Fluctuation in Interface and Electronic Structure of Single-Molecule

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Fluctuation in Interface and Electronic Structure of Single-Molecule Junctions Investigated by Current versus Bias Voltage Characteristics Yuji Isshiki, Shintaro Fujii, Tomoaki Nishino, and Manabu Kiguchi J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b13694 • Publication Date (Web): 22 Feb 2018 Downloaded from http://pubs.acs.org on February 23, 2018

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Journal of the American Chemical Society

Fluctuation Structure

in of

Interface

and

Electronic

Single-Molecule

Junctions

Investigated by Current versus Bias Voltage Characteristics Yuji Isshiki, Shintaro Fujii,* Tomoaki Nishino, and Manabu Kiguchi* Department of Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology, 2-12-1 W4-10 Ookayama, Meguro-ku, Tokyo 152-8551, Japan

ABSTRACT: Structural and electronic detail at the metal-molecule interface has a significant impact on the charge transport across the molecular junctions, but its precise understanding and control still remain elusive. On the single-molecule scale, the metalmolecule interface structures and relevant charge transport properties are subject to fluctuation, which contains fundamental science of the single-molecule transport and implication for manipulability of the transport properties in the electronic devices. Here, we present a comprehensive approach to investigate the fluctuation in the metal-molecule interface in single-molecule junctions, based on current-voltage (I-V) measurements in combination

with

first-principles

simulation. Contrary

to conventional molecular

conductance studies, this I-V approach provides a correlated statistical description of both, the degree of electronic coupling across the metal-molecule interface, and the molecular orbitalenergy level. This statistical approach was employed to study fluctuation in single-molecule

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junctions of 1,4-butanediamine (DAB), pyrazine (PY), 4,4’-bipyridine (BPY), and fullerene (C60). We demonstrate that molecular dependent fluctuation of σ-, π-, and π-plane- type interface can be captured by analyzing molecular orbital-energy (MO) level under mechanical perturbation. While the MO level of DAB with the σ-type interface shows weak distance dependence and fluctuation, the MO level of PY, BPY, and C60 features unique distance dependence and molecular dependent fluctuation against the mechanical perturbation. The MO level of PY and BPY with the σ+π-type interface increases with the increase in the stretch distance. In contrary, the MO level of C60 with the π-plane-type interface decreases with the increase in the stretching perturbation. This study provides an approach to resolve the structural and electronic fluctuation in the single-molecule junctions and insight into the molecular dependent fluctuation in the junctions.


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INTRODUCTION Understanding charge transport across the metal-molecule interface is a fundamental issue in organic devices such as organic light emitting diodes 1-4 and organic solar cells. 5, 6 In recent years, single-molecule scale studies on charge transport through the metal-molecule interface have been made possible by break junction techniques

7-9

From fundamental and practical

viewpoints in nanoscience and electronic device application, the charge transport properties through the simple and ultra-small interface in the single-molecule junction have been extensively studied last two decades.

10-14

The break junction techniques rely on statistical

distributions of large numbers of single-molecule junction-measurement, in which the electronic current is repeatedly measured and the statistical analysis of thousands of the electronic measurement provides most probable electronic conductance of the singlemolecule junction. In general, the structural detail such as the metal-molecule interface structure and the molecular conformation in the junction affects the single-molecule conductance. It has been demonstrated that structural variation in the metal-molecule interface has strong impact on the electronic conductance of the single-molecule junction. 20

15-

While the majority of the single-molecule studies have focused on the charge transport

properties of the most probable and energetically favorable interface structure of the singlemolecule junctions, the variability and the fluctuation in the interface structures and the corresponding transport properties contain characteristic information on the single-molecule scale that provides fundamental science of the simple, ultra-small, and low dimensional charge transport across the individual metal-molecule interface as well as switching functionality and manipulation of the transport properties through the metastable states, which cannot be obtained using standard technique based on the conductance measurement. Herein, on the basis of a combined computational and experimental approach, we studied molecular dependent fluctuation of the interface and the electronic structures of the


