Single-molecule Conductance Research Articles

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

Heteroatom Effects on Quantum Interference in Molecular Junctions: Modulating Antiresonances by Molecular Design.

Controlling charge transport through molecular wires by utilizing quantum interference (QI) is a growing topic in single-molecular electronics. In this article, scanning tunneling microscopy-break junction techniques and density functional theory calculations are employed to investigate the single-molecule conductance properties of four molecules that have been specifically designed to test extended curly arrow rules (ECARs) for predicting QI in molecular junctions. Specifically, for two new isomeric 1-phenylpyrrole derivatives, the conductance pathway between the gold electrodes must pass through a nitrogen atom: this novel feature is designed to maximize the influence of the heteroatom on conductance properties and has not been the subject of prior investigations of QI. It is shown, experimentally and computationally, that the presence of a nitrogen atom in the conductance pathway increases the effect of changing the position of the anchoring group on the phenyl ring from para to meta, in comparison with biphenyl analogues. This effect is explained in terms of destructive QI (DQI) for the meta-connected pyrrole and shifted DQI for the para-connected isomer. These results demonstrate modulation of antiresonances by molecular design and verify the validity of ECARs as a simple “pen-and-paper” method for predicting QI behavior. The principles offer new fundamental insights into structure–property relationships in molecular junctions and can now be exploited in a range of different heterocycles for molecular electronic applications, such as switches based on external gating, or in thermoelectric devices.

Interference Controls Conductance in Phthalocyanine Molecular Junctions

We report an experimental and theoretical study of the single-molecule conductance of two separate families of regioisomeric phthalocyanine derivatives in which the binding groups are connected at opposite or at contiguous positions about the phthalocyanine moiety. We observe that compounds with the longest distance between the anchoring groups yield molecular junctions with a higher conductance compared to those with a shorter distance. We performed both density functional theory (DFT) and tight binding calculations to understand how interference effects explain the experimental results including the role of constructive and destructive quantum interference in five phthalocyanine derivatives.

Probing Interfacial Electronic Effects on Single-Molecule Adsorption Geometry and Electron Transport at Atomically Flat Surfaces.

Clarifying interfacial electronic effects on molecular adsorption is significant in many chemical and biochemical processes. Here, we used STM breaking junction and shell-isolated nanoparticle-enhanced Raman spectroscopy to probe electron transport and adsorption geometries of 4,4'-bipyridine (4,4'-BPY) at Au(111). Modifying the surface with 1-butyl-3-methylimidazolium cation-containing ionic liquids (ILs) decreases surface electron density and stabilizes a vertical orientation of pyridine through nitrogen atom σ-bond interactions, resulting in uniform adsorption configurations for forming molecular junctions. Modulation from vertical, tilted, to flat, is achieved on adding water to ILs, leading to a new peak ascribed to CC stretching of adsorbed pyridyl ring and 316 % modulation of single-molecule conductance. The dihedral angle between adsorbed pyridyl ring and surface decreases with increasing surface electronic density, enhancing electron donation from surface to pyridyl ring.

Using the Electrode Potential to Orient Single Molecules in Nanoscale Electrical Junctions and Determine Charge Transport Mechanism

Charge transport through and between molecules is central to important processes in nature. Studying the conductivity of single molecules can contribute to a better understanding of charge transport, and also help to develop building blocks of molecular electronics, light harvesting devices, etc. We measure the conductivity of molecules using the Scanning Tunneling Microscope break-junction (STM-BJ) method that utilizes repeatedly formed circuits where one or a few molecules are trapped between two electrodes, at least one of which has nanoscale dimensions. The statistical analysis of thousands of measurements yields the conductance of single molecules.One particular interest is the role of the molecule-electrode contact and orientation in charge transport. In the simplest analysis this contact can present a substantial barrier to charge injection, which can have important consequences in devices such as dye sensitized semiconductor nanoparticle solar cells. Our most recent developments include controlling the orientation of the molecule in the junction using the electrode potential so that we can measure conductance along different molecular axes, accessing elements of the anisotropy of the molecular conductance tensor. Specifically, when the molecule is oriented flat, with the molecular plane parallel to the electrode surface, charge transport is measured perpendicular to the molecular plane. Furthermore, the electrode dependence of the molecular conductivity provides insight into the transport mechanism. This approach can be applied to molecules that do not have suitable molecular anchoring groups for traditional STM-BJ measurements, and allows us to determine the importance of anchoring groups in charge transport in molecular junctions.

