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[Masato Takei](https://orcid.org/0000-0002-3522-0033), Akira Takatsuki, [Katsunori Wakabayashi](https://orcid.org/0000-0002-9147-9939), [Satoshi Kaneko](https://orcid.org/0000-0002-0351-6681), [Hiroshi Suga](https://orcid.org/0000-0003-4333-4898), [Yasuhisa Naitoh](https://orcid.org/0000-0003-2431-9625), [Kazuhito Tsukagoshi](https://orcid.org/0000-0001-9710-2692)

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This document is the unedited Author’s version of a Submitted Work that was subsequently accepted for publication in ACS Applied Materials & Interfaces, copyright ©  2025 American Chemical Society after peer review. To access the final edited and published work see https://doi.org/10.1021/acsami.5c14847.[In Copyright](http://rightsstatements.org/vocab/InC/1.0/)

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[Conductance Switching in an Asymmetric Single-Molecule Junction](https://mdr.nims.go.jp/datasets/0dcbb554-a163-44af-848a-f0a4df538503)

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1  Conductance Switching in an Asymmetric Single-molecule 1 Junction 2  3 Masato Takei,a,b Akira Takatsuki,a,b Katsunori Wakabayashi,c Satoshi Kaneko,d 4 Hiroshi Suga,*a,b Yasuhisa Naitoh**,e and Kazuhito Tsukagoshi***,b,a 5  6 a Chiba Institute of Technology, Tsudanuma, Narashino, Chiba 275-0016, Japan 7 b International Center for Materials Nanoarchitectonics (WPI-MANA), National Institute 8 for Materials Science (NIMS), Tsukuba, Ibaraki 305-0044, Japan 9 c Department of Nanotechnology for Sustainable Energy, School of Science and 10 Technology, Kwansei Gakuin University, Sanda, Hyogo 669-1330, Japan  11 d Department of Materials Science and Engineering, School of Materials and Chemical 12 Technology, Institute of Science Tokyo, Tokyo 152-8550, Japan 13 e Core Electronics Research Institute, National Institute of Advanced Industrial Science and 14 Technology (AIST), Tsukuba, Ibaraki 305-8565, Japan 15 Keywords: fullerene, C60 pyrrolidine tris-acid, single-molecule device, metal migration, 16 single-level transport model, transition voltage spectroscopy 17 18 2  ABSTRACT 19 Electric switching in a single-molecule junction based on the asymmetric C60 pyrrolidine 20 tris-acid (CPTA) molecule was demonstrated using nanogap electrodes spin-coated with a 21 CPTA thin film. Among the embedded molecules, those closest to the cathode were 22 preferentially activated during two-terminal conductance measurements, exhibiting 23 reproducible bistable switching between low- and high-conductance states at room 24 temperature. The CPTA molecule was anchored to the cathode via a carboxyl ligand, while 25 the opposing electrode was positioned to allow modulation of the molecule–electrode 26 distance by an applied bias voltage. This configuration enabled two distinct transport 27 regimes: metal–fullerene conduction in the high-conductance state and through-space 28 tunneling across a metal–CPTA–vacuum–metal junction in the low-conductance state. 29 Analysis of the high-conductance state using the single-level tunneling transport model 30 confirmed that charge transport occurred through a single molecule, despite the film-based 31 fabrication. In the low-conductance state, transition voltage spectroscopy revealed that the 32 junction asymmetry parameter was strongly dependent on the through-space distance 33 between the fullerene cage and the opposing electrode, offering insight into structural 34 modulation during switching.  35   36 3  ■ INTRODUCTION 37 The continuous drive for miniaturization in electronic devices has spurred extensive 38 research into molecular-scale components.1–15 Among various miniaturization strategies, 39 single-molecule junctions represent the ultimate limit in size for switching elements.2,16–22 40 However, constructing such junctions remains technically demanding, primarily due to the 41 challenges in fabricating nanometer-scale electrode gaps and positioning individual 42 molecules within these gaps with atomic precision. Traditional studies on single-molecule 43 junctions have typically required the fabrication of numerous devices, followed by individual 44 electrical characterization to identify suitable candidates for detailed investigation. 45 Mechanically controllable break junction (MCBJ) and scanning tunneling microscopy 46 break junction (STMBJ) techniques are widely employed to fabricate single-molecule 47 junctions.19,21,23–26 A key advantage of these methods is their tunable electrode gap, which 48 facilitates molecular connection. However, both approaches face significant technical 49 challenges in integrating large numbers of single-molecule junctions as functional electrical 50 switching elements. The MCBJ method, which utilizes piezoelectric actuators or substrate 51 bending, requires bulky equipment and is ill suited for parallel operation of multiple devices. 