Dynamically blocking leakage current in molecular tunneling junctions

Molecular tunneling junctions based on self-assembled monolayers (SAMs) have demonstrated rectifying characteristics at the nanoscale that can hardly be achieved using traditional approaches. However, defects in SAMs result in high leakage when applying bias. The poor performance of molecular diodes compared to silicon or thin-film devices limits their further development. In this study, we show that incorporating “mixed backbones” with flexible-rigid structures into molecular junctions can dynamically block tunneling currents, which is difficult to realize using non-molecular technology. Our idea is achieved by the interaction between interfacial dipole moments and electric field, triggering structured packing in SAMs. Efficient blocking of leakage by more than an order of magnitude leads to a significant enhancement of the rectification ratio to the initial value. The rearrangement of supramolecular structures has also been verified through electrochemistry and electroluminescence measurements. Moreover, the enhanced rectification is extended to various challenging environments, including endurance measurements, bending of electrodes, and rough electrodes, thus demonstrating the feasibility of the dynamic behavior of molecules for practical electronic applications.


Introduction
][6][7][8][9][10] Molecular diodes, based on the unsymmetric accessibility of molecular orbitals (spatial or/and energetical) at different directions of applied bias, are the essential components among these elements.2][13][14][15] The magnitude of rectication is the most important parameter for evaluating molecular diodes.Most molecular diodes have a disappointing rectication ratio lower than 10 2 , which is attributed to large leakage derived from the supramolecular defects in SAMs. 16,17Previous research shows that molecular diodes with a high rectication ratio (i.e.small leakage) highly depend on ordered supramolecular structures of SAMs.][23][24] Indeed, super-ordered SAMs can efficiently block leakage of tunneling current, but unexpected diode failures still arise in ordered SAMs owing to the charge scattering at the interfaces generating Joule heating. 25Differing from silicon materials, molecules mainly made from carbons are dynamic and easy to trigger chemistry or deformation that can be used to control the packing of SAMs in situ.7][28][29] Here, we took advantage of the dynamic behavior of molecules and developed a general method to block the leakage to recuse failed molecular diodes to give high performance.We designed a series of molecules with "mixed backbones" comprising exible linear alkane and rigid aromatic backbones.These molecules could gradually diminish leakage when subjected to an external electric eld.We assume that SAMs can dynamically reach more ordered structures owing to the interaction between dipole moments and electric elds.

Electrical response of SAMs
][32] We used the Simmons equation to simply correlate tunneling current, J, with the degree of order in SAMs: Tunneling currents are highly sensitive to the tunneling barrier length, d.4][35] Thus, it is a reection of a more ordered structure in SAMs if the current across the junction diminishes owing to the exponential decay over the longer tunneling distance.
Based on our measurements, we conclude that the molecular junctions can gradually block tunneling currents only if molecules have (i) a "mixed backbone" and (ii) polarized substituents.The schematic illustrations of the structural transformation of SAMs are presented in Fig. 1a.This observation implies that the polarized substituents, such as nitrile functionality, have a propensity to align with the electric eld, effectively blocking the leakage currents in the junctions.We infer that the electrostatic energy arising from the interaction of interfacial dipoles and external electric eld can induce rearrangements in the supramolecular structures and longer tunneling distances for charge transport.When an external electric eld ( Ẽ) is correctly applied, aligning the vertical component of the molecular dipole moment with the direction of Ẽ may lead to a "pull-up" effect on the molecules.This encourages a quasi-stable supramolecular structure with a higher degree of order (Fig. 1a) and consequently yields thicker SAMs.We hypothesize that the conjugated OPE molecules in SAMs lack a degree of freedom, while the exible C14 are densely packed, requiring higher energy for conformational change.Thus, introducing mixed backbones and polar groups is crucial for dynamically diminishing tunneling currents.