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single-molecule junctions. To investigate the electronic structures of the single-molecule junctions, recently developed break junction method combined with the current-voltage (I-V) measurement19 was adapted in this study. Within the single-channel Landauer-Büttiker formalism,21,22 the I-V characteristics of the single-molecule junction provide useful information on electronic structures, 19,23-27 such as the electronic coupling between the metal and the molecule (Γ ) and the energy difference between the Fermi level energy and the conduction-orbital (ε) (Figure 1a). The Γ and ε are essential parameters needed to understand the electronic structure in the molecular junctions. Combined analysis in Γ and ε and the density functional theory (DFT)-based atomistic simulation enabled us to resolve the electronic and the relevant metal-molecule interface structures at the molecular junctions. We applied this approach to the single-molecule junctions of 1,4-butanediamine (DAB), pyradine (PY), 4,4’-bipyridine (BPY),

9,30,31

and fullerene (C60)

32,33

16,28,29

sandwiched by Au

electrodes (Figure 1a,b). To capture the fluctuation of the interface and electronic structures, I-V measurement was collected for the singe-molecule junctions with a variety of the stretching perturbation (Figure 1c). The fluctuation of the electronic structure was investigated by I-V-based analysis of the ε and Γ values in combination with the structural and electronic simulation of the junction under the mechanical perturbation. We made the direct link between the fluctuation in the electronic structures and the fluctuation of the interface of the σ−, π−, and π-plane− type interface in the single-molecule junctions. This I-V approach provides new insight into the structural and electronic fluctuation in the molecular junction on the single-molecule scale, which has been inaccessible by the standard molecular conductance measurement and the statistical conductance analysis.

EXPERIMENTAL SECTION


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Experiment. DAB, PY, and C60 were purchased from TCI Japan and BPY was from Wako Pure Chemical Industries Japan (Figure 1b). All the chemicals were used without further purification. The Au(111) substrate was prepared by thermal deposition of gold on mica at elevated temperature under high vacuum. The sample was prepared by dipping the Au substrate into a 1-10 mM ethanol or toluene solution containing the molecules for 12-24 hours. After evaporation of the solution, the substrate surface was washed with the solvent. We used a commercially available STM (Nanoscope V, Bruker, Santa Barbara, CA) operating at ambient conditions. STM tips were prepared by mechanically cutting an Au wire (Nilaco, diameter ≈ 0.3 mm, purity >99 %). The I-V curves of the single-molecule junction were obtained by the following procedure19 (Figure 1c). Firstly, an Au point contact (~10 G0, G0 = 2e2/h) was made between the STM tip and the sample surface. Secondly, the tip was withdrawn by 10 nm at a speed of ca. 40 nm/s to break the Au contact and to make a nanogap between the Au electrodes, forming the molecular junction during current monitoring at a fixed bias voltage of 20 mV. Thirdly, the tip position was fixed and one I-V curve was recorded by scanning the bias voltage from 20 to 1000, –1000 mV, and back to 20 mV within a time period of 2.5 ms at constant tip-sample separation. Finally, the junction was broken by pulling the STM tip away from the substrate. To capture possible structural variation of the junction structures, the junctions are stretched by as large as 10 nm until the junction was broken, and we cycled the molecular junction making and breaking process and reformed the junction-structure after obtaining each I-V curve. This I-V measurement-scheme was performed though a signal access module III (Bruker, Santa Barbara, CA) using an external piezo driver (E-663 LVPZT-Amplifier, Physik Instrumente) and a data-acquisition-device with LabVIEW2016 (NI PXI-4461, National Instruments). Again it should be noted here that, for each cycle of the I-V measurements, single I-V curve of the molecular junctions with a


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variety of stretch lengths was recorded (Figure 1c). To analyze I-V response to the variety of the stretch lengths, more than 1000 I-V measurements were cycled for each molecule. Theory. Electronic calculation of the single-molecule junctions were performed using cluster models where each side of the junction consists of 20 Au atoms. In the geometrical optimization, the two outermost Au layers of the left and right electrodes were fixed and the other atomic positions were allowed to relax. We used the Perdew–Burke–Ernzerhof (PBE) XC functional.

