Electrochemical Characterization of Self-Assembled Alkylsiloxane

Nov 6, 2000 - electrochemical properties of the OTMS monolayers were quantitatively analyzed in terms of resistance, dielectric thickness, diffusion c...
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J. Phys. Chem. B 2001, 105, 4270-4276

Electrochemical Characterization of Self-Assembled Alkylsiloxane Monolayers on Indium-Tin Oxide (ITO) Semiconductor Electrodes Heiko Hillebrandt and Motomu Tanaka* Physik Department, E22, Lehrstuhl fu¨ r Biophysik, Technische UniVersita¨ t Mu¨ nchen, James Franck Strasse, 85748 Garching, Germany ReceiVed: NoVember 6, 2000

Self-assembled monolayers (SAMs) of octyltrimethoxysilane (OTMS) were deposited onto indium-tin oxide (ITO) electrode surfaces. Contact angle measurements evidenced the hydrophobicity and homogeneity of the functionalized surface despite the intrinsic surface roughness of the polycrystalline ITO electrodes. The electrochemical properties of the OTMS monolayers were quantitatively analyzed in terms of resistance, dielectric thickness, diffusion constant, and defect area by cyclic voltammetry and impedance spectroscopy. It has been demonstrated that the alkylsiloxane monolayer acts as a diffusion barrier for ions in the electrolyte. Furthermore, a significant suppression of the charge transfer at the interface was observed, demonstrating the passivation effect of the monolayers against electrochemistry. In addition, the effect of alkyl chain length on the electrochemical properties was briefly analyzed by using octadecyltrimethoxysilane (ODTMS). The defect area of this self-assembled monolayer was reduced to 0.2%.

Introduction Modification of semiconductor devices with bioorganic molecular assemblies allows versatile applications toward biosensorics.1,2 Despite the rapid development in semiconductor devices, realistic applications of semiconductors in biological fields have been impeded due to the undesired nature of each material. For example, organic layers deposited onto siliconsilicon dioxide surfaces may not be detected separately from the background signals, when the capacitive contibution from the oxide layer is dominant. On the other hand, the surface of gallium arsenide (GaAs) is easily degraded under aqueous buffer conditions, which requires the high-quality electrochemical passivation of the surfaces.3 Among various semiconductor electrodes, indium-tin oxide (ITO) is a very promising material for the characterization of biological systems. ITO surfaces are stable under physiological conditions because of their polarizable properties,4,5 maintaining high sensitivity without insulating oxide layers. Furthermore, ITO is transparent to visible light, which enables multiple parameter measurements by using optical and electrical techniques. Indeed, such properties allow various technical applications such as solar cells,6 light-emitting diodes,7 and optical waveguides.8 However, there have been only a few reports on the study of biological processes on ITO surfaces.9,10 To fabricate biocompatible and functional interlayers between biomaterials and solids, organic self-assembled monolayers (SAMs) are suitable both to render the surface hydrophobic for the later modification and to form very thin insulating layers that prevent leak currents, unspecific adsorption, and the surface decomposition in aqueous electrolytes.11 To date, the most intensively studied systems along this strategy are SAMs of alkanethiols on gold electrodes, which is mainly due to the chemical stability of the interfaces. The investigations of blocking efficiencies of the SAMs against heterogeneous * Corresponding author. Phone: 0049-89-28912539. Fax: 0049-8928912469. E-mail: [email protected].

electron transfer and ion penetration enable one to determine the surface coverage quantitatively.12-17 Another class of SAMs widely used is based on alkylsiloxane monolayers on different oxide surfaces (e.g., SiO2, Al2O3, SnO2, TiO2).18,19 A variety of characterization techniques have been applied to characterize the monolayer structures, such as contact angle measurements,20,21 X-ray reflectivity, or ellipsometry.22 In addition, these molecules can provide self-assembled monolayers with different functional groups.23 There have been only a few reports concerning the electrical properties of SAMs on semiconductors. One example is the electrical characterization of alkyltrichlorosilane monolayers on Si/SiO2, discussed in terms of the suppresion of charge-carrier tunneling.24,25 Nevertheless, systematic studies under physiological conditions (in aqueous electrolytes, near neutral pH) are still missing. Previously, we reported the successful fabrication of polymer/ lipid composites on ITO electrodes. As the first step of the functionalization of the semiconductor, we deposited SAMs of octadecyltrichlorosilane (OTS).26 However, the electrical properties of OTS monolayers were not observed in buffered electrolyte. The surface coverage of these SAMs was measured in terms of the decrease in the active electrode area, yielding a surface defect area ratio of 0.9%. In this study, SAMs of alkylsilanes with trimethoxy coupling groups (octyltrimethoxysilane, OTMS) have been deposited onto ITO surfaces. The hydrophobicity and homogeneity of the surfaces are discussed by measuring contact angles between the ITO electrodes and water droplets before and after the monolayer deposition. The electrochemical properties of the SAMs (e.g., resistance, capacitance, defect area ratio, ion penetration) were measured quantitatively by using cyclic voltammetry and impedance spectroscopy. The voltammograms proved the polarizable properties of ITO electrodes in buffered electrolyte over a large potential range. In addition, cyclic voltammetry in the presence of redox couples demonstarted the passivation effect of the SAMs against surface electrochemistry. The

