Mechanically Controllable Break Junction Research Articles

Design and Synthesis of Multipath Compact Cyclophanes for Quantum Interference Studies

To investigate interference phenomena and con­duc­tance properties in mechanically controlled break junctions (MCBJs), macrocycles 1 and 2 (BMCs: for BenzeneMacroCycles), containing a meta‐substituted benzene moiety with solubilizing tert‐butyl groups, as well as structures 3 and 4 (TMCs: for ThiopheneMacroCycles), featuring 2,5‐connected 3,4‐hexyl‐thio­phene corners, were synthesized. Macrocycles 1 and 2 respect­ively 3 and 4 differ in the positions of the acetyl‐protected sulfur anchoring groups, which impacts both, the individual transport efficiency of their parallel electronic pathways and their overall molecular wire lengths. All macrocycles were synthesized based on a series of Sonogashira cross‐coupling reactions. For 3 and 4, a 2‐(4‐pyridin­yl)ethyl protecting group for the sulfur atoms was successful, while for macrocycles 1 and 2 the more common tert‐butyl protecting group did the job. To our delight, proof‐of‐concept charge transport studies conducted in an MCBJ setup demon­strated the expected trends regarding improved conductance intensities for the TMCs compared to the BMCs. Furthermore, the corres­ponding molecular plateaus from the breaking experiments were in the expected length range of the S‐S distances for all compounds. We also found that the overall conductance seems to follow a more complex transport mechanism than just the sum of contributions from both channels.

Electric Field-Induced Sequential Prototropic Tautomerism in Enzyme-like Nanopocket Created by Single Molecular Break Junction.

Mastering the control of external stimuli-induced chemical transformations with detailed insights into the mechanistic pathway is the key for developing efficient synthetic strategies and designing functional molecular systems. Enzymes, the most potent biological catalysts, efficiently utilize their built-in electric field to catalyze and control complex chemical reactions within the active site. Herein, we have demonstrated the interfacial electric field-induced prototropic tautomerization reaction in acylhydrazone entities by creating an enzymatic-like nanopocket within the atomically sharp gold electrodes using a mechanically controlled break junction (MCBJ) technique. In addition to that, the molecular system used here contains two coupled acylhydrazone reaction centers, hence demonstrating a cooperative stepwise electric field-induced reaction realized at the single molecular level. Furthermore, the mechanistic studies revealed a proton relay-assisted tautomerization showing the importance of external factors such as solvent in such electric field-driven reactions. Finally, single-molecule charge transport and energetics calculations of different molecular species at various applied electric fields using a polarizable continuum solvent model confirm and support our experimental findings. Thus, this study demonstrates that mimicking an enzymatic pocket using a single molecular junction's interfacial electric field as a trigger for chemical reactions can open new avenues to the field of synthetic chemistry.

Halide adsorption influences snapback distance in Scanning Tunnelling Microscope break junctions

‘Snapback distance’ refers to the rapid increase in the size of the gap formed immediately after breaking an atomic-scale metallic contact. It is a commonly observed phenomenon in Scanning Tunnelling Microscope break junction (STM-BJ) and mechanically controlled break junction (MCBJ) experiments. Here, we show that the snapback distance measured for a gold break junction in pure water was significantly reduced in an electrolyte containing halide anions. In the case of Br−, experiments under electrochemical control provided clear evidence that this reduction was caused by halide adsorption on the surface of the gold.

Benchmarking break-junction techniques: electric and thermoelectric characterization of naphthalenophanes.

Break-junction techniques provide the possibility to study electric and thermoelectric properties of single-molecule junctions in great detail. These techniques rely on the same principle of controllably breaking metallic contacts in order to create single-molecule junctions, whilst keeping track of the junction's conductance. Here, we compare results from mechanically controllable break junction (MCBJ) and scanning tunneling microscope (STM) methods, while characterizing conductance properties of the same novel mechanosensitive para- and meta-connected naphtalenophane compounds. In addition, thermopower measurements are carried out for both compounds using the STM break junction (STM-BJ) technique. For the conductance experiments, the same data processing using a clustering analysis is performed. We obtain to a large extent similar results for both methods, although values of conductance and stretching lengths for the STM-BJ technique are slightly larger in comparison with the MCBJ. STM-BJ thermopower experiments show similar Seebeck coefficients for both compounds. An increase in the Seebeck coefficient is revealed, whilst the conductance decreases, after which it saturates at around 10 μV K-1. This phenomenon is studied theoretically using a tight binding model. It shows that changes of molecule-electrode electronic couplings combined with shifts of the resonance energies explain the correlated behavior of conductance and Seebeck coefficient.

