Functionalization of Oxide-Free Silicon Surfaces with Redox-Active

Review pubs.acs.org/CR

Functionalization of Oxide-Free Silicon Surfaces with Redox-Active Assemblies Bruno Fabre* Institut des Sciences Chimiques de Rennes, UMR 6226 CNRS/Université de Rennes 1, Matière Condensée et Systèmes Electroactifs MaCSE, 35042 Rennes Cedex, France ABSTRACT: This review provides a comprehensive survey of the derivatization of hydrogen-terminated, oxide-free silicon surfaces with electroactive assemblies (from molecules to polymers) attached through strong interactions (covalent, electrostatic, and chimisorption). Provided that surface modification procedures are thoroughly optimized, such an approach has appeared as a promising strategy toward high-quality functional interfaces exhibiting excellent chemical and electrochemical stabilities. The attachment of electroactive molecules exhibiting either two stable redox states (e.g., ferrocene and quinones) or more than two stable redox states (e.g., metalloporphyrins, polyoxometalates, and C60) is more particularly discussed. Attention is also paid to the immobilization of electrochemically polymerizable centers. Globally, these functional interfaces have been demonstrated to show great promise for the molecular charge storage and information processing or the elaboration of the electrochemically switchable devices. Besides, there are also some relevant examples dealing with their activity for other fields of interest, such as sensing and electrochemical catalysis.

CONTENTS 1. Introduction and Scope 2. Attachment Procedures of Redox-Active Molecules on Hydrogen-Terminated Silicon Surfaces (Si−H) 2.1. One-Step versus Multistep Approaches 2.2. Fate of Unreacted Si−H Sites and Impact on the Stability of the Functionalized Surfaces 3. Electrochemistry at Redox-Active Center-Modified Silicon Surfaces 3.1. In the Dark 3.2. Under Light Irradiation 4. Attachment of Bistable Redox Molecules 4.1. Ferrocene as the Redox-Active Center 4.2. Other Metallic Complexes 4.3. Quinones 5. Attachment of Multistable Redox Molecules 5.1. Systems Exhibiting More than Two Reversible Electron Transfer Steps 5.1.1. Metal-Complexed Porphyrins 5.1.2. C60 5.1.3. Polyoxometalates 5.2. Systems Exhibiting Two Reversible Electron Transfer Steps 5.2.1. Tetrathiafulvalene (TTF) 5.2.2. Bipyridinium Cations 5.2.3. Heterobimetallic Complexes 5.2.4. Other Bimetallic Systems 6. Attachment of Electrochemically Polymerizable Centers

7. Applications Other than Charge Storage and Information Processing 8. What About Functionalized Si−H Micro/Nanostructures ? 9. Conclusions and Future Directions Author Information Corresponding Author Notes Biography References Note Added after ASAP Publication

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1. INTRODUCTION AND SCOPE One of the most important class of chemically modified surfaces stems from the irreversible immobilization of electrochemically reactive molecules on different types of (semi)conducting electrodes, namely metals, semiconductors, carbon, and metal oxides.1−5 The attachment of such molecules onto electrode surfaces has been essentially motivated in order to take advantage of their specific properties observed in solution. Procedures for immobilizing strongly and durably such reagents are numerous and include the chemisorption, the self-assembly, the covalent bonding between the redox-active reagent, and either the electrode surface oxides or a surface-bound chemically reactive unit, and the electrostatic entrapment. All of these procedures result in the formation of monolayer, multilayer, or polymer films. The monolayer approach offers a

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Received: October 8, 2015 Published: April 11, 2016 © 2016 American Chemical Society

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Scheme 1. Direct One-Step (Top Part) and Multistep (Bottom Part) Covalent Attachment of Redox-Active Monolayers to Si−H surfacesa

a

The attachment of nonredox diluent chains mixed with the electroactive chains in order to control the surface coverage of the electroactive center (functionalization level) is omitted for reasons of clarity.

attractive substrate for electrical applications owing to the ease and reproducibility of its preparation, its well-defined structure, its very low density of electrically active surface defects (the socalled surface states),16 measured at less than 1 electronic defect per 40 million surface atoms,17 and its propensity to be chemically modified with organic monolayers linked through nonpolar and robust interfacial Si−C bonds.18−22 Furthermore, the SiO2-free organic/silicon interface constitutes an almost defect-free electrical interface with a direct electronic coupling between the surface and the organic functionality. This situation is somewhat opposite to that of alkylsilane-derived monolayers bound to oxidized silicon,23−25 which are often unstable because interfacial siloxane bonds, Si−O−Si, are susceptible to hydrolysis, and thus the long-term applicability of these surfaces remains lower than that of covalently modified unoxidized silicon. Moreover, the hybrid molecule/silicon assemblies offer a number of advantages over molecular assemblies on metals, such as extensively described gold. Unlike metals, the electronic properties of silicon can be finetuned by modifying the density and the nature of the charge carriers (electrons, holes) upon light illumination. This characteristic can be relevant for the development of photochemically switchable systems. Second, the interfacial Si−C bond is much stronger than the Au−S bond between gold and organosulfur adsorbates, 447 versus 254 kJ mol−1,26 which confer a better stability to these hybrid assemblies.27 Also interestingly, the integration of redox-active molecules onto Si−

wide choice of functionalities for molecular-level control of layer structure (homogeneity, ordering of organic chains) and surface chemistry. Multilayers and polymers in spite of their usually more disordered arrangement offer some advantages in terms of higher surface coverage, larger electrochemical currents which make, for example, more efficient electrocatalysis, higher apparent electrochemical stability, and more efficient passivating layer against corrosion phenomena. All these modification approaches are relevant if the key properties of the electroactive molecule are not degraded after its transfer from the solution to the electrode surface, which is a prerequisite for the involvement of these functional surfaces in numerous technologically important (bio)electrochemical processes, such as electrocatalysis,6,7 electroanalysis, the development of biosensing devices,8,9 and the wiring of enzymes to electrodes.10−12 In parallel to these studies directed toward the development of electroanalytical devices, other investigations aim at exploiting the peculiar character of the electrochemical addressing of the immobilized redox center to widen the potential applications of electroactive molecule-modified surfaces to novel molecular electronic devices.13−15 In this context, interfacing technologically important semiconducting surfaces, such as oxide-free, hydrogen-terminated silicon (Si− H) with high-quality, and stable redox-active films has appeared as a promising strategy toward functional devices for charge storage and information processing. Si−H is a particularly 4809

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which has been much less exploited is the direct electroless grafting of aryldiazonium salts substituted by redox-active groups onto Si−H.38,39 Nevertheless, as commonly reported,40 the aryldiazonium procedure may fail to produce monomolecular films in a reproducible and controllable manner, even though recent strategies have been reported, enabling the controlled formation of functional monolayers from protected aryldiazonium derivatives.41−46 Following the hydrosilylation approach, electroactive centerterminated monolayers covalently bound to Si through Si−C interfacial bonds are produced either directly in one step or in several steps after efficient conversion of the terminal reactive group (e.g., acid, amine, alcohol, triazide, etc.) to the electroactive molecule using common solution chemistry reactions (carbodiimide, “click”, organolithium, etc. chemistries).20,34 Compared with the direct grafting method, the multistep procedure often involves milder conditions (e.g., reaction in aqueous medium at room temperature), which are more compatible with some fragile molecules. Moreover, another advantage is that the functionalization step takes place on usually densely packed and well-ordered ω-substituted alkyl/alkenyl monolayers. So, the resulting electroactive assembly often exhibits characteristics, such as ordering and density, not too different from those of the preassembled monolayer. In contrast, owing to the steric hindrance of the electroactive molecule, the direct attachment procedure usually yields less densely packed and poorly passivating monolayers with a large content of remaining Si−H sites. Besides Si−C functionalization, another synthetic approach which has been much less developed for the grafting of electroactive molecules involves reactions of alcohols and aldehydes with Si−H (Scheme 1).47,48 This results in Si−O−Clinked electroactive monolayers with electronic transport characteristics, film packing density, and surface coverage comparable with those obtained with analogous Si−C-linked electroactive monolayers.47,49,50 Nevertheless, it has been reported that the Si−C bound alkyl monolayers showed greater chemical durabilities in aqueous acidic and basic solutions than alkoxy monolayers.51

H surfaces has significant advantages in light of the extensive technologies and fabrication methods already built up around Si in the existing semiconductor industry (doping, processing, and patterning technologies). The fabrication of functional devices incorporating redox-active molecules will benefit from the current development on complementary metal oxide semiconductor (CMOS) structures and direct availability of industrial integration. This review is intended to provide an up-to-date description of advances in the functionalization of flat Si−H surfaces with electroactive assemblies (from molecules to polymers) attached through strong interactions (covalent, electrostatic, and chimisorption). Physisorbed electroactive systems are beyond the scope of this review. Moreover, even though the immobilization of electroactive centers onto other types of conducting surfaces, such as oxide (metal oxides, oxidized silicon, etc.),23−25 metallic,28 and carbon-based substrates,29,30 is not discussed in this manuscript, I would like to take the opportunity to invite the reader to refer to excellent and comprehensive reviews on this topic. Section 2 overviews the different surface chemistry reactions used for attaching redoxactive molecules to Si−H. Attention will be also paid to the fate of unreacted Si−H sites and the stability of the functionalized surfaces. Section 3 provides a brief theoretical background for a better understanding in the charge transfer characteristics at these modified Si−H surfaces in the dark and upon light illumination. Emphasis is then placed on examples of surfaces modified by electroactive molecules exhibiting either two stable redox states (bistable molecules, section 4) or more than two stable redox states (multistable molecules, section 5). The stability criterion is given to electrochemical reactions not complicated by preceding or following chemical reaction(s). Section 6 is dedicated to the particular case of the immobilization of electrochemically polymerizable centers. Contrary to the two previous sections, the electrochemical oxidation of such functional units gives rise to highly reactive electrogenerated species, the coupling of which yields thin layers of electronically conducting polymers. Section 7 focuses on applications of these functionalized surfaces other than charge storage and information processing. Finally, the functionalization of Si−H micro/nanostructures with redoxactive molecules will be discussed in section 8.

2.2. Fate of Unreacted Si−H Sites and Impact on the Stability of the Functionalized Surfaces

The quality of the formed monolayers will be critical in determining the interfacial electrical properties and the sensitivity of silicon to oxidation. In particular, high packing density is desired to obtain a stable chemically and electrochemically surface. Thus, the size of the molecules in the grafted monolayer dictates the sensitivity of the surface to oxidation by limiting the packing density. It is noteworthy that this density criterion is less critical for electroactive multilayers and polymers because these systems are intrinsically less ordered and usually protect more efficiently the underlying silicon surface against some oxidizing species. Owing to steric considerations, the surface coverage of an electroactive center immobilized in a monolayer structure will be usually lower than those reported for long-chain linear alkene- and alkyne-derived monolayers bound to Si(111), namely 50−55%37,52−54 and 60−65%,55,56 respectively. This indicates that more than 45−50% or 35−40%, respectively, of Si−H sites remaining after completion of the monolayer are susceptible to oxidation if water or oxygen can penetrate through the molecular layer via defects or pinholes. This would result in a significant density of electrically active surface defects

2. ATTACHMENT PROCEDURES OF REDOX-ACTIVE MOLECULES ON HYDROGEN-TERMINATED SILICON SURFACES (SI−H) 2.1. One-Step versus Multistep Approaches

The most popular method for grafting electroactive molecules onto oxide-free silicon [usually monocrystalline Si(100) or Si(111)] surfaces involves the covalent reaction of bifunctional molecules with Si−H which has been previously prepared by etching of an oxide-covered silicon wafer in an aqueous fluoride-containing solution (HF or NH4F). One end is the unit binding to the surface, and the other end is either the desired redox-active center or a reactive group subsequently converted to the desired functionality in high yield. In this frame, a largely developed wet chemistry method for incorporating electroactive centers to silicon uses the hydrosilylation reaction with ω-substituted 1-alkenes or 1-alkynes by thermal induction, ultraviolet (UV) light, or catalysis (Scheme 1),18,31−35 as initially proposed by Linford and Chidsey for long-chain linear alkenes.36,37 Another reaction of interest 4810

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Figure 1. Simplified energetic diagrams of modified n-type silicon surfaces in the dark (a) before and (b) after grafting of the redox-active molecule and charge equilibrium. The left and right side panels correspond to the electroactive systems characterized by a formal potential higher (usually electrochemically oxidizable systems) and smaller (electrochemically reducible systems) than the flatband potential Efb of silicon, respectively. (c) corresponds to the situation when a positive (left) or negative (right) external potential is applied to the semiconductor surface. χ, ϕ, and Eg are the semiconductor electron affinity, work function, and band gap energy (Eg = 1.1 eV for silicon), respectively.

which would influence negatively the electron transfer characteristics of redox-active modified silicon surfaces. Therefore, it is essential that the monolayer is as densely packed as possible in order to provide an efficient hydrophobic environment that is not readily penetrated by water or oxygen molecules and also to maintain a low density of surface states as close as that of the initial Si−H surface. One successful strategy to minimize the oxidation of underlying silicon is the dilution of the redox-center-terminated chains with electrochemically inert organic chains. This enables control of the surface coverage of the redox center and improvement of both the quality and the packing density of the resulting redox-active monolayer. Globally, all these grafting strategies aim at producing highdensity, electroactive monolayers with excellent passivation properties to prevent oxidation of the large number of remaining unreacted Si−H atop sites. Simplistically, a mixed electroactive monolayer can be considered of high density

when the total surface coverage (electroactive chains plus the nonredox chains) is higher than 70% of the maximum surface coverage expected for long-chain linear alkene-derived monolayers (i.e., 0.35 per surface silicon atom or 4 × 10−10 mol cm−2). Nevertheless, in spite of serious efforts to optimize the conditions of surface chemistry, it is often difficult to avoid the formation of silicon oxides when the modified surfaces are studied under ambient conditions and/or under electrochemical conditions in liquid electrolytes. Indeed, due essentially to the dynamics of the chains, heterogeneity in the spatial arrangement of the grafted chains and presence of pinholes or defects, there are numerous possible permeation paths that oxidizing species may follow to degrade the properties of the semiconductor surface. This results usually in the decrease in both chemical and electrochemical stabilities of the grafted films. Consequently, one of the greatest 4811

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characteristic energy versus position (Gerischer) diagrams presented in Figures 1 and 2 for n-type and p-type doped semiconductors, respectively.62−64 The case of semiconductors in the dark will be treated first followed by the case under irradiation.

challenges of the semiconductor functionalization, and in particular silicon, is the development of grafting strategies yielding near 100% Si−C functionalization without silicon oxidation. To date, such a surface coverage has been only rarely demonstrated. Among the successful examples, a 100% coverage of the Si(111) surface by methyl groups has been reported by Lewis’ group using a two-step chlorination/ alkylation procedure of Si(111)-H.57,58 As a matter of fact, the methyl group with its 2.2 Å van der Waals radius is the only saturated alkyl group enabling achievement of 100% coverage of the atop sites of Si(111). Due to steric constraints, the coverage decreases to 80% for an ethylated Si(111) surface. Nevertheless, the poor reactivity of the terminal methyl group impedes further secondary functionalization. Therefore, alternative termination is needed to allow the full coverage of Si(111) while enabling additional functionalization. Besides the methyl group, the acetylene group is also expected to cover 100% of Si(111) surface atoms in the light of the relatively similar specific areas of these two groups. On the basis of such considerations, ethynyl-modified Si(111) surfaces have been synthesized with a near 100% coverage59,60 and proved to be useful for anchoring electroactive groups (namely, ferrocene and benzoquinone) through a “click” reaction between an azide-substituted electroactive molecule and the surface-bound alkyne.60 Because the presence of interfacial oxide even at low levels may dramatically influence the charge transfer characteristics and the stability of redox-active modified silicon surfaces, it is necessary to measure accurately the amount of silicon oxides produced during the grafting process. The most popular surface characterization techniques used for this purpose are the X-ray photoelectron spectroscopy (XPS), Fourier transform infrared absorption spectroscopy (FTIR), and electrochemistry. For more details about the interest of such techniques to provide valuable information on both the chemical composition of the grafted layer and the electronic quality of the interface, the reader is invited to refer to a recent review on this topic.61 XPS detects the atomic core levels and is sensitive to the oxidation state of the elements. The formation of silicon oxides can be evidenced by the presence of a broad peak between 101 and 104 eV in the Si 2p region. By FTIR, silicon oxidation can be revealed by the appearance of vibration bands in the 1000− 1200 cm−1 spectral range and around 2250 cm−1, which are attributed to Si−O−Si and OSi−H bonds, respectively. Finally, electrochemical measurements are not only particularly useful to characterize the electron-transfer kinetics of the grafted electroactive center (see Section 3) but also to probe the quality of the interface between silicon and molecule. In particular, capacitance (C) − voltage (V) measurements, performed either at the solid state or in solution with a liquid electrolyte, enable the detection of surface defects, due to silicon oxides. The presence of these defects is usually indicated by the appearance of an additional capacitance peak in the Sshaped C−E curve and by a large frequency dispersion.