Molecular diodes: mechanism and supramolecular structures
We observe that as for trivial molecular junctions with mixed backbones, ordered supramolecular structures can diminish tunneling currents in both directions of bias (Fig. 1d).Similarly, leakage of molecular didoes is highly dependent on supramolecular structures.Thus, it is promising to incorporate a "mixed backbone" into molecular diodes to diminish leakage and enhance rectication (Fig. 2a).
][39][40] This well-established mechanism of rectication with Fc-based molecular junctions is schematically illustrated in Fig. 2b: under negative bias at the top electrode, the HOMO of Fc falls within the energy window and participates in the charge transport, which allows current to ow more efficiently through the junction.Under reverse bias, the HOMO falls below both Fermi levels, causing the diodes to be in the OFF state, blocking the current and leaving only a small leakage (denoted as J(+V), i.e. current density at positive bias in this work). 41From our previous studies, we found that the presence of disordered domains in SAMs signicantly increases the leakage currents. 16,42Especially, when molecules composed of Fc form SAMs on Au TS surfaces (the surface contains smaller at grains compared to Ag TS and Pt TS surfaces) only showing a disappointing rectication ratio (RR) ∼ 10 and remain consistent even under the inuence of a large applied electric eld (approximately 1-10 GV m −1 ). 39,43,44arious parameters can reect failed molecular diodes directly or indirectly, including EGaIn junctions (leakage), photoelectron spectroscopy (tilt angle, thickness, and binding energy), and cyclic voltammograms (redox peaks).Generally, high-performance Fc diodes with small leakage are dependent on highly ordered SAMs.Unfortunately, only a few molecules match the standard and yield a high-performance diode (RR > 10 2 ).In consideration of the strong correlation between leakage and supramolecular structures of SAMs, the supramolecular structures, together with the leakage, were determined once the SAM had been formed.If the supramolecular structures of SAMs become more ordered by degrees, it could decouple the leakage and initial structures of SAMs.From the abovementioned experiments, introducing mixed backbones can induce ordered SAMs with assistance from an external electric eld, which may provide a general template to dynamically yield high-performance molecular diodes.
To fully investigate the effect of mixed backbones in molecular diodes, we fabricate SAMs derived from Fc-B-SH (B represents the backbone of Fc thiols), which incorporates biphenyl (B1) and naphthalene diimides (B2) into SAMs.The schematic structure of the molecular diode is depicted in Fig. 2a.In our study, we form Fc-based SAMs on a gold substrate and perform XPS to determine the species of sulfur components (Fig. S11 †).According to the tting of the S 2p doublet, whose binding energy is ∼162.0eV, we verify that the thiols covalently form Au-S bonds on the gold substrate rather than physisorption.UPS is used to determine the HOMO of the molecules in SAMs (Fig. S12 †).The HOMO is lower than −5.0 eV for Fc molecules with different backbones, which means rectifying mechanism (Fig. 2b) is proper for these Fc molecules. 18We characterize the structures of SAMs with cyclic voltammetry (CV).The CV of SAMs shows the electrochemical behavior of Fc termini in relation to their supramolecular structures.Surface coverage (G) can be determined following equations reported in previous research, where Q tot is total charge when redox, F is Faraday's constant and A is charging area. 21 SAMs formed with Fc-B1-SH and Fc-B2-SH have surface coverage G = 3.79 × 10 −10 and 2.97 × 10 −10 mol cm −2 , respectively, slightly lower than the value derived from ferrocene alkanethiols.A mismatch of the size between the backbone and Fc unit and roughness of the substrates results in defects in SAMs.Peak deconvolution of the anodic peaks reects three different types of structures in SAMs: 45   packed, disordered, and buried domains (Fig. 2c and d).The ratio of densely packed domains is 51.43% and 40.51% for Au TS -S-B1-Fc and Au TS -S-B2-Fc, respectively.Disordered and buried domains form thin areas in SAMs, which induce higher leakage and lead to the molecular diodes failing. 16o measure the electrical characteristics of molecular diodes comprising B1 and B2, we used EGaIn top electrodes to form tunneling junctions with SAM.The rectication ratio is given by eqn (3), where we dene the current density J (−1.8 V) owing across the junction with "hopping" as ON current while J (+1.8 V) is leakage.
We applied 150 continuous sweeps at one junction and measured ∼20 junctions in total for statistical analysis of 3000 sweeps for one molecule.Fig. 2e and f demonstrates the average logjJ(V)j sweeps of the rst four sweeps (blue) and last four sweeps (pink).We observe that the high leakage results in the failed molecular diodes (blue, RR < 10), in line with defects in SAMs demonstrated through surface characterization.Nonetheless, aer 150 sweeps, the leakage of the molecular diodes decreases remarkably, resulting in a higher rectication ratio (pink, RR > 100).We plot the histograms of log(RR) collected from 150 sweeps of Au TS -S-B-Fc//Ga 2 O 3 /EGaIn junctions, which show a broad distribution.The noteworthy discreteness in the log(RR) distributions for the rst four and last four sweeps demonstrates a signicant enhancement of rectication for molecular diodes with mixed backbones.
According to eqn (1), dynamically blocking leakage suggests a longer tunneling distance (d), that is, yielding thicker SAMs in the tunneling junction.Thin areas (disordered, buried domains) inducing leakage in molecular diodes can gradually convert to densely packed domains under an electric eld.To validate the more ordered SAMs under an electric eld, we biased the bottom electrode to apply an electric eld on Au TS -S-B1-Fc in aqueous solution and analyze its electrochemical behavior.To avoid cleavage of the Au-S bond and redox reaction of Fc units, we used 1.0 mol L −1 Na 2 SO 4 as the electrolyte and applied bias from +0.3 to −1.3 V, followed by removing Na 2 SO 4 solution and carrying out CV measurements in HClO 4 .The peak split shows that the portion of ordered domains increases to 61.52% from 51.43% aer applying an electric eld on SAMs in aqueous solution, suggesting the formation of more ordered structures under an electric eld (Fig. 2g).

