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Non-Ideal Electrochemical Behavior of Ferrocenyl-Alkanethiolate SAMs Maps the Microenvironment of the Redox Unit Nisachol Nerngchamnong, Damien Thompson, Liang Cao, Li Yuan, Li Jiang, Max Roemer, and Christian A. Nijhuis J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b05137 • Publication Date (Web): 31 Aug 2015 Downloaded from http://pubs.acs.org on September 3, 2015

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The Journal of Physical Chemistry

Non-Ideal Electrochemical Behavior of Ferrocenyl-Alkanethiolate SAMs Maps the Microenvironment of the Redox Unit Nisachol Nerngchamnong1, Damien Thompson2, Liang Cao1, Li Yuan1, Li Jiang1, Max Roemer1, and Christian A. Nijhuis1,3,4* 1 Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543 2 Materials and Surface Science Institute and Department of Physics and Energy, University of Limerick, Ireland 3 Centre for Advanced 2D Materials and Graphene Research Centre, National University of Singapore, 6 Science Drive 2, Singapore 1175464 4 Solar Energy Research Institute of Singapore, 7 Engineering Drive 1, Singapore 117574

*corresponding author: Tel.: (+65) 6516 2667 e-mail: [email protected]

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Abstract: We studied the electrochemical behavior of self-assembled monolayers (SAMs) of n-alkanethiolates with ferrocenyl (Fc) termini on gold (SCnFc, n = 0 – 15) in relation to their supramolecular structures (characterized by photoemission spectroscopy (PES) supported by molecular dynamics (MD) simulations) to elucidate the origin of non-ideal features commonly observed in cyclic voltammograms (CVs) of these SAMs by systematically changing n from 0 to 15. For all SAMs the CV data are dominated by a main peak for Fc units directly in contact with the electrolyte solution and interacting with neighboring Fc units. A second peak is observed for SAMs with n ≥ 2 ascribed to partially buried Fc units as a result of lattice strain due to the different sizes of the lattices of the Fc units and the sulfur atoms. For thick SAMs with strong molecule-molecule interactions, the strain is large resulting in a third peak due to Fc units that are well shielded from the electrolyte by other Fc units. Our results do not agree with widely used explanations to account for peak splitting involving isolated Fc units vs. clustered Fc units, disordered vs. ordered domains, or back bending (which require the formation of Gauche defects). In contrast, we show that the peak splitting and peak broadening is an inherent property of densely packed SCnFc SAMs with Fc units in different electrochemical environments to release the build-up of strain. Based on our results we propose a simple model to explain all the non-ideal features observed in CV data. In addition, we show that this model can also explain the abnormal shapes of CV recorded on S(CH2)11Fc SAMs formed on very rough and defective electrodes, or derived from the corresponding disulfide, i.e., (S(CH2)11Fc)2.


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Introduction Understanding the mechanisms of charge transport across organic matter and molecule— electrode interfaces is a fundamental issue in applications including molecular1 and organic electronics,2,3 energy storage,4 photovoltaics,5,6 catalysis,7 and biological processes.8 The electrochemical behavior of self-assembled monolayers (SAMs) of n-alkanethiolates with ferrocene (Fc) termini on gold, S(CH2)nFc (or in short SCnFc) has been studied intensively, because these SAMs serve as model systems to study the redox behavior of immobilized redoxcouples on electrodes. The interpretation of the electrochemical data generated by these SAMs in relation to their structure is still problematic because the origins of commonly observed nonideal electrochemical behavior in cyclic voltammograms (CVs) are poorly understood. Deviations from ideal Nernstian behavior9 include, for example, peak broadening10,11,12,13 or narrowing (so-called spikes) ,14 appearance of multiple redox waves (or so-called peak splitting),12,15,16,17 and shifts in the peak oxidation and reduction potentials with respect to the same (or similar) redox species free in solution (see Background on pages S2-5 for a brief overview of factors that affect the electrochemical response of redox-active SAMs).18,19 Here we show that deviations from ideal electrochemical behavior of SCnFc SAMs are characteristic of functionalised SAMs and that peak splitting and broadening is the result of the build-up of strain due to the mismatch in size of the Fc unit and the alkyl chain diameter. In this system, commonly used explanations involving back-bending of Fc units (which require energetically unfavorable strained molecular conformations known as Gauche defects) or disordered SAMs do not apply. Here we show that strain caused by the mismatch in diameters between the Fc units and CH2 chains result in partially buried Fc units that are shielded from the electrolyte by other Fc units causing peak splitting and broadening.. In addition, we observed a new redox-wave at low oxidation potentials for



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highly disordered SAMs when disulfides were present in the thiol precursors, and very broad redox-waves when rough electrodes were used. Electrochemical reactions can be reversible, quasi-reversible, or irreversible.9 Cyclic voltammograms show a single and symmetrical redox wave for a one-step one-electron process that is reversible. This so-called electrochemical ideal behavior is observed when the redoxactive groups in the SAMs do not interact which each other and the rate of electron transfer is fast on the experimental time-scales.9 The CV of an ideal one-electron process has five characteristics: i) the difference between the oxidation peak potential, Epa, and reduction peak potential, Epc, ∆Ep = 0, ii) the ratio of the anodic peak current, Ipa, and cathodic peak current, Ipc, is unity, iii) the values of Ipa and Ipc are proportional to the scan rate, υ (in contrast to the υ1/2 dependence for solution based redox-processes9), iv) the full width at half-maximum of the voltammetric peak, Efwhm, is 90.6 mV at 25 oC , and v) only one reversible redox wave is present.20 In general, ideal behavior is rarely observed and non-ideal electrochemical behavior is usually ascribed to inhomogeneity of the SAMs with the terminal group, in this case Fc unit, in different electrochemical environments. Reversible CV data are obtained for the SAMs wherein the redox-active component, e.g., SCnFc, is highly diluted by a non-redox-active component, e.g., n-alkanethiolates (SCn).21,22 In these diluted SAMs the Fc units are believed to be in the same electrochemical environments and do not interact which each other. Chidsey et al.23 found reversible CV behavior when SCn(OOC)Fc:SCn SAMs were formed at low mole fractions of SCnFc (χFc ≤ 0.25) and reported a Efwhm of about 90 mV, and ∆Ep of close to zero. Rowe et al.24 reported reversible electrochemical behavior, a Ipa/Ipc of about 1, a small ∆Ep of 5 mV, and CV shape independent of υ, for SAMs of SC6Fc:SC6 (χFc ≤ 0.05). However, the value of the Efwhm of


