Molecule Contacts for Efficient

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Electrical Matching at Metal/Molecule Contacts for Efficient Heterogeneous Charge Transfer Shino Sato, Shigeru Iwase, Kotaro Namba, Tomoya Ono, Kenji Hara, Atsushi Fukuoka, Kohei Uosaki, and Katsuyoshi Ikeda ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.7b07223 • Publication Date (Web): 11 Jan 2018 Downloaded from http://pubs.acs.org on January 12, 2018

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Electrical Matching at Metal/Molecule Contacts for Efficient Heterogeneous Charge Transfer Shino Sato,†,1,2 Shigeru Iwase,3 Kotaro Namba,1 Tomoya Ono,3,4 Kenji Hara,5 Atsushi Fukuoka,6 Kohei Uosaki,1,2 and Katsuyoshi Ikeda*,2,7,8 1

Graduate School of Chemical Sciences and Engineering, Hokkaido University, Sapporo 060-

0810, Japan. 2

Global Research Center for Environment and Energy based on Nanomaterials Science

(GREEN), National Institute for Ma-terials Science (NIMS), Tsukuba 305-0044, Japan. 3

Graduate School of Pure and Applied Sciences, University of Tsukuba, Tsukuba 305-8571,

Japan. 4

Center for Computational Sciences, University of Tsukuba, Tsukuba 305-8577, Japan.

5

Department of Applied Chemistry, School of Engineering, Tokyo University of Technology,

1404-1 Katakura, Hachioji, Tokyo 060-0810, Japan. 6

Institute for Catalysis, Hokkaido University, Sapporo 001-0021, Japan.

7

Department of Physical Science and Engineering, Nagoya Institute of Technology, Gokiso,

Showa, Nagoya 466-8555, Japan.

ACS Paragon Plus Environment

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Frontier Research Institute for Materials Science (FRIMS), Nagoya Institute of Technology,

Gokiso, Showa, Nagoya 466-8555, Japan.

Corresponding Author *[email protected]. Present Addresses †JNC Petrochemical Co., 5-1, Goikaigan, Ichihara, Chiba 290-8551, Japan.


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ABSTRACT.

In a metal/molecule hybrid system, unavoidable electrical mismatch exists between metal continuum states and frontier molecular orbitals.

This causes energy loss in the electron

conduction across the metal/molecule interface. For efficient use of energy in a metal/molecule hybrid system, it is necessary to control interfacial electronic structures. Here we demonstrate that electrical matching between a gold substrate and π-conjugated molecular wires can be obtained by using monoatomic foreign metal interlayers, which can change the degree of d-π* back donation at metal/anchor contacts. This interfacial control leads to energy level alignment between the Fermi level of the metal electrode and conduction molecular orbitals, resulting in resonant electron conduction in the metal/molecule hybrid system. When this method is applied to molecule-modified electrocatalysts, the heterogeneous electrochemical reaction rate is considerably improved with significant suppression of energy loss at the internal electron conduction.

KEYWORDS. Surface binding structure, energy level alignment, resonant electron tunneling, electrocatalyst, oxygen reduction reaction


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Charge transfer at metal/molecule interfaces is of prime importance in various fields such as electrocatalysis, organic photovoltaics, and molecular electronics.

When electrons relocate

between delocalized metal states and localized molecular orbitals, their kinetic behavior is influenced by the nature of the interface. For example, heterogeneous electron transfer kinetics between a metal electrode and reactant molecules is sensitive to atomic surface features of the electrode; electron transfer reactions often proceed when reactants are adsorbed at specific local surface sites with catalytically active atomic arrangements.1-4