52 Similarly, STMBJ relies on atomic-scale probe manipulation, making it impractical for large-53 scale fabrication. Despite these limitations, both techniques have contributed significantly to 54 the fundamental demonstration of molecular electronic devices.23,27–29 55 Efficient incorporation of molecules into metallic junctions within solid-state devices is 56 critical for future molecular electronics applications.1,30 Conventional static nanogap 57 electrodes fabricated on Si substrates suffer from fixed electrode spacing, leading to low 58 molecular insertion efficiency and hindering further development. While the integration of 59 billions of molecular switches still remains a challenge, we have developed an alternative 60 strategy based on molecular self-assembly combined with nanogap tunability to enable 61 reliable switching operation. 62 In this study, C60 pyrrolidine tris-acid (CPTA) molecules were incorporated into nanogap 63 electrodes via spin-coating of a CPTA solution. The carboxyl ligands of CPTA served to 64 anchor the molecule to the cathode. The electrical connection between the C60 cage and the 65 opposing electrode was modulated through applied bias voltage. Cyclic voltage sweeps 66 induced repeatable, bistable conductance switching between high-conductance state (HCS) 67 and low-conductance state (LCS). The assembled molecular device exhibited stable 68 4  switching over more than 100 cycles, with an on/off ratio exceeding 100. The HCS, 69 corresponding to a metal–CPTA–metal junction, was analyzed using the single-level 70 tunneling transport (SLTT) model. This revealed that electron tunneling was governed by an 71 energy offset (ε0) of 0.82 eV between the metal Fermi level and the molecular conduction 72 orbital. The LCS, attributed to through-space tunneling across a metal–CPTA–vacuum–metal 73 junction, was evaluated using transition voltage spectroscopy (TVS).31–36 The ratio of the 74 transition voltage (Vtrans) to ε0 in the Fowler–Nordheim (FN) plot reflected the junction’s 75 overall asymmetry, allowing electrical monitoring of structural changes.37 Variations in the 76 asymmetry parameter (η) indicated an increased transport distance between the molecule and 77 the anode in the LCS, consistent with metal atom migration at the anode. Moreover, the 78 continuous modulation of η demonstrated the device’s capability for multistate conductance 79 switching. 80 ■ EXPERIMENTAL 81 The molecular switch was fabricated by combining electron-beam lithography, controlled 82 electrical metal migration, and spin-coating of a CPTA molecular solution (Figure 1a). To 83 ensure uniform coating of the CPTA film, a 2-nm-thick AlOx layer was deposited on a Si 84 substrate with a 250-nm thermal oxide layer via atomic layer deposition. Subsequently, a 85 200-nm-wide metal wire was patterned using electron-beam lithography. A 10-nm-thick 86 AuPd (8:2) alloy was then deposited to form the metal wire on the substrate. 87 Electromigration was employed to create nanoscale electrodes separated by a nanogap, 88 enabling through-space electron tunneling. A continuous metal wire was intentionally broken 89 by applying a high-density current under feedback-regulated voltage control.38–40 The initial 90 application of bias voltage under vacuum allowed electron transport through the nanogap, 91 with the tunneling current dependent on both the gap length and the electrode surface work 92 function. Fitting the resulting current–voltage (I–V) characteristics using the Simmons model 93 estimated the gap length to be approximately 0.7 nm or slightly larger.41 It is well established 94 that as a result of electromigration, the anode side of the nanogap tends to undergo more 95 pronounced atomic migration, while the cathode side remains relatively stable.42–45 96 5   97  98 Figure 1. (a) Schematic of the CPTA junction. (b) Single-molecule switch model based on 99 electromigration and field migration. (c) Observed conductance switching between two 100 states: high-conductance state (HCS) and low-conductance state (LCS). Each point 101 represents an average over 10 cycles. 