34

The electronic wave functions were expanded in a double-numeric

polarized basis set with a real-space cutoff of 0.4 nm using Dmol3 code.35 To simulate the stretching process of the molecular junctions, the Au electrodes (the clusters of 20 Au atoms) were separated stepwise (in steps of 0.02~0.05 nm) and the junction geometry was relaxed at every step.

RESULTS AND DISCUSSION Conductance measurement and analysis. Prior to the investigation on the fluctuation in the interface and electronic structures based on the I-V analysis, the standard STM-based break junction technique

9

was used to determine the single-molecule conductance and to see

molecular dependent fluctuation of the single-molecule conductance during the stretching process of the junctions of DAB, PY, BPY, and C60 (Figure 1b). DAB binds to the Au electrodes and forms (Nsp3H2)-Au point interfaces,28 while PY and BPY forms (Nsp2)-Au point interfaces. Here, Nsp3 and Nsp2 represent the hybridizations of the atomic orbitals of the N atoms. The (Nsp3H2)-Au point interface is consist of a σ-type bonding, while the (Nsp2)-Au point interface has both σ- and π-type bondings30 and therefore additional effect of the π−electron (π-binding) to the interface formation appears in PY and BPY. From a simple chemistry viewpoint, a lone pair of the Nsp3 atom in DAB forms the σ-type Nsp3-Au bond,


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while a lone pair of the Nsp2 atom in PY and BPY participates the σ-type bond and additionally, a π-electron of the Nsp2 atom, that is in resonance with the delocalized electrons in the aromatic pyridine ring, can form the π-type bonding. In contrast to the point-like interfaces of DAB, PY, and BPY, C60 forms plane-like interface by direct binding between the π-plane of C60 and the Au electrode. Figure 2a-d shows two dimensional (2D) histograms of the conductance traces

36

of the single-molecule junctions, in which electronic current of

the single-molecule junctions are repeatedly measured during the stretching process of the junctions by applying the fixed bias voltage between the Au sample electrodes and the AuSTM tip. In the 2D histograms, the stretching distance where the conductance drops blew 0.7 G0 was set to the distance = 0, and the conductance traces were overlapped each other. Therefore, the statistical distribution in the 2D histograms indicates probable length and conductance of the single-molecule junctions during the stretching process. In the 2D histograms, DAB is characterized by a single distribution, GDAB (Figure 2a), while two distinct distributions are apparent for PY, BPY, and C60, which are denoted by GPY_H and GPY_L (Figure 2b), GBPY_H and GBPY_L (Figure 2c), and GC60_H and GC60_L (Figure 2d). The two distinct distributions are due to the formation of two preferential metal-molecule interface structures as discussed in the following section. Length analysis for each of the molecular junction indicated that the stretch length was molecular dependent and the average stretch lengths were 0.13, 0.10, 0.24, and 0.38 nm for DAB, PY, BPY, and C60, respectively (Supporting Information 1). To determine most probable conductance values, conductance histograms were constructed from the same data set used for the 2D histograms (Figure 2a-d) by getting rid of the distance information, which are shown in Figure 2e. Each molecule features single-molecule conductance peaks in the histograms and the single-molecule conductance was determined to be (GBDA = 0.8 mG0), (GPY_H = 1.0 mG0, GPY_L = 0.3 mG0), (GBPY_H = 0.6 mG0, GBPY_L = 0.2 mG0), (GC60_H = 32.0 mG0, GC60_L = 3.0 mG0), which are


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listed in Table 1. The obtained most probable single-molecule continuance values are in agreement with those in literature for GBDA (1.5 mG0,16 and 1.0 mG028), GBPY_H (0.6 mG0),5 GBPY_L (0.2 mG0),5 GC60_H (33 mG0),37 and GC60_L (3 mG037 and 6 mG033). It should be noted here that additional standard break junction experiment was performed for a series of the single-molecule junctions with the pyridine anchoring group including 4-(4-pyridin-4ylphenyl)pyridine and the observed length dependence of the single-molecule conductance was in agreement with those in literature, which assess applicability of our measurement method (Supporting Information 1). The closer examination of the molecular dependent conductance distribution in the 2D histograms (Figure 2a-d) indicates that the molecular conductance decays along with increasing the mechanical stretching distance and the slope of the conductance versus distance distribution becomes steeper in the order of DAB, PY~BPY, and C60. The averaged slope of ∆[Log(conductance/G0)]/ ∆(distance/nm) is listed in Table 1 (For a detail, see Supporting Information 1). A detailed discussion is described in the following section.