Figure 2. As in Figure 1 but for a p-type silicon surface.

3.1. In the Dark

Unlike metals, semiconductors have two types of charge carriers, namely electrons and holes. For a n-type doped conductive semiconductor, the electrons are the majority charge carriers because their concentration is much higher than that of holes and the electrochemical potential of the semiconductor, the so-called Fermi level EF, lies just below the conduction band (CB) edge. For a p-type semiconductor, the holes are the majority charge carriers because their concentration is much higher than that of electrons, and EF now lies just above the valence band (VB) edge. Before contact with the electrolyte solution, the energy bands and the charge-carrier profiles are uniform, and the entire semiconductor is neutral (Figures 1a and 2a). After contact and charge equilibrium, a charge flow occurs between the silicon surface and the electroactive molecule until the initial difference between EF and E0redox is neutralized. Herein, we will consider the simplest

3. ELECTROCHEMISTRY AT REDOX-ACTIVE CENTER-MODIFIED SILICON SURFACES In order to facilitate the discussion of electrochemistry at redoxactive molecule-modified silicon surfaces, it is worth recalling some basic principles associated with electron transfer at bare semiconductor electrodes in contact with an electrolyte solution containing an electroactive molecule characterized by its formal potential, E0redox. This can be illustrated using the 4812

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Figure 3. Determination of some electron transfer characteristics from cyclic voltammetry measurements at electroactive molecule-modified conducting surfaces.

case of depletion is called inversion. In the depletion regime, the surface concentration of majority carriers being very small and the minority carrier concentration remaining negligible, the charge transfer in the depleted electrode is usually inhibited.62,63 Contrary, in the accumulation regime, the concentration of majority carriers at the silicon surface is very important and the electrode usually displays metal-like properties. Determination of EF is thus crucial to find the position of the band edges and build the corresponding interfacial energetic diagram. EF is generally determined experimentally from the flat-band potential Efb (EF = qEfb) [i.e., the electrical potential difference between the semiconducting electrode and a reference electrode for which there is no excess charge near the semiconductor surface (no band bending)]. This parameter can be estimated from electrochemical impedance spectroscopy (EIS) measurements, and in particular, from the dependence of the semiconductor depletion layer capacitance (Csc) on the applied potential E. Under depletion conditions, a plot of Csc−2 versus E (Mott−Schottky plot) should be linear with a slope proportional to the dopant density and an intercept corresponding to Efb (eq 2).62−64

model of charge distribution and exchange at the interface uncomplicated by the existence of surface states16 lying in the forbidden band gap region of the semiconductor. With dependence on the relative position of EF and E0redox, different situations are encountered. If E0redox > EF and in the case of ntype silicon (Figure 1b), the majority charge carriers (i.e., electrons) will flow from the conduction band to the redox species. In the case of p-type silicon (Figure 2b), holes will be injected from the redox species into the valence band of the semiconductor. If E0redox < EF, reverse charge movements will occur. This charge movement generates an electric field localized near the surface of the semiconductor, which is represented by curvature of the bands (band bending) and characterized by a built-in potential ψsc. The region where this curvature exists is referred to as the space charge region. The concept of band bending is crucial to the understanding of (photo)electrochemical reactions at semiconductor surfaces.65 In the case of n-type semiconductor, majority carriers (electrons) are either depleted or accumulated in the space charge region if E0redox > EF or E0redox < EF, respectively, which results in excess of either positive (upward band bending) or negative (downward band bending) charge, respectively. These regions are therefore called depletion and accumulation regions, respectively. The opposite situation is encountered for a p-type semiconductor with the depletion or accumulation of holes in the space charge region if E0redox < EF or E0redox > EF, respectively. The width of the depletion region W can be approximated as ⎛ 2εε0ψsc ⎞1/2 W=⎜ ⎟ ⎝ qN ⎠

Csc−2 =

⎛ 2 kT ⎞ E − Efb − ⎟ 2⎜ q ⎠ qεε0NA ⎝

(2)

where A is the surface area of the electrode, k is the Boltzmann constant, and T is temperature. Interface energetics for a silicon surface derivatized with an electroactive molecule can be sketched as a semiconductorredox electrolyte interface.66 The presence of an insulating molecular linker between silicon and the electroactive molecule will not affect to a large extent the charge distribution at the interface. Indeed, the thickness of this molecular layer is usually in the range of 10−20 Å, which is much smaller than the widths of the silicon depletion and accumulation layers. To summarize, under accumulation conditions, the electrode will fastly exchange electrons (n-type) or holes (p-type) in the two directions with the grafted electroactive molecule by tunneling through the linker. Under such conditions, the problem can be theoretically treated using characteristic electrochemical equations available for electroactive molecule-modified conducting electrodes. Therefore, it is worth recalling some basic expressions extracted from cyclic voltammetry which has been the method most commonly employed to study the charge transfer characteristics of electroactive films bound to Si−H surfaces. In the case of a surface-confined reversible redox system, the anodic and cathodic voltammetric peaks are ideally symmetrical (i.e., there is no splitting between the anodic and cathodic peak potentials, Epa and Epc), and the corresponding

(1)

where ε is the relative permittivity of silicon (11.7), ε0 is the permittivity of free space, N is the dopant density of the semiconductor (expressed as Nd, the donor density for a n-type semiconductor, or Na, the acceptor density for a p-type semiconductor), and q is the electron charge. The corresponding charge density (in C cm−3) in this region is qN. W is usually on the order of micrometers while the width of an accumulation layer is very small (typically less than 100 Å) due to the high density of charges in the surface. Now, when a bias is applied to the interface versus a reference electrode, the Fermi level of the semiconductor moves, and the band bending can either decrease or increase with respect to the polarization (Figure 1c and 2c) because the majority carrier concentration changes at the surface of the semiconductor. By applying sufficiently positive (n-type) or negative (p-type) bias, the Fermi level can approach either the valence (n-type) or the conduction (p-type) band so that the concentration of minority carriers becomes sufficiently important to promote charge transfer reactions. This extreme 4813

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Figure 4. Determination of kapp from AC voltammetry and EIS measurements at electroactive molecule-modified conducting surfaces. (Top) Randles equivalent electrical circuit where Rsol is the solution resistance and CDL and Cads are the double-layer capacitance and adsorption pseudocapacitance, respectively, and Rct is the charge transfer resistance. To account for the nonideality of the electrochemical interfaces, in particular the frequency-dependent capacitive behavior, CDL and Cads are sometimes replaced by constant-phase elements (CPEs) for which the impedance is equal to C(jω)ϕ, in which C is the capacitance, ω is the angular frequency (ω = 2πf), and ϕ is an exponential term with a value between 0 and 1 (ϕ = 1 for an ideal capacitor).

100 V s−1 at least) with efficient compensation of the ohmic drop, two linear Epa, Epc − log v regions are usually observed at low and high v: a horizontal part at low v and a linear part with a slope equal to 2.3 RT/{(1-α)nF} (for Epa) or −2.3 RT/αnF (for Epc) at high v (Figure 3c). The intersect of these two linear regions at va (for Epa) or vc (for Epc) enables kapp to be determined using eq 6.

peak current intensities (Ipa and Ipc) are directly proportional to the potential scan rate v (Figure 3, eq 3).67 Ip =

n 2F 2 vA Γ 4RT

(3)

where n is the total number of exchanged electrons, F the Faraday constant, R the ideal gas constant, and Γ the surface coverage of bound electroactive units, which is commonly estimated from the area under either the anodic or cathodic voltammetry peak (eq 4).

Γ=

kapp = αnFvc/RT = (1 − α)nFva /RT

Another electrochemical technique sometimes used for the determination of kapp at electroactive molecule-modified silicon surfaces is the alternating current (AC) voltammetry initially developed by Creager and Wooster.71 A series of AC voltammograms are collected for a range of AC frequencies (typically, between 100 kHz and 0.1 Hz), where the ratio of the peak current, Ip, to the background current, Ib, is determined for each frequency f (Figure 4a). kapp is determined from the fitting of the experimental Ip/Ib versus log of the AC frequency plot using a simple Randles equivalent circuit model commonly applied for redox species attached to a monolayer (Figure 4b). One of the advantages of this method, due to its high sensitivity, is the determination of kapp for very low surface coverages of redox species. Another derived impedance approach which is also based on small AC sinusoidal signals overimposed on the potential waveform consists in the analysis of the frequency response of the modified electrode by measuring both imaginary Z″ and real Z′ parts of the total impedance at a constant potential.72 The frequency response of the impedance is then plotted in the form of Nyquist diagrams (−Z″ vs Z′) and fitted to the equivalent circuit model (usually Randles circuit including capacitors or constant-phase elements) in order to extract kapp (Figure 4c).73,74 In the case where the impedance data are obtained at E = E0redox, kapp can be simply expressed as

∫ IdE (4)

nFAv

Experimentally, a few tens of millivolts splitting (less than 50 mV) has been usually observed for electroactive moleculemodified silicon surfaces with the peak width at half-maximum either larger or smaller than the theoretical value of 90.6/n mV, indicative of either repulsive or attractive interactions between the immobilized redox centers.68,69 When the electron transfer is slow (quasi-reversible and irreversible cases), the cyclic voltammetry waves become distorted with a large peak-to-peak separation and again Ip is expected to vary linearly with v according to67 Ipa =

(1 − α)nnaF 2 vA Γ; 2.718RT

Ipc =

αnncF 2 vA Γ 2.718RT

(6)

(5)

where na (nc) is the number of electrons involved in the ratedetermining anodic (cathodic) step and α the charge transfer coefficient. Another important parameter extracted from cyclic voltammetry measurements is the apparent rate constant for electron transfer at the attached redox center, kapp. This can be commonly calculated using Laviron’s formalism (eq 6)70 based on classical Butler−Volmer theory. Upon increasing v, Epa and Epc corresponding to the redox process shift in positive and negative directions relative to E0redox, respectively, which reflects control of the voltammetry by the rate of electron transfer at the redox center (Figure 3c). Provided that a sufficiently broad v range is applied (typically between 0.1 and

kapp =

1 2RT = 2 2 2RctCads n F A ΓR ct

(7)

3.2. Under Light Irradiation

Now, we pay attention to the situation of the electrochemical interface under irradiation conditions. When the modified 4814

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the dopant concentration, the extent of the carrier recombination processes, and the illumination level. The irradiation of a depleted electrode under low light levels results in the distortion of the cyclic voltammetry wave.

electrode is irradiated with light of adequate wavelength (higher than the silicon gap of 1.1 eV), an electron−hole pair is generated in the semiconductor resulting in a delocalized electron in the CB and a delocalized hole in the VB.63,64,75,76 The key point is that the concentration of minority carriers is strongly modified under illumination, whereas that of majority carriers is not significantly altered. Consequently, photoeffects are generally not observed at accumulated n-type and p-type silicon electrodes for redox couples located at potentials negative and positive of Efb, respectively. In both cases, an (electron or hole, respectively) accumulation layer forms and the illuminated semiconductor behavior approaches that of an inert metal electrode. In depleted electrodes, the direction of the electric field at the interface is such that the minority carriers are swept to the surface and the majority carriers are driven to the rear ohmic contact. Therefore, the oxidation of bound reduced species by photogenerated surface holes or the reduction of bound oxidized species by photogenerated surface electrons will be primarily responsible for the photocurrents observed at illuminated modified n- or p-type silicon electrodes, respectively (Figure 5). For such reasons, the illuminated

4. ATTACHMENT OF BISTABLE REDOX MOLECULES From an electrochemical point of view, bistable redox molecules refer to molecules exhibiting two stable redox states in solution (i.e., both oxidized and reduced species are not degraded by a chemical reaction in the course of an electrochemical study). The attachment of electroactive molecules exhibiting more than two stable redox states (e.g., metal-complexed porphyrins and C60) will be detailed in the following section. 4.1. Ferrocene as the Redox-Active Center

Seminal works on ferrocene (Fc) self-assembled monolayers on gold have demonstrated that these electroactive structures can serve as excellent model systems for studying electron transfer at surfaces, the properties of the electrical double-layer, and the microenvironmental effects on long-range interfacial electron transfer kinetics.78−82 Owing to the attractive electrochemical characteristics exhibited by Fc (namely, fast electron-transfer rate, low oxidation potential, and stability of the neutral Fc and oxidized ferrocenium Fc+ species),83 its introduction in organic monolayers can also be of interest for the development of electrocatalytic, electroanalysis and biosensing devices, and “wired” enzyme electrodes.10,84,85 Pioneering work in the area of the derivatization of semiconducting surfaces (Ge, GaAs, and Si) with Fc was performed by Wrighton and co-workers.86−92 They reacted a dichlorosilane-substituted derivative with hydroxyl groups that were present on the oxidized surface of these semiconductors. The pursued goal was to use the grafted electroactive system to efficiently capture the photogenerated holes under illumination of these semiconductors and consequently protect them against the corrosion phenomenon. This surface silanization reaction has been then extended to attach other alkylsilane-substituted ferrocenes93 and other alkoxysilane-substituted electroactive molecules94,95 to oxidized silicon surfaces. The limited hydrolytic stability of these electroactive layers and the lack of reproducibility in their preparation have however seriously hampered their use in a variety of devices. Conversely, electroactive monolayers of both higher quality and higher stability can be prepared from the covalent attachment of Fc to oxide-free silicon substrates. Direct and multistep synthetic approaches have been used to functionalize such surfaces.96 Following the first approach, Fc substituted by vinyl 1a,97−109 ethynyl 2,103−106,110−112 and OHcontaining (methanol,113 carboxaldehyde 12,99,101 benzyl alcohol 13,49,114−116 and phosphines 14117) linkers have been grafted in a one-step procedure to Si(100) and Si(111) surfaces via the formation of an interfacial Si−C or Si−O bond (Scheme 2). Fc-modified Si(100) surface (3) was also prepared in onestep through aryldiazonium chemistry using a phenyltriazenederived Fc.118 Fc monolayers on silicon have also been prepared in several steps from reactive preassembled monolayers through organolithium (surface 1b119), “click” (4,120 5,121−124 and 6111,112,125,126), carbodiimide (8,127,128 9a129, and 9b130,131), acetylenic coupling 10132 and Heck coupling 11133 chemistries. Using thiol−ene click chemistry, Ravoo and co-workers have presented a versatile approach for the covalent layer-by-layer assembly of Fc on Si(100) surfaces, 7.134 This constitutes the first example of controlled

Figure 5. Photoinduced charge transfer processes at electroactive molecule-modified n-type (left) and p-type (right) silicon electrodes under depletion conditions.

depleted n- and p-type semiconductor electrodes are commonly called photocathode and photoanode, respectively. It is worth stressing that the observed photocurrents are affected not only by the energy of the incident photon but also by the extent of electron−hole recombination (annihilation of an electron and a hole) events diminishing concentrations of usable charge carriers and reducing the rates of electrochemical processes.77 Since carrier recombination is favored at defect sites on the electrode surface (i.e., surface states), care must be taken to prepare oxide-free functionalized surfaces to efficiently collect the photogenerated charge carriers and promote efficiently photoelectrochemical processes. At sufficiently high illumination levels (typically, in the range of 30−100 mW cm−2, i.e., ∼ 1/3−1 sun under Air Mass 1.5 G conditions), the cyclic voltammetry shape of an illuminated modified semiconductor electrode is expected to be very similar to that of a metal electrode but shifted along the potential axis. Under these conditions, the same electrochemical treatment can be applied. Moreover, the formal potential observed under illumination at an electroactive molecule-bound depleted silicon electrode is often shifted compared to the case of a similarly modified accumulated electrode in the dark or under illumination. This photovoltage is due to the magnitude of the voltage drop in the silicon which is significantly dependent on 4815

