Dynamic molecular tunnel junctions based on self-assembled monolayers for high tunneling current triboelectricity generation†

Lu Zhang ^ab, Awad Shalabny ^b, Chen Su ^a, Xin Cui ^c, Amro Sweedan ^b, Kefan Zhang ^b, Sherina Harilal ^b, Sumesh Sadhujan ^b and Muhammad Y. Bashouti *^bd
aSchool of Advanced Materials and Nanotechnology, Xidian University, Shaanxi 710126, P.R. China
^bDepartment of Solar Energy and Environmental Physics, Swiss Institute for Dryland Environmental and Energy Research, J. Blaustein Institutes for Desert Research, Ben-Gurion University of the Negev, Midreshset Ben-Gurion, Building 26, 8499000, Israel. E-mail: Bashouti@bgu.ac.il
^cSchool of Physics, University of Electronic Science and Technology of China, Chengdu 610054, P.R. China
^dThe IISe-Katz Institute for Nanoscale Science & Technology, Ben-Gurion University of the Negev, POB 653, Beer-Sheba Campus, Building 51, 8410501, Israel

First published on 30th January 2023

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Introduction

Triboelectricity, one of the oldest topics involving electron loss and gain occurring between different materials, has been pursued as an emerging technology for developing energy harvesting devices through induced mechanical friction.^1–4 It is well known that conventional triboelectric nanogenerators (TENGs) based on the coupling of triboelectrification and electrostatic induction effects can directly convert mechanical energy into electricity. However, the fundamental physical mechanism of the charge transfer process during friction is still not clear.^5,6 In conventional TENGs, the dielectric displacement current generated during the contact electrification process is constrained by the high intrinsic impedance of polymer-based systems. All inductive charges have ignorable interactions with the insulated friction layer resulting in limited current density (0.1–1 A m⁻²) and regular a.c. output characteristics.^7,8 In order to solve these issues, semiconductor-based tribotunneling nanogenerators have been discovered and have shown promising prospects ascribed to their high current density (as high as 100 A m⁻²), and continuous and d.c. electrical output characteristics, e.g., the moving Schottky diode (MS) between a Pt tip and molybdenum disulfide (MoS₂)⁹ or between graphene and Si wafer based on the built-in electric field for a tribotunneling DC generator,¹⁰ the sliding metal–insulator–semiconductor (MIS) depending on quantum mechanical tunneling for sustained high current density,¹¹ and other dedicated moving heterojunctions.^12,13 However, high internal impedance (MΩ) and low tribovoltage resulting from friction between the metal and semiconductor or the oxide layer constrain the electrical output performance to be further improved.

Nowadays, the in-depth study of forefront molecular electronics provides valuable guidance for challenges faced by conventional TENGs, which have been widely applied in optical switching, molecular wire behavior, molecular rectification, spin-polarized transport, and mechanistic change due to the modulation of electron transport in molecules and the alternative molecular structure.¹⁴ In a typical metal–insulator–metal (MIM) junction, a self-assembled monolayer (SAM) is contacted at either terminal by the metal electrode, which has always been used to understand the mechanism of electrons flowing through molecules fully.^15,16 Previous studies generally indicated that the electronic properties of molecules have been correlated with the molecular electronic structure and molecule–electrode contacts. Specifically, for a given low voltage, the tunneling current (J) subjected to a simplified model of the Simmons equation can be expressed as the exponential dependence on the molecular length (L), i.e., J = J₀ exp (βL), where J₀ is determined by the barrier height or contact resistance, and β is the tunneling decay constant correlated to the molecular backbone.¹⁷ Likewise, the β values have been identified in the range 0.8–1.5 Å⁻¹ for alkane thiolate SAMs, and 0.2–0.5 Å⁻¹ for oligoacene thiolate SAMs.¹⁸ The assumption is that, for a dynamic MIM molecular tunnel junction that can disappear and establish during the movement between a metal electrode (e.g., Au and Pt) and a certain SAM modified electrode, the triboelectric charges will be directly driven by the interfacial tribo-voltage to form tunneling current. In order to improve the electrical output performance, the SAMs can be varied with low attenuation (β), and also the terminal groups of the SAM can be tuned with desired head groups (e.g., –CF₃) with an excellent ability to gain electrons, facilitating the higher tribo-voltage compared with inorganic semiconductors and oxide layers.¹⁹

