Molecular Tunneling Research Articles

Do Molecules Tunnel through Nanoporous Graphene?

The molecular transport and quantum tunneling of H2 and H2O molecules through nanoporous graphene is studied using computational modeling and first-principles density functional theory. It is demonstrated that molecules with sufficiently high kinetic energies can tunnel through nanopores. It is also demonstrated that molecules can be trapped in front of a nanopore or behind it. These investigations help us learn the behavior of molecules in and around the nanopores of graphene. They also help us learn the fundamentals of molecular tunneling. We believe nanoporous graphene can play important roles for gas separation and nanofiltration.

The complexities of ligand/receptor interactions: Exploring the role of molecular vibrations and quantum tunnelling.

Molecular vibrations and quantum tunneling may link ligand binding to the function of pharmacological receptors. The well-established lock-and-key model explains a ligand's binding and recognition by a receptor; however, a general mechanism by which receptors translate binding into activation, inactivation, or modulation remains elusive. The Vibration Theory of Olfaction was proposed in the 1930s to explain this subset of receptor-mediated phenomena by correlating odorant molecular vibrations to smell, but a mechanism was lacking. In the 1990s, inelastic electron tunneling was proposed as a plausible mechanism for translating molecular vibration to odorant physiology. More recently, studies of ligands' vibrational spectra and the use of deuterated ligand analogs have provided helpful information to study this admittedly controversial hypothesis in metabotropic receptors other than olfactory receptors. In the present work, based in part on published experiments from our laboratory using planarians as an experimental organism, I will present a rationale and possible experimental approach for extending this idea to ligand-gated ion channels.

Photosystem I complexes form remarkably stable self-assembled tunneling junctions.

This paper describes large-area molecular tunneling junctions comprising self-assembled monolayers (SAMs) of light-harvesting protein complexes using eutectic Ga-In (EGaIn) as a top contact. The complexes, which are readily isolable in large quantities from spinach leaves, self-assemble on top of SAMs of [6,6]-phenyl-C61-butyric acid (PCBA) on gold (Au) supported by mica substrates (AuMica), which induces them to adopt a preferred orientation with respect to the electron transport chain that runs across the short axis of each complex, leading to temperature-independent rectification. We compared trimeric protein complexes isolated from thermophilic cyanobacteria to monomeric complexes extracted from spinach leaves by measuring charge-transport at variable temperatures and over the course of at least three months. Transport is independent of temperature in the range of 130 to 310 K for both protein complexes, affirming that the likely mechanism is non-resonant tunneling. The junctions rectified current and were stable for at least three months when stored at room temperature in ambient conditions, with the yield of working junctions falling from 100% to 97% over that time. These results demonstrate a straightforward strategy for forming remarkably robust molecular junctions, avoiding the fragility that is common in molecular electronics.

Fabrication of multi-functional molecular tunnelling junctions by click chemistry.

Continuous advancement in molecular electronics demands increasing functionality and diversity of integrated molecular junctions; however, single-functional molecular junctions fail to meet these requirements. In this article, we propose the use of a widely applicable and efficient click reaction on the surface to modify self-assembled monolayers (SAMs) to achieve multifunctional molecular tunnelling junctions, current rectification and memristance, on a single chip. This approach has allowed us to meet the growing demand for versatility and functionality in molecular electronic devices.

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.

Charge Transport across Proteins inside Proteins: Tunneling across Encapsulin Protein Cages and the Effect of Cargo Proteins.

