Micropatterned Ferrocenyl Monolayers Covalently Bound to Hydrogen

Article pubs.acs.org/Langmuir

Micropatterned Ferrocenyl Monolayers Covalently Bound to Hydrogen-Terminated Silicon Surfaces: Effects of Pattern Size on the Cyclic Voltammetry and Capacitance Characteristics Bruno Fabre,*,† Sidharam P. Pujari,‡ Luc Scheres,‡,§ and Han Zuilhof*,‡,∥ †

Institut des Sciences Chimiques de Rennes, UMR 6226 CNRS/Université de Rennes 1, Matière Condensée et Systèmes Electroactifs (MaCSE), Campus de Beaulieu, 35042 Rennes Cedex, France ‡ Laboratory of Organic Chemistry, Wageningen University, Dreijenplein 8, 6703 HB Wageningen, The Netherlands § Surfix B.V., Dreijenplein 8, 6703 HB Wageningen, The Netherlands ∥ Department of Chemical and Materials Engineering, King Abdulaziz University, Jeddah, Saudi Arabia S Supporting Information *

ABSTRACT: The effect of the size of patterns of micropatterned ferrocene (Fc)-functionalized, oxide-free n-type Si(111) surfaces was systematically investigated by electrochemical methods. Microcontact printing with amine-functionalized Fc derivatives was performed on a homogeneous acid fluoride-terminated alkenyl monolayer covalently bound to ntype H-terminated Si surfaces to give Fc patterns of different sizes (5 × 5, 10 × 10, and 20 × 20 μm2), followed by backfilling with n-butylamine. These Fc-micropatterned surfaces were characterized by static water contact angle measurements, ellipsometry, X-ray photoelectron spectroscopy (XPS), infrared reflection−absorption spectroscopy (IRRAS), atomic force microscopy (AFM), and scanning electron microscopy (SEM). The charge-transfer process between the Fc-micropatterned and underlying Si interface was subsequently studied by cyclic voltammetry and capacitance. By electrochemical studies, it is evident that the smallest electroactive ferrocenyl patterns (i.e., 5 × 5 μm2 squares) show ideal surface electrochemistry, which is characterized by narrow, perfectly symmetric, and intense cyclic voltammetry and capacitance peaks. In this respect, strategies are briefly discussed to further improve the development of photoswitchable charge storage microcells using the produced redoxactive monolayers.

1. INTRODUCTION Interfacing technologically important semiconducting surfaces, such as oxide-free, hydrogen-terminated silicon (H−Si) with high-quality and stable redox-active monomolecular films has appeared as a promising strategy toward functional devices for charge storage and information processing.1,2 Two features of H−Si are of specific interest: (1) its very low density of electrically active surface defects (the so-called surface states)3 and (2) its propensity to be chemically modified with organic monolayers linked through nonpolar and robust interfacial Si− C bonds,4,5 which renders it particularly attractive for electrical applications. While the electrical properties of the Si substrate itself can also be fine tuned by covalent surface modifications,6,7 the stable attachment of reversibly electroactive moieties can actually add functionality to the Si surface. Among the electroactive molecules grafted on H−Si, ferrocene,2,8−20 metal-complexed porphyrins 1,16,21−24 and polyoxometalates25−28 have received considerable attention due to the retention of their attractive electrochemical characteristics after the immobilization step. Provided that the grafting conditions are optimized, such H−Si-confined systems exhibit electron transfer within an easily accessible and nondamaging potential window with high charge-transfer rate constants and various © 2014 American Chemical Society

chemically stable redox states. The integration of multiredox metalloporphyrins and polyoxometalates with silicon is appealing for (and relevant to) multibit information storage media, in which electrical charge is stored in the different redox states of the bound molecules. In spite of these stimulating opportunities, the principal drawback of these electroactive molecules is their relatively large size, namely, a 15−20 Å average diameter for the tetraphenylporphyrin29 and 10 to 50 Å diameters for the simple Keggin type to more sophisticated polyoxometallates.30 This results in a low surface coverage of the resulting films on H−Si (lower than 1.0 × 10−10 mol cm−2). Furthermore, if a one-step direct attachment procedure is used, then a large number of H−Si sites that are susceptible to oxidation remain after the completion of the monolayer. As a consequence, the resulting modified surfaces show a significant density of electrically active surface defects. Compared to such functional systems, ferrocene (Fc) is a much smaller molecule (average diameter of 6.6 Å31) and is immobilized on H−Si with a surface coverage higher than 3.0 × 10−10 mol cm−2, which Received: April 7, 2014 Revised: May 28, 2014 Published: June 2, 2014 7235