In this study, to demonstrate unique d.c. triboelectric generation based on a dynamic MIM molecular tunnel junction, an atomic force microscope (AFM) with Pt-coated silicon electrode was moved and conducted on a SAM modified Au layer. The mechanism in our case, which differs from conventional TENGs illustrated above, can be attributed to the tribovoltaic effect,^20–22 where the tribovoltage spanning the entire molecular junction from the dynamic MIM interface is capable of driving the electrons to pass through the monolayer and form a tunneling current. We were able to obtain a tunneling current density output of up to 7.6 × 10⁶ A m⁻² with the aid of this method. Its theoretical viability has been rigorously tested through experiments, and its potential is further expanded by macroscopic application. Prospectively, this fundamental understanding of tribo-tunneling has been beneficial not only for exploring novel material candidates and heterojunction structures, but also for scaling up and optimizing the concept for energy conversion and enabling variable promising practical applications.

Results

Fabrication of the DMIMG and electrical outputs

To demonstrate the representative dynamic MIM generator (DMIMG), the proposed schematic diagram of the moving AFM probe over a SAM chemically anchored on a Au layer prepared by magnetron sputtering on a heavily doped p-type Si wafer is illustrated in Fig. 1a. The main structure of a DMIMG consists of three components: the AFM Pt-coated tip is acting as a moving metal electrode, the SAM is the key part for carrier transportation, and the Au covered Si wafer is used as the counter electrode. Based on the triboelectrification and molecular tunnel junction principles, three critical factors should be considered in pursuit of highly efficient DMIMGs: low attenuation and contact resistance for less energy consumption, good electron capture ability to obtain more external electrons, and appropriate candidates for dry lubricants. Therefore, the oligoacene 4-(trifluoromethyl)benzenethiol (PhSHCF₃) in our experiment was chosen as the SAM due to its low attenuation coefficient of about 0.5 Å^−1,23 and head groups (–CF₃) with strong ability to capture electrons.¹⁹Fig. 1b shows a typical AFM topographic image of the flattened p-Si wafer (left part) and evenly distributed sputtered Au layer on the p-Si wafer (right part) bounded by a red dashed line, and the cross-sectional line analysis in Fig. 1e shows the thickness of the Au layer to be approximately 35 nm. Its corresponding surface potential map in single-pass Kelvin force microscopy (HD-KPFM) mode is shown in Fig. 1c; the distinguishable difference between the Si wafer and Au layer can be observed spanning the original red boundary line. According to the readout cross-sectional line in Fig. 1f, the specific work function (WF) can be calculated using equation CPD = SP_tip – SP_sample for the Si wafer (4.70 ± 0.02 eV) and Au (5.14 ± 0.01 eV), respectively, both of which agree well with previously reported studies.^24,25 It should be noted that the WF of the Pt-coated tip (5.74 ± 0.01 eV) has been calibrated in ESI Fig. S1.† Afterward, the PhSHCF₃ SAM was chemically grown on the Au surface by the solution method at room temperature. The detailed procedures and X-ray photoelectron spectra (XPS) characterization studies that have been conducted to identify the SAM. A successful covalent bond (Au–S) between the Au surface and the SAM is presented in the Methods section and ESI.† As is shown in the surface potential map of the PhSHCF₃/Au/Si wafer (Fig. 1d) and its readout cross-sectional line (Fig. 1g), the WF of the PhSHCF₃ SAM modified substrate has reached 5.40 ± 0.01 eV by a 0.26 eV increment because of the coupling effects of the formation of a bond dipole associated with the Au–S bond and the intrinsic polarity of electric dipole moment of the C–F bond.^26–28 In contrast, the contact angle (CA) for the wettability test is significantly improved from 66.7 ± 0.1° (inset of Fig. 1c) to 79.8 ± 0.1° (inset of Fig. 1d), which further verifies the fluorine groups covered on the surface of the substrate.

