Understanding the Role of Parallel Pathways via In‐Situ Switching of Quantum Interference in Molecular Tunneling Junctions

This study describes the modulation of tunneling probabilities in molecular junctions by switching one of two parallel intramolecular pathways. A linearly conjugated molecular wire provides a rigid framework that allows a second, cross‐conjugated pathway to be effectively switched on and off by protonation, affecting the total conductance of the junction. This approach works because a traversing electron interacts with the entire quantum‐mechanical circuit simultaneously; Kirchhoff's rules do not apply. We confirm this concept by comparing the conductances of a series of compounds with single or parallel pathways in large‐area junctions using EGaIn contacts and single‐molecule break junctions using gold contacts. We affect switching selectively in one of two parallel pathways by converting a cross‐conjugated carbonyl carbon into a trivalent carbocation, which replaces destructive quantum interference with a symmetrical resonance, causing an increase in transmission in the bias window.     

Quantum Interference Enhanced Chemical Responsivity in Single-Molecule Dithienoborepin Junctions

Providing a chemical control over charge transport through molecular junctions is vital to developing sensing applications at the single-molecule scale. Quantum-interference effects that affect the charge transport through molecules offer a unique chance to enhance the chemical control. Here, we investigate how interference effects can be harnessed to optimize the response of single molecule dithienoborepin (DTB) junctions to the specific coordination of a fluoride ion in solution. The single-molecule conductance of two DTB isomers is measured using scanning tunneling microscopy break-junction (STM-BJ) before and after fluoride ion exposure. We find a significant change of conductance before and after the capture of a fluoride ion, the magnitude of which depends on the position of the boron atom in the molecular structure. This single-molecule sensor exhibits switching ratios of up to four orders of magnitudes, suggesting that the boron–fluoride coordination can lead to quantum-interference effects. This is confirmed by a quantum chemical characterization, pointing toward a cross-conjugated path through the molecular structure as the origin of the effect.     

Electrochemical electron transfer and its relation to charge transport in single molecule junctions

Ability to control charge transport at nanometer scale lies in the heart of design of fast reliable electronic devices. Molecular electronics thrive to use functional molecules for such transport. If the molecule contains redox center(s), a diode-like or transistor-like behavior can be easily explored by controlling not only the voltage difference between two metallic contacts of the molecular junction but also the potential of one of the contacting electrodes with respect to some reference. Thus, one needs to understand the relationship between electrochemical electron transfer and charge transport in metal–molecule–metal junctions. This review presents latest theoretical approaches toward understanding of such relationship and discusses pivotal experimental works to validate them. Tunneling and hopping pathways may operate in parallel (two-channel model), but experimental conditions dictate the channel preference.     

Functionality in single-molecule devices: model calculations and applications of the inelastic electron tunneling signal in molecular junctions

We analyze how functionality could be obtained within single-molecule devices by using a combination of non-equilibrium Green's functions and ab initio calculations to study the inelastic transport properties of single-molecule junctions. First, we apply a full non-equilibrium Green's function technique to a model system with electron-vibration coupling. We show that the features in the inelastic electron tunneling spectra (IETS) of the molecular junctions are virtually independent of the nature of the molecule-lead contacts. Since the contacts are not easily reproducible from one device to another, this is a very useful property. The IETS signal is much more robust versus modifications at the contacts and hence can be used to build functional nanodevices. Second, we consider a realistic model of a organic conjugated molecule. We use ab initio calculations to study how the vibronic properties of the molecule can be controlled by an external electric field which acts as a gate voltage. The control, through the gate voltage, of the vibron frequencies and (more importantly) of the electron-vibron coupling enables the construction of functionality: nonlinear amplification and/or switching is obtained from the IETS signal within a single-molecule device.     

All-carbon molecular tunnel junctions

This Article explores the idea of using nonmetallic contacts for molecular electronics. Metal-free, all-carbon molecular electronic junctions were fabricated by orienting a layer of organic molecules between two carbon conductors with high yield (>90%) and good reproducibility (rsd of current density at 0.5 V <30%). These all-carbon devices exhibit current density-voltage (J-V) behavior similar to those with metallic Cu top contacts. However, the all-carbon devices display enhanced stability to bias extremes and greatly improved thermal stability. Completed carbon/nitroazobenzene(NAB)/carbon junctions can sustain temperatures up to 300 °C in vacuum for 30 min and can be scanned at ±1 V for at least 1.2 × 10(9) cycles in air at 100 °C without a significant change in J-V characteristics. Furthermore, these all-carbon devices can withstand much higher voltages and current densities than can Cu-containing junctions, which fail upon oxidation and/or electromigration of the copper. The advantages of carbon contacts stem mainly from the strong covalent bonding in the disordered carbon materials, which resists electromigration or penetration into the molecular layer, and provides enhanced stability. These results highlight the significance of nonmetallic contacts for molecular electronics and the potential for integration of all-carbon molecular junctions with conventional microelectronics.     

