Anthracene and Anthracene:C60 Adduct-Terminated Monolayers

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Anthracene and Anthracene:C60 Adduct-Terminated Monolayers Covalently Bound to Hydrogen-Terminated Silicon Surfaces Bruno Fabre,*,† Dario M. Bassani,‡ Chih-Kai Liang,‡ Debdas Ray,‡ Fei Hui,† and Philippe Hapiot† †

Sciences Chimiques de Rennes, CNRS UMR 6226, Mati
ere Condensee et Syst
emes Electroactifs (MaCSE), Universite Rennes 1, 35042 Rennes Cedex, France ‡ Institut des Sciences Moleculaires, CNRS UMR 5255, Universite Bordeaux 1, 33405 Talence, France

bS Supporting Information ABSTRACT: Anthracene monolayers covalently bound to hydrogen-terminated n-type Si(100) surfaces have been prepared from the attachment of an aminesubstituted anthracene derivative to a preassembled acid-terminated alkyl monolayer using carbodiimide coupling. The anthracene headgroups were then used as anchoring sites for C60 following [4 + 2] Diels
Alder cycloaddition. After cycloaddition of C60 on the anthracene layer, the surface roughness determined by atomic force microscopy increased from 3.0 ( 0.7 to 5.0 ( 1.0 Å and the morphology showed uniformly distributed globular features a few nanometers high. Cyclic voltammograms of the anthracene-modified monolayer in the dark were characterized by an ill-defined reversible system at E°0 =
2.05 V vs saturated calomel electrode (SCE), which compares well with the value determined for the anthracene derivative in solution on a platinum electrode. Furthermore, the surface coverage of attached anthracene units was estimated to be (4.6 ( 0.3)  10
10 mol cm
2, which is consistent with a densely packed monolayer. In contrast, the voltammogram of the C60-modified monolayer did not show multiple reversible one-electron transfers characteristic of the anthracene:C60 adduct. Instead, one irreversible cathodic peak at
1.50 V followed by a reversible system at
2.15 V was observed. These electrochemical differences between surface-confined and dissolved species are assigned to reduced charge transfer kinetics between the underlying semiconductor and bound C60 within a certain potential range. This hypothesis is consistent with the flat band potential Efb value of
0.80 ( 0.05 V vs SCE, determined from capacitance measurements. Moreover, scanning electrochemical microscopy (SECM) measurements in feedback mode provided clear evidence for the electroactive properties of bound C60. The SECM approach curves suggest that both the anthracene and anthracene:C60 layers displayed good conductivity, presumably by electron hopping between adjacent redox sites.

1. INTRODUCTION The attachment of functional self-assembled monolayers (SAMs) onto silicon oxide (glass, quartz, oxidized silicon),1,2 semiconducting,3
5 metallic,6 or carbon7,8-based substrates constitutes an attractive approach for building novel interfaces for molecular electronic devices with applications ranging from solar energy conversion to chemical/biological sensing. Semiconducting surfaces such as doped silicon are particularly relevant for the development of novel electrically addressable and switchable functional devices.3,5,9 Indeed, unlike metallic substrates, the electronic properties of silicon are tunable by changing the nature and/or the concentration of dopants, or by generating electron
hole pairs under illumination. Moreover, silicon-based devices can be directly integrated within existing electronic circuitry10,11 and the numerous existing technological processes used for the micro- and nanopatterning of silicon are mature enough for producing highly miniaturized functional electronic components. For these reasons, the functionalization of oxide-free hydrogen-terminated silicon surfaces (henceforth referred to as Si
H) using the covalent attachment of organic monolayers r 2011 American Chemical Society

has received intense attention due to the numerous potential applications of controlled and robust organic/Si interfaces. Furthermore, Si
C linked monolayers prepared using, e.g., linear 1-alkenes and 1-alkynes3
5,12,13 are thermally and chemically very stable owing to the nonpolar character of the strong interfacial Si
C bond, and the absence of a SiO2 layer results in an almost defect-free electrical interface with direct electronic coupling between the surface and the organic functionality. In this context, the derivatization of Si
H surfaces with redox-active molecules (ferrocene,14
20 metal-complexed porphyrins,21
24 etc.) constitutes a powerful approach to the fabrication of integrated systems devoted to information processing (molecular memories and diodes). Herein, we describe our results on the derivatization of Si(100)
H surfaces with alkyl monolayers terminated by an anthracene redox-active unit. The latter presents particular interest Received: March 4, 2011 Revised: June 22, 2011 Published: June 22, 2011 14786

