Single-molecule Conductance Research Articles

Transition from stochastic events to deterministic ensemble average in electron transfer reactions revealed by single-molecule conductance measurement

Electron transfer reactions can now be followed at the single-molecule level, but the connection between the microscopic and macroscopic data remains to be understood. By monitoring the conductance of a single molecule, we show that the individual electron transfer reaction events are stochastic and manifested as large conductance fluctuations. The fluctuation probability follows first-order kinetics with potential dependent rate constants described by the Butler-Volmer relation. Ensemble averaging of many individual reaction events leads to a deterministic dependence of the conductance on the external electrochemical potential that follows the Nernst equation. This study discloses a systematic transition from stochastic kinetics of individual reaction events to deterministic thermodynamics of ensemble averages and provides insights into electron transfer processes of small systems, consisting of a single molecule or a small number of molecules.

Investigation of the charge transport in model single molecule junctions based on expanded bipyridinium molecular conductors

In this work the charge transport and energetics of a photochemically addressable single molecule switch based on the expanded bipyridinium core linked to an anchoring group with a controlled torsion angle θ was investigated. Electrochemical and UV-VIS absorption spectroscopy techniques complemented by the analysis based on density functional theory (DFT) show that for selected molecules the energy and shape of the LUMO is insensitive to the value of θ, but the difference in torsion angle θ leads to a sizable shift of the LUMO energy and single molecule conductance value in a metal-molecule-metal junction for these molecules as shown by the combination of experimental single molecule break junction technique and theoretical non-equilibrium Green's function (NEGF)/DFT approach. The conductance switching ratio calculated from the cos2 θ law is in a perfect agreement with the value obtained from the NEGF approach. Our combined experimental and theoretical approach paves the way for investigating expanded bipyridinium systems with multiple photochemically addressable units potentially achieving greater conductance switching ratios.

Tuneable single-molecule electronic conductance of C60 by encapsulation.

It has been demonstrated that the single-molecule transport properties of fullerene C60 can be modulated by encapsulating endohedral species, i.e. Li+ and H2O, which exhibit different degrees of van der Waals interactions with the C60 cage. Single-molecule junctions were prepared between the gaps of Au electrodes using a break junction technique. Encapsulation of H2O inside the cage caused a slight decrease in the electronic conductivity relative to that of pristine C60. This is in sharp contrast to Li+ encapsulation, which results in a twofold-to-fourfold increase in the conductivity. The electronic couplings between the C60 cage and the Au electrodes were weakly dependent on the endohedral species in the cage, though the molecular orbital energy levels were remarkably modulated upon encapsulation.

Single-molecule conductance oscillations in alkane rings

We investigate the single-molecule electrical conductance of alkane rings connected to gold electrodes and demonstrate that their logarithmic conductances are ocillatory functions of length.

Single-molecule conductance investigation of BDT derivatives: an additional pattern found to induce through-space channels beyond π-π stacking.

The through-space conductance of individual molecules is supposed to improve the macroscopic carrier movement, but the most widely acclaimed through-space conductance channel just existed in sufficiently close π-π stacked benzene rings. As a breakthrough to this primary cognition, additional conducting channels were confirmed to exist in non-strict face-to-face aligned thiophenes or phenyl-thiophene in BDT derivatives for the first time.

Methods of Synthesis and Characterization of Conductive DNA Nanowires Based on Metal Ion-Mediated Base Pairing for Single-Molecule Electronics

Advances in the field of molecular electronics have made possible the directmeasurement of charge transport across single molecules. In particular, workon DNA oligomers has demonstrated that this weakly-conducting biomoleculecan be functionalized through metal-mediated nucleobase pairing in order tosignificantly increase electron mobility across the molecule. The introductionof interacting stacks of single metal ions inside the DNA helix is an attractiveplatform for assay and optimization; for this reason we present a protocolfor the production and processing of nanowires with a metal base pair forsingle-molecule applications. In particular, we describe the construction of DNA duplex wires with a cytosine-Ag+-cytosine base pair (dC:Ag+:dC).A thorough investigation of buffer components suggests the use of divalentmagnesium counterions to stabilize highly mismatched oligonucleotides insolution. We further analyse cleaning and processing of thin gold films forbatch-fabrication of conductive imaging substrates for use in conductivescanning probe assays of single-molecule conductivity. With a clear path toelectrical assays, we suggest that the C:Ag+:C orthogonal nucleotide pair andother similar chemistries may provide a foundation for molecular electroniccomponents in integrated devices