102  103 6  A CPTA film approximately 2 nm thick was spin-coated from a dimethylformamide 104 solution, uniformly covering both the electrodes and the insulating surface (Figure 1a). CPTA 105 was chosen as the switching element due to its structural asymmetry and site-selective 106 bonding characteristics. The carboxyl ligands of CPTA can form stable bonds with oxygen-107 deficient surface sites (Figure 1b).46 This stability is likely facilitated by oxidation of surface 108 Pd atoms, enabling robust anchoring of CPTA molecules. Among the CPTA molecules 109 present within the nanogap, the one closest to the electrodes typically dominates electrical 110 conduction when a bias voltage is applied. Upon initial conduction, the current increases 111 nonlinearly and reaches a relatively high level. Subsequently, due to electromigration, the 112 anode surface tends to retract from the CPTA molecule—especially when the molecule is 113 anchored to the cathode.4,43 This displacement creates a through-space gap, thereby 114 suppressing the tunneling current. 115 Once the through-space gap forms, applying a bias voltage from zero can induce surface 116 ionization under a strong electric field, triggering a process known as field migration.4,43 This 117 drives the metal edge to shift back toward the CPTA molecule (Supporting Information S1), 118 re-establishing conduction through the molecular junction. The through-space separation in 119 the junction can thus be externally modulated by the applied bias voltage, enabling cyclic 120 switching behavior (Figure 1c, Supporting Information S2). Such reversible shifts of the 121 anode surface—governed by the interplay of electromigration and field migration—are also 122 observed in scanning tunneling microscope systems.43 123 The LCS, characterized by an open through-space gap, is defined as the OFF state, while 124 the HCS, where the gap is closed, corresponds to the ON state (Figure 1c). Transitioning 125 from the OFF (LCS) to the ON (HCS) state is achieved by applying a small bias voltage, 126 which induces a nonlinear increase in current and promotes field migration, thereby closing 127 the through-space. When a higher voltage is applied, an overcurrent can occur, resulting in 128 the appearance of negative differential resistance (NDR) in the I–V curve. This overcurrent 129 triggers the reopening of the through-space, switching the junction back to the OFF state 130 (LCS).  131 7  Conversely, when the CPTA molecule is anchored to the anode surface, the initial current 132 induces electromigration, potentially causing the anode metal to retract and create a through-133 space gap. Once this gap forms, the CPTA molecule—now positioned on the anode side—134 no longer contributes effectively to tunneling conduction due to its unfavorable spatial 135 configuration, despite lowering the local work function compared to the bare metal 136 surface.47–49 As a result, the overall current through the junction is significantly reduced. 137 In this model, only approximately half of the CPTA molecular junctions are statistically 138 expected to participate in cyclic switching, due to the asymmetry in electrode anchoring. 139 However, in practical devices, conduction can be taken over by the CPTA molecule 140 positioned at the second-shortest electrode distance if the primary molecule (at the first-141 shortest electrode distance) is deactivated on the anode side. If this second molecule is also 142 anchored to the anode, it too becomes inactive, allowing a cathode-anchored molecule to 143 potentially assume the switching role.  144 To support this proposed switching mechanism, two key factors must be considered: (1) 145 the involvement of single-molecule transport, and (2) the activation and switching 146 functionality of asymmetrically anchored CPTA molecules on the cathode electrode. 147 To confirm single-molecule transport, the metal–CPTA–metal junction was characterized 148 in the HCS. Under HCS conditions, the nanogap length was systematically varied 149 (Supporting Information S1), and junction conductance was recorded to construct a 150 conductance histogram (Figure 2). The occurrence frequency (φ) of the metal–CPTA–metal 151 junction and that of a reference nanogap junction without CPTA were plotted as a function 152 of junction conductance over 2000 switching cycles. Both junction types exhibited a peak in 153 occurrence frequency; however, their most probable conductance values differed 154 significantly. The reference junction without CPTA showed higher conductance, indicating 155 that the presence of CPTA increases the overall junction resistance (Figure 2). The differential 156 conductance characteristics between the CPTA and reference junctions were extracted (inset, 157 Figure 2), confirming that the insertion of the resistive CPTA molecule reduces overall 158 conductance—supporting the conclusion that single-molecule transport is responsible for the 159 observed behavior.  