Fabrication of molecular nanoscale junctions with a junction area of 7 × 7 nm² and their structural and electrical properties

Abstract

Molecular electronics has received considerable attention because molecular devices can provide several unique properties, such as giant magnetoresistance, a large Seebeck effect, and nonvolatile switching properties. These unique properties, including enhanced performances, have been observed in molecular nanoscale devices. Therefore, the miniaturization of molecular devices is a key issue for their practical use as well as for the development of fundamental science. In a previous study, we proposed a new nanojunction fabrication method using thin-film edges and successfully fabricated Ni₇₈Fe₂₂/2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT)/Ni₇₈Fe₂₂ nanojunctions with a junction area of 42 × 42 nm². In this study, toward the realization of a smaller junction area, we fabricate Ni₇₈Fe₂₂/C8-BTBT/Ni₇₈Fe₂₂ nanojunctions using our advanced method. As electrodes in our nanojunctions, 7-nm-thick Ni₇₈Fe₂₂ thin films sandwiched between low-softening-point glasses can be fabricated using the thermal pressing technique. The area of the nanojunctions is determined from the thickness of the Ni₇₈Fe₂₂ thin film. Using these electrodes, we have successfully fabricated Ni₇₈Fe₂₂/C8-BTBT/Ni₇₈Fe₂₂ nanojunctions with a junction area of 7 × 7 nm², which is the minimum value ever reported for edge-to-edge nanodevices, and observed electrical conduction through C8-BTBT molecules in the devices. Our study provides a novel nanofabrication technique and opens new opportunities for research in molecular nanoelectronics.

Supplementary Information

The online version contains supplementary material available at 10.1186/s11671-025-04354-z.

Introduction

Molecular electronics has attracted considerable interest owing to its unique properties and potential applications. One of the advantages of molecular devices is the large variety of molecules that can be customized in terms of composition, size, geometry, and functionality via chemical synthesis. Thus, suitable molecules can be selected for each device application. Another advantage is the low fabrication cost of these devices. Compared with conventional inorganic devices, organic devices can be fabricated at a lower cost because expensive and complicated facilities are not required for fabrication. In addition, attractive phenomena such as giant magnetoresistance (MR) [1, 2], a large Seebeck effect, nonvolatile properties [3, 4], and large on/off switching characteristics [5, 6] can be observed in nanoscale molecular devices. For example, first-principles calculations predict a large MR of 600% in Ni/1,4-3-phenyl-dithiolate (1,4-tricene-dithiolate)/Ni nanoscale junctions [2]. Experimentally, a large MR ratio of 300% is observed in La_0.7Sr_0.3MnO₃ (LSMO)/tris-(8-hydroxy-quinoline)aluminum (Alq₃)/Co nanoscale junctions at 2 K [1], whereas the MR ratio is 40% in LSMO/Alq₃/Co milliscale junctions at 11 K [7]. The large MR effect in nanoscale junctions originates from effective orbital hybridization due to the spin-hybridization-induced polarized states (SHIPS) at the metal/molecule interfaces [1]. In addition, the molecular layer with homogeneity can be formed within the entire region in nanoscale junctions owing to the suppression of long-range fluctuation in structures. Thus, nanoscale spin devices can provide better performance than micro- or milliscale devices. Low-dimensional materials, including nanoscale materials, are promising candidates for thermoelectric applications [8]. Theoretical studies predict that molecular junctions exhibit a large thermoelectric figure of merit ZT, such as 1.1 for rubrene with three unit lengths [9], and 1200 for poly(guanine)–poly(cytosine) eoxyribonucleic acid (DNA) molecules with four chain lengths [10]. The experimental results demonstrate that the Seebeck effect is observed in single molecules such as fullerene [11, 12], benzenedithiol [11, 13], and oligo(phenyleneethynylene)-9,10-anthracenyl molecules with dihydrobenzo[b]thiophene anchoring groups (DHBT-OPE3-An) [14]. Recent experimental results have demonstrated that the bulk organic molecules show extremely high Seebeck coefficients under ultra-high vacuum (< 10⁻⁷ Pa), such as 3.5‒70 mV/K for C8-BTBT and 20‒100 mV/K for 2,9-didecyldinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (C10-DNTT) [15]. The reported results show the high potential of organic molecules for use as thermoelectric materials. In ambient air, oligo(Ru(terpyridine)₂) also exhibits a large Seebeck coefficient of 1.027 mV/K, which is the highest value ever reported in molecular thermoelectric studies [16]. Moreover, studies on single-molecule junctions suggest that it is important to control the metal/molecule interface because the Seebeck effect is enhanced owing to orbital hybridization in the metal/molecule interface and/or the presence of a slight difference between the work function of the metal and the highest occupied molecular orbital (HOMO) or lowest unoccupied molecular orbital (LUMO) energy level of the molecule [11, 13]. Therefore, a large Seebeck effect can be observed in nanoscale thermoelectric devices using organic molecules fabricated by controlling the metal/molecule interfaces. It is well known that intriguing phenomena, such as nonvolatile properties and large on/off switching characteristics, are also observed in a single molecular monolayer of bistable [2]rotaxanes [3] and self-assembled monolayer containing a nitroamine redox center (2′-amino-4-ethynylphenyl-4′-ethynylphenyl-5′-nitro-1-benzenethiol) [5], respectively. Thus, various studies have been conducted on molecular nanoscale devices to demonstrate their unique properties and potential.

