Gating of single molecule junction conductance by charge transfer complex formation

Long-Range Gating in Single-Molecule One-Dimensional Topological Insulators

Single-molecule one-dimensional topological insulator (1D TI) is a class of molecular wires that exhibit increasing conductance with wire length. This unique trend is due to the coupling between the two low-lying topological edge states of 1D TIs described by the Su-Schrieffer-Heeger model. In principle, this quantum phenomenon within 1D TIs can be utilized to achieve long-range gating in molecular conductors. Here, we study electron transport through a single-edge state of doubly oxidized oligophenylene bis(triarylamine) to understand the effect of the edge state coupling on conductance. We find that conductance is elevated by approximately 1 order of magnitude compared to a control molecule with the same conductance pathway. Density function theory calculations further support that the increase in conductance is due to the interaction between the edge states of 1D TIs. This work demonstrates a new gating paradigm in molecular electronics, while also providing a deeper understanding of how edge states interact and affect electron transport within 1D TIs.

Fano resonance in molecular junctions of spin crossover complexes

In this paper, we introduce a novel molecular switch paradigm that integrates spin crossover complexes with the Fano resonance effect. Specifically, by performing density-functional theory calculations, the feasibility of achieving Fano resonance using spin crossover complexes is demonstrated in our designed molecular junctions using the complex {Fe[H2B(pz)2]2[Bp(bipy)]} [pz = 1-pyrazolyl, Bp(bipy) = bis(phenylethynyl)(2,2'-bipyridine)]. It is further revealed that the Fano resonance, particularly the Fano dip, is most prominent in the junction with cobalt tips among all the schemes, together with the spin-filtering effect. Most importantly, this junction of cobalt tips is able to exhibit three distinct conductance states, which are controlled by the modulation of Fano resonance due to the spin-state transition of the complex and the applied gate voltage. Such a molecular switch paradigm holds potential for applications in logic gates, memory units, sensors, thermoelectrics, and beyond.

Decoding the mechanical conductance switching behaviors of dipyridyl molecular junctions

Dipyridyl molecular junctions often show intriguing conductance switching behaviors with mechanical modulations, but the mechanisms are still not completely revealed. By applying the ab initio-based adiabatic simulation method, the configuration evolution and electron transport properties of dipyridyl molecular junctions in stretching and compressing processes are systematically investigated. The numerical results reveal that the dipyridyl molecular junctions tend to form specific contact configurations during formation processes. In small electrode gaps, the pyridyls almost vertically adsorb on the second Au layers of the tip electrodes by pushing the top Au atoms aside. These specific contact configurations result in stronger molecule-electrode couplings and larger electronic incident cross-sectional areas, which consequently lead to large breaking forces and high conductance. On further elongating the molecular junctions, the pyridyls shift to the top Au atoms of the tip electrodes. The additional scattering of the top Au atoms dramatically decreases the conductance and switches the molecular junctions to the lower conductive states. Perfect cyclical conductance switches are obtained as observed in the experiments by repeatedly stretching and compressing the molecular junctions. The O atom in the side-group tends to hinder the pyridyl from adsorbing on the second Au layer and further inhibits the conductance switch of the dipyridyl molecular junction.

Characterization and Application of Supramolecular Junctions

The convergence of supramolecular chemistry and single-molecule electronics offers a new perspective on supramolecular electronics, and provides a new avenue toward understanding and application of intermolecular charge transport at the molecular level. In this review, we will provide an overview of the advances in the characterization technique for the investigation of intermolecular charge transport, and summarize the experimental investigation of several non-covalent interactions, including π-π stacking interactions, hydrogen bonding, host-guest interactions and σ-σ interactions at the single-molecule level. We will also provide a perspective on supramolecular electronics and discuss the potential applications and future challenges.