I-V analysis and fluctuation in the electronic structures during the stretching process of the single-molecule junctions. The single-molecule conductance analysis revealed molecular dependent conductance values and fluctuation. Next, we investigated molecular dependent fluctuation in the electronic structures of the single-molecule junctions based on the I-V analysis. Within the Landauer-Büttiker formalism,

21,22

current through the single-channel single-

molecule junctions can be represented by +∞

2e eV eV   I (V ) = dE ⋅ τ ( E )  f ( E − ) − f ( E + ) , ∫ h −∞ 2 2  

τ (E) =

4 ΓL ΓR

( Γ L + Γ R ) 2 + (E − ε )

2

equation (1)

equation (2)

,


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where τ and ε are the transmission probability and the energy of the conduction channel, and ΓL (ΓR) is the electronic coupling energy between the molecule and the left (right) electrode. Here, we set the Fermi level, EF, to zero. Because tunneling behavior is reasonably insensitive to temperature, it is convenient to work in the limit of zero temperature, where the Fermi functions become step functions. Then, the current through the single-molecule junction is written by

I (V ) =

 8e αeV − ε   (1 − α )eV + ε   + tan −1  α (1 − α )Γ tan −1    , h Γ  Γ    

equation (3)

where Γ and α are defined as Γ = ΓL + ΓR, α = ΓL/Γ. Note that the temperature effect of the Fermi-Dirac distribution is several percent of I(V) at 300 K.

19

By fitting experimental I-V

curves of the single-molecule junctions with various stretching distances by the equation (3), the essential parameters of Γ and ε needed to understand the electronic structure can be obtained. We found that the parameter α was almost 0.5 (i.e., ΓL = ΓR) for all the singlemolecule junctions studied here (Figure S4) and therefore, analysis and discussion on the metal-molecule coupling is based the Γ (= ΓL + ΓR) value in the following sections. As described in the experimental section, the single-molecule junction was stretched by as large as 10 nm until the junction was broken, during which an I-V of the singlemolecule junction was recorded. We repeatedly collected the I-V until statistically significant data set was obtained. Figure 3a-d shows 2D mapping of the I-Vs for the single-molecule junctions of (a) DAB, (b) PY, (c) BPY, (d) C60. The I-V mappings were constructed by more than 1000 data measured at the various stretching distances. The 2D I-V mapping of DAB is characterized by one band (Figure 3a), while two distinct bands are visible for PY, BPY, and C60 in the 2D I-V mappings (Figure 3b-d). The most probable low bias conductance at 10~100 mV in the I-Vs was calculated for DAB, PY, BPY, and C60 as listed in Table 1, which is in reasonable agreement with the results obtained from the standard break junction


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experiments (Figure 2e). Detailed description of the I-V analysis is available in the Supporting Information 2. At the high bias voltage, nonlinearity of the I-Vs are clearly visible for C60 (Figure 3d), which reflects the arctangent function (equation (3)) and can be intuitively understood as a result of a closer lying energy-level of the conduction orbital of C60 and resultant resonance in energy between the conduction electron and the molecular orbital at the lower bias region than those for DAB, PY, and BPY. Figure 3e-h shows molecular dependent 2D histograms of the Γ and ε values obtained from the fitting of the I-Vs using the equation (3). Compared with the results of the I-V mapping (Figure 3a-d), the molecular dependent distribution is much more apparent in the 2D (ε-Γ) histograms (Figure 3e-h). The 2D histogram of DAB consists of a single distribution, while we can identify nodal main distribution where two distributions vertically aligned each other for PY and BPY (see circles in Figure 3). Well-separated two main distributions are apparent for C60. The corresponding statistically probable values of (ε, Γ) are listed in Table 1 for DAB, PY, BPY, and C60. Closer examination of the 2D (ε-Γ) histograms revealed remarkable differences in molecular dependent distribution and fluctuation in the histograms. The two main distributions of C60 reflect negative correlation between ε and Γ, (Figure 3h), which is in sharp contrast to the case of BPY (Figure 3g), in which the increase in the ε values leads to the increase in Γ values (i.e., positive correlation between ε and Γ). A detailed discussion is described in the following section.