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Scheme 1. Micropatterned Fc-Terminated Monolayers Covalently Bound to n-Type Si(111) Surfaces

gives rise to higher charge densities.2 High-quality ferrocenyl monolayers prepared in our groups exhibited extremely fast metal-like electron communication between the electroactive headgroups.10 Interestingly, the 2D charge propagation rate could be finely tuned by diluting the electroactive chains with electrochemically inert alkenyl chains.9 Very recently, we have demonstrated that tailor-made 5 × 5 μm2-sized patterns of such redox-active monolayers deposited on n-type Si behave as lightactivated molecular memory cells with unprecedented capacitance performance, thanks to both the quality of the functional monolayers and the length of the insulating arm between Fc and the underlying surface (>1.5 nm).32 This photoinduced activity was caused by the reversible photoswitching of the electrical properties of Si from its insulating to conducting state, which enables the controlled oxidation/ reduction of bound Fc/Fc+. Following these preliminary investigations, in the current paper we present a more detailed study of different ferrocenemicropatterned Si surfaces (Scheme 1). X-ray photoelectron spectroscopy (XPS), infrared reflection−absorption spectroscopy (IRRAS), and atomic force microscopy (AFM) measurements were used to obtain a precise view of the chemical composition and the ordering of the grafted molecular chains and the identification of the produced patterns. On the basis of this characterization, an interpretation can be provided of the unprecedented observation of the effect of pattern size on the electrochemical (cyclic voltammetry and capacitance) properties of micropatterned surfaces.

dispensed, and the contact angles were determined by a Tangent 2 fitting model. The error in the determined contact angles is approximately 1°. 2.2.2. Ellipsometry. The ellipsometric thicknesses were measured with a Sentech Instruments (type SE-400) ellipsometer operating at 632.8 nm (He−Ne laser) and an angle of incidence of 70°. First the optical constants of the substrate were determined with a piece of freshly etched H−Si(111) (n = 3.821 and k = 0.051). The thicknesses of the monolayers were determined with a planar three-layer (ambient, organic monolayer, substrate) isotropic model with assumed refractive indices of 1.00 and 1.46 for ambient and the organic monolayer, respectively. The reported values are the averages of at least eight measurements taken at different locations on several samples, and the error is less than 1 Å. 2.2.3. X-ray Photoelectron Spectroscopy (XPS) Analysis. XPS analysis was performed using a JPS-9200 photoelectron spectrometer (JEOL, Japan). High-resolution spectra were obtained under UHV conditions using monochromatic Al Kα X-ray radiation at 12 kV and 25 mA using an analyzer pass energy of 10 eV. All high-resolution spectra were corrected with a linear background before fitting. Experimental C 1s resolved spectra were fitted using Gaussian− Lorentzian mixed peaks (with a 70%/30% ratio) for the different C atoms on the surface. 2.2.4. Infrared Reflection−Absorption Spectroscopy (IRRAS). IRRAS was performed on a Bruker Tensor 27 using a Harrick Auto Seagull sample holder and an MCT (mercury, cadmium, telluride) detector. Measurements were made using Auto Seagull Pro v1.50 software. The p-polarized spectra were recorded at a mirror angle of 68°. Per measurement, 10 000 scans were recorded at a resolution of 4 cm−1. The spectra were analyzed using the Opus 6.5 software. The final spectra were obtained as the raw data divided by the data recorded on a piranha-oxidized reference surface as the background. All spectra were recorded at room temperature in a dry nitrogen atmosphere. A linear baseline correction was applied. 2.2.5. Atomic Force Microscopy (AFM). Atomic force microscopy (AFM) topography and phase images were recorded in tapping mode on an Asylum Research AFM, MFP-3D Xop v27 up6. Measurements were made using an NSC35/AIBS cantilever with a spring constant of 14 N/m. As settings, a scan size of ∼50 × 50 μm2, a scan speed of 1.00 Hz/line, and 256 pixels were chosen. Igor Pro 6.03A was used to analyze the spectra. 2.2.6. Scanning Electron Microscopy (SEM). The morphology of ferrocene micropatterns was confirmed by using scanning electron microscopy (JAMP-9500F, JEOL Tokyo) with an acceleration voltage of 5.0 kV. 2.2.7. Electrochemical Characterization. Cyclic voltammetry and impedance spectroscopy measurements were performed with an Autolab electrochemical analyzer (PGSTAT 302N potentiostat/ galvanostat from Eco Chemie B.V.) equipped with GPES and FRA software in a self-designed three-electrode Teflon cell. The working electrode, modified Si(111), was pressed against an opening in the cell