According to the previous studies, various techniques have been developed for the fabrication of molecular nanodevices [17, 18]. For example, a mechanically controllable break-junction method can provide a single or a few molecular junctions [19–21]. In this method, a metal wire is pulled until it breaks in the solution containing the target molecules. During the breaking process, a nanogap is formed between the broken metal wires, and a molecule, or a few molecules, are cross-linked within the nanogap. A molecule or a few molecules cross-linked between broken wires can be considered as single or few-molecule junctions. In this method, the breaking process should be repeated thousands of times because the electrical conductance is unstable. The conductance value is obtained from the peak of the histogram of conductance versus repeated counts. On the other hand, the most popular fabrication technique is lithography, such as electron-beam lithography, extreme ultraviolet lithography, and focused ion beam lithography, which is capable of generating nanosized features with excellent reproducibility. Using lithography, nanogap structures can be fabricated between metal electrodes on substrates in the lateral direction. Metal/molecule/metal structures can then be created by depositing molecules onto fabricated nanogap electrodes [22, 23]. In lateral devices, extremely small features with precisely controlled dimensions and accurate alignments are required for the width of the metal electrodes to form nanoscale junctions. Another lithographic approach allows the fabrication of nanoscale crossbar junctions consisting of metal/molecule/metal vertical structures, in which molecules are sandwiched between two metal electrodes with crossed electrodes. In vertical devices, the molecules are damaged during the development and removal of resist films using lithographic techniques. The deposition of top electrodes can also result in the diffusion of metal atoms into the molecular layer [24]. Thus, although many fabrication methods for molecular nanodevices have been proposed, each method has advantages and disadvantages.

Previously, we fabricated molecular nanoscale junctions with a junction area of 42 × 42 nm², consisting of the organic molecule 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT) sandwiched between two Ni₇₈Fe₂₂ thin films with crossed edges [25]. We focused on high-mobility molecules and selected C8-BTBT. The high-mobility molecules can suppress spin scattering, which is suitable for the use as a spacer of organic spin valves (OSVs). In this device structure, the junction area is determined by the thickness of the Ni₇₈Fe₂₂ thin film. This implies that the junction area can be controlled by the film thickness; that is, 10-nm-thick films can produce 10 × 10 nm² nanojunctions. In most studies on molecular electronics, the objective is to investigate the electrical characteristics of individual molecules. Because the influence of the electrodes should be ruled out in electrical measurement, the 4-probe method is an effective technique for the precise electrical measurement of molecules. The proposed nanojunctions can be readily applied to the 4-probe method due to their unique device structure consisting of two crossed thin films. Thus, our method is suitable for investigating nanoscale molecules. Moreover, molecular nanojunctions can be fabricated without physical vapor deposition, such as sputtering, laser ablation, or electron-beam evaporation, during the fabrication of the top layer on the molecules. Therefore, there is no diffusion of metal atoms into the molecular layers of the device. The reason why we selected Ni₇₈Fe₂₂ as the electrodes of our devices is that Ni₇₈Fe₂₂ can be applied for not only the electrodes of molecular electronic devices but also those of spintronic devices. Ni₇₈Fe₂₂ is one of the representative ferromagnetic materials. Notably, Ni₇₈Fe₂₂ has a small coercivity. Therefore, the devices using Ni₇₈Fe₂₂ can exhibit a high magnetic sensitivity. In addition to the small coercivity, Ni₇₈Fe₂₂ has a high Curie temperature of approximately 850 K. This means that Ni₇₈Fe₂₂ shows ferromagnetic nature at room temperature, which led to the observation of the room-temperature MR effect in Ni₇₈Fe₂₂/C8-BTBT/Ni₇₈Fe₂₂ devices presented in this study and our previous one [25]. Our previous study also demonstrated that the coercivity of the Ni₇₈Fe₂₂ electrode can be controlled by the pressure during thermal pressing process [25].