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Scheme 2. Si−C (Top) and Si−O (Bottom) Linked Ferrocenyl Monolayers and Multilayer onto Si−H

The electrochemical characteristics of Fc monolayers prepared following the one- and multistep procedures are summarized in Table 1. Some representative cyclic voltammograms of Fc-modified surfaces that differ from the nature and the length of the linker are shown in Figure 6. From data in Table 1, it can be noticed that silicon orientation has no significant effect on both the surface coverage of Fc (ΓFc) and the electroactive properties of the modified surfaces. For singlecomponent Fc monolayers (i.e., composed only of Fcterminated chains), ΓFc was found to vary over almost 2 orders of magnitude, from 6 × 10−12 to 5 × 10−10 mol cm−2. If one considers the ferrocene molecules as spheres with a diameter of 6.6 Å,136 the theoretical maximum ΓFc can be estimated at 4.8 × 10−10 mol cm−2. Experimental values measured for 1a,101 5,121,122 8,127,128 9a,129 9b,130,131 12,99 and 1349 are therefore indicative of highly densely packed electroactive monolayers.

preparation of Fc multilayers on Si. Compared with the onestep grafting method, the multistep procedure often involves milder conditions (e.g., reaction in aqueous medium at room temperature). Moreover, another advantage is that the functionalization step takes place on usually densely packed, well-ordered long-chain monolayers while the one-step attachment of substituted ferrocenes onto Si−H can produce both less ordered and less passivating monolayers owing to the steric hindrance of Fc. For some surfaces (1c,135 5,121,124 8,127 9a,129 11,133 and 1349), the dilution of Fc-terminated chains with nonredox organic chains has proved to be a useful strategy for controlling the concentration of bound Fc while maintaining the molecular orientation and ordering of the monolayer. Interestingly, in these mixed monolayers, the diluting chains keep the Fc groups well-separated from one another. 4816

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Table 1. Electrochemical Data of Ferrocene Monolayers Covalently Bound to Si−Ha surface 1a

1b 1c 2

3 4 5

6

7 8 9a 9b 10 11 12

13 14

silicon type and orientation p-(100) p-(100) p-(100)m n-(100) n-(111) n-(111) n-(111) p-(100) p-(111) p-(100) p-(100) p-(100) p-(100) n-(111) p-(100) p-(100) p-(100) p-(100) p-(100) p-(100) p-(100) p-(111) n-(111) n-(111) n-(111) n-(111)o p-(100) n-(111) p-(100) n-(100) n-(111) p-(100) p-(100)

surface coverage of ferrocene/mol cm−2 −10

(1.6 ± 0.8) × 10 6.7 × 10−12 d d 7.5 × 10−10 1.4 × 10−10 0.03−1.30 × 10−10 (4.2 ± 0.6) × 10−11 > 1.0 × 10−9e 3.1 × 10−11 3.4 × 10−11 d 2.8 × 10−9e 6.5 × 10−12 0.3−3.4 × 10−10 (2.2 ± 0.4) × 10−11 0.5−2.3 × 10−10 1.0 × 10−10 (1.1 ± 0.1) × 10−10 d 6 × 10−11e,j 0.2−3.5 × 10−10 3.5 × 10−10 0.25-5.0 × 10−10 (4.8 ± 0.4) × 10−10 (4.4 ± 0.7) × 10−10 (4.0 ± 1.7) × 10−11 d 5.0 × 10−10 d 2.1 × 10−10 2 × 10−12−4.2 × 10−10 1.2 × 10−10

E°′/V vs SCEb

kapp/s−1c

ref

0.38 0.36 0.40n 0.24f 0.36f 0.35f 0.37f,I 0.40 0.40 0.61 0.44 0.40−0.44l −0.24 0.11f 0.35 0.30−0.35 0.35−0.40 0.47 0.30−0.35 0.38−0.44l ∼ −0.20k 0.50 0.25f 0.13f 0.07f 0.07−0.18f 0.38 0.43f 0.50 0.05f 0.33f 0.62−0.77g 0.34−0.40h

130 62 d d d d 2−44g d d 160 104 d 160 d 5.5 8±2 d 30 d d d 50 d d d d 376 ± 58 d 10 d d 7 × 102−6 × 104g d

99, 100, 102 103, 105 97 99 101, 107 108 109 119 135 110 103, 105 111 118 120 121, 122 123 124 125 126 111 134 127 128 129 130 131 132 133 99 99 101 49 117

a

The mixed monolayers with Fc-terminated chains diluted with nonredox chains are highlighted in italics. bSaturated Calomel electrode. cApparent charge transfer rate constant for the bound ferrocenium/ferrocene couple. dNot reported. eFormation of multilayers occurred. fUnder illumination; no redox response was observed in the dark. gDepends on the surface coverage. hDepends on the substituent on the phosphorus atom. IVersus a Pd−H reference electrode. jSurface coverage per Fc layer. kBroad and strongly separated redox process corresponding to bound decasubstituted Fc. l Versus a Pt pseudoreference electrode. mPorous silicon. nAnodic peak potential, its cathodic counterpart being not observed. oMicropatterned Fc surfaces with pattern-size-dependent electrochemical behavior.

with low ΓFc were thought to be caused by the large number of Si−H sites remaining after the monolayer preparation which are susceptible to oxidation during the electrochemical measurements, as evidenced by, for example, postelectrochemistry X-ray photoelectron spectroscopy (XPS) measurements. Unlike single-component monolayers, mixed monolayers with low ΓFc show ideal electrochemical characteristics and high stability, provided that the total surface coverage (Fcterminated chains and nonredox chains) is close to the theoretical maximum value [i.e., (3−5) × 10−10 mol cm−2] and approximately constant in all prepared mixed monolayers. From Table 1, two other features about the electrochemical behavior of Fc monolayers can be noticed. Both E°′ and kapp for bound Fc are found to be highly dependent on the doping type of the underlying semiconductor but also on the length and the chemical composition of the organic linker. First, for most monolayers bound to p-type silicon, the E°′ of the bound Fc was within 50 mV close to the value measured for the molecules in solution (i.e., 0.40 V vs SCE for unsubstituted Fc+/Fc couple when studied in a common organic solvent).83 For some monolayers (2,110 8,127 12,99 and 1349), the E°′ of

When studied by cyclic voltammetry, such monolayers bound to p-type silicon show usually electrochemical features close to the ideal case for metallic surface-confined Nernstian redox species,67 namely, a small separation between Epa and Epc (a few tens of mV), a ratio between Ipa and Ipc around 1, and a fullwidth at half-maximum (fwhm) of the voltammetric peak close to 100 mV, indicating the presence of weak lateral interactions between the ferrocenyl centers.69 Moreover, their electrochemical stability over repeated cycling was relatively high, ranging from 103 cycles up to the remarkable number of 108 cycles reported for surface 13. At the opposite, values of ΓFc much higher than (4−5) × 10−10 mol cm−2 or smaller than 10−10 mol cm−2 are consistent with the formation of either uncontrolled Fc multilayers (except the controlled example of surface 7) or poorly densely packed monolayers, respectively. Both exhibit clearly nonideal surface electrochemistry characterized by a large peak-to-peak separation and fwhm higher than 200 mV (e.g., surface 4, Figure 6b). Additionally, their electrochemical stability was also reduced with an electroactivity loss of more than 15% after a few hundreds of cycles. Such features observed for single-component Fc monolayers 4817

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Figure 6. Representative cyclic voltammograms of Fc-modified Si surfaces. (a) Surface 1a on p-type Si(100) in CH3CN + 0.1 M NEt4ClO4 at 5 V/s (ΓFc = 2.4 × 10−10 mol cm−2). Reprinted with permission from from ref 99. Copyright 2005 Elsevier. (b) Surface 4 on p-type Si(111) at 0.1 V/s (ΓFc = 6.5 × 10−12 mol cm−2). Reprinted from ref 120. Copyright 2006 American Chemical Society. (c) Surface 5 on p-type Si(100) in 1.0 M HClO4 at 0.02 V/s (ΓFc = 3.2 × 10−10 mol cm−2). Reprinted from ref 121. Copyright 2009 American Chemical Society. (d) Surface 8 on p-type Si(111) in CH3CN + 0.1 M NBu4ClO4 (ΓFc = 3.0 × 10−10 mol cm−2). The scan rates are 0.1, 0.2, 0.4, 0.6, and 1 V/s. Reprinted from ref 127. Copyright 2006 American Chemical Society.

bound Fc was 100−250 mV positively shifted relative to that observed in solution. In the case of densely packed Fc monolayers, such a behavior can be explained by space-charge and counteranions/solvent exclusion effects at the monolayersolution interface.49 Moreover, as expected for a p-type semiconductor in accumulation conditions, the voltammetric characteristics of Fc modified p-type surfaces were found not to be sensitive to illumination. In contrast, no oxidation current or weakly intense oxidation/reduction currents with a large peakto-peak separation was observed for similarly modified n-type surfaces in the dark as expected for a semiconductor under depletion conditions, that is, when few majority carriers are available for charge transfer.63 Upon illumination with a sufficient light intensity (higher than ca. 30 mW cm−2),137 the full electrochemical reversibility of the bound Fc+/Fc couple could be reached owing to the fast oxidation of Fc by the photogenerated electron−hole pairs. As a result of the photogenerated electrons-induced activation of the redox process, the redox potential of Fc bound to n-type silicon under illumination is between 200 and 400 mV lower than that observed for Fc bound to p-type silicon in the dark or under illumination (Figure 7). Now, the effects of both the length and the chemical composition of the organic linker on kapp for bound Fc are somewhat difficult to rationalize. Nevertheless, some trends can be drawn. For short linkers, the presence of π-conjugated bond(s) in the organic chains results in increased kapp if one compares values higher than 100 s−1 reported for surfaces 2, 3, and 13 with those obtained for surfaces 1a and 12, in agreement with previous works on gold-bound Fc monolayers possessing oligo(phenyleneethynylene) and oligo(phenylenevinylene) bridges.80,138 The case of surface 13 is

Figure 7. CV in CH3CN + 0.1 M NEt4ClO4 at 10 V/s of surface 1c on n-type Si(111) under illumination (blue) and on p-type Si(111) in the dark (black). The photovoltage observed was 0.44 V. Reprinted from ref 135. Copyright 2013 American Chemical Society.

atypical because this is the only reported example of mixed Fc monolayers with a strong coverage dependence of kapp.49 Such an effect could be evidenced only when ΓFc was varied over more than 2 orders of magnitude, between 1 × 10−12 and 4 × 10−10 mol cm−2. At very low ΓFc ( 2 × 104 s−1), but as ΓFc is increased, a dramatic decrease in kapp was observed until reaching a limiting value of ca. 7 × 102 s−1 for ΓFc > 4 × 10−11 mol cm−2. This behavior that is observed only for extremely diluted mixed Fc monolayers has been attributed to spacecharge and counteranions/solvent exclusion effects at the 4818

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Figure 8. (a) Fc-micropatterned silicon surfaces for all-solid AND molecular logic gate using the capacitance response as the output signal. (b) Phase-mode atomic force microscopy (AFM) image of the surface micropatterned with 5 × 5 μm2-squares of Fc-terminated chains separated by 5 μm of butylamide-terminated areas. (c) Capacitance-time profiles at 50 Hz measured at different applied potentials during several dark (OFF state)/ illumination (ON state) switching cycles. Electrolyte: CH3CN + 0.1 M Bu4NClO4. Reprinted with permission from ref 130. Copyright 2013 WileyVCH.

monolayer-solution interface.49 It is worth noting that this trend is however contradictory to that observed in other analogous studies showing either no or weak dependence within a narrow ΓFc range (from 2 × 10−11 to 3 × 10−10 mol cm−2)109,127 or an increase with ΓFc within a broader ΓFc range (from 2 × 10−12 to 3 × 10−10 mol cm−2).139 For longer linkers, the presence of a 1,2,3-triazole group resulting from the Fc grafting by click coupling chemistry yields monolayers with slower rates (compare surfaces 4, 5, and 6 with 8 and 10). This is probably ascribed to the differences in the molecular packing and tunneling distance rather than electronic effects because it has been reported that this type of linkage had a small to negligible effect on the rate of electron transfer.140,141 Globally, values of kapp reported for Fc-modified silicon surfaces were found to be systematically smaller than those of analogous monolayers bound to gold.78,82 Several assumptions can be involved to explain this trend. The first one is related to differences in the electronic coupling between the surface and the grafted organic molecules with a stronger electronic coupling between gold and the orbitals of the organic chains. The second arises from the band structure of silicon. Due to the energy band gap of silicon, the charge-transfer kinetics is expected to be slower than that on metal surfaces owing to the applied energy dependence of the concentration of charge carriers (electrons and holes) to promote the oxidation/ reduction processes. For metals, the number of electronic states (i.e., the density of states DOS) is high and, consequently, there is a large concentration of electrons available to participate in electron transfer reactions.62 Finally, the occurrence of charge transfer via surface states (e.g., patches of silicon oxide) produced during the electrochemical investigations can not be ruled out and might also account for the smaller rate constants. The stable and ideal electrochemical characteristics exhibited by some Fc-functionalized silicon surfaces have aroused peculiar attention for the development of charge storage devices in which the two redox states of the grafted electroactive molecule would constitute the two memory binary states. Indeed, these molecular interfaces could provide some benefits in the current memory devices, in terms of redox stability, charge-retention time, and capacitance. Moreover, because the potential required to oxidize ferrocene is much lower than 1.0 V, the charge could be stored with lower power consumption. Higher charge densities are also expected because the surface coverages reached for high-quality ferrocenyl monolayers are in the range (1.0−4.0) × 10−10 mol cm−2,

which corresponds to charge densities in the range of 10−40 μC cm−2, these values being much higher than those measured for Si/SiO2 capacitors currently used in dynamic random access memories (DRAM). The charge storage capabilities of model Fc-functionalized surfaces have been evaluated from capacitance−voltage and chronoamperometry-derived measurements. First, differential capacitance measurements of some Fcterminated monolayers bound to p-type silicon showed capacitance peaks at a potential relatively close to E°′ of the bound Fc+/Fc couple, the amplitude of which increased with decreasing the measurement frequency. These peaks were clearly attributed to the charging/discharging currents associated with the oxidation/reduction of Fc. So, capacitance maxima in the range of 20−30 μF cm−2 and higher than 100 μF cm−2 have been measured at 100 Hz for micrometer-sized capacitor structures and millimeter-sized electrodes in conventional electrochemical cells, respectively. To compare, the capacitance density of today’s DRAM storage cell is higher than 100 fF μm−2 (10 μF cm−2) with a benchmark cell size below the square-micrometer range.142 The second essential parameter for memory systems is the rate of charge dissipation. The charge-retention time of an electroactive monolayer-modified surface can be expressed as the half-life time t1/2 [i.e., the time required for converting the half of the electrogenerated oxidized species (Fc+) to the reduced species (Fc) after the electrical disconnection of the modified surface]. So, t1/2 values in the range of 150−200 s have been measured by chronoamperometry for surfaces 8143 and 1349 at high ΓFc (>5 × 10−11 mol cm−2) and were found to decrease until a few tens of seconds for much more diluted monolayers. Even though comparison of such values determined in liquid electrolytes with those measured for solid state devices may be somewhat misleading, it is worth mentioning that the charge in a typical DRAM cell is retained for ca. 64 ms because the dielectric used in the DRAM capacitor is not able to stabilize the charged state over a longer time.142 Another important issue that must be addressed concerns the thermal stability of Fc-functionalized surfaces. Indeed, high-temperature conditions (higher than 400 °C) are often used during the fabrication steps of the electronic components. Even though it has been reported that unsubstituted Fc was decomposed only above 450 °C,144 experimental evidence must be provided that Fcmodified surfaces show not only a comparable or higher thermal resistance but also the film structure, and its electrochemical characteristics are not significantly degraded after high-temperature treatment. 4819

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Scheme 3. Other Redox-Active Metallic Complexes Covalently Bound to Oxide-Free Silicon Surfaces (Ph = Phenyl)

approach for constructing an AND gate is quite unique and really different from that based on other outstanding redoxactive, functionalized surfaces and reported by van der Boom and co-workers.145,146 As a matter of fact, this monofunctional layer-based, all solid-state device is entirely optical and electrical addressable and does not require the addition of inputs in solution to function. This concept of light-activated electrochemical addressing of grafted electroactive centers has been refined and exploited for localizing electrochemical reactions on monolithic silicon surfaces. This promising strategy combining a high quality Fc monolayer and a focused light source has been demonstrated with the photochemical switching of a heterogeneous catalytic reaction at a local scale from the surface 5.150 Thanks to differences in redox potentials between surface-bound and dissolved redox species, the oxidation of n-type silicon-bound Fc to Fc+ under illumination promoted the catalytic oxidation of ferrocyanide to ferricyanide in solution. This original approach will undoubtedly allow the widening of this research topic scope toward the elaboration of functional micropatterns on surfaces, spatially controlled and light-directed sensing, and electrocatalysis.

