Break-junction Measurements Research Articles

Effects of cis-trans Conformation between Thiophene Rings on Conductance of Oligothiophenes.

Oligothiophenes have been established as important π-conjugated frameworks in organic electronics and molecular electronics. Although oligothiophenes possess the rotational flexibility of thiophene rings, the effects of cis-trans conformations on their electrical conductance have not been investigated yet. To investigate the effects of cis-trans conformations between thiophene rings on the conductance of oligothiophenes, we performed first-principles transport calculations. The conductance of the cis-oligothiophene was calculated to be higher than that of trans-oligothiophene, because the highest occupied molecular orbital was closer to the Fermi level of the gold electrode in the cis isomer than the trans isomer. This prediction was confirmed through mechanically controllable break junction measurements and fitting of the current-voltage characteristics for the newly synthesized, insulated oligothiophenes with controlled cis-trans conformations. This study demonstrates that cis- and trans-conformations can affect the electrical properties of oligothiophene frameworks and can potentially be used to control the electronic structure of long oligothiophene molecular wires.

Analytical modeling of the junction evolution in single-molecule break junctions: towards quantitative characterization of the time-dependent process

The conductance through single-molecule junctions characterized by the break junction techniques consists of the through-space tunneling and through-molecule tunneling conductance, and the existence of through-space tunneling between the electrodes makes the quantitative extraction of the intrinsic molecular signals of single-molecule junctions challenging. Here, we established an analytic model to describe the evolution of the conductance of a single molecule in break junction measurements. The experimental data for a series of oligo(aryleneethynylene) derivatives validate the proposed model, which provides a modeling insight into the conductance evolution for the opening process in a “real” break junction experiment. Further modulations revealed that the junction formation probability and rupture distance of the molecular junction, which reflect the junction stability, will significantly influence the amplitude and position of the obtained conductance peak. We further extend our model to a diffusion and a chemical reaction process, for which the simulation results show that the break junction technique offers a quantitative understanding of these time-dependent systems, suggesting the potential of break junction techniques in the quantitative characterization of physical and chemical processes at the single-molecule scale.

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.

Near Length-Independent Conductance in Polymethine Molecular Wires

Polymethine dyes are linear π-conjugated compounds with an odd number of carbons that display a much greater delocalization in comparison to polyenes that have an even number of carbon atoms in their main chain. Herein, we perform scanning tunneling microscope based break-junction measurements on a series of three cyanine dyes of increasing length. We demonstrate, at the single molecule level, that these short chain polymethine systems exhibit a substantially smaller decay in conductance with length (attenuation factor β = 0.04 Å-1) compared to traditional polyenes (β ≈ 0.2 Å-1). Furthermore, we show that by changing solvent we are able to shift the β value, demonstrating a remarkable negative β value, with conductance increasing with molecular length. First principle calculations provide support for the experimentally observed near-uniform length dependent conductance and further suggest that the variations in β with solvent are due to solvent-induced changes in the alignment of the frontier molecular orbitals relative to the Fermi energy of the leads. A simplified Hückel model suggests that the smaller decay in conductance correlates with the smaller degree of bond order alternation present in polymethine compounds compared to polyenes. These findings may enable the design of molecular wires without a length-dependent decay for efficient electron transport at the nanoscale.

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.

Quantum interference and heteroaromaticity of para- and meta-linked bridged biphenyl units in single molecular conductance measurements

Is there a correlation between the (hetero)aromaticity of the core of a molecule and its conductance in a single molecular junction? To address this question, which is of fundamental interest in molecular electronics, oligo(arylene-ethynylene) (OAE) molecular wires have been synthesized with core units comprising dibenzothiophene, carbazole, dibenzofuran and fluorene. The biphenyl core has been studied for comparison. Two isomeric series have been obtained with 4-ethynylpyridine units linked to the core either at para-para positions (para series 1–5) or meta-meta positions (meta series 6–10). A combined experimental and computational study, using mechanically controlled break junction measurements and density functional theory calculations, demonstrates consistently higher conductance in the para series compared to the meta series: this is in agreement with increased conjugation of the π–system in the para series. Within the para series conductance increases in the order of decreasing heteroaromaticity (dibenzothiophene < carbazole < dibenzofuran). However, the sequence is very different in the meta series, where dibenzothiophene ≈ dibenzofuran < carbazole. Excellent agreement between theoretical and experimental conductance values is obtained. Our study establishes that both quantum interference and heteroaromaticity in the molecular core units play important and inter-related roles in determining the conductance of single molecular junctions.

Effects of the Backbone and Chemical Linker on the Molecular Conductance of Nucleic Acid Duplexes.