Parity symmetry in a number of problems of quantum and structural chemistry

A synthetic review and new results are given of the alternant symmetry theory and its applications within a unified approach. It is based on J–symmetry (parity) operators. Unlike usual commutation rules, these symmetry operators anticommute with Hamiltonians or other relevant quantities. In the J–symmetry terms we treat a variety of problems and topics, mainly related to π-shells of conjugated molecules. In particular, various orbital theories are outlined with a systematic use of block-matrix technique (density matrices, operator functions etc.). Noval π‑models and their J–symmetry are studied within the current context of single-molecule conductance and the relevant problems concerning Green’s function and electron transmission evaluation. We stress on the key importance of account for π-electron correlation for describing correctly transmission π-spectra. We discuss electron-structure peculiarities of alternant radical states and the validity of the Lieb-Ovchinnikov spin rule resulting from the J–symmetry and electron correlation effects. It is shown how the simplified (based on Hückel’s MOs) spin-polarized theory provides a correct number of effectively unpaired electrons in polyradicaloid alternant molecules. Another type of problems is concerned with chirality (generllly, structural asymmetry) problems. By spectral analysys of the previously defined chirality operator we could reinterpret the problem in terms of J–symmetry. It allowed us to construct here the noval chirality operator which is nonnegative definite and vanishes on achiral structures. Its simplest invariant, the matrix trace, surves us as a quantitative measure of the structural (electronic) chirality. Preliminary calculations tell us that the new chirality index behaves reasonably even for the difficult (high-symmetry) chiral systems.

Quantum Interference Effects in Charge Transport through Single-Molecule Junctions: Detection, Manipulation, and Application.

Quantum interference effects (QIEs), which offer unique opportunities for the fine-tuning of charge transport through molecular building blocks by constructive or destructive quantum interference, have become an emerging area in single-molecule electronics. Benefiting from the QIEs, charge transport through molecular systems can be controlled through minor structural and environmental variations, which cause various charge transport states to be significantly changed from conductive to insulative states and offer promising applications in future functional single-molecule devices. Although QIEs were predicted by theoreticians more than two decades ago, only since 2011 have the challenges in ultralow conductance detection originating from destructive quantum interference been overcome experimentally. Currently, a series of single-molecule conductance investigations have been carried out experimentally to detect constructive and destructive QIEs in charge transport through various types of molecular junctions by altering molecular patterns and connectivities. Furthermore, the use of QIEs to tune the properties of charge transport through single-molecule junctions using external gating shows vital potential in future molecular electronic devices. The experimental and theoretical investigations of QIEs offer new fundamental understanding of the structural-electronic relationships in molecular devices and materials at the nanoscale. In this Account, we discuss our progress toward the experimental detection, manipulation, and further application of QIEs in charge transport through single-molecule junctions. These experiments were carried out continuously in our previous group at the University of Bern and in our lab at Xiamen University. As a result of the development of mechanically controllable break junction (MCBJ) and scanning tunneling microscope break junction (STM-BJ) techniques, we could detect ultralow charge transport through the cross-conjugated anthraquinone center, which was one of the earliest experimental studies of QIEs. In close cooperation with organic chemists and theoretical physicists, we systematically investigated charge transport through single-molecule junctions originating from QIEs in conjugated centers ranging from simple single benzene to polycyclic aromatic hydrocarbons (PAHs), heteroaromatics, and even complicated metalla-aromatics at room temperature. Then we further investigated the quantitative correlation between molecular structure and quantum interference by altering different molecular patterns and connectivities in homologous series of PAHs and heteroatom systems. Additionally, external chemical and electrochemical gating of single-molecule devices can be used for direct QIE manipulation via not only tuning molecular conjugation but also shifting the electrode Fermi level. Our study further suggested that distinguishable differences in conductance resulting from QIEs offer opportunities to detect photothermal reaction kinetics and to recognize isomers at the single-molecule scale. These investigations demonstrate the universality of QIEs in charge transport through various molecular building blocks. Moreover, effective manipulation of QIEs leads to various novel phenomena and promising applications in molecular electronic devices.