Efficient Intermolecular Charge Transport in π-Stacked Pyridinium Dimers Using Cucurbit[8]uril Supramolecular Complexes

Intermolecular charge transport through π-conjugated molecules plays an essential role in biochemical redox processes and energy storage applications. In this work, we observe highly efficient intermolecular charge transport upon dimerization of pyridinium molecules in the cavity of a synthetic host (cucurbit[8]uril, CB[8]). Stable, homoternary complexes are formed between pyridinium molecules and CB[8] with high binding affinity, resulting in an offset stacked geometry of two pyridiniums inside the host cavity. The charge transport properties of free and dimerized pyridiniums are characterized using a scanning tunneling microscope-break junction (STM-BJ) technique. Our results show that π-stacked pyridinium dimers exhibit comparable molecular conductance to isolated, single pyridinium molecules, despite a longer transport pathway and a switch from intra- to intermolecular charge transport. Control experiments using a CB[8] homologue (cucurbit[7]uril, CB[7]) show that the synthetic host primarily serves to facilitate dimer formation and plays a minimal role on molecular conductance. Molecular modeling using density functional theory (DFT) reveals that pyridinium molecules are planarized upon dimerization inside the host cavity, which facilitates charge transport. In addition, the π-stacked pyridinium dimers possess large intermolecular LUMO-LUMO couplings, leading to enhanced intermolecular charge transport. Overall, this work demonstrates that supramolecular assembly can be used to control intermolecular charge transport in π-stacked molecules.

Tracking Confined Reaction Based on Host-Guest Interaction Using Single-Molecule Conductance Measurement

The host-guest interaction acts as an essential part of supramolecular chemistry, which can be applied in confined reaction. However, it is challenging to obtain the dynamic process during confined reactions below micromolar concentrations. In this work, a new method is provided to characterize the dimerization process of the guest 1,2-bis(4-pyridinyl) ethylene in host cucurbit[8]curil using scanning tunneling microscope-break junction (STM-BJ) technique. The guest reaction kinetics is quantitatively by nuclear magnetic resonance (NMR) and in situ single-molecule junctions. It is found that in the single-molecule conductance measurements, the electrical signals of the reactants with a concentration as low as 5 × 10^-6  m are clearly detected, and the reaction kinetics at micromolar concentrations are further obtained. However, in NMR measurements, the characteristic peak signal of the reactants is undetectable when the concentration of the reactants is lower than 0.5 × 10^-3  m and it cannot be quantified. In addition, the strong electric field from the nanogap accelerates the reaction. This work reveals that single-molecule STM-BJ techniques are more sensitive for tracking confined reactions than that by NMR techniques and can be used to study effect of extremely strong electric field on kinetics.

Determining plasmonic hot-carrier energy distributions via single-molecule transport measurements

Hot carriers in plasmonic nanostructures, generated via plasmon decay, play key roles in applications such as photocatalysis and in photodetectors that circumvent bandgap limitations. However, direct experimental quantification of steady-state energy distributions of hot carriers in nanostructures has so far been lacking. We present transport measurements from single-molecule junctions, created by trapping suitably chosen single molecules between an ultrathin gold film supporting surface plasmon polaritons and a scanning probe tip, that can provide quantification of plasmonic hot-carrier distributions. Our results show that Landau damping is the dominant physical mechanism of hot-carrier generation in nanoscale systems with strong confinement. The technique developed in this work will enable quantification of plasmonic hot-carrier distributions in nanophotonic and plasmonic devices.

Synthetic Control of Quantum Interference by Regulating Charge on a Single Atom in Heteroaromatic Molecular Junctions

A key area of activity in contemporary molecular electronics is the chemical control of conductance of molecular junctions and devices. Here we study and modify a range of pyrrolodipyridines (carbazole-like) molecular wires. We are able to change the electrical conductance and quantum interference patterns by chemically regulating the bridging nitrogen atom in the tricyclic ring system. A series of eight different N-substituted pyrrolodipyridines has been synthesized and subjected to single-molecule electrical characterization using an STM break junction. Correlations of these experimental data with theoretical calculations underline the importance of the pyrrolic nitrogen in facilitating conductance across the molecular bridge and controlling quantum interference. The large chemical modulation for the meta-connected series is not apparent for the para-series, showing the competition between (i) meta-connectivity quantum interference phenomena and (ii) the ability of the pyrrolic nitrogen to facilitate conductance, that can be modulated by chemical substitution.