In a metal/molecule/metal

junction, the electron conduction is influenced not only by conductance of the molecular wires but also by the adsorption geometry of the anchor group.5-7 Although the influences of the wire length and π-conjugation to molecular conduction is well recognized,8-14 there is still a dearth of information about the relation between efficient charge transfer and binding structures of metal/molecule contacts.15-20 It is of great interest to design electrical matching between metal substrate and highly conductive π-conjugated molecular wires to develop highly efficient molecular systems. In the field of electrocatalysis, one of research interests is how catalytic surface sites are introduced to a conducting substrate. Many attempts have been made to fabricate efficient electrocatalysts by direct attachment of molecular catalysts onto a conducting electrode such as a carbon, graphite, or metal substrate.21,22 The overall kinetic behavior of such a system should be dependent on electronic connection between the substrate and catalytic local sites as well as the activity of the catalytic sites.8 Two theoretical approximation conditions can be assumed for the interaction between the substrate and catalytic sites, depending on the coupling strength. In the approximation of the strong coupling limit, the activity of the catalytic sites is not independent of the electronic structure of the conducting substrate, and thus theoretical design of catalytic sites


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is rather complicated.23,24

The strong interaction of molecular catalysts with a conducting

surface often reduce the activity due to the substrate effect.25,26 In contrast, when molecular catalysts are weakly connected with the substrate via molecular wires, that is, the weak coupling limit is assumed,27-29 the overall reaction rate may be limited by the internal electron conduction through the molecular wires.8 In this case, the activity of the molecular catalysts remains intact, and hence, theoretical design of molecular catalysts is more straightforward, compared with the strong coupling condition. If efficient charge transfer is achieved between a conducting substrate and catalytic sites under the weak coupling condition, design strategy for electrocatalysts should be much simplified. In this report, we experimentally demonstrate that heterogeneous electron transfers in a metal/molecule hybrid system can be improved by electrical matching at metal/molecule contacts. As a model for the hybrid system, cobalt(II)-tetraphenylporphyrin (CoTPP) catalytic molecules are immobilized to an Au substrate using π-conjugated molecular wires with isocyanide (CN) anchor group. The binding structure of the CN group is changed by tuning the surface electronic structure, which is conducted by forming foreign metal monolayers on the Au surface. The rate of oxygen reduction reaction (ORR), catalyzed by CoTPP, is compared by changing the binding structure. The result clearly shows that the electrical matching at the metal/anchor contacts can enhance the overall reaction rate considerably.

Importantly, the

enhancement effect is not observed for saturated molecular wires but for π-conjugated molecular wires.

This finding provides a design approach not only for electrocatalysts in energy

conversion but also for molecular circuits in molecular electronics.


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RESULTS AND DISCUSSION Control of binding structures. Figure 1 shows a layer-by-layer method for preparing CoTPP-modified electrodes.8

Details for the sample preparation are described in Method

section. Briefly, an atomically planar Au disk was employed as a conducting substrate. Then, it was covered with a self-assembled monolayer (SAM) of molecular wires of various lengths and structures. The binding structure of the CN anchor was changed by forming monoatomic foreign metal overlayers of palladium (PdML) or platinum (PtML) on the Au disk prior to the SAM formation.30,31

Finally, CoTPP catalytic adlayers were formed on top of the SAM using

coordination between the Co central ion and the terminal group of the molecular wires.8,27-29,32,33 Hereinafter, the modified electrodes are denoted as CoTPP/wire/Au, CoTPP/wire/PdML/Au, and CoTPP/wire/PtML/Au, where the “wire” is replaced by individual molecular wires shown in the bottom panel of Fig. 1. Here, PDI, TPDI, OPEDI, C6DI, and ImC10SH denote 1,4-phenylene diisocyanide, 4,4’-terphenyl diisocyanide, 1,4-bis[2-(4-isocyanophenyl)ethynyl]benzene, 1,6hexane diisocyanide, and imidazole-tagged decanethiol, respectively. ImC10SH was purchased from ProChimia Surfaces, and PDI and C6DI were purchased from Aldrich. TPDI and OPEDI were synthesized by reported methods with slight modification.33,34


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Figure 1. Schematic illustration of the model electrodes with layer-by-layer structures. The Au disk with and without PtML or PdML was modified with various molecular wires. CoTPP catalytic adlayers were then formed on top of the monolayers.