The Bicyclo[2.2.2]octane Motif: A Class of Saturated Group 14 Quantum Interference Based Single-Molecule Insulators.

The electronic transmission through σ-conjugated molecules can be fully suppressed by destructive quantum interference, which makes them potential candidates for single-molecule insulators. The first molecule with clear suppression of the single-molecule conductance due to σ-interference was recently found in the form of a functionalized bicyclo[2.2.2]octasilane. Here we continue the search for potential single-molecule insulators based on saturated group 14 molecules. Using a high-throughput screening approach, we assess the electron transport properties of the bicyclo[2.2.2]octane class by systematically varying the constituent atoms between carbon, silicon, and germanium, thus exploring the full chemical space of 771 different molecules. The majority of the molecules in the bicyclo[2.2.2]octane class are found to be highly insulating molecules. Though the all-silicon molecule is a clear-cut case of σ-interference, it is not unique within its class and there are many potential molecules that we predict to be more insulating. The finding of this class of quantum interference based single-molecule insulators indicates that a broad range of highly insulating saturated group 14 molecules are likely to exist.

Detection and identification of genetic material via single-molecule conductance.

The ongoing discoveries of RNA modalities (for example, non-coding, micro and enhancer) have resulted in an increased desire for detecting, sequencing and identifying RNA segments for applications in food safety, water and environmental protection, plant and animal pathology, clinical diagnosis and research, and bio-security. Here, we demonstrate that single-molecule conductance techniques can be used to extract biologically relevant information from short RNA oligonucleotides, that these measurements are sensitive to attomolar target concentrations, that they are capable of being multiplexed, and that they can detect targets of interest in the presence of other, possibly interfering, RNA sequences. We also demonstrate that the charge transport properties of RNA:DNA hybrids are sensitive to single-nucleotide polymorphisms, thus enabling differentiation between specific serotypes of Escherichia coli. Using a combination of spectroscopic and computational approaches, we determine that the conductance sensitivity primarily arises from the effects that the mutations have on the conformational structure of the molecules, rather than from the direct chemical substitutions. We believe that this approach can be further developed to make an electrically based sensor for diagnostic purposes.

Large Variations in the Single-Molecule Conductance of Cyclic and Bicyclic Silanes.

Linear silanes are efficient molecular wires due to strong σ-conjugation in the transoid conformation; however, the structure-function relationship for the conformational dependence of the single-molecule conductance of silanes remains untested. Here we report the syntheses, electrical measurements, and theoretical characterization of four series of functionalized cyclic and bicyclic silanes including a cyclotetrasilane, a cyclopentasilane, a bicyclo[2.2.1]heptasilane, and a bicyclo[2.2.2]octasilane, which are all extended by linear silicon linkers of varying length. We find an unusual variation of the single-molecule conductance among the four series at each linker length. We determine the relative conductance of the (bi)cyclic silicon structures by using the common length dependence of the four series rather than comparing the conductance at a single length. In contrast with the cyclic π-conjugated molecules, the conductance of σ-conjugated (bi)cyclic silanes is dominated by a single path through the molecule and is controlled by the dihedral angles along this path. This strong sensitivity to molecular conformation dictates the single-molecule conductance of σ-conjugated silanes and allows for systematic control of the conductance through molecular design.

