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[Masato Takei](https://orcid.org/0000-0002-3522-0033), Takuma Hirama, [Hiroshi Suga](https://orcid.org/0000-0003-4333-4898), [Katsunori Wakabayashi](https://orcid.org/0000-0002-9147-9939), [Kazuhito Tsukagoshi](https://orcid.org/0000-0001-9710-2692)

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[Effective Conduction Path of a C<sub>60</sub> Chain in a Nanogap Electrode](https://mdr.nims.go.jp/datasets/b413ddff-cd0f-495d-8c38-ee7c248cd7b8)

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Effective Conduction Path of a C60 Chain in a Nanogap ElectrodeEffective Conduction Path of a C60 Chain in a Nanogap ElectrodeMasato Takei, Takuma Hirama, Hiroshi Suga,* Katsunori Wakabayashi, and Kazuhito Tsukagoshi*Cite This: ACS Appl. Electron. Mater. 2024, 6, 1740−1745 Read OnlineACCESS Metrics & More Article Recommendations *sı Supporting InformationABSTRACT: The electrical current path of fullerene-derived films between microfabricatedsharpened metal electrodes (gaps of 10−20 nm) was evaluated. When a high voltage wasapplied, a space-charge-limited current (SCLC) flowed through the fullerene film. The effectiveconduction path length estimated from the SCLC voltage was slightly longer than the shortestdistance between the electrodes. This indicates that the effective current path was tortuousbecause of the amorphous packing of the spherical fullerenes. Correlations between theeffective path length and switching parameters such as operating voltages were used to showthat the minimum operating power required for a minimum-sized fullerene switch is expectedto be in the subpicowatt range.KEYWORDS: fullerene, polymerization, C60 pyrrolidine tris-acid (CPTA), resistance switching, nanogap electrode, fullerene derivative,Poole−Frenkel effect, space-charge-limited current (SCLC)■ INTRODUCTIONA fundamental understanding of the effective electrical currentpath in electronic devices is useful for designing devicestructures and developing advanced functions.1−3 For con-ductive channel materials with a sufficiently large size, currentpaths can be visualized using discharge phenomena caused byhigh-frequency magnetic fields.4 When an electrical bias isapplied, current is induced in the material, and its path isdeformed. The deformed material along the current path canbe observed by scanning electron microscopy (SEM) ortransmission electron microscopy (TEM) imaging.2,5−7 Inaddition, the deformed material trace can be spatially mappedusing Raman spectroscopy.8 For conductive media at thesingle-molecule scale, TEM images provide a direct visual-ization of the conductive channel, such as metallic atoms9bridging the gap between two metallic contacts or a fullerenedimer within the gap.10 Computational image constructionbased on energy spectra is currently a powerful tool toprecisely define the molecular-scale conduction channel.11,12However, when the conduction channel is placed on aninsulating substrate and no clear trace of current flow in thematerial remains, it is extremely difficult to clarify the effectivecurrent path between the two electrodes.13,14 In particular,material with a length of several nanometers exceeds the rangeof computational image construction, even though thephenomena occurring between electrodes can be understood.When current flows between two sharpened electrodes, it iswidely believed that the current is simply injected from the topof the electrode and flows through the medium at the shortestdistance between the two electrodes. This assumption can beapplied to large-scale channels and media with homogeneouselemental distributions. For media with countable particleswithin the conduction space, the current flows through themedia via connections between adjacent particles.In this study, we estimated the current path length byperforming space-charge-limited current (SCLC) character-ization of C60 chains within electrode gaps of different widths.The effective path length evaluated from the SCLC voltage waslonger than that obtained from the SEM images. This isattributed to the tortuous path formed by the amorphouspacking of spherical fullerenes. The characteristics of a verysmall switch element were inferred from the current pathlength, with respect to the switch characteristics observed forthe C60 chains.