Mechanical Break Junctions Research Articles

2,5-Diaryl 6-hydroxyphenalenones for Single-Molecule Junctions

A modular access to 2,5-diaryl 6-hydroxyphenalenone derivatives is developed and demonstrated by a small series of 5 molecules. Within this series, the structures 1 and 2 expose terminal methylsulfanyl anchor groups, enabling their integration in a single-molecule junction. The modular synthesis is based on Suzuki cross-coupling of the aryl substituents as boronic acid precursors with 5,8-dibromo-2-(tert-butyl)-4,9-dimethoxy-2,3-dihydro-1H-phenalen-1-one, and the subsequent transformation of the product to the desired 2,5-diaryl 6-hydroxyphenalenone in a reduction/deprotection sequence. The new structures are fully characterized and their optical and electrochemical properties are analysed. For the derivatives 1 and 2 suitable for single-molecule junctions, the corresponding oxophenalenoxyl radicals 1R and 2R were obtained by oxidation and analysed by electron paramagnetic resonance spectroscopy. Preliminary mechanical break junction experiments with 1 display the structureʼs ability to form transient single-molecule junctions. The intention behind the molecular design is to profit from the various redox states of the structure (including the neutral radical) as a molecular switch in an electrochemically triggered single-molecule transport experiment.

Dependence of Molecular Diode Behaviors on Aromaticity.

A molecule-scale diode is an essential component for the concept of molecular electronics. Here we report on heterogeneous contact-mediated rectifying behavior in single-molecule junctions. We performed massive current versus voltage characteristics measurements of metal-molecule-metal structures under stretching by a mechanical break junction method. In-situ deformations of the molecular bridges were revealed to induce stochastic switching of the rectifying direction to varying rectification ratio derived from the induced asymmetry in the contact motifs at the molecule termini. Aromatic molecules were found to enable stronger rectifications via the more pronounced Fermi pinning effect to shift the molecular orbital levels by the applied voltage. Dissimilar anchoring groups also served to stabilize the single-molecule diode properties by bestowing a chemically defined difference in the electronic coupling strengths at the electrode-molecule links. The present findings provide a guide to design diodes with the smallest and simplest structures.

A Mechanically Tunable Quantum Dot in a Graphene Break Junction.

Graphene quantum dots (QDs) are intensively studied as platforms for the next generation of quantum electronic devices. Fine tuning of the transport properties in monolayer graphene QDs, in particular with respect to the independent modulation of the tunnel barrier transparencies, remains challenging and is typically addressed using electrostatic gating. We investigate charge transport in back-gated graphene mechanical break junctions and reveal Coulomb blockade physics characteristic of a single, high-quality QD when a nanogap is opened in a graphene constriction. By mechanically controlling the distance across the newly formed graphene nanogap, we achieve reversible tunability of the tunnel coupling to the drain electrode by 5 orders of magnitude, while keeping the source-QD tunnel coupling constant. The break junction device can therefore become a powerful platform to study the physical parameters that are crucial to the development of future graphene-based devices, including energy converters and quantum calorimeters.

Quantum Transport through a Single Conjugated Rigid Molecule, a Mechanical Break Junction Study.