An experimental set-up to probe the quantum transport through a single atomic$$/$$molecular junction at room temperature

Understanding the transport characteristics at the atomic limit is the prerequisite for futuristic nanoelectronic applications. Among various experimental procedures, mechanically controllable break junction (MCBJ) is a well-adopted experimental technique to study and control atomic or molecular scale devices. Here, we present the details of developing a piezo-controlled table-top MCBJ set-up, working at ambient conditions, along with the necessary data acquisition technique and analysis of the data. We performed the conductance experiment on a macroscopic gold wire, which exhibits a quantised conductance plateau upon pulling the wire. Conductance peak up to ~ 20G0 (G0 = 2e2\(/\)h where e is the electronic charge and h is the Planck’s constant) could be resolved at room temperature. A well-known test bed molecule, 4,4′-bipyridine, was introduced between the gold electrodes and the conductance histogram exhibits two distinctive conductance peaks, confirming the formation of a single molecular junction, in line with the previous reports. This demonstrates that our custom-designed MCBJ set-up can measure the quantum transport of a single molecular junction at ambient conditions.

Optical Trapping of a Single Molecule of Length Sub-1 nm in Solution

Optical Trapping of a Single Molecule of Length Sub-1 nm in Solution

Over-the-Gap Conductance Oscillations in Superconducting Vanadium Nanocontacts Induced by Hydrogen Impurities.

We have investigated the current-voltage (I-V) characteristics of Josephson junctions made by a low-temperature superconductor of vanadium (V) with a small amount of hydrogen (H) and deuterium (D) impurities using a mechanically controllable break junction (MCBJ) technique. Below the superconducting transition temperature TC, the differential conductance dI/dV spectra show distinct peaks within the superconducting gap, known as the subgap structure, which result from multiple Andreev reflections in pure V nanocontacts. Additionally, the H and D impurities in V nanocontacts induce several new conductance peaks outside the gap, referred to as an over-the-gap structures (OGSs). We found that the OGS peaks exhibit strong temperature dependence until the temperature is close to TC and follow the gap function predicted by BCS theory. When the contact diameter is changed at low temperature using the MCBJ technique, the OGS anomalies change almost linearly with the inverse of the contact diameter. In addition, the gap anomalies are significantly shifted outward by changing the positions of the H atoms through application of a high bias voltage between the contacts. The above features suggest that the OGS peaks are caused by the superconducting quasiparticle interference induced by H or D impurities and/or inhomogeneous structures in the nanocontacts.

Conductance Growth of Single-Cluster Junctions with Increasing Sizes.

Quantum-tunneling-based nanoelectronics has the potential for the miniaturization of electronics toward the sub-5 nm scale. However, the nature of phase-coherent quantum tunneling leads to the rapid decays of the electrical conductance with tunneling transport distance, especially in organic molecule-based nanodevices. In this work, we investigated the conductance of the single-cluster junctions of a series of atomically well-defined silver nanoclusters, with varying sizes from 0.9 to 3.0 nm, using the mechanically controllable break junction (MCBJ) technique combined with quantum transport theory. Our charge transport investigations of these single-cluster junctions revealed that the conductance grows with increasing cluster size. The conductance decay constant was determined to be ∼-0.4 nm-1, which is of opposite sign to that of organic molecules. Comparison between experiment and theory reveals that although charge transport through the silver single-cluster junctions occurs via phase-coherent tunneling, this is compensated by a rapid decrease in the energy gap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital (HOMO-LUMO gap) with size and the increase in the electrode-cluster coupling, which results in their conductance increase up to lengths of ∼3.0 nm. These results demonstrate that such families of nanoclusters provide unique bottom-up building blocks for the fabrication of nanodevices in the sub-5 nm size range.