10.1021/jp004062n CCC: $20.00 © 2001 American Chemical Society Published on Web 04/12/2001

Self-Assembled Alkylsiloxane Monolayers impedance spectrum of the SAMs exhibited an obvious difference compared to that of the pure electrode in the frequency region between 1 kHz and 1 Hz. We suppose that the monolayer behaves as a diffusion barrier for ions in the electrolyte. The effects of alkyl chain length on the electrochemical properties were also studied using a trimethoxysilane with a longer chain (octadecyltrimethoxysilane, ODTMS). The impedance measurements in the presence of redox couples yielded a defect area ratio of as low as 0.2% for the ODTMS monolayer. Therefore, using trimethoxysilane derivatives instead of trichlorosilane derivatives26 reduced the defect area ratio by nearly a factor of 5. In addition, the reproducibility of the preparation was remarkably increased. Experimental Section Chemicals. Octyltrimethoxysilane (OTMS) was purchased from ABCR (Karlsruhe, Germany). All other chemicals were purchased from Fluka (Neu Ulm, Germany) and were used without further purification. Ultrapure water was used throughout this study (Millipore Milli-Q-Systems, Molsheim, France; R > 18 MΩ cm-1, pH ) 5.5). The standard electrolyte was degassed 10 mM Hepes (4-(2-hydroxyethyl)piperazine-1ethansulfonic acid) titrated with NaOH to pH ) 7.5. For the study of electron transfer at the semiconductor interface, 1 mM K3/4Fe(CN)6 was added to the standard electrolyte. ITO Electrodes. Glass slides coated with 110 nm thick ITO were purchased from Balzers (Balzers, Lichtenstein). The lithographic preparation process and the geometry of the electrode arrays used in this study had been described elsewhere.26 First, the etched substrates were cleaned with acetone and ethanol. Then they were immersed into a solution of 1:1:5 (v/v) H2O2 (30%)/NH4OH (30%)/H2O for 5 min under ultrasonication and soaked another 30 min at 60 °C. Finally, they were rinsed 10 times with water. Before the silanization, the samples were thoroughly dried in a vacuum chamber. Silanization. The coupling reaction of the silanes was achieved by a 60 min sonication of the substrates in a 5 vol % solution of the silane in dry toluene (packaged with molecular shieves, water content < 0.005%) using n-butylamine (0.5 vol %) as a catalyst.27 Then, the samples were incubated for another 30 min in the same solution. After the coupling reaction the substrates were sonicated in pure toluene for 2 min to remove the physisorbed silanes from the surface. To optimize the quality of the monolayers, the temperature during the deposition was kept below T < 293 K for ODTMS and T < 280 K for OTMS. These temperature conditions were chosen according to the threshold temperatures for the formation of ordered alkyl chain packing reported previously.21,28 Characterization Methods. Contact angles of water droplets were measured using a self-developed goniometer apparatus coupled to a CCD camera (aqua TV HR480, Kempten, Germany). Measurements were carried out in an ambient atmosphere at room temperature. Sessile drops (volume ∼ 10 µL) for the static contact angles were placed on the substrates by a micropipet. The advancing contact angles were determined during the continuous growth of the droplet using a micropipet, while the receding angles were measured during the volume reduction by suction. The overall accuracy throughout the contact angle measurements was within the range of (3°. Cyclic voltammetry experiments were carried out with a conventional electrochemical setup (VoltaLab 40, RadiometerAnalytical, Lyon, France). The three-electrode setup used in this study consists of an ITO electrode as working electrode, a Au counter electrode, and a Ag/AgCl reference electrode