In this study, toward the realization of a smaller junction area, we fabricate Ni₇₈Fe₂₂/C8-BTBT/Ni₇₈Fe₂₂ nanojunctions by using our advanced method. Consequently, we have successfully fabricated Ni₇₈Fe₂₂/C8-BTBT/Ni₇₈Fe₂₂ nanojunctions with small junction areas of 7 × 7 and 20 × 20 nm² as shown in Fig. 1, and observed electrical conduction through C8-BTBT molecules in the junctions. Hence, this study provides a novel nanofabrication technique that can open up new opportunities for research in molecular electronics.

Fig. 1.

Schematic of Ni₇₈Fe₂₂/C8-BTBT/Ni₇₈Fe₂₂ nanojunctions utilizing thin-film edges

Methods

Device fabrication

Ni₇₈Fe₂₂/C8-BTBT/Ni₇₈Fe₂₂ nanojunctions were fabricated by sputtering, thermal pressing, mechanical cutting, polishing techniques, and off-center spin-coating. The detailed fabrication method for nanojunctions is described in our previous paper with its schematic illustrations [25]. Prior to the sputtering of the Ni₇₈Fe₂₂ thin films, both sides of the low-softening-point (LSP) glasses were polished to make two chamfered edges. Ni₇₈Fe₂₂ thin films were then sputtered on LSP glass substrates. Subsequently, Au thin films with a thickness of 14 nm were sputtered onto the chamfered edges of the sputtered Ni₇₈Fe₂₂ thin films, in which Au thin films were to provide access for electrical contacts. LSP glasses were stacked on Ni₇₈Fe₂₂ thin films by thermal pressing techniques in a N₂ atmosphere at a temperature of 513 °C and pressure of 0.15–1.0 MPa. The thicknesses of the Ni₇₈Fe₂₂ thin films were 7 and 20 nm. The Ni₇₈Fe₂₂ thin films with thicknesses of 7 and 20 nm can produce 7 × 7 and 20 × 20 nm² nanojunctions, respectively. After cutting the thermally pressed glass/Ni₇₈Fe₂₂/glass samples, the cut surfaces were treated by the mechanical and chemical mechanical polishing. Onto the polished surfaces of the glass/Ni₇₈Fe₂₂/glass samples, the C8-BTBT thin film was formed by an off-center spin-coating method with a 10 mg mL⁻¹ solution. Finally, two glass/Ni₇₈Fe₂₂/glass samples coated with C8-BTBT films were carefully stacked by pressing under a pressure of 0.05 MPa from the top electrode using the control knob. The pressure was measured by a digital force gauge. The precise control of this pressure enabled to flow the electrical current from the one-side electrode to the other electrode, resulting in the successful measurement of electrical transport (current–voltage (I–V) characteristics). Thus, to obtain the electrical conduction through molecules, our device needs to be carefully fixed. Thus, Ni₇₈Fe₂₂/C8-BTBT/Ni₇₈Fe₂₂ nanojunctions were fabricated by sandwiching the C8-BTBT organic layers between two glass/Ni₇₈Fe₂₂/glass samples with the Ni₇₈Fe₂₂ thin-film edges crossed, as shown in Fig. 1.