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the mixed SAMs was found to be larger (> 90 mV) than the ideal value even at low χFc (χFc ≤ 0.25). This peak broadening has been frequently observed by others25,26,27 e.g., Guo et al.28 reported a Efwhm of 103 mV for SAMs of SC11Fc/SC8 (χFc ≤ 0.10) and Voicu et al.29 reported a Efwhm of 145 mV for SAMs of SC11(CO2)Fc/SC10 (χFc ≤ 0.25). In densely packed monolayers multiple peaks are often observed which have been attributed to the presence of disordered phases, back-bending of the Fc units into the SAM, Fc–Fc interactions and Fc+–counter-ion interactions.23,24 Figure 1 gives an example of a CV with an abnormal anodic peak shift and broadening of a SC2Fc SAM on ultra-flat template-stripped gold (AuTS) relative to that for a SAM of SC11Fc (see Results and Discussion for details and examples of CVs with multiple peaks that are discernible by eye). The schematic shows the intermolecular and the Fc-electrode interactions that determine the microenvironments of the Fc units. The CV peak appears to consist of a single broad redoxwave but closer examination reveals the presence of a second peak. In this paper we use this model based on intermolecular and molecule-electrode interactions to rationalize the shapes of the CVs recorded for SAMs of SCnFc with n = 0, 1, 2, …, 15 on AuTS electrodes. By varying the value of n, we controlled supramolecular interactions i – v as indicated in Fig. 1, i.e., the Fc-Fc (i), Fc-alkyl chain (Fc-Cn; ii), alkyl-alkyl chain (Cn-Cn; iii), and the non-covalent (iv) and covalent Fc-Au (v) electrode interactions. The interplay of these interactions determines the outcome of the self-assembly process and the structure of the SAMs. Figure 1 shows an idealized schematic with the Fc units in the same electrochemical environment, but below we show that SAMs in reality always consist of Fc units in different electrochemical environments due to mismatch in diameters between the Fc units and the alkyl chains. In addition, we investigated how the topography of the electrode and the purity of the SAM precursor change the shape of the



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CVs. These experiments allowed us to relate the shape of the cyclic voltammogram to the supramolecular structure of the SAMs.

Figure 1: Schematic illustration of SAMs of SCnFc with n = 2 and 11. The arrows labeled with i to v indicate the supramolecular interactions: i) Fc-Fc interactions, ii) Fc-Cn interactions, iii) CnCn interaction, iv) Fc-Au interactions (van der Waals interaction), and v) Fc-Au interactions (orbital-hybridization). The intermolecular interactions change as n changes, and this influences the electrochemical behavior of the SAMs, observed as peak broadening, peak splitting, peak shifting, and packing densities. The electrochemical parameters are indicated: peak oxidation potential (Epa), peak reduction potential (Epc), full-width at half maximum (Efwhm), anodic peak current (Ipa), cathodic peak current (Ipc), and total charge (Qtot) determined by integration of the complete redox wave.

The shape of the CV is quantitatively discussed using common electrochemical parameters (Ipa, Ipc, Efwhm, Epa, Epc, and ∆Ep) which are defined in Figure 1. The surface coverage of the Fc 6 ACS Paragon Plus Environment

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group (ΓFc, mol/cm2) can be calculated by eq. 1, where Qtot is the total charge determined by integration of the redox peak, n is the number of electrons per mole of reaction, F is the Faraday constant, and A is the surface area of the electrode exposed to the electrolyte solution (0.33 cm2).17 ΓFc = Qtot/nFA

(1)

For SAMs of SCnFc on AuTS, the theoretical value of ΓFc is 4.5 × 10-10 mol/cm2 assuming a hexagonal packing with Fc treated as spheres with a diameter of 6.7 Å. Lee et al.27 reported an anodic peak deconvoltuion methods using Gaussian and Lorentzian functions to probe the heterogeneity of the SAMs. We used a similar procedure in the analysis of our CV data to determine the number of redox-waves and the surface area underneath these waves. Here we show that the value of n, the roughness of the Au electrode, and the purity of thiol SAM precursors, all play a crucial role in the shape of the CVs. To relate the supramolecular structure of the SAMs to the electrochemical data, we characterized the SAMs in detail by ultraviolet photoelectron spectroscopy (UPS), near edge X-ray absorption fine structure (NEXAFS) spectroscopy, and molecular dynamics (MD) and density functional theory (DFT) calculations. We deconvoluted the signals of the CV data and identified four different redoxpeaks and assigned them to the local supramolecular structures of the SAMs: (i) local disorder involving flat-lying molecules caused by impurities or defects in the electrode material, (ii) the nature of the Fc—electrode interaction which is non-covalent (i.e., van der Waals interactions are important) when n ≥ 3 but is covalent in character (i.e., orbital-hybridization is important) when n = 0 – 2, (iii) Fc units that are in direct contact with the electrolyte, and (iv) Fc units that are partially shielded from the electrolyte by other Fc units. The redox potentials for each peak depend on the strength of the Fc—electrode and intermolecular interactions. We believe that the



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results reported here will help with the interpretation of CV data of future studies concerning Fc SAMs and in studies involving other types of redox-active SAMs. Experimental section We synthesized the HSCnFc, n = 0 – 15 compounds following previously reported procedures in literatures.18,30,31,32,33 The synthetic routes and structural characterizations of HSCnFc with n = 0 – 15 are described in the Supporting Information (the synthetic details of compounds with n = 6 – 15 were reported previously34). We formed the SAMs using ethanolic solutions (3 mM) of the corresponding thiols under an N2 atmosphere over a period of time of three hours. To minimize contamination from the ambient, we immersed the Au surfaces into the solution immediately after fabrication. The thiols decompose to their corresponding disulfides and therefore we only used freshly prepared or purified thiols (stored under an atmosphere of N2 at 4 °C). We used a custom built electrochemical cell equipped with platinum counter electrode, a Ag/AgCl reference electrode and the Au served as a working electrode. The CVs were recorded in an aqueous solution of 1.0 M HClO4, between -0.1 to 0.9 V at a scan rate of 1.00 V/s using an AUTOLAB PGSTAT302N with NOVA 1.10 software. The CV data were analyzed using a combination of Gaussians and Lorentzians using OriginPro 9.0 software. The anodic peaks were deconvoluted after background correction following a procedure similar to that described in Lee et al.27 To study the effect of surface topography of the bottom-electrode on the electrochemical properties of the SAMs, we formed SAMs on four different types of Au substrates: i) templatestripped Au (AuTS)35Error! Bookmark not defined., ii) polished Au electrode (AuPL)36, iii) ebeam metal deposited (AuED)37, and iv) sputtered Au (AuSP).38 We reported the preparation