J. Phys. Chem. B, Vol. 105, No. 19, 2001 4271 (XR110, Radiometer-Analytical, Lyon, France). The bias potential was varied between -500 and +500 mV with respect to the reference electrode. Voltammograms were measured with a constant sweep rate of 25 mV/s. Five complete cycles were monitored for each sample in order to verify the electrochemical stability of the interface. Two different setups were used for impedance spectroscopy. The impedance spectra without bias potential were measured using an impedance analyzer (SI 1260, Schlumberger, England), where the spectral data were collected using a series connection of two identical electrodes (A ) 0.09 cm2) immersed in degassed buffer. Because of the polarizable properties of ITO, a reference electrode was not required for Ubias ) 0 mV. In the case of the impedance spectra for the Mott-Schottky analysis, where different bias potentials are nessesary, the three-electrode setup of the cyclic voltammetry was applied. All impedance measurements were performed using sinusoidal voltages with an amplitude of 50 mV and frequencies in the range between 50 kHz and 90 mHz. The amplitude of the applied voltage is small enough, so that linear response of the system can be preserved. The current response of the system was measured in terms of the absolute value of the complex impedance |Z| and the phase shift φ. For the presentation of the data, we chose Bode plots of |Z| and φ as a function of frequency. The spectra were analyzed in terms of equivalent circuits consisting of simple electrical elements, such as resistances, capacitances, and diffusion elements. In addition, constant phase elements (CPE),29,30 were used to account for nonlinearities and inhomogeneities. Details of our impedance procedures were introduced elsewhere.5,26 The Mott-Schottky plots to evaluate the flatband potential of the semiconductor space charge region were obtained by measuring impedance spectra from 50 kHz to 90 mHz at different bias potentials (potential steps of 100 mV for Ubias between -500 mV and +500 mV). By applying equivalent circuits, the capacitance of the semiconductor space charge region before and after the monolayer deposition as a function of bias potential was estimated. Results and Discussion I. Surface Properties. The hydrophobicity and the homogeneity of the silanized ITO electrodes were examined by contact angle measurements with water in air. The static contact angle of bare, freshly cleaned ITO was θst(ITO) ) 32°. This relatively high value is attributed to the intrinsic surface roughness of the polycrystalline ITO. The mean square roughness of the ITO surface was determined by atomic force microscopy (AFM), Rms ) 2.5 nm. Several mechanical and wetchemical treatments were attempted to reduce this intrinsic roughness, but no relevant changes were observed (unpublished results). Indeed, the advancing contact angle, θad(ITO), varied between 30° and 50°, because the movement of the wetting front was disturbed by a large amount of pinning centers. The receding angle was about θre(ITO) ) 5°. Such a significant hysteresis between advancing and receding contact angle, θad(ITO) - θre(ITO) ≈ 35°, can be phenomenologically interpreted due to the presence of chemical and/or phycical heterogeneity.31,32 After the OTMS monolayer deposition, the static contact angle increased to θst(OTMS) ) 95°.This value is only slightly smaller than contact angles reported for OTS monolayers on silicon dioxide surfaces.21,33 The advancing contact angle increased to θad(OTMS) ) 104°, while the receding angle was θre(OTMS) ) 84°. This hysteresis corresponds to the indicative quantity, cos θad(OTMS) - cos θre(OTMS) ) 0.3. This value

4272 J. Phys. Chem. B, Vol. 105, No. 19, 2001

Figure 1. Cyclic voltammogramm of ITO electrodes at a scan rate of 25 mV/s. Solid line, bare ITO in 10 mM Hepes buffer (pH ) 7.5); broken line, bare ITO in 10 mM Hepes buffer containing 1 mM K3/4Fe(CN)6 (pH ) 7.5); dotted line, ITO coated with OTMS monolayer in redox buffer (pH ) 7.5). The voltage was applied versus a Ag/ AgCl reference electrode.

is still in a plausible range of those previously reported,20,34 but approximately 1 order of magnitude larger than that for SAMs of the highest quality.21,35 However, the reduced hysteresis suggests that the chemical and/or physical heterogeneity of the ITO surface was remarkably “healed” by the chemically robust alkoxysilane monolayers. II. Electrochemical Characterization. Cyclic Voltammetry. The cyclic voltammograms of an ITO electrode in buffer containing redox couple before (broken line) and after the deposition with OTMS (dotted line) are presented in Figure 1. For comparison, a voltammogram of a bare ITO electrode in standard buffer is also included (solid line in Figure 1). The voltage was applied versus a Ag/AgCl reference electrode with a scan rate of 25 mV/s for all curves. The bare ITO electrode behaved as a polarizable electrode in Hepes buffer for -500 mV e Ubias e +500 mV. The maxium current observed was only |Imax| ) 3 µA cm-2. Actually, it had been reported that the decomposition of ITO only occurs at potentials larger than (0.8 V.4,5,10 In the presence of 1 mM redox couple, the voltammogram of the bare ITO electrode exhibited the typical shape for diffusion-limited electron-transfer processes (broken line in Figure 1).36 The observed voltammogram shape with current peaks occurs for the electrolysis rate larger than the diffusion rate. If the diffusion rate of the redox species toward the electrode is too small, their concentration close to the surface will be reduced due to the electrochemical reaction. This induces a decrease in electrolysis current after reaching a local maximum. The reduction and the oxidation peak currents were about |Imax| ) 150 ( 10 µA cm-2 with a peak separation of 80 mV. This is a characteristic value for a reversible one-electrontransfer process with linear diffusion.13,15 After the ITO electrode was coated with a monolayer of OTMS, the global shape of the voltammogram trace (dotted line in Figure 1) was significantly changed. The Faradaic current was reduced by nearly 1 order of magnitude to |Imax| ) 20 µA cm-2, and no oxidation or reduction peaks were detectable. This change in current response is consistent with a high coverage of the electrode surface by the OTMS monolayer, revealing only a small amount of defects that can serve as active areas for electrochemistry. The voltammorgams were independent from scan direction, suggesting that the charge transfer was now kinetically controlled at steady state and that there was no significant depletion of the redox species in the diffusion