In order to verify the conduction mechanism, we formed the MIM junction for current–voltage measurements by contacting an AFM conductive tip with a specific load (contact mode in AFM) directly on SAM-modified Au substrates. We initiated these I–V studies with conductive AFM (C-AFM) system calibration and details are shown in Fig. S3.† For comparison, multiple I–V data are shown in Fig. 1h and i with respect to Pt– Au/Si and Pt-PhSHCF₃/Au/Si structures with a load of 10 nN. To fully understand the properties of the new structure we fabricated, an equivalent circuit (inset of Fig. 1i) is suggested where a resistor is in series with a unit composed of a resistor and a capacitor in parallel, contributing to the ability of the SAM to behave as a space charge region. In contrast, the direct two-metal contact is typically ohmic and does not have any accumulated charge, hence it is equivalent to two resistors (intrinsic resistance and contact resistance) connected in series (inset of Fig. 1h). As a result, it is proposed that the charge distribution at the SAM will alter the system response to voltage. The I–V spectra in Fig. 1h reveal that the Pt-coated tip and Au have a clear, typical ohmic contact with a marginal internal resistance of about 6.2 kΩ. The example I–V curve for the Pt-PhSHCF₃/Au/Si structure is shown in Fig. 1i, and it is nearly sigmoidal and is symmetrical with respect to an external 0 V bias. For the low bias regime of about ±0.5 V, the current (I) is in a nearly linear relationship with the applied voltage (V), which is attributed to the carrier transport mechanism of low bias nonresonant tunneling.²³ Nevertheless, the tunneling current increases exponentially with the applied voltage during a relatively high voltage regime (0.5 V to 2 V) since its potential is comparable to the average tunneling barrier.²⁹ Note that this discrepancy among the I–V test for multiple points may result from subtle fluctuations on different Au grains and oxidation vulnerability of the Si substrate.

Working principle of the DMIMG

Compared with the basic working principle of conventional TENGs based on triboelectrification and electrostatic induction effects, the carrier transport mechanism of the DMIMG in our experiment can be briefly explained by two processes, including the working principle of the conventional TENG, i.e., contact electrification under friction and electrostatic induction, and molecular junction nonresonant tunneling excited by tribovoltage as illustrated in Fig. 2. In the initial state (Fig. 2a), the Pt-coated tip is brought into physical contact with the PhSHCF₃ SAM with a certain load pressure and sustained still. According to the triboelectric series,^30,31 the electrons will flow from the Pt tip to the sufficient fluorine group terminated monolayer leading to the positively charged Pt tip and negatively charged monolayer without inductive charges. At this moment, the ideal asymmetric MIM structure based on evident WF_Au = 5.14 ± 0.01 eV and WF_Pt = 5.74 ± 0.01 eV, reaches a thermodynamic equilibrium state, and the corresponding band energy alignment and barrier formation are shown in Fig. 2b. It is worth noting that the temperature effect, the mirror force effect, and the field permeability effect in the metal electrode can be ignored. However, when the AFM tip starts to slide along one direction on the surface of the monolayer, the thermodynamic equilibrium state will break (Fig. 2c). The phenomenon of friction-induced electron tunneling occurs under the synergistic effect of the interfacial potential difference (qΔΦ) and inductive charge-induced tribovoltage (V_tribo). As a result, the charge will pass through the MIM interface to form the tunneling current (Fig. 2d). It should be noted that, when the tip is located at a certain point on the surface of the SAM, the charge transfer occurring in the external circuit should be the sum of the inductive charge and tunneling charge. Regardless of the direction and velocity of the probe, the direction of electron tunneling is determined by tailored frictional molecules. Finally, the AFM tip stops moving at one end of the monolayer surface, the flow of the friction-induced charge will vanish, and the surface equilibrium state is re-established (Fig. 2e and f).

