Molecular Tunnel Junctions Research Articles

Molecular Electronics: The Drive Force of Electrical Breakdown of Large‐Area Molecular Tunnel Junctions (Adv. Funct. Mater. 28/2018)

Molecular Electronics: The Drive Force of Electrical Breakdown of Large‐Area Molecular Tunnel Junctions (Adv. Funct. Mater. 28/2018)

The Drive Force of Electrical Breakdown of Large‐Area Molecular Tunnel Junctions

AbstractThe origin of electrical breakdown of molecular tunnel junctions is systematically studied by determination of the breakdown voltages as a function of six types of bottom‐electrodes (Au, Ag, Pt, Pd, Ni, and Cu) and different thickness of self‐assembled monolayers of n‐alkanethiolates, S(CH2)n–1CH3 with n = 2, 4, …, 18, with GaOx/EGaIn top contacts. It is found that at positive bias, the migration of metallic atoms is dominated by the wind force, but, at negative bias, both the wind force and direct force are involved in the mechanism of filament formation. Remarkably, the breakdown voltage is independent of the molecular length for short molecules (n < 10), and the breakdown field could be improved by a factor of ≈2 from 0.80 to 1.5 GV m−1 by replacing the Ag with Pt (or Ni) bottom electrodes. These findings give insights into the design of stable molecular junctions.

Transition from direct to inverted charge transport Marcus regions in molecular junctions via molecular orbital gating.

Solid-state molecular tunnel junctions are often assumed to operate in the Landauer regime, which describes essentially activationless coherent tunnelling processes. In solution, on the other hand, charge transfer is described by Marcus theory, which accounts for thermally activated processes. In practice, however, thermally activated transport phenomena are frequently observed also in solid-state molecular junctions but remain poorly understood. Here, we show experimentally the transition from the Marcus to the inverted Marcus region in a solid-state molecular tunnel junction by means of intra-molecular orbital gating that can be tuned via the chemical structure of the molecule and applied bias. In the inverted Marcus region, charge transport is incoherent, yet virtually independent of temperature. Our experimental results fit well to a theoretical model that combines Landauer and Marcus theories and may have implications for the interpretation of temperature-dependent charge transport measurements in molecular junctions.

Monitoring the Dynamic Process of Formation of Plasmonic Molecular Junctions during Single Nanoparticle Collisions.

The capability to study the dynamic formation of plasmonic molecular junction is of fundamental importance, and it will provide new insights into molecular electronics/plasmonics, single-entity electrochemistry, and nanooptoelectronics. Here, a facile method to form plasmonic molecular junctions is reported by utilizing single gold nanoparticle (NP) collision events at a highly curved gold nanoelectrode modified with a self-assembled monolayer. By using time-resolved electrochemical current measurement and surface-enhanced Raman scattering spectroscopy, the current changes and the evolution of interfacial chemical bonding are successfully observed in the newly formed molecular tunnel junctions during and after the gold NP "hit-n-stay" and "hit-n-run" collision events. The results lead to an in-depth understanding of the single NP motion and the associated molecular level changes during the formation of the plasmonic molecular junctions in a single NP collision event. This method also provides a new platform to study molecular changes at the single molecule level during electron transport in a dynamic molecular tunnel junction.

The transport and optoelectronic properties of γ-graphyne-based molecular magnetic tunnel junctions

By cutting γ-graphyne along different directions, two kinds of γ-graphyne nanodots (γ-GYNDs) can be acquired. In present study, using the γ-GYNDs we theoretically designed and investigated the spin-dependent transport properties of two kinds of molecular magnetic tunnel junctions (MMTJs). Depending on the orientation of γ-GYND and the connection way between γ-GYND and electrodes, our results show that two kinds of MMTJs have different microscopic transport mechanisms. Significant single or dual spin-filtering effects, giant magnetoresistances (reach 108%) and spin negative differential resistances can be observed in the MMTJs. In addition, the spin-polarized optoelectronic properties of the MMTJs have also been discussed, and the results indicate that the spin-polarized photocurrents are dependent on the polarization direction of light and magnetization directions of the electrodes. Especially, two different light-generated spins can be simultaneously produced in the MMTJs if the incident photons have given energies and they flow along opposite directions. The above findings show that the γ-graphyne-based MMTJs can be used as spintronic devices or opto-spintronic devices.