Electron-Catalyzed Dehydrogenation in a Single-Molecule Junction.

Investigating how electrons propagate through a single molecule is one of the missions of molecular electronics. Electrons, however, are also efficient catalysts for conducting radical reactions, a property that is often overlooked by chemists. Special attention should be paid to electron catalysis when interpreting single-molecule conductance results for the simple reason that an unexpected reaction mediated or triggered by electrons might take place in the single-molecule junction. Here, we describe a counterintuitive structure-property relationship that molecules, both linear and cyclic, employing a saturated bipyridinium-ethane backbone, display a similar conductance signature when compared to junctions formed with molecules containing conjugated bipyridinium-ethene backbones. We describe an ethane-to-ethene transformation, which proceeds in the single-molecule junction by an electron-catalyzed dehydrogenation. Electrochemically based ensemble experiments and theoretical calculations have revealed that the electrons trigger the redox process, and the electric field promotes the dehydrogenation. This finding not only demonstrates the importance of electron catalysis when interpreting experimental results, but also charts a pathway to gaining more insight into the mechanism of electrocatalytic hydrogen production at the single-molecule level.

Organometallic Molecular Wires with Thioacetylene Backbones, trans-{RS-(C≡C)n }2 Ru(phosphine)4 : High Conductance through Non-Aromatic Bridging Linkers.

In this work, the design, synthesis, and single-molecule conductance of ethynyl- and butadiynyl-ruthenium molecular wires with thioether anchor groups [RS=n-C6 H13 S, p-tert-Bu-C6 H4 S), trans-{RS-(C≡C)n }2 Ru(dppe)2 (n=1 (1R ), 2 (2R ); dppe: 1,2-bis(diphenylphosphino)ethane) and trans-(n-C6 H13 S-C≡C)2 Ru{P(OMe)3 }4 3hex ] are reported. Scanning tunneling microscope break-junction study has revealed conductance of the organometallic molecular wires with the thioacetylene backbones higher than that of the related organometallic wires having arylethynylruthenium linkages with the sulfur anchor groups, trans-{p-MeS-C6 H4 -(C≡C)n }2 Ru(phosphine)4 4n (n=1, 2) and trans-(Th-C≡C)2 Ru(phosphine)4 5 (Th=3-thienyl). It should be noted that the molecular junctions constructed from the butadiynyl wire 2R , trans-{Au-RS-(C≡C)2 }2 Ru(dppe)2 (Au: gold metal electrode), show conductance comparable to that of the covalently linked polyynyl wire with the similar molecular length, trans-{Au-(C≡C)3 }2 Ru(dppe)2 63 . The DFT non-equilibrium Green's function (NEGF) study supports the highly conducting nature of the thioacetylene molecular wires through HOMO orbitals.

STM studies of electron transfer through single molecules at electrode-electrolyte interfaces

Electrochemical scanning tunnelling microscopy (EC-STM) is one of the most important tools in modern interfacial electrochemistry. Over several decades it has proven its worth by providing in-situ atomic and molecular scale visualisations of electrode surfaces, but its value goes far beyond topographic imaging. This article discusses the use of EC-STM as a tool for quantitatively studying mechanisms of electron transfer (ET) at electrode-electrolyte interfaces. In combination with theoretical modelling, it has been revealing mechanistic details of charge transfer across molecules at electrochemical interfaces which are of broad interest to a wide range of electrochemists. This article discusses different ways in which the EC-STM can be deployed to monitor charge flow through single molecules and thereby analyse ET mechanisms. Mechanisms covered range from superexchange to 2-step hopping ones such as the Kuznetsov Ulstrup (KU) mechanism. Literature examples of a variety of redox and non-redox molecular targets are used to describe how measurements of single molecule conductance as a function of electrode potential can be used to analyse charge transfer through single molecules. The review finishes by highlighting some of the most recent work and new developments which will ensure that EC-STM will continue to be an important tool for studying ET mechanisms at electrode-electrolyte interfaces.