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methods of these surfaces previously34 (see Supporting Information for experimental details). The topography and roughness of the substrates were characterized by atomic force microscopy (AFM); the topographies of the surface were similar to those reported in references.35-38 The AFM images were recorded on Bruker Dimension FastScan AFM by using tapping mode tips with intermittent contact (FASTSCAN-A, resonant frequency: 1.4 MHz, force constant: 18 N/m; see Supporting Information for more details). The ultraviolet photoelectron spectroscopy (UPS) and near edge X-ray absorption fine structure (NEXAFS) spectroscopy, were recorded at the SINS beam-line of the Singapore Synchrotron Light Source (SSLS) following previously reported procedures34 which are briefly summarized in the Supporting Information. The molecular dynamics (MD) models and methods, supported by quantum mechanical density functional theory (DFT) models at low n values, are described in detail elsewhere34,44 and summarized in the Supporting Information.

Results and Discussion Structural Characterization of the SAMs. The HSCnFc SAM precursors were synthesized, and the SAMs were formed, using well established procedures.18 To relate the supramolecular structures of the SAMs to their electrochemical behavior, we characterized the SAMs experimentally by ultraviolet photoelectron spectroscopy (UPS), near edge X-ray absorption fine structure (NEXAFS) spectroscopy, and theoretically by molecular dynamics (MD). Table 1 lists the structural and electronic characteristics of the SAMs determined by aforementioned techniques. We performed the MD simulations using the same procedures as reported in reference 34. The NEXAFS and MD data for SAMs with n ≥ 6 were previously reported34, but here we provide a more detailed



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analysis using a complete data set for n = 0 – 15 (see Supporting Information for details), which reveals new features of the SAM properties. Table 1. The structural and electronic characteristics of the SCnFc SAMs on AuTS surfaces Title angleb (o) δEMEc WFc IPd Fc-Fc Fc-Cn Cn-Cn Full Fc Full mol. (eV) (eV) (eV) NEXAFS MD 0 54.5 0.32 4.36 4.68 1 59.2 0.31 4.37 4.68 2 -5.14 -3.59 -1.12 -8.74 -9.86 49.9 57 0.35 4.38 4.73 3 -5.17 -3.42 -1.84 -8.60 -10.43 60.2 69 0.39 4.39 4.78 4 -5.17 -3.90 -2.77 -9.06 -11.84 45.4 54 0.57 4.35 4.92 5 -5.11 -4.12 -3.63 -9.24 -12.87 59.3 68 0.60 4.36 4.96 6 -5.01 -4.40 -5.24 -9.41 -14.65 50.0 56 0.71 4.33 5.04 7 -5.18 -4.56 -6.42 -9.74 -16.16 58.1 65 0.86 4.34 5.20 8 -4.93 -4.45 -7.59 -9.38 -16.96 53.5 57 0.98 4.20 5.18 9 -4.81 -4.76 -7.98 -9.57 -17.54 59.5 63 1.02 4.07 5.09 10 -4.96 -4.69 -9.60 -9.66 -19.25 52.1 58 1.10 4.10 5.20 11 -4.78 -4.86 -10.37 -9.63 -20.01 58.0 64 1.09 4.02 5.11 12 -4.88 -4.85 -11.68 -9.73 -21.41 53.5 50 1.24 4.05 5.29 13 -4.49 -5.14 -12.58 -9.63 -22.21 59.5 63 1.21 4.01 5.22 14 -4.51 -5.46 -13.50 -9.97 -23.47 54.5 58 1.19 4.09 5.28 15 -4.85 -5.05 -14.67 -9.89 -24.57 58.0 59 1.08 4.15 5.23 a The Epack of each intermolecular interactions were calculated by MD simulation.. b The tilt angle of the Fc units with respect to the surface normal were derived from NEXAFS and calculated by MD. c The δEME and WF were determined by UPS with an instrumental error of ±0.05 eV. d The IP = δEME + WF SCnFc

Epacka (Kcal/mol)

Figure 2A shows the computed SAM packing energies (Epack) of each type of intermolecular interaction: Fc-Fc (i), Fc-Cn (ii) and Cn-Cn (iii) interactions for n = 2 – 15, along with the sum of the interactions of the Fc units (i + ii) and the full molecule packing energies. This Figure shows that the interactions of the Fc unit, Fc-Fc (i) and Fc-Cn (ii) interactions are nearly constant as a function of n, while the interactions between alkyl chains, iii, increase with increasing n. The FcFc interaction dominates the packing structure when n < 5, whereas the Cn-Cn interaction starts to dominate when n ≥ 5. The UPS data show that Fc-Au interactions (iv and v) are important when n ≤ 5 (see below). Figure 2B presents the odd-even effect in the value of Epack for the full molecule for SAMs with n = 2 – 15. This so-called odd-even effect originates from the Au-S-C bond angle which is 10 ACS Paragon Plus Environment

