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Effects of Interface Electronic Structures on Transition Voltage Spectroscopy of Alkanethiol Molecular Junctions Daisuke Nose,† Kouki Dote,† Tomoya Sato,† Makoto Yamamoto,† Hisao Ishii,†,‡ and Yutaka Noguchi*,§ †

Graduate School of Advanced Integration Science, Chiba University, Chiba 263-8522, Japan Center for Frontier Science, Chiba University, Chiba 263-8522, Japan § School of Science and Technology, Meiji University, Kawasaki 214-8571, Japan ‡

S Supporting Information *

ABSTRACT: We investigated the charge transport characteristics of alkanemonothiol (CnH2n+1SH, n = number of carbons) molecular junctions by means of transition voltage spectroscopy (TVS) based on the observations of the interface electronic structures. The minimum in the Fowler−Nordheim plot was observed at the average positive and negative sample biases of +1.23 and −1.44 V. These voltages (Vmin) were insensitive to the molecular length. The low-energy ultraviolet photoelectron spectroscopy (LE-UPS) measurements revealed the presence of an Au−S bond at a binding energy of 1.4 eV with reference to the Fermi level of the Au substrates. The binding energy of the interface electronic state was independent of the molecular length. The TVS results were analyzed based on the LE-UPS results, including the differences in the measurement conditions. The results were consistently explained by the Au−S bond being responsible for Vmin at the negative bias. In addition, another interface state was suggested to be responsible for Vmin at the positive bias. The effects of the interface electronic structures besides the apparent barrier height should be considered to understand TVS of molecular junctions with wide energy gap molecules.

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INTRODUCTION The properties of the charge transport through single molecules between a pair of electrodes have attracted much interest in single-molecular electronics, where single molecules are used as active electronic components.1−3 The energy offset between the Fermi level of the electrode (EF) and the nearest molecular orbital in the molecular junction often dominates the charge transport characteristics. The accurate evaluation of the energy level alignment at the molecular junction is important for understanding the charge transport properties. Transition voltage spectroscopy (TVS) is a powerful tool for investigating the correlations between the current−voltage (I−V) characteristics and the energy level alignment at molecular junctions.4−12 TVS analyzes the energy barrier height in the molecular junctions from the voltage, Vmin, at the minimum of the Fowler−Nordheim (F−N) curve. Beebe et al. found that Vmin is proportional to the energy barrier height ΔE (= EF − εh) in the molecular junctions with π-conjugated molecules, where εh is the energy of the highest occupied molecular orbital level estimated by ultraviolet photoelectron spectroscopy (UPS).4 Because of its simplicity and reproducibility, TVS is becoming a standard tool for estimating the apparent energy barrier height in molecular junctions. Physical interpretations of TVS have been proposed based on the Landauer formula.13−18 The current through the molecular junction is described as I=

2e h

where e is the elementary charge, h is Planck’s constant, T(E) is the transmission function, and f L(E) and f R(E) are the Fermi− Dirac distributions for the left and right electrodes, respectively. Assuming a single-level model, T(E) is given by a Lorentzian function13−15,17,19 T (E ) =

© 2015 American Chemical Society

[E − (ε0 + ηeV )]2 + Γ 2/4

(2)

where ε0 is the energy level responsible for the electric current, Γ is a broadening factor originating from the orbital coupling at the molecule/electrode interfaces, η is the asymmetry factor, and f is a constant to account for multiple molecules at the junction.15 If the potential reference is defined at the center of the junction, the asymmetry factor varies within the range −0.5 ≤ η ≤ 0.5. Chen et al. reported that η plays an important role in TVS, although η is often assumed to be 0, corresponding to the symmetric junctions in previous studies.15 Vmin appears when T(E) in the bias window grows rapidly. TVS includes ambiguity in the determination of the energy offset of the junction because the responsible energy level is not directly obtained from Vmin without evaluating Γ and η. To determine T(E) experimentally, complementary studies of interface electronic structures and charge transport properties at the corresponding molecular junctions are required. Moreover, evaluation of Vmin Received: April 6, 2015 Revised: May 10, 2015 Published: May 11, 2015

∞

∫−∞ T(E)[fL (E) − fR (E)] dE

f

(1) 12765

DOI: 10.1021/acs.jpcc.5b03296 J. Phys. Chem. C 2015, 119, 12765−12771

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The Journal of Physical Chemistry C