Besides the reversible storage of information at the molecular level, Fc-modified silicon surfaces offer also novel opportunities for the elaboration of photochemically switchable electrical devices. Keeping in mind that the electrical properties of n-type silicon can be reversibly switched from an insulating to conducting state upon light irradiation for oxidation processes, this provides a unique way to communicate efficiently with the bound Fc redox-active centers when the light is turned on. Based on this idea, it has been demonstrated that the capacitance at 50 Hz of a n-type silicon electrode micropatterned with Fc-functionalized squares could be switched from a few 10−1 μF cm−2 in the dark to ca. 100 μF cm−2 under illumination when the applied potential was close to the E°′ of bound Fc, ca. 0.10 V versus SCE (Figure 8).130,131 Consistent with the potential-dependent response, the capacitance switching was much lower for potentials located before and after E°′. The high capacitance state was retained as long as the voltage was applied and remained constant over numerous ON/OFF switching cycles. A ca. 10% decrease in the maximum photocapacitance was observed after 103 ON/OFF switching cycles. Such characteristics were thought to be ascribed to the unprecedented high rate of lateral charge propagation between the Fc headgroups.128 These are of great scientific interest for data-processing applications, such as redox-based Boolean logic gates.145−149 As a matter of fact, the switching of the capacitance upon light irradiation that is observed only at a certain electrical potential can be applied for constructing a prototypical two-input AND logic gate, the two inputs being defined as the illumination level and the applied electrical potential. Thus, the output signal (capacitance) of the Fc layer increased by 3 orders of magnitude only when the two inputs are in their state “1”, namely upon light irradiation and an applied potential of ca. 0.10 V. It is worth stressing that this

4.2. Other Metallic Complexes

In parallel to studies devoted to Fc-containing redox assemblies, redox-active electron-rich organometallic acetylides of the group 8 metals (iron and ruthenium in particular) have been demonstrated to possess a strong potential for the reversible storage of information at the molecular level.151−159 For instance, in the field of optics, the high stability of these compounds under several redox states allows for simple electrochemical generation of families of highly polarizable compounds exhibiting widely different electronic transitions in 4820

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Figure 9. Cyclic voltammograms of (a) 15a−15c at 0.2 V s−1, (b) 18 at different scan rates (between 0.025 and 1 V s−1), and (c) 20a−20j as a function of the number of the coordination cycles n with (d) the corresponding linear coverage dependence on n. The slope is ca. 3.5 × 10−11 mol cm−2 per layer. Electrolytic solution: (a) CH3CN + 0.1 M Bu4NClO4; (b and c) CH2Cl2 + 1 M Bu4NClO4. Reprinted from ref 174. Copyright 2013 American Chemical Society. Reprinted with permission from ref 176. Copyright 2014 Wiley-VCH. Reprinted from ref 179. Copyright 2012 American Chemical Society.

coverage of bound iron was electrochemically estimated at (2.1−2.2) × 10−10 mol cm−2, irrespective of the length of the bound complex. Given the steric hindrance of these molecules, this indicates a dense packing of the metal center-containing chains on the silicon surface. The charge transfer rate constants for the bound mononuclear complexes were of some hundreds of s−1 with a slight increase in kapp with the increasing length of the conjugated spacer, from 350 s−1 (15a) to 600 s−1 (15c). Such a trend was thought to be ascribed to the improvement of the overall order of the monolayer structure.175 Finally, as expected, the monolayers prepared from the derivatives possessing an alkyne unit in meta position and the alkene unit exhibit lower charge transfer rates (kapp = 55 and 140 s−1 for 16 and 17, respectively) because of smaller degree of conjugation between the molecule and the silicon surface. Recently, the Nishihara’s group has proposed a bottom-up versatile approach to fabricate redox-active metal complex oligomer wires on Si(111)-H surfaces.176−179 This was based on the formation of coordination complexes between terpyridine (tpy) ligands and iron(II) cation. Such complexes were attached to silicon through either a π-conjugated phenylcontaining linker (18,176 20,179 and 21178) or a phenyloxy tether (19176) and the resulting monolayers showed a perfectly reversible one-electron system ascribed to the bound Fe(III)/ Fe(II) couple at relatively positive potentials, ca. 0.80 V vs Ag+/ Ag (i.e., 1.10 V vs SCE) (Figure 9b). The packing density of these films was comparable (Γ ≈ 4.7 × 10−11 mol cm−2), but the interfacial electron-transfer rate constant determined for the Si−O-linked monolayer 19 was significantly higher than that measured for 18 (310 s−1 against 480 s−1), which might seem surprising in view of the degree of conjugation of the bridging unit. From density functional theory (DFT) calculations, the authors explained this trend by a better electronic coupling between the [Fe(tpy)2] unit and silicon in 19,176 in line with

the different redox states, which also translate into widely different linear and nonlinear optical properties.160−162 Moreover, in contrast to most organic derivatives, their peculiar redox chemistry offers a unique possibility for reversible redoxswitching between these states. Thus, several groups have shown that ruthenium-containing representatives of such molecules when covalently anchored on (semi)conducting surfaces, such as halogenated silicon163,164 and gold,165−171 exhibited a molecular conductance which could be electrochemically gated. It is worth noting that the use of halogenated silicon as the immobilization platform led to SiN-linked organometallic monolayers exhibiting small interfacial electron-transfer rate, essentially owing to the insulating character of the bridge between the surface and the redox-active complex and/or the fragility of the interfacial bond.163,164 Such a feature might prove critical for developing electrochemically switchable devices exhibiting fast clocking rates. Since “π-conjugation” between the organometallic species and the surface can in principle be maintained when the grafting is conducted from an alkyne-terminated molecular precursor and Si−H, Fabre, Paul, and co-workers have prepared Si−H-bound monolayers from mononuclear iron(II) complexes possessing conjugated oligo(phenyleneethynylene) bridges of variable length and a terminal alkyne unit (15a−15c, Scheme 3).172−174 The molecular precursors with an alkyne unit in the meta position (17) and a terminal alkene unit (16) have been also considered in order to examine the effects of both the position and the nature of the linking unit on the rate of charge transfer of the resulting assemblies. Cyclic voltammograms of the resulting modified surfaces showed a perfectly reversible one-electron redox system attributed to the iron(II)/iron(III) couple of the terminal metal centers, at relatively low potentials, close to −0.1 V versus SCE (Figure 9a) and in agreement with the redox potentials determined for the dissolved complexes. The surface 4821

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straightforward strategy was developed. Indeed, Re6-functionalized surfaces were obtained by a simple water elimination reaction between the OH units of the metal cluster and carboxylic groups of an undecanoic acid monolayer bound to silicon (25, Scheme 3).189 This enabled the formation of strong covalent Re6−O bonds, resulting in a stable organic chain/ cluster interface. Interestingly, the surface coverage of the Mo6 or Re6 cluster could be finely controlled by preparing mixed monolayers containing reactive chains diluted with inert alkyl chains with a predefined surface chain ratio. Different surface characterization techniques, such as scanning tunneling microscopy (STM, Figure 10), spectroscopic ellipsometry,

the fast electron transfer previously reported for the benzyloxylinked Fc-modified surface 13 (Scheme 2).49 Interestingly, Nishihara’s approach enables redox-active assemblies to be sequentially grown by successive coordination of tpy ligands with Fe(II) without affecting their electrochemical characteristics (Figure 9c). A linear coverage dependence on the number of [Fe(tpy)2] complex layer was observed with a slope from 2.8 × 10−11 (21) to 3.5 × 10−11 (20) mol cm−2 per complex layer. The electroactive wires in 21 were diluted with short nonelectroactive vinylbenzene units to protect the remaining Si−H sites against possible oxidation and, consequently, to ensure the stability of the resulting mixed monolayers.178 A branched electroactive architecture 22 was also developed by the same group, but a nonlinear coverage dependence on the number of coordination cycles was observed which was ascribed to the formation of cross-linked segments inside these redox architectures.178 The formation of coordination complexes between surfaceconfined 2,2′-bipyridyl (bpy) ligands and metal cations has been used by Lattimer et al. as the driving force to immobilize some metal reagents on unoxidized silicon surfaces. Mixed monolayers consisting of short bpy-terminated chains diluted with methyl groups have been successfully modified by different VIIIa-group metal cations exhibiting efficient catalytic activity in solution, such as Rh, Ru, and Ir (23a, Scheme 3).180 The Rhand Ru-metallized monolayers were electroactive and showed a single quasi-reversible redox process at −1.1 V and −0.48 V vs Fc+/Fc, respectively, attributed to either the two-electron reduction of bound Rh(III) to Rh(I) or the one-electron oxidation of bound Ru(II) to Ru(III). In contrast, any electroactivity was not observed for the Ir-metallized surface. Nevertheless, due to both a low surface coverage of electroactive Rh and Ru (only 0.2% of a monolayer) as well as a limited stability over electrochemical cycling, scientifically and technologically relevant (photo)electrochemical catalysis experiments have not been undertaken with the redox-active metallized surfaces. In contrast, successful catalytic results for H2 generation have been obtained with the surface 23b incorporating a nickel-phosphine complex.181 Besides the immobilization of coordination metallic complexes, the grafting of metal atom clusters, and in particular octahedral metal clusters M6Li8La6 (with M = Mo, Re, W, Nb,...; L = halogen and/or chalcogen ligand; i = inner and a = apical), onto silicon surfaces has been motivated to take advantage of luminescent, magnetic, and redox properties shown by this family of clusters at the solid state.182−185 Interestingly, the electronic, optical, and magnetic properties of these metal clusters are highly dependent on the nature of the metal and ligands to which they are bonded as well as to the geometry of the units. In solution, they show redox oxidation processes that may influence their luminescent or magnetic properties. Moreover, these metallic clusters can be functionalized by exchange of halogen in apical position with a functional donor ligand. Thus, two approaches have been developed for the grafting of these metal clusters onto silicon surfaces. The first one that has been used for the anchoring of a hexamolybdenum cluster was based on the substitution reaction of one or several apical CF3SO3− group(s) of [(Mo6Ii8) (CF3SO3)a6]2− by the pyridine units end-capping an organic monolayer covalently bound to a n- or p-type Si(111)-H surface (24, Scheme 3).186,187 For the immobilization of hexarhenium clusters and thanks to stability and the reactivity of the [(Re6Sei8)(tertbutylpyridine)a4(OH)a2] with carboxylic acids,188 a more

Figure 10. STM images of the undecanoic acid/dodecyl-modified Si(111) (a) before and after grafting of Re6-based cluster (surface 25) at different surface Re6 coverages: (b) (4.5 ± 0.3) × 10−11 and (c) (1.7 ± 0.3) × 10−11 mol cm−2. Size: (a) 500 × 500 nm2 and (b and c) 200 × 200 nm2. Atomic steps of the underlying Si(111) substrate are clearly visible in (a). (d) Cyclic voltammograms at 0.1 V s−1 in CH3CN + 0.1 M Bu4NClO4 of 2 mM [(n-C4H9N)2Mo6I14] on a 1 mm diameter platinum disk electrode (i), and surface 24 with a surface Mo6 coverage of (ii) (4.0 ± 0.5) × 10−11 or (iii) (1.0 ± 0.5) × 10−11 mol cm−2. The irreversible one-electron oxidation peak observed at 0.97 V vs SCE corresponds to the clusters oxidation, from 24 to 23 electrons per Mo6 unit. For easier comparison, the current intensity scale in curve (i) has been multiplied by 15. Reprinted from ref 189. Copyright 2010 American Chemical Society. Reprinted with permission from ref 186. Copyright 2007 Wiley-VCH.

and XPS, were used to demonstrate the covalent immobilization of clusters. Furthermore, their integrity within the monolayers was confirmed by their vibrational Raman (characteristic bands around 200−250 cm−1) and electrochemical (Figure 10d) signatures. Importantly, the electronic and charge transport properties of these novel modified electrodes were found to be controlled by the surface coverage of the metal cluster with a clear evidence of the charge storage in the metal cluster for cluster coverages higher than ca. 6.5 × 10−11 mol cm−2. 4.3. Quinones

Quinones are a fascinating class of electroactive molecules because of their typical proton-coupled electron transfer 4822

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Scheme 4. Quinone-Functionalized Si−H Surfaces

Figure 11. Effects of pH and the light intensity on the electrochemical response of the anthraquinone-terminated monolayer bound to poorly doped p-type Si(100)-H (surface Q3, Scheme 4). (a) pH was varied from 10 to 5; light intensity: 94.2 mW cm−2. (b) The light intensity was varied from 10.3 to 94.2 mW cm−2; pH 10.1. All CVs were performed in Britton Robinson buffer at 2 V s−1. Reprinted from ref 197. Copyright 2016 American Chemical Society.

process in aqueous medium,190 their electrochemical/chemical activation to further attach biomolecules (e.g., via Diels−Alder reaction),191 and their key role in vital cellular processes. In an aqueous environment, quinone and derivatives (naphtoquinone and anthraquinone) can be reversibly reduced into their corresponding hydroquinone form through an overall electrochemical process involving two protons and two electrons.192−194 Owing to these attractive characteristics, surfaces functionalized with such moieties have been demonstrated to show great promise for diverse applications, ranging from directed growth or micropatterning of cells and proteins to biosensing. Hydroquinone monolayers have been covalently attached to Si−H surfaces from the UV-induced hydrosilylation reaction with an alkene ω-substituted by a protected quinone, followed by a deprotection step of the terminal moiety (surface Q1,

Scheme 4).195 In a phosphate buffer medium (pH 7.4), the grafted hydroquinone moieties could be electrochemically oxidized at potentials above 0.6 V versus Ag/AgCl and the electrogenerated quinone units were then used for the covalent attachment of a functional cyclopentadiene or thiol via a Diels− Alder reaction or Michael addition. This functionalization route has been exploited for the electrochemically driven, spatially selective immobilization of proteins such as streptavidin. Nonspecific adsorption of proteins with a high sensitivity to oxidation of the underlying surface was however observed when Si(100) was used as the starting substrate instead of Si(111). Such a result was believed to be caused by a weakly densely packed electroactive monolayer (the surface coverage of hydroquinone on Si(100) was ca. 1.0 × 10−10 mol cm−2). The selectivity of this postgrafting quinone-based functionalization approach has been recently improved by branching directly 4823

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Scheme 5. Metal-Complexed Porphyrins Covalently Bound to Oxide-Free Silicon Surfaces

to lactone system together with the release of the oligo(ethylene oxide) chain. Interestingly, the surface Q2a could be chemically regenerated in three steps from the oxidation of

a short antifouling oligo(ethylene oxide) molecule to the grafted quinone (surface Q2a).196 The electrochemical reduction of the surface Q2a yielded the conversion of quinone 4824

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5.1. Systems Exhibiting More than Two Reversible Electron Transfer Steps

confined lactone to benzoquinone acid then formation of the activated ester followed by a carbodiimide coupling. Due to the presence of antifouling chains, the surface Q2a was found to be resistant to the cell adhesion while adhesion was greatly enhanced on the surface Q2b (see Section 7). In the two previous examples, the grafted quinones were used to dynamically switch the surface properties of modified silicon. The immobilization of these redox-active units on Si−H can also give rise to original electrochemical events. In a recent work, it has been elegantly demonstrated that the redox processes associated with the grafted quinone could be selectively modulated either by changing the electrolyte pH or by varying the light intensity.197 Such electrochemical features were not observed at metallic electrodes and therefore were clearly due to the semiconducting character of the electrode. For pH-dependent redox systems like quinones studied at metals, both the anodic and cathodic processes are expected to shift in potential with a theoretical slope of −59 mV pH−1 (for a two electrons and two protons system), whatever the illumination conditions. In the case of the anthraquinone-terminated monolayer covalently bound to Si− H (as shown in Scheme 4), typical pH- and light-dependent electrochemical changes occur only if the redox moiety is immobilized on a poorly doped p-type silicon (∼10 Ω cm resistivity). Indeed, it is essential that silicon is under charge depletion conditions in the dark when reductive potentials are applied. As a matter of fact, this situation is somewhat analogous to that of oxidizable ferrocene immobilized on n-type silicon.96,130 With the surface Q3 in hand, it becomes thus possible to selectively facilitate either the oxidation process of the grafted anthraquinone by increasing pH or its related reduction process by increasing the light intensity at a given pH (Figure 11). Such modifications which were ascribed to the differences in the nature of charge carriers (photogenerated electrons and holes) involved in each redox process may hold promise for the controlled manipulation of electrocatalytic processes.