Charge transport across proteins can be surprisingly efficient over long distances-so-called long-range tunneling-but it is still unclear as to why and under which conditions (e.g., presence of co-factors, type of cargo) the long-range tunneling regime can be accessed. This paper describes molecular tunneling junctions based on an encapsulin (Enc), which is a large protein cage with a diameter of 24 nm that can be loaded with various types of (small) proteins, also referred to as "cargo". We demonstrate with dynamic light scattering, transmission electron microscopy, and atomic force microscopy that Enc, with and without cargo, can be made stable in solution and immobilized on metal electrodes without aggregation. We investigated the electronic properties of Enc in EGaIn-based tunnel junctions (EGaIn = eutectic alloy of Ga and In that is widely used to contact (bio)molecular monolayers) by measuring the current density for a large range of applied bias of ±2.5 V. The encapsulated cargo has an important effect on the electrical properties of the junctions. The measured current densities are higher for junctions with Enc loaded with redox-active cargo (ferritin-like protein) than those junctions without cargo or redox-inactive cargo (green fluorescent protein). These findings open the door to charge transport studies across complex biomolecular hierarchical structures.

Reversing Molecular Dilution Effect by Varying the Bottom Electrode

AbstractThis paper describes charge tunneling across mixed self‐assembled monolayers (SAMs) and investigates the effect of bottom electrode on molecular dilution and tunneling function in large‐area molecular junctions. Binary mixed SAMs are formed with organic molecular diode (2,2′‐bipyridyl‐terminated n‐undecanethiol (SC11BIPY)) and nonrectifying n‐alkanethiolate diluent (SCn; n = 8, 10, 16, 18) via coadsorption on different electrodes, namely Au and Ag. By taking advantage of self‐referencing feature of rectification, the tunneling–molecular dilution relationship is reliably examined. Notably, rectification ratio of mixed SAMs on the Ag electrode decreases with increasing the diluent's length, whereas that of analogous SAMs on the Au electrode oppositely increases. Simulation of rectification using a parallel circuit model and surface analysis with contact angle goniometry evidence that such an opposite trend is dominated by the supramolecular structural order, which arises from the difference in interthiolate distance between Au and Ag. This work demonstrates that the molecular dilution effect on tunneling can be reversed by varying the bottom electrode.

AFM Manipulation of EGaIn Microdroplets to Generate Controlled, On-Demand Contacts on Molecular Self-Assembled Monolayers.

Liquid metal droplets, such as eutectic gallium-indium (EGaIn), are important in many research areas, such as soft electronics, catalysis, and energy storage. Droplet contact on solid surfaces is typically achieved without control over the applied force and without optimizing the wetting properties in different environments (e.g., in air or liquid), resulting in poorly defined contact areas. In this work, we demonstrate the direct manipulation of EGaIn microdroplets using an atomic force microscope (AFM) to generate repeated, on-demand making and breaking of contact on self-assembled monolayers (SAMs) of alkanethiols. The nanoscale positional control and feedback loop in an AFM allow us to control the contact force at the nanonewton level and, consequently, tune the droplet contact areas at the micrometer length scale in both air and ethanol. When submerged in ethanol, the droplets are highly nonwetting, resulting in hysteresis-free contact forces and minimal adhesion; as a result, we are able to create reproducible geometric contact areas of 0.8-4.5 μm2 with the alkanethiolate SAMs in ethanol. In contrast, there is a larger hysteresis in the contact forces and larger adhesion for the same EGaIn droplet in air, which reduced the control over the contact area (4-12 μm2). We demonstrate the usefulness of the technique and of the gained insights in EGaIn contact mechanics by making well-defined molecular tunneling junctions based on alkanethiolate SAMs with small geometric contact areas of between 4 and 12 μm2 in air, 1 to 2 orders of magnitude smaller than previously achieved.

Demonstration of Molecular Tunneling Junctions Based on Vertically Stacked Graphene Heterostructures

We demonstrate the fabrication and complete characterization of vertical molecular tunneling junctions based on graphene heterostructures, which incorporate a control series of arylalkane molecules acting as charge transport barriers. Raman spectroscopy and atomic force microscopy were employed to identify the formation of the molecular monolayer via an electrophilic diazonium reaction on a pre-patterned bottom graphene electrode. The top graphene electrode was transferred to the deposited molecular layer to form a stable electrical connection without filamentary damage. Then, we showed proof of intrinsic charge carrier transport through the arylalkane molecule in the vertical tunneling junctions by carrying out multiprobe approaches combining complementary transport characterization methods, which included length- and temperature-dependent charge transport measurements and transition voltage spectroscopy. Interpretation of all the electrical characterizations was conducted on the basis of intact statistical analysis using a total of 294 fabricated devices. Our results and analysis can provide an objective criterion to validate molecular electronic devices fabricated with graphene electrodes and establish statistically representative junction properties. Since many of the experimental test beds used to examine molecular junctions have generated large variation in the measured data, such a statistical approach is advantageous to identify the meaningful parameters with the data population and describe how the results can be used to characterize the graphene-based molecular junctions.