Seiko Instruments, Inc.) under ambient conditions. An Aucoated Si cantilever (Si-AF01, Epolead Service, Inc.) was used for the measurement, and the radius of the tip was less than 50 nm. The CP-AFM tip was brought into soft mechanical contact with the CnT SAMs on the Au substrate. The contact force was set as 2 nN. The active contact area was estimated to be 10 nm2. A bias voltage was applied to the Au substrate with reference to the tip within −2 to 2 V. The I−V curves were measured at more than 300 points for each sample. The measurement points were distributed over the area of ∼10 mm2. All the measured I−V curves were fitted by smooth curves of a third-order polynomial of the applied bias. The fitted curves were used for determining Vmin and the differential conductances. LE-UPS Measurements. The LE-UPS measurements were performed by using a home-built system with a base pressure below 5 × 10−7 Pa. The excitation photons from a 150 W deuterium lamp were monochromatized by a doubled grating system (M25GTM-DZ, Bunkoukeiki). The incident photon energy was 7.7 eV, and the incident angle was set at 55° from the surface normal. The spot size of the incident photon was 12 mm2. The use of a low energy incident photon is advantageous for observing the electronic structures at the molecule/metal interface around the Fermi level because it decreases the amount of secondary electrons and has a deeper probing depth compared with conventional He(I) UPS.22 The kinetic energy (Ek) distribution of the photoelectrons was acquired by a concentric hemispherical analyzer (Resolve120, PSP) in the surface normal direction. All the measurements were conducted under a sample bias of −10 V.

at positive and negative biases is essential because they are different in relation to η.20 The TVS results have often been explained by the energy level alignment of the self-assembled monolayer (SAM)/ substrate systems determined by UPS.4−6,8 However, comparison between the TVS and UPS results are not straightforward because these measurements are usually performed under different conditions, such as measurement area, presence of the top electrode, applied electric field, and surface sensitivity. These differences should be discussed to understand the charge transport properties of the molecular junctions accurately in terms of their energy level alignment. Moreover, the effects of interface electronic structures including the contributions from the substrate to TVS are not well understood. In this study, we performed conducting probe atomic force microscopy (CP-AFM) and low-energy (LE)-UPS measurements of the alkanemonothiol (CnT) SAMs on the Au substrate, where n is the number of carbons. Although CnT SAMs on Au is a standard system for molecular junctions, there is a large discrepancy between the energy offsets estimated by TVS and UPS.6,10 We analyzed the charge transport properties of the CnT molecular junctions based on the interface electronic structure measured by LE-UPS, including the energy offset and the spectrum broadening. The results indicate that the Au−S bond instead of the alkyl chain could be the state responsible for Vmin at the negative voltage. We estimated η and Γ in the single-level model from the experimental results of TVS and UPS. The value of η indicates that the responsible state couples more strongly with the Au substrate side. Consequently, the single-level model showed that Vmin at the positive bias originated from another interface state above the Fermi level of Au, e.g., the Au−S antibonding state. Our results suggest that TVS can probe the interface electronic states instead of the apparent barrier height if the interface states appear in between the “energy gap” of the molecular junction.

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RESULTS Charge Transport Characteristics. Figure 1a shows the averaged I−V curves of the CnT molecular junctions. The raw I−V curves are presented in the Supporting Information. The number of molecules at the molecular junctions was less than 102 estimated from the active contact area, where a density of 4.6 nm−2 was assumed for the alkanethiols.23 The differential conductances at zero bias are shown in Figure 1b. The zero bias conductances exponentially decay as the number of carbons in the alkyl chain (nc) increases. The tunneling decay constant, β, was evaluated as 0.91, which agrees well with the reported value.9,23−25 The results indicate that the I−V characteristics of the CnT molecular junctions were properly examined. The measured I−V curves were analyzed in the F−N plot. A typical F−N curve of the C7T junction is shown in Figure 2a. The voltage at the minimum point of the F−N curve at the positive and negative biases is defined as the transition voltage, Vmin(+) and Vmin(−), respectively. Figure 2b shows the transition voltage histograms. The most probable transition voltage and the standard deviation were determined by fitting the histograms with a Gaussian function. Here, the fluctuation of the transition voltage is mainly attributed to the fluctuation of the energy level responsible owing to the local contact details.17,26 Figure 2c shows Vmin as a function of nc. Vmin(+) increases slightly for longer molecules, whereas Vmin(−) is almost constant. Interface Electronic Structures. Figure 3a shows the full LE-UPS spectra of C6T, C8T, C10T, and C12T SAMs on the Au substrate. The intensity was normalized by the value at a binding energy of 2.7 eV. The LE-UPS spectrum of the bare Au substrate is also shown. All these samples were prepared through the same method as for the charge transport