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The Journal of Physical Chemistry C in view of constructing hierarchically ordered multilayered devices as it offers numerous opportunities for direct photolithography (through its photoinduced oxidation25,26 or photodimerization reactions27
29) of the substrates. More importantly, surface-bound anthracene can be used as an anchoring site of more complex architectures, taking advantage of the particular affinity of anthracene with fullerene derivatives using [4 + 2] Diels
Alder cycloaddition.30
39 We have recently demonstrated the viability of this approach through the grafting of fullerene derivatives onto oxidized silicon substrates.40 In the present study, the anthracene-modified silicon surfaces are obtained by coupling between an amino-substituted anthracene derivative and activated ester groups end-capping an alkyl monolayer covalently bound to Si
H. The resulting assemblies are thus expected to exhibit film characteristics, especially ordering and density, that are not too different from those of the preformed carboxylate-terminated monolayer and significantly better than those of an Si
C linked monolayer prepared in one step from an anthracene-substituted alkene, as recently reported by Michelswirth et al.41 The functional anthracene monolayers and their ensuing fullerene adducts have been characterized by various experimental techniques including contact angle goniometry, atomic force microscopy (AFM), and transmission FTIR spectroscopy. Detailed electrochemical investigations of these redox-active interfaces, using cyclic voltammetry, electrochemical impedance spectroscopy, and scanning electrochemical microscopy (SECM), provide also a complete view of charge transfer mechanism in these assemblies.

2. EXPERIMENTAL SECTION 2.1. Reagents. Acetone (MOS electronic grade, Erbatron from Carlo Erba), anhydrous ethanol (RSE electronic grade, Erbatron from Carlo Erba), trichloroethylene (VLSI electronic grade from Carlo Erba), N,N0 -dimethylformamide DMF (puriss over molecular sieve, min 99.5% from Sigma-Aldrich), toluene (HPLC grade, Hipersolv Chromanorm from VWR), and acetic acid (99.7%, Acros) were used without further purification. Dichloromethane (min 99.5%, AnalaR Normapur from VWR) and acetonitrile from SDS were distilled over phosphorus pentoxide (Sicapent from Merck) before use. The chemicals used for cleaning and etching of silicon wafer pieces (30% H2O2, 96
97% H2SO4, and 50% HF solutions) were of VLSI semiconductor grade (Riedel-de-Ha€en). Undecylenic acid (Acros, 99%) was passed through a neutral, activated alumina column to remove residual water and peroxides. N-Hydroxysuccinimide (NHS) (Acros, 98+%), 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide (EDC) (Acros, 98+%), and fullerene (C60) (MTR, 99.5%) were used as received. Preparation of anthracene derivatives was carried out under an atmosphere of dry nitrogen and in the dark to minimize anthracene photooxidation. 1H NMR and 13C NMR spectra were recorded on a Bruker DPX 300 or DPX 400 MHz spectrometer and are referenced in parts per million with respect to TMS using the residual solvent signal as internal reference. Mass spectral analyses were carried out on a VG Autospec Q mass spectrometer. Synthesis of 1-(1-Anthryloxy)-6-aminohexane (An). Sodium hydride (0.13 g, 5.41 mmol) and 1.5 g (5.03 mmol) of 6-bromohexylphthalimide were consecutively added to a stirred solution of 1-hydroxyanthracene40 (0.7 g, 3.6 mmol) in 20 mL of dry DMF at room temperature. After addition the mixture was stirred for 24 h at 70 °C. After the usual workup the residue was purified