2. EXPERIMENTAL SECTION 2.1. Preparation of the Micropatterned Ferrocene-Modified Si(111) Surfaces. The preparation of the micropatterned surfaces using microcontact printing with poly(dimethylsiloxane) (PDMS) stamps has been described in our previous paper for 5 × 5 μm2-sized patterns32 and is identical for the other arrays considered in this work. Briefly, the micropatterned surfaces were prepared from the reaction of an amino-substituted Fc with a preformed, reactive, acid fluorideterminated alkenyl monolayer covalently bound to n-type H−Si(111) (Scheme 1). The PDMS-based lithographic method resulted in 5 × 5 (SiFc5), 10 × 10 (SiFc10), and 20 × 20 (SiFc20) μm2 Fcfunctionalized squares separated by 5, 10, and 20 μm, respectively, of butylamide-terminated areas. For comparison, surfaces totally modified with single-component ferrocene (SiFctotal) and butylamide-terminated monolayers were also prepared, as previously described.32 2.2. Surface Characterization. 2.2.1. Contact Angle Measurements. Static water contact angles were measured with an automated Krüss DSA 100 goniometer. At least six small droplets (3.0 μL) were 7236

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Figure 1. AFM phase images of 5 × 5 μm2 (SiFc5, a) and 10 × 10 μm2 (SiFc10, b) Fc-functionalized squares separated by 5 and 10 μm, respectively, of butylamide-terminated areas. Scan size: 45 × 45 μm2.

Figure 2. SEM images of 5 × 5 μm2 (SiFc5, a), 10 × 10 μm2 (SiFc10, b), and 20 × 20 μm2 (SiFc20, c) Fc-functionalized squares separated by 5, 10, and 20 μm, respectively, of butylamide-terminated areas. bottom using an FETFE (Aldrich) O-ring seal. Ohmic contact was made on the previously polished rear side of the sample by applying a drop of an In−Ga eutectic (Alfa-Aesar, 99.99%). The total surface area of Si(111) (∼0.15 cm2) was estimated by measuring the charge under the voltammetric peak corresponding to the ferrocene oxidation on unpatterned Si(111)-H and compared to that obtained with a 1 cm2 Pt electrode using the same electrolytic conditions. The counter electrode was a platinum grid, and system 10−2 M Ag+|Ag in acetonitrile was used as the reference electrode (+0.29 V vs aqueous KCl saturated calomel electrode SCE). All reported potentials are referenced to the SCE (uncertainty ±5 mV). Tetra-n-butylammonium perchlorate, Bu4NClO4 (Fluka, puriss, electrochemical grade, 0.1 M), was used as the supporting electrolyte in acetonitrile (anhydrous, analytical grade from SDS). The (CH3CN + 0.1 M Bu4NClO4) electrolytic medium was dried over activated, neutral alumina (Merck) for 30 min under stirring and under argon. About 20 mL of this solution was transferred with a syringe into the electrochemical cell prior to experiments. All electrochemical measurements were carried out inside a homemade Faraday cage at room temperature (20 ± 2 °C) and under a constant flow of argon, either in the dark or under illumination using an optical fiber (Olympus, Highlight 2100, maximum power). For impedance spectroscopy measurements, the amplitude of the ac signal was 10 mV. The differential capacitance C was determined from the imaginary part of the complex impedance Z′′ (C = −1/2πf Z′′) in the frequency range f (1 kHz to 50 Hz) in which the phase angle of the complex impedance was greater than 80°, i.e., the range for which the system behaved primarily as a combination of capacitive circuit elements.