Structural and electronic simulation during the stretching process of the single-molecule junctions. The analysis in the I-Vs allowed us to study the molecular dependent distribution and fluctuation in the electronic structures with a variety of the stretching distances of the molecular junctions. Next, to identify the origin of the molecular dependent fluctuation in the electronic and interface structures, we performed computational simulation, in which the


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electronic structures were calculated by stretching the single-molecule junctions in a stepwise fashion (Supporting Information 3). Based on the total energy calculation as a function of the stretching distance (Figure S8), metastable structures were identified as shown in the structural models (see the insets in Figure 4a-d and Figure S10). While DAB showed one energetically stable structure, two metastable structures could be found for PY, BPY, and C60. The two metastable structures of PY and BPY are originated from preferential molecular orientations where the aromatic ring(s) adopts tilted- and upright- orientations with respect to the charge transport direction at the small and large stretching distances, respectively. The two preferential configurations of the pyridine-Au interface and their electronic conductance has been reported in the combined experimental and theoretical study by Louie, Hybertsen, Neaton, and Venkatraman coworkers for BPY.30 The simulated result of C60 is consistent with that reported by Cuevas and co-workers.38 In their DFT-based transport study, electronic calculations were performed during stretching process of a junction consisting a C60 molecule sandwiched by two Au clusters. Following their results, we found two metastable structures where the C60 molecule located the proximity of the side surface of the Au clusters at the small stretching distance and, at the large stretching distance, the C60 molecule places itself in the middle of the junction, adopting a geometry in which the top gold atom is bound to a single C atom.

38

It

should be note here that there have been combined theoretical and experimental STM studies on single C60 molecules on metal electrodes,

39-41

in which single-molecule C60 junctions

were formed by gently contacting a STM tip to individual C60 molecules. Formation and evolution of such molecular junctions prepared in the compressing process is essentially different from those prepared in the stretching and breaking process of the junctions studied here. Therefore we limited our discussion to that about the break junctions.


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Comparison between theoretical and experimental results and identification of the molecular dependent fluctuation of the electronic and interface structures of the singlemolecule junctions. In the previous sections, the experimental fluctuation of the electronic structures and the theoretical electronic and structural simulation of the single-molecules junctions were investigated. Next, we combined the experimental and theoretical findings to understand the origin of the observed molecular dependent fluctuation in the electronic structures and the relevant fluctuation of the interface structures. In line with this purpose, we analyzed the (ε-Γ) distribution obtained from the I-V analysis. As described in the first section (Figure 2 and Figure S1), the molecular conductance (i.e., logG) indicated clear linear dependence on the stretch distance for the molecular junctions studied in this study, therefore, the ε and Γ values are plotted against logG (Note that G = (2e2/h) ×τ, here τ is the transmission probability. See also equation (2)). We found that the Γ values are almost linearly dependent on the logG values (i.e., the stretch distance) for all the molecular junctions (Figure S12). This result indicates that the electronic coupling between the molecule and the electrodes decreases as the junctions are stretched although the degree or extent of the Γ-decay is molecular dependent. In contrast to the dependence of Γ on logG (i.e., the stretch distance), the ε value featured non-monotonic dependence on logG (i.e., the stretch distance) as shown in Figure 4e-h. DAB is characterized by a single homogeneous distribution (Figure 4e), while we can identify characteristic shapes for the distributions of PY, BPY, and C60 in the 2D histograms (Figure 4f-h). The characteristic shapes in the (εlogG) histograms reflect the molecular dependent fluctuation in the conduction orbital energy-level (ε) against the stretching distance. As shown in Figure 4g, the distribution of BPY displays non-linear dependence where ε decreases quickly at the small distance region(i) (i.e., at the large logG region), while ε stays constant at the large distance region(ii). We consider that the distribution of PY shows similar to that of BPY, but due to the small