Controlled Hysteresis of Conductance in Molecular Tunneling Junctions.

The problem this paper addresses is the origin of the hysteretic behavior in two-terminal molecular junctions made from an EGaIn electrode and self-assembled monolayers of alkanethiolates terminated in chelates (transition metal dichlorides complexed with 2,2'-bipyridine; BIPY-MCl2). The hysteresis of conductance displayed by these BIPY-MCl2 junctions changes in magnitude depending on the identity of the metal ion (M) and the window of the applied voltage across the junction. The hysteretic behavior of conductance in these junctions appears only in an incoherent (Fowler-Nordheim) tunneling regime. When the complexed metal ion is Mn(II), Fe(II), Co(II), or Ni(II), both incoherent tunneling and hysteresis are observed for a voltage range between +1.0 V and -1.0 V. When the metal ion is Cr(II) or Cu(II), however, only resonant (one-step) tunneling is observed, and the junctions exhibit no hysteresis and do not enter the incoherent tunneling regime. Using this correlation, the conductance characteristics of BIPY-MCl2 junctions can be controlled. This voltage-induced change of conductance demonstrates a simple, fast, and reversible way (i.e., by changing the applied voltage) to modulate conductance in molecular tunneling junctions.

The influence of water on the charge transport through self-assembled monolayers junctions fabricated by EGaIn technique

Eutectic Gallium-Indium (EGaIn) is a promising technique to fabricate self-assembled monolayers (SAMs)-based molecular devices. However, several abnormal signals had been reported in previous measurements on the as-fabricated devices, like the offset of zero-bias current and others. To date, the limited study has been taken to explore the origin of the offset, which prevents the understanding of electrical transport through large-area molecular tunneling junctions, and hinders the development of high-performance devices. To explore the offset of zero-bias current, we built a new EGaIn automatic instrument platform, which allowed a systematic study on the electrical behavior of SAMs of n-alkanethiolate (n = 14, 16, 18). The measurements showed that the ambient water affected charge transport through as-fabricated molecular junctions. With the presence of water, a primary cell was established at the interface between SAMs and EGaIn/GaOx, which generated an internal potential and caused the offset of zero-bias current when the geometry contact area of the molecular junction is smaller than 600 μm2.

Promotion and suppression of single-molecule conductance by quantum interference in macrocyclic circuits

Promotion and suppression of single-molecule conductance by quantum interference in macrocyclic circuits

A single atom change turns insulating saturated wires into molecular conductors

We present an efficient strategy to modulate tunnelling in molecular junctions by changing the tunnelling decay coefficient, β, by terminal-atom substitution which avoids altering the molecular backbone. By varying X = H, F, Cl, Br, I in junctions with S(CH2)(10-18)X, current densities (J) increase >4 orders of magnitude, creating molecular conductors via reduction of β from 0.75 to 0.25 Å−1. Impedance measurements show tripled dielectric constants (εr) with X = I, reduced HOMO-LUMO gaps and tunnelling-barrier heights, and 5-times reduced contact resistance. These effects alone cannot explain the large change in β. Density-functional theory shows highly localized, X-dependent potential drops at the S(CH2)nX//electrode interface that modifies the tunnelling barrier shape. Commonly-used tunnelling models neglect localized potential drops and changes in εr. Here, we demonstrate experimentally that beta propto 1/sqrt{{varepsilon }_{r}}, suggesting highly-polarizable terminal-atoms act as charge traps and highlighting the need for new charge transport models that account for dielectric effects in molecular tunnelling junctions.