■ EXPERIMENTAL SECTIONWe investigated the effective current path based on the SCLCconduction principle of fullerene.15−23 This mechanism is reprodu-cibly observed as a current flowing through a fullerene-derived filmbetween electrodes under a high applied electric field.24 Within thelow-voltage regime, the dominant conduction mechanism of nano-scale C60 chains is Poole−Frenkel (PF) conduction, characterized byhopping conduction. When larger voltages are applied, SCLC isobserved.24 An SCLC occurs when the electron injection from theelectrical contact to the conductive media exceeds the equilibriumconcentration of the electrical current flow in the channel, limitingfurther electron injection.Source and drain electrodes with nanogaps between them werefabricated using an electron beam lithography process and electro-migration.25 Gapless wire electrodes were fabricated by using thermalevaporation and lift-off techniques and were subsequently dis-connected by electromigration (Figure 1a). A 10 nm-thick narrowReceived: November 25, 2023Revised: February 7, 2024Accepted: February 7, 2024Published: February 26, 2024Articlepubs.acs.org/acsaelm© 2024 The Authors. Published byAmerican Chemical Society1740https://doi.org/10.1021/acsaelm.3c01656ACS Appl. Electron. Mater. 2024, 6, 1740−1745This article is licensed under CC-BY-NC-ND 4.0Downloaded via NATL INST FOR MATLS SCIENCE (NIMS) on March 28, 2024 at 08:16:48 (UTC).See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.https://pubs.acs.org/action/doSearch?field1=Contrib&text1="Masato+Takei"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Takuma+Hirama"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Hiroshi+Suga"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Katsunori+Wakabayashi"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Kazuhito+Tsukagoshi"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/showCitFormats?doi=10.1021/acsaelm.3c01656&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaelm.3c01656?ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaelm.3c01656?goto=articleMetrics&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaelm.3c01656?goto=recommendations&?ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaelm.3c01656?goto=supporting-info&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaelm.3c01656?fig=tgr1&ref=pdfhttps://pubs.acs.org/toc/aaembp/6/3?ref=pdfhttps://pubs.acs.org/toc/aaembp/6/3?ref=pdfhttps://pubs.acs.org/toc/aaembp/6/3?ref=pdfhttps://pubs.acs.org/toc/aaembp/6/3?ref=pdfpubs.acs.org/acsaelm?ref=pdfhttps://pubs.acs.org?ref=pdfhttps://pubs.acs.org?ref=pdfhttps://doi.org/10.1021/acsaelm.3c01656?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-ashttps://pubs.acs.org/acsaelm?ref=pdfhttps://pubs.acs.org/acsaelm?ref=pdfhttps://acsopenscience.org/researchers/open-access/https://creativecommons.org/licenses/by-nc-nd/4.0/https://creativecommons.org/licenses/by-nc-nd/4.0/https://creativecommons.org/licenses/by-nc-nd/4.0/https://creativecommons.org/licenses/by-nc-nd/4.0/https://creativecommons.org/licenses/by-nc-nd/4.0/AuPd wire (Au:Pd = 8:2) was prepared. Electromigration using thefeedback scheme was then used to gently open the gap, and itsfeedback parameters, such as current limitation, were used to adjustthe width of the gap. A variable gap is useful for characterizing the C60nanochain length dependence. Three lengths of 11, 17, and 20 nm (asmeasured by SEM) were prepared. These three gaps are denoted asthe short (S), medium (M), and long (L) channels. Although the gapwidth was carefully measured from the highly magnified SEM image,typical SEM images show a dim edge of a subnanometer object on athick insulating substrate (Figure S1).26 In addition to the surfaceconditions of the metallic electrode, the actual contact point with thenanoscale spherical fullerene could potentially be selected by usinganother current injection point near the top of the tip. Therefore, it isdifficult to precisely define the actual current path from SEM imagesof the nanoscale electrode.A Si substrate with a 250 nm SiO2 film was coated with a 2 nm filmof AlOx by atomic layer deposition to allow C60 pyrrolidine tris-acid(CPTA) to be applied uniformly on the substrate surface.14 CPTAdispersed in dimethylformamide was deposited by spin coating.27 Thethickness of a typical CPTA layer was 5 nm. The nanogap showed nocurrent during I−V characterization before CPTA coating andshowed reproducible currents after coating (Figure