Single-molecule level control of host-guest interactions in metallocycle-C60 complexes

Host−guest interactions are of central importance in many biological and chemical processes. However, the investigation of the formation and decomplexation of host−guest systems at the single-molecule level has been a challenging task. Here we show that the single-molecule conductance of organoplatinum(II) metallocycle hosts can be enhanced by an order of magnitude by the incorporation of a C₆₀ guest molecule. Mechanically stretching the metallocycle-C₆₀ junction with a scanning tunneling microscopy break junction technique causes the release of the C₆₀ guest from the metallocycle, and consequently the conductance switches back to the free-host level. Metallocycle hosts with different shapes and cavity sizes show different degrees of flexibility to accommodate the C₆₀ guest in response to mechanical stretching. DFT calculations provide further insights into the electronic structures and charge transport properties of the molecular junctions based on metallocycles and the metallocycle-C₆₀ complexes.

Studying the single-molecule behavior of host-guest complexes can provide fundamental insights into their supramolecular interactions. Here, the authors use the scanning tunneling microscopy break junction technique to show that encapsulation of a C₆₀ molecule significantly enhances the conductance of an organoplatinum metallocycle; mechanical stretching of the junction releases the guest, returning the conductance to free-host level.

Charge transfer complexation boosts molecular conductance through Fermi level pinning

Interference features in the transmission spectra can dominate charge transport in metal-molecule-metal junctions when they occur close to the contact Fermi energy (E F). Here, we show that by forming a charge-transfer complex with tetracyanoethylene (TCNE) we can introduce new constructive interference features in the transmission profile of electron-rich, thiophene-based molecular wires that almost coincide with E F. Complexation can result in a large enhancement of junction conductance, with very efficient charge transport even at relatively large molecular lengths. For instance, we report a conductance of 10-3 G 0 (∼78 nS) for the ∼2 nm long α-quaterthiophene:TCNE complex, almost two orders of magnitude higher than the conductance of the bare molecular wire. As the conductance of the complexes is remarkably independent of features such as the molecular backbone and the nature of the contacts to the electrodes, our results strongly suggest that the interference features are consistently pinned near to the Fermi energy of the metallic leads. Theoretical studies indicate that the semi-occupied nature of the charge-transfer orbital is not only important in giving rise to the latter effect, but also could result in spin-dependent transport for the charge-transfer complexes. These results therefore present a simple yet effective way to increase charge transport efficiency in long and poorly conductive molecular wires, with important repercussions in single-entity thermoelectronics and spintronics.

Stable-radicals increase the conductance and Seebeck coefficient of graphene nanoconstrictions

Nanoscale thermoelectricity is an attractive target technology, because it can convert ambient heat into electricity for powering embedded devices in the internet of things. We demonstrate that the thermoelectric performance of graphene nanoconstrictions can be significantly enhanced by the presence of stable radical adsorbates, because radical molecules adsorbed on the graphene nanoconstrictions create singly-occupied orbitals in the vicinity of Fermi energy. This in turn leads to sharp features in their transmission functions close to Fermi energy, which increases the electrical conductance and Seebeck coefficient of the nanoconstrictions. This is a generic feature of radical adsorbates and can be employed in the design of new thermoelectric devices and materials.

Metal bis(acetylide) complex molecular wires: concepts and design strategies

The past decade has seen a remarkable surge in studies of thin-film and single-molecule electronics, due in no small part to the development and advancement of experimental methods for the construction and measurement of metal|molecule|metal junctions. Within the plethora of molecular structures that have been investigated, metal complexes of general form trans-M(C[triple bond, length as m-dash]CR)2(Ln) have attracted attention from the inorganic and organometallic chemistry community in the search for efficient molecular wires due to the potential π-d-π orbital mixing along the molecular backbone. In this article progress towards this goal will be summarised, and design strategies for future molecular components discussed.

Probing Lewis acid-base interactions in single-molecule junctions

A novel strategy to regulate the tunneling mechanism for charge transport through an organoborane wire via Lewis acid-base interactions has been developed. A change from LUMO- to HOMO-dominated charge transport upon the addition of the fluoride is verified both experimentally and theoretically.