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molecular size of PY, the two regions of (i) and (ii) found in BPY are overlapped each other for PY and the non-linear dependence can be weakened in the histogram (Figure 4f). Interestingly, the distribution of C60 features mirror symmetry of the BPY-distribution across the X-axis. At the small distance region(i), ε increases rapidly and reached to stationary state at the large distance region(ii) (Figure 4h). To compare the experimental results and the DFT simulation, the calculated energy level of the conduction orbitals (LUMO (lowest unoccupied molecular orbital ) or HOMO (highest occupied molecular orbital)) were plotted against the LogG as show in Figure 4a-d (see also Supporting Information 1 and 3). The point like distribution of DAB with the small energy dispersion originates from the point like σ-type interface because the σ-type bonding is directional and has small structural degree of freedom, which results in small variation in ε against the mechanical and structural perturbation (Figure 4a). Venkataraman and co-workers demonstrated DFT simulation of the formation and evolution of single DAB junctions during the junction stretching process.

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Calculated transmission probability was 1~3 mG0 and was

almost independent on the stretch length. This theoretical result in agreement with ours, in which the small variations in LUMO energy against the stretch perturbation is attributable to the origin of the almost constant transmission regardless the stretch perturbation. The clear (weak) nonlinear distribution of BPY (PY) with the wide energy range is characteristics of the point like π-type interface. In contrast to the σ-type binding, the π-type binding has large structural degree of freedom. Depending on the structural variation in the rotational angle of the pz-orbital with respect to the Au electrode surface, the Npz-Au interaction and ε can be modulated (Figure 4b,c). The clear nonlinear energy distribution of C60 is characteristics of the plane like π-type interface with the large structural degree of freedom (Figure 4d). The all simulated data (Figure 4a-d) of DAB, PY, BPY, and C60


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qualitatively matches with the experimental ε-logG (i.e., ε-Distance) distributions in the histograms (Figure 4e-h). Next, we focus on the clear distance (logG) dependence of the frontier molecular orbital (MO) levels of BPY and C60, which were confirmed in both of the experiment (Figure 4g,h) and the simulation (Figure 4c,d). As shown in Figure 4c,g, the MO level (ε) of BPY decreases with increasing the stretch distance (i.e., decreasing logG). On the other hand, as shown in Figure 4d,h, the MO level of C60 increases with increasing the stretch distance (i.e., decreasing logG). At the short distance region (at the large logG region), the BPY molecule has tilted molecular orientation (inset in Figure 4c), and the pz-orbital of the N atom that orients perpendicular to the aromatic ring and also the Au electrode surface can effectively interacts with the Au electrode. On the other hand, at the large distance, the BPY molecule has upright orientation (inset in Figure 4c), and therefore, the pz-orbital orients parallel to the Au electrode surface, which results in the decreased in the Npz-Au interaction. The down shift of the ε value with decreasing the N-Au interaction can be intuitively understood by considering the energy level diagram during mixing of the orbitals between the metal (Au) and the molecule. In the distance dependence of the energy levels of the bonding and antibonding orbitals, the unoccupied antibonding-level (i.e., LUMO level) downshifts with increasing the distance and decreasing the interaction between the two orbitals. As discussed above, the point-like π-type interface causes fluctuation in the MO level (ε) under the stretching perturbation. At the large distance, instead of the Npz-Au interaction, Nlonepair-Au interaction contributes the interface formation. The lone pair of the N atom is orthogonal to the pz-orbital and forms σ-type Nlonepair-Au bonding, which is directional and has small structural degree of freedom, which results in small variation and fluctuation in ε against the mechanical and structural perturbation. The interplay between the π- and σ- type bonding under the mechanical perturbation is the origin of the nonlinear ε-logG (ε-distance)


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distribution of BPY (PY). Beside the experimental analysis in the electronic transition under the mechanical perturbation, manipulability of the electronic state and the transmission probability of BPY has been successfully demonstrated by Louie, Hybertsen, Neaton, and Venkatraman co-workers.