Blocking leakage in failed molecular diodes
For comparison, we introduce molecular diodes containing only exible (B3) or only rigid (B4) backbones (Fig. 3a).Fig.   demonstrates the trend of log(RR) in 150 continuous sweeps (plots with error bars are available in ESI Section 5 †).Table 1 compiles the mean and standard deviations of the initial and ultimate log(RR).The initial RR is about 10 for all the molecular diodes.However, we observe an increase of RR by more than one order of magnitude with molecules B1 and B2, which is absent for molecules B3 and B4.This observation manifests that the addition of mixed backbones can efficiently improve the performance of failed molecular diodes.Furthermore, the corresponding logjJ(V)j reveals that the enhancement of log(RR) is exclusively attributed to the decline of leakage currents (Fig. 3c).The corresponding values of ON currents (logjJ(ON)j) decrease slightly in 150 sweeps.The leakage, however, decreases more signicantly for B1 and B2, in comparison with B3 and B4.We investigate the attenuation of ON currents and block of leakage independently, which rely on electronic structures and supramolecular structures respectively of molecular diodes.

Attenuation of ON currents when applying an electric eld
As mentioned above, ON currents of molecular diodes rely mostly on a proper energy alignment between molecular orbitals and the Fermi level of the electrodes.The continuous application of such a large electric eld at molecular junctions may change the properties of the SAM and SAM//EGaIn interface.Thus, we infer that the attenuation is attributed to the oxidation of the EGaIn tip and can be veried by the transition voltage spectrum.The growing Ga 2 O 3 layer can form a Schottky contact between the top electrode and SAMs and increase the tunneling barrier length, in turn decreasing the current. 46o elucidate the effect of the electric eld on the electronic structure of SAMs, we applied a constant bias of +1.8 V or −1.8 V on the junction for 10 minutes and performed J(V) measurements every 1 minute to study the trend of logjJ(±1.8V)j.Fig. 4 shows that the leakage can be blocked under downward and upward Ẽ.However, under upward Ẽ, there is a slight attenuation of the ON current, logjJ(−1.8V)j, which is absent when applying downward Ẽ.We assume it originates from the oxidation of the EGaIn tip aer a long time scanning under ambient conditions.The transition voltage remains the same when applying downward Ẽ (Fig. 4a). 47In contrast, the absolute value of the transition voltage increases from −0.45 V to −0.63 V aer applying upward Ẽ for 10 min, representing a slight increase in the height of the tunneling barrier (Fig. 4b).Therefore, the attenuation of ON currents originates from an upward electric eld.According to the constant tunneling barrier height, we can safely exclude the effect of the Ga 2 O 3 layer in the blocking of leakage when applying downward Ẽ.