Probing charge transfer dynamics in a single iron tetraphenylporphyrin dyad adsorbed on an insulating surface

Although the dynamics of charge transfer (CT) processes can be probed with ultimate lifetime resolution, the inability to control CT at the nanoscale is one of the most important roadblocks to revealing some of its deep fundamental aspects. In this work, we present an investigation of CT dynamics in a single iron tetraphenylporphyrin (Fe-TPP) donor/acceptor dyad adsorbed on a CaF2/Si(100) insulating surface. The tip of a scanning tunneling microscope (STM) is used to create local ionic states in one fragment of the dyad. The CT process is monitored by imaging subsequent changes in the neighbor acceptor molecule and its efficiency is mapped, revealing the influence of the initial excited state in the donor molecule. In the validation of the experiments, simulations based on density functional theory show that holes have a higher donor-acceptor CT rate compared to electrons and highlight a noticeable initial state dependence on the CT process. We leverage the unprecedented spatial resolution achieved in our experiments to show that the CT process in the dyad is governed via molecule-molecule coherent tunneling with negligible surface-mediated character.

Influence of Quantum Interference on the Thermoelectric Properties of Molecular Junctions

Molecular junctions offer unique opportunities for controlling charge transport on the atomic scale and for studying energy conversion. For example, quantum interference effects in molecular junctions have been proposed as an avenue for highly efficient thermoelectric power conversion at room temperature. Toward this goal, we investigated the effect of quantum interference on the thermoelectric properties of molecular junctions. Specifically, we employed oligo(phenylene ethynylene) (OPE) derivatives with a para-connected central phenyl ring ( para-OPE3) and meta-connected central ring ( meta-OPE3), which both covalently bind to gold via sulfur anchoring atoms located at their ends. In agreement with predictions from ab initio modeling, our experiments on both single molecules and monolayers show that meta-OPE3 junctions, which are expected to exhibit destructive interference effects, yield a higher thermopower (with ∼20 μV/K) compared with para-OPE3 (with ∼10 μV/K). Our results show that quantum interference effects can indeed be employed to enhance the thermoelectric properties of molecular junctions.

Precise tuning of single molecule conductance in an electrochemical environment

Tuning of molecular conductance in a liquid environment is a hot topic in molecular electronics. In this article, we explore a new concept where the Fermi level positions of the metallic ends are varied simply by modifying the electroactive salt concentration in solution. We rely on the electrochemical scanning tunneling microscope break junction method that allows the construction in solution of copper atomic contacts that can be then bridged by single molecules. The experimental conductance evolution is first confronted with an analytical formulation that allows the deduction of the molecule's LUMO position and electronic coupling factors. These parameters are in close agreement with those obtained by independent DFT calculations.

Current's Fluctuations through Molecular Wires Composed of Thiophene Rings

We study theoretically the electronic transport and quantum fluctuations in single-molecule systems using thiophene rings as integrated elementary functions, as well as the dependence of these properties with the increase of the coupled rings, i.e., as a quantum wire. In order to analyze the current flow through these molecular systems, the thiophene rings are considered to be connected to metal contacts, which, in general terms, will be related to the application of voltages (bias voltages or gate voltages) to generate non-equilibrium behavior between the contacts. Due to the nonlinear behavior that is generated when said voltages are applied, it is possible to observe quantum fluctuations in the transport properties of these molecular wires. For the calculation of the transport properties, we applied a tight-binding approach using the Landauer–Büttiker formalism and the Fischer–Lee relationship, by means of a semi-analytic Green’s function method within a real-space renormalization (decimation procedure). Our results showed an excellent agreement with results using a tight-binding model with a minimal number of parameters reported so far for these molecular systems.

Gateway state-mediated, long-range tunnelling in molecular wires

If the factors controlling the decay in single-molecule electrical conductance G with molecular length L could be understood and controlled, then this would be a significant step forward in the design of high-conductance molecular wires. For a wide variety of molecules conducting by phase coherent tunnelling, conductance G decays with length following the relationship G = Ae^-βL. It is widely accepted that the attenuation coefficient β is determined by the position of the Fermi energy of the electrodes relative to the energy of frontier orbitals of the molecular bridge, whereas the terminal anchor groups which bind to the molecule to the electrodes contribute to the pre-exponential factor A. We examine this premise for several series of molecules which contain a central conjugated moiety (phenyl, viologen or α-terthiophene) connected on either side to alkane chains of varying length, with each end terminated by thiol or thiomethyl anchor groups. In contrast with this expectation, we demonstrate both experimentally and theoretically that additional electronic states located on thiol anchor groups can significantly decrease the value of β, by giving rise to resonances close to E_F through coupling to the bridge moiety. This interplay between the gateway states and their coupling to a central conjugated moiety in the molecular bridges creates a new design strategy for realising higher-transmission molecular wires by taking advantage of the electrode-molecule interface properties.