Scanning tunneling microscope break junction measurements are used to examine how the molecular conductance of nucleic acids depends on the composition of their backbone and the linker group to the electrodes. Molecular conductances of 10 base pair long homoduplexes of DNA, aeg-PNA, γ-PNA, and a heteroduplex of DNA/aeg-PNA with identical nucleobase sequence were measured. The molecular conductance was found to vary by 12 to 13 times with the change in backbone. Computational studies show that the molecular conductance differences between nucleic acids of different backbones correlate with differences in backbone structural flexibility. The molecular conductance was also measured for duplexes connected to the electrode through two different linkers, one directly to the backbone and one directly to the nucleobase stack. While the linker causes an order-of-magnitude increase in the overall conductance for a particular duplex, the differences in the electrical conductance with backbone composition are preserved. The highest molecular conductance value, 0.06G0, was measured for aeg-PNA duplexes with a base stack linker. These findings reveal an important new strategy for creating longer and more complex electroactive, nucleic acid assemblies.

Temporal correlations and structural memory effects in break junction measurements

We review data analysis techniques that can be used to study temporal correlations among conductance traces in break junction measurements. We show that temporal histograms are a simple but efficient tool to check the temporal homogeneity of the conductance traces, or to follow spontaneous or triggered temporal variations, like structural modifications in trained contacts, or the emergence of single-molecule signatures after molecule dosing. To statistically analyze the presence and the decay time of temporal correlations, we introduce shifted correlation plots. Finally, we demonstrate that correlations between the opening and subsequent closing traces may indicate structural memory effects in atomic-sized metallic and molecular junctions. Applying these methods on measured and simulated gold metallic contacts as a test system, we show that the surface diffusion induced flattening of the broken junctions helps to produce statistically independent conductance traces at room temperature, whereas at low temperature repeating tendencies are observed as long as the contacts are not closed to sufficiently high conductance setpoints. Applying opening-closing correlation analysis on Pt–CO–Pt single-molecule junctions, we demonstrate pronounced contact memory effects and recovery of the molecule for junctions breaking before atomic chains are formed. However, if chains are pulled the random relaxation of the chain and molecule after rupture prevents opening-closing correlations.

The Orbital Selection Rule for Molecular Conductance as Manifested in Tetraphenyl-Based Molecular Junctions.

Using two tetraphenylbenzene isomers differing only by the anchoring points to the gold electrodes, we investigate the influence of quantum interference on the single molecule charge transport. The distinct anchor points are realized by selective halogen-mediated binding to the electrodes by formation of surface-stabilized isomers after iodine cleavage. Both isomers are essentially chemically identical and only weakly perturbed by the electrodes avoiding largely parasitic effects, which allows us to focus solely on the relation between quantum interference and the intrinsic molecular properties. The conductance of the two isomers differs by over 1 order of magnitude and is attributed to constructive and destructive interference. Our ab initio based transport calculations compare very well with the accompanying scanning tunneling microscope break junction measurements of the conductance. The findings are rationalized using a two level model, which shows that the interorbital coupling plays the decisive role for the interference effects.

Synthesis and Single-Molecule Conductances of Neutral and Cationic Indenofluorene-Extended Tetrathiafulvalenes: Kondo Effect Molecules.

Development of molecules that can switch between redox states with paired and unpaired electrons is important for molecular electronics and spintronics. In this work, a selection of redox-active indenofluorene-extended tetrathiafulvalenes (IF-TTFs) with thioacetate end groups was prepared from a readily obtainable dibromo-functionalized IF-TTF building block using palladium-catalyzed cross-coupling reactions, such as the Suzuki reaction. The end groups served as electrode anchoring groups for single-molecule conductance studies, and the molecules were subjected to mechanically controlled break-junction measurements with gold contacts and to low-bias charge transport measurements in gated three-terminal electromigration junctions. The neutral molecules showed clear conductance signatures, and somewhat surprisingly, we found that a meta–meta anchoring configuration gave a higher conductance than a para–meta configuration. We explain this behavior by “through-space” coupling between the gold electrode and the phenyl on which the anchoring group is attached. Upon charging the molecule in a gated junction, we found reproducibly a Kondo effect (zero-bias conductance) attributed to a net spin. Ready generation of radical cations was supported by cyclic voltammetry measurements, revealing stepwise formation of radical cation and dication species in solution. The first oxidation event was accompanied by association reactions as the appearance of the first oxidation peak was strongly concentration dependent.

Controlling the rectification properties of molecular junctions through molecule-electrode coupling.