Benchmark Study of Alkane Molecular Chains

Alkanes serve an important role as benchmark system in molecular electronics. However, a large variation in the conductance values is reported in the literature. To better understand these fluctuations, in this study we measure large molecular data sets (up to 100 000 breaking traces) of a series of alkanes with different lengths and anchoring groups using the mechanically controlled break junction (MCBJ) technique. Employing an unsupervised learning algorithm, we investigate both the time evolution and the distance dependence of the measured traces. For all the molecules considered, we have been able to identify the single-molecule conductance value for the fully stretched molecular configuration. For alkanedithiols, the corresponding extracted β decay constant of 1.05 ± 0.08 per CH2 group agrees well with literature values. In the case of the stronger thiol bonding, additional peaks in the conductance histograms are found, suggesting the formation of molecular junctions containing a single molecule plus additional gold/molecule unit(s). The results shine light on the dispersion in reported conductance values and show that the evolution of the molecule as a function of stretching and time contains crucial information in determining the molecular junction configuration in MCBJs.

Electronic transport through single-molecule oligophenyl-diethynyl junctions with direct gold-carbon bonds formed at low temperature.

We report on the first systematic transport study of alkynyl-ended oligophenyl-diethynyl (OPA) single-molecule junctions with direct Au–C anchoring scheme at low temperature using the mechanically controlled break junction technique. Through quantitative statistical analysis of opening traces, conductance histograms and density functional theory studies, we identified different types of junctions, classified by their conductance and stretching behavior, for OPA molecules between Au electrodes with two to four phenyl rings. We performed inelastic electron tunneling spectroscopy and observed the excitation of Au–C vibrational modes confirming the existence of Au–C bonds at low temperature and compared the stability of molecule junctions upon mechanical stretching. Our findings reveal the huge potential for future functional molecule transport studies at low temperature using alkynyl endgroups.

Atomistic modeling to predict and improve the strength of doped Sn-Cu solder interfaces

Abstract The reliability of solder joints depends upon the strength of the interface where two materials are joined. The strengthening of solder joints has been routinely achieved via doping with other elements, though this is not yet well understood at a fundamental level, and typically accomplished by trial and error. In the present work, we have used atomistic modeling based on density functional theory (DFT) and ab initio molecular dynamics (AIMD) to study the mechanical strength of the Sn-Cu interface under various conditions. We have investigated the cleavage energy (CE) of the Sn-Cu interface, and how it changes with various dopants (Ag, Au, Bi, Cu, Ni, Zn) to determine the benefit (or detriment) to the strength of this simulated solder joint. We have also tested multiple faces of Sn ([001], [100] and [110]) as potential interfaces with Cu. Our simulations show that each of the dopants considered, except for Bi, increases the strength of the interface. In all our constructed Sn-Bi interfaces, a single atomic layer of Sn atoms deposits on the Cu and binds strongly to it. The weakest point of the interface is located between the deposited Sn layer and the remaining bulk Sn. For the undoped [001] Sn-Cu system, the cleavage energy between the Sn and Cu layers is 1.63 J/m2, whereas the cleavage energy between the deposited layer of Sn and the remaining Sn bulk is considerably lower: 0.54 J/m2; this is likely the location of joint failure, and the focus of our investigation. We observed this trend in each interface that we studied. As expected, the strength of the Sn-Cu interface can be modified with dopants; the CE of the weakest point can be increased (strengthening it) by 0.1–0.2 J/m2 when doping it with Ag, Au, Cu, Ni and Zn, though Bi results in a decrease (weakening it) by 0.15 J/m2. As a complementary method for investigating this interface, we have used AIMD to simulate a mechanically controlled break junction (MCBJ) of the solder interface by gradually increasing the separation between the two ends of the simulated junction. The process is continued until the junction completely breaks, yielding an energy vs. distance curve, which provides information about the strength of the solder joint that is similar to a stress-strain curve. We observe that, as the Sn is moved away from the Cu, there is a relatively steady increase in energy until the system begins to separate. From this separation point, the energy curve plateaus as the interaction between the two halves of the system vanishes. The location of the breaking point is in excellent accordance with our CE calculations; there is also a correlation between the CE and the amount of distance that the system must be stretched before reaching the breaking point. Furthermore, we have adapted the MCBJ method in AIMD to simulate shearing of the Sn-Cu interface. In these simulations, as opposed to moving Sn atoms away from the Cu atoms perpendicularly to the interface, we are sliding the Sn atoms laterally across the interface relative to the Cu atoms. The deposited Sn layer remains strongly bonded to the Cu surface, with most of the shearing occurring inside the bulk Sn region. The first principles nature of the methods employed makes them highly transferable and applicable to any chemical composition. These atomistic simulations provide valuable insights that can be used to design stronger solder joints based on physical understanding.