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the same for all SAMs (~109.5°)37,39 which results in an odd-even effect in the tilt angle, α (°), of the Fc units with respect of the surface normal of 7 ± 3° (Figure 2F). The Fc units stand-up and repel each other less when α is small for SAMs with neven resulting in larger-magnitude Epack values than for SAM with nodd. Figure 2C shows the calculated Fe height distributions indicating the presence of sub-surface layers of Fc units in the SAMs. Figure 2D shows the thickness of the SAMs determined by angular dependent XPS as a function of n. The computed Fe height distributions in Fig. 2C are in agreement within the ranges of the error bars in the experimental thickness in Fig. 2D. To confirm the odd-even effect in the value of α, we determined α experimentally by NEXAFS.40,41 Figure 2E and 2F show the NEXAFS spectra for SAMs of SC10Fc and SC11Fc recorded at an incident angle of θ = 90o and a grazing angle of θ = 20o and the value of α obtained by NEXAFS and MD as a function of n. The odd-even effects in the theoretically and experimentally determined values of α are in good qualitative agreement from which we conclude that MD simulations represent our SAMs well. For SAMs with n > 5, the Fc groups with neven have a smaller α value and stand up more than with nodd by 5 ± 2° on average. This allows SAMs with neven to pack better by 0.4 ± 0.6 kcal/mol according to MD (Figure B inset) in agreement with the CV data due to more favourable interaction iii. Moreover, the oddeven effect of α value is more pronounced (~11 ± 4°) for the short SAMs (n ≤ 5), for which the structures are dominated by Fc-Fc interaction over Cn-Cn interactions (see below for more details), than for tall SAMs.



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Figure 2: A) the Epack of each particular intermolecular interaction of Fc-Fc (i), Fc-Cn (ii), and Cn-Cn (iii) in the SAMs of SCnFc, n = 2 – 15 as a function of n calculated by MD, showing Epack of the interactions i – iii, total Epack of Fc unit (i + ii), and full molecules (i + ii + iii), where interactions i-iii are as defined in Figure 1.. B) The Epack of the SAMs for n = 2 – 15 with neven and nodd with the odd-even differences ∆Epack of the SAMs at each n (inset), C) The calculated Fe 12 ACS Paragon Plus Environment

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height distributions above the Au bottom electrode, average over ~25,000 molecules for each SAM for n = 2 – 15. Representative distributions are shown for n = 2, 6, 11 and 15. The full dataset with computed height distributions for all values of n = 2 – 15 is given in Fig S7. D) The thickness of the SAMs determined by XPS, E) The NEXAFS spectra for SAMs of SC10Fc and SC11Fc recorded at an incident angle of θ = 90o and a grazing angle of θ = 20o, F) The average tilt angle α of Fc unit as a function of n, derived from NEXAFS spectra and calculated by MD (data for n = 6 – 15 is taken from reference34).

Electronic Structure of the SAMs. Figure 3A shows the UPS spectra of the SAMs on AuTS and we assigned the signal around 1 – 2.5 eV to the HOMO centered at the Fc.42,43 We used the same procedure to analyze the UPS spectra as reported in reference 44 in order to extract the values of the HOMO-onset values (δEME), the work function (WF), and the ionization potential (IP = WF + δEME). Figure 3A shows the HOMO peak (from which δEME was derived) and Fig. S8 shows the low kinetic energy secondary electron cut-off spectra (from which WF was derived). Figure 3B shows the values of IP and WF as a function of n. The IP values slightly decrease from 5.3 eV to 5.1 eV with decreasing n from 15 to 7, but decrease to 4.6 eV when n decreases from 7 to 1. In addition, Figure 3A shows that the intensity of the signal decreases with decreasing n when n < 7. We attribute these observations to changes in the Fc—electrode interactions. The broad and weak signal of the HOMO when n ≤ 2 indicates hybridization of the HOMO with Au d5 orbitals of the metal electrode.44 Our results indicate that apparently two, or more, CH2 units effectively prevent hybridization of the HOMO with the Au surface by keeping Fc at a van der Waals separation from Au45 and the HOMO localized on the Fc units. MD shows that a significant fraction of the



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Fc units for the SAMs with n = 3-5 are close enough to the Au surface to form van der Waals interactions.44 The Fc-Au van der Waals interaction strength of 173 ± 12 meV has been estimated by Grimmes46 corrected DFT calculations by us before.44 We believe that this van der Waals interaction shifts of the HOMO energy level toward the Fermi level of the electrode for 2 < n < 7 as observed in the UPS data (Fig. 3B); similar observation were made for a different type of SAMs.44

Figure 3: A) The valence band spectra close to the HOMO centered at the Fc for SAMs of SCnFc. B) The IP (eV) and WF (eV) as a function of n for the SAMs of SCnFc determined by UPS.

Electrochemical Characterization of the SAMs. The role of the alkyl chain length. Figure 4A – D shows the CV data of SCnFc SAMs on AuTS as a function of n at a scan rate of 1.00 V/s (Table S1 lists the electrochemical characteristics).


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To determine the origin of the different electrochemical behavior of these SAMs, we deconstructed the contributions to the anodic signal of the CVs by fitting the curves to one or more Gaussian(s) and one Lorentzian. Figure 5A – F shows the results for SAMs of SCnFc, n = 0, 2, 10, 11, 14, and 15 (see Table S2 and Fig. S2 for all data). As indicated in Figure 4, we identified three regimes at which the electrochemical behavior changes significantly (regime 1 with n = 0 – 4, regime 2 with n = 5 – 13, and regime 3 with n = 14 – 15). The CV data fit well to the models which include one, two, or three peaks (R2 = 0.997-0.999) and below we discuss the origin of these peaks in detail for each regime.



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Figure 4: Electrochemical characterization of the SAMs of SCnFc, n = 0 – 15 on AuTS, with aqueous 1.0 M HClO4 as the electrolyte solution, a Ag/AgCl reference electrode, recorded at a scan rate of 1.0 V/s. CVs of SAMs of SCnFc of A) very short chain (n = 0, 1), B) short chain (n = 2, 3 for example), C) medium chain (n = 10, 11 for example), and D) long chain (n = 14, 15). E) Epa and Epc, F) E1/2 = (Epa+Epc)/2, G) ∆Ep and Ipa/Ipc (inset), and H) Efwhm , and the ratio Ipa/Ipc (inset) of the SAMs of SCnFc as a function of n. 16 ACS Paragon Plus Environment

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Figure 5: The peak deconvolution of the anodic peaks of SAMs of SCnFc with n = A) 0, B) 2, C) 10, D) 11, E) 14, F) 15. G) The Epa and H) the ΓFc of the individual peaks as a function of n.