Structural and Charge Transport Properties of Molecular Tunneling Junctions with Single-Layer Graphene Electrodes

Herein, we present Raman spectroscopy of chemically grafting aryl diazonium molecules onto single-layer graphene (SLG) and charge transport characteristics of the aryl alkane monolayers sandwiched between the SLG electrodes in an electrode/monolayers/electrode motif. Our spectroscopic characterization shows that the aryl component molecules were incorporated as an effective tunnel barrier into the SLG electrodes-based molecular junctions. The temperature-variable and length-dependent transport measurements clearly demonstrate that tunneling occurs through the SLG/monolayers/SLG junctions and the current density can be modulated with a correct decay coefficient (β) by the length of a molecular tunneling barrier.

Stable Molecular Diodes Based on π-π Interactions of the Molecular Frontier Orbitals with Graphene Electrodes.

In molecular electronics, it is important to control the strength of the molecule-electrode interaction to balance the trade-off between electronic coupling strength and broadening of the molecular frontier orbitals: too strong coupling results in severe broadening of the molecular orbitals while the molecular orbitals cannot follow the changes in the Fermi levels under applied bias when the coupling is too weak. Here, a platform based on graphene bottom electrodes to which molecules can bind via π-π interactions is reported. These interactions are strong enough to induce electronic function (rectification) while minimizing broadening of the molecular frontier orbitals. Molecular tunnel junctions are fabricated based on self-assembled monolayers (SAMs) of Fc(CH2 )11 X (Fc = ferrocenyl, X = NH2 , Br, or H) on graphene bottom electrodes contacted to eutectic alloy of gallium and indium top electrodes. The Fc units interact more strongly with graphene than the X units resulting in SAMs with the Fc at the bottom of the SAM. The molecular diodes perform well with rectification ratios of 30-40, and they are stable against bias stressing under ambient conditions. Thus, tunnel junctions based on graphene with π-π molecule-electrode coupling are promising platforms to fabricate stable and well-performing molecular diodes.

Controlling destructive quantum interference in tunneling junctions comprising self-assembled monolayers via bond topology and functional groups.

Quantum interference effects (QI) are of interest in nano-scale devices based on molecular tunneling junctions because they can affect conductance exponentially through minor structural changes. However, their utilization requires the prediction and deterministic control over the position and magnitude of QI features, which remains a significant challenge. In this context, we designed and synthesized three benzodithiophenes based molecular wires; one linearly-conjugated, one cross-conjugated and one cross-conjugated quinone. Using eutectic Ga-In (EGaIn) and CP-AFM, we compared them to a well-known anthraquinone in molecular junctions comprising self-assembled monolayers (SAMs). By combining density functional theory and transition voltage spectroscopy, we show that the presence of an interference feature and its position can be controlled independently by manipulating bond topology and electronegativity. This is the first study to separate these two parameters experimentally, demonstrating that the conductance of a tunneling junction depends on the position and depth of a QI feature, both of which can be controlled synthetically.

Understanding interface (odd-even) effects in charge tunneling using a polished EGaIn electrode.

Charge transport across large area molecular tunneling junctions is widely studied due to its potential in the development of quantum electronic devices. Large area junctions based on eutectic gallium indium (used in the form of a conical tip top electrode) have emerged as a reliable platform for delineating structure-property relationships. Discrepancies, however, arise from different tip-morphologies and fabrication techniques. It can be, therefore, challenging to make reliable conclusions based on molecular features. Of particular note is the discrepancy between the behaviors of hydrocarbons containing odd and even numbered carbons across different EGaIn electrodes. Moreover, inconsistencies in tip roughness and oxide thickness can lead to more than a 100× increase in current densities with narrow distribution in data. Besides effects on the precision vs. accuracy of data, a theoretically predicted length-dependent limit to observation of the odd-even effect has not been realized experimentally. We developed a method to chemically polish the EGaIn tip to allow formation of smooth conformal contact due to re-establishment of liquid character at the point of contact though tension-driven reconstruction of a thin oxide layer. To evaluate the polished tip, we measured charge transport behavior across n-alkanethiolate SAMs and observed good correlation in the odd-even oscillation behavior to that observed from wetting studies. Since these molecules are homologues of each other, only differing in the orientation of the terminal CH2CH3 moiety, the odd-even effects are governed by orientation induced differences in the absences of SAM (gauche) defects. Comparison of obtained data with the literature shows significant difference between odd-numbered SAMs across Ag and Au.