5.1.1. Metal-Complexed Porphyrins. When studied in solution, metalloporphyrins show very exciting electrochemical characteristics, such as multiple electron transfer steps at relatively low potentials, chemical stability of the different redox forms under ambient conditions, and a versatility of their redox properties depending on the nature of the complexed metal.198 Their immobilization and their control at surfaces could be exploited for the elaboration of functional nanostructures, organized layers, and bioinspired systems.199 Moreover, their integration to electronically relevant silicon could provide a real breakthrough in the field of molecular-based information storage.200−202 Indeed, metalloporphyrin-modified silicon surfaces could be used as multibit information storage media with high charge density in which electrical charge is stored in multiple redox states of the bound molecules. Extensive efforts by the Bocian and Lindsey groups200 have been devoted to the preparation and direct grafting of numerous zinc-, cobalt-, copper-, and nickel-complexed porphyrins incorporating different anchoring atoms (O, S, and Se) and organic bridges (benzyl and alkyl) in order to analyze their effects on both the electrochemical characteristics (electron-transfer rates, position of the redox potential, stability over a repeated cycling) and the properties of charge retention of the resulting assemblies on silicon (Scheme 5). Only p-type surfaces (100) have been considered owing to the ease and swiftness of the preparation of Si−H (etching in diluted HF for a few min) but also to avoid photogenerated charge carrier effects. One-step covalent grafting of porphyrin monolayers onto Si(100)-H surfaces was achieved by using a high-temperature (400 °C), short-time (2 min) attachment procedure. This unusual very-hightemperature attachment strategy yielded robust and densely packed porphyrin monolayers with a surface coverage only slightly lower than the saturation coverage (∼10−10 mol cm−2).203 The structural and electron-transfer characteristics of Zn(II)-complexed porphyrin monolayers were found to not depend on the nature of the anchoring atom (C, O, S, or Se) but rather on the nature of the organic linker (Table 2). At oxidizing potentials, these monolayers exhibited two welldefined reversible one-electron voltammetric waves attributed to the formation of the mono- and dication porphyrin πradicals (Figure 12). The associated electron-transfer rate constants kapp showed a surface coverage dependence similar to that observed for some ferrocene-terminated monolayers49 (surface 13), ranging in the cases of 26204 and 27205 from smaller than 104 s−1 to 105 s−1 upon decreasing Γ from ca. 5 × 10−11 to 10−12 mol cm−2. Such inverse correlation between the electron-transfer rates and the porphyrin surface coverage has already been reported for porphyrin monolayers on gold206 and was attributed to space-charge effects among the oxidized molecules.207 Furthermore, the kapp values calculated for monolayers bearing a benzyl linker (28,208 29,204 and 30205,209,210) were approximately twice smaller than those of monolayers without this linker. Such a trend was consistently explained by a longer distance between the redox-active center and the surface.205 The surface coverage of the metalloporphyrin center on silicon could be varied in a controlled fashion by either changing the reactive porphyrin concentration (from a few micromolar to millimolar) used for the preparation of the single-component monolayer or coattaching porphyrinterminated chains with nonredox 4-biphenylmethoxy diluting chains, as in the case of 31.49,211 Even though the first approach

5. ATTACHMENT OF MULTISTABLE REDOX MOLECULES Si−H surfaces functionalized with molecules undergoing multiple reversible electron transfer processes in solution have aroused an increasing interest owing to their potential for the design of high storage capacity molecular memories and the development of sophisticated molecular logic gate platforms. Furthermore, if the targeted molecule is a molecular catalyst, we can hypothesize that these attractive electrochemical characteristics could be exploited in supported catalytic reactions, in particular, to fine-tune the catalytic activity (efficiency and selectivity) of the modified semiconducting electrode depending on the applied electrical potential, in other words on the redox state of the immobilized catalyst. In this thematic area, Si−H surfaces modified by systems exhibiting usually three or more reversible electron transfer steps in solution (e.g., metalloporphyrins and polyoxometalates) have been more largely documented and will be therefore described first in this section. Such a trend can be essentially explained by the opportunity to access electronically richer and more versatile electrochemically active interfaces compared with surfaces modified by systems exhibiting “only” two reversible electron transfer steps that will be treated later. 4825

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Table 2. Electrochemical Data of Metalloporphyrin Monolayers Covalently Bound to Si−H surface

metal

interfacial bond

26b 27c

Zn Zn Zn Zn Zn Zn Zn

Co

Si−(CH2)n− Si−O−CH2− Si−S− Si−Se− Si−(CH)2− Si−(CH2)n− Si−O−CH2; R1 = −CH3 Si−O−CH2; R1 = −CH3 Si−O−CH2; R1 = −CH3 Si−S−; R1 = −CH3 Si−S−; R1 = −CH3 Si−Se−; R1 = −CH3 Si−S−

31c

Zn

Si−O−

32c

Zn Zn

28c 29b 30c

Zn Zn Zn Zn Zn

32b

Cu Zn

33e

Co

Si−(CH2)n− Si−(CH2)n−; R1 = R2 = −CH3 Si−(CH2)n−; R1 = −CH3; R2 = −CN Si−(CH2)n−; R1 = −CN; R2 = −CH3 Si−(CH2)n− Si−(CH2)n−; R1 = R2 = −CH3 Si−Ph

34 35e 36c

Zn Ni Zn

Si−(CH2)n− Si−(CH)2− Si−(CH2)n−

37c

Eu

Si−O−CH2−

38b

Eu

Si−(CH2)n−

Zn Zn

c

a

E°′ (V ) 0.56; 0.64; 0.64; 0.61; 0.88; 0.62; 0.67;

0.91 0.96 0.96 0.96 1.22 0.98 1.03

∼0.40; ∼0.75 0.79; 1.15 0.72; 1.02 0.79; 1.15 0.70; 1.06

surface coverage (mol cm−2)

ref

10−11 10−11 10−11 10−11 10−11 10−11 10−11

204 205 205 205 208 204 205

4.5 × 10−11

210

6.0 5.5 5.7 5.5 9.8 9.2 6.9

× × × × × × ×

__d

209

6.2 × 10−11 __d

7.4 × 10−11

205 209 205

__d

209

0.60; 0.90 0.53; 0.90

0.1−6.4 × 10−11 1.8 × 10−10 1.9 × 10−10

49, 211 213 214

0.76; 1.06

3.2 × 10−10

214

0.73; 1.04

2.5 × 10−10

214

1.02; 1.25; 1.52 0.90; 1.30

−10

0.90; 1.10 0.50; 0.83

3.1 × 10 2.2 × 10−10

214 215

−0.55; −1.80 0.61; 0.99f 1.26 ∼0.52; ∼0.87 0.51; 0.82; 1.28; 1.47 0.42; 0.67; 1.05; 1.30h

1.0 × 10−9g

216

__d

6.0 × 10−11 2.0−5.1 × 10−10 ∼2.5 × 10−11 6.3−8.0 × 10−11

Figure 12. Representative fast-scan (100 V s−1) cyclic voltammograms of Zn(II)-complexed porphyrin monolayer-modified Si(100) surfaces: 30 (R1 = CH3; left panel side) and 27 (R1 = R2 = H; right panel side). (a and d) X = O; (b and e) X = S, and (c and f) X = Se. Reprinted from ref 205. Copyright 2004 American Chemical Society.

111 217 218

multibit information storage interfaces exhibiting three cationic redox states at distinct voltages.212 This relative simple and successful approach allows for the avoidance of tedious and time-consuming synthesis of a single molecule that would exhibit the same number of distinct redox states. Because ferrocene and porphyrin are structurally very different, it is however expected that their homogeneous distribution in the mixed monolayer is not straightforward to control. In order to increase the packing density in the porphyrin monolayers, porphyrins containing a tripodal carbon linker were covalently anchored on Si(100) through the “baking” (400 °C for 2 min) hydrosilylation reaction between the allyl tripod and Si−H sites (surface 32).213−215 These surfaces exhibited charge transfer rates and charge retention times which were comparable to those of surfaces derived from porphyrins with monopodal tethers.215 Nevertheless, as expected, the packing density of tripodal porphyrin monolayers was considerably enhanced as supported by the corresponding surface coverage values which were in the range of 2.0−3.0 × 10−10 mol cm−2 (Table 2) (i.e., approximately 3-fold higher than those obtained for monopodal porphyrin monolayers). Surprisingly, this characteristic was not due to a more upright orientation of porphyrins on the surface insofar as the average tilt angle of the porphyrin ring with respect to the surface normal was found to not depend on the number of the anchoring sites of the porphyrin molecule.215 A plausible explanation for this increased packing density could be the presence of torsional constraints imposed by the tripod.

209 220

a

Formal potentials corresponding to reversible one-electron redox processes, referred to Ag+/Ag unless otherwise stated. bCH2Cl2 + 1.0 M Bu4NPF6. cPropylene carbonate +1.0 M Bu4NPF6. dNot indicated. e CH3CN + 0.1 M Bu4NClO4 or Bu4NPF6. fV vs Pt pseudoreference. g Formation of multilayers occurred. hThe redox potentials were not really dependent on the location of the tripod on the triple-decker complex.

yielded electroactive monolayers with electron transfer rates comparable to those of the mixed monolayers, the second approach afforded a remarkable improvement in the robustness of the modified surfaces which could be repeatedly cycled over more than 1010 redox cycles (ca. 30 days) against some thousands of cycles for the first approach. Also interestingly for the design of molecular memories, the charge-retention characteristics of the mixed monolayers were noticeably higher than those of single-component monolayers, with t1/2 values reaching 200 s49 against 40 s204 at a surface coverage of porphyrin of 6 × 10−11 mol cm−2. The preparation of 13/31bifunctionalized surfaces without nonredox 4-biphenylmethanol diluent has been achieved with the purpose of achieving 4826

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Besides this considerable body of work developed by the Bocian and Lindsey groups, there are few other examples of covalent grafting chemistry of redox-active porphyrins onto Si− H surfaces. In the first example, Tour and co-workers reported the direct grafting of metalloporphyrins bearing in situ generated aryldiazonium groups onto Si(100)-H surfaces (33).216 Compared with the high-temperature, short-time hydrosilylation procedure, this grafting method worked under ambient conditions. But as commonly observed for other systems immobilized onto semiconductor or metal surfaces through diazonium chemistry,40 this approach failed to produce systematically and reproducibly good quality monolayers, in terms of ordering and electron-transfer characteristics. Instead, the multilayer formation was observed with surface coverages of ca. 1 × 10−9 mol cm−2 (i.e., approximately 10-fold higher than the saturation coverage). A click chemistry approach has also been used to attach porphyrin monolayers on Si−H surfaces. This was based on the reaction of azido217 or acetylenic111 groups in the porphyrin with an acetylene- or azide-terminated alkyl monolayer previously deposited on Si−H. The surface coverage of porphyrin in 34 and 35 was found to be ca. 6 × 10−11 mol cm−2 and, as a consequence of the long alkyl linker between silicon and the bound redox-active center, electron transfer was much slower than that occurring in the electroactive monolayers wherein the porphyrin was linked closer to the silicon surface, such as 28, 29, or 30. Consistent with that, further electrical measurements with 34-based porphyrin capacitors also showed slower charging/discharging processes of the bound molecules.111 It should be noted that while the great majority of studies on Si-bound porphyrin monolayers were devoted to Zn(II)complexed porphyrins exhibiting two reversible oxidation states, two approaches have been proposed to increase the memory density of porphyrin-based assemblies on silicon for the development of useful multibit information storage devices. These were based on either an increase in the surface coverage of Zn(II)-complexed porphyrin or an increase in the number of reversible oxidation states of the bound porphyrins complexed with a metal different from Zn(II). The first approach consisted in the covalent anchoring of vertically aligned porphyrin oligomers via imide units (36).218 Following this method, elaborate redox-active architectures of well-controlled thickness could be grown in a stepwise fashion (until 5 porphyrins) with a surface coverage ranging from 2.0 to 5.0 × 10−10 mol cm−2 (which corresponds to 40−90 μC cm−2 total charge densities). The second approach focused on the covalent anchoring of Co(II)-complexed porphyrin monolayers and triple-decker lanthanide sandwich complexes. Co(II)-complexed porphyrin monolayers, such as 30,209 exhibited three stable oxidation states in the 0.5−1.5 V versus Ag/Ag+ range associated with the Co(III)/Co(II) metal couple and the mono- and dicationic species of the porphyrin in the order of increasing potentials (Figure 13). Triple-decker coordination complexes, such as Europium sandwich complexes composed of phthalocyanin and porphyrin ligands 37200,209,219 and 38,220 afforded four cationic states (from mono- to tetracations). Compared with mono- and multiporphyrin layers, the electrochemical stability of these triple-decker complex-modified surfaces were relatively similar.219 Nevertheless, smaller charge retention times and charge densities were observed when these complexes were linked to silicon through a benzyloxy unit (37). In particular, the calculated charge-retention half-life t1/2 values were found to increase monotonically from ca. 10 to 100 s as the oxidation

Figure 13. Fast-scan (100 V s−1) cyclic voltammograms of (A) 30 (M = Co2+, R1 = −CH3, X = S; Scheme 5) and (B) 37 surfaces in propylene carbonate containing 1.0 M Bu4NPF6. Reprinted from ref 209. Copyright 2004 American Chemical Society.

state increased and the charge density per oxidation state was lower than 1−2 μC cm−2. Such results could be explained by the large footprint (∼670 Å2)221 occupied by the bound tripledecker complex stemming from its large size and the overall nonvertical orientation of the molecular assembly. Interestingly, the surface coverage and consequently the charge density of these assemblies could be strongly enhanced if a tripodal tether was used as the anchoring site of these porphyrin complexes. Therefore, surfaces 38 showed a surface coverage close to 8 × 10−11 mol cm−2 which was approximately 4-fold higher than that obtained for the monopodal 37 surface.220 The origin of this increase in the surface packing density was partially explained by a more upright orientation of the triple-decker complex thanks to the tripod. But surprisingly, the surface packing density of these assemblies was found to not depend significantly on the location of the tripodal tether on the complex. 5.1.2. C60. Similar to some metalloporphyrins exhibiting multiple reversible redox states, fullerene C60 shows remarkable electrochemical properties characterized by the reversible stepwise addition of up to six electrons leading to C606− within a scanning range from −0.45 to −2.70 V versus SCE.222−225 It is worth stressing that the observation of such features required scrupulous optimization of the electrochemical conditions, namely low temperature and thoroughly dried electrolytic medium.224,225 In general, due to the high reactivity of multicharged species, only the first four reversible reduction steps were frequently observed. In contrast to reduction, the oxidation of C60 is less attractive and yields usually the formation of the highly unstable monocation species.223,224 The advantageous electron-accepting character of C60 and its optical properties as well have thus motivated many groups to elaborate C60-functionalized monolayers on a variety of conductive (essentially gold) and semiconductive substrates.226 These electroactive and photoactive monolayers could find useful applications for the development of electronic (e.g., multibit memories) and photonic molecular devices. Furthermore, in the case of silicon, the immobilized C60 moieties could strongly influence the superficial properties (namely work function and band bending) of the semiconductor as a consequence of the interaction between the frontier orbitals of the carbon spheres with the valence and conduction bands of Si. 4827

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Scheme 6. C60-Functionalized Si−H Surfaces