Characterization of DMIMG electrical output performance

Fig. 3 shows the systematic investigation of the electrical output characteristics of the DMIMG. The working parameters involving the lateral sliding velocity and load pressure have been investigated. At first, a randomly selected area (5 × 5 μm²) modified with the PhSHCF₃ monolayer is characterized by a surface roughness of about 618 pm (Fig. 3a). The experiment in C-AFM mode was carried out using the default setup of 3.75 μm s⁻¹ velocity and 75 nN load pressure. As shown in Fig. 3b, the tunneling current can be intuitively observed to be evenly distributed within the scope of the scanning field. Note that numerous red dots with respect to the high current value are in good consistency with the topology in Fig. 3a. The contact interface in this experiment belongs to point contacts of many protrusions from Au nanoparticles, resulting in excessive local stress and high tunneling current when the tip continuously scans the whole surface. In addition, the inset curve in Fig. 3b displays the readout cross-sectional line analysis, which shows that the tunneling current can reach about 1.5 nA. Its corresponding voltage is approximately 10 mV extrapolated from the evident I–V curve in Fig. 1i. In contrast, the Pt-coated tip directly rubbed on the Au layer without any treatment is set as the control group performed with identical parameters, and the output current is always maintained on the order of tens of femtoamperes as shown in ESI Fig. S4,† which means the absence of the electron tunneling effect.

The dependence of the output tunneling current on the tip load pressure is subsequently studied. For the intuitive and accurate observation of the tunneling current in the same field of vision, similar parameters have been investigated in this set of measurements except for the variation of load pressure, which increases by 25 nN every micrometer from top to bottom (red dashed line marked as the boundary). As shown in Fig. 3c and d, by continuously increasing the load pressure from 0 to 100 nN with a particular velocity of 3.75 μm s⁻¹ during the scan, the tunneling current response increases from 0 to 1.5 nA. It then slightly decreases (see Video 1 for details in the ESI†). This phenomenon can explain the insufficient tip–sample contact at the initial load pressure of 0 nN, but good contact can be achieved when the load pressure increases to 25 nN and the tunneling current finally reaches saturation. In addition, the final slight decrease may result from the overload pressure.

Furthermore, we also explored the effect of the tip moving velocity on the tunneling current output with the similar approach mentioned above. Fig. 3e demonstrates the current mapping tendency with a variation of velocity from 2.50 to 7.50 μm s⁻¹ by a step of 1.25 μm s⁻¹ (red dashed line marked as the boundary) and load pressure maintained at 75 nN. Fig. 3f shows its corresponding cross-sectional line analysis extracted from the marked black line in Fig. 3e (see the ESI†). It can be seen that the spontaneous tunneling current shows an insensitive response to the increasing tip velocity with a value of 1.5 nA, which may be explained by the relatively low sliding velocity compared with the friction of different macroscopic objects resulting in insufficient frictional energy.²² When viewed as a whole, the velocity and load pressure-dependent data obtained during numerous iterations of scanning reveal that the tunneling current output can be obtained repeatedly, and is insensitive to the tip velocity in the tested range.