This Account provides an overview of our recent efforts to unravel charge transport characteristics of a metal-molecule-metal junction containing an individual π-conjugated molecule. The model system of our choice is an oligo(phenylene-ethynylene) consisting of three rings, in short OPE3, which represents a paradigmatic model system for molecular-scale electronics. Members of the OPE family are among the most studied in the field thanks to their simple and rigid structure, the possibility of chemically functionalizing them, and their clear transport characteristics. When investigating charge transport in molecular systems, two general directions can be distinguished: one in which assemblies composed of many molecules contacted in parallel are studied, while in the other a single molecule is investigated at a time. In the former approach, molecule-molecule interactions and ensemble-averaged quantities may play a role, thereby introducing broadening of spectral features and hindering the study of the behavior of individual molecules making it more difficult to deconvolute local and intrinsic molecular effects from collective ones. In contrast, single-molecule experiments directly probe individual molecular features and, when they are repeated many times, allow build up of a statistical representation of the changes introduced by, e.g., different junction configurations. Especially in recent years, experimental techniques have advanced such that now large sets of individual events can be measured and analyzed with statistical tools. To study individual single-molecule junctions, we use the break junction technique, in which two sharp movable electrodes are formed by breaking a thin metallic wire and used to contact a single or few molecules. By probing thousands of single-molecule junctions in different conditions, we show that their creation involves independent events justifying the statistical tools that are used. By combining room- and low-temperature data, we show that the dominant transport mechanism for electrons through the OPE3 molecule is off-resonant tunneling. The simplest model capturing transport details in this case is a single-level model characterized by three parameters: the level alignment of the frontier orbital with the Fermi energy of the leads and the electronic couplings to the leads. Variations in these parameters give a broad distribution (1 order of magnitude) in the observed conductance values, indicating that at the microscopic level both the hybridization with the metallic electrodes and the molecular electronic configuration can fluctuate. The low-temperature data show that these variations are due to abrupt changes in the configuration of the molecule in the junction leading to changes in either one of these parameters or both at the same time. The complementary information gained from different experiments is needed to build up a consistent and extended picture of the variability of molecular configurations, omnipresent in single-molecule studies. Knowledge of this variability can help one to better understand the behavior of molecules at the atomic level and at the metal-molecule interface in particular.

Abrupt Switching of Crystal Fields during Formation of Molecular Contacts.

Magnetic molecules have the potential to be used as building blocks for bits in quantum computers. The spin states of the magnetic ion in the molecule can be represented by the effective spin Hamiltonian describing the zero field splitting (ZFS) of the magnetic states. We determined the ZFS of mechanically flexible metal-chelate molecules (Co, Ni, and Cu as metal ions) adsorbed on Cu2N/Cu(100) by inelastic tunneling spectroscopy at temperatures down to 30 mK. When moving the tip toward the molecule, the tunneling current abruptly jumps to higher values, indicating the sudden deformation of the molecule bridging the tunneling junction. Hand in hand with the formation of the contact, an abrupt change of the ZFS occurs. This work also implies that ZFS expected in mechanical break junctions can drastically deviate from that of adsorbed molecules probed by other techniques.

Conformational Smear Characterization and Binning of Single-Molecule Conductance Measurements for Enhanced Molecular Recognition.

Electronic conduction or charge transport through single molecules depends primarily on molecular structure and anchoring groups and forms the basis for a wide range of studies from molecular electronics to DNA sequencing. Several high-throughput nanoelectronic methods such as mechanical break junctions, nanopores, conductive atomic force microscopy, scanning tunneling break junctions, and static nanoscale electrodes are often used for measuring single-molecule conductance. In these measurements, "smearing" due to conformational changes and other entropic factors leads to large variances in the observed molecular conductance, especially in individual measurements. Here, we show a method for characterizing smear in single-molecule conductance measurements and demonstrate how binning measurements according to smear can significantly enhance the use of individual conductance measurements for molecular recognition. Using quantum point contact measurements on single nucleotides within DNA macromolecules, we demonstrate that the distance over which molecular junctions are maintained is a measure of smear, and the resulting variance in unbiased single measurements depends on this smear parameter. Our ability to identify individual DNA nucleotides at 20× coverage increases from 81.3% accuracy without smear analysis to 93.9% with smear characterization and binning (SCRIB). Furthermore, merely 7 conductance measurements (7× coverage) are needed to achieve 97.8% accuracy for DNA nucleotide recognition when only low molecular smear measurements are used, which represents a significant improvement over contemporary sequencing methods. These results have important implications in a broad range of molecular electronics applications from designing robust molecular switches to nanoelectronic DNA sequencing.