Figure 14. (a) C60 film grafted into a 7 nm silicon nanogap and corresponding electrical current measurements from 10 device cycles in which one cycle corresponds to one write (−6 V, 10 s), five reads (4 V, 1 μs each), one erase (6 V, 10 s), and five reads (4 V, 1 μs each). Reprinted from ref 229. Copyright 2010 American Chemical Society.

anthracene was used as an anchoring site of C60 following [4 + 2] Diels−Alder cycloaddition233,234 and the resulting modified surface 44 exhibited a very weak density of surface states, less than 0.05% of the total surface. Furthermore, scanning electrochemical microscopy (SECM) measurements revealed that the anthracene:C60 layer displayed good conductivity, presumably by electron hopping between adjacent redox sites. From an electrochemical point of view, the multiredox activity exhibited by fullerenes in solution was however not retained when immobilized on Si−H. As a matter of fact, only a single reduction step appearing more or less reversible was visible for C60-modified Si−H surfaces.227,228,231,233 Such differences between the electrochemical behavior of surface-confined and dissolved species are not uncommon and have already been reported for fullerene self-assembled monolayers (SAMs) attached to gold.226,235−239 In the case of gold, it has been claimed that the compensating ion transport during the reduction process of bound C60 would be inhibited owing to the high packing density of the fullerene SAMs. For siliconconfined C60, the ion transport is expected to be not affected to a large extent as the C60 monolayers were globally poorly densely packed. Another plausible explanation could be related to the reduced charge transfer kinetics between the underlying depleted semiconductor and bound C60 within a certain potential range. 5.1.3. Polyoxometalates. The polymetallic clusters such as polyoxometalates (POMs) with the general formula

The direct attachment of C60 to Si−H surfaces has been achieved via a hydrosilylation reaction with the CC bonds on C60 following either solvent casting or vapor deposition of C60 at ca. 200 °C.227,228 However, this method yielded poorly dense C60 monolayers (39, Scheme 6). The surface coverage of C60 was only 10% of the estimated one full monolayer coverage (1.9 × 10−10 mol cm−2).228 The use of aromatic solvents with high boiling point such as mesitylene as the deposition solvent was found to enhance dramatically the surface coverage of C60. Under these conditions, 4−5 nm-thick, multilayer C60 films consisting of a large number of C60 oligomers were produced and grafted into 7 nm silicon nanogap devices.229 The resulting molecularly modified nanogap devices showed charge-based memory behavior with high ON/OFF ratios (maximum ∼1500) and a good stability (>100 cycles without device degradation) (Figure 14). Multistep attachment methods of C60 were also reported and based on the reaction of pristine C60 or C60 conjugates with reactive preassembled organic monolayers covalently bound to Si−H surfaces. Accordingly, reactions of N-methylfullerenepyrrolidone with a benzylalcohol monolayer (40),230 aminosubstituted C60 with an acid-terminated monolayer (43a)231 and pristine C60 with an aryldiazonium- (41),230 amino(42),232 aldehyde- (43b),231 or azide- (43c)231 terminated monolayer, yielded electroactive and robust C60-terminated monolayers with a low surface coverage of ca. 2.5 × 10−11 mol cm−2.231 In another report from our group, silicon-bound 4828

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Scheme 7. POM-Functionalized Si−H Surfacesa

a

The blue octahedra in 47, 48, and 49 represent the metal oxide cores.

waves at −0.73 and −1.45 V versus Ag/Ag+ attributed to the stepwise reduction of one and two Mo(VI) to Mo(V) atom(s). The corresponding surface coverage was estimated at 1.8 × 10−11 mol of POM cm−2, which is considerably smaller than the theoretical maximum value calculated for one [Mo6O19]2− monolayer (i.e., 3.2 × 10−10 mol cm−2) by considering the [Mo6O19]2− anions as spheres with a diameter of 8.1 Å.255 Moreover, a kapp value of 25 s−1 was calculated, which is somewhat unexpectedly low with respect to the presence of a conjugated bridge between POM and silicon. Such molecular systems were successfully attached onto the channel region of pseudo metal-oxide-semiconductor field-effect transistors (MOSFETs) in view of molecularly controlling the electronic performances in these devices.256 Besides the immobilization of Lindqvist-type structures, the functionalization of Si−H surfaces with Keggin-type anions [XM12O40]n‑ and its lacunary derivatives [XM12‑oO40‑p]q− has also been considered with the goal of producing modified surfaces endowed with more tunable redox properties. The first tested approach consisted in an electrostatic immobilization of [SiW12O40]4− with a cationic trimethylammonium-terminated organic monolayer covalently bound to Si−H (47).257 The film was then covered with a cationic polymer in order to stabilize the assembly and to avoid the release of the POMs. The interest of this method is that it allows the easy introduction of commercially available POMs and does not require the time-consuming synthesis of POMs substituted by anchoring groups compatible with Si−H. The surface coverage of POMs estimated by XPS was ca. 1.3 × 10−10 mol cm−2, indicating a dense packing of POMs if one refers to the theoretical maximum coverage of 2.0 × 10−10 mol cm−2 calculated from the size of [SiW12O40]4−.258 Nevertheless, the redox signature of [SiW12O40]4− was not observed as the CV of the POM-modified surface showed only a single irreversible cathodic peak, whereas, in solution, [SiW12O40]4− usually exhibited between three and five reversible redox processes assigned to multiple reduction steps of W(VI) to W(V) centers.259 In contrast, the redox integrity of POMs was retained when these were immobilized on Si−H through a covalent link, as in the work from Joo et al.260 The acetylenesubstituted Keggin-type [PW9O34(tBuSiO)3Ge]3− POM was

[XxMyOz]n− (where X is a heteroatom such as P, Si, etc. and M is a metal such as Mo, W, etc.) have been widely used in analysis, catalysis, biology, and materials science owing to essentially their remarkable and versatile topological and electronic characteristics.240−244 The ability to modify, almost at will, the redox and chemical properties of POMs by replacing in their structure one or many elements renders them particularly attractive for catalytic and electrocatalytic applications.243−245 They have been often considered as inorganic analogues of metalloporphyrins (in particular, they also exhibit multiple reversible redox states) with the advantage of being more resistant toward an oxidizing environment and more thermally stable.243,246 Therefore, the use of POMs as multiredox components in monolayers would be more compatible with the fabrication techniques of molecular-based information storage devices. Indeed, temperatures as high as 400 °C can be used in the manufacturing processes of CMOS technologies while some metalloporphyrins can begin to degrade from 200 °C. Some studies have demonstrated the potential of these molecular systems for the design and fabrication of SiO2-based electronic devices, such as flash247−249 and quantum250,251 memories. The functionalization of Si−H surfaces with POMs has started in 2005 with the covalent metal alkoxide bonding between the Lindqvist-type [(CH3O)TiW5O18]3− heteropolyanion and a preassembled alkanol monolayer-modified Si(111) surface (45, Scheme 7).252 Unfortunately, this modification route yielded the formation of POMs aggregates, probably as a result of electrostatic interactions between POMs. Furthermore, the electrochemical properties of these modified surfaces have not been investigated. In another example, electroactive monolayers and multilayers of Lindqvist-type isopolyanions (containing only transition metals and no heteroatom X) linked to silicon through conjugated bridges were produced from the direct grafting of aryldiazoniumderived [Mo6O19]2− onto Si(111)-H (46).253,254 The thickness of the grafted films could be varied from ca. 1.0 to 6.0 nm by controlling the concentration of the reactive POM and the reaction time.253 The cyclic voltammetry study of a 1.8 nmthick hexamolybdate film showed two reversible reduction 4829

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Figure 15. (a) Anodic electrografting procedure used for the preparation of the POM-modified surface 48. (b) CVs of 48 at 20, 50, 100, 200, and 500 mV s−1 in CH3CN + 0.1 M Bu4NBF4. Reprinted with permission from ref 260. Copyright 2010 Wiley-VCH.

Scheme 8. TTF-Functionalized Si−H Surfaces

Figure 16. (a) CVs in CH3CN + 0.1 M Bu4NClO4 at 0.1 V s−1 of mixed TTF monolayer-modified surfaces 51 prepared from the ethyne-substituted TTF alone (solid trace) and mixed with 1-decyne: molar ratios of 0.33 (dashed trace) and 0.16 (dotted trace). (b) Corresponding variation of ΓTTF with the molar fraction of the ethyne-substituted TTF in the initial alkyne mixture. (c) CV at 0.01 V s−1 of 52b. Reprinted from refs 278 and 279. Copyright 2012 American Chemical Society.

5.2. Systems Exhibiting Two Reversible Electron Transfer Steps

covalently attached to Si(100)-H through the anodic electrografting of the terminal alkyne unit following a protocol described by Buriak and co-workers.261 The electrochemical response in acetonitrile medium of the [PW9O34(tBuSiO)3Ge]3−-modified surface 48 was characterized by three reversible one-electron reduction processes within the range from −0.7 to −1.9 V versus SCE, consistent with the response observed for the POM in solution (Figure 15).260 Furthermore, electron transfer between the immobilized POM and silicon was found to be faster than that for silicon-bound [Mo6O19]2−.253 In another example, a Keggin-type lacunary POM has also been attached in one-step to a n-type Si(100) surface via the electroreduction of its terminal aryldiazonium group (49).262 Densely packed monolayers were produced (ΓPOM ∼ 1 × 10−10 mol cm−2) and characterized by a first oneelectron reduction wave ascribed to the grafted POMs. However, the corresponding kapp was low (kapp ∼ 5 s−1) which could originate from the presence of traces of interfacial silicon oxides, as supported by XPS.

5.2.1. Tetrathiafulvalene (TTF). Another fascinating type of both electrochemically and optically active molecule that has given rise to various molecular materials displaying a wide range of applications (sensors, receptors, switches, and conductors) is tetrathiafulvalene (TTF).263−267 Compared with metalloporphyrins and polyoxometallates, TTF is much less sterically hindered and shows a multiredox activity in oxidation characterized by three stable redox states (namely, neutral, radical cation, and dication). Interestingly, the anchoring of this organic donor on (semi)conducting surfaces could also give rise to surface-bound organic conductors and organic/inorganic hybrid materials endowed with stimulating properties (electrocatalysis, magnetism, etc.). In this area, a large body of literature has been devoted to TTF SAMs bound to gold268−273 and optically transparent semiconducting274−276 surfaces in view of exploiting their electrochemical characteristics for the develop4830

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Scheme 9. V2+ Functionalized Si−H Surfacesa

a

53: Reprinted with permission from ref 299. Copyright 2013 Wiley-VCH.

because of the presence of the interfacial vinyl bond and to a better structural organization of the monolayers. The successful grafting under mild thermal conditions of the ethyne-substituted TTF derivative and the higher quality of the produced monolayers have been the driving force to extend this surface functionalization route to TTF derivatives bearing a diacetylenic group as the anchoring site.279 The presence of a second acetylenic unit offers the unique opportunity to produce both redox-active TTF monomolecular and polymer films in a reproducible and controllable manner, using a temperaturecontrolled grafting procedure. Indeed, TTF monolayermodified silicon surfaces (52a) were produced when low grafting temperatures were used (typically 45 °C), whereas higher temperatures (e.g., 90 °C) yielded TTF polymermodified surfaces (52b). The TTF monolayers prepared at 45 °C were densely packed with a surface coverage of ca. 5.4 × 10−10 mol of TTF per cm2 and were linked to the surface through enyne bonds, as supported by IR spectroscopy measurements. Furthermore, they were electrochemically stable with an electroactivity loss of only 15% after 200 redox cycles, and the measured rate constants for electron transfer of bound TTF units were high (>200 s−1), as a consequence of the presence of the conjugated bridge. The polymer generated at 90 °C was believed to result from the polymerization of terminal diacetylenic moieties and/or silicon-bound enyne moieties, in agreement with other reports on the polymerization of diacetylene monomers 280−283 and aromatic enynes.284,285 The so-produced electroactive films exhibited an equivalent surface coverage of TTF exceeding 2.0 × 10−8 mol cm−2 (Figure 16c). Such a polymerization route involving butadiyne segments is particularly attractive because it provides convenient access to surface-bound electroactive polymers with considerably enhanced chemical and thermal stability, as

ment of ion sensors and molecular electronics devices (e.g., memories and molecular wires). In contrast, there are only a few examples related to the immobilization of TTF on Si−H surfaces. Si−O-linked TTFterminated monolayers have been prepared from the reaction of an alcohol-substituted TTF derivative with Si(111)-H (50, Scheme 8).277 The cyclic voltammetry response of 50 showed as expected two reversible one-electron systems corresponding to the TTF•+/TTF and TTF2+/TTF•+ couples at formal potentials relatively close to those observed for the molecule in solution. The surface coverage of the TTF units was electrochemically estimated at 1.9 × 10−10 mol cm−2, which was consistent with a poorly densely packed monolayer as a theoretical maximum coverage of 4.0−5.0 × 10−10 mol cm−2 was calculated for TTF monomolecular films, by considering the TTF molecules as rectangular blocks.270,273 Moreover, the electron-transfer rate constant for the first redox system kapp1 was relatively low (∼25 s−1) with respect to the small distance between the electroactive center and the silicon surface. More densely packed and ordered TTF monolayers were produced from hydrosilylation chemistry at relatively low temperature (90 °C) using an acetylene-substituted TTF derivative (51).278 These Si−CC− linked monolayers exhibited the three-state redox signature of the bound TTF units at formal potentials E°′1 = 0.40 V and E°′2 = 0.75 V vs SCE (Figure 16). Furthermore, the amount of immobilized TTF units could be finely tuned in the range from 1.7 × 10−10 to 5.2 × 10−10 mol cm−2 by diluting the TTF-terminated chains with inert ndecenyl chains. The packing density and ordering characteristics of these mixed monolayers were somewhat similar to those of the single-component monolayer. Compared with 50, the bound TTF units in 51 were electrooxidized at higher rates 4831

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Figure 17. (a) CVs in 0.1 M Na2SO4 aqueous solution at 50 mV s−1 of V2+-free n-type Si(111) (thin solid line), 54 before the introduction of PtCl42− anion with n = 0 (dotted line), 2 (dashed line), and 4 (thick solid line). (b) Corresponding surface coverage of V2+, and thickness measured by ellipsometry as a function of the number of V2+ layers (n). Reprinted from ref 301. Copyright 2008 American Chemical Society.