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EXPERIMENTAL SECTION Sample Preparation. Flat Au substrates were prepared by the template stripping method.21 The fabrication procedure started from the Au deposition on the template substrate, which was a Si(110) wafer with a native oxide layer. The substrate was sonicated twice for 5 min each in pure acetone and in isopropanol successively. After ultraviolet-ozone treatment for 10 min, the substrate was immediately placed in the evaporation chamber. Au was deposited at a typical thickness of 500 nm by a standard thermal evaporation technique with a base pressure of 2 × 10−4 Pa. Another Si wafer or glass substrate was glued onto the surface of the Au-deposited substrate with an epoxy adhesive. Finally, the top Si wafer was peeled off together with the Au layer. The root-mean-square roughness of the resultant Au substrate was less than 4 Å over an area of 4 μm2. The SAM formation on the Au substrate was conducted in a nitrogen-filled glovebox (H2O, O2 < 1 ppm). The Au substrates were peeled from the template and immediately immersed in an ethanolic solution of CnT for 1 day. The concentration of the CnT solutions was typically 2 mM. After the SAM formation, the sample substrates were rinsed with pure ethanol to remove excess molecules on the sample surface. Finally, the substrates were dried under a flow of nitrogen gas. CP-AFM Measurement. The I−V measurement was performed by using the CP-AFM mode of a conventional scanning probe microscopy system (SPA400 with SPI3800N, 12766

DOI: 10.1021/acs.jpcc.5b03296 J. Phys. Chem. C 2015, 119, 12765−12771

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Figure 3. LE-UPS spectra of the C6T, C8T, C10T, and C12T SAMs on the Au substrate. The energy of the incident photon was 7.7 eV. The spot size of the incident photon was 12 mm2. The spectra are vertically offset for each sample. (a) Full spectra. The intensity is normalized at 2.7 eV. The peak at 2.7 eV is assigned to the emission from Au(5d). The work function of the Au substrate was 4.7 eV. (b) LE-UPS spectra in the SECO region. The SECO shift to the negative side was induced by the CnT adsorption. (c) LE-UPS spectra around the Fermi edge. An additional peak (indicated by the small bars) appeared around the binding energy of 1.4 eV owing to the CnT SAMs. These peaks originate from the Au−S bond.

Figure 1. (a) I−V curves of CnT molecular junctions. The average currents detected from more than 300 points of the sample surface are displayed. The top contact was the CP-AFM tip with a contact force of 2 nN. The junction active area was estimated to be approximately 10 nm2. (b) Averaged differential conductance at zero bias as a function of the number of carbons. The decay constant, β, was estimated to be 0.91.

measurements. The work function of the Au substrate was typically 4.7 eV. The secondary electron cutoff (SECO) shift to the higher binding energy was observed (Figure 3b) because of the electron donating properties of sulfur at the CnT SAMs/Au contacts.27 The SECO shift was 0.33 eV for C6T/Au, C8T/Au, and C10T/Au, whereas it was 0.47 eV for C12T/Au. These values are less than those previously reported for the alkanethiol SAMs on a polycrystalline Au foil (1.01 to 1.35 eV).27 A possible explanation of the discrepancy is the differences in the molecular density and the alignment of SAMs because of the different experimental conditions, such as the substrate preparation. Direct observation of the electronic

structure of samples prepared in the same manner for the charge transport measurements is important for estimating the energy level alignment at the molecular junctions accurately. In Figure 3c, the expanded spectra near the Fermi level are shown. In the LE-UPS spectrum of the bare Au substrate, a flat terrace extends from the Fermi edge to the binding energy of around 1.8 eV. The secondary electron emissions and the emissions from Au(5d) are overlapped on the spectrum at higher binding energies. An additional structure clearly

Figure 2. (a) Typical F−N curves of C7T molecular junctions. The transition voltages appeared at the positive and negative voltages. The solid curves show the fitting curves of the third order polynomial. (b) Transition voltage histograms for C6T, C8T, C10T, and C12T molecular junctions. The solid curves were the Gaussian function determined by a curve fitting method. (c) Most probable transition voltages of CnT molecular junctions. The transition voltages at the positive and negative biases are 1.23 and −1.44 V on average, respectively. 12767