Influence of the Contact Area on the Current Density across Molecular Tunneling Junctions Measured with EGaIn Top-Electrodes

This paper describes the relationship between the rates of charge transport (by tunneling) across self-assembled monolayers (SAMs) in a metal/SAM//Ga2O3/EGaIn junction and the geometric contact area (Ag) between the conical Ga2O3/EGaIn top-electrode and the bottom-electrode. Measurements of current density, J(V), across SAMs of decanethiolate on silver demonstrate that J(V) increases with Ag when the contact area is small (Ag < 1000 μm2), but reaches a plateau between 1000 and 4000 μm2, where J(0.5 V) ≈ 10–0.52±0.10 A/cm2. The method used to fabricate Ga2O3/EGaIn electrodes generates a tip whose apex is thicker and rougher than its thin, smoother sides. When Ag is small, the Ga2O3/EGaIn electrode contacts the bottom-electrode principally over this rough apex and forms irreproducible areas of electrical contact. When Ag is large, the contact is through the smoother regions peripheral to the apex and is much more reproducible. Measurements of contact pressure between conical EGaIn electrodes and atomic forc...

Maskless Arbitrary Writing of Molecular Tunnel Junctions.

Since fabricating geometrically well-defined, noninvasive, and compliant electrical contacts over molecular monolayers is difficult, creating molecular-scale electronic devices that function in high yield with good reproducibility is challenging. Moreover, none of the previously reported methods to form organic-electrode contacts at the nanometer and micrometer scales have resulted in directly addressable contacts in an untethered form under ambient conditions without the use of cumbersome equipment and nanolithography. Here we show that in situ encapsulation of a liquid metal (eutectic Ga-In alloy) microelectrode, which is used for junction formation, with a convenient photocurable polymeric scaffold enables untethering of the electrode and direct writing of arbitrary arrays of high-yielding molecular junctions under ambient conditions in a maskless fashion. The formed junctions function in quantitative yields and can afford tunneling currents with high reproducibility; they also function at low temperatures and under bent. The results reported here promise a massively parallel printing technology to construct integrated circuits based on molecular junctions with soft top contacts.

Theory of magnetoresistance of organic molecular tunnel junctions with nonmagnetic electrodes

Large room-temperature magnetoresistance observed for devices composed of self-assembled monolayers of different oligophenylene thiols sandwiched between gold contacts has recently been reported [Z. Xie, S. Shi, F. Liu, D. L. Smith, P. P. Ruden, and C. D. Frisbie, ACS Nano 10, 8571 (2016)]. The transport mechanism through the organic molecules was determined to be nonresonant tunneling. To explain this kind of magnetoresistance, we develop an analytical model based on the interaction of the tunneling charge carrier with an unpaired charge carrier populating a contact-molecule interface state. The Coulomb interaction between carriers causes the transmission coefficients to depend on their relative spin orientation. Singlet and triplet pairing of the tunneling and the interface carriers thus correspond to separate conduction channels with different transmission probabilities. Spin relaxation enabling transitions between the different channels, and therefore tending to maximize the tunneling current for a given applied bias, can be suppressed by relatively small magnetic fields, leading to large magnetoresistance. Our model elucidates how the Coulomb interaction gives rise to transmission probabilities that depend on spin and how an applied magnetic field can inhibit transitions between different spin configurations.

Supramolecular Structure of the Monolayer Triggers Odd–Even Effects in the Tunneling Rates across Noncovalent Junctions on Graphene

Molecular electronics aims to control charge transport at the molecular level, but it is not always clear which factors are important to control because it is difficult to distinguish interface effects from molecular effects. Investigating so-called “odd–even” effects in molecular tunnel junctions provides an opportunity to study molecular effects while keeping the nature of the molecule–electrode interactions unchanged. Odd–even effects in charge tunneling rates have been observed in self-assembled monolayer (SAM)-based tunnel junctions with strong covalent molecule-bottom electrode interactions, but it is not clear whether these odd–even effects originate from the intrinsic properties of the SAM or strong molecule-electrode interactions. Herein, we report tunnel junctions based on SAMs that form on graphene through weak noncovalent interactions (i.e., van der Waals interactions) and also form a van der Waals contact with the top electrode. We found that odd–even effects in charge tunneling rates persist...