described procedure allowed the layer-by-layer growth of V2+ multilayers from the preformed V2+ monolayer with a surface coverage controllable in the range from ca. 2.0 × 10−10 to 1.4 × 10−9 mol cm−2 (Figure 17).301,302 In another paper, Uosaki and colleagues have prepared bifunctional monolayers covalently bound to Si(111)-H incorporating V2+ and a photochromic diarylethene molecule in the same chain structure (55).303 Diarylethenes and in particular dithienylethenes (DTEs) have been largely studied for their ability to be reversibly converted from a nonconjugated open isomeric form to a π-conjugated closed isomeric form upon UV light irradiation (for ring closure) or visible light irradiation (for ring opening).304,305 Therefore, this gives the unique opportunity to photoswitch the electron transfer between the underlying silicon surface and the terminal redox-active V2+ center, as in a recent example of gold-confined bifunctional DTE and redox-active Ru complex monolayer.165 The cathodic current corresponding to the first reduction wave of bound V2+/V•+ was found to be more intense after UV irradiation than after visible irradiation, as a result of an easier electron transfer between V2+ and silicon due to the DTE ring closure. Although the measured currents were small (below 0.3 μA cm−2) suggesting a poorly densely packed bifunctional monolayer, the current ratio between the two states was ca. 3 and higher than the photochemically induced tunnel current changes measured by conductive AFM with a single DTE monolayer covalently bound to Si−H.306 This demonstrates the beneficial effect of combining DTE with a redox-active unit for the modulation of charge transport through molecular junctions, in agreement with a recent report on conjugated polymer molecular junctions incorporating TTF units as the redox center.307 5.2.3. Heterobimetallic Complexes. Investigations from our group have demonstrated that carbon-rich group 8 homonuclear organometallics were relevant candidates for producing redox-active densely packed and ordered monolayers covalently bound to Si−H through robust Si−CC− bonds.172−174 Furthermore, these systems exhibited remarkably fast charge-transfer kinetics with the underlying silicon surface.174 On the basis of these observations, semiconducting interfaces incorporating related heterobinuclear complexes should not only possess similar attractive features but also show higher charge density owing to the presence of two different metal centers. Therefore, heterobinuclear complexes possessing pendant ethynyl bonds have been covalently attached to Si−H surfaces using the same grafting strategy as

already demonstrated for gold-grafted polydiacetylenic monolayers.286,287 5.2.2. Bipyridinium Cations. 4,4′-Bipyridinium dications (also called viologen V2+ because of the violet color of their reduction products) have aroused a great interest in the community of electrochemists essentially owing to their highly reversible electrochemical characteristics, their excellent electron acceptor character, and their cationic nature but also to their optical properties and their relatively easy synthetic accessibility.288,289 Such stimulating properties have been therefore exploited in electrochemical catalysis, for the study of electron transfer in molecular junctions, the triggering off redox-induced molecular motions, the electrostatic entrapment of metallic complexes, and the elaboration of electroactive monolayers.289−297 From an electrochemical point of view, V2+ and its alkylated derivatives (e.g., 1,1′-dimethyl-4,4′-bipyridinium or methyl viologen MV2+) show two perfectly reversible one-electron reduction steps corresponding to the formation of the radical cation V•+ (MV•+) and neutral V (MV) forms, respectively.288,298 Depending on the nature of the electrolyte anion, these are usually observed at formal potentials ranging from −0.4 to −0.7 V and −0.7 to −1.0 V versus SCE for the first and second systems, respectively. The immobilization of this redox-active molecule on Si−H surfaces has been driven by different types of interactions. In the first example reported by Boccia et al.,299 the noncovalent assembly of V2+ units on Si−H was achieved via the pseudorotaxane formation. The ditopic guest incorporating V2+ and another pyridinium ring was first immobilized through a host−guest complexation reaction with a host calix[6]arene monolayer covalently bound to Si(100)-H (53, Scheme 9). In a second step, gold nanoparticles (NPs) protected with a thiolate calix[4]arene ligand were assembled on Si(100)-H thanks to a second host−guest complexation with the terminal pyridinium ring bound to V2+. Such a supramolecular hierarchical assembly was used for the electrocontrolled release of gold NPs. As a matter of fact, the application of a suitable voltage enabling the electrochemical reduction of pyridinium groups (i.e., −0.4 V vs SCE) promoted the decomplexation of the pyridinium and/or V2+ unit(s) and consequently the release of NPs. The reversibility of this NPs diassembly/assembly process upon the reduction−oxidation of the electroactive units has however not been demonstrated. Besides the immobilization approach by host−guest complexation, the covalent attachment of Si−Clinked V2+ monolayers has been carried out by Uosaki and coworkers for catalytic purposes (54).300 Interestingly, the 4832

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two redox couples were higher than 200 s−1, and the surface coverage of the molecular chains was electrochemically estimated at ca. 10−10 mol cm−2. Nevertheless, the surface 56 was found to be poorly stable upon electrochemical cycling as a loss of electroactivity of 40% was observed after 102 scans over the first Fe-based redox process. The stability of the first oxidized state was greatly enhanced when the organoiron(II) center was replaced by a Fc one. Moreover, the kapp values for the Fc+/Fc couple in 57 and 58 were around 600 s−1 and thus considerably higher than those previously reported for ferrocene monolayers (Table 1) and for mononuclear systems bound to Si surfaces through a saturated spacer.164,174 Additionally, while the coverage calculated for 57 was similar to that calculated for 56, the coverage achieved for 58 was significantly lower (4.3 × 10−11 mol cm−2). This result was ascribed to the presence of a flexible linker between the redox sites enabling lateral motion and rotation around the Si−C axis (Figure 18c). At the opposite, the lateral motions of the complexes in 56 and 57 are restricted owing to the rigidity of the silicon-bound phenyleneethynylene group. In contrast with the relatively high electrochemical stability of 57 and 58, when potential was scanned over the first Fc+/Fc redox process, an enhanced fragility was observed for these two modified surfaces when more oxidizing potentials were applied up to the second Ru(III)/Ru(II) process. Such a trend was thought to originate from a larger spin density present on the Fc+ unit of the grafted dications, as evidenced by molecular modeling calculations.308 5.2.4. Other Bimetallic Systems. Bimetallic networks on silicon have been elaborated from a preassembled organic monolayer terminated by a pyridine-containing tridentate chelating ligand.309,310 In a first step, a nickel(II) complex was formed with the terminal tridentate ligand. The complexed sites were further used as anchoring sites to grow sequentially Prussian Blue-like phases by repeated immersion in [FeII(CN)6]4− then [NiII(H2O)6]2+ solutions (59). This simple and fast approach allowed for the growth of sub-10 nm-thick

that used for homonuclear complexes.172,308 The electrochemical characterization of redox-active Si−CC-linked monolayer produced from the Fe(II)/Ru(II) arylacetylide complex (56, Scheme 10) evidenced the redox signature of Scheme 10. Si−H-Attached Heterobimetallic Complexes (Ph = Phenyl)

the two bound metallic centers [i.e., Fe(III)/Fe(II) and Ru(III)/Ru(II)] at formal potentials close to those measured for the complex in solution (Figure 18).308 It also provides evidence for generation of the mixed-valence complex at the silicon surface that could be exploited for the elaboration of electrochemically switched optical gates. Indeed, the same dinuclear complex in solution exhibited linear and nonlinear optical properties which could be changed upon the applied potential.162 Furthermore, the kapp values determined for the

Figure 18. CVs in CH3CN + 0.1 M Bu4NClO4 of 56 at 0.2 V s−1, (a) first (dashed trace) and second (solid trace) scans, (b) 57 at 0.1, 0.2, 0.4, 0.6, and 1 V s−1, and (c) illustration of the rotational flexibility of 58. Reprinted from ref 308. Copyright 2014 American Chemical Society. 4833

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metallized films with a fine control on the density of deposited material and the film morphology. It is worth recalling that the elaboration of cyanide-bridged networks from Prussian Blue analogues as those envisioned in this work is of great interest in the field of materials science, essentially owing to the versatility of the sequential growth procedure, attractive magnetic, photomagnetic, electrochemical, and catalytic characteristics exhibited by such functional materials.311−314 Nevertheless, the electrochemical properties of aforementioned metallic filmsmodified silicon surfaces have not been explored so far, although each used metallic complex exhibited redox activity in solution either in oxidation or reduction.

(nature of the solvent and the electrolyte, dryness of the electrolytic solution, and applied potential), these unstable species undergo coupling reactions that eventually yield electronically conducting polymer films (e.g., polypyrrole and polythiophene) through successive electrochemical (E) and chemical (C) steps according to a general E(CE) n scheme.315−317 These conducting polymer films have aroused great interest in the field of large-area molecular junctions as a viable alternative to contact electrically surface-confined organic molecules for further charge transport measurements. It is worth stressing that contact formation is a key issue in molecular scale devices.318 Many fabricated devices employed noble metal deposition (e.g., Au) by evaporation to form a top contact. A potential difficulty of this approach is penetration and/or damage of the molecular layer by the incoming metal atoms.319−321 Different strategies have been used to overcome this difficulty such as the atomic layer deposition,322 metal transfer using a polymer-assisted lift-off process,323 the deposition of a liquid metal (Hg324,325 or eutectic gallium− indium alloy326−329), and the use of a conducting polymer. Among them, the intercalation of a conducting polymer [e.g., poly(3,4-ethylenedioxythiophene) stabilized with poly(4-styrenesulfonic acid)] between the monolayer and the upper metal layer in metal/monolayer/metal or metal/monolayer/semiconductor junctions has appeared as a highly promising approach for the production of nonshorting stable and reproducible devices.330−333 Also interesting to manipulate the electronic properties of devices, the conductivity of these materials can be tuned over a large range between an insulating neutral state and a conducting oxidized state either by electrochemical doping or to a lesser extent by the nature of the compensating anion.316 The formation of conducting polymer/monolayer/silicon junctions has been initially reported by Wrighton and coworkers in the 80’s with the aim of efficiently protecting the semiconductor against the photocorrosion phenomenon.334 A

6. ATTACHMENT OF ELECTROCHEMICALLY POLYMERIZABLE CENTERS In sections 4 and 5, the considered redox-active systems and their corresponding electrogenerated oxidized and/or reduced species are known to be stable at the cyclic voltammetry time scale when studied in solution. In contrast, the systems discussed in this section give rise to highly reactive electrogenerated species (radical cations) upon electrochemical oxidation. When some experimental conditions are fulfilled

Scheme 11. (Top) Electrochemically Polymerizable Aromatic Ring-Modified Si−H Surfaces and (Bottom) Electrochemical Deposition of Conducting Polymer Films from Monomer-Terminated Surfaces, Here in the Case of Polypyrrole

4834

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Figure 19. (Left) Schematic of the localized electropolymerization of terminal pyrrole rings immobilized on 61 using a scanning conducting AFM tip. (Right) Corresponding surface potential map after the electrochemical patterning. The brighter 400 × 400 nm2 squares show a surface potential higher by ∼80 mV and correspond to the scanned areas which were electrogenerated with a bias of 5.5 V. Adapted from ref 348. Copyright 2012 American Chemical Society.

±0.5 V was >100 against ∼1) and a smaller ideality factor n (1.5 against 2.3) determined for the polypyrrole/60b junction.347 Such results are believed to result from an improved charge transfer between the conducting material and silicon owing to more intimate contact between the two phases. The presence of surface-confined electropolymerizable centers also gives the opportunity to locally pattern the silicon surface with conducting material features, using, for example, a conducting AFM tip. Such a lateral polymerization reaction has been attempted with surfaces 61 and 62. Application of biases higher than 4 V between the conducting AFM tip and the surface 61 resulted in the in situ polymerization of the terminal pyrrole groups, as clearly evidenced by the surface potential map (Figure 19).348 The conductance enhancement in the scanned areas was due to the formation of more conjugated polymer/oligomer segments. Nevertheless, the principal drawback of this approach is that the electropolymerization process is only limited to pyrrole headgroups and thus does not allow the growth of a conducting material of controllable size. Moreover, it requires a densely packed monolayer with closely spaced monomer units in order to allow the propagation of the electropolymerization reaction and to generate polymer/ oligomer segments of sufficient length. A more versatile approach has been proposed by Fabre and co-workers.349 It was based on a scanning probe microscopy method combining direct-write dip-pen nanolithography and electrochemistry.350,351 By applying an appropriate voltage between a pyrrole-coated conducting AFM tip and the surface 62, strongly adherent polypyrrole dots with diameters ranging from 75 to 200 nm and sub-200 nm-wide lines could be electrogenerated. Interestingly, this type of localized electropolymerization reaction could allow access to multifunctional conducting structures only by changing the substitution of the monomer ink coating the AFM tip. Besides surface-confined pyrrole, thiophene, and aniline derivatives have also been covalently immobilized on Si−H surfaces in order to generate adherent conducting polymer films exhibiting different conduction properties and electroactivity range. In a work by Fabre et al., di(2-thienyl)carbinol moieties end-capping an alkyl monolayer covalently bound to Si−H (63) were electropolymerized in the presence of thiophene to yield robust and smooth doped polythiophene films.352−354 These films showed a perfectly and stable reversible electron transfer process at ca. 0.40 V versus SCE corresponding to the conversion of the insulating, neutral

pyrrole derivative was first covalently anchored to SiO2/Si via a silanization reaction, and the bound monomer rings were further used as nucleation sites for the electrochemical growth of a polypyrrole film. Such a procedure was then extended to the deposition of other conducting polymers, such as polythiophene335−339 and polyaniline,340−342 onto monomermodified oxidized silicon surfaces. Due to the presence of a covalent linkage, the organic polymer films were found to be strongly adherent to the electrode surface and much more adherent than conducting polymer films directly deposited on the semiconductor surface without an organic link.343,344 Another advantage of the method is that the coverage of polymer and thus its thickness could be finely controlled from the electrical charge consumed during the electropolymerization reaction. Nevertheless, the presence of an interfacial oxide layer in such junctions precluded the investigation of their electrical characteristics. Therefore, the covalent anchoring of conducting polymers to oxide-free silicon surfaces is highly desirable and appears as a more promising route to hybrid junctions with enhanced physical and electrical properties. Furthermore, the electrical characteristics, and in particular the current rectification properties of the so-modified surfaces, should be in principle controlled by the thickness of the organic link between the underlying silicon and the conducting material. Pyrrole-terminated monolayers have been covalently bound to Si−H surfaces in one step using pyrrole derivatives Nsubstituted with an alkene345 or alkyllithium346,347 chain. The surfaces 60a−60c showed an electrochemical response characterized by the irreversible oxidation of the bound pyrrole ring beginning at 0.9 V versus SCE (Scheme 11). Expectedly, oxidation of the bound monomer was found to be slower upon increasing the length of the alkyl chain. The electrochemical copolymerization of the immobilized pyrrole units with pyrrole in solution yielded conducting polypyrrole films which were strongly adherent to the electrode surface,345,346 in agreement with data reported for similar films deposited on oxidized silicon. Thanks to the covalent link, polypyrrole deposited on 60a−60c was neither removed nor damaged by sonication in an organic solvent. In contrast, polypyrrole deposited on Si−H was totally removed simply by rinsing. As another consequence of the presence of the covalent link, the electrical characteristics of the polypyrrole films deposited on pyrrole-modified surfaces were greatly improved. Polypyrrole/n-Si junctions formed on 60b and on unmodified Si−H behaved as Schottky diodes with higher forward currents, more rectification (the current ratio at 4835

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trifluoroacetic acid at −0.67 V versus normal hydrogen electrode NHE (Figure 20a). Such a performance was ascribed

polymer matrix to its conducting, doped form. The resulting doped polythiophene/decyl/Si(111) structures exhibited properties similar to other more conventional metal−insulatorsemiconductor (MIS) junctions, namely, Schottky diode behavior with 102−103 rectification ratios.355 In another report, conjugated oligothiophenes have been successfully attached to Si−H using a two-step halogenation and thienylation route (64).356 Unfortunately, the produced layers were poorly densely packed with a substantial amount of unreacted halogen and surface oxides which precluded the investigation of their charge transport characteristics. Oxide-free molecular junctions based on such systems could be of wide interest for redox-gated charge transport experiments, as demonstrated with similar oligomers or parent polymers deposited on carbon or metallic substrates.357−359 Among the conducting polymers, polyaniline is of particular interest because it can be switched reversibly between three distinct oxidation states (namely leucoemeraldine, emeraldine, and pernigraniline) depending on its protonation state and the applied electrical potential.360 Thus, polyaniline and its oligomeric derivatives appear as promising candidates for pHand potential-dependent molecular switches. Aniline monomer (65)361,362 and oligomers (66)363 have been grafted on Si−H using hydrosilylation and aryldiazonium reactions, respectively. As expected, the produced layers were adherent to the semiconducting surface. The integration of some of these molecules into silicon-based testbed electronic devices resulted in an intriguing hysteretic memory effect, which was totally molecularly relevant.363,364

Figure 20. (a) Catalytic CVs of the illuminated surface 23b in the presence of increasing concentrations of trifluoroacetic acid (TFA) in CH3CN + 0.2 M LiClO4 under N2 (glovebox). Inset: TFA dependence at −0.67 V versus NHE. Conditions: illumination with a LED (33 mW cm−2), 100 mV s−1 scan rate. (b) CVs in 0.1 M Na2SO4 aqueous solution at 50 mV s−1 of reference (4-ethylbenzyl)triethylammonium- (EBTEA, □) and Pt NPs-EBTEA- (■) modified n-type Si(111), 54: n = 0 without (Δ) and with (▲) Pt NPs, n = 2 (◊) and 4 (⧫) with Pt NPs. Reprinted with permission from ref 181. Copyright 2015 Royal Society of Chemistry. Reprinted from ref 301. Copyright 2008 American Chemical Society.