Synthesis of Functional Carbo-benzenes with Functional Properties: The C2 Tether Key

Beyond demonstration of conceptual relevance and synthetic feasibility of aryl/alkyl-substituted representatives, carbo-benzene molecules started to gain prospects of broader impact through the emergence of alkynyl derivatives. This is first illustrated by examples of di- and hexaalkynyl-carbo-benzenes, a carbo-naphthalene, a carbo-biphenyl, and two carbo-terphenyls. A focus is then given to dialkynyl derivatives by reference to the peripherally C2-extruded parents. In the centro­symmetric quadrupolar series, the C2 expansion or ethynylogation effect is more particularly considered for 9H-fluoren-2-yl, tris(O-n-­alkyl)pyrogallyl, indol-3-yl, 4-anilinyl, and tetraphenyl-carbo-phenyl substituents on the following respective properties: two-photon absorption, chemical stability, columnar mesogenicity, on-surface photo­induced charge separation vs single-molecule conductance, and reduction potential. Topical results and prospects of application are discussed on the basis of crystallographic, spectroscopic, and electrochemical analyses vs DFT-calculated nuclear and electronic structures. For the sake of the discussion consistency, complementary experimental and computational results are disclosed in the dianilinyl series. Overall, it is shown that combined advances in strategy, protocols, and substrate scope of acetylenic synthesis remain crucial for the development of yet poorly explored but promising types of molecular materials.1 Introduction2 Hexaalkynyl-carbo-benzene3 ortho-Dialkynyl-carbo-benzene4 para-Dialkynyl-carbo-benzenes4.1 Bistrimethylsilylethynyl-carbo-benzene4.2 Bisfluorenylethynyl-carbo-benzene4.3 Bistrialkoxyarylethynyl-carbo-benzenes4.4 Bisindolylethynyl-carbo-benzene4.5 Bisanilinylethynyl-carbo-benzene5 Carbo-oligo(phenyleneethynylene)s6 Conclusions

Technical Effects of Molecule–Electrode Contacts in Graphene-Based Molecular Junctions

This study focuses on comparing methods for capturing and measuring the charge transport properties of single molecules in gold–graphene contact gaps. We have attempted to measure the single-molecule conductance of a series of 1,n-alkanedithiols (n = 4, 6, 8) tethered between a gold and a graphene contact with three different methods. The conducting probe atomic force microscopy break junction (CP-AFM BJ), scanning tunneling microscopy (STM) break junction (STM BJ), and STM-based I(s) techniques for forming molecular junctions with graphene lower contacts were compared. In each case, the upper contact was gold, with a gold-coated AFM probe in the CP-AFM BJ method and a gold STM tip for both the STM BJ and I(s) techniques. Both the CP-AFM BJ and the STM-based I(s) methods yielded similar values for the conductance decay constant values, with βN = 0.56 and 0.40, respectively. In line with previous observations, these are much smaller than values recorded for the same alkanedithiol series in symmetric gold–m...

Protein Motion and Configurations in a Form-Fitting Nanopore: Avidin in ClyA

We probe the molecular dynamics and states of an avidin protein as it is captured and trapped in a voltage-biased cytolysin A nanopore using time-resolved single-molecule electrical conductance signals. The data for very large numbers of single-molecule events are analyzed and presented by a new method that provides clear visual insight into the molecular scale processes. Avidin in cytolysin A has surprisingly rich conductance spectra that reveal transient and more permanently trapped protein configurations in the pore and how they evolve into one another. We identify a long-lasting, stable, and low-noise configuration of avidin in the nanopore into which avidin can be reliably trapped and released. This may prove useful for single-molecule studies of other proteins that can be biotinylated and then transported by avidin to the pore via their coupling to avidin with biotin-avidin linking. We demonstrate the sensitivity of this system with detection of biotin attached to avidin captured by the pore.

Reverse Bond-Length Alternation in Cumulenes: Candidates for Increasing Electronic Transmission with Length

Single-molecule conductance generally decays exponentially with the length of the molecule when the transport mechanism is a coherent tunneling process. However, it was recently found that this length dependence can be reversed in linear conjugated molecules if the bond-length alternation is reversed. In this work we show that even-carbon cumulenes show this behavior as the bond lengths are reversed for the dominant π-system compared to the equivalent polyenes and polyynes. We explore the electronic origins of the reversed bond-length alternation in cumulenes and its relation to the length dependence of the electronic transmission. Through density functional theory and nonequilibrium Greens function calculations we predict that cumulenic wires have reverse decay of transmission with length; that is, the decay constant β is found to be negative. As a direct consequence of the reversed bond-length alternation, the electronic transmission increases with length as the highest occupied molecular orbital–lowest...

