Electrically Controlled Bimetallic Junctions for Atomic-Scale Electronics

Forming atomic-scale contacts with attractive geometries and material compositions is a long-term goal of nanotechnology. Here, we show that a rich family of bimetallic atomic-contacts can be fabricated in break-junction setups. The structure and material composition of these contacts can be controlled by atomically precise electromigration, where the metal types of the electron-injecting and sink electrodes determine the type of atoms added to, or subtracted from, the contact structure. The formed bimetallic structures include, for example, platinum and aluminum electrodes bridged by an atomic chain composed of platinum and aluminum atoms as well as iron-nickel single-atom contacts that act as a spin-valve break junction without the need for sophisticated spin-valve geometries. The versatile nature of atomic contacts in bimetallic junctions and the ability to control their structure by electromigration can be used to expand the structural variety of atomic and molecular junctions and their span of properties.

Single-Atom Control of Single-Molecule van der Waals Junctions with Semimetallic Transition Metal Dichalcogenide Electrodes

Electrodes play an essential role in controlling electrode-molecule coupling. However, conventional metal electrodes require linkers to anchor the molecule. Van der Waals interaction offers a versatile strategy to connect the electrode and molecule without anchor groups. Except for graphene, the potential of other materials as electrodes to fabricate van der Waals molecular junctions remains unexplored. Herein, we utilize semimetallic transition metal dichalcogenides (TMDCs) 1T'-WTe₂ as electrodes to fabricate WTe₂/metalated tetraphenylporphyrin (M-TPP)/WTe₂ junctions via van der Waals interaction. Compared with chemically bonded Au/M-TPP/Au junctions, the conductance of these M-TPP van der Waals molecular junctions is enhanced by ∼736%. More importantly, WTe₂/M-TPP/WTe₂ junctions exhibit the tunable conductance from 10^-3.29 to 10^-4.44 G₀ (1.15 orders of magnitude) via single-atom control, recording the widest tunable range of conductance for M-TPP molecular junctions. Our work demonstrates the potential of two-dimensional TMDCs for constructing highly tunable and conductive molecular devices.

Structural Regulation of Mechanical Gating in Molecular Junctions

In contrast to silicon-based transistors, single-molecule junctions can be gated by simple mechanical means. Specifically, charge can be transferred between the junction's electrodes and its molecular bridge when the interelectrode distance is modified, leading to variations in the electronic transport properties of the junction. While this effect has been studied extensively, the influence of the molecular orientation on mechanical gating has not been addressed, despite its potential influence on the gating effectiveness. Here, we show that the same molecular junction can experience either clear mechanical gating or none, depending on the molecular orientation in the junctions. The effect is found in silver-ferrocene-silver break junctions and analyzed in view of ab initio and transport calculations, where the influence of the molecular orbital geometry on charge transfer to or from the molecule is revealed. The molecular orientation is thus a new degree of freedom that can be used to optimize mechanically gated molecular junctions.

Single-Molecule Electrochemical Transistors

Single-molecule electrochemical transistors are a type of novel molecular devices in which the tunneling current through the single-molecule junction is modulated by the electrochemical gate, and is considered a promising candidate to be employed in molecular integrated circuits for building the future "molecular computers." Benefiting from the particular interfacial electrical double layer, the current modulation process can be realized through direct orbital gating as well as electrochemical electron transfer driven by electrode potential, thus significantly enriching the functions of the transistor devices. This review focuses on the transfer characteristics and the performance of several typical types of single-molecule electrochemical transistors and the prospects for the fabrication toward integrated devices.

Electrical conductance measurement of HgII-mediated DNA duplex in buffered aqueous solution

Electrical properties of metal-mediated DNA duplexes (metallo-DNA) have been of particular interest because of their potential applications in DNA-based nanoelectronics. We prepared Hg^II-mediated DNA duplex with NH₂ anchors and measured the electrical conductance of single-molecule metallo-DNA via scanning tunneling microscopy-based break junction method in the buffered solution. Three conductance values were observed that may correspond to different conformations of the metallo-DNA molecule bridged over metallic electrodes.

An Electromechanical Approach to Understanding Binding Configurations in Single-Molecule Devices

The configuration of the molecule-electrode contact region plays an important role in determining the conductance of a single-molecule junction, and the variety of possible contact configurations have yielded multiple conductance values for a number of molecular families. In this report, we perform simultaneous conductance and electromechanical coupling parameter measurements on a series of oligophenylene-dithiol single-molecule junctions. These molecules show two distinct conductance values, and by examining the conductance changes, the electromechanical coupling, and the changes in the I- V characteristics coupled with a combination of analytical mechanical models and density functional theory (DFT) structure calculations, we are able to determine the most-probable binding configuration in each of the conductance states. We find that the lower-conductance state is likely due to the thiols binding to each electrode at a gold top site, and in the higher-conductance state, the phenylene π orbitals interact with electrodes, drastically modifying the transport behavior. This approach provides an expanded methodology for exploring the relationship between the molecule-electrode contact configuration and molecular conductance.

Measuring Single-Molecule Conductance at An Ultra-Low Molecular Concentration in Vacuum

We report on systematic investigation of single-molecule detection mechanisms in break junction experiments in vacuum. We found molecular feature in the conductance traces at an extremely low concentration of molecules of 10 nM. This was attributed to condensation of the molecular solution on the junction surface upon evaporation of the solvent during evacuation. Furthermore, statistical analyses of the temporal dependence of molecular junction formation probabilities suggested accumulation effects of the contact mechanics to concentrate molecules absorbed on a remote area to the tunneling current sensing zone, which also contributed to the capability of molecular detections at the low concentration condition. The present findings can be used as a useful guide to implement break junction measurements for studying electron and heat transport through single molecules in vacuum.