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by flash chromatography (SiO2, CH2Cl2) to afford N-[6-(1anthryloxy)hexyl]phthalimide as a green-yellow solid (1.2 g, 78%). Mp 67 °C. 1H NMR (CDCl3, 300 MHz) δ: 1.49
1.55 (m, 2H,
CH2), 1.58
1.81 (m, 4H,
CH2), 1.96
2.04 (m, 2H, CH2), 3.74 (t, J = 7.15 Hz, 2H,
NCH2CH2
), 4.19 (t, J = 6.3 Hz, 2H,
OCH2CH2
), 6.70 (d, J = 7.17 Hz, 1Har), 7.34 (d, J = 7.35 Hz, 1Har), 7.43
7.47 (m, 2Har), 7.56 (d, J = 9.03 Hz, 1Har), 7.68 (m, 2Har), 7.82 (m, 2Har), 7.97 (d, J = 9.6 Hz, 1Har), 8.07 (d, J = 9.6 Hz, 1Har), 8.34 (s, 1Har), 8.83 (s, 1Har). 13C NMR (CDCl3, 75 MHz) δ: 25.6, 26.3, 28.2, 28.9, 37.6, 67.6, 101.9, 117.8, 119.8, 120.7, 122.8, 124.7, 125.0, 125.2, 127.0, 127.5, 128.5, 130.8, 131.5, 131.8, 132.3, 133.5, 154.5, 168.1. Anal. Calcd (%) for C28H25NO3: C, 79.4; H, 5.9; N, 3.3. Found: C, 78.6; H, 6.2, N, 3.2%. IR (cm
1): 2930.7, 2867.8, 1771.9, 1702.6, 1464.4, 1399.6, 1267.8, 1209.2, 1144.4, 1053.0, 983.3, 897.5, 881.1, 834.8, 780.4, 755.3, 720.5, 549.7, 539.4, 529.9, 513.1. To a suspension of phthalimide (0.7 g, 0.16 mmol) in absolute EtOH (50 mL) was added 51% hydrazine monohydrate (10 mL, excess), and the mixture was heated at 90 °C for 4 h. The resulting solid was removed by filtration, and the filtrate was evaporated to dryness under reduced pressure. Chloroform (100 mL) was added to the residue, and the mixture was stirred for 0.5 h and then filtered through Celite. The precipitate was washed with CH2Cl2 (100 mL), and the combined organic fractions were washed with 10% NaOH solution, followed by brine (3  100 mL). The combined organic phase was dried with anhydrous Na2SO4, filtered, and concentrated. Flash chromatography (CH2Cl2/MeOH: 4:1) yielded 0.312 g (69.4%) of An as a yellow semisolid. 1H NMR (300 MHz, CDCl3) δ: 1.44
1.71 (m, 6H,
CH2), 1.94
2.08 (m, 2H,
CH2), 2.48 (bs, 2H, NH), 2.75 (bs, 2H,
CH2), 3.98 (t, J = 4.36 Hz, 2H,
NCH2CH2
), 4.19 (t, J = 6.36 Hz, 2H,
OCH2CH2
), 6.70 (d, J = 7.38 Hz, 1Har), 7.30
7.55 (m, 4Har), 7.99 (dd, J1 = 7.01 Hz, J2 = 4.87 Hz, 1Har), 8.07 (dd, J1 = 6.5 Hz, J2 = 4.7 1Har), 8.36 (s, 1Har), 8.83 (s, 1Har). 13 C NMR (75 MHz, CDCl3) δ: 25.8, 26.3, 28.2, 28.9, 37.4, 67.6, 101.93, 111.8, 119.8, 120.7, 124.7, 125.0, 125.2, 127.3, 128.4, 129.4, 130.8, 131.5, 132.3, 154.4. Anal. Calcd (%) for C20H23NO: C, 81.8; H, 7.9; N, 4.7. Found: C, 81.1; H, 8.1, N, 4.6. ESI-MS m/z calcd 293.1780 for [C20H23NO] and found 294.1841 [M + H]. 2.2. Preparation of Modified Flat n-Type Si(100) Surfaces. 2.2.1. Si
H. Double side polished silicon Si(100) samples (n-type, phosphorus doped, 1
5 Ω cm, Siltronix) were cut into 1.5  1.5 cm2 pieces and sonicated for 10 min in acetone, ethanol, and 18.2 MΩ cm ultrapure water. They were then cleaned in 3:1 v/v concentrated H2SO4/30% H2O2 at 100 °C for 30 min, followed by copious rinsing with ultrapure water. Caution: The concentrated H2SO4:H2O2 (aq) piranha solution is very dangerous, particularly in contact with organic materials, and should be handled extremely carefully. The surface was dipped in ca. 10% HF for 2 min and dried under an argon stream without rinsing.3 2.2.2. Undecanoic Acid-Modified Si(100) (Si
Ac). Freshly prepared Si
H samples were transferred immediately into Pyrex Schlenk tubes containing ca. 10 mL of neat undecylenic acid previously deoxygenated at 100 °C for 1 h at least and then allowed to cool down to ca. 30
40 °C before introducing the Si
H substrates. After Ar was bubbled through the solution with Ar for 30 min, the Si
H surface was irradiated in a Rayonet photochemical reactor (300 nm) for 3.5 h. The COOH-modified silicon surfaces (Si
Ac) were rinsed copiously with dichloromethane and trichloroethylene and then dipped into hot acetic 14787