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Regime 1. Figure 4A and B show the CVs of the SAMs in region 1 for n = 0 – 4. We observed a single but broad redox peak when n = 0 or 1 (Efwhm = 237 ± 5 and 321 ± 12 meV, respectively), and the values of Efhwm decrease with increasing n (Fig. 4H). Figure 4E shows that the values of Epa and Epc shifts anodically with decreasing chain length from n = 4 to 0 by ~171 and 147 meV, respectively. The CV data for n = 0 (Fig. 5A) and 1 (Fig. S2B) was fitted to a single broad Gaussian with high values of Efwhm of 228 ± 6 and 297 ± 2 meV (Table S2). As explained above, the UPS data indicate that the HOMO is hybridized with Au 5d orbitals when n ≤ 2. Therefore, the shapes of the CVs of these SAMs are dominated by the orbital hybridization of Fc and Au and we assigned this peak (labeled as peak Iʹ) to covalent Fc-Au interactions (interaction v in Fig. 1). For SAMs with n = 2 – 4, the data were fitted to two peaks labeled as peak Iʹ and peak II (Fig. 5B). We fitted the peak Iʹ to a Gaussian with decreasing value of Efwhm when n increases (from 177 to 110 mV and then remains roughly constant for n > 4) and peak II fitted well to a Laurentzian. We attribute the change in the value of Efwhm to weakening of the Fc—electrode interaction and that for SAMs with n > 2 this interaction becomes non-covalent (i.e., van der Waals) in nature. To indicate this difference in the nature of the Fc—electrode interactions, we labeled using the prime to indicate covalent character (interaction v in Fig. 1) and peak I refers to the non-covalent Fc – Au interaction (interaction iv) for n > 2. We examined the near-Fermi electron density distributions in occupied states at the top of the valence band in quantum mechanical models of the Fc-alkanethiolate complexes on gold and found increasingly large S p-orbital contributions to the Fe-predominated HOMO as Fc approaches the bottom electode, i.e., as n is decreased down to 0 (Fig. 6). This supports the


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interpretation of the Regime 1 data as weakening of the Fc—electrode interaction as n increases (becoming non-covalent for n>2).

Figure 6: Computed near-Fermi electron densities in the energy range Ef → (Ef –1.2) eV, shown as solid black surfaces overlaid on the computed ferrocenyl-alkanethiolate SAM structures (hydrogen atoms are omitted for clarity). The blue vertical lines mark the horizontal periodic cell boundaries in the x and y directions. For the two-molecule cells in panels a-f and h-i, we used a three-layer gold surface containing 48 atoms with cell surface area of 1.154 nm × 0.990 nm. This gives a surface coverage of 1.74 molecules/nm2 or 2.89 × 10-10 mol/cm2, typical of the coverage measured in the experiments. We used a 10 nm vacuum gap perpendicular to the gold surface; hence the periodic cell boundaries in the z direction are not shown. The energy range and resolution (the electron density is displayed using an

isovalue of 0.05 a.u.) was chosen to plot simultaneously the Fc and S contributions at the top of



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the valence band. Panels a-f show the electron density surfaces for SAMs of SCnFc where n is increased from 0 to 5. These cells were used in an earlier study to compute SAM work functions.44 Panel g shows a model used to test for any dependence of the near-Fermi densities on molecule packing density and gold roughness. This larger cell has a lower surface coverage of molecules and a gold surface ridge feature, and is shown in two alternative orientations with the electron density calculated over the same range and resolution as for models a-f, together with a third plot (lower right hand subpanel with the isovalue resolution doubled to 0.10 a.u. and hydrogen atoms shown

explicitly). Panels h and i show electron density plots for n=6 and n=7.

The calculated electronic structures shown in Fig. 6 reveal that the S contribution at the top of the valence band gets smaller as the number of methylene groups under Fc is increased. The general features of the energy levels at the top of the valence band, i.e., strong Fe d-orbital HOMO with a small contribution from S p-orbitals that becomes weaker as n increases, is also evident in the larger cell and in agreement with recent calculations using a similar model.44 Increasing the resolution to 0.10 atomic units (a.u.) shows a 100% Fe d-orbital HOMO for all SAMs with n > 2, emphasising that the S contribution is very low for all but SAMs with n = 0 2. It is striking that the density isovalue required to remove the S contribution from the electron density plots scales near-linearly with decreasing n, from 0.10 atomic units for n = 3 up to 0.14 a.u. for n = 2, 0.21 a.u. for n = 1 and 0.26 a.u. for n = 0, indicating the larger contribution of sulfur to the HOMO at low n that, through Fc---S-Au hybridization, changes the CV of the SAMs (Fig. 5). We relate peak II to Fc units that have strong Fc-Cn interactions (interaction ii); these Fc units are partially buried by other Fc units (Fig. 2C) and shielded from the electrolyte solution which agrees well with the observation that peak II always appears at higher oxidation potential 20 ACS Paragon Plus Environment

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than peak I or Iʹ. In a separate study47 we showed by scanning tunneling microscopy that SAMs of SC3Fc and SC4Fc have row defects caused by the build-up of lattice stain induced by the mismatch in sizes between the Fc head groups and sulfur atoms. These row defects result in Fc moieties that are more buried in the SAMs and this study confirms the presence of partially shielded Fc units in the SAMs. Regime 2. For the SAMs with values of n = 5 – 13, Figure 4E shows that the Epa and Epc values of the main peak are 25 ± 9 mV higher for SAMs with neven than for nodd. To elucidate the nature of this odd-even effect in more detail, we deconvoluted the main peak into two or three peaks. The results for SAMs with n = 10 or 11 are shown in Figure 5C and D respectively. The CVs of the SAMs with n = 5 – 7, 9, 11, or 13, are composed of two peaks I and II, but the CVs of the SAMs with n = 8, 10, or 12, contain an additional peak (labeled as peak III). From the fitting results, we integrated each peak from which we determined the value of ГFc for each peak and ГFc,tot (= ГFc,I + ГFc,II + ГFc,III) using equation 1. Figure 5H shows that ГFc,tot increases with increasing n, except for SAMs with n = 1 for which we observed a significantly lower ГFc,tot (0.89 ± 0.30 mol/cm2) compared to the other SAMs. We believe that this lower ГFc,tot value is caused by the cleavage of S-C bond to produce +C1Fc carbocation which is known to be stabilized by resonance with the Cp ring (see Fig. S9).48 ГFc,I remains constant at roughly 2.4 ± 0.5 × 10-10 mol/cm2 with an odd-even effect when n = 5 – 13 of 0.7 ± 0.2 × 10-10 mol/cm2. The value of ГFc,II remains nearly constant at 0.8 ± 0.2 × 10-10 mol/cm2 for the SAMs of n = 2 – 15. For n > 7, we observed the appearance of peak III for neven. The value of Epa,III is larger than Epa,I and Epa,II, and ГFc,III increases with n. We rationalize these observations as follows. Above we showed that both MD and NEXAFS reveal that the odd-even effect in the Epack values originates from an odd-even effect in α values: the Fc units are standing up more for SAMs with neven