Measurement setup

The interfacial features of the glass/Ni₇₈Fe₂₂/glass samples were examined by transmission electron microscopy (TEM; TECNAI G2, FEI) and energy-dispersive X-ray spectroscopy (EDS; TECNAI Osiris, FEI). Cross-sectional TEM specimens were prepared using the focused ion beam (FIB) technique. The surface morphologies and roughnesses of the polished glass/Ni₇₈Fe₂₂/glass substrates with and without C8-BTBT were analyzed using atomic force microscopy (AFM; Nanocute, SII Nano Technology Inc.). The electrical properties of the Ni₇₈Fe₂₂ thin-film edges with and without C8-BTBT were evaluated using a conductive-AFM (c-AFM) system equipped with a Rh-coated cantilever (Si–DF3–R, SII Nano Technology Inc.). The I–V characteristics of the fabricated nanojunctions were evaluated using a 4-probe method at room temperature. MR effect was measured by a 4-probe method under magnetic field at room temperature. The magnetic field was applied in the in-plane direction with respect to both the Ni₇₈Fe₂₂ electrodes.

Results and discussion

Fabrication of the Ni₇₈Fe₂₂ electrodes

Figure 2 shows cross-sectional TEM images of 7 and 20-nm-thick Ni₇₈Fe₂₂ films sandwiched between two LSP glasses. Films with smooth, well-defined interfaces have been successfully formed. Figure 3 shows high-angle annular dark field (HAADF)–EDS mapping images of the same samples. The films exhibit smooth and clear interfaces without diffusion of Ni or Fe atoms into the bulk glass. These results indicate that glass/Ni₇₈Fe₂₂ (7 and 20 nm)/glass can be successfully fabricated by thermal pressing. Figure 4a and b show the AFM images of the polished cross-sectional surfaces of the glass/Ni₇₈Fe₂₂ (7 and 20 nm)/glass substrates. The roughness of the polished surface measured over the same scanning area of 10 × 20 µm², shown in Fig. 4a and b, is measured to be 0.71 and 0.66 nm, respectively. Figure 4c and d show c-AFM images of the same specimens, which indicate uniform electrical conduction along the Ni₇₈Fe₂₂ edges. Figure 4e shows the I–V characteristics obtained at arbitrary positions on the Ni₇₈Fe₂₂ edges under optimized polishing conditions. The I–V curve exhibits ohmic behavior, suggesting that the Ni₇₈Fe₂₂ edge is not oxidized. The AFM and c-AFM results reveal that 7 and 20-nm-thick Ni₇₈Fe₂₂ thin films sandwiched between glasses can be used as electrodes in the proposed nanojunctions.

Fig. 2.

Cross-sectional TEM images obtained for a 7 and b 20-nm-thick Ni₇₈Fe₂₂ films sandwiched between the two LSP glasses

Fig. 3.

HAADF–EDS mapping images obtained for a 7 and b 20-nm-thick Ni₇₈Fe₂₂ films sandwiched between the two LSP glasses

Fig. 4.

a, b AFM and c, d c-AFM images of the polished surfaces of the glass/Ni₇₈Fe₂₂ (7 and 20 nm)/glass substrates. e I–V characteristics of the Ni₇₈Fe₂₂ edge obtained under optimized polishing conditions

C8-BTBT thin films on the electrodes

After the formation of the C8-BTBT films on the polished surfaces of the glass/Ni₇₈Fe₂₂ (7 and 20 nm)/glass substrates, the roughness of the surfaces of the C8-BTBT films is measured to be 0.91 and 1.07 nm, respectively, over a scanning area of 10 × 20 µm² using AFM (Fig. 5a and b). Uniform electrical conduction along the Ni₇₈Fe₂₂ edges is observed in the c-AFM images (Fig. 5c and d). Figure 5e shows a cross-sectional TEM image of 2-nm-thick C8-BTBT film on the polished surface of glass/Ni₇₈Fe₂₂ (7 nm)/glass substrates. A film with smooth and clear interfaces is successfully fabricated on the Ni₇₈Fe₂₂ edges. Figure 5f shows the local I–V characteristics obtained by c-AFM of the C8-BTBT films on the polished Ni₇₈Fe₂₂ edges. The I–V curve exhibits a clear nonlinear behavior (I ∝ V²) at higher voltages. These results indicate the successful formation of C8-BTBT films on the Ni₇₈Fe₂₂ edge.