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The Journal of Physical Chemistry C acid (70 °C) for 2  20 min and dried under an argon stream. It has been demonstrated that the rinsing in hot acetic acid leaves the functionalized surface smooth and perfectly free of physisorbed contaminants.42 2.2.3. Anthracene-Modified Si(100) (Si-An). The terminal COOH groups were activated with NHS by immersing the Si
Ac substrates for 2.5 h in a freshly prepared mixture of a deaerated solution of EDC at 0.2 M in anhydrous DMF (5 mL) and a deaerated solution of NHS at 0.1 M in anhydrous DMF (5 mL). The mixture was gently purged by bubbling with argon at room temperature. The surface was then rinsed with DMF, dried under an argon stream, and used immediately for amide formation. The covalent attachment of anthracene units on Si(100) was performed by immersing the NHS-activated silicon surface in a deaerated distilled dichloromethane solution containing ca. 1.5  10
2 M of An at room temperature in the dark overnight. The anthracene-modified substrates (Si-An) were rinsed with dichloromethane, ethanol, and trichloroethylene, then sonicated in dichloromethane for 5 min, and dried under argon. 2.2.4. Cycloaddition Reaction of C60 on Anthracene-Modified Si(100)(Si-An:C60). The Si-An substrates were immersed in a distilled toluene solution containing ca. 1  10
3 M of C60, previously sonicated for 10 min and deoxygenated for 30 min. The reaction was allowed to proceed in the dark for 3.5 days at 45 °C. The modified substrates thus obtained (Si-An:C60) were rinsed with toluene, dichloromethane, and trichloroethylene and dried under argon. 2.3. Preparation of Modified Porous n-Type Si. A flat Si
H surface was prepared as described in section 2.2.1. It was pressed against an opening in the cell bottom using a FETFE (Aldrich) O-ring seal and Ohmic contact was made on the polished rear side of the sample with the steel bottom cap (care was taken to avoid surface contamination for subsequent FTIR investigation). A platinum counter electrode was used. The hydrogenterminated porous Si surface was produced by applying a current density of 20 mA cm
2 for 5 min in 50% HF/ethanol/ultrapure 18.2 MΩ cm water (2:2:1 vol) under illumination using an optical fiber (Olympus Highlight 2100 with max power) or a halogen bulb. The surface was then rinsed with ethanol and dried under an argon stream. After the FTIR spectrum of hydrogen-terminated porous Si was monitored, the sample was again dipped in ca. 10% HF for 2 min and dried under an argon stream without rinsing. It was immediately transferred into a Pyrex Schlenk tube containing 10 mL of deoxygenated neat undecylenic acid. The solution was thoroughly purged with argon for 30 min and then heated at 110 °C under argon overnight.43 The undecanoic acidmodified porous surface was rinsed as described for the flat Si(100) surface. The grafting of anthracene and further cycloaddition reaction of C60 on porous Si were similar to those described in sections 2.2.3 and 2.2.4 for flat Si(100). 2.4. Characterization Techniques. Electrochemical Characterizations. Cyclic voltammetry and impedance spectroscopy measurements were performed with an Autolab electrochemical analyzer (PGSTAT 30 potentiostat/galvanostat from Eco Chemie B.V.) equipped with GPES and FRA software in a selfdesigned three-electrode Teflon cell. To avoid photoinduced electron transfer processes at silicon surfaces, all the electrochemical measurements on Si(100)
H and modified Si(100) surfaces have been performed in the dark. The working electrode, modified Si(100), was pressed against an opening in the cell bottom using a FETFE (Aldrich) O-ring seal. Ohmic contact was made on the previously polished rear side of the sample by

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applying a drop of an In
Ga eutectic (Alfa-Aesar, 99.99%). The electrochemically active area of the Si(100) surface (namely 0.14 cm2) was estimated by measuring the charge under the voltammetric peak corresponding to the ferrocene oxidation on Si(100)
H and compared to that obtained with a 1 cm2 Pt electrode under the same conditions. The counter electrode was a platinum grid and 10
2 M Ag+|Ag in acetonitrile was used as the reference electrode (+0.29 V versus aqueous SCE). All reported potentials are referred to SCE (uncertainty (0.01 V). Tetra-nbutylammonium perchlorate (Bu4NClO4) was purchased from Fluka (puriss, electrochemical grade) and was used at 0.1 mol L
1 as supporting electrolyte in acetonitrile or mixture of acetonitrile/toluene (3/1 vol). The 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. 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 Z00 (C =
1/2πfZ00 ) in the frequency range f (typically 50 kHz to 500 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. Scanning Electrochemical Microscopy (SECM) Measurements. Measurements were performed using a CHI900B instrument from CH-Instruments equipped with an adjustable stage for the tilt angle correction. Because of the sensitivity of the experiments to impurities and oxygen traces, the electrochemical cell was protected by a glass envelop to work under an Ar atmosphere. A typical three-electrode configuration, with a platinum counter electrode and an Ag/AgNO3 reference electrode was used. All SECM experiments were performed at room temperature under unbiased conditions (the substrate is not electrically connected). Typical SECM experimentation consists of recording approach curves where the normalized current IT = i/iinf is plotted versus the normalized distance L = d/a, where i is the current at the tip electrode localized at a distance d from the substrate, iinf is the steady-state current when the tip is at an infinite distance from the substrate iinf = 4nFDCa, with n the number of electrons transferred per species, F the Faraday constant, D and C the diffusion coefficient and the initial concentration of the mediator, and a the radius of the ultramicroelectrode (UME).44 The UME tip was a homemade 3 μm radius gold disk with a typical RG = 10 (RG is the ratio of the total electrode radius including the glass insulator over UME radius) prepared by sealing a gold wire in a glass tube.44 The UME were characterized by cyclic voltammetry and by typical approach curves recorded on conducting and insulating surfaces. The applied potential at the UME is chosen as being sufficiently negative to ensure a fast electron reduction at the UME (diffusion plateau of the mediator). In these conditions, the global charge transfer mechanism was characterized by the apparent charge transfer rate constant kel that is the apparent constant for the reaction between the reduced mediator and the surface under analysis.44 The kel values were derived from adjustments between the experimental approach curves and dimensionless theoretical curves assuming irreversible electron transfer kinetics for which semiempirical solutions have been published.44
46 Following the Bard
Mirkin formalism, these fittings provide the dimensionless parameter k = kela/D. An important point concerns the zero 14788