Electronic conduction during the formation stages of a single-molecule junction

Single-molecule junctions are versatile test beds for electronic transport at the atomic scale. However, not much is known about the early formation steps of such junctions. Here, we study the electronic transport properties of premature junction configurations before the realization of a single-molecule bridge based on vanadocene molecules and silver electrodes. With the aid of conductance measurements, inelastic electron spectroscopy and shot noise analysis, we identify the formation of a single-molecule junction in parallel to a single-atom junction and examine the interplay between these two conductance pathways. Furthermore, the role of this structure in the formation of single-molecule junctions is studied. Our findings reveal the conductance and structural properties of premature molecular junction configurations and uncover the different scenarios in which a single-molecule junction is formed. Future control over such processes may pave the way for directed formation of preferred junction structures.

Quantum Interference in Graphene Nanoconstrictions

We report quantum interference effects in the electrical conductance of chemical vapor deposited graphene nanoconstrictions fabricated using feedback controlled electroburning. The observed multimode Fabry-Pérot interferences can be attributed to reflections at potential steps inside the channel. Sharp antiresonance features with a Fano line shape are observed. Theoretical modeling reveals that these Fano resonances are due to localized states inside the constriction, which couple to the delocalized states that also give rise to the Fabry-Pérot interference patterns. This study provides new insight into the interplay between two fundamental forms of quantum interference in graphene nanoconstrictions.

High electronic couplings of single mesitylene molecular junctions

We report on an experimental analysis of the charge transport properties of single mesitylene (1,3,5-trimethylbenzene) molecular junctions. The electronic conductance and the current-voltage characteristics of mesitylene molecules wired into Au electrodes were measured by a scanning tunnelling microscopy-based break-junction method at room temperature in a liquid environment. We found the molecular junctions exhibited two distinct conductance states with high conductance values of ca. 10(-1) G 0 and of more than 10(-3) G 0 (G 0 = 2e (2)/h) in the electronic conductance measurements. We further performed a statistical analysis of the current-voltage characteristics of the molecular junctions in the two states. Within a single channel resonant tunnelling model, we obtained electronic couplings in the molecular junctions by fitting the current-voltage characteristics to the single channel model. The origin of the high conductance was attributed to experimentally obtained large electronic couplings of the direct π-bonded molecular junctions (ca. 0.15 eV). Based on analysis of the stretch length of the molecular junctions and the large electronic couplings obtained from the I-V analysis, we proposed two structural models, in which (i) mesitylene binds to the Au electrode perpendicular to the charge transport direction and (ii) mesitylene has tilted from the perpendicular orientation.

Alternative types of molecule-decorated atomic chains in Au-CO-Au single-molecule junctions

We investigate the formation and evolution of Au-CO single-molecule break junctions. The conductance histogram exhibits two distinct molecular configurations, which are further investigated by a combined statistical analysis. According to conditional histogram and correlation analysis these molecular configurations show strong anticorrelations with each other and with pure Au monoatomic junctions and atomic chains. We identify molecular precursor configurations with somewhat higher conductance, which are formed prior to single-molecule junctions. According to detailed length analysis two distinct types of molecule-affected chain-formation processes are observed, and we compare these results to former theoretical calculations considering bridge- and atop-type molecular configurations where the latter has reduced conductance due to destructive Fano interference.

Kondo effect in a neutral and stable all organic radical single molecule break junction

Organic radicals are neutral, purely organic molecules exhibiting an intrinsic magnetic moment due to the presence of an unpaired electron in the molecule in its ground state. This property, added to the low spin-orbit coupling and weak hyperfine interactions, make neutral organic radicals good candidates for molecular spintronics insofar as the radical character is stable in solid state electronic devices. Here we show that the paramagnetism of the polychlorotriphenylmethyl radical molecule in the form of a Kondo anomaly is preserved in two- and three-terminal solid-state devices, regardless of mechanical and electrostatic changes. Indeed, our results demonstrate that the Kondo anomaly is robust under electrodes displacement and changes of the electrostatic environment, pointing to a localized orbital in the radical as the source of magnetism. Strong support to this picture is provided by density functional calculations and measurements of the corresponding nonradical species. These results pave the way toward the use of all-organic neutral radical molecules in spintronics devices and open the door to further investigations into Kondo physics.

Effect of mechanical stretching on DNA conductance

Studying the structural and charge transport properties in DNA is important for unraveling molecular scale processes and developing device applications of DNA molecules. Here we study the effect of mechanical stretching-induced structural changes on charge transport in single DNA molecules. The charge transport follows the hopping mechanism for DNA molecules with lengths varying from 6 to 26 base pairs, but the conductance is highly sensitive to mechanical stretching, showing an abrupt decrease at surprisingly short stretching distances and weak dependence on DNA length. We attribute this force-induced conductance decrease to the breaking of hydrogen bonds in the base pairs at the end of the sequence and describe the data with a mechanical model.

Breaking Force and Conductance of Gold Nanojunctions: Effect of Humidity

Forces acting on elongated gold nanojunctions and their electric conductance were simultaneously measured by current-sensing force spectroscopy in an atmosphere with controlled humidity. The breaking force of "thick" nanojunctions with conductance >20G0 is not affected by the environmental humidity. The presence of ambient water stabilizes "thin" nanojunctions with conductance <15G0, whose breaking force of 10-15 nN was higher than that in a dry atmosphere due to the capillary forces. The observed effect of humidity would not be possible to distinguish by techniques measuring only forces or only conductance in nanojunctions.