Orientation‐Controlled Single‐Molecule Junctions

The conductivity of a single aromatic ring, perpendicular to its plane, is determined using a new strategy under ambient conditions and at room temperature by a combination of molecular assembly, scanning tunneling microscopy (STM) imaging, and STM break junction (STM‐BJ) techniques. The construction of such molecular junctions exploits the formation of highly ordered structures of flat‐oriented mesitylene molecules on Au(111) to enable direct tip/π contacts, a result that is not possible by conventional methods. The measured conductance of Au/π/Au junction is about 0.1 G_o , two orders of magnitude higher than the conductance of phenyl rings connected to the electrodes by standard anchoring groups. Our experiments suggest that long‐range ordered structures, which hold the aromatic ring in place and parallel to the surface, are essential to increase probability of the formation of orientation‐controlled molecular junctions.     

Interaction-induced negative differential resistance in asymmetric molecular junctions

Combining insights from quantum chemistry calculations with master equations, we discuss a mechanism for negative differential resistance (NDR) in molecular junctions, operated in the regime of weak tunnel coupling. The NDR originates from an interplay of orbital spatial asymmetry and strong electron-electron interaction, which causes the molecule to become trapped in a nonconducting state above a voltage threshold. We show how the desired asymmetry can be selectively introduced in individual orbitals in, e.g., oligo(phenyleneethynylene)-type molecules by functionalization with a suitable side group, which is in linear conjugation to one end of the molecule and cross-conjugated to the other end.     

Conjugated thiol linker for enhanced electrical conduction of gold-molecule contacts

Single-molecule electrical conduction studies are used to evaluate how the molecular linking unit influences the tunneling efficiency in metal-molecule-metal (m-M-m) junctions. This work uses conducting-probe atomic force microscopy (CP-AFM) to compare the molecular conduction of two pi-bonded molecules: one with a single thiol linker, and another with a conjugated double thiol linker at both ends of the molecules. The results demonstrate that the molecule with conjugated double thiol linkers displays higher conduction in gold-molecule-gold junctions than nonconjugated single thiol-gold contacts.     

Structure-property relationships in redox-gated single molecule junctions--a comparison of pyrrolo-tetrathiafulvalene and viologen redox groups

We demonstrate that the electrical "switching" behavior of single molecules connected between two electrode contacts can be controlled by altering their structure and electrochemical characteristics. The electrical properties of gold|molecule|gold single molecule junctions incorporating HS(CH2)6-X-(CH2)6SH, where X = viologen (4,4'-bipyridinium) or pyrrolotetrathiafulvalene, are determined using a scanning tunneling microscopy based technique. The switching behavior, controlled through a tuneable electrochemical gate, changes from an on-off response (viologen) to an off-on-off response (pyrrolotetrathiafulvalene) on changing the central redox group. In contrast, the electrical properties of junctions incorporating redox-inactive HS(CH2)6-1,4-C6H4-(CH2)6SH do not alter significantly as a function of applied potential.     

Molecular optoelectronics: the interaction of molecular conduction junctions with light

The interaction of light with molecular conduction junctions is attracting growing interest as a challenging experimental and theoretical problem on one hand, and because of its potential application as a characterization and control tool on the other. It stands at the interface between two important fields, molecular electronics and molecular plasmonics and has attracted attention as a challenging scientific problem with potentially important technological consequences. Here we review the present state of the art of this field, focusing on several key phenomena and applications: using light as a switching device, using light to control junction transport in the adiabatic and non-adiabatic regimes, light generation in biased junctions and Raman scattering from such systems. This field has seen remarkable progress in the past decade, and the growing availability of scanning tip configurations that can combine optical and electrical probes suggests that further progress towards the goal of realizing molecular optoelectronics on the nanoscale is imminent.     

Effect of length and contact chemistry on the electronic structure and thermoelectric properties of molecular junctions

We present a combined experimental and computational study that probes the thermoelectric and electrical transport properties of molecular junctions. Experiments were performed on junctions created by trapping aromatic molecules between gold electrodes. The end groups (-SH, -NC) of the aromatic molecules were systematically varied to study the effect of contact coupling strength and contact chemistry. When the coupling of the molecule with one of the electrodes was reduced by switching the terminal chemistry from -SH to -H, the electrical conductance of molecular junctions decreased by an order of magnitude, whereas the thermopower varied by only a few percent. This has been predicted computationally in the past and is experimentally demonstrated for the first time. Further, our experiments and computational modeling indicate the prospect of tuning thermoelectric properties at the molecular scale. In particular, the thiol-terminated aromatic molecular junctions revealed a positive thermopower that increased linearly with length. This positive thermopower is associated with charge transport primarily through the highest occupied molecular orbital, as shown by our computational results. In contrast, a negative thermopower was observed for a corresponding molecular junction terminated by an isocyanide group due to charge transport primarily through the lowest unoccupied molecular orbital.     