The binding structures of the CN anchor groups on the metal surfaces were evaluated by polarization-modulation infrared reflection absorption spectroscopy (PM-IRRAS) (Thermofisher scientific, iS-50R) with a resolution of 4 cm-1 and 1024-integration. Figure 2 shows typical PMIRRAS spectra of TPDI on Au, PtML/Au, and PdML/Au. All of these spectra show a distinct peak at 2123 cm-1, which is assigned to the stretching of the unbound CN (νCNfree), indicating that TPDI molecules are adsorbed with edge-on configuration.35,36 The stretching mode of the bound CN is found at 2175 cm-1 on Au, 2152 cm-1 on PtML, and 2004 cm-1 on PdML. The peaks on Au and PtML are assigned to νCN with the atop configuration (νCNatop).37 The broader peak on PdML is attributed to νCN with the bridge configuration (νCNbridge).30,31,38,39 As shown in Table S1,


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density functional theory (DFT)40 calculations of νCN with different adsorption configurations well reproduce the experimental results. It is known that the σ donation induces the blue shift of νCN from νCNfree while the d-π* back donation contributes the red-shift.30,31,37,41 Therefore, the degree of d-π* back donation is expected to be the order of TPDIbridge/PdML/Au > TPDIatop/PtML/Au > TPDIatop/Au. A similar tendency of the biding structures is confirmed for PDI, OPEDE, and C6DI, as shown in Figure S1.

Figure 2. PM-IRRAS spectra of TPDI on Au, PtML/Au, and PdML/Au. The binding structures are also illustrated. The structure of the metal/anchor contacts can be changed from atop to bridge using PdML.

ORR kinetics. The rate of the electrochemically driven charge transfer from Au substrate to oxygen molecules (O2), catalyzed by CoTPP, was measured in O2-saturated 0.5 M H2SO4 solution. The mass transfer of O2 was controlled during the measurements using the rotating


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disk electrode (RDE) method.8 In the present experiments, the CoTPP-to-O2 charge transfer is governed by the activity of CoTPP.28 On the other hand, the internal conduction, i.e., Au-toCoTPP electron transmission, is influenced by the conductance of molecular wires and metal/anchor contacts.8,9

In our previous report, the contributions of the molecular wire

conductance and CoTPP activity were disentangled from the overall charge transfer by analyzing the RDE polarization curves measured for various molecular wires using Koutecky-Levich equation.8 In this study, the contribution of the metal/anchor contacts was discussed from the comparison of the ORR kinetics among different binding structures. For more details, see Method section. Figure 3 shows RDE polarization curves for ORR, which were measured on the CoTPP/TPDI systems with three different metal/anchor contacts. Clearly, there is a significant difference between the bridge and atop configurations; the polarization curve measured on CoTPP/TPDIbridge/PdML/Au is significantly shifted to the positive direction, compared with the other two structures. It is here noted that no ORR is observed in this potential region without CoTPP adlayers. This means that direct ORR on PtML or PdML is perfectly hindered by the presence of the TPDI-SAMs. That is, ORR proceeds only on CoTPP adlayers in the present layered system. It is also known that CoTPP can catalyze two-electron ORR pathway: O2 + 2e+ 2H+ → H2O2.8,22 From the Koutecky-Levich analysis of the polarization curves measured at various rotation rates, the exchange of two electron per O2 molecules is indeed confirmed in these three samples. Importantly, the surface density of CoTPP is estimated to be similar among them, as shown in Supporting Information. Therefore, the positive shift of the polarization curve, i.e., more efficient ORR, is thought to be due to higher electron conduction at the TPDIbridge/PdML/Au junction than at TPDIatop/PtML/Au and TPDIatop/Au junctions.


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Figure

3.