to the excellent stability of the Ni(II/I) redox couple over electrochemical cycling and the high surface coverage of the Nibased catalyst (2.5 × 10−10 mol cm−2). In another work, the surface 54 modified with V2+/Pt NPs layers was found to electrocatalyze efficiently the H2 generation and the rate of this process was dramatically enhanced upon increasing the number of V2+/Pt layers (Figure 20b).301,302 Such catalytic films were produced by chemical reduction of PtCl42−, which was previously electrostatically entrapped inside the V2+ layers (Scheme 9). In this electrocatalytic reaction, the immobilized V•+ reduced species acted as an efficient electron mediator for the hydrogen evolution at the Pt NPs. Compared with the first study reported by Dominey et al. in the 80’s and dealing with a similar Pt NPs-V2+ assembly from a V2+-modified siloxane monolayer,95 the system described by Uosaki and co-workers showed an improved catalytic efficiency owing to a higher concentration in deposited Pt NPs. In the field of sensing and electroanalysis, the only rare examples of effective functional silicon surfaces have been reported by the Gooding’s group. In their first work, cytochrome c was immobilized on pyridine moieties covalently bound to Si(100) surfaces376 using a grafting strategy similar to that reported for the immobilization of this redox biomolecule on gold surfaces.377,378 The strong interaction between the heme of cytochrome c and the terminal pyridine allowed the protein’s electron transfer kinetics to be electrochemically characterized. The reversible one-electron process of the Fe(III/II) couple of the heme was observed at −0.15 V versus Ag/AgCl, and the related electron-transfer rate constant was estimated at ca. 6.0 s−1. Globally, these electrochemical findings were comparable to those obtained with cytochrome c immobilized on pyridine-modified gold. Compared with the covalent attachment of cytochrome c, it has been hypothesized that the pyridine-directed anchoring allowed for the production of more homogeneous films and retained advantageously the conformational freedom of the protein. In their second study, a quinone-based modular assembly was developed for electro-

7. APPLICATIONS OTHER THAN CHARGE STORAGE AND INFORMATION PROCESSING Besides the potential applications of these functional interfaces for molecular charge storage and information processing, there are very few examples dealing with their activity for other fields of interest, such as sensing and electrochemical catalysis. In the latter field, the use of electroactive molecule-functionalized Si− H surfaces as photocathodes could however provide a real benefit for technologically and economically important reactions, such as CO2 reduction and hydrogen evolution. The key difference between semiconducting photoelectrodes and working electrodes traditionally used for electrocatalytic applications (e.g., metals and carbon) is that, for the semiconductors, light serves as an important source of energy input.365 Such a property based on photovoltaic conversion can achieve an energy-saving route to electrochemical catalysis. Although various redox-active molecular systems have been demonstrated to be efficient electrocatalysts in solution at Si− H photocathodes,366−374 their immobilization on flat Si−H surfaces through strong interactions has however been surprisingly seldom developed.181,301,302,375 In that context, it is worth recalling that the surface coverage of the catalyst and the electrochemical stability of the catalyst and/or the surface are two crucial parameters to be considered to operate an efficient supported redox catalysis. As an illustrative example, the surface 23a incorporating various coordination metal complexes (Rh, Ru, and Ir) was not catalytically active owing to both a low surface coverage and a poor electrochemical stability.180 In contrast, recent catalytic results obtained with an immobilized nickel-phosphine complex were promising.181 The surface 23b electrocatalyzed the H2 generation in acid acetonitrile medium with a maximum turnover frequency (TOF) of 285 s−1 calculated for ca. 90 mM 4836

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chemically switching the cell adhesion.196 The key component in this assembly consisted of a redox-active quinone moiety bearing an oligo(ethylene oxide) (OEO) chain as a celladhesion-resistant molecule. The application of a reducing potential of −1.8 V versus Ag/AgCl to the surface Q2a (Scheme 4) led to the conversion of the quinone functionality to the lactone group and the concomitant cleavage of the OEO chain (surface Q2b), therefore making the surface amenable to cell adhesion. An interest of this strategy is that the initial structure could be chemically regenerated. The switching properties of the surface Q2a was probed by fluorescence microscopy in the presence of mammalian cells. Due to the presence of the OEO chains, the surface Q2a showed as expected antifouling properties with a number of adherent cells of ca. 100 mm−2 (Figure 21). After electrochemical reduction

the covalent integration of electroactive units, such as Fc,389−391 quinones,195 and triphenylamine derivatives,392 into porous silicon and Si NWs has appeared as a viable route toward highdensity and stable functionalized surfaces. These could find applications in the field of high-density molecular memories, highly effective redox catalysis, and sensitive electrochemical sensors. In another recent work, thiophene rings have been attached to Si MWs with the goal of efficiently protecting the silicon surface and improving the electrical contact with a conducting polymer poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate).393 This type of functionalization constitutes a promising approach for solar-driven membrane-supported fuel cells with minimized electrical losses.

9. CONCLUSIONS AND FUTURE DIRECTIONS The modification of oxide-free Si−H surfaces at the molecular level with redox-active mono- and multimolecular films constitutes a powerful approach for the fabrication of smart interfaces, which could be of high interest for applications in the realization of integrated systems for information storage/ transfer or the elaboration of the electrochemically switchable molecular junctions. Nevertheless, the transition to integrated systems is not an effortless step and requires that the modified surfaces are of high quality and extremely stable, both chemically and electrochemically. Therefore, the access to such dream surfaces involves that the experimental conditions of the used surface chemistry reactions, such as the choice of the surface chemistry (e.g., light-, thermally-, or catalytically driven hydrosilylation, two-step halogenation/alkylation) and the number of grafting steps (direct or multistep grafting), are thoroughly optimized in order to produce densely packed and robust electroactive film-functionalized oxide-free surfaces showing ideal electrochemical characteristics. Despite much progress made in improving the compactness, ordering, and robustness of the electroactive films, efforts must also be concentrated on a fine control of the surface coverage of the electroactive centers and the long-term applicability of the functionalized surfaces. The first criterion that is particularly relevant in molecular catalysis can be fulfilled by mixing the electroactive chains with inert nonelectroactive chains which are chemically as close as possible to the linker bearing the redox center. The second criterion that is particularly relevant for information storage applications is more difficult to reach. It requires that the redox-active assemblies immobilized on Si−H are sufficiently passivating to avoid the diffusion of water and oxygen through the pinholes/defects of the molecular layer until the underlying semiconducting surface and the subsequent oxidation of remaining Si−H sites. The second successful strategy pioneered by Lewis and co-workers consists in the near 100% functionalization of Si−H sites with short-chain reactive molecules which are susceptible to be efficiently converted to electroactive centers.57,58 Besides these chemical considerations, the long-term applicability of these redox interfaces can also be dependent on their thermal stability. In that context, studies focusing on the thermal stability of modified silicon surfaces under high-temperature conditions (>400 °C) commonly used during the fabrication of charge storage devices are lacking. Additionally, the degradation or not of the redox activity of the immobilized molecule after such a high-temperature treatment needs also to be checked. Moreover, the use of solid electrolytes, such as high-κ dielectric materials (e.g., aluminum nitride, AlN) or thermally resistant polymer electrolytes, instead of liquid electrolytes is highly

Figure 21. Fluorescence micrographs of bovine aortic endothelial cells cultured on the surface Q2a incorporating antifouling (OEO) chains (a) before and (b) after electrochemical reduction at −1.8 V versus Ag/AgCl (surface Q2b). Reprinted from ref 196. Copyright 2012 American Chemical Society.

and cleavage of the OEO chains, the number of adhesive cells was significantly enhanced (ca. 500 mm−2). Even though an allelectrochemical activation/inhibition of the cell adhesion properties would have strengthened the impact of this work, the proposed modular strategy constitutes however an important step toward the controlled manipulation and growth of cells and could be extended to other biological systems of interest.

8. WHAT ABOUT FUNCTIONALIZED Si−H MICRO/NANOSTRUCTURES ? The different surface modification strategies used for the covalent grafting of the molecular films onto flat Si−H can be basically transposed to silicon micro/nanostructures, such as porous silicon379 and silicon micro/nanowires (Si MWs/Si NWs).380−382 From an electrochemical point of view, these high-aspect-ratio structures could produce larger (photo)currents and establish charge transfer at less negative potentials,383−385 compared with flat silicon. Nevertheless, two main limitations have hampered the interest of the scientific community for the chemical derivatization of such structures. First, the grafting efficiency is often lower with nanostructured silicon substrates. Indeed, the reagents need to diffuse effectively within the pores and wire network to react with the hydrogenated sites. Second, because of their highsurface-area and their complex morphology, hydrogenated nanostructured silicon is often more sensitive to oxidation than flat silicon. For these reasons, the functionalization of hydrogenated porous Si, Si MWs, and Si NWs with electroactive molecules has been only marginally reported. Besides some studies devoted to the immobilization by physisorption of electroactive catalysts in silicon arrays,386−388 4837

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Stable Synthetic ‘Metallic’ Polymer. J. Chem. Soc., Chem. Commun. 1979, 854−855. (2) Pham, M.-C.; Lacaze, P.-C.; Dubois, J.-E. Obtaining Thin Films of ″Reactive Polymers″ on Metal Surfaces By Electrochemical Polymerization. Part I. Reactivity of Functional Groups in a Carbonyl Substituted Polyphenylene Oxide Film. J. Electroanal. Chem. Interfacial Electrochem. 1978, 86, 147−157. (3) Murray, R. W. Chemically Modified Electrodes. Acc. Chem. Res. 1980, 13, 135−141. (4) Murray, R. W. Molecular Design of Electrode Surfaces, Techniques of Chemistry; John Wiley & Sons: New York, NY, 1992; Vol. XXII. (5) Ulgut, B.; Abruna, H. D. Electron Transfer through Molecules and Assemblies at Electrode Surfaces. Chem. Rev. 2008, 108, 2721− 2736. (6) Savéant, J.-M. Molecular Catalysis of Electrochemical Reactions. Mechanistic Aspects. Chem. Rev. 2008, 108, 2348−2378. (7) Savéant, J.-M. Elements of Molecular and Biomolecular Electrochemistry; Wiley-Interscience: New York, 2006. (8) Nijhuis, C. A.; Ravoo, B. J.; Huskens, J.; Reinhoudt, D. N. RedoxActive Supramolecular Systems. Coord. Chem. Rev. 2007, 251, 1761− 1780. (9) Rurack, K.; Martinez-Manez, R.; Rozkiewicz, D. I.; Ravoo, B. J.; Reinhoudt, D. N. Immobilization and Patterning of Biomolecules on Surfaces. In The Supramolecular Chemistry of Organic-Inorganic Hybrid Materials; Rurack, K., Martinez-Manez, R., Eds.; J. Wiley & Sons, Inc.: Hoboken, 2010; pp 433−466. (10) Willner, I.; Katz, E. Integration of layered redox proteins and conductive supports for bioelectronic applications. Angew. Chem., Int. Ed. 2000, 39, 1180−1218. (11) Willner, B.; Willner, I. Reconstituted Redox Proteins on Surfaces for Bioelectronic Applications. In Bioinorganic Electrochemistry; Hammerich, O., Ulstrup, J., Eds.; Springer; Dordrecht, 2008; pp 37−90. (12) Léger, C.; Bertrand, P. Direct Electrochemistry of Redox Enzymes as a Tool for Mechanistic Studies. Chem. Rev. 2008, 108, 2379−2438. (13) Jortner, J., Ratner, M. A. Molecular Electronics: A Chemistry for the 21st Century Monograph; Blackwell Science: Malden, MA, 1997. (14) Lindsay, S. M.; Ratner, M. A. Molecular Transport Junctions: Clearing Mists. Adv. Mater. 2007, 19, 23−31. (15) Ghosh, A. W. Electronics with Molecules, Comprehensive Semiconductor Science and Technology; Elsevier: Amsterdam, 2011; Chapter 5.09, pp 383−479. (16) The surface states are electronic energy levels created in the forbidden gap region of silicon and are capable of transferring charges with the semiconductor and contact solution. (17) Yablonovitch, E.; Allara, D. L.; Chang, C. C.; Gmitter, T.; Bright, T. B. Unusually low surface-recombination velocity on silicon and germanium surfaces. Phys. Rev. Lett. 1986, 57, 249−252. (18) DeBenedetti, W. J. I.; Chabal, Y. J. Functionalization of OxideFree Silicon Surfaces. J. Vac. Sci. Technol., A 2013, 31, 050826. (19) Thissen, P.; Seitz, O.; Chabal, Y. J. Wet Chemical Surface Functionalization of Oxide-Free Silicon. Prog. Surf. Sci. 2012, 87, 272− 290. (20) Ciampi, S.; Harper, J. B.; Gooding, J. J. Wet Chemical Routes to the Assembly of Organic Monolayers on Silicon Surfaces via the Formation of Si-C Bonds: Surface Preparation, Passivation and Functionalization. Chem. Soc. Rev. 2010, 39, 2158−2183. (21) Li, Y.; Calder, S.; Yaffe, O.; Cahen, D.; Haick, H.; Kronik, L.; Zuilhof, H. Hybrids of Organic Molecules and Flat, Oxide-Free Silicon: High-Density Monolayers, Electronic Properties, and Functionalization. Langmuir 2012, 28, 9920−9929. (22) Cummings, S. P.; Savchenko, J.; Ren, T. Functionalization of Flat Si Surfaces with Inorganic Compounds - Towards Molecular CMOS Hybrid Devices. Coord. Chem. Rev. 2011, 255, 1587−1602. (23) Pujari, S. P.; Scheres, L.; Marcelis, A. T. M.; Zuilhof, H. Covalent Surface Modification of Oxide Surfaces. Angew. Chem., Int. Ed. 2014, 53, 6322−6356.

desirable in order to satisfy such thermal requirements and to implement really all-solid silicon-based molecular electronic devices. Another aspect to be considered affecting both the quality and the molecular and electronic properties of the redox-active assemblies grafted on silicon is the nature of the interfacial bond between the semiconducting surface and the electroactive molecule. Most examples presented in this review focus on Si− C- or Si−O-linked electroactive monolayers. Experimental works from Bocian and co-workers,205,209 and more recently from Buriak and co-workers394 as well as a computational work395 have suggested that interfacial Si-E bonds (with E = S, Se) could be of interest for the development of electrochemically active interfaces with controllable electronic properties. Finally, future development in the areas of bioanalytical devices (e.g., sensing and adhesion/manipulation of living cells) and electrochemical catalysis also merit exploration. Moreover, the transposition of well-mastered surface modification strategies to different silicon micro/nanostructures, such as hydrogenated porous silicon and silicon micro/nanowires, could constitute a really promising avenue toward electrochemical sensors and solar-driven fuel cells with high electrocatalytic activity and sensitivity. This thematic area is still in its infancy, and the few promising studies aimed at integrating electroactive catalysts into silicon arrays195,386−391,392,393 should prompt other research groups to pursue this route toward novel modified photoelectrodes. There is also little doubt that such redox center-functionalized nanoarchitectures will find potential applications in electrochemical energy storage (batteries and supercapacitors) in the near future.396−399

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Tel: +33 2 23 23 65 50. Fax: +33 2 23 23 67 32. Notes

The author declares no competing financial interest. Biography Bruno Fabre received his engineer diploma and M.S. degree in electrochemistry from the Ecole Nationale Supérieure d’Electrochimie et d’Electrométallurgie de Grenoble (ENSEEG, University J. Fourier, Grenoble, France) in 1990. After a Ph.D. under the guidance of Dr. Gérard Bidan (Commissariat à l’Energie Atomique CEA, Grenoble, France) in 1994 and a postdoctoral stay at the Département de Médecine Expérimentale (Lyon, France), he joined the laboratory of Molecular Electrochemistry in Rennes (France) as Chargé de Recherche at Centre National de la Recherche Scientifique (CNRS) in 1995. He spent one year as an invited researcher (NATO felloswship) in Ottawa (Canada) in Prof. Dan Wayner’s lab at the Steacie Institute for Molecular Sciences (National Research Council of Canada NRCC) in 2002. Since 2008, he has been Directeur de Recherche at CNRS working in the Institut des Sciences Chimiques de Rennes (France). His current research interests lie in the areas of chemically modified electrodes, the functionalization and micro- or nanopatterning of silicon surfaces, and the development of functionalized semiconducting photoelectrodes for catalytic purposes.

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DOI: 10.1021/acs.chemrev.5b00595 Chem. Rev. 2016, 116, 4808−4849

Chemical Reviews

Review

NOTE ADDED AFTER ASAP PUBLICATION This paper was published ASAP on April 11, 2016 with an error in the author's name. The corrected version was reposted to the web on April 13, 2016.

4849

DOI: 10.1021/acs.chemrev.5b00595 Chem. Rev. 2016, 116, 4808−4849