Three‐State Single‐Molecule Naphthalenediimide Switch: Integration of a Pendant Redox Unit for Conductance Tuning

We studied charge transport through core‐substituted naphthalenediimide (NDI) single‐molecule junctions using the electrochemical STM‐based break‐junction technique in combination with DFT calculations. Conductance switching among three well‐defined states was demonstrated by electrochemically controlling the redox state of the pendent diimide unit of the molecule in an ionic liquid. The electrical conductances of the dianion and neutral states differ by more than one order of magnitude. The potential‐dependence of the charge‐transport characteristics of the NDI molecules was confirmed by DFT calculations, which account for electrochemical double‐layer effects on the conductance of the NDI junctions. This study suggests that integration of a pendant redox unit with strong coupling to a molecular backbone enables the tuning of charge transport through single‐molecule devices by controlling their redox states.     

Influence of binding groups on molecular junction formation

We study the formation mechanism of molecular junctions using break-junction experiments. We explore the contribution of gold-atom rearrangements in the electrodes by analyzing the junction stretching length, the length of individual plateaus, and the length of the gold one-atom contacts. Comparing the results for alkane dithiols and diamines, we conclude that thiols affect gold electrode dynamics significantly more than amines. This is a vital factor to be considered when comparing different binding groups.     

Heat conduction across molecular junctions between nanoparticles

We investigate the problem of heat conduction across molecular junctions connecting two nanoparticles, both in vacuum and in a liquid environment, using classical molecular dynamics simulations. In vacuum, the well-known result of a length independent conductance is recovered; its precise value, however, is found to depend sensitively on the overlap between the vibrational spectrum of the junction and the density of states of the nanoparticles that act as thermal contacts. In a liquid environment, the conductance is constant up to a crossover length, above which a standard Fourier regime is recovered.     

Electron transport and redox reactions in carbon-based molecular electronic junctions

A unique molecular junction design is described, consisting of a molecular mono- or multilayer oriented between a conducting carbon substrate and a metallic top contact. The sp2 hybridized graphitic carbon substrate (pyrolyzed photoresist film, PPF) is flat on the scale of the molecular dimensions, and the molecular layer is bonded to the substrate via diazonium ion reduction to yield a strong, conjugated C-C bond. Molecular junctions were completed by electron-beam deposition of copper, titanium oxide, or aluminium oxide followed by a final conducting layer of gold. Vibrational spectroscopy and XPS of completed junctions showed minimal damage to the molecular layer by metal deposition, although some electron transfer to the molecular layer resulted in partial reduction in some cases. Device yield was high (>80%), and the standard deviations of junction electronic properties such as low voltage resistance were typically in the range of 10-20%. The resistance of PPF/molecule/Cu/Au junctions exhibited a strong dependence on the structure and thickness of the molecular layer, ranging from 0.13 ohms cm2 for a nitrobiphenyl monolayer, to 4.46 ohms cm2 for a biphenyl monolayer, and 160 ohms cm2 for a 4.3 nm thick nitrobiphenyl multilayer. Junctions containing titanium or aluminium oxide had dramatically lower conductance than their PPF/molecule/Cu counterparts, with aluminium oxide junctions exhibiting essentially insulating behavior. However, in situ Raman spectroscopy of PPF/nitroazobenzene/AlO(x)/Au junctions with partially transparent metal contacts revealed that redox reactions occurred under bias, with nitroazobenzene (NAB) reduction occurring when the PPF was biased negative relative to the Au. Similar redox reactions were observed in PPF/NAB/TiO(x)/Au molecular junctions, but they were accompanied by major effects on electronic behavior, such as rectification and persistent conductance switching. Such switching was evident following polarization of PPF/molecule/TiO2/Au junctions by positive or negative potential pulses, and the resulting conductance changes persisted for several minutes at room temperature. The "memory" effect implied by these observations is attributed to a combination of the molecular layer and the TiO2 properties, namely metastable "trapping" of electrons in the TiO2 when the Au is negatively biased.     