RDE

polarization

curves

for

ORR

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measured

on

CoTPP/TPDIatop/Au,

CoTPP/TPDIatop/PtML/Au, and CoTPP/TPDIbridge/PdML/Au in O2-saturated 0.5 M H2SO4 aqueous solution with the rotation rate of 1000 rpm. The positive shift of the curve indicates that the ORR rate is higher on CoTPP/TPDIbridge/PdML/Au than on the other structures

The increase of the ORR efficiency in the presence of PdML is indeed observed for other πconjugated phenyl diisocyanide molecules such as PDI and OPEDI. Figure 4 shows Tafel plots of ORR measured for these molecular wires with CoTPP, which were converted from the ORR polarization curves using the Koutecky-Levich analysis.8 When the binding structure of CN is the atop in the absence of PdML, the reaction rate decreases with increasing the wire length, as expected from the difference in the molecular conductance.8 The slight change of the Tafel slope is also consistently explained by the difference of the wire length. On the other hand, when the binding structure of CN is altered to the bridge in the presence of PdML, these Tafel lines are equally shifted to the positive direction without changing their Tafel slopes. These variations correspond to the increase of the rate constant for the exchange current at the equilibrium by a


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factor of about 50 and no change in the transfer number (also see Figure S2). The fact that the transfer number is invariant with and without PdML implies that the activity of CoTPP alone is not influenced by the presence of PdML. It is, therefore, believed that the presence of PdML can improve the Au-to-CoTPP internal electron transmission probability via the alternation of the binding structure.

Figure 4. Tafel plots of ORR measured on CoTPP adlayers attached to PDI, TPDI, and OPEDI monolayers formed on Au electrodes with and without PdML interlayers.

iac denotes the

activation controlled current density, which was obtained by the Koutecky-Levich analysis of the RDE polarization curves. The positive shift of Tafel lines without changing the Tafel slope indicates that PdML improves the internal electron transmission probability without changing the catalytic activity of CoTPP.

If the effect of PdML for the π-conjugated phenyl diisocyanides is explained by a simple model of a series circuit consisting of the interface resistance and molecular wire resistance, a similar effect should be observed for alkyl diisocyanide molecules consisting of a saturated wire


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and π-conjugated CN anchors. However, this is not the case, as shown in Figure 5a. When C6DI is

utilized

as

a

molecular

wire,

the

rate

of

ORR

becomes

rather

low

on

CoTPP/C6DIbridge/PdML/Au than on CoTPP/C6DIatop/Au; this behavior is opposite from the results for the π-conjugated phenyl diisocyanides, shown in Figures. 3 and 4. Incidentally, such an opposite tendency is similarly seen when saturated alkylthiols are utilized as a molecular wire. For example, when ImC10SH is utilized as a molecular wire, the rate of ORR is lower on CoTPP/ImC10SH/PdML/Au than on CoTPP/ImC10SH/Au, as shown in Figure 5b. It is again noted that the π-backbonding significantly contributes to the formation of the bridge configuration of CN on PdML while the σ-bonding is dominant for the atop configuration.30,37-39 In the case of the thiol anchor group, no contribution of π-backbonding is expected, although the exact binding structure on Pd is not resolved. Thus, the combination of π-conjugated wire body and π-backbonding at the metal/CN junction seems to be essential for improving the Au-toCoTPP internal conduction.

Presumably, the electron tunneling probability is resonantly

enhanced when both the molecular wires and metal/anchor contacts are π-conjugated.


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Figure

5.

ORR

polarization

curves

measured

for

CoTPP/C6DIatop/Au

and

CoTPP/C6DIbridge/PdML/Au (a) and for CoTPP/ImC10SH/Au and CoTPP/ImC10SH/PdML/Au (b) using the RDE method at the rotation rate of 1000 rpm.