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which enhance the Cn-Cn interactions, resulting in stronger lateral interaction among Fc units. During oxidation of the SAMs, the Fc units bind to the perchlorate counter-ions and the supramolecular structure is rearranged. For SAMs that pack better (neven), this reorganization of the SAMs is more difficult resulting in an increase of the Epa,I. We believe that peak III originated from Fc units that are well-shielded from the electrolyte solution by other Fc units and these Fc units may be present in different conformations which would explain that Epa,III is larger than Epa,I and Epa,II and the large Efwhm,III of ~200 mV, respectively. This reasoning is consistent with the observation that peak III is not observed for the more loosely packed SAMs with nodd. Regime 3. For the SAMs with n = 14 or 15, two peaks are clearly visible by eye (Figure 4D) and also these CVs were deconvoluted into three peaks (Figure 5E and F). As described above, we attribute peaks I and II to Fc units predominantly interacting with other Fc units or CH2 chains. Usually back-bending10,26 (back folding of the Fc moieties toward the electrode) is used to explain the appearance of the new peak (peak III) by eye, but our results do not agree with this hypothesis for the following three reasons. i) The CVs always consist of at least two peaks (except when the molecular orbitals hybridize with the electrode for n = 0 – 1), including when the length of the CH2 chain is smaller than the dimension of the Fc units (i.e., back bending is not possible because of these size constraints). ii) The strong van der Waals interactions between CH2 chains (for large n) make it energetically unfavorable for the Fc units to back-bend which would require interruption of these large van der Waals interactions by the smaller Fc-Cn interaction ii (Cn-Cn interactions iii are larger than Fc-Fc + Fc-Cn interactions (i + ii) when n > 10; Figure 2A). iii) Back-bending would require the formation of destabilizing Gauche defects in the CH2 chains but MD simulations indicate a refutation of back-bending with increasing n (see MD data in SI Fig. S10).


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To explain the origin of peak III, we propose that the mismatch in size between the Fc moieties and CH2 units results in the build-up of strain which can be relieved by bending of the alkyl chain (without the formation of Gauche effects). Due to strong Cn-Cn interaction iii over Fc-Fc + Fc-Cn interactions (i + ii), Fc units are forced to come closer, but they cannot be in the same plane due to steric hindrance of Fc groups. Bending of the CH2 chains only causes a small energy penalty but allows the Fc units to pack underneath each other. These results in a fraction of Fc units directly exposed to the electrolyte predominantly interacting with neighboring Fc units, and a fraction of Fc units interacting with CH2 units which are shielded from the electrolyte, resulting in the appearance of the new peak (peak III). The appearance of sub-surface layers of Fc units is in agreement with the broad distribution of Fc positions, especially in the SAMs for n = 14 and 15, calculated by MD (Figure 2C). Also, Figure 5H shows that the ГFc,tot of SAMs with n = 14, 15 is about 1.2 times larger than the theoretical value (4.5 × 10-10 mol/cm2; a close-packed hexagonal structure of Fc, estimated by Chidsey et al.10) which supports this hypothesis. We note that peak III in regime 2 is not obviously observed by eye but the value of ГFc,III steadily increases from n = 8 to 15 while ГFc,I and ГFc,II remain roughly constant. Thus the increase of ГFc,tot is primarily caused by the increase of the number of buried Fc units. The Role of Surface Roughness. Different methods are used to fabricate and/or clean the gold surfaces prior to self-assembly of the monolayer.49 We50 and other51 showed that the surface roughness of the metal surface affect behaviors of the SAMs, such as the performance as a molecular diode of SCnFc SAMs and the hydrophobicity of SCn SAMs. However, the effect of the quality of the metal surface, e.g., surface roughness, size of the grains, or size of the grain boundaries, are not clear because mostly the CV data are reported without characterization of the



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topography of the gold electrode. To investigate how the surface topography affects the shape of the CVs, we prepared Au substrates by four different methods: i) template-stripping (TS), ii) sputtering (SP), iii) e-beam metal depositing (ED), and iv) mechanical polishing by slurry alumina (PL). Figure 7A – D shows the CVs recorded for SAMs of SC11Fc immobilized on AuTS, AuSP, AuED, and AuPL with the AFM images of these surfaces. From the AFM data we derived the root-mean-square (rms) surface roughness values of 1.0, 1.8, 3.8, and 17.5 nm, respectively, over an area of 1.0 × 1.0 µm2. Table S3 lists the electrochemical parameters. The values of Efwhm and ΓFc increase with increasing rms, from which we conclude that SAMs on rough surfaces are inhomogeneous. To quantify the inhomogeneity of the SAMs more precisely, we deconvoluted the CV peaks into peaks I and II (we did not observe more peaks), using the same procedure as described above. Figure 7E – H shows the fitting results and the Table S4 lists the electrochemical data obtained from the fits.


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Figure 7. CVs and peak deconvolution of the anodic peaks of the SAMs on AuTS (A and e), AuSP (B and F), AuED (C and G) and AuPL (D and H) with the AFM images and the rms values of of 1.0, 1.8, 3.8, and 17.5 nm, respectively.