The tunneling magnetoresistance and spin-polarized optoelectronic properties of graphyne-based molecular magnetic tunnel junctions

Using density functional theory and the non-equilibrium Green’s function method, we investigate the spin-dependent transport and optoelectronic properties of the graphyne-based molecular magnetic tunnel junctions (MMTJs). We find that these MMTJs exhibit an outstanding tunneling magnetoresistance (TMR) effect. The TMR value is as high as 106%. When the magnetization directions of two electrodes are antiparallel under positive or negative bias voltages, two kinds of pure spin currents can be obtained in the systems. Furthermore, under the irradiation of infrared, visible or ultraviolet light, spin-polarized photocurrents can be generated in the MMTJs, but the corresponding microscopic mechanisms are different. More importantly, if the magnetization directions of two electrodes are antiparallel, the photocurrents with different spins are spatially separated, appearing at different electrodes. This phenomenon provides a new way to simultaneously generate two spin currents.

Effect of Heteroatom Substitution on Transport in Alkanedithiol-Based Molecular Tunnel Junctions: Evidence for Universal Behavior.

The transport properties of molecular junctions based on alkanedithiols with three different methylene chain lengths were compared with junctions based on similar chains wherein every third -CH2- was replaced with O or S, that is, following the general formula HS(CH2CH2X)nCH2CH2SH, where X = CH2, O, or S and n = 1, 2, or 3. Conducting probe atomic force microscopy revealed that the low bias resistance of the chains increased upon substitution in the order CH2 < O < S. This change in resistance is ascribed to the observed identical trend in contact resistance, Rc, whereas the exponential prefactor β (length sensitivity) was essentially the same for all chains. Using an established, analytical single-level model, we computed the effective energy offset εh (i.e., Fermi level relative to the effective HOMO level) and the electronic coupling strength Γ from the current-voltage (I-V) data. The εh values were only weakly affected by heteroatom substitution, whereas the interface coupling strength Γ varied by over an order of magnitude. Consequently, we ascribe the strong variation in Rc to the systematic change in Γ. Quantum chemical calculations reveal that the HOMO density shifts from the terminal SH groups for the alkanedithiols to the heteroatoms in the substituted chains, which provides a plausible explanation for the marked decrease in Γ for the dithiols with electron-rich heteroatoms. The results indicate that the electronic coupling and thus the resistance of alkanedithiols can be tuned by substitution of even a single atom in the middle of the molecule. Importantly, when appropriately normalized, the experimental I-V curves were accurately simulated over the full bias range (±1.5 V) using the single-level model with no adjustable parameters. The data could be collapsed to a single universal curve predicted by the model, providing clear evidence that the essential physics is captured by this analytical approach and supporting its utility for molecular electronics.

Large Magnetoresistance at Room Temperature in Organic Molecular Tunnel Junctions with Nonmagnetic Electrodes.

We report room-temperature resistance changes of up to 30% under weak magnetic fields (0.1 T) for molecular tunnel junctions composed of oligophenylene thiol molecules, 1-2 nm in length, sandwiched between gold contacts. The magnetoresistance (MR) is independent of field orientation and the length of the molecule; it appears to be an interface effect. Theoretical analysis suggests that the source of the MR is a two-carrier (two-hole) interaction at the interface, resulting in spin coupling between the tunneling hole and a localized hole at the Au/molecule contact. Such coupling leads to significantly different singlet and triplet transmission barriers at the interface. Even weak magnetic fields impede spin relaxation processes and thus modify the ratio of holes tunneling via the singlet state versus the triplet state, which leads to the large MR. Overall, the experiments and analysis suggest significant opportunities to explore large MR effects in molecular tunnel junctions based on widely available molecules.

Even the Odd Numbers Help: Failure Modes of SAM-Based Tunnel Junctions Probed via Odd-Even Effects Revealed in Synchrotrons and Supercomputers.