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Scheme 1. Preparation of Anthracene and Anthracene:C60 Adduct-Functionalized Si(100) Surfacesa

a Reagents and conditions: (i) neat undecylenic acid, 300 nm, 3.5 h; (ii) NHS at 5  10
2 M + EDC at 10
1 M in anhydrous DMF, 2.5 h, then An at 1.5  10
2 M in CH2Cl2, room temperature, dark, overnight; (iii) C60 at 1  10
3 M in toluene, 45 °C, dark, 3.5 days.

adjustment. For a defined sample, all approach curves are adjusted using the same zero origin. This was achieved by carefully changing the solution containing the mediator between experiments without moving the UME. Absolute zero offset was estimated with the ferrocene curve that was first fitted to the insulating case and then slightly adjusted to take into account the low but nonzero k value. FTIR Spectroscopy. FTIR spectra were acquired using a Br€uker Optics Vertex 70 FT-IR spectrometer in the transmission mode (100 scans, 2 cm
1 resolution and automatic gain) using a DTGS detector. The porous silicon was mounted on a homemade Teflon sample mount. Compared with flat silicon, the advantage of porous silicon is its very large surface area which greatly facilitates the spectroscopic identification of surface species using a classical FTIR spectrometer in transmission mode. In contrast, the IR identification of molecules grafted on hydrogen-terminated flat silicon requires to use 45° beveled attenuated total reflectance (ATR) silicon crystals and a more sophisticated FTIR spectrometer equipped with a sensitive liquid-nitrogen-cooled MCT photovoltaic detector. Atomic Force Microscopy (AFM). AFM images were recorded in contact mode with a PicoSPM II microscope from Molecular Imaging using nitride silicon tips (k ∼ 0.06 N m
1) from ScienTec-Nanosensors. Contact Angle Measurements. Static water contact angles were measured with an automated Kr€uss easy drop goniometer. At least three droplets of 2.0 μL of ultrapure water were dispensed, and the contact angles were determined using a Tangent 2 fitting model.

3. RESULTS AND DISCUSSION The covalent derivatization of oxide-free, hydrogen-terminated Si(100) surfaces (Si
H) by an anthracene-functionalized monolayer is depicted in Scheme 1. In the first step, a SiC-linked organic monolayer terminated by carboxyl groups is prepared from the photochemical reaction (λ = 300 nm) of Si
H with neat undecylenic acid.42,47
52 After activation of the COOH

Figure 1. Transmission FTIR spectra of porous Si crystals: Si
H (black), Si
Ac (red), Si
An (purple) and Si
An:C60 (magenta). The bands attributed to bound C60 are highlighted by the filled circles.

headgroups with NHS,43,47,48,51,53 the anthracene moieties were introduced by amide formation with an amino-substituted anthracene derivative. Compared with the one-step attachment from 1-alkene ω-substituted anthracene, we have found that this multistep procedure yielded more densely packed monolayers with much less oxidation of the underlying silicon surface.54 Static contact angles measured with water reveal, as expected, that the acid-modified surface is more hydrophilic than the anthracene and anthracene:C60 adduct-modified surfaces, 55 ( 2°, 77 ( 2°, and 75 ( 2°, respectively. Such values are in agreement 14789

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Figure 2. Contact-mode AFM images and corresponding cross-section profiles (taken in the middle of the images) of Si
Ac (A), Si
An (B) and Si
An:C60 (C). Scan areas: 1  1 μm2.