The Control of Intramolecular Through-Bond and Through-Space Coupling in Single-Molecule Junctions

The Control of Intramolecular Through-Bond and Through-Space Coupling in Single-Molecule Junctions

Tuning Single-Molecule Conductance by Controlled Electric Field-Induced trans-to-cis Isomerisation

External electric fields (EEFs) have proven to be very efficient in catalysing chemical reactions, even those inaccessible via wet-chemical synthesis. At the single-molecule level, oriented EEFs have been successfully used to promote in situ single-molecule reactions in the absence of chemical catalysts. Here, we elucidate the effect of an EEFs on the structure and conductance of a molecular junction. Employing scanning tunnelling microscopy break junction (STM-BJ) experiments, we form and electrically characterize single-molecule junctions of two tetramethyl carotene isomers. Two discrete conductance signatures show up more prominently at low and high applied voltages which are univocally ascribed to the trans and cis isomers of the carotenoid, respectively. The difference in conductance between both cis-/trans- isomers is in concordance with previous predictions considering π-quantum interference due to the presence of a single gauche defect in the trans isomer. Electronic structure calculations suggest that the electric field polarizes the molecule and mixes the excited states. The mixed states have a (spectroscopically) allowed transition and, therefore, can both promote the cis-isomerization of the molecule and participate in electron transport. Our work opens new routes for the in situ control of isomerisation reactions in single-molecule contacts.

Single-molecule Electric Switching Induced by Acid-Base Reaction

Electric switches are one of the most fundamental electric circuitries. Recent developments in single-molecule techniques allow us to study various electric-switching phenomena on the single-molecule scale. In this study, the switch of the current through a single molecule of a biphenothiazine derivative was investigated using the break junction technique. The biphenothiazine derivative undergoes acid-base reaction-induced electronic modulation due to the reversible structural transformation between a closed shell and an open shell, resulting in a change in the electronic gap and effective tunneling barrier height. We succeeded in controlling the single-molecule electric conductivity of the biphenothiazine derivative wired to two Au electrodes by allowing the reaction to proceed reversibly on the electrode.

Giant single-molecule conductance enhancement achieved by strengthening through-space conjugation with thienyls

Summary Exploring single-molecule wires with high conductance is of significant importance for constructing efficient molecular electronic devices. Contrary to the general method of increasing through-bond conjugation in conventional molecular wires, strengthening through-space conjugation is proposed herein to improve molecular conductance. A series of through-space conjugated molecular wires based on hexaphenylbenzene (HPB) are synthesized, and their crystal and electronic structures, conductance behaviors, and working mechanisms are investigated. The scanning tunneling microscopy-break junction technique discloses that, by replacing phenyls with thienyls in HPB, the molecular conductance is apparently boosted, with up to ∼239-fold enhancement, and becomes larger than that of the through-bond conjugated control molecule. The flicker noise analyses and theoretical calculation confirm that improved through-space conjugation with thienyls contributes significantly to high conductance. These findings demonstrate that the introduction of strong through-space conjugation is an effective and feasible approach to attain high molecular conductance.