Rectification in Molecular Tunneling Junctions Based on Alkanethiolates with Bipyridine-Metal Complexes.

This paper addresses the mechanism for rectification in molecular tunneling junctions based on alkanethiolates terminated by a bipyridine group complexed with a metal ion, that is, having the structure AuTS-S(CH2)11BIPY-MCl2 (where M = Co or Cu) with a eutectic indium-gallium alloy top contact (EGaIn, 75.5% Ga 24.5% In). Here, AuTS-S(CH2)11BIPY is a self-assembled monolayer (SAM) of an alkanethiolate with 4-methyl-2,2'-bipyridine (BIPY) head groups, on template-stripped gold (AuTS). When the SAM is exposed to cobalt(II) chloride, SAMs of the form AuTS-S(CH2)11BIPY-CoCl2 rectify current with a rectification ratio of r+ = 82.0 at ±1.0 V. The rectification, however, disappears (r+ = 1.0) when the SAM is exposed to copper(II) chloride instead of cobalt. We draw the following conclusions from our experimental results: (i) AuTS-S(CH2)11BIPY-CoCl2 junctions rectify current because only at positive bias (+1.0 V) is there an accessible molecular orbital (the LUMO) on the BIPY-CoCl2 moiety, while at negative bias (-1.0 V), neither the energy level of the HOMO or the LUMO lies between the Fermi levels of the electrodes. (ii) AuTS-S(CH2)11BIPY-CuCl2 junctions do not rectify current because there is an accessible molecular orbital on the BIPY-CuCl2 moiety at both negative and positive bias (the HOMO is accessible at negative bias, and the LUMO is accessible at positive bias). The difference in accessibility of the HOMO levels at -1.0 V causes charge transfer-at negative bias-to take place via Fowler-Nordheim tunneling in BIPY-CoCl2 junctions, and via direct tunneling in BIPY-CuCl2 junctions. This difference in tunneling mechanism at negative bias is the origin of the difference in rectification ratio between BIPY-CoCl2 and BIPY-CuCl2 junctions.

Investigation of dissociative photoionization pathways for orientation-selected molecules by phase-controlled laser fields

Intense (5.0 × 1012 W/cm2) nanosecond phase-controlled laser fields consisting of fundamental and second-harmonic light induce orientation-selective molecular tunneling ionization in a randomly orientated molecular ensemble in a gas phase. The selection of oriented molecules enables one to elucidate dissociative photoionization pathways while eliminating loss of information due to orientational averaging. Here, we have investigated the dissociative ionization of hydrofluorocarbon molecules induced by phase-controlled two-color laser fields. From the phase-dependent behavior of photofragment emission from orientation-selected molecules, dissociation pathways were elucidated experimentally. Bond dissociation energies obtained by quantum chemical calculations support the identified dissociation pathways.

Toward Printed Molecular Electronics: Direct Printing of Liquid Metal Microelectrode on Self‐Assembled Monolayers

AbstractDirect printing of conductive material on ultrathin organic films such as molecular monolayers in a non‐invasive and high‐yielding manner can potentially offer a route to unprecedented merging of molecular electronics with printed electronics. This paper reports on an approach to fabricate arrays of molecular tunnel junctions via automated direct printing of liquid‐metal microelectrode comprising eutectic gallium‐indium onto self‐assembled monolayers of n‐alkanethiolates, as a proof‐of‐concept. The printing design enables production of 350 molecular tunneling junctions per chip of 2 cm × 2 cm template‐stripped substrate and 2800 molecular tunneling junctions per batch of 4 in. diameter Si wafer, which exhibit high yields of working junctions (>80%). Printed molecular junctions have uniform geometrical contact area of ≈4.7 × 103 µm2 and guarantees reproducibility in tunneling performance. The junctions exhibit exponential decay of current density (J, A cm−2) as a function of molecular length, obeying the simplified Simmons model, and single, narrow log‐normal distributions of current density (σlog|J| = ≈0.2–0.5). Importantly, there are no significant differences in tunneling decay coefficient and charge injection current density not only between collections of junctions per printed electrode, but also between batches. Results reported herein promise high‐throughput fabrication of molecular‐scale devices via printed molecular electronics.