Fig. 5.

a, b AFM and c, d c-AFM images of the polished glass/Ni₇₈Fe₂₂ (7 and 20 nm)/glass substrates coated with C8-BTBT films. e Cross-sectional TEM image obtained for 2-nm-thick C8-BTBT film on the polished surface of the glass/Ni₇₈Fe₂₂ (7 nm)/glass substrates. f I–V characteristics of the polished Ni₇₈Fe₂₂ edge after the formation of C8-BTBT films

Electrical properties of the fabricated nanojunctions

Using the described methods, we fabricated (i) Ni₇₈Fe₂₂/Ni₇₈Fe₂₂ nanojunctions, in which two edges of the Ni₇₈Fe₂₂ thin films were in direct contact, and (ii) Ni₇₈Fe₂₂/C8-BTBT/Ni₇₈Fe₂₂ nanojunctions, in which 4-nm-thick C8-BTBT molecules are sandwiched between the two edges of the Ni₇₈Fe₂₂ thin films with junction areas of 7 × 7 and 20 × 20 nm². In our device, the C8-BTBT layer consists of both the C8-BTBT thin film on the top electrode and that on the bottom one. The thickness of the one-side C8-BTBT thin film was approximately 2 nm obtained from Fig. 5e. Estimating from the unit cell size of C8-BTBT and the thickness of the C8-BTBT, the one-side C8-BTBT thin film may consist of monolayer. Therefore, the C8-BTBT layers are considered to be two monolayers of C8-BTBT in the device. The I–V curves of the fabricated nanojunctions are shown in Fig. 6. The pink lines represent the results of the calculation for obtaining the resistance R using V = RI, where V is the bias voltage and I is the electrical current. Figure 6a has more data points than those in Fig. 6b–d. Fig. S1 in the Supplementary Information (SI) shows the 1/4 plots of the original data. The number of the plots in Fig. S1 is almost same as those in Fig. 6b–d. The resistances of Ni₇₈Fe₂₂/Ni₇₈Fe₂₂ are 1.22 kΩ and 179 Ω (Fig. 6a and b), which correspond to the resistances of the electrode short circuit. The resistances of the Ni₇₈Fe₂₂/C8-BTBT (4 nm)/Ni₇₈Fe₂₂ nanojunctions are 5.26 kΩ and 396 Ω (Fig. 6c and d), which are larger than the resistances of Ni₇₈Fe₂₂/Ni₇₈Fe₂₂ (1.22 kΩ and 179 Ω). Here, it should be noted that these I–V curves (Fig. 6) have been obtained by a 4-probe method in the Ni₇₈Fe₂₂/Ni₇₈Fe₂₂ and Ni₇₈Fe₂₂/C8-BTBT (4 nm)/Ni₇₈Fe₂₂ devices while the local I–V curves (Figs. 4e and 5f) have been obtained by a 2-probe method using c-AFM system. Therefore, they cannot be directly compared due to the difference in experimental conditions. From the electrical properties, we obtain a resistance R vs. junction area S plot (R‒S plot), as shown in Fig. 7. The resistance increases with decreasing junction area in both the Ni₇₈Fe₂₂/Ni₇₈Fe₂₂ and Ni₇₈Fe₂₂/C8-BTBT/Ni₇₈Fe₂₂ nanojunctions. Here, it is found that the resistance increases by sandwiching C8-BTBT molecules in each device. The increase in resistance is one of the pieces of evidence to rule out the possibility of electrode short circuit, i.e., to observe the conductance through C8-BTBT molecules. This increase is much smaller than that estimated from the resistivity of molecules. As we discussed in our previous study, although the reason has not been clarified, these small resistance values can be caused by our unique fabrication method and device structure [25]. If we investigate the device resistance in nanojunctions with various junction areas including larger areas such as 100 × 100 and 200 × 200 nm², we could contribute to clarify the mechanism of the low device resistance in our future work. A detailed discussion of the R‒S plot is provided in the SI (Fig. S2). In addition, we investigate the magnetization curves and spin-transport properties of the fabricated nanojunctions. Ni₇₈Fe₂₂ thin films of the glass/Ni₇₈Fe₂₂ (7 and 20 nm)/glass samples are magnetized, as shown in Figs. S3 and S4. We also investigated MR effect in the fabricated devices. Figure 8 shows the MR effect observed in Ni₇₈Fe₂₂/Ni₇₈Fe₂₂ and Ni₇₈Fe₂₂/C8-BTBT/Ni₇₈Fe₂₂ nanojunctions with a junction area of 20 × 20 nm² at room temperature. The blue (red) plots represent the results obtained under the forward (reverse) sweeping field. In Ni₇₈Fe₂₂/Ni₇₈Fe₂₂ devices, the anisotropic MR (AMR) effect is observed as a negative MR effect, as shown in Fig. 8a. The observed AMR effect can be explained by the coupling of magnetization states in both the top and bottom Ni₇₈Fe₂₂ thin films. The detailed interpretation of AMR effect in Ni₇₈Fe₂₂/Ni₇₈Fe₂₂ nanojunctions is shown in our previous study [25]. In Ni₇₈Fe₂₂/C8-BTBT (4 nm)/Ni₇₈Fe₂₂ nanojunctions, a positive MR effect is observed at room temperature as shown in Fig. 8b. The four negative peaks are considered as AMR effect of Ni₇₈Fe₂₂ thin films. Since a positive MR effect cannot be explained by AMR effect in the devices, the positive MR effect indicates that we can successfully observe a spin signal through C8-BTBT molecules. The observation of MR effect is one of the evidence that the electrical current flows through the C8-BTBT molecules.