Effect of surface contact potential in atomic-size contacts

We present a study of the effect of surface contact potential in a mechanical break junction experiment. Using amplitude-modulated Kelvin probe microscopy (KPM), we show that the surface potential of a real metal is highly non-uniform and is strongly distance-dependent. Based on our KPM results, we propose a model in which a current is induced from the capacitive coupling of the surface potential and accounts for much of the observed shifts of the conductance peaks from integer multiples. The significance of our results in other areas of physics is also discussed.

Direct observation of ballistic Andreev reflection

An overview is presented of experiments on ballistic electrical transport in inhomogeneous superconducting systems which are controlled by the process of Andreev reflection. The initial experiments based on the coexistence of a normal phase and a superconducting phase in the intermediate state led to the concept itself. It was followed by a focus on geometrically inhomogeneous systems like point contacts, which provided a very clear manifestation of the energy and direction dependence of the Andreev reflection process. The point contacts have recently evolved towards the atomic scale owing to the use of mechanical break-junctions, revealing a very detailed dependence of Andreev reflection on the macroscopic phase of the superconducting state. In present-day research, the superconducting in homogeneity is constructed by clean room technology and combines superconducting materials, for example, with low-dimensional materials and topological insulators. Alternatively, the superconductor is combined with nano-objects, such as graphene, carbon nanotubes, or semiconducting nanowires. Each of these “inhomogeneous systems” provides a very interesting range of properties, all rooted in some manifestation of Andreev reflection.

Kondo conductance across the smallest spin 1/2 radical molecule

Molecular contacts are generally poorly conducting because their energy levels tend to lie far from the Fermi energy of the metal contact, necessitating undesirably large gate and bias voltages in molecular electronics applications. Molecular radicals are an exception because their partly filled orbitals undergo Kondo screening, opening the way to electron passage even at zero bias. Whereas that phenomenon has been experimentally demonstrated for several complex organic radicals, quantitative theoretical predictions have not been attempted so far. It is therefore an open question whether and to what extent an ab initio-based theory is able to make accurate predictions for Kondo temperatures and conductance lineshapes. Choosing nitric oxide (NO) as a simple and exemplary spin 1/2 molecular radical, we present calculations based on a combination of density functional theory and numerical renormalization group (DFT+NRG), predicting a zero bias spectral anomaly with a Kondo temperature of 15 K for NO/Au(111). A scanning tunneling spectroscopy study is subsequently carried out to verify the prediction, and a striking zero bias Kondo anomaly is confirmed, still quite visible at liquid nitrogen temperatures. Comparison shows that the experimental Kondo temperature of about 43 K is larger than the theoretical one, whereas the inverted Fano lineshape implies a strong source of interference not included in the model. These discrepancies are not a surprise, providing in fact an instructive measure of the approximations used in the modeling, which supports and qualifies the viability of the density functional theory and numerical renormalization group approach to the prediction of conductance anomalies in larger molecular radicals.

Nanogap structures: combining enhanced Raman spectroscopy and electronic transport

Surface-enhanced Raman spectroscopy (SERS) is an experimental tool for accessing vibrational and chemical information, down to the single molecule level. SERS typically relies on plasmon excitations in metal nanostructures to concentrate the incident radiation and to provide an enhanced photon density of states to couple emitted radiation to the far field. Many common SERS platforms involve metal nanoparticles to generate the required electromagnetic enhancements. Here we concentrate on an alternative approach, in which the relevant plasmon excitations are supported at a truly nanoscale gap between extended electrodes, rather than discrete subwavelength nanoparticles. The ability to fabricate precise gaps on demand, and in some cases to tune the gap size in situ, combined with the additional capability of simultaneous electronic transport measurements of the nanogap, provides access to information not previously available in standard SERS. We summarize the rich plasmonic physics at work in these extended systems and highlight the recent state of the art including tip-enhanced Raman spectroscopy (TERS) and the application of mechanical break junctions and electromigrated junctions. We describe in detail how we have performed in situ gap-enhanced Raman measurements of molecular-scale junctions while simultaneously subjecting these structures to electronic transport. These extended electrode structures allow us to study the pumping of vibrational modes by the flow of tunneling electrons, as well as the shifting of vibrational energies due to the applied bias. These experiments extend SERS into a tool for examining fundamental processes of dissipation, and provide insight into the mechanisms behind SERS spectral diffusion. We conclude with a brief discussion of future directions.