Functionalized oligoynes: comparison of theoretical parameters with experimental single molecule conductance

The single molecule conductance values of five polyyne molecules (BT[4], BTh[4], CN[4], Py[4], NO2[4]) were reported experimentally previously. In the present study, we compared the experimentally obtained single molecule conductance values of these five molecules with the theoretical parameters which define their single molecule conductance. Molecular orbitals, bond length alternation (BLA), transport barrier, reorganization energy, HOMO-LUMO gap, and excitation energy were the considered theoretical parameters since these parameters were reported to have the potential to influence the single molecule conductance. A modification in BLA occurs when interacting with metal surface due to charge transfer; it was seen that this modified BLA correlates well with the reported single molecule conductance values. Also, BT[4] and Py[4] showed higher binding energies compared to the other cases showing good correlation with the experimental results.

Molecular Structure-(Thermo)electric Property Relationships in Single-Molecule Junctions and Comparisons with Single- and Multiple-Parameter Models.

The most probable single-molecule conductance of each member of a series of 12 conjugated molecular wires, 6 of which contain either a ruthenium or platinum center centrally placed within the backbone, has been determined. The measurement of a small, positive Seebeck coefficient has established that transmission through these molecules takes place by tunneling through the tail of the HOMO resonance near the middle of the HOMO-LUMO gap in each case. Despite the general similarities in the molecular lengths and frontier-orbital compositions, experimental and computationally determined trends in molecular conductance values across this series cannot be satisfactorily explained in terms of commonly discussed "single-parameter" models of junction conductance. Rather, the trends in molecular conductance are better rationalized from consideration of the complete molecular junction, with conductance values well described by transport calculations carried out at the DFT level of theory, on the basis of the Landauer-Büttiker model.

Z-Piezo Pulse-Modulated STM Break Junction: Toward Single-Molecule Rectifiers with Dissimilar Metal Electrodes.

Fabricating single-molecule junctions with asymmetric metal electrodes is significant for realizing single-molecule diodes, but it remains a big challenge. Herein, we develop a z-piezo pulse-modulated scanning tunneling microscopy break junction (STM-BJ) technique to construct a robust asymmetric junction with different metal electrodes. The asymmetric Ag/BPY-EE/Au single-molecule junctions exhibit a middle conductance value in between those of the two individual symmetric metal electrode junctions, which is consistent with the order of calculated energy-dependent transmission coefficient T(E) of the asymmetric junctions at EF. Furthermore, the single-molecule conductance of Ag/BPY-EE/Au decreases by about 70% when reversing the bias voltage from 100 to -100 mV, and a clear asymmetric I-V feature at the single-molecule level is observed for these junctions. This rectifying behavior could be ascribed to a different interfacial coupling of molecules at the two end electrodes, which is confirmed by the different displacement of T(E) at the two bias voltages. Other asymmetric junctions exhibit similar rectifying behavior. The current work provides a feasible way to fabricate hybrid junctions based on asymmetric metal electrodes and investigate their electron transport toward the design of molecular rectifiers.

Single-Molecule Conductance of 1,4-Azaborine Derivatives as Models of BN-doped PAHs.

The single-molecule conductance of a series of BN-acene-like derivatives has been measured by using scanning tunneling break-junction techniques. A strategic design of the target molecules has allowed us to include azaborine units in positions that unambiguously ensure electron transport through both heteroatoms, which is relevant for the development of customized BN-doped nanographenes. We show that the conductance of the anthracene azaborine derivative is comparable to that of the pristine all-carbon anthracene compound. Notably, this heteroatom substitution has also allowed us to perform similar measurements on the corresponding pentacene-like compound, which is found to have a similar conductance, thus evidencing that B-N doping could also be used to stabilize and characterize larger acenes for molecular electronics applications. Our conclusions are supported by state-of-the-art transport calculations.