Theoretically, the electron tunneling phenomenon in C-AFM mode has been ascribed to the intense local electric field (E) beneath the AFM tip and the low attenuation of the SAM. By using the finite element method (FEM), it can be seen that a simulated electric field is uniformly distributed at the tip–sample interface (Fig. 4a) and its 2D model is presented in ESI Fig. S5.† Obviously, the Pt-coated tip after friction can generate a super high electric field (E = 0.8 × 10⁵ V m⁻¹) across the whole monolayer and therefore yield remarkable tunneling current as expected.^32,33 In order to calculate J, an actual contact area (i.e., an actual radius (r_act)) should be involved due to the AFM tip's special circular geometry. Fig. 4b shows that E manifests itself as intense electric field at the contact edge and attenuates to its one-tenth at d = 3.3r_act, where r_act is the radius of the actual contact area. According to the previous report,³³ this distance (d) can be theoretically defined as the effective radius (r_eff) in our case for the tunneling channel between the tip and sample. Therefore, the actual J can be calculated using equation (()), and it reaches a high value of 7.6 × 10⁶ A m⁻², which is several orders of magnitude higher than that of typical contact electrification and comparable with other effects, as illustrated in ESI Table S1.†

In order to verify the PhSHCF₃ molecule as a good paradigm in DMIMG, several typical thiol molecules have been selected as the control group including oligoacene thiols (thiophenol (PhSH), 4-tert-butylbenzenethiol (PhSHCH₃), 4-(dimethylamino)benzenethiol (PhSHNCH₃) and PhSHCF₃) and alkane thiols (1-decanethiol (C₁₀SH) and 1-octadecanethiol (C₁₈SH)). As shown in Fig. 4c and ESI Fig. S6,† as well as its partial enlargement of Fig. 4d, the typical I–V characteristics generally show that the junction resistance values of oligoacene thiols are nearly 2 orders of magnitude lower than those of both alkane thiols used in our case due to their different attenuation factors and contact resistance, which are in good agreement with reported studies.^23,34 Even for oligoacene thiols with similar molecular backbones, they still show different junction resistances (R_PhSH < R_PhSHCH3 < R_PhSHCF3< R_PhSHNCH3) caused by the different molecular lengths and terminal groups. The usual topographic image and the features of the most recent mapping are shown in Fig. 4e and f. Due to the poorer capacity of the terminal groups to acquire electrons than that of the –CF₃ groups, the tunneling current is only approximately 0.8 nA. The aforementioned experiments demonstrate that the value of the tunneling current depends on both the capacity to capture electrons from other materials as well as the intrinsic carrier transport mechanism. Moreover, the pressure resistance of the PhSHCF₃ modified Au layer was also tested in our experiment. Superior electrical property stability is seen in Fig S7a–e† with an applied force ranging from 30 to 200 nN (equivalent to 0.14 to 0.99 GPa). Even after 10 full cycles of scanning at 200 nN, the morphology of the PhSHCF₃ modified Au layer still remains unaltered (Fig. S7f†). The PhSHNCH₃ SAM modified Au layer exhibits generally stable electrical characteristics and good pressure resistance. However, compared with the initial morphology of the PhSHCF₃ SAM modified on the sputtered Au layer (Fig. S8a†), the remarkable surface damage (e.g., peeling off and scratching) can be intuitively observed in the morphology characterization (Fig. S8b†) after tens of intensive repetition scanning, where the thickness of the PhSHCF₃ monolayer can be extracted from Fig. S8b† marked as a green line indicating the monolayer thickness of about 0.92 nm (Fig. S8c†).

Practical applications for tunneling current imaging and scale-up energy conversion

According to the evident working principle of the DMIMG, it can be further employed in our experiment for tunneling current imaging with a specific geometry.

In place of the sputtered polycrystalline Au layer, triangle Au nanoflakes with a thickness of 55 nm and a surface roughness of about 135 pm are used for the DMIMG. The Methods section and Fig. S9a–d† show the synthetic procedure and characterization. The potential surface maps before (Fig. 5b) and after (Fig. 5c) the PhSHCF₃ monolayer modification are contrasted and their corresponding extracted line analysis is shown in Fig. 5d. After modification, there is a distinct increase of about 195 mV, which is completely consistent with the variation trend based on the sputtered Au layer. Fig. S9e† shows typical ohmic contact without the PhSHCF₃ monolayer; conversely, it shows a similar sigmoidal and symmetrical shape with respect to external 0 V bias (Fig. S9f†). With the working parameters of 3.75 μm s⁻¹ velocity and 25 nN load pressure, the tunneling current mapping demonstrates a clear triangular profile with a value of about 1.8 nA, and its corresponding 3D image is shown in Fig. 5f. The good crystallinity of Au nanoflakes may have reduced the contact barrier between gold nanoparticles, resulting in the improved tunneling current.