S2).The actual diameter of C60 is 0.7 nm with a space of 0.37 nmbetween the C60 molecules,28 which is similar to the typical layerspacing of graphite. Then, the C60 period was identified asFigure 1. (a) SEM image (50 kV) of an 11 nm nanogap electrode (S channel device) formed by electron beam lithography. (b) Schematic of C60chains bridging the electrode gap. I−V characteristics of (c) set with turn-on and (d) reset with NDR. Inset in panel (d): I−V2 plot to visualizeSCLC characteristics. (e) Sequential resistance changes were observed over 500 cycles of the input for panels (c, d).Figure 2. (a, b) Set and (c, d) reset characteristics in the M channel device. I−V plots for the (a) set and (c) reset operations plotted over 1000measurements. (a) In the set operation, the current transition occurred at 3.4 V as a turn-on point from the LRS to HRS. (b) Turn-on voltage as afunction of the gap width measured by the SEM. (c) Inflection point in the I−V plots at 4.5 V was taken as VSCLC. (d) Average VNDR as a functionof the gap width measured by SEM.ACS Applied Electronic Materials pubs.acs.org/acsaelm Articlehttps://doi.org/10.1021/acsaelm.3c01656ACS Appl. Electron. Mater. 2024, 6, 1740−17451741https://pubs.acs.org/doi/suppl/10.1021/acsaelm.3c01656/suppl_file/el3c01656_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acsaelm.3c01656/suppl_file/el3c01656_si_001.pdfhttps://pubs.acs.org/doi/10.1021/acsaelm.3c01656?fig=fig1&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaelm.3c01656?fig=fig1&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaelm.3c01656?fig=fig1&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaelm.3c01656?fig=fig1&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaelm.3c01656?fig=fig2&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaelm.3c01656?fig=fig2&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaelm.3c01656?fig=fig2&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaelm.3c01656?fig=fig2&ref=pdfpubs.acs.org/acsaelm?ref=pdfhttps://doi.org/10.1021/acsaelm.3c01656?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asapproximately 1.1 nm, indicating that the number of C60 molecules inthe 17 nm gap was 16 or less.In this study, a fullerene derivative of CPTA was used as theconductive channel material. Simple fullerenes such as C60 and C70and variable endohedral fullerenes have been employed as single-molecule29−33 or film13−16,34−45 devices. In comparison with thesefullerenes, CPTA has advantages such as oversolvability in solutionand uniform adhesion on the substrate surface.46 Thus, CPTA hasbeen widely used as an efficient electron-transport layer in perovskitesolar cells.45 Because of the nanoscale short-range path of verticaltransport in solar cells and in-plane transport in the case of ournanogap experiment, hopping conduction due to the PF effect couldoccur to generate a current flow.Measurements (Figure 1b) were performed at room temperatureand a pressure of 5.0 × 10−3 Pa. Electrical conductivity measurementswere performed in a light-shielded chamber to avoid unintendedexposure of the device to light. For the preset switching operation, avoltage was gradually applied between the electrodes by increasing itfrom zero. The current injected into the CPTA film from theelectrode results in hopping conduction23 because of the PF effect.47It should be noted that the CPTA film placed between sharpenedmetal electrodes with a 10−20 nm gap width did not require electronbeam irradiation to preset the initial film conduction, whereas a large-area fullerene film required a preset polymerization process.When a further voltage was applied, the current increasednonlinearly, promoting fullerene polymerization in the film betweenthe two electrodes.48,49 When a larger current is applied, a step-likecurrent reduction in the I−V characteristics occurs and the resistancebetween the electrodes becomes high. This decrease in current istermed negative differential resistance (NDR) and is denoted as areset process. NDR can be understood as the current decrease whenthe C60 chain is disconnected, which is induced by the heat generatedby the high current density, resulting in an increase in the two-terminal resistance. This condition can be regarded as a high-resistance state (HRS). When the voltage was applied again from zero,the current showed a step-like increase, and the two-terminalresistance changed