMBF) under the DIP Program for cooperation between Israeli and German scientists and by the Israeli Ministry of Science and Technology within the framework of infrastructure research. We also gratefully acknowledge the input and insight of one of the referees of this paper. LA0006768

Highly Doped Silicon Electrodes for the ... - ACS Publications

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