The macroscopic demonstration of this concept is shown in Fig. 6a by manually rubbing a flexible electrode covered in a golden nanowire array and stuck to a finger (see the left corner inset of Fig. 6a) in contact with a layer of PhCHCF₃-modified Au (1 × 1 cm²) in contact-separation mode.³⁵ More detailed instructions and characterization studies are shown in the Methods section and ESI Fig. S10.† The flexible electrode substituting AFM probe was electrically connected with the sample surface through an electrical test system (Paios characterization suite). During the driving process, the finger with the electrode slightly touched the sample surface to prevent unexpected physical damage. As shown in Fig. 6e, the output d.c. short circuit current timely responds to finger movements, i.e., contact and release, which can reach about 40 nA and the unstable output value results from the different load pressures applied by the finger at each contact. Its corresponding open circuit voltage is about 35 mV. Although the output performance of the DMIMG in macroscopic applications cannot be on par with that of the conventional TENG, it has proven feasible and promising for scaling up and further applications.

Summary

In summary, we have demonstrated a novel d.c. dynamic metal–insulator–metal generator (DMIMG) for efficiently converting mechanical energy into electric energy, which is theoretically based on the molecular junction for tunneling current. Specifically, the intense local electric field induced by tip–sample nanoscale interfacial friction boosts the frictional charge to move across through the SAM with a high tunneling current density of 7.6 × 10⁶ A m⁻². The proposed mechanism has been systematically verified through a series of experiments presented herein, showing the validity of different material structures, such as nanowires, two-dimensional materials, and thin layers. It can be found that the molecular backbones and terminal groups play a vital role in carrier transformation efficiency. Although this structure is at the proof-of-concept stage, its excellent performance and potential for scale-up fabrication have broad application prospects in portable electronics, molecular electronics, smart sensors, and other fields.

Methods

Materials

Thiophenol (PhSH, 97%), 1-decanethiol (C10SH, 96%), 1-octadecanethiol (C18SH, 98%), gold(III) chloride trihydrate (HAuCl₄·3H₂O, 99.9%), ethyl acetate (99.8%), poly(methyl methacrylate) (PMMA), 4-mercaptobenzoic acid (MBA, 99%), (3-aminopropyl)triethoxysilane (APTES, 99%) L-ascorbic acid (LAA, 99%), tri-sodium citrate (99%), and sodium borohydride (NaBH₄, 99.9%) were purchased from Sigma-Aldrich. In addition, 4-(trifluoromethyl)benzenethiol (PhSHCF₃, 98%), 4-tert-butylbenzenethiol (PhSHCH₃, 97%), and 4-(dimethylamino)-benzenethiol (PhSHNCH₃, 98%) were purchased from TCI America. The Sylgard 184 silicane elastomer was purchased from Dow Chemical Company. All chemicals were used as received without further purification. The p-Si<001> wafers with resistivity 0.0008–0.001 ohm cm were purchased from Silicon Materials company.

SAM modification

The Si substrates (1 × 1 cm²) were first etched in dilute HF buffer solution to remove the native oxide, and subsequently, an Au film with desired thickness was sputtered on the substrate. The metal surfaces were immersed in 20 mL of 0.1 mM absolute ethanol solution of the oligoacene thiols and 1.0 mM absolute ethanol solution of alkane thiols in argon-purged absolute ethanol for 24 h at ambient temperature, respectively. Finally, the residual solvent was dried using a vigorous stream of N₂.