Hillebrandt and Tanaka

Figure 2. Absolute impedance and phase shift of the ITO electrodes before (4) and after (2) the deposition of the OTMS monolayer in 10 mM Hepes buffer (pH ) 7.5). The symbols represent the measured data, while the lines correspond to the fits using the equivalent circuit models from Figure 3. The bare ITO surface could be analyzed by the equivalent circuit I (solid lines). Fitting of the spectrum after the silanization with the ideal equivalent circuit II from Figure 3 shows clear deviations from the measured data (solid lines). However, by introducing a Randles circuit for the monolayer (equivalent circuit III in Figure 3), the fit could be improved (dotted lines).

layer.12,13,37,38 The electrolysis rate was reduced to the range of the diffusion rate due to the much smaller active electrode area. For octadecylthiol monolayers on gold electrodes, it had been demonstrated that no plateau or peak currents were apparent in cyclic voltammograms, when the defect area ratio of the monolayer was extremely small (0.6%), peak currents can be observed in the voltammogramms, which could be explained in terms of isolated microelectrodes.15,16 Nevertheless, the electrode reactions can still be controlled by linear diffusion, if the distance of these microdefects is small enough for a total overlap of their diffusion profiles.39,40 A quantitative estimation of the integrity of the SAM out of the cyclic voltammetry data is not valid, if the observed charge transfer is controlled by linear diffusion.15,39 However, ac impedance spectroscopy is expected to be more sensitive for the estimation of such small defect ratios, because it can be performed without bias potentials and the ac amplitude can be very small. Indeed, large overpotentials across the interface may affect the monolayer conformation and increase the tunneling probability through the monolayer.16 Impedance Spectroscopy. The impedance spectra of ITO electrodes before (4) and after the OTMS deposition (2) in a standard Hepes buffer are presented in Figure 2. The impedance spectrum of the bare ITO electrode could be interpreted by the equivalent circuit I in Figure 3. The serial resistance, R0 ) 9 × 103 Ω, corresponds to the ohmic behavior of the electrolyte and the contacts in the high-frequency regime. The capacitance, CIF ) 8.0 × 10-6 F cm-2, represents the capacitive contribution from the ITO/electrolyte interface. This capacitance is dominated by the capacitance of the semiconductor space charge region. The contribution from the electrochemical double layer can be neglected, since it is about 1 × 10-4 F cm-2 in our experimental system. The capacitive contribution from the surface states has a minor effect for impedance spectroscopy as long as the bias potential is far away from the flatband potential UFB.41 To analyze the impedance of ITO electrodes after the deposition of OTMS monolayers (2 in Figure 2), we first introduced an additional RC element to the equivalent circuit,

Self-Assembled Alkylsiloxane Monolayers

J. Phys. Chem. B, Vol. 105, No. 19, 2001 4273 al.12 reported that semiinifinte linear diffusion can still be assumed even for a thin monolayer with distributed pinhole defects, if their average distance is small enough for an overlap of their spherical diffusion layers. Indeed, the fitting curve from this approach (dotted lines in Figure 2) represents the measured impedance data very well. The phase transfer resistance RPT, which corresponds to the interface resistance between the electrolyte and the SAM, was 3.3 × 103 Ω cm2. The electrolyte SAM/electrolyte interface capacitance was CS ) 1.8 × 10-5 F cm-2. The Warburg impedance W(ω)36 can be represented by

W(ω) ) (σ + 1/σ)ω-1/2 Figure 3. Summary of the equivalent circuits used in this study. Equivalent elements: Ro, electrolyte resistance; CIF, semiconductor/ electrolyte interface capacitance; RS, SAM resistance; CS, SAM capacitance; CPE, constant phase element; RPT, phase-transfer resistance between SAM and electrolyte; ZW, Warburg impedance; Rct, chargetransfer resistance.