Figure 3. Cyclic voltammograms in the dark at 0.1 V s
1 of An at 2 mM on Si(100)
H (A) and 1 mm diameter platinum disk electrode (inset in A), Si
An in CH3CN + 10
1 M Bu4NClO4 (black line) and Si
An: C60 in CH3CN/toluene (3/1 vol.) + 10
1 M Bu4NClO4 (gray line) (B). The inset in B shows the jpc
v plot for the Si
An surface with jpc corrected from the background current.

with previous literature data on acid-terminated monolayers bound to Si
H,48,55
57 anthracene-terminated monolayers bound to oxidized silicon58 and gold,27 and C60-terminated monolayers deposited on gold.59 Nevertheless, the value measured for the acid-terminated monolayer is significantly higher than those reported for similar monolayers deposited on gold ( Efb in the dark), it is expected that the electrochemical reduction of the two first one-electron transfers of C60 is either not visible or appears irreversible at Si
An:C60 (vide supra). Under conditions where accumulation occurs (E < Efb), the equivalent capacitance C of the modified silicon surfaces can be described by eq 3 if one considers that the equivalent electrical circuit of the silicon
molecular film
electrolyte interface is similar to that commonly used for the silicon
alkyl monolayer
electrolyte interface.75
78 In this general expression, the presence of surface states in silicon is explicitly considered C
1 ¼ ðCsc þ Css Þ
1 þ Cf
1 þ CH
1

ð3Þ

where Css, Cf, and CH are the capacitance of the surface states, functional monolayer, and the Helmholtz layer, respectively. 14791

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Figure 4. (A, C) C
E curves at 1 kHz and (B, D) Mott
Schottky Csc
2
E plots at 50 kHz in the dark of Si
An in CH3CN + 10
1 M Bu4NClO4 (A, B) and Si
An:C60 in CH3CN/toluene (3/1 vol.) + 10
1 M Bu4NClO4 (C, D). Insets in panels A and C are capacitance peaks corresponding to the surface states obtained after subtraction of an sigmoid baseline (dashed line) from the experimental C
E curve.

Csc is the only potential-dependent capacitance and can be approximated by75 !1=2   q2 εε0 N qðEfb
EÞ ð4Þ Csc ¼ exp 2kT 2kT Consequently, under strong accumulation conditions (E , Efb), Csc
1 can be neglected and the C
E curves obtained for Si
An and Si
An:C60 show a small plateau (Figure 4A,C) that is determined only by the total capacitance of CH and Cf, provided that the density of surface states is low. The capacitance values corresponding to these plateaus are 1.8 and 1.4 μF cm
2 for Si
An and Si
An:C60, respectively. Assuming that CH is not changed in the presence of the organic monolayer, these results indicate a smaller capacitance of the An:C60 adduct-terminated monolayer, which is consistent with a thicker film if one considers the An and An:C60 films as ideal capacitors with similar dielectric properties.79 As mentioned above, the derivatization of silicon surfaces is thought to introduce surface states that are probably due to some unavoidable oxidation of Si(100) and/or the possible presence of interfacial alkoxy species. The presence of these surface states is indicated by the appearance of an additional capacitance peak in the C
E curves at electrode potentials near Efb.75,80 To separate parallel capacitance due to surface states from the underlying Csc, a baseline subtraction was made for each C
E curve. From the obtained capacitance peaks (insets in panels A and C of Figure 4), the total density of the surface states can be calculated (eq 5) 1 e2 Stot ð5Þ 4 kT Stot is estimated at 2.6  1011 and 2.9  1011 cm
2 for Si
An and Si
An:C60, respectively. These densities correspond to one

Cp ðmaxÞ ¼

surface state per ca. 2600 and 2300 surface silicon atoms, respectively, considering the atomic density of Si(100) of 6.78  1014 cm
2,81 i.e., less than 0.05% of the total surface. SECM Measurements. As explained in the previous section, the analysis by cyclic voltammetry of the Si(100) surfaces modified by redox-active head groups is somewhat complicated by the fact that both the underlying substrate and the bound electroactive units could contribute in the overall charge transport mechanism.16 To obtain complementary information about the redox properties of the Si
An and Si
An:C60 surfaces, we used SECM in feedback mode under conditions where the surface is in the dark and not electrically connected (unbiased conditions). SECM has been shown to be a powerful tool to analyze redoxactive monolayer or polymer-modified surfaces.44,82,83 The major difference from other electrochemical methods is that in SECM the surface is locally probed from the solution side with a ultramicroelectrode (UME) and a dissolved redox mediator. Such a configuration permits the characterization of electroactive layers deposited on conducting as well as insulating surfaces. Briefly, the principle of SECM is based on the electrochemical interactions of a redox species (the mediator) electrogenerated at a UME and the substrate under investigation. After diffusion of the electrogenerated species to the substrate, an electrochemical reaction is possible on a localized spot on the surface (the socalled diffusion cone of the UME) where the initial form of the mediator can be regenerated, resulting in an enhancement of the current at the UME. More specifically for our study, the contribution of underlying silicon in the charge transport mechanism can be eliminated provided that the organic chains bearing the electroactive groups are totally blocking toward the electron transfer (tunneling) or that diffusion of the redox probe inside the layer to the semiconducting surface is absent. When one of these conditions is fulfilled, SECM probes the redox reactivity of the bound electroactive headgroups.14
16,84 For this purpose, 14792