A Brief Review of the Current Technologies Used for the Fabrication of Metal-Molecule-Metal Junction Electrodes

Fabrication techniques for Metal-molecule-metal junction electrodes suitable to study electron tunneling through metal junctions are reviewed. The applications of current technologies such as mechanical break junction, electromigration, shadow mask lithography, focused ion beam deposition, chemical and electrochemical plating, electron-beam lithography, in fabricating vacant junction electrodes are briefly described. For biomolecular sensing applications, the size of the junction electrodes must be small enough to allow the biomolecule inserted into the junction space to connect both leads to keep the molecules in a relaxed and undistorted state. A significant advantage of using Metal-molecule-metal junction electrodes devices is that the junction can be characterized with and without the molecule in place. Any electrical artifacts introduced by the electrode fabrication process are more easily deconvoluted from the intrinsic properties of the molecule.

Few-Hundred GHz Carbon Nanotube Nanoelectromechanical Systems (NEMS)

We study 23-30 nm long suspended single-wall carbon nanotube quantum dots and observe both their stretching and bending vibrational modes. We use low-temperature DC electron transport to excite and measure the tubes' bending mode by making use of a positive feedback mechanism between their vibrations and the tunneling electrons. In these nanoelectromechanical systems (NEMS), we measure fundamental bending frequencies f(bend) ≈ 75-280 GHz and extract quality factors Q ∼ 10(6). The NEMS's frequencies can be tuned by a factor of 2 with tension induced by mechanical breakjunctions actuated by an electrostatic force or tension from bent suspended electrodes.

Mechanical Break Junctions: Enormous Information in a Nanoscale Package

Mechanical break junctions, particularly those in which a metal tip is repeatedly moved in and out of contact with a metal film, have provided many insights into electronic conduction at the atomic and molecular scale, most often by averaging over many possible junction configurations. This averaging throws away a great deal of information, and Makk et al. in this issue of ACS Nano demonstrate that, with both simulated and real experimental data, more sophisticated two-dimensional analysis methods can reveal information otherwise obscured in simple histograms. As additional measured quantities come into play in break junction experiments, including thermopower, noise, and optical response, these more sophisticated analytic approaches are likely to become even more powerful. While break junctions are not directly practical for useful electronic devices, they are incredibly valuable tools for unraveling the electronic transport physics relevant for ultrascaled nanoelectronics.

Switching the Conductance of Dy Nanocontacts by Magnetostriction

The electrical conductance G of mechanical break-junctions fabricated from the rare-earth metal dysprosium has been investigated at 4.2 K where Dy is in the ferromagnetic state. In addition to the usual variation of the conductance while breaking the wire mechanically, the conductance can be changed reproducibly by variation of the magnetic field H, due to the large magnetostriction of Dy. For a number of contacts, we observe discrete changes in G(H) in the range of several G(0) = 2e(2)/h. The behavior of G(H) and its angular dependence can be quantitatively understood by taking into account the magnetostrictive properties of Dy. This realization of a magnetostrictive few-atom switch demonstrates the possibility of reproducibly tuning the conductance of magnetic nanocontacts by a magnetic field.