In Operando Modulation of Rectification in Molecular Tunneling Junctions Comprising Reconfigurable Molecular Self-Assemblies.

The reconfiguration of molecular tunneling junctions during operation via the self-assembly of bilayers of glycol ethers is described. Well-established functional groups are used to modulate the magnitude and direction of rectification in assembled tunneling junctions by exposing them to solutions containing different glycol ethers. Variable-temperature measurements confirm that rectification occurs by the expected bias-dependent tunneling-hopping mechanism for these functional groups and that glycol ethers, besides being an unusually efficient tunneling medium, behave similarly to alkanes. Memory bits are fabricated from crossbar junctions prepared by injecting eutectic Ga-In (EGaIn) into microfluidic channels. The states of two 8-bit registers were set by trains of droplets such that they are able to perform logical AND operations on bit strings encoded into chemical packets that alter the composition of the crossbar junctions through self-assembly to effect memristor-like properties. This proof-of-concept work demonstrates the potential for fieldable devices based on molecular tunneling junctions comprising self-assembled monolayers andbilayers.

Molecular Fingerprint Detection: Molecular Fingerprint Detection Using Portable Water‐Compatible Electronic Tunneling Spectroscopy Device (Adv. Mater. Interfaces 19/2020)

Nanofloret-based electronic tunneling spectroscopy devices enable detection of molecules by direct measurement of tunneling currents through molecules under ambient conditions. Molecular fingerprint tunneling data is obtained by employing a robust, scalable, portable and water-compatible device. More details can be found in article number 2000605 by Yossi Paltiel, Roie Yerushalmi and co-workers.

Interplay of Fermi Level Pinning, Marcus Inverted Transport, and Orbital Gating in Molecular Tunneling Junctions.

This Letter examines the interplay of important tunneling mechanisms-Fermi level pinning, Marcus inverted transport, and orbital gating-in a molecular rectifier. The temperature dependence of the rectifying molecular junction containing 2,2'-bipyridyl terminated n-alkanethiolate was investigated. A bell-shaped trend of activation energy as a function of applied bias evidenced the dominant occurrence of unusual Marcus inverted transport, while retention of rectification at low temperatures implied that the rectification obeyed the resonant tunneling regime. The results allowed reconciling two separately developed transport models, Marcus-Landauer energetics and Fermi level pinning-based rectification. Our work shows that the internal orbital gating can be substituted with the pinning effect, which pushes the transport mechanism into the Marcus inverted regime.

Intercalation and hybrid heterostructure integration of two-dimensional atomic crystals with functional organic semiconductor molecules

Van der Waals (vdW) integration affords semiconductor heterostructures without constrains of lattice matching and opens up a new realm of functional devices by design. A particularly interesting approach is the electrochemical intercalation of two-dimensional (2D) atomic crystal and formation of superlattices, which can provide scalable production of novel vdW heterostructures. However, this approach has been limited to the use of organic cations with non-functional aliphatic chains, therefore failed to take the advantage of the vast potentials in molecular functionalities (electronic, photonic, magnetic, etc.). Here we report the integration of 2D crystal (MoS2, WS2, highly oriented pyrolytic graphite (HOPG), WSe2 as model systems) with electrochemically inert organic molecules that possess semiconducting characteristics (including perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA), pentacene and fullerene), through on-chip electrochemical intercalation. An unprecedented long-range spatial feature of intercalation has been achieved, which allowed facile assembly of a vertical MoS2-PTCDA-Si junction. The intercalated heterostructure shows significant modulation of the lateral transport, and leads to a molecular tunneling characteristic at the vertical direction. The general intercalation of charge neutral and functional molecules defines a versatile platform of inorganic/organic hybrid vdW heterostructures with significantly extended molecular functional building blocks, holding great promise in future design of nano/quantum devices.