Figure 8A shows the value of Epa,I and Epa,II as a function of rms. For the relatively smooth surfaces, i.e., AuTS, AuSP, and AuED (rms roughness of surface is 1.0, 1.8, and 3.8 nm, respectively), the values of Epa,I and Epa,II are nearly constant with an average of 309 ± 4 and 407 25 ACS Paragon Plus Environment


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± 9 mV, respectively. It seems that the values of Epa,I and Epa,II increase to 344 ± 15 and 429 ± 37 mV, respectively, for rough AuPL surfaces but we also note that the error bars are large which reflect the decrease in reproducibility due to the increase in topography variations. Figure 8B shows that the value of Efwhm,I and Efwhm,II increase as a function of rms (especially the Efwhm,II increases from 109 ± 10 to 244 ± 22 mV), indicating stronger interactions (interactions ii and/or iv) and/or the presence of Fc groups in different environments. Figure 8C shows that the value of ΓFc,tot increases linearly with increasing rms which we relate to an increase of the effective electrode area with increasing rms. Hence, our experiments did not indication potential of differences in accessibility of the electrolyte to Fc moiety in the SAMs with increasing rms due to capillary effects. Figure 8B also shows that the ΓFc,II increases, while ΓFc,I remains roughly constant as a function of rms, indicating that the value of ΓFc,II more significantly increases with increasing rms than that of ΓFc,I. We therefore also plotted the relative values of ΓFc,I and ΓFc,II (ΓFc,II/ ΓFc,I; Figure 8D) for clarity which increases with increasing roughness. We explain these observations as follows. We propose that because of defects (SAMs at grain boundaries cannot pack well), Fc-Cn interactions, besides Fc-Fc interactions, are important. As described above to explain the odd-even effects, here disorder in the SAMs induced by grain boundaries at which SAMs cannot pack well results in domains with loosely packed molecules where the Fc units can interact with the alkyl chains and are shielded from the electrolyte solutions resulting in peak II. Fc units that are in direct contact with the electrolyte are still present and contribute to peak I. The Fc units in the disordered domains of the SAMs may be present in a large number of microenvironments which explains why Efwhm,II is larger than Efwhm,I. Thus, the surface fraction of exposed grain boundaries at which SAMs cannot pack well


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increases with increasing rms resulting in peak broadening and an increase of the ΓFc,II/ ΓFc,I ratio.

Figure 8. A) the Epa,I and Epc,II, B) the Efwhm, C) the ГFc,I, ГFc,II, and ГFc,tot (the horizontal line is the theoretical surface coverage) and D) the ratio of ГFc,II/ГFc,I as a function of surface rms roughness.

The Role of Impurities. The effect of impurities on the shape of the CV data is poorly understood. We cannot investigate the influence of all potential impurities, but we choose to study here a large potential impurity, which is the effect of the corresponding disulfide (FcC11SSC11Fc). Thiols form disulfides in ambient conditions and so the disulfide analogues are



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likely the most common impurities encountered in most studies. Figure 9 shows the changes of electrochemical behavior and deconvoluted CV data of SAMs of SC11Fc with different fractions of the disulfide χSS in the range of 0.015 to 1.0 (see Fig. S5 and Table S5 for all data). These data show that even for small values of χSS ≥ 0.03 the shape of the CV changes significantly and a new peak appears at cathodic potentials with respect to peak I (labeled as peak IV; Figure 9E). We found that peak IV fitted well to a Gaussian. Figure 9C shows the Epa,I, Epa,II, and Epa,IV, as a function of χSS, and that SAMs derived from the disulfide precursor require less energy to oxidize that SAMs derived from pure thiol precursor ( Epa,I and Epa,II are lower by 8 and 39 mV). The Epa,I, EpaII, and Epa,IV values are nearly constant as a function of χSS (328 ± 11, 425 ± 16 mV, and 262 ± 9, respectively). Figure 9F shows the ΓFc,I, ΓFc,II, ΓFc,IV, and ΓFc,tot as a function of χSS. The value of ΓFc,tot decreases from 4.79 ± 0.02 to 3.79 ± 0.04 × 10-10 mol/cm2 when χSS increased from 0 to 1.0 indicating that the SAMs derived from FcC11SSC11Fc are more loosely packed and disordered than those derived from SC11Fc. The value of ΓFc,IV is constant at 19 ± 4% of the total surface coverage (2.3 ± 0.4 × 10-10 mol/cm2) when χSS ranges from 0.03 to 0.80. Recently we reported angle resolved X-ray photoelectron spectra (XPS) for SAMs derived from FcC11SSC11Fc and found that the presence of peak IV correlates with a Au-S bonding mode of a disordered chemisorbed species.52 These XPS data also indicate that these SAMs have lower surface coverages and average heights than SAMs derived from the corresponding thiols. Thus, these SAMs contain two different domains of chemisorbed species – one that is ordered with the molecules standing-up, and the other that is disordered with molecules lying-down on the surface. For these reasons, we assign peak IV to Fc units in a disordered chemisorbed phase likely consisting of molecules that lie flat on the Au surface and only weakly interact with the


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neighboring molecules. We propose that the Fc units in this domain are readily accessible to the counter-ions from the electrolyte and molecules readily re-arrange once they are oxidized and interact with the perchlorate counter-ions.

Figure 9. CVs and peak deconvolution of SAMs of SC11Fc with different fractions of its corresponding disulfide, χSS = 0.0 (A and B), and 0.80 (D and E). C) The Epa,I, Epa,II and Epa,IV as a function of χSS. F) The ГFc,I, ГFc,II, ГFc,IV and ГFc,tot as a function of χSS . The total ГFc value, ГFc,tot = ГFc,I + ГFc,II + ГFc,IV. 29 ACS Paragon Plus Environment


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The Model. Figure 10 summarizes the peak assignments and relates the structure of the SAMs to the overall shape of the CVs. Peak Iʹ originates from covalent Fc—Au interactions (i.e., orbital hybridization for n = 0 – 2). Peak Iʹ shifts cathodically, and Efwhm decreases, with increasing value of n (n > 2) as the Fc—Au interactions weaken and become non-covalent in nature resulting in peak I. We assign peak I to Fc units directly exposed to the electrolyte solution and predominantly interacting with other Fc units (Fc-Fc interactions) but peak I is always accompanied by peak II at high oxidation potentials. Peak II corresponds to Fc units that are slightly buried into the SAM and partially shielded from the electrolyte solution because the Fc units cannot pack all in the same plane due to a mismatch in size between the diameters of the Fc units and the alkyl chains. This strain can be relieved by bending of the alkyl chains without the need for forming energetically unfavorable Gauche defects which allows a fraction of the Fc units to pack underneath each other and, consequently, are more shielded from the electrolyte; these Fc groups interact with neighboring molecules via Fc-Cn interactions. Peak III originates from strong Cn-Cn interactions (i.e., stronger than the sum of the Fc-Fc + Fc-Cn interactions). We only observed this peak for SAMs for neven, or when n = 14 and 15, resulting in tightly packed SAMs with a fraction of the Fc units well-shielded from the electrolyte. Peak IV was only observed for disordered SAMs which contains two domains of chemisorbed species derived from precursors that contain a fraction of the disulfide derivative.