Sandwich-type gated mechanical break junctions

We introduce a new device architecture for the independent mechanical and electrostatictuning of nanoscale charge transport. In contrast to previous gated mechanicalbreak junctions with suspended source–drain electrodes, the devices presentedhere prevent an electromechanical tuning of the electrode gap by the gate. Thissignificant improvement originates from a direct deposition of the source and the drainelectrodes on the gate dielectric. The plasma-enhanced native oxide on the aluminumgate electrode enables measurements at gate voltages up to 1.8 V at cryogenictemperatures. Throughout the bending-controlled tuning of the source–drain distance, theelectrical continuity of the gate electrode is maintained. A nanoscale island in theCoulomb blockade regime serves as a first experimental test system for the devices, inwhich the mechanical and electrical control of charge transport is demonstrated.

Molecular junctions based on intermolecular electrostatic coupling

Electrostatically linked ferrocene junctions were assembled and their conductance was investigated on the level of individual molecules by means of an electrochemical mechanical break junction. Conductance histograms of repeatedly formed molecular junctions showed peaks at multiples of 3.4 × 10(-5) G(0), which is one order of magnitude smaller than the conductance reported for covalently linked cysteamine-ferrocene-cysteamine.

Controlled Stability of Molecular Junctions

Influence of interfaces on thiol-terminated molecular wires consisting of three phenyl (P3) or three thiophene (T3) rings was monitored by a new statistical approach based on mechanical controllable break junction technique (see SEM image of the junction and schematic of molecular arrangement). The P3 molecule exhibits stochastic variations in conductance G, in contrast to the T3 molecule.

Conductance and I-V characteristics of Au/BPY/Au single molecule junctions

We have investigated the low bias conductance and I-V characteristics of single 4,4′-bipyridine (BPY) molecule connected to gold electrodes at room temperature. Exploiting the mechanical controllable break junction technique we statistically determined the most preferred conductance value of Au/BPY/Au single molecule junctions. The measured conductance, 0.01G0 (G0 is the conductance quantum unit), is in good agreement with two of the previous experiments using scanning tunnel microscopy break junctions. The I-V characteristics of the Au/BPY/Au junction have been directly measured by varying the bias up to ±0.5 V while holding the junction at its single molecule state. By comparing the obtained I-V curves with those of the Au/1,4-benzenedithiol (BDT)/Au junction, we observed differences in the energy level alignment of BPY and BDT molecules with respect to the Au electrode.

A Nanoelectromechanical Single-Atom Switch

We have exploited the electromechanical properties of gated mechanical break junctions to form single-atom relays. The gate voltage can be used to reversibly switch between a monatomic contact with a conductance around 2e(2)/h and the tunneling regime. In tunneling, the source-drain conductance varies smoothly with gate voltage. The characteristics of the devices can be understood within a simple continuum model. It indicates that the elastic properties of the substrate facilitate the electromechanical tuning and that the details of the switching depend sensitively on the nanoscale geometry of the electrode tips.

Alligator clips to molecular dimensions

Techniques for fabricating nanospaced electrodes suitable for studying electron tunnelingthrough metal–molecule–metal junctions are described. In one approach, top contacts aredeposited/placed on a self-assembled monolayer or Langmuir–Blodgett film resting on aconducting substrate, the bottom contact. The molecular component serves as a permanentspacer that controls and limits the electrode separations. The top contact can be athermally deposited metal film, liquid mercury drop, scanning probe tip, metallic wire orparticle. Introduction of the top contact can greatly affect the electrical conductance ofthe intervening molecular film by chemical reaction, exerting pressure, or simplymigrating through the organic layer. Alternatively, vacant nanogaps can be fabricatedand the molecular component subsequently inserted. Strategies for constructingvacant nanogaps include mechanical break junction, electromigration, shadowmask lithography, focused ion beam deposition, chemical and electrochemicalplating techniques, electron-beam lithography, and molecular and atomic rulers.The size of the nanogaps must be small enough to allow the molecule to connectboth leads and large enough to keep the molecules in a relaxed and undistortedstate. A significant advantage of using vacant nanogaps in the construction ofmetal–molecule–metal devices is that the junction can be characterized with and withoutthe molecule in place. Any electrical artifacts introduced by the electrode fabricationprocess are more easily deconvoluted from the intrinsic properties of the molecule.