The development of molecular components functioning as switches, rectifiers or amplifiers is a great challenge in molecular electronics. A desirable property of such components is functional robustness, meaning that the intrinsic functionality of components must be preserved regardless of the strategy used to integrate them into the final assemblies. Here, this issue is investigated for molecular diodes based on N-phenylbenzamide (NPBA) backbones. The transport properties of molecular junctions derived from NPBA are characterized while varying the nature of the functional groups interfacing the backbone and the gold electrodes required for break-junction measurements. Combining experimental and theoretical methods, it is shown that at low bias (<0.85 V) transport is determined by the same frontier molecular orbital originating from the NPBA core, regardless of the anchoring group employed. The magnitude of rectification, however, is strongly dependent on the strength of the electronic coupling at the gold-NPBA interface and on the spatial distribution of the local density of states of the dominant transport channel of the molecular junction.

Solvent-dependent conductance decay constants in single cluster junctions.

Single-molecule conductance measurements have focused primarily on organic molecular systems. Here, we carry out scanning tunneling microscope-based break-junction measurements on a series of metal chalcogenide Co6Se8 clusters capped with conducting ligands of varying lengths. We compare these measurements with those of individual free ligands and find that the conductance of these clusters and the free ligands have different decay constants with increasing ligand length. We also show, through measurements in two different solvents, 1-bromonaphthalene and 1,2,4-trichlorobenzene, that the conductance decay of the clusters depends on the solvent environment. We discuss several mechanisms to explain our observations.

Hybridization gaps and antiferromagnetic gap in the Kondo semiconductorsCeT2Al10(T=Feand Os) observed by break-junction tunneling spectroscopy

${\mathrm{CeOs}}_{2}{\mathrm{Al}}_{10}$ exhibits Kondo semiconducting properties, yet orders antiferromagnetically at a rather high temperature, ${T}_{\mathrm{N}}=28.5$ K. We have performed break-junction tunneling measurements on single crystals of ${\mathrm{CeOs}}_{2}{\mathrm{Al}}_{10}$ and the nonmagnetic Kondo semiconductor ${\mathrm{CeFe}}_{2}{\mathrm{Al}}_{10}$. Upon cooling ${\mathrm{CeFe}}_{2}{\mathrm{Al}}_{10}$, two hybridization gaps successively appear in the tunneling spectra, while another gap opens in ${\mathrm{CeOs}}_{2}{\mathrm{Al}}_{10}$ below ${T}_{\mathrm{N}}$. For both compounds, the ratio of the two hybridization gap widths is approximately 4, in agreement with the hybridization gap model for the crystal-field ground state of ${\mathrm{Ce}}^{3+}$ with $|{J}_{z}\ensuremath{\rangle}=|\ifmmode\pm\else\textpm\fi{}3/2\ensuremath{\rangle}$. Furthermore, we found that the gap widths are well scaled by the Kondo temperature among Ce-based Kondo semimetals/semiconductors with orthorhombic structures.

Quasiparticle current along thecaxis in junctions involvingd-wave superconductors partially gapped by charge density waves

Quasiparticle tunnel current either between identical $d$-wave superconductors partially gapped by charge density waves (SCDWs) or between an SCDW and a normal metal was calculated. The cases of unidirectional and checkerboard CDWs were considered. The tunnel conductance was found in both cases to possess a number of peculiarities, which cannot be described by introducing a single combined gap. The results are in qualitative agreement with experimental data obtained for a number of cuprates by the scanning tunnel spectroscopy, intrinsic-tunneling, and break-junction measurements. The difference between the experiment and the theory seems to stem from the spread of gap values occurring due to the intrinsic spatial inhomogeneity of nonstoichiometric oxides and reflected in the cuprate tunnel spectra.

Thermoelectricity at the molecular scale: a large Seebeck effect in endohedral metallofullerenes.

Single molecule devices provide a unique system to study the thermoelectric energy conversion at an atomistic level and can provide valuable information for the design of organic thermoelectric materials. Here we present a comprehensive study of the thermoelectric transport properties of molecular junctions based on C(82), Gd@C(82), and Ce@C(82). We combine precise scanning tunneling microscope break-junction measurements of the thermopower and conductance with quantitatively accurate self-energy-corrected first-principles transport calculations. We find that all three fullerene derivatives give rise to a negative thermopower (n-conducting). The absolute value, however, is much larger for the Gd@C(82) and Ce@C(82) junctions. The conductance, on the other hand, remains comparable for all three systems. The power factor determined for the Gd@C(82) based junction is so far the highest obtained for a single-molecule device. Although the encapsulated metal atom does not directly contribute to the transport, we show that the observed enhancement of the thermopower for Gd@C(82) and Ce@C(82) is elucidated by the substantial changes in the electronic- and geometrical structure of the fullerene molecule induced by the encapsulated metal atom.

A comprehensive study of extended tetrathiafulvalene cruciform molecules for molecular electronics: synthesis and electrical transport measurements.