to a low-resistance state (LRS) (Figure 1c). Inthis process, the C60 chain was reconnected between the twoelectrodes; when the voltage was applied to the LRS, the NDRoccurred again (Figure 1e). This switching sequence can be cyclicallyrepeated as the voltage input was cycled (Figure S3).13,14,42,50 Furtherdetails can be found in a previous report.24Figure 1c illustrates the I−V characteristics of the set process,where the voltage was reapplied after the resistance increased owingto the NDR. During voltage application, SCLC occurred on the high-voltage regime following the PF effect in the low-voltage regime.SCLC and PF conduction were distinguished based on their differentvoltage characteristics. PF conduction was identified using a linear I−V1/2 plot, whereas SCLC was characterized using a linear I−V2 plot(inset of Figure 1d). The SCLC transition voltage (VSCLC) is definedby the pronounced inflection point in the I−V curves.To obtain the statistical average VSCLC value, the switch operation(Figure 1d) was repeated 1000 times, and the I−V characteristics ofthe reset process were plotted (Figure 2a). All I−V curves exhibitedinflection points used to determine VSCLC (marked by an arrow),proving that SCLC occurred in the narrow gap. The parts of the I−Vcurves between VSCLC and VNDR (the voltage at which NDR occurs)exhibit SCLC properties, as confirmed by the I−V2 plots (Figure 1d,inset). The average value for VSCLC was derived from the second-orderdifferentiation of 1000 I−V curves. For a 20 nm gap, the averageVSCLC was 5.5 ± 0.8 V.This highly reproducible VSCLC suggests that SCLC between thefixed nanogap electrodes occurred with reasonable reproducibility,depending on the nanogap width. The current in the PF regimeexhibited only small variations, whereas large variations in the currentwere observed after switching to SCLC. However, the cause for thisvariation remains unknown.The average VNDR is plotted as a function of the gap width(measured using SEM) in Figure 2b. As the gap width increases, VNDRincreases, supporting the understanding that NDR is generated by thelocal heating effect through the conduction channel in an LRS.The I−V curves obtained from 1000 repeated measurements of theset process were also plotted, and the turn-on transition voltage(Vturn‑on) was determined (Figure 2c). Although there were largedifferences in the initial currents, Vturn‑on was consistently 3.3 V andwas independent of the gap width (Figure 2d). This suggests that theturn-on transition occurs because of the applied voltage between thedisconnected conducting chains of the electrodes.To analyze the reset process, I−V plots measured for devices withdifferent gap widths were characterized (Figure S4 in the SupportingInformation). From these plots, VSCLC values of 5.5 ± 0.8, 4.5 ± 0.6,and 4.0 ± 0.4 V were obtained for SEM-determined lengths of 20, 17,and 11 nm, respectively. These VSCLC values were used to calculatethe conduction path length (d) of the SCLC medium using thefollowing formula:51,52Vqd n2SCLC2t= (1)where nt is the trap density, ε is the permittivity, and q is theelementary charge. The conduction path lengths for the M and Schannel devices were calculated by normalizing the data with respectto the L channel device (assuming a nominal gap width of 20 nm).The trap densities of the three channels were assumed to be the samebecause the CPTA spin-coating conditions were the same. Then, thefollowing relationship was used:dVVd x( M or S)xx2 SCLC20 SCLC202x20= =(2)where d20, ε20, and VSCLC20 are the conduction path length,permittivity, and starting voltage of SCLC in the L (20 nm) channel,respectively. We did not use a specific value for the trap densitybecause this was a relative length estimation. Although thepermittivity (εx) in the CPTA films of all samples should be similar,experimental values were used to estimate dx. The experimental εxvalues were extracted from the inverse gradients of the I−V curve inthe PF regime,53 which varied slightly in each I−V curve. The slightvariations in the I−V curves in the PF regime are attributed to localfluctuations in the degree of polymerization in and around thenanogap channel.The extracted effective path lengths for the M and S channels werelonger than the SEM-determined lengths (Table 1). This implies thatthe current did not flow through the shortest straight path in theimage, where the electric