clearly revealed by AFM phase imaging (Figure 1) and SEM (Figure 2), owing to the large differences in the chemical compositions and corresponding wetting properties between the butylamide- and ferrocene-terminated areas. In contrast, due to the small differences in height between the two regions, only a weak contrast is observed by AFM topography. Further information on both the average chemical composition of the grafted molecular films and the oxidation state of the underlying silicon surface was obtained using XPS. First, the XPS survey spectrum of the starting acid fluorideterminated monolayer reveals characteristic peaks from the Si substrate itself and from the C 1s, F 1s, and O 1s core levels of the attached molecule (Figure S1 in the Supporting Information). The C 1s narrow-scan spectrum shows components at 283.9, 285.0, 286.3, and 290.9 eV, assigned to the carbon covalently bonded to silicon (Si−CH), the aliphatic carbons (CH2), carbon adjacent to the acid fluoride group (CH2−(CO)F), and the acid fluoride carbon ((C O)F), respectively. The peak areas are nearly consistent with the chain structure, and the assignment is in line with theoretical predictions based on DFT calculations.33 The experimental ratio between the areas under the C and F peaks is 10.2 ± 0.2, i.e., close to the 11 C/1 F theoretical ratio (Table S1). Moreover, the absence of signal in the 102−104 eV range in the Si 2p spectrum indicates that the surface is not or is only very weakly oxidized. After microcontact printing of Fc and subsequent backfilling of the noncontact areas with nbutylamine, XPS analysis of the resulting surfacesSiFc5, SiFc10 (Figures S2 and S3), and SiFc20 (Figures S4 and S5) shows additional elements, namely, Fe and N, together with the disappearance of the F 1s signal, which demonstrates the complete conversion of the acid fluoride headgroups (Figure 3). The C 1s spectra are reasonably fitted with four components located at 283.7, 285.0, 286.3, and 288.0 ± 0.2 eV, irrespective of the size of patterns, and assigned to the

3. RESULTS AND DISCUSSION 3.1. AFM and XPS Characterizations of the Micropatterned Ferrocene-Modified Surfaces. The microcontact printing used in this work resulted in 106, 2.5 × 105, and 6.25 × 104 Fc-containing microstructures per cm2 of the electrode for SiFc5, SiFc10, and SiFc20, respectively. It is noteworthy that the area of Fc-modified regions represents one-quarter of that for the total area for all three arrays. The produced patterns with the expected lateral dimensions are 7237

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Figure 3. XPS spectra of 10 × 10 μm2 Fc-functionalized squares separated by 10 μm of butylamide-terminated areas of SiFc10: (a) wide scan, (b) C 1s, (c) Si 2p, (d) Fe 2p3/2, (e) N 1s, and (f) F 1s narrow scans.

of these two IR bands was found not to be modified with the pattern size. Therefore, all of these IR data are consistent with similar monolayer ordering for the three arrays. 3.3. Ellipsometric and Water Contact Angle Measurements. In addition, the ellipsometric thickness values were similar to all patterned surfaces with respect to what was reported for SiFc5.32 The thickness of the acid fluorideterminated monolayer was 1.3 ± 0.1 nm, and ferrocenepatterned as well as butylamine-backfilled surfaces were 2.1 ± 0.1 nm, respectively (Table S1). After micropatterning with ferrocene and further backfilling with butylamine, the water contact angles increased from 91° (SiFc5)32 to 101° (SiFc20, Table S1). It must be recalled that the surfaces modified by single-component ferrocene- and butylamide-terminated monolayers had contact angles of 80 and 102°, respectively.32 Because XPS measurements reveal almost identical atomic