Correlations between molecular structure and single-junction conductance: a case study with oligo(phenylene-ethynylene)-type wires

The charge transport characteristics of 11 tailor-made dithiol-terminated oligo(phenylene-ethynylene) (OPE)-type molecules attached to two gold electrodes were studied at a solid/liquid interface in a combined approach using an STM break junction (STM-BJ) and a mechanically controlled break junction (MCBJ) setup. We designed and characterized 11 structurally distinct dithiol-terminated OPE-type molecules with varied length and HOMO/LUMO energy. Increase of the molecular length and/or of the HOMO-LUMO gap leads to a decrease of the single-junction conductance of the linearly conjugate acenes. The experimental data and simulations suggest a nonresonant tunneling mechanism involving hole transport through the molecular HOMO, with a decay constant β = 3.4 ± 0.1 nm(-1) and a contact resistance R(c) = 40 kΩ per Au-S bond. The introduction of a cross-conjugated anthraquinone or a dihydroanthracene central unit results in lower conductance values, which are attributed to a destructive quantum interference phenomenon for the former and a broken π-conjugation for the latter. The statistical analysis of conductance-distance and current-voltage traces revealed details of evolution and breaking of molecular junctions. In particular, we explored the effect of stretching rate and junction stability. We compare our experimental results with DFT calculations using the ab initio code SMEAGOL and discuss how the structure of the molecular wires affects the conductance values.     

Effect of metal-molecule contact roughness on electronic transport: bacteriorhodopsin-based, metal-insulator-metal planar junctions

Molecular electronics is very much about contacts, and thus understanding of any generic contact effect is essential to its advance. For example, it is still not obvious in how far variations in electrode roughness of macroscopic contacts can lead to rectification. Here we report an investigation of this contact effect on electronic transport properties using metal-insulator-metal planar junctions with a 5 nm thick bacteriorhodopsin-based insulator as model system. We demonstrate that the experimentally observed rectifying behavior is not an intrinsic property of the molecules used, but rather of the local contact quality. Even a slight increase in surface roughness of the bottom electrode gives rise to distinct rectifying behavior in these and, by extrapolation, possibly other molecular junctions.     

Contact resistance in metal-molecule-metal junctions based on aliphatic SAMs: effects of surface linker and metal work function

Using conducting probe atomic force microscopy (CP-AFM), we have formed molecular tunnel junctions consisting of alkanethiols and alkane isonitrile self-assembled monolayers sandwiched between gold, platinum, silver, and palladium contacts. We have measured the resistance of these junctions at low bias (dV/dI |V=0) as a function of alkane chain length. Extrapolation to zero chain length gives the contact resistance, R0 . R0 is strongly dependent on the type of metal used for the contacts and decreases with increasing metal work function; that is, R0,Ag > R0,Au > R0,Pd > R0,Pt. R0 is approximately 10% smaller for Au junctions with isonitrile versus thiol surface linkers. We conclude that the Fermi level of the junction lies much closer to the HOMO than to the LUMO.     

Enhanced hopping conductivity in low band gap donor-acceptor molecular wires Up to 20 nm in length

We have measured the current-voltage characteristics of conjugated oligo-tetrathiafulvalene-pyromelliticdiimide-imine (OTPI) wires ranging in length from 2.5 to 20.2 nm, contacted by Au electrodes. OTPI wires were built from Au substrates using alternating donor (tetrathiafulvalene, TTF) and acceptor (pyromelliticdiimide, PMDI) building blocks linked via aryl imine groups. Metal-molecule-metal junctions consisting of approximately 100 wires in parallel were prepared by contacting the wire films with an Au-coated atomic force microscope tip. The long OTPI wires exhibit a narrow band gap (<1.5 eV) and multiple redox states, which facilitate carrier injection from the Au contacts for hopping transport. We observe the theoretically predicted change in direct current (DC) transport from tunneling to hopping as a function of systematically controlled wire length, as well as strongly enhanced wire conductivity (0.02 S/cm) in the hopping regime. Hopping conduction is confirmed by length-, temperature-, and field-dependent transport measurements. These nanoscale transport measurements illuminate the role of molecular length and bond architecture on molecular conductivity and open opportunities for greater understanding of hopping transport in conjugated polymer films.     

Anisotropic Conductivity at the Single‐Molecule Scale

In most junctions built by wiring a single molecule between two electrodes, the electrons flow along only one axis: between the two anchoring groups. However, molecules can be anisotropic, and an orientation‐dependent conductance is expected. Here, we fabricated single‐molecule junctions by using the electrode potential to control the molecular orientation and access individual elements of the conductivity tensor. We measured the conductance in two directions, along the molecular plane as the benzene ring bridges two electrodes using anchoring groups (upright) and orthogonal to the molecular plane with the molecule lying flat on the substrate (planar). The perpendicular (planar) conductance is about 400 times higher than that along the molecular plane (upright). This offers a new method for designing a reversible room‐temperature single‐molecule electromechanical switch that controllably employs the electrode potential to orient the molecule in the junction in either “ON” or “OFF” conductance states.