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Figure 10. Schematic illustration of the SAMs of SCnFc showing four different CV peaks which can be described in terms of packing structures induced by intermolecular interactions which influence five distinct electrochemical behaviors assigned as peak Iʹ (covalent Fc-Au interaction), peak I (Fc units exposed to electrolyte), peak II (partially buried Fc units), peak III (buried Fc units), and peak IV (disordered chemisorbed phase). The difference in the tilt angle, α, of the Fc units between nodd and neven is indicated. Gold, sulfur and iron atoms are shown as pale yellow, dark yellow, and orange space-filling spheres, CH2 groups are shown as blue sticks.

Conclusions Although Fc SAMs have been investigated since the early 1990’s, the appearance of multiple peaks and abnormal peak broadening remained an open question. By investigating a series of SAMs of SCnFc with n = 0 – 15 we were able to shed light on the origins of the widely observed deviations from ideal electrochemical behavior. We found that the origin of the peak broadening and the presence of multiple redox-waves can be ascribed to Fc units in different microenvironments which in turn depend on the intermolecular Fc-Fc, Fc-Cn, and Cn-Cn 31 ACS Paragon Plus Environment


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interactions, and the nature (i.e., covalent or non-covalent) of the Fc-Au interactions. Our findings could not be explained by the commonly used explanations that peak splitting originates from isolated Fc units (i.e., Fc units that do not interact with other Fc units) and Fc units present in clusters, or from disordered and ordered domains. Our results show that the peak splitting (defined as peaks I, II and III) and peak broadening is an inherent property of densely packed SCnFc SAMs on Au that has Fc units in different electrochemical environment due to build-up of strain in densely packed films and an additional peak at low oxidation potential (defined as peak IV) is the loosely packed disordered SAMs caused by a disulfide impurity of the SAMs precursor. The roughness of the Au electrode is important as an increase of the rms value caused a large increase in the widths of the redox-waves, primarily resulting in changes in the ratios of peaks I and II and peak broadening. We have shown elsewhere that a detailed understanding of the SAM structure, in terms of chain length odd-even effects34, impurities in the SAMs-precursor52, and surface roughness of the supporting electrode50, affects the performance of molecular diodes. Here, our results help to improve understanding of the electrochemical behavior of the redox-active SAMs in relation to their local supramolecular structures, i.e., electrochemical environment, of the redox-active group within the SAMs. The richness of the electrochemical behavior allows us to develop an atomic-scale model for SAM structure which will in turn inform efforts to use redox-active SAMs as advanced functional materials for devices.


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Associated Content Author Information. Corresponding Author: [email protected] Notes. The authors declare no competing financial interest. Acknowledgements. The Singapore National Research Foundation (NRF Award No.NRF-RF 2010-03 to C.A.N.) is kindly acknowledged for the supporting this research. Prime Minister’s Office, Singapore under its Medium sized centre program is kindly acknowledged for supporting this research. D.T. thanks Science Foundation Ireland (SFI) for financial support under Grant Number 11/SIRG/B2111, and for provision of computing resources at the SFI/Higher Education Authority Irish Centre for High-End Computing (ICHEC). Supporting Information. Background, experimental details of fabrication of Au substrates, surface characterization of the substrates by AFM, photoelectron spectroscopy (UPS and NEXAFS), and theoretical models; the peak deconvolution of CVs of the SAMs with residuals plot; the synthesis of HSCnFc. This material is available free of charge via the Internet at http://pubs.acs.org.

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(3) Heimel, G.; Romaner, L.; Zojer, E.; Bredas, J.-L. The interface energetics of self-assembled monolayers on metals. Acc. Chem. Res. 2008, 41, 721-729. (4) Wang, Q.; Evans, N; Zakeeruddin, S. M.; Exnar, I.; Gratzel, M. Molelcular wiring of insulators: charging and discharging electrode materials for high-energy lithium-ion batteries by molecular charge transport layers. J. Am. Chem. Soc. 2007, 129, 3163-3167. (5) Kim, J. S.; Park, J. H.; Lee, J. H. Control of the electrode work function and active layer morphology via surface modification of indium tin oxide for high efficiency organic photovoltaics. Appl. Phys. Lett. 2007, 91, 112111. (6) Knesting, K. M.; Hotchkiss, P. J.; MacLeod, B. A.; Marder, S. R.; Ginger, D. S. Spatially modulating interfacial properties of transparent conductive oxides: patterning work function with phosphonic acid self-assembled monolayers. Adv. Mater. 2012, 24, 642-646. (7) Song, Y. F.; McMillan, N.; Long, D.-L.; Kane, S.; Malm, J.; Riehle, M. O.; Pradeep, C. P.; Gadegaard, N.; Cronin, L. Micropatterned surface with covalently grafted unsymmetrical polyoxometalate-hybrid clusters lead to selective cell adhesion. J. Am. Chem. Soc. 2009, 131, 1340-1341. (8) Murgida, D. H.; Hildebrandt, P. Heterogeneous electron transfer of cytochrome c on coated silver electrode. Electric field effects on structure and redox potential. J. Phys. Chem. B 2001, 105, 1578-1586. (9) Bard, A. J.; Faulkner, L. R. Electrochemical methods: fundamentals and application. Wiley: New York, US, 1980 (10) Chidsey, C. E. D.; Bertozzi, C. R.; Putvinski, T. M.; Mujsce, A. M. Coadsorption of ferrocene-terminated and unsubstituted alkanethiols on gold: electroactive self-assembled monolayers. J. Am. Chem. Soc. 1990, 112, 4301-4306. 34 ACS Paragon Plus Environment

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Nonideal Electrochemical Behavior of Ferrocenyl ... - ACS Publications

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