Fig. 6.

I–V characteristics in a, b Ni₇₈Fe₂₂/Ni₇₈Fe₂₂ and c, d Ni₇₈Fe₂₂/C8-BTBT/Ni₇₈Fe₂₂ nanojunctions with junction areas of 7 × 7 and 20 × 20 nm² at room temperature

Fig. 7.

R–S plot in Ni₇₈Fe₂₂/Ni₇₈Fe₂₂ and Ni₇₈Fe₂₂/C8-BTBT/Ni₇₈Fe₂₂ nanojunctions at room temperature

Fig. 8.

MR effect in a Ni₇₈Fe₂₂/Ni₇₈Fe₂₂ and b Ni₇₈Fe₂₂/C8-BTBT (4 nm)/Ni₇₈Fe₂₂ nanojunctions with a junction area of 20 × 20 nm² at room temperature

Importance and advantages of nanofabrication techniques using thin-film edges

Finally, we discuss the importance of nanofabrication techniques and their prospects in molecular nanoelectronics. In this study, we emphasize that a junction area of 7 × 7 nm² in the fabricated devices is the minimum value ever reported for devices fabricated using our proposed method. In spintronic research, the chirality-induced spin selectivity (CISS) effect has been observed in chiral molecules [26, 27] and has attracted attention for applications in OSVs, which exhibit the MR effect at room temperature. Although chiral molecules provide high spin polarization in magnetic c-AFM studies at room temperature [26, 28–35], the reported MR ratio is relatively low (0.03%–10%) in OSVs using chiral molecules at room temperature [30–40]. This low MR ratio may have been caused by electrons passing through the pinholes in the chiral molecular layer. To solve this problem, our proposed nanojunctions using chiral molecules are considered to be useful for improving the MR ratio owing to their ultrasmall junction area, in which the possibility of pinholes can be suppressed. However, the MR ratio was not improved in the nanoscale MR device fabricated using a chiral molecule [41]. This implies that there should be other causes for the low MR ratio, except for the leakage current through pinholes in chiral molecules. This suggestion was obtained from the experiments on the nanodevice fabricated by our method. In addition, our fabrication method can be applied to nanoscale magnetic tunnel junctions (MTJs), in which the insulating layer is sandwiched between the two edges of magnetic thin films. MTJs exhibit a large tunnel magnetoresistance (TMR) at room temperature [42–45]. We can fabricate them by sandwiching an insulating layer, such as Al oxide or MgO, between the two edges of the magnetic thin films. Nanoscale MTJs may contribute to the observation of a large TMR ratio owing to coherent tunneling [46] and suppression of pinholes. Our recent study also demonstrated the fabrication of the Au electrode and new nanodevices with a chiral molecule sandwiched between the Au and Ni₇₈Fe₂₂ electrodes [41]. Thus, our fabrication method can be applied for wide range of devices with various kinds of molecules and electrodes. As another application, we plan to realize the molecular nanoscale thermoelectric device in our future work.