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We propose a method for estimating η and Γ from the experimental results of TVS and UPS. Initially, we deduced the interface DOS originating from the Au−S bond. LE-UPS measured the occupied DOS (n(E)) at the CnT/Au interface. The spectra include contributions from the DOS of the Au substrate (gAu(E)) as well as the CnT SAMs, although gAu(E) is almost constant in the energy range from the Fermi level to 1.8 eV (Figure 3a,c). The total occupied DOS in this energy range can be described as

appeared in the spectra at a binding energy of around 1.4 eV in all samples except the bare Au substrate. This structure was induced by the CnT adsorption. The binding energy of the additional structure was almost constant. These signals were not attributed to ionization of the alkyl chains, but to the Au−S bond.27,28 The energy of the Au−S bond is denoted as εs. The highest energy of the occupied molecular orbital of the alkyl chain (εc) is located around 4.5 eV below the Fermi level of Au,27,29 although it is out of the range of our spectra.

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n(E) = [gs(E) + gAu(E)]f (E)

DISCUSSION The differential conductance (G) at zero bias decayed exponentially with increasing alkyl chain length, and thus G ∝ exp(−βnc).23−25,30 In addition, the transition voltages of the CnT molecular junctions were insensitive to the alkyl chain length. The results are consistent with the charges traveling through the energy barrier of the alkyl chain, where the energy barrier height is independent of the molecular length.6,27 Previous UPS studies on the CnT SAMs on Au also support the charge transport properties of a constant energy offset at the CnT/Au contact with various alkyl chain lengths.27 However, εc is located 4.5 eV below the Fermi level of Au. This energy level seems too far from the Fermi level to induce the transition voltages around −1.44 and +1.23 V. Note here that the UPS evaluates only the occupied states; however, a similar discussion is valid for the unoccupied state of the alkyl chain because it is located 3.35 eV above the Fermi level.29 The transition voltage intrinsically appears at a bias lower than the resonant condition, which indicates that the energy level responsible becomes accessible by the bias window.13−15 This feature is attributed to the broadening factor (Γ) of the transmission function induced by the molecule/metal contacts. However, for CnT molecular junctions, a large gap remains between the TVS and UPS, which results in considering Γ and η, if εc is assumed to be responsible for the transition voltages.10,13 Instead of the alkyl chain, the Au−S bond, which has a binding energy of 1.4 eV is more plausible as the state responsible for the transition voltage of the CnT molecular junctions. The charge transport through the molecular junctions is characterized by the transmission function. The transmission function is proportional to the interface density of states (DOS) at the molecule/substrate contact when the coupling of the molecule with the substrate is dominant.19 Because the interactions between the CP-AFM tip and the surface of the CnT SAMs are expected to be weak, we assumed here that the LE-UPS spectra are proportional to the transmission function. The situation is similar to that of scanning tunneling spectroscopy (STS), where the interface DOS modifies the tunneling current through the vacuum. In the case of CnT molecular junctions, the Au−S bond contributes to TVS, but the alkyl chain orbitals dominate the tunneling decay constant. Because of the differences in the experimental conditions, comparison between the TVS and UPS results are not straightforward. For instance, large area measurements in LEUPS (12 mm2 in our experimental setup) involve the energy level fluctuation originating from a variety of the local electrostatic environment. Thus, a further broadening appears in the spectra in addition to the energy level broadening, Γ, the CnT/Au contacts in the CP-AFM measurements (measurement area ∼ 10 nm2). In addition, the applied external biases change the energy offset at the molecular junctions depending on the asymmetry factor η.

(3)

Here, gs(E) is the DOS arising from the Au−S bond, and f(E) is the Fermi−Dirac distribution. gs(E) can be described by the Voigt function (a convolution of a Gaussian and a Lorentzian function), as ⎡ (ε ′ − E )2 ⎤ 1 0 ⎥ exp⎢ 0 2πσUPS ⎣ 2σUPS2 ⎦ 1 × dε0′ (E − ε0′)2 + Γ 2/4 ∞

g s (E ) ∝

∫−∞

(4)

Here, E0 is the peak energy of the LE-UPS spectrum and σUPS is the standard deviation describing the energy level fluctuations. The Lorentzian function describes the contribution from the single Au−S contact, and the Gaussian function expresses the energy level fluctuations induced by the contact properties (discussed later). Figure 4 shows the LE-UPS spectra of the

Figure 4. LE-UPS spectra of the C6T SAM on the Au substrate and the fitting curve in the Fermi edge region. The function for the fitting curve is eq 3. The LE-UPS spectrum of the bare Au substrate is also shown as a reference. The intensity was normalized at the Fermi edge.