Synthesis of Au nanoflakes

In a typical procedure,³⁶ 200 μL of 0.1 M aqueous solution of HAuCl₄·3H₂O was transferred into 20 mL EG solution, and vigorously stirred to get a homogeneous solution. Meanwhile, one cleaned glass slide (2.5 cm × 1 cm) was placed and stuck in the glass bottle slantingly to prevent numerous particles from precipitating at the bottom of the glass. And then, the glass bottle was placed in a preset oven at 90 °C for 18 h to grow Au nanoflakes dispersed in solution. After the growth, the slide was carefully taken out from the solution.

Fabrication of the vertical Au NWs/PDMS flexible electrode

A modified seed-mediated approach depicted in reported studies was used for the preparation of the Au NWs/PDMS flexible electrode.³⁷ First, 500 mg of PMMA was dissolved in 10 mL of ethyl acetate solution and then spin-coated on a clean glass slide acting as a sacrificial layer at 2000 rpm for 50 s and baked at 150 °C for 2 minutes. Then, the substrates were immersed in 5 mM APTES solution for 1 hour for amino group functionalization. After rinsing with ethyl alcohol and drying with N₂ flow, the substrates were subsequently immersed in gold suspension to anchor the Au seed layer on the surface. Briefly, the suspension was prepared by adding 125 μL of 25 mM HAuCl₄·3H₂O and 75 μL of 34 mM tri-sodium citrate into 10 mL of DI water under vigorous stirring. Afterwards, 300 μL of 0.1 M NaBH₄ solution was added and stirred at least for 5 min. After the Au seed deposition process, the substrates were washed with DI water to remove the residual solvent and particles. The next step was immersing the substrates into the Au NW growth solution for 2–4 min, which was prepared by dissolving HAuCl₄·3H₂O (12 mM), LAA (30 mM) and MBA (1.1 mM) in ethanol/DI (1:1.2 v/v) water mixed solution. The substrates were rinsed with ethanol and blown with ultrahigh-purity N₂ gas. Finally, the PDMS solution (PDMS base and a curing agent mixed in 10:1 w/w) was spin-coated on the substrates at 500 rpm for 1 min and degassed in a vacuum oven at 60 °C for at least 2 h. After sonication in acetone solution for 20 s to remove the sacrificial layer, the PDMS layer can be easily peeled off from the substrate. Therefore, the Au NWs were successfully transferred and embedded in the PDMS film and the vertical Au NWs/PDMS flexible electrode was obtained.

Characterization

The SEM images were recorded using a Thermo Fisher Verios 460L field-emission scanning electron microscope (FESEM) operating at a voltage of 5 kV. To test the electrical output performance of the macroscopic application, the two electrode ends were attached to a Pios test system. For the evidence of the SAM anchoring on the Au layer, XPS (Thermo Fisher scientific ESCALAB 250) was conducted to characterize the correlating chemical bonds. Furthermore, the unique HD-KPFM mode developed by CSI instruments was applied to test surface potential with high resolution and sensitivity in this work. The Pt/Ir coated conductive tips were purchased from App Nano. The I–V curves and dynamic tunneling current mapping of MIM structures were measured using a CSI AFM in ResiScope mode (C-AFM) specialized for electrical characterization.

Conflicts of interest

The authors declare no competing financial interest.

Acknowledgements

M. B. is thankful for the BGU grant of faculty members and the technical support from the CSI company. L. Z. is thankful for the SEEDER scholarship support of a postdoctoral fellowship from the Jacob Blaustein Center for Scientific Cooperation, and also acknowledges the National Natural Science Foundation of China (No. 61904134 and 62204191). C. S. is thankful to the Shanghai Key Lab for Urban Ecological Processes and Eco-Restoration (SHUES2021A06). A. S. acknowledges the institutional scholarship for PhD students that he received from the Ben-Gurion University of Negev. In addition, A. S. is thankful for the TZIN scholarship for outstanding PhD students. We also acknowledge the Instrumental Analysis Centre of Xidian University for providing the testing equipment.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d2ta10048g

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