30

They showed that BPY single-molecule junctions can be

reversibly switched between two conductance states (i.e., GBPY_H and GBPY_H in Figure 2e) using mechanical perturbation of repeated junction elongation and compression. Based on ﬁrst-principles calculations, they attributed the two conductance states to distinct junction structures: conductance is low when the N-Au bonding is perpendicular to the conducting πsystem, and high otherwise. 30 They calculated transition of the transmission probably during the elongation process of the BPY junctions, in which ε as well as Γ becomes smaller along with the junction elongation. This theoretical result in the electronic transition during the junction stretching is in agreement with our experimental finding in the ε-logG (ε-distance) distribution discussed above (Figure 4g). To understand the unique distance (logG) dependence of the MO level of C60 (Figure 4d,h), electronic charge of the C60 molecule in the junction was calculated (Figure S10). The C60 displayed substantial charge transfer (i.e, back-donation) during the junction stretching process, which is in contrast to BPY. The back-donation can cause a shifting of the LUMO resonance upward and away from EF. At the small distance region where the C60 molecule locates on the side surface of the Au electrode and forms plane-like interface (Figure 4d), the formation of the plane-like interface results in the substantial charge transfer and the distance dependence of the back-donation (Figure S12) causes the upward shift of the LUMO level along with the stretching perturbation, which is visible in the experimental ε-logG (εdistance) distribution (Figure 4h). At the large distance region (ii), the C60 molecule places itself on the top Au atom and forms a point-like σ-type interface, which results in small variation and fluctuation in ε against the stretching perturbation. The transition from plane-


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like interface to the point-like interface is the origin of the unique nonlinear ε-logG (εdistance) distribution of C60. Finally, we comment on the intuitive explanation of the observed distance dependence of the MO levels of BPY and C60 in terms of the molecular shapes. The BPY molecule has a long, narrow, and rigid plate-like shape. Therefore, BPY is able to tilt its molecular axis and leans against the side wall of the Au electrodes during the junction stretching process. This molecular response results in the gradual decrease in the interaction between two orbitals of the metal and the molecule, and resultant downshift of the antibonding orbital. In contrast, the C60 molecule has a rigid spherical shape and can just accommodate into the nanogap in the junction during the gap opening and the junction stretching process (from the side wall adsorption to the on-top adsorption (see insets in Fig. 4d)). This response leads to increase in the interaction between two orbitals of the metal and the molecule, which is in contrast to the case of BPY. These molecular shape dependent modulation and fluctuation of the interaction against the mechanical perturbation can explained the observed opposite MO shifts of BPY and C60.

CONCLUSIONS In conclusion, on the basis of the combined break junction experiments and ab-initio simulation of the single-molecule junction under mechanical perturbation, the electronic and structural fluctuation in the junctions was investigated for the molecules with the interface of the σ, π, and plane- type interface. This approach provides new insight into the structural and electronic fluctuation in the molecular junction on the single-molecule scale that had been inaccessible by the standard molecular conductance measurement and the statistical


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conductance analysis, and puts forward the manipulability of the transport on the singlemolecule scale.

ASSOCIATED CONTENT Supporting information The Supporting Information is available free of charge on the ACS Publications website at DOI: XXXXX. Conductance histograms of the single-molecule junctions, analysis of the I-Vs of the singlemolecule junctions, DFT simulation of the single-molecule junctions, and 2D histograms of Γ values against logarithmic conductance (LogG)

AUTHOR INFORMATION Corresponding authors E-mail: [email protected] (SF), [email protected] (MK) Notes The authors declare no competing financial interest.