160 8   161 Figure 2. Conductance histogram obtained in the junction with and without CPTA. Detailed 162 procedures are mentioned in Supporting Information S1. The vertical axis (𝜑) represents the 163 occurrence frequency. Inset: Difference histogram of occurrence of the junctions with and 164 without CPTA. The occurrence frequency of the CPTA junction exhibited a peak at −2.3 G/G0. 165  166 A standard analytical approach based on the SLTT model was employed to evaluate the 167 molecular junction, assuming electron transport occurs via a specific molecular energy 168 level.29,42 The SLTT model describes the current I(V) through the junction as 169 𝐼(𝑉) =8𝑒ℎΓ1Γ2Γ1+Γ2{tan−1⁡ (Γ2Γ1+Γ2𝑒𝑉−𝜀0Γ1+Γ2) + tan−1⁡ (Γ1Γ1+Γ2𝑒𝑉+𝜀0Γ1+Γ2)},  (1) 170 where ε0  is the energy offset between the Fermi level of the electrodes and the molecular 171 conducting orbital, and Γ1 and Γ2 are the coupling strengths of the molecule to the left and 172 right electrodes, respectively (Figure 3a). The value of ε0 was estimated from conductance 173 measurements obtained under varying nanogap conditions across 2000 switching cycles 174 (Supporting Information S1). The total coupling strength Γ = Γ1 + Γ2 was extracted from the 175 corresponding current values in each cycle. The extracted values of ε0 and Γ are plotted in 176 Figure 3b, showing consistent and reproducible overlap across all measurements. Statistical 177 analysis yielded average values of ε0 and Γ are 0.82 ± 0.024 eV and 0.08 ± 0.004. Notably, 178 the extracted ε0 is comparable to that reported for single C60 molecular junctions (ε0 ≈ 0.60 179 eV), further supporting the conclusion that charge transport occurs through a single CPTA 180 molecule.29 181   182 9   183 Figure 3. (a) Schematic of the resonant tunneling model mediated by molecular orbitals. In 184 the SLTT model, the tunneling process is described using the coupling Γ and ε0 between the 185 conducting orbital and Fermi level. (b) ε0 and Γ values extracted using the SLTT model from 186 I–V characteristics obtained in the range of −1 to +1 V in the HCS. 187  188 Previous studies using MCBJ have reported ε0 ≈ 0.60 eV for single C60 molecular 189 junctions.29 The conducting orbital (lowest unoccupied molecular orbital) of CPTA is known 190 to lie approximately 0.2 eV higher than that of C60.46,50 Additionally, the Fermi level of the 191 AuPd (8:2) alloy used in this study may be slightly higher than that of pure Au, potentially 192 leading to an underestimation of ε0 by up to 0.1 eV.51 Taking into account these factors and 193 the measurement fluctuations, the observed ε0 value is consistent with charge transport 194 through a single CPTA molecule.  195 In the LCS, the junction adopts a metal–CPTA–vacuum–metal configuration, which is 196 inherently structurally asymmetric. The experimental observations in this state were analyzed 197 using the TVS model.31–37 TVS correlates the transition voltage (Vₜᵣₐₙₛ)—identified by a 198 change in the slope of the FN plot—with the energy offset and the asymmetry parameter (η) 199 of the junction, providing insights into structural variations. Replotting the experimental I–V 200 characteristics across junctions with varying conductance in FN coordinates revealed that FN 201 tunneling features were absent at a conductance of 3 mG0, and Vₜᵣₐₙₛ systematically shifted to 202 lower values as conductance decreased. This trend indicates an increasing through-space gap 203 10  in lower-conductance junctions (dashed line, Figure 4a), consistent with a structurally 204 evolving junction geometry in the LCS.  205  206 Figure 4. (a) TVS analysis of the CPTA device. The dashed lines in the figure guide readers 207 to see the transition voltage of FN tunneling. (b) Extracted asymmetry parameter using Vtrans 208 obtained in (a). 209  210 To evaluate the structural dependence of charge transport in the molecular junction, η was 211 extracted using TVS analysis (Figure 4b).36 The asymmetry parameter is defined as 212 𝜂 =𝜀−0.85×𝑉trans2.3×𝑉trans,    (2) 213 where η ranges between −0.5 and 0.5. In a fully asymmetric junction, η approaches ±0.5. A 214 positive η indicates a larger voltage drop on the anode side, while a negative η suggests a 215 larger drop on the cathode side. The value 𝜀  used in Equation (2) was obtained from 216 photoelectron spectroscopy data.46 The extracted trend shows that η decreases from 217 approximately 0.3 to 0 as the transition voltage increases (Figure 4b). This indicates that 218 lower-conductance CPTA junctions correspond to higher structural asymmetry—associated 219 with a larger through-space—while higher-conductance junctions exhibit reduced asymmetry. 220 This structural dependence is also reflected in the corresponding I–V characteristics 221 (Supporting Information S3). These results provide a consistent interpretation of structural 222 asymmetry in the CPTA junction, as inferred from electrical measurements. 