This Account describes a body of research in atomic level design, synthesis, physicochemical characterization, and macroscopic electrical testing of molecular devices made from ferrocene-functionalized alkanethiol molecules, which are molecular diodes, with the aim to identify, and resolve, the failure modes that cause leakage currents. The mismatch in size between the ferrocene headgroup and alkane rod makes waxlike highly dynamic self-assembled monolayers (SAMs) on coinage metals that show remarkable atomic-scale sensitivity in their electrical properties. Our results make clear that molecular tunnel junction devices provide an excellent testbed to probe the electronic and supramolecular structures of SAMs on inorganic substrates. Contacting these SAMs to a eutectic "EGaIn" alloy top-electrode, we designed highly stable long-lived molecular switches of the form electrode-SAM-electrode with robust rectification ratios of up to 3 orders of magnitude. The graphic that accompanies this conspectus displays a computed SAM packing structure, illustrating the lollipop shape of the molecules that gives dynamic SAM supramolecular structures and also the molecule-electrode van der Waals (vdW) contacts that must be controlled to form good SAM-based devices. In this Account, we first trace the evolution of SAM-based electronic devices and rationalize their operation using energy level diagrams. We describe the measurement of device properties using near edge X-ray absorption fine structure spectroscopy, cyclic voltammetry, and X-ray photoelectron spectroscopy complemented by molecular dynamics and electronic structure calculations together with large numbers of electrical measurements. We discuss how data obtained from these combined experimental/simulation codesign studies demonstrate control over the supramolecular and electronic structure of the devices, tuning odd-even effects to optimize inherent packing tendencies of the molecules in order to minimize leakage currents in the junctions. It is now possible, but still very costly to create atomically smooth electrodes and we discuss progress toward masking electrode imperfections using cooperative molecule-electrode contacts that are only accessible by dynamic SAM structures. Finally, the unique ability of SAM devices to achieve simultaneously high and atom-sensitive electrical switching is summarized and discussed. While putting these structures to work as real world electronic devices remains very challenging, we speculate on the scientific and technological advances that are required to further improve electronic and supramolecular structure, toward the creation of high yields of long-lived molecular devices with (very) large, reproducible rectification ratios.

A Single-Level Tunnel Model to Account for Electrical Transport through Single Molecule- and Self-Assembled Monolayer-based Junctions.

We present a theoretical analysis aimed at understanding electrical conduction in molecular tunnel junctions. We focus on discussing the validity of coherent versus incoherent theoretical formulations for single-level tunneling to explain experimental results obtained under a wide range of experimental conditions, including measurements in individual molecules connecting the leads of electromigrated single-electron transistors and junctions of self-assembled monolayers (SAM) of molecules sandwiched between two macroscopic contacts. We show that the restriction of transport through a single level in solid state junctions (no solvent) makes coherent and incoherent tunneling formalisms indistinguishable when only one level participates in transport. Similar to Marcus relaxation processes in wet electrochemistry, the thermal broadening of the Fermi distribution describing the electronic occupation energies in the electrodes accounts for the exponential dependence of the tunneling current on temperature. We demonstrate that a single-level tunnel model satisfactorily explains experimental results obtained in three different molecular junctions (both single-molecule and SAM-based) formed by ferrocene-based molecules. Among other things, we use the model to map the electrostatic potential profile in EGaIn-based SAM junctions in which the ferrocene unit is placed at different positions within the molecule, and we find that electrical screening gives rise to a strongly non-linear profile across the junction.

Electrostatic control over temperature-dependent tunnelling across a single-molecule junction.

Understanding how the mechanism of charge transport through molecular tunnel junctions depends on temperature is crucial to control electronic function in molecular electronic devices. With just a few systems investigated as a function of bias and temperature so far, thermal effects in molecular tunnel junctions remain poorly understood. Here we report a detailed charge transport study of an individual redox-active ferrocene-based molecule over a wide range of temperatures and applied potentials. The results show the temperature dependence of the current to vary strongly as a function of the gate voltage. Specifically, the current across the molecule exponentially increases in the Coulomb blockade regime and decreases at the charge degeneracy points, while remaining temperature-independent at resonance. Our observations can be well accounted for by a formal single-level tunnelling model where the temperature dependence relies on the thermal broadening of the Fermi distributions of the electrons in the leads.

Tunneling Electron Induced Fluorescence from Single Porphyrin Molecules Decoupled by Striped-Phase Octanethiol Self-assembled Monolayer

We investigate tunneling electron induced luminescence from isolated single porphyrin molecules that are decoupled by striped-phase self-assembled monolayer of octanethiol from the underneath Au(111) substrate. Intrinsic single-molecule electroluminescence has been realized by such decoupling at both bias polarities. The photon emission intensity acquired from the molecular lobe is found stronger than that from the molecular center. These results provide useful information on the understanding of electroluminescent behavior and mechanism in molecular tunnel junctions.