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The Journal of Physical Chemistry C Scheme 2. Redox Potential of the Probes and Corresponding Reduction Potential of the Grafted Species

Figure 5. SECM approach curves in the dark obtained at a gold disk UME tip on Si
Ac using DEA (black), AB (red), 4NB (green), or TCNQ (blue) at a concentration of 10
3 mol L
1 in DMF + 10
1 mol L
1 Bu4NPF6. The solid line is the theoretical curve expected for the totally insulating substrate case.

Figure 6. (a) SECM approach curves in the dark obtained at a gold disk UME tip on Si
An: DEA (black), AB (red), 4NB (green), and TCNQ (blue) at 10
3 mol L
1 in DMF + 10
1 mol L
1 Bu4NPF6. The solid line is the theoretical curve expected for the totally insulating substrate case. (b) SECM approach curves on Si
An as a function of the DEA concentration in DMF solution: 1 (black), 2 (red), and 5 (green)  10
3 mol L
1.

our strategy relies, on the one hand, on the variations of the driving force of the electron transfer between the mediator in solution and the substrate and, on the other hand, on a step-by-step analysis of the different layer constituents. In the following studies, four different redox mediators exhibiting reversible one-electron reduction processes with decreasing formal potentials have been considered for the SECM measurements in DMF: 9,10-diethoxyanthracene (DEA), azobenzene (AB), 4-nitrobenzonitrile (4NB), and tetracyanoquinodimethane

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(TCNQ) (Scheme 2). Considering the thermodynamic E° values, the radical anion of DEA is able to reduce both An and An:C60 moieties, whereas AB•
and 4NB•
can only transfer electrons to An:C60. Finally, TCNQ
is a too weak reducing agent for reducing any of the immobilized species. In the first experiment, we examined the reference non-redox-active Si
Ac surface where no redox attached group is present. As shown in Figure 5, the normalized current IT, at the gold UME tip approaching the Si
Ac surface diminishes for all four mediators investigated when the distance between the UME and the surface decreases. This reflects the absence of regeneration of the oxidized form of the mediator at the interface (negative feedback).44 Moreover, all the experimental approach curves display a good agreement with the theoretical variation expected for an insulating substrate, demonstrating the excellent blocking properties of the monolayer under these conditions. For the Si
An substrates, two different responses are observed, depending on the nature of the mediator (Figure 6A). In the case of AB, 4NB, and TCNQ, a negative feedback behavior is obtained. As for the previous nonelectroactive layer, the experimental curves match the theoretical behavior expected for the insulating substrate. In contrast, the use of DEA as a mediator results in the approach curve displaying a clear positive feedback, indicating the occurrence of continuous electron transfer between the electrogenerated DEA radical anion produced at the UME and the interface. The presence of small fluctuations in the positive feedback curve at L values around 2
3 can be observed. This phenomenon is attributed to the instability of the radical anion of DEA in DMF at the time scale of the SECM experiment due to the presence of traces of oxygen and water despite our experimental precautions. The same experimental artifact is observed in a test experiment performed on a conductive metallic surface.85 Despite these difficulties, the experiments show large electron transfer rates characterized by estimated k values around 2 or higher (see Supporting Information). Because the radical anion of DEA is the strongest reducing agent of the series, it is the only one able to reduce the bound anthracene units (Scheme 2), whereas all other mediators have redox potential too positive for reacting with grafted anthracene. As we discussed before, under steady state conditions and because the substrate is not electrically connected, large values of k imply the occurrence of the following steps: (i) a fast reduction of the anthracene moieties in contact with the solution by the radical anion of DEA; (ii) a fast transport of the injected negative charges inside and by the electroactive layer; (iii) the return of the charges to neutral DEA to balance the injection of negative charges that occurs on large areas outside the diffusion cone of the UME. If one of these steps is not fast enough, negative feedback is expected. To discriminate between these different steps, it is interesting to investigate the effect of increasing mediator concentration. Higher concentrations of mediator result in higher charge injection flux that could allow the observation of other limitations, notably due to the charge transport inside the layer. The results of such experiments are depicted in Figure 6B. It is noticeable that upon increasing mediator concentration, the resulting approach curves tend toward negative feedback behavior, thus allowing the limitation due to the charge conduction by the anthracene headgroups to be attained. The estimated k values decrease from 2.0 to 0.4 when the mediator concentration increases from 10
3 to 5  10
3 mol L
1 (Table S1 in the Supporting Information). 14793