assuming the dielectric properties of the SAM (circuit II in Figure 3). The obtained electric parameters of the SAM were CS ) 4.0 × 10-6 F cm-2 for the capacitance and RS ) 9 × 102 Ω cm2 for the resistance. However, the quality of the fit using this model (solid lines in Figure 2) was quite poor in the frequency region between 1 kHz and 1 Hz, this being especially obvious for the phase shift plot. Indeed, the obtained capacitance CS yielded a dielectric thickness of around d ) 6 Å by assuming a dielectric constant of fluid alkyl chains,  ) 2.0-2.3.42,43 This value is unreasonably lower compared to the reported ellipsometric thickness for octyltrichlorosilane monolayers on oxidized silicon wafers, d ) 13-14 Å.20 Even though the contribution of the Si-O bonds on the dielectric properties is not well understood and different chain conformations are possible, this discrepancy is certainly too large. These results strongly suggest that the interpretation of the SAM as an ideal, dielectric layer (represented by a parallel connection of a resistance and a capacitance) is not valid. Therefore, we replaced the ideal capacitance CS in circuit II (Figure 3) by a constant phase element in a second approach, to account for nonlinearities. This approach yielded a CPE frequency exponent of R ) 0.67. The deviation from the theoretical value for a capacitance, R ) 1, strongly indicates that the SAM does not behave as a dielectric layer. Actually, this frequency exponent obtained here suggests that the monolayer behaves more like a diffusion element, where the theoretically predicted frequency exponent is 0.5.36 Thus, the equivalent circuit III in Figure 3 was introduced in order to express the SAM as a diffusion barrier. In this approach, the SAM is represented by the Randles circuit44 with a phase transfer resistance, RPT; an interface capacitance, CS; and a Warburg element, W.36 The last element describes the diffusion normal to the electrode surface through the monolayer. Originally, this circuit was introduced to describe the electron transfer from an electrolyte to an electrode.44 However, this model circuit can be also applied for the diffusion of metal into an oxide electrode, if the diffusion is driven by a gradient in the composition and not by an electric field.45 Moreover, it should be noted that the application of the Warburg element for the diffusion through a layer with a defined thickness is only valid as an approximation, since this element normally discribes linear diffusion in a semiinfinite space. Although there had been some equivalent circuit models taking into account the defined geometry of practical systems,45,46 we decided to use the equivalent circuit III in Figure 3 to explain the results in this study. Finklea et

(1)

where ω is the frequency and σ the Warburg parameter

σ)

1 x2n F AG xD 4RT

2 2

(2)

A is the active electrode area, D is the diffusion constant of the ions, and F is their concentration at the interface. The constants R, T, n, and F have their usual meanings. If one assumes that the concentration of the ions at the interface is about equal to that in the bulk solution, a diffusion constant of Dbuffer(OTMS) ∼ 1 × 10-2 µm2/s can be derived from the obtained Warburg parameter, σ ) 8.7 × 104 V/A s1/2. Compared to the diffusion constant of ions in aqueous electrolytes of D ∼ 1 × 103 µm2/s, the calculated value seems to be quite small. On the other hand, a diffusion constant of 2 × 10-4 µm2/s has been reported for the diffusion of lithium ions in tungsten trioxide films.45 Unfortunately, to our knowledge, there have been no studies concerning the diffusion of aqueous ions through SAMs without any electroactive species. Nevertheless, the estimated value for Dbuffer(OTMS) suggests phenomenologically that the SAM behaves as a diffusion barrier against aqueous ions. This is in contrast to our previous study,26 where the deposition of an OTS monolayer did not change the electrical properties of ITO electrodes in a 10 mM Hepes buffer. One reason for this phenomenological difference could be that the OTMS monolayer has less defect area and a higher packing density than the OTS monolayer, despite the shorter chain length. To estimate the integrity of the OTMS monolayer more quantitatively, we performed impedance spectroscopy using a buffer containing 1 mM K3/4Fe(CN)6. The impedance spectra of an ITO electrode before (4) and after the deposition of the OTMS monolayer (2) are presented in Figure 4. The impedance of the bare ITO electrode in contact with the redox couple can be interpreted by the equivalent circuit IV in Figure 3. The nonlinear Faraday impedance, given by a series connection of a charge-transfer resistance Rct and a Warburg impedance W, describes the charge transfer at the semiconductor/electrolyte interface due to the redox couple. From the Warburg parameter σ ) 2.3 × 103 V/A s1/2 the diffusion constant of the redox species was calculated, Dredox(ITO) ) 1.1 × 103 µm2/s, which is in the same region as values reported for diffusion constants of redox species to gold surfaces.15,16 The charge-transfer resistance at the bare electrode was Rcto ) 110 Ω cm2. After the surface was covered with the SAM, the system was represented by the equivalent circuit V in Figure 3, where an additional RC element was used. The Randles circuit describes the electron-transfer reaction at active areas of the electrode, while the RC element represents the complete blocking of electrochemistry by the SAM. The deposition of the SAM led to a large increase in the charge-transfer resistance, Rct ) 1.8 × 104 Ω cm2. This increase of Rct due to the monolayer