223  224 ■CONCLUSION 225 11  The performance and characteristics of single-molecule switching devices based on the 226 structurally asymmetric CPTA molecule were systematically investigated. The CPTA 227 molecular junction exhibited bistable conductance switching, in which conductance was 228 reversibly modulated between LCS and HCS through the formation and closure of through-229 space gaps within the junction. 230 Quantitative analysis using the SLTT model elucidated the roles of molecular energy levels 231 and electrode coupling strengths in determining conductance. Complementary TVS analysis 232 provided insight into the asymmetry parameter associated with each conductance state. 233 Together, these analyses revealed a direct correlation between structural modulation of the 234 molecular junction and its electronic transport behavior. Notably, the results indicated that 235 CPTA molecules anchored to the cathode electrode predominantly governed the switching 236 mechanism. 237 The developed fabrication method—based on spin-coating CPTA onto nanogap 238 electrodes—offers a simple and scalable strategy for integrating switching-capable molecules 239 into nanoscale junctions. The intrinsic asymmetry of the CPTA molecule improves both 240 molecular insertion efficiency and cyclic switching performance, presenting a promising 241 route toward scalable, high-performance molecular electronic devices. 242  243 ■ ASSOCIATED CONTENT 244 Supporting Information 245 Additional experimental details support the experimental result (PDF): 246 Figure S1. Nanogap distance tuning and conductance measurement by varying nanogap 247 distances: electromigration and field migration in an electric field. 248 Figure S2. Voltage sequence for the cyclic operation of binary conductance switching. 249 Figure S3. Current imbalance between the positive and negative bias in the low-conductance 250 state. 251  252  253 ■ AUTHOR INFORMATION 254 Corresponding Authors  255 Hiroshi Suga − Chiba Institute of Technology, Tsudanuma, Narashino, Chiba 275-0016, 256 Japan; orcid.org/ 0000-0003-4333-4898; Email: hiroshi-suga@it-chiba.ac.jp 257 mailto:hiroshi-suga@it-chiba.ac.jp12  Yasuhisa Naitoh − Core Electronics Research Institute, National Institute of Advanced 258 Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8565, Japan; 259 orcid.org/0000-0003-2431-9625; Email: ys-naitou@aist.go.jp 260 Kazuhito Tsukagoshi − International Center for Materials Nanoarchitectonics, National 261 Institute for Materials Science, Tsukuba 305-0044 Ibaraki, Japan; orcid.org/0000-0001-9710-262 2692; Email: TSUKAGOSHI.Kazuhito@nims.go.jp 263  264 Authors  265 Masato Takei − Chiba Institute of Technology, Narashino, Chiba 275-0016, Japan; 266 International Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for 267 Materials Science (NIMS), Tsukuba, Ibaraki 305-0044, Japan; orcid.org/0000-0002-3522-268 0033 269 Akira Takatsuki − Chiba Institute of Technology, Narashino, Chiba 275-0016, Japan; 270 International Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for 271 Materials Science (NIMS), Tsukuba, Ibaraki 305-0044, Japan; orcid.org/0000-0002-6422-272 6899 273 Katsunori Wakabayashi − Department of Nanotechnology for Sustainable Energy, School 274 of Science and Technology, Kwansei Gakuin University, Sanda, Hyogo 669-1330, Japan; 275 orcid.org/0000-0002-9147-9939 276 Satoshi Kaneko − Department of Materials Science and Engineering, School of Materials 277 and Chemical Technology, Institute of Science Tokyo, Tokyo 152-8550, Japan; 278 orcid.org/0000-0002-0351-6681 279  280 Author Contributions 281 M. Takei, A. Takatsuki, K. Tsukagoshi, and H, Suga fabricated and performed electrical 282 measurements. M. Takei, H. Suga. S. Kaneko. K. Wakabayashi, Y. Naitoh, and K. Tsukagoshi 283 analyzed the conduction properties of the CPTA single-molecule junction system. M. Takei, 284 H. Suga, and K. Tsukagoshi conducted the experiments and prepared the manuscript. 285 All authors reviewed and approved the final version of the manuscript. The authors 286 contributed equally to this work. 287  288 ■ ACKNOWLEDGMENTS  289 mailto:TSUKAGOSHI.Kazuhito@nims.go.jp13  The authors thank K. Homma (Institute of Science Tokyo) for the valuable discussions on 290 single-molecule junctions. This work was supported by JSPS KAKENHI Grant Number 291 20K05291, Japan. 292  293  294 ■ REFERENCES 295 (1) Park, H.; Park, J.; Lim, A. K. L.; Anderson, E. H.; Alivisatos, A. P.; McEuen, P. L. 296 Nanomechanical Oscillations in a Single-C60 Transistor. 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