field was intuitively concentrated. Instead, itmeanders through tortuous conductive media composed of 1 nmspherical nanoparticles. This was attributed to the disorderedfullerene arrangement in the amorphous film between the electrodes(in contrast to the ordered crystalline structure). Alternatively, it ispossible that the point at which the current was injected into themedia was the same as that assumed to calculate the shortest distancebetween the electrodess from the SEM images and that the otheractual surface tip could not be resolved well by SEM.VNDR and the required current (INDR) were extracted for each gapand plotted as histograms (Figure 3a,b). As the channel lengthTable 1. SEM-Determined Length, Average VSCLC, andEffective Current Path Lengths Determined from VSCLCaSEM-determinedlength (nm)averageVSCLC (V)path lengths extractedfrom VSCLC (nm)L (long) 20 5.5 ± 0.8 (20)M(medium)17 4.5 ± 0.6 18S (short) 11 4.2 ± 0.4 14aThe effective current path is a relative length based on the measuredwidth of the long channel. The average VSCLC was derived fromsecond-order differentiation of 1000 I−V curves.ACS Applied Electronic Materials pubs.acs.org/acsaelm Articlehttps://doi.org/10.1021/acsaelm.3c01656ACS Appl. Electron. Mater. 2024, 6, 1740−17451742https://pubs.acs.org/doi/suppl/10.1021/acsaelm.3c01656/suppl_file/el3c01656_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acsaelm.3c01656/suppl_file/el3c01656_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acsaelm.3c01656/suppl_file/el3c01656_si_001.pdfpubs.acs.org/acsaelm?ref=pdfhttps://doi.org/10.1021/acsaelm.3c01656?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asincreases, the VNDR and INDR also increase. The required NDR power(VNDR × INDR) is also plotted (Figure 3c). As expected from VNDR andINDR, a longer channel requires greater power, and the electric powerincreases nonlinearly with the effective current path length (Figure 4).This suggests that shorter current paths result in more efficient localheating and that the bonds in the C60 chain can be resolved bydepolymerization. The gap-dependent power was linear when plottedlogarithmically (Figure 4, inset), although the mechanism remainsunknown. Extrapolation to the nanometer scale predicts that therequired power is on the order of nanowatts. For example, a 2 nm gapshould reduce the power to 0.24 pW. In contrast, the turn-on voltagewas always observed at approximately 3.3 V regardless of the gap(Figure 3d), but there was no clear channel length dependence of theturn-on current (Figure 3e). These voltage and current results showthat, although there is no clear channel length dependence, therequired turn-on power is 1 order of magnitude lower than the NDRpower (Figure 3f).The effective conduction path length of the fullerene polymerchains introduced between the nanoscale electrodes was estimated byusing SCLC. The effective path length was evaluated by averaging1000 measurements of the voltage at which the SCLC appeared. Thiseffective path length tends to be longer than the shortest distancebetween the electrodes obtained from the SEM images, presumablybecause of the tortuous path through the spherical fullerenesdistributed in the amorphous film. Furthermore, the correlationbetween the path length and power required for NDR generated byheat near the channel was found to be logarithmic, indicating thatnanowatt-scale (or lower) switch element operations can be expectedin optimized nanometer-scale switching devices.■ ASSOCIATED CONTENT*sı Supporting InformationThe Supporting Information is available free of charge athttps://pubs.acs.org/doi/10.1021/acsaelm.3c01656.Additional experimental details and theoretical supportof the experimental results. Figure S1. Comparison ofSEM systems to confirm the nanogap size. Figure S2. I−V characteristics of nanogaps without or with a CPTAcoating. Figure S3. Operating principles of fullerenenanochain switching. Figure S4. Gap width dependenceof SCLC transition voltage (VSCLC) (PDF)■ AUTHOR INFORMATIONCorresponding AuthorsHiroshi Suga − Faculty of Engineering, Chiba Institute ofTechnology, Narashino, Chiba 275-0016, Japan; ResearchCenter for Materials Nanoarchitectonics (MANA), NationalInstitute for Materials Science (NIMS), Tsukuba, Ibaraki305-0044, Japan; orcid.org/0000-0003-4333-4898;Email: hiroshi-suga@it-chiba.ac.jpKazuhito Tsukagoshi − Research Center for MaterialsNanoarchitectonics (MANA), National Institute forMaterials Science (NIMS), Tsukuba, Ibaraki 305-0044,Japan; Faculty of Engineering, Chiba Institute of Technology,Narashino, Chiba 275-0016, Japan; orcid.org/0000-0001-9710-2692; Email: TSUKAGOSHI.Kazuhito@nims.go.jpAuthorsMasato Takei − Faculty of Engineering, Chiba Institute ofTechnology, Narashino, Chiba 275-0016, Japan; ResearchCenter for Materials Nanoarchitectonics (MANA), NationalInstitute for Materials Science (NIMS), Tsukuba, Ibaraki305-0044, Japan; orcid.org/0000-0002-3522-0033Takuma Hirama − Faculty of Engineering, Chiba Institute ofTechnology, Narashino, Chiba 275-0016, Japan; ResearchCenter for Materials Nanoarchitectonics (MANA), NationalFigure 3. Turn-on properties and NDR distributions of devices with short (S), medium (M), and long (L) channels. (a) NDR voltage distribution,(b) current distribution, and (c) required power. (d) Turn-on voltage distribution, (e) current distribution, and (f) required power.Figure 4. NDR generation power was calculated from VNDR and INDRas a function of the effective current path length in the nanogap. Inset:log plot of NDR generation power used to estimate the requiredpower for a minimum fullerene chain length.ACS Applied Electronic Materials pubs.acs.org/acsaelm Articlehttps://doi.org/10.1021/acsaelm.3c01656ACS Appl. Electron. Mater. 2024, 6, 1740−17451743https://pubs.acs.org/doi/10.1021/acsaelm.3c01656?goto=supporting-infohttps://pubs.acs.org/doi/suppl/10.1021/acsaelm.3c01656/suppl_file/el3c01656_si_001.pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Hiroshi+Suga"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://orcid.org/0000-0003-4333-4898mailto:hiroshi-suga@it-chiba.ac.jphttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Kazuhito+Tsukagoshi"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://orcid.org/0000-0001-9710-2692https://orcid.org/0000-0001-9710-2692mailto:TSUKAGOSHI.Kazuhito@nims.go.jpmailto:TSUKAGOSHI.Kazuhito@nims.go.jphttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Masato+Takei"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://orcid.org/0000-0002-3522-0033https://pubs.acs.org/action/doSearch?field1=Contrib&text1="Takuma+Hirama"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaelm.3c01656?fig=fig3&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaelm.3c01656?fig=fig3&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaelm.3c01656?fig=fig3&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaelm.3c01656?fig=fig3&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaelm.3c01656?fig=fig4&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaelm.3c01656?fig=fig4&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaelm.3c01656?fig=fig4&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsaelm.3c01656?fig=fig4&ref=pdfpubs.acs.org/acsaelm?ref=pdfhttps://doi.org/10.1021/acsaelm.3c01656?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asInstitute for Materials Science (NIMS), Tsukuba, Ibaraki305-0044, JapanKatsunori Wakabayashi − Department of Nanotechnology forSustainable Energy, School of Science and Technology,Kwansei Gakuin University, Sanda, Hyogo 669-1330,Japan; orcid.org/0000-0002-9147-9939Complete contact information is available at:https://pubs.acs.org/10.1021/acsaelm.3c01656Author ContributionsM.T., T.H., and H.S. fabricated and measured the devices.K.W. and K.T. analyzed the conduction properties of a C60polymer system. H.S. and K.T. conducted the experiments andprepared the manuscript. The manuscript was written withcontributions from all the authors. All authors approved thefinal version of the manuscript. All authors contributed equallyto this study.NotesThe authors declare no competing financial interest.■ ACKNOWLEDGMENTSThe authors would like to thank Y. Naitoh (AIST) for valuablediscussions on binary switching operations. This study wassupported by the JSPS KAKENHI (grant numbers 20K05291,23K17869, and 19H05460), Japan.■ ABBREVIATIONSCPTA, C60 pyrrolidine tris-acid; PF, Poole−Frenkel; SCLC,space-charge-limited current; NDR, negative differentialresistance; SEM, scanning electron microscopy; TEM, trans-mission electron microscopy; LRS, low-resistance state; HRS,high-resistance state■ REFERENCES(1) Ji, D.; Li, T.; Liu, J.; Amirjalayer, S.; Zhong, M.; Zhang, Z.-Y.;Huang, X.; Wei, Z.; Dong, H.; Hu, W.; Fuchs, H. 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