4274 J. Phys. Chem. B, Vol. 105, No. 19, 2001

Hillebrandt and Tanaka

Figure 4. Absolute impedance and phase shift of the ITO electrodes before (4) and after (2) the deposition of the OTMS monolayer with 1 mM K3/4Fe(CN)6 in 10 mM Hepes buffer (pH ) 7.5). The symbols represent the measured data, while the solid lines show the fitting curves. The bare ITO electrodes and the electrodes covered with the SAM could be analyzed by the equivalent circuits IV and V, respectively. The deposition of the SAM led to a significant increase in the charge-transfer resistance from Rcto ) 110 Ω cm2 to Rct ) 18 kΩ cm2, yielding the local defect area ratio of 0.6%.

deposition can be related to the fraction of accessible area for the redox species,15,16 which can be described as

(1 - Θ) )

Rcto Rct

(3)

Rcto and Rct are the charge-transfer resistance before and after the deposition, respectively. From the measured values of Rcto ) 110 Ω cm2 and Rct ) 1.8 × 104 Ω cm2, the local defect area ratio of the OTMS monolayer was estimated, (1 - Θ) ) 0.6%. The obtained value is less than our previous result for the OTS monolayer, (1 - Θ) ) 0.9%.26 Considering the intrinsic roughness of the polycrystalline ITO, a surface coverage of 99.4% is certainly high. The resistance and the capacitance of the SAM were RS ) 4 × 103 Ω cm2 and CS ) 1.6 × 10-5 F cm-2, respectively. The Warburg parameter σ of the ITO electrode covered by the SAM was σ ) 8.7 × 103 V/A s1/2, yielding a diffusion constant of Dredox(OTMS) ) 80 µm2/s. Therefore, the deposition of the OTMS monolayer reduces the diffusion constant of the redox species by more than 1 order of magnitude compared to the bare electrode surface. Previously, a reduction of the diffusion constant of the same redox species by a factor of 3 had been reported for an aromatic thiol monolayer on gold.16 The significant reduction of the diffusion constant observed here is in good agreement with a small amount of local defect area. It is worth noting that the diffusion constant for the OTMS monolayer in contact with the standard buffer, Dbuffer(OTMS) ) 0.01 µm2/s, is much smaller than the corresponding diffusion constant with redox species, Dredox(OTMS) ) 80 µm2/s. Without any electroactive species, the observed diffusion is dominated by the penetration of ions into the SAM. However, in the presence of the redox couple, the diffusion of redox species takes place through the local pinhole defects. Therefore, it is reasonable that the diffusion through pinhole defects is much faster than the diffusion through an alkyl layer. Compared to our previous experiments with OTS,26 the deposition of the OTMS monolayer resulted not only in a higher surface coverage, but also in a much better reproducibility of the preparations. These differences can be explained by two reasons: (1) the very high reactivity of the trichlorosilane

Figure 5. Mott-Schottky plots of an ITO electrode with (2) and without (9) the OTMS monolayer. The bias potential was applied with respect to a Ag/AgCl reference electrode. The measured interface capacitance CIF of the electrode surface is dominated by the capacitance of the semiconductor space charge region CSC. The flatband potential UFB is the intercept of the extrapolated linear part of the slope with the x-axis. As a result of the monolayer deposition, UFB changed from UFB(ITO) ) - 560 mV to UFB(OTMS) ) -510 mV.