Insulated molecular wires: inhibiting orthogonal contacts in metal complex based molecular junctions

Metal complexes are receiving increased attention as molecular wires in fundamental studies of the transport properties of metal|molecule|metal junctions. In this context we report the single-molecule conductance of a systematic series of d⁸ square-planar platinum(ii) trans-bis(alkynyl) complexes with terminal trimethylsilylethynyl (C[triple bond, length as m-dash]CSiMe₃) contacting groups, e.g. trans-Pt{C[triple bond, length as m-dash]CC₆H₄C[triple bond, length as m-dash]CSiMe₃}₂(PR₃)₂ (R = Ph or Et), using a combination of scanning tunneling microscopy (STM) experiments in solution and theoretical calculations using density functional theory and non-equilibrium Green's function formalism. The measured conductance values of the complexes (ca. 3-5 × 10^-5G₀) are commensurate with similarly structured all-organic oligo(phenylene ethynylene) and oligo(yne) compounds. Based on conductance and break-off distance data, we demonstrate that a PPh₃ supporting ligand in the platinum complexes can provide an alternative contact point for the STM tip in the molecular junctions, orthogonal to the terminal C[triple bond, length as m-dash]CSiMe₃ group. The attachment of hexyloxy side chains to the diethynylbenzene ligands, e.g. trans-Pt{C[triple bond, length as m-dash]CC₆H₂(Ohex)₂C[triple bond, length as m-dash]CSiMe₃}₂(PPh₃)₂ (Ohex = OC₆H₁₃), hinders contact of the STM tip to the PPh₃ groups and effectively insulates the molecule, allowing the conductance through the full length of the backbone to be reliably measured. The use of trialkylphosphine (PEt₃), rather than triarylphosphine (PPh₃), ancillary ligands at platinum also eliminates these orthogonal contacts. These results have significant implications for the future design of organometallic complexes for studies in molecular junctions.

Supramolecular Systems and Chemical Reactions in Single-Molecule Break Junctions

The major challenges of molecular electronics are the understanding and manipulation of the electron transport through the single-molecule junction. With the single-molecule break junction techniques, including scanning tunneling microscope break junction technique and mechanically controllable break junction technique, the charge transport through various single-molecule and supramolecular junctions has been studied during the dynamic fabrication and continuous characterization of molecular junctions. This review starts from the charge transport characterization of supramolecular junctions through a variety of noncovalent interactions, such as hydrogen bond, π-π interaction, and electrostatic force. We further review the recent progress in constructing highly conductive molecular junctions via chemical reactions, the response of molecular junctions to external stimuli, as well as the application of break junction techniques in controlling and monitoring chemical reactions in situ. We suggest that beyond the measurement of single molecular conductance, the single-molecule break junction techniques provide a promising access to study molecular assembly and chemical reactions at the single-molecule scale.

Discriminating single-molecule sensing by crown-ether-based molecular junctions

Crown-ether molecules are well known to selectively bind alkali atoms, so by incorporating these within wires, any change in electrical conductance of the wire upon binding leads to discriminating sensing. Using a density functional theory-based approach to quantum transport, we investigate the potential sensing capabilities of single-molecule junctions formed from crown ethers attached to anthraquinone units, which are in turn attached to gold electrodes via alkyl chains. We calculate the change in electrical conductance for binding of three different alkali ions (lithium, sodium, and potassium). Depending on the nature of the ionic analyte, the conductance is enhanced by different amounts. This change in electrical conductance is due to charge transfer from the ion to molecular wire causing the molecular resonances to shift closer to the electrode Fermi energy.

Single-Molecule Transport at a Rectifying GaAs Contact

In most single- or few-molecule devices, the contact electrodes are simple ohmic resistors. Here we describe a new type of single-molecule device in which metal and semiconductor contact electrodes impart a function, namely, current rectification, which is then modified by a molecule bridging the gap. We study junctions with the structure Au STM tip/X/n-GaAs substrate, where "X" is either a simple alkanedithiol or a conjugated unit bearing thiol/methylthiol contacts, and we detect current jumps corresponding to the attachment and detachment of single molecules. From the magnitudes of the current jumps we can deduce values for the conductance decay constant with molecule length that agree well with values determined from Au/molecule/Au junctions. The ability to impart functionality to a single-molecule device through the properties of the contacts as well as through the properties of the molecule represents a significant extension of the single-molecule electronics "tool-box".