Guiding Students to Understand the Nanoscale Charge Transport by the Mechanically Controllable Break Junction Technique

Fabrication of molecular devices will meet the demands of the miniaturization of traditional electronic devices. Although microfabrication based on the complementary metal-oxide–semiconductor process can fabricate high-quality single-molecule devices, the cost is high, leading to a challenge to provide a hands-on experiment in the teaching practice of charge transport through single-molecule devices. Here, we demonstrate the low cost and simple procedures associated with a mechanically controllable break junction (MCBJ) technique to fabricate single-molecule devices. On the basis of the MCBJ technique, the conductance of gold–gold atomic contacts and single-molecule junctions can be readily characterized. The teaching practice helps students understand charge transport at the single-molecule level, that is, the quantum tunneling mechanisms. Moreover, the MCBJ technique study provides students the opportunity to develop interdisciplinary thinking, which is vital for understanding, designing, and developing single-molecule electronics.

Conductance quantization in atomic-sized gold contacts using a low-cost mechanically controllable break junction setup

The mechanically controllable break junction (MCBJ) experimental setup is one of the main techniques employed in the study of electronic transport properties at the atomic and molecular scales. This work presents the construction of an inexpensive and simple but robust setup that shows the emergence of conductance quantization as a macroscopic gold wire is pulled to atomic dimensions. The homemade device is based on the MCBJ principle and allows repeatedly forming and breaking the metallic contact to perform a statistical analysis of the data extracting the most frequent electron transport structure. The histogram built from conductance measurements, at room temperature in air, show that the quality of the MCBJ equipment developed here is comparable to that of more sophisticated devices used in research laboratories. It is able to resolve up to three conductance peaks associated with gold nanowires reported in the literature. Such an experiment is suggested to be implemented as a powerful pedagogical tool in modern undergraduate physics labs.

Intermolecular Effects on Tunneling through Acenes in Large-Area and Single-Molecule Junctions

This paper describes the conductance of single-molecules and self-assembled monolayers comprising an oligophenyleneethynylene core, functionalized with acenes of increasing length that extend conjugation perpendicular to the path of tunneling electrons. In the Mechanically Controlled Break Junction (MCBJ) experiment, multiple conductance plateaus were identified. The high conductance plateau, which we attribute to the single molecule conformation, shows an increase of conductance as a function of acene length, in good agreement with theoretical predictions. The lower plateau is attributed to multiple molecules bridging the junctions with intermolecular interactions playing a role. In junctions comprising a self-assembled monolayer with eutectic Ga–In top-contacts (EGaIn), the pentacene derivative exhibits unusually low conductance, which we ascribe to the inability of these molecules to pack in a monolayer without introducing significant intermolecular contacts. This hypothesis is supported by the MCBJ data and theoretical calculations showing suppressed conductance through the PC films. These results highlight the role of intermolecular effects and junction geometries in the observed fluctuations of conductance values between single-molecule and ensemble junctions, and the importance of studying molecules in both platforms.

Structure‐Independent Conductance of Thiophene‐Based Single‐Stacking Junctions

AbstractThe experimental investigation of intermolecular charge transport in π‐conjugated materials is challenging. Herein, we describe the investigation of charge transport through intermolecular and intramolecular paths in single‐molecule and single‐stacking thiophene junctions by the mechanically controllable break junction (MCBJ) technique. We found that the ability for intermolecular charge transport through different single‐stacking junctions was approximately independent of the molecular structure, which contrasts with the strong length dependence of conductance in single‐molecule junctions with the same building blocks, and the dominant charge‐transport path of molecules with two anchors transited from an intramolecular to an intermolecular path when the degree of conjugation increased. An increase in conjugation further led to higher binding probability owing to the variation in binding energies, as supported by DFT calculations.