different carbon atoms present in the two molecular chains grafted inside and outside the patterns, namely, Si−C, C−C + CC, C−N + C−O, and C(O)NH, respectively.33 As expected, the N 1s spectrum shows a single component at 400.2 eV corresponding to the amide bonds. Moreover, the Fe 2p3/2 level is observed at 708.5 eV for the three micropatterned surfaces, in excellent agreement with values previously reported for some ferrocene derivatives.34,35 From the peak areas, the experimental N/Fe ratios have been estimated to be 4:1 and 3.7:1 for SiFc10 and SiFc20, respectively (Table S1), which is perfectly in line with the theoretically expected ratio of 4:1 based on the respective areas of Fc- and butylamide-terminated regions. Interestingly, the microcontact printing and backfilling steps were found not to introduce surface oxidation (no observable peak at 102−104 eV), indicating the high passivation quality of the initially formed acid fluoride monolayer. Globally, the shape and intensity of the XPS spectra as well as their relative positions in energy are found to be similar for the three micropatterned surfaces, in line with the identical overall molecular compositions of the three arrays (1/4 Fc and 3/4 butylamide-terminated chains). 3.2. IR Spectroscopy Analysis. The IRRAS analysis of the three micropatterned surfaces shows peaks due to the stretching and deformation of the CH2 groups of the ferroceneand butyl-connected alkyl tails at 2923 ± 1 and 2952 ± 1 cm−1, respectively (Figure 4). The positions of the antisymmetric (νa CH2) and symmetric (νs CH2) methylene stretching can be used to distinguish between monolayers that display shortrange order (2918 cm−1/2850 cm−1) or disorder (2928 cm−1/ 2854 cm−1).36,37 The position of these vibration bands was not significantly dependent on the pattern size and suggests that these monolayers were slightly disordered, as expected for this length of chain with functional headgroups.38,39 Two additional bands were observed at 3104 ± 1 and 3312 ± 1 cm−1. Comparison with IR spectra of the control surfaces, namely, totally Fc- and butylamide-terminated surfaces (Figure S6), enabled these bands to be ascribed to the C−H stretching modes of the ferrocene ring40−42 and to the N−H stretching absorption of the amide group, respectively. Again, the position

Figure 4. IRRAS spectra of 5 × 5 μm2 (SiFc5), 10 × 10 μm2 (SiFc10), and 20 × 20 μm2 (SiFc20) Fc-functionalized squares separated by 5, 10, and 20 μm, respectively, of butylamide-terminated areas. The right side corresponds to the IR region of the N−H stretching band, which has been enlarged 50-fold. 7238