In situ visualization of ordered SAMs in molecular diodes
Previous studies have shown that the tunneling electrons can excite surface plasmons.The number of electroluminescent spots is directly proportional to the number of molecules that conduct current inside SAMs. 19Ordered SAMs can produce more upright molecules to contact the top electrode serving as channels.Electroluminescent measurements provide evidence of the correlation between the applied electrical eld and number of conducting molecules, as indicated by electroluminescent spots.Fig. 5a shows a schematic illustration of the home-built testbeds for conducting electrical and optical measurements simultaneously.The footprint of the EGaIn tip with SAMs is captured through the inverted objective circled in dash line. 48Light emission images were acquired at −1.8 V with an acquisition time of 30 seconds.Initially, for molecular diodes with conformational defects, high leakage currents and a lower rectication ratio are observed (black curve in Fig. 5b), alongside negligible electroluminescent spots and emission intensity (Fig. 5c).However, aer applying downward Ẽ for 1 minute, the number of light-emitting spots increases (Fig. 5d), along with the blocking of leakage and a higher rectication ratio (blue curve in Fig. 5b).The augmentation of conducting molecules is indicative of the dynamic rearrangement of molecules in response to the electric eld.Leakage is more sensitive to the supramolecular structures, showing a more pronounced decrease than J(−1.8V).Therefore, we conclude that, aer applying downward Ẽ, more molecules are forced to adopt upright conformation and become involved with charge transport, thereby blocking leakage more efficiently.
From the preceding section, tunneling junctions with mixed backbones can gradually block tunneling currents through transition into more ordered structures.As for exible molecules with an alkyl chain, the strong chain-chain entanglement increases the total packing energy of SAMs, which requires high energy for conformational transformation.As for rigid backbones, the lack of degrees of freedom of the molecules is unfavorable for conformational change.It is worth noting that dipole moments of SAMs are indispensable for the dynamic blocking of leakage current.The interfacial dipole moments of molecules tend to align perpendicularly with the electric eld, resulting in a "pulling effect" on the molecules to yield thicker SAMs.Thus, molecular diodes can block leakage current gradually under an electric eld.Under upward Ẽ, the Fc units are oxidized to form Fc + .Molecules are inclined to rearrange to minimize Fc + -Fc + electrostatic repulsion, stabilizing the overall energy of junctions.This rearrangement forces the molecules to be more "upright", increasing the tunneling barrier length. 19,21,49Besides, Fc + can rotate to approach the negatively charged top electrode by electrostatic attraction, further increasing the tunneling distance as well. 19In summary, from both scenarios, interfacial dipole moments interact with the electric eld, leading to the rearrangement of molecules and formation of thicker lms in SAMs.Leakage currents, which are highly sensitive to the supramolecular structures of SAMs, are dynamically blocked under the inuence of an electric eld, thereby effectively enhancing the rectication ratio.

Application of rectication enhancement
To thoroughly assess the rectication enhancement and demonstrate that the "mixed backbone" can be used as a general method for the structural design of molecular diodes, we present three distinct experiments that address different scenarios for the application of molecular diodes: (i) Endurance test for enhanced rectication.To determine whether the conformational transformation of SAMs induces a persistent RR enhancement aer the removal of Ẽ, we initially performed 100 sweeps (represented by black spots in Fig. 6b), and subsequently, we lied the tip to break the junction.We then pressed to re-form the junction in the same location and measured 10 J(V) sweeps from the newly formed junctions (represented by colored spots in Fig. 6b).The rectication ratio of these newly formed junctions was found to be similar to the ultimate value achieved aer the initial 100 sweeps.This compelling result suggests that the more ordered structures of SAMs are persistent, leading to a robust RR enhancement.
(ii) Flexible device application.To explore the possibility of using molecular diodes in exible electronic applications, we utilized a home-built bending testbed (Fig. S10 †).In this experiment, we employed PET (polyethylene terephthalate)supported template-stripped gold electrodes, which were bent in a mechanically controlled device with a bending radius of 17.0 mm, as the bottom electrode.By comparing the performance of exible Au-S-B3-Fc//Ga 2 O 3 /EGaIn junctions with that of Au-S-B1-Fc//Ga 2 O 3 /EGaIn molecular diodes, we observed an RR enhancement of approximately ve times for the latter (Fig. 6c).This outcome indicates that molecular diodes exhibit promising characteristics for a wide range of exible and durable electronic applications.
(iii) Feasibility on rough surface.While it is well known that ultra-at bottom electrodes can efficiently block leakage currents in molecular diodes, direct-deposition bottom electrodes (Au DE ), which possess rougher surfaces compared to template-stripped bottom electrodes, oen struggle to effectively block leakage currents.Consequently, the rectication ratio of diodes constructed on such rough electrodes typically remains close to unity. 16In contrast, molecular diodes featuring exible-rigid structures demonstrate more efficient blocking of leakage currents under the inuence of an electric eld.This feature is associated with a signicant enhancement in the rectication ratio, as illustrated in Fig. 6d.