Structure‐Independent Conductance of Thiophene‐Based Single‐Stacking Junctions

The experimental investigation of intermolecular charge transport in π-conjugated materials is challenging. Herein, we describe the investigation of charge transport through intermolecular and intramolecular paths in single-molecule and single-stacking thiophene junctions by the mechanically controllable break junction (MCBJ) technique. We found that the ability for intermolecular charge transport through different single-stacking junctions was approximately independent of the molecular structure, which contrasts with the strong length dependence of conductance in single-molecule junctions with the same building blocks, and the dominant charge-transport path of molecules with two anchors transited from an intramolecular to an intermolecular path when the degree of conjugation increased. An increase in conjugation further led to higher binding probability owing to the variation in binding energies, as supported by DFT calculations.

Electric-Field-Induced Connectivity Switching in Single-Molecule Junctions.

SummaryThe manipulation of molecule-electrode interaction is essential for the fabrication of molecular devices and determines the connectivity from electrodes to molecular components. Although the connectivity of molecular devices could be controlled by molecular design to place anchor groups in different positions of molecule backbones, the reversible switching of such connectivities remains challenging. Here, we develop an electric-field-induced strategy to switch the connectivity of single-molecule junctions reversibly, leading to the manipulation of different connectivities in the same molecular backbone. Our results offer a new concept of single-molecule manipulation and provide a feasible strategy to regulate molecule-electrode interaction.

Evolution of Magnetic Anisotropy of an Organometallic Molecule in a Mechanically Controlled Break Junction: The Roles of Connecting Electrodes

The precise control of local spin states in magnetic organometallic molecules by means of a mechanically controlled break junction (MCBJ) is a cutting edge frontier of single molecule science. For instance, a fine-tuning of the magnetic anisotropy of a Co(tpy–SH)2 molecule has been achieved experimentally by sandwiching the molecule between two electrodes and gradually enlarging their separation. However, an accurate simulation of such a process has remained rather challenging, which is largely due to the complex structure of the composite junction as well as the static electron correlation originating from the nearly degenerate d-orbitals of the cobalt atom. In this work, by employing an embedding method which combines the high-level complete active space self-consistent field (CASSCF) method and a density functional theory (DFT) method, we carefully simulate the evolution of magnetic anisotropy of the Au–Co(tpy–S)2–Au junction during the mechanical stretching process. It is found that the presence of th...

Can One Define the Conductance of Amino Acids?

We studied the electron-transport properties of ten different amino acids and one dimer (di-methionine) using the mechanically controlled break-junction (MCBJ) technique. For methionine and cysteine, additional measurements were performed with the scanning tunneling microscope break-junction (STM-BJ) technique. By means of a statistical clustering technique, we identified several conductance groups for each of the molecules considered. Ab initio calculations revealed that the observed broad conductance distribution stems from the possibility of various binding geometries which can be formed during stretching combined with a multitude of possible conformational changes. The results suggest that it would be helpful to explore different experimental techniques such as recognition tunneling and conditions to help identify the nature of amino-acid-based junctions even further, for example, with the goal to establish a firm platform for their unambiguous recognition by tunneling break-junction experiments.

A reference-free clustering method for the analysis of molecular break-junction measurements

Single-molecule break-junction measurements are intrinsically stochastic in nature, requiring the acquisition of large datasets of “breaking traces” to gain insight into the generic electronic properties of the molecule under study. For example, the most probable conductance value of the molecule is often extracted from the conductance histogram built from these traces. In this letter, we present an unsupervised and reference-free machine learning tool to improve the determination of the conductance of oligo(phenylene ethynylene)dithiol from mechanically controlled break-junction (MCBJ) measurements. Our method allows for the classification of individual breaking traces based on an image recognition technique. Moreover, applying this technique to multiple merged datasets makes it possible to identify common breaking behaviors present across different samples, and therefore to recognize global trends. In particular, we find that the variation in the extracted molecular conductance can be significantly reduced resulting in a more reliable estimation of molecular conductance values from MCBJ datasets. Finally, our approach can be more widely applied to different measurement types which can be converted to two-dimensional images.