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to the oxidation of Fc to ferrocenium (Fc+) promoted by the captured photogenerated holes, as the cyclic voltammogram of the nonelectroactive butylamide-terminated surface does not show any redox system either in the dark or under illumination. Both the anodic and cathodic peak photocurrents, Ipa and Ipc, corresponding to the Fc+/Fc redox couple are found to be proportional to the potential scan rate v for the three arrays (Figure 6), as expected for surface-confined reversible redox species.45 We also observe that the full width at half-maximum (fwhm) of the anodic peak at low scan rates ranges from 115 to 125 mV for SiFc5 and SiFc10 to 205 mV for SiFc20 (Table 1). Such values are higher than the 90 mV predicted theoretically for a monoelectronic system,45 indicating the presence of some weak (SiFc5 and SiFc10) and strong (SiFc20) lateral interactions between the electroactive molecules in these films. The shift in redox potential to more positive potentials and the broadening of the cyclic voltammograms upon increasing the size of the patterns are somewhat intriguing and have never been mentioned in electrochemical studies on other micropatterned Fc-terminated self-assembled monolayers essentially bound to Au surfaces.46−49 It must be noticed that such a trend is reproducible and is observed for three separately prepared surfaces of each patterned sample. Such observations cannot be ascribed to differences in the electrical properties of the underlying Si(111) surface among the three arrays (vide infra) or to large differences in the packing density of the grafted redox monolayers resulting from differences in the Fc coverage. Indeed, the total surface density of attached Fc moieties was estimated by integrating the cyclic voltammetry peak at 0.2−0.4 V s−1 to be (1.1 ± 0.2) × 10−10 mol per cm2 of electrode, somewhat irrespective of the size of the patterns. Considering the fractional coverage of the electroactive sites, namely, a quarter of the total area, this value corresponds to (4.4 ± 0.8) × 10−10 mol per cm2 of the electroactive patterns, which is perfectly in line with the ball-like shape of the Fc molecules with a diameter of 6.6 Å and indicative of a densely packed Fc monolayer inside the patterns. Moreover, such values are perfectly in agreement with those usually reported for similar high-quality ferrocenyl monolayer-modified millimeter-sized silicon surfaces.2 As pointed out by Laviron and coworkers in their seminal work on the electrochemistry at chemically modified electrodes,50 the existence of electrostatic attractions and repulsions between the adjacent Fc/Fc+ centers may cause changes in the shape, the magnitude, and the width of the cyclic voltammograms. The broadening of the cyclic voltammograms observed at SiFc20 but also at the totally Fc-modified surface, SiFctotal (inset in Figure 5), reflects the unhomogeneity of the grafted ferrocene sites arising from a distribution of its formal potential and/or its apparent rate constant of charge transfer. This nonideal electrochemical behavior for the case of larger patterns suggests the formation of more aggregated domains or inhomogeneous electroactive sites caused by variations in local structure or environment, as previously reported for other ferrocene-terminated monolayers deposited on gold surfaces.31,51 This pattern-size-dependent behavior would be rather consistent with the fact that some Fc units close to the outer edges of the redox-active patterns are in an environment different from that of Fc units inside the patterns. Therefore, this suggests that there are critical dimensions for which the organization and the dynamics of the electroactive chains in the liquid medium change from a perfectly homogeneous to a more heterogeneous state. Such a situation is thought to be more prominent in the case of electrochemical studies in liquid

concentrations of the three arrays, the increase in hydrophobicity with pattern size cannot be ascribed to differences in molecular compositions. Rather, the observed trend is believed to be attributed to zero eccentricity, i.e., center-to-center offset distance, and increased relative spacing between these micropatterns from 5 × 5 to 20 × 20 μm2.43 3.4. Electrochemical Properties of the Micropatterned Ferrocene-Modified Surfaces. 3.4.1. Cyclic Voltammetry. Voltammetric analysis of the micropatterned Fc-modified surfaces in the dark shows the absence of a significant oxidation current response, as expected for a semiconductor under depletion conditions,44 i.e., when only a few majority charge carriers are available for charge transfer (vide infra). Upon illumination with white light, a well-defined reversible wave appears at a formal potential of E°′ ranging from 0.07 to 0.18 ± 0.01 V vs SCE upon increasing the size of the patterns (Figure 5 and Table 1). Such a system was found to be electrochemi-

Figure 5. Cyclic voltammograms in CH3CN + 0.1 M Bu4NClO4 under irradiation of the micropatterned 5 × 5 μm2 (SiFc5, a), 10 × 10 μm2 (SiFc10, b), and 20 × 20 μm2 (SiFc20, c) Fc-functionalized squares separated by 5, 10, and 20 μm, respectively, of butylamide-terminated areas. (d) The dotted line corresponds to the CV of the butylamideterminated surface. (Inset) CV in the dark (dotted line) and under irradiation (solid line) of the totally Fc-modified surface, SiFctotal. Potential scan rate: 0.4 V s−1.

Table 1. Electrochemical Data Obtained at Different Ferrocenyl Monolayer-Modified Si(111) Surfaces in CH3CN + 0.1 M Bu4NClO4 surface

Efb/V vs SCEa

butyl SiFc5 SiFc10 SiFc20 SiFctotal

−0.77 −0.77 −0.77 −0.90 −0.87

E°′/V vs SCEb

ΔEp/ mV

fwhm/ mV

Γ/mol cm−2 c

d

d

d

d

0.07 0.105 0.185 0.08

25 30 30 30

125 115 205 160

(4.8 (4.2 (4.6 (4.8

± ± ± ±

0.4) 0.5) 0.4) 0.5)

× × × ×

10−10 10−10 10−10 10−10

Determined from Mott−Schottky C−2−E plots. bAverage of anodic and cathodic peak potentials measured at low scan rates (