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Figure 7. (a) SECM approach curves in the dark obtained at a gold disk UME tip on Si
An:C60: DEA (black), AB (red), 4NB (green), and TCNQ (blue) at 10
3 mol L
1 in DMF + 10
1 mol L
1 Bu4NPF6. The solid line is the theoretical curve expected for the totally insulating substrate case. (b) SECM approach curves on Si
An:C60 as a function of the AB concentration in DMF solution: 1 (black), 2 (red), and 5 (green)  10
3 mol L
1.

Similar experiments were performed with the Si
An:C60 substrates. When the mediator is DEA or AB, clear positive feedbacks are observed showing that the An:C60 adduct could be reduced by either of these two radical anions whereas negative feedback behavior is obtained with 4NB and TCNQ (Figure 7A). The experimental curves fit well with the theoretical curve calculated for a rate constant on the order of k = 0.05 for 4NB and that expected for an insulating substrate for TCNQ. The value calculated for 4NB indicates that the regeneration of the oxidized form of the mediator at the Si
An:C60 surface occurs at a much slower rate than that observed with DEA or AB. Considering the blocking properties of the reactive preassembled monolayer (i.e., Si
Ac), the results obtained for Si
An and Si
An:C60 demonstrate that the regeneration of the mediator involves only the attached electroactive units. An additional proof is provided by the dependence of the SECM approach curves at different mediator concentrations (Figure 7B). For both Si
An and Si
An:C60, feedback becomes less positive upon increasing the mediator concentration, indicating limitations by the redox transport inside the electroactive films.86,87 Compared with Si
An, the different behavior observed at Si
An:C60 can be ascribed to additional electron transfer steps provided by the bound C60 units. Interestingly, SECM provides clear evidence for the electroactive character of bound C60, which has not been the case for cyclic voltammetry measurements. The clear “positive feedback” character of the approach curves implies that both An:C60 and An layers display good conductivity, presumably by electron hopping between adjacent redox sites.

4. CONCLUSIONS In this study, redox-active anthracene units have been covalently bound to hydrogen-terminated Si(100) surfaces using an amidation reaction between an amino-substituted anthracene derivative and a reactive preassembled acid-terminated organic monolayer. This multistep procedure proved to be more reliable

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than a one-step procedure involving the direct grafting of an alkene-terminated anthracene derivative and resulted in the formation of densely packed monolayers with a surface coverage of (4.6 ( 0.3)  10
10 mol of anthracene per cm2. The anthracenemodified surfaces proved reactive toward fullerene C60, leading to the direct formation of fullerene monolayers, presumably through thermal Diels
Alder cycloaddition of C60 and the surface-bound anthracene. Cyclic voltammetry and SECM measurements proved complementary and showed that the anthracene and the anthracene:C60 adduct retain their redox activity after anchoring on the semiconducting surface. The immobilization of these electroactive molecules to the silicon surface introduced only a very weak density of surface states, less than 0.05% of the total surface. Interestingly, the results obtained from SECM indicate that the dissipation of charges on the surface by the organic monolayer is efficient, suggesting that the monolayers thus obtained can be of interest for the construction of miniaturized molecule-based electronic devices that could be further endowed with functionality (e.g., molecular recognition88) using modified fullerene derivatives.

’ ASSOCIATED CONTENT

bS

Supporting Information. Spectral characterizations of An (NMR, MS, and FTIR), electrochemical cell used for the preparation of porous silicon, AFM image of the Si
An:C60 surface and details of the estimations of the kinetics rate constants in the SECM experiments. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT Financial support from the CNRS and the Agence Nationale de la Recherche (ANR-08-BLAN-0161) is gratefully acknowledged. ’ REFERENCES (1) Onclin, S.; Ravoo, B. J.; Reinhoudt, D. N. Angew. Chem., Int. Ed. 2005, 44, 6282–6304. (2) Herzer, N.; Hoeppener, S.; Schubert, U. S. Chem. Commun. 2010, 46, 5634–5652. (3) Buriak, J. M. Chem. Rev. 2002, 102, 1271–1308. (4) Wayner, D. D. M.; Wolkow, R. A. J. Chem. Soc., Perkin Trans. 2 2002, 23–34. (5) Ciampi, S.; Harper, J. B.; Gooding, J. J. Chem. Soc. Rev. 2010, 39, 2158–2183. (6) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Chem. Rev. 2005, 105, 1103–1169. (7) McCreery, R. L. Chem. Rev. 2008, 108, 2646–2687. (8) Barri
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