Theoretical calculations. In general, the influence of metal/molecule contacts to the charge transfer rates is explained by the strength of metal/molecule coupling at the interface.12 Indeed, the bond strength of metal/CN is stronger in the bridge configuration than in the atop


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configuration, due to the contribution of π-backbonding. However, this cannot explain the difference of the PdML effect between the π-conjugated wires (Figure 4) and saturated wires (Figure 5a). Thus, the electronic structures in the metal/molecule hybrid model is here evaluated by theoretical calculations of LDOS for three different types of the interfaces: Au/PDIatop, Au/PdML/PDIatop, and Au/PdML/PDIbridge. (The atop adsorption on PdML has also been observed in a specific condition.31) The calculations of the electronic structures at the interfaces were carried out using the RSPACE code,42-44 which is based on the real-space grid method45 within the framework of DFT40 (see Method section). Figure 6 shows that the HOMO level of the phenyl ring is upshifted in the Au/PdML/PDIbridge structure, compared with the other two structures. This is apparently related to the difference of the binding structure between atop and bridge. In general, the local charge of a metal surface is redistributed due to bond formation, which can be referred to as bond dipole.6 The potential of the molecular layer relative to the Fermi level of the metal is shifted by the presence of the bond dipole. Since the magnitude and direction of the bond dipole depend on the binding structure, the energy level alignment can be tuned by alternating the binding structure at the interface. It is here note that the LDOS projected on the bound CN appears just below the Fermi level in the case of the Au/PdML/PDIbridge structure.

This is preferable to electron transport from the metal to molecules, which

corresponds to cathodic electron transfer in an electrochemical system.

Accordingly, the

electrical matching is expected between the conducting substrate and π-conjugated molecular wires when the bridge configuration is formed at the junction. That is, the energy difference between the Fermi level and conduction orbital can be decreased by tuning the bond dipole of the metal/anchor contacts. Since the HOMO-LUMO gap is much larger in saturated alkyl wires than


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in π-conjugated phenyl wires,13,14 such electrical matching is not expected for the saturated wires even if the CN anchor is adsorbed with the bridge configuration.

Figure 6. Calculated LDOS maps of PDIatop/Au, PDIatop/PdML/Au, PDIbridge/PdML/Au interfaces. HOMOs of phenyl ring (Ph) and CN anchor are upshifted in the case of the PDIbridge/PdML/Au.

To confirm that the HOMO level dominates electron transport in the present case, electron transmission functions for metal/PDI/metal junctions under application of zero bias voltage are also calculated using the first-principles electron-transport calculation method.44,46

This

calculation is effective for evaluation of the internal electron conduction between Au and CoTPP, although the electron transfer from CoTPP to O2 is not taken into account. Figure 7 shows transmission functions calculated for Au/PDIatop/Au, Au/PdML/PDIatop/PdML/Au, and Au/PdML/PDIbridge/PdML/Au. The LDOS maps of these sandwich structures are shown in Figure S5, indicating the energetic alignment in the Au/PdML/PDIbridge/PdML/Au structure. It is noted that nonzero transmission is found in the region of E-EF < 0 in the case of the bridge binding


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structure. This means that the HOMO level can contribute to electron conduction in this binding structure. Consequently, the bridge configuration on PdML plays an important role to improve the electron transfer rate in metal/molecule hybrid system as a result of the resonant electron tunneling. Although the potential E-EF is not correlated with the electrochemical potential at the electrified metal/electrolyte interface, this tendency implies that cathodic heterogeneous electron transfer from Au electrode to O2 molecules is also more efficiently induced under electrochemical polarization.

Figure 7. Transmission functions under application of zero bias voltage, calculated for the sandwich

structures

of

Au/PDIatop/Au,

Au/PdML/PDIatop/PdML/Au,

and

Au/PdML/PDIbridge/PdML/Au. The transmission probability of the bridge configuration increases with increasing the negative potential shift, suggesting that the bridge binding structure is more suitable for electron conduction from Au to CoTPP.