C6T SAM on the Au substrate and the fitting curve, where gAu(E) was assumed to be constant. gs(E) was then calculated with the fitting parameters. The finite energy resolution of the measurement system also causes broadening of the UPS spectra. From the spectrum shape of the Au substrate around the Fermi edge, the instrumental function was derived as a Gaussian function with fwhm = 0.32 eV.31 The instrumental function of our measurement system caused the 40 meV broadening of the intrinsic peak shape at fwhm, although it was deconvoluted from the raw UPS spectra for the analyses. The contributions from the Au−S bond and the Au substrate to the tunneling current should be considered because gAu(E) offsets the interface DOS. The probing depth of LE-UPS is several nanometers,22 indicating that the contributions from the Au underneath the CnT/Au interface are also included in the spectra. However, CP-AFM is very sensitive to the outermost DOS in the bias window because of the tunneling current.32 Thus, the contributions of the Au substrate are suppressed in 12768

DOI: 10.1021/acs.jpcc.5b03296 J. Phys. Chem. C 2015, 119, 12765−12771

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The Journal of Physical Chemistry C the charge transport measurements rather than in the LE-UPS measurements. For the first approximation, we omitted the contributions of gAu(E) to T(E) in the following analyses; this is discussed in the Supporting Information. In the CP-AFM measurements, the I−V curves for a small area were acquired at different points on the sample surface. Thus, the energy level fluctuation caused by variations in the local contact details can be estimated from the transition voltage histograms (Figure 2b). Because the transition voltage is insensitive to Γ compared with the energy level alignment,15 the distribution of the transition voltage can be mainly attributed to the fluctuations in the responsible energy level. The transition voltage histograms can be fitted by the Gaussian function with a standard deviation of σTVS. However, the fluctuation in the transition voltage histograms is affected by the external applied voltage.17,26 The applied voltage induces the energy level shift, and thus, the width of the transition voltage histogram changes depending on η. Figure 5 shows the relationship between the transition voltage and energy offset in the initial state (without applied voltage) calculated by using the single-level model.

Figure 6. η and Γ obtained from the results of TVS and UPS by the self-consistent method. η = 0.16 indicates that the responsible state couples more strongly with the substrate side of the junction.

average η value of the CnT molecular junctions was estimated to be 0.16, indicating that the state responsible formed a stronger coupling with the substrate side of the junction. These results are consistent with the Au−S bond being responsible for the transition voltage. However, η ≈ 0.16 seems to be small for the Au−S bond, which is expected to couple strongly with the substrate. A possible explanation for the underestimation is the contribution of gAu(E) to T(E), although it was omitted from the calculations. If we assume finite contributions of gAu(E) to T(E) a larger value of η was obtained, indicating the stronger coupling to the substrate side (see Supporting Information). However, the obtained Γ value is much larger than the reported value in the alkanedithiol molecular junctions (26.5 meV).26 The discrepancy may arise from the differences in the test system; our experiments were performed on the SAM on an Au substrate, whereas the previous study was on isolated single molecules connected to Au by using the break junction technique.9 Our experimental system includes the effects of multiple molecules in the film form. The interactions between the molecules can induce energy level broadening, leading to a large Γ. In addition, the LE-UPS spectra may include other broadening factors, such as inelastic scattering of photoelectrons, and polarization energy.33 Although the model needs to be improved, we demonstrated a method for the direct comparison between the TVS and UPS results for the first approximation. The single-level model showed that the transition voltage appears at +3 V, if εs is also responsible for the transition voltage at the positive bias. Here, the obtained parameters, η = 0.16 and Γ = 0.9 were substituted into the transmission function. The estimated transition voltage is significantly larger than the measured value (average of 1.23 V). The result suggests that another interface state, e.g., the Au−S antibonding state34 or H2O induced state at the CP-AFM tip, contributes to the transition voltage at the positive bias. Because of the large energy gap between the saturated alkanes, the transition voltages at the positive and negative biases have been analyzed by assuming the nearest state, namely, the highest occupied state or lowest unoccupied state originating from the alkyl chain.11 However, when the interface state induced at the Au−S contact plays a significant role on TVS, the different states are responsible for the transition voltages at positive and negative biases. The presence of filled and empty interface states within the “energy gap” of the alkyl chain has also been suggested by theoretical and experimental studies.29,34,35 Nakaya et al. performed a STS study on the C8T SAM on the Au(111)