Effective suppression of conductance in multichannel molecular wires

Although quantum effects have introduced fascinating properties to molecular circuits, exploring new features for molecular circuits remains a challenge. In this work, effective suppression of conductance in the parallel intramolecular circuit is identified, going beyond the well-known superposition effect. By introducing oligo(phenylene ethynylene) (OPE) as a high-conductivity channel (HCC) and [cis-Pt(PEt3)2]2+ coordinated with m-phenylethynyl as a low-conductivity channel (LCC), the single-molecule conductance measured by the scanning tunneling microscopy break junction (STM-BJ) technique demonstrates that, in the multichannel molecular wires, the conductance decreases progressively with the addition of the LCC. A theoretical investigation indicates that all channels participate in electron transport. The conductance suppression in the multichannel structures is ascribable to the higher contribution of the LCC than of the HCC to the conductive orbitals. This finding improves the understanding of electron transport theory at the molecular scale and provides a strategy to design parallel intramolecular circuits.

The Effect of Anchor Group on the Phonon Thermal Conductance of Single Molecule Junctions

There is a worldwide race to convert waste heat to useful energy using thermoelectric materials. Molecules are attractive candidates for thermoelectricity because they can be synthesised with the atomic precision, and intriguing properties due to quantum effects such as quantum interference can be induced at room temperature. Molecules are also expected to show a low thermal conductance that is needed to enhance the performance of thermoelectric materials. Recently, the technological challenge of measuring the thermal conductance of single molecules was overcome. Therefore, it is timely to develop strategies to reduce their thermal conductance for high performance thermoelectricity. In this paper and for the first time, we exploit systematically the effect of anchor groups on the phonon thermal conductance of oligo (phenylene ethynylene) (OPE3) molecules connected to gold electrodes via pyridyl, thiol, methyl sulphide and carbodithioate anchor groups. We show that thermal conductance is affected significantly by the choice of anchor group. The lowest and highest thermal conductances were obtained in the OPE3 with methyl sulphide and carbodithioate anchor groups, respectively. The thermal conductance of OPE3 with thiol anchor was higher than that with methyl sulphide but lower than the OPE3 with pyridyl anchor group.

Selective Anchoring Groups for Molecular Electronic Junctions with ITO Electrodes.

Indium tin oxide (ITO) is an attractive substrate for single-molecule electronics since it is transparent while maintaining electrical conductivity. Although it has been used before as a contacting electrode in single-molecule electrical studies, these studies have been limited to the use of carboxylic acid terminal groups for binding molecular wires to the ITO substrates. There is thus the need to investigate other anchoring groups with potential for binding effectively to ITO. With this aim, we have investigated the single-molecule conductance of a series of eight tolane or "tolane-like" molecular wires with a variety of surface binding groups. We first used gold-molecule-gold junctions to identify promising targets for ITO selectivity. We then assessed the propensity and selectivity of carboxylic acid, cyanoacrylic acid, and pyridinium-squarate to bind to ITO and promote the formation of molecular heterojunctions. We found that pyridinium squarate zwitterions display excellent selectivity for binding to ITO over gold surfaces, with contact resistivity comparable to that of carboxylic acids. These single-molecule experiments are complemented by surface chemical characterization with X-ray photoelectron spectroscopy, quartz crystal microbalance, contact angle determination, and nanolithography using an atomic force miscroscope. Finally, we report the first density-functional theory calculations involving ITO electrodes to model charge transport through ITO-molecule-gold heterojunctions.

Environmental Control of Single-Molecule Junction Evolution and Conductance: A Case Study of Expanded Pyridinium Wiring.

Environmental control of single‐molecule junction evolution and conductance was demonstrated for expanded pyridinium molecules by scanning tunneling microscopy break junction method and interpreted by quantum transport calculations including solvent molecules explicitly. Fully extended and highly conducting molecular junctions prevail in water environment as opposed to short and less conducting junctions formed in non‐solvating mesitylene. A theoretical approach correctly models single‐molecule conductance values considering the experimental junction length. Most pronounced difference in the molecular junction formation and conductance was identified for a molecule with the highest stabilization energy on the gold substrate confirming the importance of molecule–electrode interactions. Presented concept of tuning conductance through molecule–electrode interactions in the solvent‐driven junctions can be used in the development of new molecular electronic devices.