CONCLUSION


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In summary, the binding structure of the CN anchor group was controlled between atop and bridge so that the electron conduction was able to be improved in the metal/molecule hybrid system. When the body of the molecular wire was π-conjugated, the bridge binding with the stronger d-π* back donation increased the electron tunneling probability owing to the electrical matching at the interface. Metal/molecule interactions alone have been extensively studied in terms of catalysis; for example, the interaction strength on a specific metal surface is well described by the so-called d-band theory.47,48 On the other hand, the energy level alignment between the Fermi level of metal and molecular orbitals has been described using the concept of the bond dipole in the context of molecular electronics.6 In this study, the combination of these concepts from the different fields enabled us to achieve the electric matching through controlling the interfacial electronic structures. When this metal/molecule hybrid system was connected with the molecular catalysts, the electrochemical reaction proceeded with the substantially reduced overpotential. In principle, this electrical matching effect is independent of catalytic molecular species. Therefore, this result allows us to design various catalytic surface sites without considering the substrate effect.

This will lead to a dramatic advancement in the

theoretical treatment of electrocatalysts. In the single molecule conductance experiments, the influence of anchor groups to the electron transmission is hotly debated;15-20 for example, the π-conjugated CN anchor group is expected to be better than the saturated thiol anchor, but there is no broad consensus whether the former can indeed provide better transmission probability. This is in part because the geometry of metal/anchor junctions has not been controlled at the atomic scale in the single molecule experiments. In this work, we demonstrated that the combination of the CN anchor and PdML provided the “seamless” π-conjugated connection between metal substrate and aromatic


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molecular wires, accompanied by the shift-up of the HOMO level. The electrical matching for π-conjugated molecules, which are a key component for molecular devices, should be useful for improving the efficient use of energy in various molecule-based functional devises.

METHODS Preparation of chemically modified electrodes. A (111)-like Au disk with a diameter of 10 mm and a roughness factor of less than 1.2 was utilized as a substrate. The Au disk was firstly cleaned using piranha solution and hydrogen flame annealing before use. When the Au disk was utilized with surface modification of PdML or PtML, these monolayers were formed on the Au surface using the underpotential deposition (UPD).30,31 The UPD of PdML was conducted in 0.1 M H2SO4 aqueous solution with 1 mM PdCl2 and 2 mM HCl at the scan rate of 1 mV·sec-1. For the PtML formation, CuML was initially formed using UPD of Cu in 0.1 M H2SO4 aqueous solution with 1 mM CuSO4. Then, surface limited redox replacement from Cu to Pt was conducted in 0.1 M HClO4 solution with 5 mM K2PtCl4. Next, each substrate was covered with organic monolayers of molecular wires, which were self-assembly formed in a solution such as 10 mM PDI in tetrahydrofuran, 1 mM TPDI in dichloromethane, 1mM OPEDI in dichloromethane, 1 mM C6DI in methanol, or 1 mM ImC10SH in ethanol. For the molecules with the isocyanide group, this procedure was conducted in a glovebox filled with Ar. Finally, CoTPP adlayers were formed on the modified surfaces by immersing these substrates into chloroform solution with 0.1 mM CoTPP for three days. Analytical procedure of RDE polarization curves. In the RDE measurement, the current density of ORR, iORR, is obtained as a function of both ω and E, where ω is the angular velocity