Cruciform-like molecules with two orthogonally placed π-conjugated systems have in recent years attracted significant interest for their potential use as molecular wires in molecular electronics. Here we present synthetic protocols for a large selection of cruciform molecules based on oligo(phenyleneethynylene) (OPE) and tetrathiafulvalene (TTF) scaffolds, end-capped with acetyl-protected thiolates as electrode anchoring groups. The molecules were subjected to a comprehensive study of their conducting properties as well as their photophysical and electrochemical properties in solution. The complex nature of the molecules and their possible binding in different configurations in junctions called for different techniques of conductance measurements: (1) conducting-probe atomic force microscopy (CP-AFM) measurements on self-assembled monolayers (SAMs), (2) mechanically controlled break-junction (MCBJ) measurements, and (3) scanning tunneling microscopy break-junction (STM-BJ) measurements. The CP-AFM measurements showed structure-property relationships from SAMs of series of OPE3 and OPE5 cruciform molecules; the conductance of the SAM increased with the number of dithiafulvene (DTF) units (0, 1, 2) along the wire, and it increased when substituting two arylethynyl end groups of the OPE3 backbone with two DTF units. The MCBJ and STM-BJ studies on single molecules both showed that DTFs decreased the junction formation probability, but, in contrast, no significant influence on the single-molecule conductance was observed. We suggest that the origins of the difference between SAM and single-molecule measurements lie in the nature of the molecule-electrode interface as well as in effects arising from molecular packing in the SAMs. This comprehensive study shows that for complex molecules care should be taken when directly comparing single-molecule measurements and measurements of SAMs and solid-state devices thereof.

Random telegraph signals in molecular junctions

We investigate conductance fluctuations in molecular junctions using a mechanically controllable break junction setup in a liquid environment. In contrast to conventional break junction measurements, time-dependent conductance signals were recorded while reducing the gap size between the two contact electrodes. Only small amplitude fluctuations of the conductance are observed when measuring in pure solvent. Conductance traces recorded in solutions containing alkanedithiols show significantly larger fluctuations which can take the form of random telegraph signals. Such signals emerge in a limited conductance range, which corresponds well to the known molecular conductance of the compounds investigated. These large-amplitude fluctuations are attributed to the formation and thermally driven breaking of bonds between a molecule and a metal electrode and provide a still poorly explored source of information on the dynamics of molecular junctions formation. The lifetimes of the high and low conductance states are found to vary between 0.1 ms and 0.1 s.

Molecular diodes enabled by quantum interference.

We use scanning tunneling microscope break-junction (STM-BJ) measurements to study the low-bias conductance and high-bias current-voltage (IV) characteristics of a series of asymmetric para-meta connected diphenyl-oligoenes. From tight-binding calculations, we determine that the quantum interference features inherent in our molecular design result in a 'through-bond' coupling on the para-side, and through-space coupling on the meta-side. We show that these molecular junctions form single molecule diodes, and show that the rectification results from a difference in the voltage dependence of the coupling strength on the through-bond and the through-space side. The interplay between the applied voltage and the molecule-metal coupling results from the asymmetric polarizability of the conducting orbital under an external field.

Silicon Ring Strain Creates High-Conductance Pathways in Single-Molecule Circuits

Here we demonstrate for the first time that strained silanes couple directly to gold electrodes in break-junction conductance measurements. We find that strained silicon molecular wires terminated by alkyl sulfide aurophiles behave effectively as single-molecule parallel circuits with competing sulfur-to-sulfur (low G) and sulfur-to-silacycle (high G) pathways. We can switch off the high conducting sulfur-to-silacycle pathway by altering the environment of the electrode surface to disable the Au-silacycle coupling. Additionally, we can switch between conductive pathways in a single molecular junction by modulating the tip-substrate electrode distance. This study provides a new molecular design to control electronics in silicon-based single molecule wires.

Mechanically controllable bi-stable states in a highly conductive single pyrazine molecular junction

We report the fabrication of a highly conductive single pyrazine molecular junction with Pt leads. Mechanically controllable break-junction measurements at low temperatures show two distinct high and low conductance states. These conductance values are two orders of magnitude larger than those of a conventional single molecular junction with anchoring groups because of direct binding of the π conjugated molecule to a metal electrode with large density of states at the Fermi energy. Inelastic electron tunneling spectroscopy combined with density functional theory calculations highlights the vibration modes of the system for the two regimes. Theory allows us to assign the high and low conductance states of the molecular junction to two configurations in which the pyrazine axis is tilted and parallel with respect to the junction axis, respectively. Finally, we show that the pyrazine junction can be reversibly switched between the two bi-stable conductance states by mechanically stretching and relaxing the junction.