Figure 5. Relationship between the transition voltage and peak energy of the transmission function in the single-level model with Γ = 0.9, where η is varied from 0 to 0.5. The energy level fluctuation at the zero bias (σUPS) resulted in fluctuations of the transition voltage (σTVS) depending on η.

We estimated probable η and Γ values by using selfconsistent calculations. First, to determine the tentative Γ value, the LE-UPS spectrum originating from the Au−S bond was fitted by the convolution of T(E) and the Gaussian function, assuming that the distribution of ε0 in T(E) obeys the Gaussian function with σTVS obtained from the transition voltage histogram. The transition voltages were calculated from the single-level model with ε0 as a variable, where η = 0.5 was set as the initial value. We obtained the relation between the transition voltage and the responsible energy level in the initial state (Figure 5). σUPS in the Gaussian function was obtained by using this relation, and consequently, a new Γ value was obtained. The new Γ value provided a new value of η in the single-level model. Probable Γ and η values were obtained by repeating this calculation flow until η converged. Γ and η were assumed to be independent from ε0 for simplicity. Figure 6 shows the η and Γ values for the CnT molecular junctions. η and Γ are insensitive to the molecular length. The 12769

DOI: 10.1021/acs.jpcc.5b03296 J. Phys. Chem. C 2015, 119, 12765−12771

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The Journal of Physical Chemistry C surface where the filled and empty states of the Au−S bond were responsible for the charge transport at the positive and negative biases, respectively.35 The density peak of the filled state was observed at 1.3 eV below the Fermi level, whereas that of the empty state was 1.4 eV above the Fermi level. The STS study measures the local DOS with η = 0.5, and thus, the bias voltage directly corresponds to the binding energy of the UPS spectra. The peak energy of the Au−S bond agrees well with that of our LE-UPS results (average of 1.4 eV). In addition, Qi et al. reported that the presence of the filled and empty states between the “energy gap” region measured by UPS and inverse photoemission spectroscopy, where the empty states are located near the Fermi level rather than the filled states.29 Although the effects of the interface electronic states between the “energy gap” on the TVS have not been thoroughly investigated, they should be taken into account, particularly if wide energy gap molecules are involved.

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CONCLUSIONS We investigated the charge transport properties of alkanethiol molecular junctions by means of CP-AFM and LE-UPS. The charge transport properties were analyzed based on the interface electronic structure measured by LE-UPS. Comparing the TVS and UPS results suggested that the Au−S bond was responsible for the transition voltage at the negative bias. In addition, we proposed a method to estimate η and Γ in the single-level model from the experimental results of TVS and UPS. The obtained value of η indicates that the state responsible for the transition voltage at the negative bias coupled more strongly with the substrate side. Consequently, the single-level model showed that Vmin at the positive bias should originate from another interface state above the Fermi level of Au although an identical state has been commonly assumed to be the origin of Vmin at both bias polarities. Our results show that the interface electronic states should be considered to understand the charge transport properties of molecular junctions, particularly for wide energy gap molecules.

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ASSOCIATED CONTENT

S Supporting Information *

Additional discussions for the contributions of the substrate to TVS and I−V histograms. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b03296.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We would like to thank Yasuo Nakayama for fruitful discussions. M.Y. and Y.N. would like to thank C. Daniel Frisbie, Yanfei Wu, and Davood Taherinia for their guidance with the CP-AFM experiments. This research was supported by KAKENHI (Grant Nos. 24510148 and 25288114).

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REFERENCES

(1) Lindsay, S.; Ratner, M. A. Molecular Transport Junctions: Clearing Mists. Adv. Mater. 2007, 19, 23−31. 12770

DOI: 10.1021/acs.jpcc.5b03296 J. Phys. Chem. C 2015, 119, 12765−12771

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DOI: 10.1021/acs.jpcc.5b03296 J. Phys. Chem. C 2015, 119, 12765−12771