groups, which may make polymeric siloxane structures,20 and (2) decomposition of the ITO surface by a subproduct of the coupling reaction, HCl. The silanol coupling reactions of trimethoxysilanes to the surface hydroxyl groups are slower than those of trichlorosilanes, which enables one to control the reaction conditions more accurately. In addition, the coupling under ultrasonication certainly reduces the disordered physisorption to the surfaces. Moreover, the subproduct of the coupling reaction methanol does not damage the ITO surfaces. Mott-Schottky Analysis. The Mott-Schottky analysis is a procedure to detect the flatband potential UFB of semiconductor electrodes. For this purpose, one needs the capacitance of the semiconductor space charge region CSC in dependence of the applied bias potential. If 1/CSC2 is plotted against the potential and the linear part of the slope is interpolated to 1/CSC2 ) 0, the interception with the x-axis is equal to the flatband potential of the semiconductor UFB. The flatband potential of a semiconductor electrode in contact with electrolytes can be influenced by (i) formation of oxide layers, (ii) pinning of the fermi level, (iii) the pH of electrolyte, and (iv) the existence of surface states.47 A covalent coupling of the monolayer is supposed to change the distribution of surface states at the ITO surface. To investigate changes in the surface state density of the ITO electrodes due to the deposition of the OTMS monolayer, we performed a Mott-Schottky analysis of electrodes with and without the SAM. To achieve high accuracy in determining the space charge region capacitance CSC (dominating CIF in the equivalent circuits of Figure 3), we measured the impedance spectra in the frequency region from 50 kHz to 90 mHz for different bias potentials. The bias potential was varied in steps of 100 mV from -500 to +500 mV versus an Ag/AgCl reference electrode. The values for the space charge region capacitance were determined by applying the equivalent circuits I and III for the electrode before and after the SAM deposition, respectively. The results obtained for a bare ITO electrode (9) and for an electrode coated with the OTMS monolayer (2) are shown in Figure 5. For a bare electrode, we obtained a flatband potential of UFB(ITO) ) -560 mV (9 in Figure 5), which seems to be in a reasonable range of previous reports.5 After the monolayer deposition (2), the linear part of the curve was shifted toward possitive potentials with an almost identical slope. The flatband potential of UFB(OTMS) ) -510 mV was estimated from the intercept. The shift in flatband potential of ∆UFB )

Self-Assembled Alkylsiloxane Monolayers

J. Phys. Chem. B, Vol. 105, No. 19, 2001 4275

Figure 6. Absolute impedance and phase shift of the ITO electrodes before (4) and after ([) the deposition of ODTMS monolayer in 10 mM Hepes buffer (pH ) 7.5). The symbols represent the measured data, while the lines correspond to fits using the equivalent circuit models circuit I for the bare electrode and circuit III for the monolayer.

50 mV observed here strongly suggests that the surface state distribution was altered by the covalent coupling of the OTMS to the surface hydroxyl group. An electrostatic effect on the flatband potential can be excluded, because the net charge of the OTMS molecule is zero. From the slope of the linear part of the Mott-Schottky curves, the charge carrier density (electron density ND for n-type semiconductors) of the semiconductor electrodes can be determined. When the contribution from the electrochemical double layer is neglectable, the capacitance of the semiconductor space charge region is about equal to the measured capacitance of the semiconductor/electrolyte interface. According to Arden and Fromherz,48 this capacitance is given by

1/C2(U) )

U - UFB ND EE0e0 2

(4)

 is the dielectric constant of ITO,  ) 3.6,49 while the other constants have their usual meaning. The donor concentration ND can be derived from

ND )

2 0e0b

(5)

where b is the slope of the linear part of the Mott-Schottky plots, b ) 1.9 × 102 m4 F-2 V-1. The charge carrier density of the ITO electrodes used in this study was ND ) 2 × 1021 cm-3. III. Effect of Chain Length. The effect of the alkyl chain length on the electrical properties of the SAMs has been briefly examined by depositing monolayers of octadecyltrimethoxysilane (ODTMS) on ITO electrodes. The electrical properties of the ODTMS layer were studied with impedance spectroscopy in the same manner as for OTMS. The impedance spectrum of the ODTMS monolayer in the standard Hepes buffer ([ in Figure 6) was quite similar to that of the OTMS monolayer (2 in Figure 2). This is in agreement with the similar wetting behavior observed by contact angle measurements (data not shown). The analysis based on the equivalent circuit II (Figure 3) showed again a clear deviation from the measured data (curve not shown), and the CPE approach yielded a frequency exponent of only 0.56. Therefore, we applied the equivalent circuit III (Figure 3) for the interpretation of the ODTMS monolayer, too. Indeed, the corresponding fitting curve (solid lines in Figure 6)

Figure 7. Absolute impedance and phase shift of the ITO electrodes before (4) and after ([) the deposition of the ODTMS monolayer with 1 mM K3/4Fe(CN)6 in 10 mM Hepes buffer (pH ) 7.5). The measured impedance data were analyzed by the equivalent circuits IV and V. The deposition the ODTMS monolayer led to an increase in the chargetransfer resistance from Rcto ) 110 Ω cm2 to Rct ) 50 kΩ cm2, yielding the local defect area ratio of 0.2%.