Modulation and Control of Charge Transport Through Single-Molecule Junctions

The ability to modulate and control charge transport though single-molecule junction devices is crucial to achieving the ultimate goal of molecular electronics: constructing real-world-applicable electronic components from single molecules. This review aims to highlight the progress made in single-molecule electronics, emphasizing the development of molecular junction electronics in recent years. Among many techniques that attempt to wire a molecule to metallic electrodes, the single-molecule break junction (SMBJ) technique is one of the most reliable and tunable experimental platforms for achieving metal-molecule-metal configurations. It also provides great freedom to tune charge transport through the junction. Soon after the SMBJ technique was introduced, it was extensively used to measure the conductances of individual molecules; however, different conductances were obtained for the same molecule, and it proved difficult to interpret this wide distribution of experimental data. This phenomenon was later found to be mainly due to a lack of precise experimental control and advanced data analysis methods. In recent years, researchers have directed considerable effort into advancing the SMBJ technique by gaining a deeper physical understanding of charge transport through single molecules and thus enhancing its potential applicability in functional molecular-scale electronic devices, such as molecular diodes and molecular transistors. In parallel with that research, novel data analysis methods and approaches that enable the discovery of hidden yet important features in the data are being developed. This review discusses various aspects of molecular junction electronics, from the initial goal of molecular electronics, the development of experimental techniques for creating single-molecule junctions and determining single-molecule conductance, to the characterization of functional current-voltage features and the investigation of physical properties other than charge transport. In addition, the development of advanced data analysis methods is considered, as they are critical to gaining detailed physical insight into the underlying transport mechanisms.

Single-Molecule Conductance of Viologen-Cucurbit[8]uril Host-Guest Complexes

The local molecular environment is a critical factor which should be taken into account when measuring single-molecule electrical properties in condensed media or in the design of future molecular electronic or single molecule sensing devices. Supramolecular interactions can be used to control the local environment in molecular assemblies and have been used to create microenvironments, for instance, for chemical reactions. Here, we use supramolecular interactions to create microenvironments which influence the electrical conductance of single molecule wires. Cucurbit[8]uril (CB[8]) with a large hydrophobic cavity was used to host the viologen (bipyridinium) molecular wires forming a 1:1 supramolecular complex. Significant increases in the viologen wire single molecule conductances are observed when it is threaded into CB[8] due to large changes of the molecular microenvironment. The results were interpreted within the framework of a Marcus-type model for electron transfer as arising from a reduction in outer-sphere reorganization energy when the viologen is confined within the hydrophobic CB[8] cavity.

Evidence for a hopping mechanism in metal|single molecule|metal junctions involving conjugated metal–terpyridyl complexes; potential-dependent conductances of complexes [M(pyterpy)2]2+ (M = Co and Fe; pyterpy = 4′-(pyridin-4-yl)-2,2′:6′,2′′-terpyridine) in ionic liquid

Extensive studies of various families of conjugated molecules in metal|molecule|metal junctions suggest that the mechanism of conductance is usually tunnelling for molecular lengths < ca. 4 nm, and that for longer molecules, coherence is lost as a hopping element becomes more significant. In this work we present evidence that, for a family of conjugated, redox-active metal complexes, hopping may be a significant factor for even the shortest molecule studied (ca. 1 nm between contact atoms). The length dependence of conductance for two series of such complexes which differ essentially in the number of conjugated 1,4-C₆H₄- rings in the structures has been studied, and it is found that the junction conductances vary linearly with molecular length, consistent with a hopping mechanism, whereas there is significant deviation from linearity in plots of log(conductance) vs. length that would be characteristic of tunnelling, and the slopes of the log(conductance)–length plots are much smaller than expected for an oligophenyl system. Moreover, the conductances of molecular junctions involving the redox–active molecules, [M(pyterpy)₂]^2+/3+ (M = Co, Fe) have been studied as a function of electrochemical potential in ionic liquid electrolyte, and the conductance–overpotential relationship is found to fit well with the Kuznetsov–Ulstrup relationship, which is essentially a hopping description.