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of the RDE in rad·s-1 and E is the potential in V vs. RHE. The measured iORR(ω, E) can be then analysed according to the Koutecky-Levich equation, 1/iORR(ω, E) = 1/iac(E) + 1/iL(ω) = 1/nFkORR(E)CO2 + 1/0.62nFD2/3ν-1/6CO2ω1/2, where iac and iL denote the activation-controlled current density and the mass-transport-limited current density, respectively, n is the number of electrons transferred per O2, F the Faraday constant, kORR(E) the reaction rate constant, CO2 the concentration of dissolved O2, D the diffusion coefficient of O2, ν the viscosity of solvent. For these parameters, the following values were utilized in the analyses: CO2 = 1.2×10-6 mol·cm-3, D = 1.9×10-5 cm2·s-1, ν = 1.1×10-2 cm2·s-1. The obtained iac(E) can be further analysed according to the Tafel equation, η = a + b log |iac|, where η = E – E0 denotes the overpotential for the redox reaction of O2/H2O2, i.e., energy loss in the reaction. Here, the Tafel slope and intercept are b = 2.3RT/(αnF) and -a/b = log |i0| = log |nFk0CO2|, respectively. The transfer number, α, is assumed to be potential-independent. k0 is the rate constant for the exchange current at the equilibrium, k0 = kORR (η = 0 V). Theoretical calculations. Theoretical calculations of the electronic structures at interfaces were carried out by the RSPACE code,42-44 which is based on the framework of DFT40 and uses the real-space grid method42-45 to eliminate the unfavorable effect from the incompleteness of atomic basis sets.49 For the calculation of the electronic structures of the PDI on the Au surface, tetragonal supercell with the dimensions Lx = 8.65 Å, Ly = 5.00 Å, and Lz = 34.16 Å was employed, where Lx and Ly are the lengths in the x- and y-directions parallel to the Au surface, respectively, and Lz is the length in the z-direction. The supercell contains four Au(111) layers and PDI molecule.

The periodic boundary condition was applied to all directions and a

sufficiently thick vacuum region of 17.11 Å was inserted. The grid spacing was set at 0.16 Å


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and the nine-point finite-difference formula45 was employed for the kinetic-energy operator in Kohn-Sham equation. The integration over the two dimensional Brillouin zone was carried out using a 2×4 k-point mesh. The electron-ion interactions were treated using the norm-conserving pseudopotential of Troullier and Martins50,51 and the exchange-correlation effect is approximated by the local density approximation.52 When the effect of PdML was evaluated, the atoms in the first layer of the Au surface were replaced with Pd atoms. The structural optimization for atomic geometry was performed while the atoms in the bottom Au layer were fixed. The distributions of the LDOS were calculated as

, = ∑ , ∬ | , , , | × e, , where εi,k are the eigenvalues of the wave function , , with indexes i and k denoting the eigenstate and the k point, respectively. = 2/ is the normalization factor, where α is the smearing factor, here set to 13.5 eV-2. For the calculation of transmission functions of the PDI, the scattering region was sandwiched between semi-infinite Au electrode regions. The dimension of the supercell for the scattering region, which consisted of seven Au(111) atomic layers of electrode, PDI, and six (111) atomic layers, was Lx = 8.65 Å, Ly = 5.00 Å, and Lz = 40.23 Å. We suspended the PDI between two facing Au electrodes by attaching the opposite electrode to the PDI on the Au surface used to compute the LDOS so that the system was symmetric along the z-direction. The semi-infinite Au electrode were treated by the fully energy dependent self-energy,53 which enables us to minimize the number of the extra buffer layers in the scattering region.54 The scattering wave functions continuing from one electrode to the other were computed by the overbridging boundary-matching method.46

The transmission functions were then computed using the


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Landauer-Büttiker formula,55 in which transmission coefficients were obtained by scattering wave functions. We have assured that the number of the sampling k points, the exchange-correlation functional, and the computational model size do not affect our conclusion (see Fig. S4).

ASSOCIATED CONTENT Supporting Information. Characterization of chemically modified electrodes, Quantitative comparison of k0 and α, and Validation of the computational parameters. This material is available free of charge via the Internet at http://pubs.acs.org.” The authors declare no competing financial interest.

ACKNOWLEDGMENT This research was partially supported by PRESTO, Japan Science and Technology Agency (JST) (No. JPMJPR11C1), Grant-in-Aid for Scientific Research (c) (No. 15K05372) from JSPS, Japan, and also by program for Development of Environmental Technology using Nanotechnology from Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. Dr. G. Yu (Catalysis Research Center, Hokkaido University) is acknowledged for experimental help.


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Table of Contents artwork


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