In Situ Formation of N-Heterocyclic Carbene-Bound Single-Molecule Junctions.

Self-assembled monolayers (SAMs) formed using N-heterocyclic carbenes (NHCs) have recently emerged as thermally and chemically ultrastable alternatives to those formed from thiols. The rich chemistry and strong σ-donating ability of NHCs offer unique prospects for applications in nanoelectronics, sensing, and electrochemistry. Although stable in SAMs, free carbenes are notoriously reactive, making their electronic characterization challenging. Here we report the first investigation of electron transport across single NHC-bound molecules using the scanning tunneling microscope-based break junction (STM-BJ) technique. We develop a series of air-stable metal NHC complexes that can be electrochemically reduced in situ to form NHC-electrode contacts, enabling reliable single-molecule conductance measurements of NHCs under ambient conditions. Using this approach, we show that the conductance of an NHC depends on the identity of the single metal atom to which it is coordinated in the junction. Our observations are supported by density functional theory (DFT) calculations, which also firmly establish the contributions of the NHC linker to the junction transport characteristics. Our work demonstrates a powerful method to probe electron transfer across NHC-electrode interfaces; more generally, it opens the door to the exploitation of surface-bound NHCs in constructing novel, functionalized electrodes and/or nanoelectronic devices.

"Doping" of Polyyne with an Organometallic Fragment Leads to Highly Conductive Metallapolyyne Molecular Wire.

Exploration of highly conductive molecules is essential to achieve single-molecule electronic devices. The present paper describes the results on single-molecule conductance study of polyyne wires doped with the organometallic Ru(dppe)2 fragment, X-(C≡C) n-Ru(dppe)2-(C≡C) n-X. The metallapolyyne wires end-capped with the gold fragments (X = AuL) are subjected to single-molecule conductance measurements with the STM break junction technique, which reveal the high conductance (10-3-10-2 G0; n = 2-4) with the low attenuation factor (0.25 Å-1) and the low contact resistance (33 kΩ). A unique "'doping'" effect of Ru(dppe)2 fragment was found to lead to the high performance as suggested by the hybrid density functional theory-nonequilibrium green function calculation.

Evaluation of the Kinetic Property of Single-Molecule Junctions by Tunneling Current Measurements.

We investigated the formation and breaking of single-molecule junctions of two kinds of dithiol molecules by time-resolved tunneling current measurements in a metal nanogap. The resulting current trajectory was statistically analyzed to determine the single-molecule conductance and, more importantly, to reveal the kinetic property of the single-molecular junction. These results suggested that combining a measurement of the single-molecule conductance and statistical analysis is a promising method to uncover the kinetic properties of the single-molecule junction.

Heteroatom-Induced Molecular Asymmetry Tunes Quantum Interference in Charge Transport through Single-Molecule Junctions

We studied the interplay between quantum interference (QI) and molecular asymmetry in charge transport through a single molecule. Eight compounds with five-membered core rings were synthesized, and their single-molecule conductances were characterized using the mechanically controllable break junction technique. It is found that the symmetric molecules are more conductive than their asymmetric isomers and that there is no statistically significant dependence on the aromaticity of the core. In contrast, we find experimental evidence of destructive QI in five-membered rings, which can be tuned by implanting different heteroatoms into the core ring. Our findings are rationalized by the presence of antiresonance features in the transmission curves calculated using nonequilibrium Green’s functions. This novel mechanism for modulating QI effects in charge transport via tuning of molecular asymmetry will lead to promising applications in the design of single-molecule devices.

Quantitative analysis of DNA with single-molecule sequencing

Cancer can be diagnosed by identifying DNA and microRNA base sequences that have the same base length yet differ in a few base sequences, if the abundance ratios of these slightly deviant base sequences can be determined. However, such quantitative analyses cannot be performed using the current DNA sequencers. Here we determine entire base sequences of four types of DNA corresponding to the let-7 microRNA, which is a 22-base cancer marker. We record the single-molecule conductances of the base molecules using current-tunneling measurements. In addition, we count the numbers of molecules in a solution to determine the abundance ratios of two DNA strands that differ by a single base sequence.