ACKNOWLEDGEMTNTS This work was financially supported by Grants-in-Aid for Scientific Research in Innovative Areas (Nos. 26102013) and Grants-in-Aid for Scientific Research (B) (No. 21340074), (C) (No. 17K04971) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan, and Tokuyama, Kato, Precise measurement technology, Hitachi metals, and Yazaki memorial foundation.


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Figures

Figure 1. (a) Schematic illustration of the electronic structure of a single molecule junction where ε and Γ are molecular orbital energy level and metal-molecule electronic coupling (b) Chemical structure of molecules; 1,4-butanediamine (DAB), pyrazine (PY), 4,4’-bipyridine (BPY), and fullerene (C60). (c) Schematic illustration of the experimental setup where I-Vs of single-molecule junctions with different stretch lengths (L1, L2, and L3) are measured.


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Figure 2. (a-d) 2D conductance histograms of the conductance trace at the bias voltage of 100 mV for (a) DAB, (b) PY, (c) BPY, and (d) C60. The histograms (a-d) are, respectively constructed from 4012, 4150, 4109, and 2104 of conductance traces. The main distributions are indicated by arrows (GDAB for DAB, GPY_H and GPY_L for PY, GBPY_H and GBPY_L for BPY, and GC60_H and GC60_L for C60). Bin size of ∆log(G/G0) is 0.01. (e-f) Conductance histograms of the single-molecule junctions. (e) DAB, (f) PY, (g) BPY, (h) C60. The peak positions are indicated by arrows. A linear X-bin-size of 0.003 nm and a logarithmic Y-bin-size (∆log(G/G0)) of 0.02 were used.


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Figure 3. (a-d) 2D mapping of the I-V curves of the single-molecule junctions of (a) DAB, (b) PY, (c) BPY, and (d) C60. The histograms of (a)-(d) are respectively constructed from 2184, 5456, 3902, and 1306 I-Vs. X-bin-sizes are 1, 0.8, 0.8, and 10 nA for DAB, PY, BPY, and C60. The same Y-bin-size of 0.01 V was used for all the system. The arrows indicate averaged I-Vs (Supporting information 2). (e-h) 2D histograms of the I-V fitting results of ε and Γ for (e) DAB, (f) PY, (g) BPY, and (h) C60. Dotted circles are guide for eyes and indicate the preferential distributions. X(Y)-bin-sizes 60(3), 60(1), 50(1), and 7(5) meV were used for DAB, PY, BPY, and C60.


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Figure 4. (a-d) Calculated energy levels of the conduction orbitals (HOMO or LUMO) as a function of the logarithmic conductance. The insets indicate metastable junction-structures. The gray, while, blue, and yellow balls correspond to C, H, N, and Au atoms respectively. (ef) 2D histograms of ε values against logarithmic conductance (LogG). The decrease in the logG value corresponds to the increase in the stretching distance because logG is proportional to the stretching distance (For details, see main text and Supporting Information 1 and 3).


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Conductance/mG0

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Slope/(LogG0/nm)

GIV/mG0

Γ/meV

ε/eV

DAB

GDAB

0.8(0.08)

–0.7

1.0(0.4)

35(13)

1.3(0.3)

PY

GPY_H

1(0.2)

–1.0

0.6(0.2)

20(10)

0.96(0.2)

GPY_L

0.32(0.07)

–1.5

0.2(0.1)

15(5)

0.96(0.2)

GBPY_H

0.63(0.20)

–2.2

0.5(0.2)

20(5)

0.90(0.2)

GBPY_L

0.20(0.04)

–1.9

0.2(0.1)

9(3)

0.75(0.06)

GC60_H

32(21)

–4.6

50(2)

100(20)

0.55(0.02)

GC60_L

3.2(0.2)

–3.0

5(2)

24(10)

0.60(0.03)

BPY

C60

Table 1. List of conductance, slope, GIV, Γ, and ε0 values. The standard deviation is shown in parentheses. The slope is averaged slope of the conductance trances and GIV is the most probable conductance at the low bias region of 10~100 mV in the I-Vs (Figure 3a-d).


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Fluctuation in Interface and Electronic Structure of Single-Molecule

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