Here, we compare our fabrication method with conventional lithography methods such as electron-beam lithography and photolithography. Using conventional lithography, the resistance of bar electrodes with nanoscale widths is high because the aspect ratio of the bar electrode is low. In contrast, our fabrication method reduces electrode resistance. In our nanojunctions with thin-film edges, the resistance of the electrodes is extremely low because the structural aspect ratio of the electrodes is as high as 10⁵:1. The low resistance of the electrodes is beneficial for achieving low power consumption, high signal-to-noise ratio, and high-frequency applications. The low power consumption is due to the suppression of Joule heat generation. The high signal-to-noise ratio is because the resistance of the electrodes is lower than that of the sandwiched materials. In high-frequency applications, the low resistance of the electrodes ensures a short time constant because the time constant is proportional to the resistance. In addition, according to the International Roadmap for Devices and Systems (IRDS) 2023, the introduction of extreme ultraviolet (EUV) lithography with a high numerical aperture (NA) of 0.55 into manufacturing in 2025 assures the viability of reaching a metal pitch of 22 nm (including printing lines and spaces), and a metal pitch of 14–18 nm, corresponding to metal lines with a width of approximately 7 nm, has been targeted for 2033 [47, 48]. Prior to the 7-nm target, our fabrication technique can contribute to the development of fundamental nanoscience in various research fields, such as thermoelectrics, molecular nanoelectronics, and spintronics, and the creation of next-generation nanoscale devices with low power consumption, high signal-to-noise ratio, and high-frequency operation.

Conclusion

To achieve a smaller junction area, we fabricate Ni₇₈Fe₂₂/C8-BTBT/Ni₇₈Fe₂₂ nanojunctions using the proposed method. For the electrodes in our proposed nanojunctions, glass/Ni₇₈Fe₂₂ (7 and 20 nm)/glass can be successfully fabricated using a thermal pressing technique. Using these electrodes, we have successfully fabricated Ni₇₈Fe₂₂/C8-BTBT/Ni₇₈Fe₂₂ nanojunctions with areas of 7 × 7 and 20 × 20 nm². The junction area of 7 × 7 nm² in the fabricated devices is the minimum value ever reported for edge-to-edge nanodevices. Our fabrication method can contribute to the development of fundamental nanoscience in various research fields such as thermoelectrics, spintronics, and nanoelectronics. These results demonstrate a novel nanofabrication technique that can open new opportunities for research in molecular electronics.

Supplementary Information

Below is the link to the electronic supplementary material.

Author contributions

M. M. and H. K. conceived and designed the experiments. M. M., R. M., Z. Z., and K. S. established the measurement setup to investigate the electrical properties of the devices. M. M. performed the device fabrication, structural, electrical, and magnetic analyses, as well as theoretical calculations. M. M. wrote the original draft, and H. K., R. M., Z. Z., and K. S. contributed to the review and editing of the manuscript. H. K. supervised the study. All authors contributed to the data interpretation.

Funding

This research was supported by a Grant-in-Aid for Scientific Research (B) (Nos. 24K00948 and 23H01839) and a Grant-in-Aid for the Japan Society for the Promotion of Science (JSPS) Fellows (No. 24KJ1953) funded by the JSPS, Japan Science and Technology Agency Support for Pioneering Research Initiated by the Next Generation (JST SPRING; No. JPMJSP2123), and Center for Spintronics Research Network (CSRN) at Keio University.

Data availability

All relevant data are within the manuscript and its SI. Additional data are available from the corresponding author upon reasonable request.

Declarations

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Clinical trial number

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Competing interests

The authors declare no competing interests.