Conclusion
In summary, we demonstrate that the introduction of mixed backbones into SAMs can dynamically block the tunneling currents within molecular junctions.Thin areas in SAMs induce large tunneling currents of molecular junctions.This effect arises from the "pull-up" conformation of molecules under an applied electric eld, which is maintained aer the electric eld is removed.Our CV measurements and in situ electrical elddriven plasmonic excitation measurements provide direct evidence of the rearrangement of SAM structures.CV measurements manifest the increasing portion of ordered phase in SAMs aer applying electric eld in aqueous solution.The increasing number and intensity of electroluminescent spots indicates conformational changes in the SAM structure.Leakage of failed molecular diodes is successfully blocked when applying an electric eld, together with rectication enhancement.Introducing "mixed backbones" compromises the packing energy and degrees of freedom in SAMs, making conformational changes of molecules possible.The blocking of leakage is achieved through the formation of more ordered SAMs structures, facilitated by the electrostatic interaction between dipole moments and electric eld.
Therefore, backbone rearrangement, leading to optimal supramolecular structures, efficiently blocks leakage currents, as we expected, and we can showcase that the incorporation of mixed backbones has extended the applicability of molecular diodes to challenging environments, such as rough surfaces and bending testbeds.

Fig. 1
Fig. 1 (a) Schematic illustration of Au TS -SAM//Ga 2 O 3 /EGaIn molecular junctions.(b) Chemical structures of the thiol precursors.(c) Plots of tunneling current logjJ(+1.0V)j versus sweep number for the molecular junctions.(d-g) logjJ(V)j plots for the molecular junctions at the first sweep and last sweep. densely

Fig. 2
Fig. 2 (a) Schematic illustration of SAM-based molecular diodes.Thiols are composed of mixed backbones with an Fc functional group.(b) Energy level diagram and charge transport of Fc-based diodes.CV measurements and deconvolution of the anodic peaks of (c) Au-S-B1-Fc and (d) Au-S-B2-Fc.Demonstration of the blocking of leakage and corresponding rectification ratio enhancement for (e) Au-S-B1-Fc and (f) Au-S-B2-Fc.Average logjJ(V)j plots of molecular junctions in the first four sweeps and last four sweeps.The distribution of log(RR) for all 150 sweeps and corresponding log (RR) in the first four sweeps and last four sweeps.(g) CV measurements on SAMs are carried out to reflect the transition from thin areas (disordered, buried domains) to ordered phase.Peak split shows the ratio of ordered phase in SAMs. 3b

Fig. 4
Fig. 4 Plots of logjJ(±1.8V)j measured every 1 min when applying (a) downward and (b) upward Ẽ on Au-S-B1-Fc//Ga 2 O 3 /EGaIn junctions and corresponding transition voltage spectrum.The dashed lines correspond to the transition voltage, reflecting the tunneling barrier height in the SAMs//Ga 2 O 3 interface.

Fig. 5
Fig. 5 (a) Schematic illustration of home-built inverted wide-field microscope to conduct electrical and optical measurements simultaneously.Footprint of EGaIn tip with SAMs under bright field inside the dashed circle.Outside area is regarded as background area.(b) J(V) sweeps of Au-S-B1-Fc//Ga 2 O 3 /EGaIn junction before and after applying downward Ẽ. (c) Image of plasmons excited in the molecular junction with conformational defects, whose current rectification ratio is 5.94.Cross-section intensity profiles of plasmons are plotted along the white lines in the background and tip contact areas.(d) Image of plasmons excited in a more ordered phase with much brighter electroluminescent spots, whose current rectification ratio is 27.84.Line scans show the electroluminescent intensity in the background area and EGaIn footprint area.

Fig. 6
Fig. 6 (a) Simple CPK models of Au-S-B1-Fc dimer under different substrates.(b) Successful enhancement of rectification on molecular diodes of Au TS -S-B1-Fc//Ga 2 O 3 /EGaIn in 100 sweeps (black scatters), followed by breaking and forming junctions at the same place five times (colored scatters).(c) Enhancement of log(RR) on bending testbed with bending radius of 17.0 mm.(d) Enhancement of log(RR) can be observed on rough electrodes Au DE (RMS = 1.0 nm) as well.

Table 1
Initial and ultimate log(RR) and corresponding current density