represents the measured data very well. The phase transfer resistance was RPT ) 1.6 × 103 Ω cm2 and the obtained Warburg parameter σ ) 2.5 × 104 V/A s1/2 corresponds to a diffusion constant of Dbuffer(ODTMS) ) 0.1 µm2/s. This diffusion constant for the ODTMS monolayer is by a factor of 10 larger than that of the OTMS monolayer, although the diffusion constant is expected to be independent from the layer thickness. Taking into account that the Warburg element is usually applied to discribe linear diffusion in a semiinfinite space, this difference can be explained in terms of geometric effects. Therefore, the value for the thicker ODTMS layer should be the more accurate value to describe the diffusion of ions in the alkyl layer. In Figure 7, the impedance spectrum of the ODTMS monolayer in contact with redox species is presented ([). The measured data were interpreted using the equivalent circuit V (solid lines). The charge-transfer resistance obtained for this SAM, Rct ) 5 × 104 Ω cm2, demonstrated that the ODTMS monolayer could achieve an even higher surface coverage than OTMS. The suppression of the charge-transfer is related to a defect area ratio of (1 - Θ) ) 0.2%. Previously, Porter et al.13 reported that the packing density of an alkanethiol monolayer on gold was increased according to the alkyl chain length. The diffusion constant estimated from the Warburg parameter σ, Dredox(ODTMS) ) 26 µm2/s, is about a factor of 3 smaller than for the OTMS monolayer. As the diffusion of the redox species is depending on the size and the distribution of the pinhole defects,12 such a difference can be attributed to the geometric differences of the SAMs. Even a dependence of the diffusion constant on monolayer thickness is possible, since the used equivalent circuit implies semiinfinite diffusion. Summary In the present study, electrochemical characterization of selfassembled alkylsiloxane monolayers on ITO semiconductor electrodes were performed in aqueous electrolytes. The monolayers of OTMS and ODTMS on ITO electrodes behave as diffusion barriers against the access of ions in electrolytes. Such an observation is in contrast to our recent results for OTS monolayers,26 where no electric properties of the SAM were detectable. This difference can be explained by a higher grafting density of the trimethoxysilane derivatives, demonstrated by

4276 J. Phys. Chem. B, Vol. 105, No. 19, 2001 impedance spectroscopy in the presence of electroactive species. The maximum surface coverage of the SAMs amounted to 99.8%. Indeed, the cyclic voltammetry showed significant passivation effects of the OTMS monolayer against electrochemistry. The combined electrochemical studies suggested that the electrolysis current occurs via diffusion of redox species through pinhole defects in the monolayers. The Mott-Schottky analysis enabled us to detect changes in the surface state density due to the covalent coupling of the SAMs. The contact angle measurements demonstrated the fabrication of hydrophobic and homogeneous surfaces by the deposition of the monolayers, despite the intrinsic roughness of the ITO electrodes. The deposition of SAMs on semiconductor devices allows a precise engineering of the surface properties to design highly organized composite films toward biosensoric applications. Acknowledgment. The work was supported by the Deutsche Forschungs Gemeinschaft (SFB 266/SFB 563) and by the Fond der Chemischen Industrie. One of the authors (M.T.) is an Alexander von Humboldt Fellow. The authors are most grateful to Prof. Erich Sackmann for his enthusiastic support to perform this research. References and Notes (1) Sackmann, E. Science 1996, 271, 43-48. (2) Sackmann, E.; Tanaka, M. TIBTECH 2000, 18, 58-64. (3) Adlkofer, K.; Tanaka, M.; Hillebrandt, H.; Wiegand, G.; Sackmann, E.; Bolom, T.; Deutschmann, R.; Abstreiter, G. Appl. Phys. Lett. 2000, 76, 3313-3315. (4) Chyan, O. M.-R.; Rajeshwar, K. J. Electrochem. Soc. 1985, 132, 2109-2115. (5) Gritsch, S.; Nollert, P.; Ja¨hnig, F.; Sackmann, E. Langmuir 1998, 14, 3118-3125. (6) Hodes, G.; Thompson, L.; DuBow, J.; Rajeshwar, K. J. Am. Chem. Soc. 1983, 105, 324-330. (7) Ho, P. K. H.; Granstroem, M.; Friend, R. H.; Greenham, N. C. AdV. Mater. 1998, 10, 769-774. (8) Piraud, C.; E. K., M.; Yao, J.; O’Dwyer, K.; Schiffrin, D. J.; Wilkinson, J. S. J. LightwaVe Technol. 1992, 10, 693-699. (9) Fromherz, P.; Arden, W. J. Am. Chem. Soc. 1980, 102, 62116218. (10) Asanov, A. N.; Wilson, W. W.; Oldham, P. B. Anal. Chem. 1998, 70, 1156-1163. (11) Ulman, A. An introduction to ultrathin organic films: From Langmuir-Blodgett to self-assembly; Academic Press: Boston, 1991. (12) Finklea, H. O.; Snider, D. A.; Fedyk, J.; Sabatani, E.; Gafni, Y.; Rubinstein, I. Langmuir 1993, 9, 3660-3667. (13) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559-3568. (14) Miller, C.; Cuendet, P.; Gra¨tzel, M. J. Phys. Chem. 1991, 95, 877886.

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