Molecular Transport Junctions Research Articles

The merger of electrochemistry and molecular electronics

Molecular Electronics has the potential to greatly enhance existing silicon-based microelectronics to realize new functions, higher device density, lower power consumption, and lower cost. Although the investigation of electron transport through single molecules and molecular monolayers in "molecular junctions" is a recent development, many of the relevant concepts and phenomena are derived from electrochemistry, as practiced for the past several decades. The past 10+ years have seen an explosion of research activity directed toward how the structure of molecules affects electron transport in molecular junctions, with the ultimate objective of "rational design" of molecular components with new electronic functions, such as chemical sensing, interactions with light, and low-cost, low-power consumer electronics. In order to achieve these scientifically and commercially important objectives, the factors controlling charge transport in molecules "connected" to conducting contacts must be understood, and methods for massively parallel manufacturing of molecular circuits must be developed. This Personal Account describes the development of reproducible and robust molecular electronic devices, starting with modified electrodes used in electrochemistry and progressing to manufacturable molecular junctions. Although the field faced some early difficulties in reliability and characterization, the pieces are now in place for rapid advances in understanding charge transport at the molecular level. Inherent in the field of Molecular Electronics are many electrochemical concepts, including tunneling, redox exchange, activated electron transfer, and electron coupling between molecules and conducting contacts.

Molecular Tunnel Junctions Based on π-Conjugated Oligoacene Thiols and Dithiols between Ag, Au, and Pt Contacts: Effect of Surface Linking Group and Metal Work Function

The tunneling resistance and electronic structure of metal-molecule-metal junctions based on oligoacene (benzene, naphthalene, anthracene, and tetracene) thiol and dithiol molecules were measured and correlated using conducting probe atomic force microscopy (CP-AFM) in conjunction with ultraviolet photoelectron spectroscopy (UPS). Nanoscopic tunnel junctions (~10 nm(2)) were formed by contacting oligoacene self-assembled monolayers (SAMs) on flat Ag, Au, or Pt substrates with metalized AFM tips (Ag, Au, or Pt). The low bias (<0.2 V) junction resistance (R) increased exponentially with molecular length (s), i.e., R = R(0) exp(βs), where R(0) is the contact resistance and β is the tunneling attenuation factor. The R(0) values for oligoacene dithiols were 2 orders of magnitude less than those of oligoacene thiols. Likewise, the β value was 0.5 per ring (0.2 Å(-1)) for the dithiol series and 1.0 per ring (0.5 Å(-1)) for the monothiol series, demonstrating that β is not simply a characteristic of the molecular backbone but is strongly affected by the number of chemical (metal-S) contacts. R(0) decreased strongly as the contact work function (Φ) increased for both monothiol and dithiol junctions, whereas β was independent of Φ within error. This divergent behavior was explained in terms of the metal-S bond dipoles and the electronic structure of the junction; namely, β is independent of contact type because of weak Fermi level pinning (UPS revealed E(F) - E(HOMO) varied only weakly with Φ), but R(0) varies strongly with contact type because of the strong metal-S bond dipoles that are responsible for the Fermi level pinning. A previously published triple barrier model for molecular junctions was invoked to rationalize these results in which R(0) is determined by the contact barriers, which are proportional to the size of the interfacial bond dipoles, and β is determined by the bridge barrier, E(F) - E(HOMO). Current-voltage (I-V) characteristics obtained over a larger voltage range 0-1 V revealed a characteristic transition voltage V(trans) at which the current increased more sharply with voltage. V(trans) values were generally >0.5 V and were well correlated with the bridge barrier E(F) - E(HOMO). Overall, the combination of electronic structure determination by UPS with length- and work function-dependent transport measurements provides a remarkably comprehensive picture of tunneling transport in molecular junctions based on oligoacenes.

Molecular orbital concept on spin-flip transport in molecular junctions

The spin-dependent electron transport correlated with spin-flip dynamics in a molecular junction was investigated in the wave-packet and Green’s function approaches. The molecular junction adopted in this work is described by a simple one-dimensional tight-binding chain including a localized spin. The spin exchange coupling J between the localized and conduction electron spins was taken into account through the s-d Hamiltonian. The wave-packet simulations showed that the transmission probabilities in both the spin-flip and no-flip processes show large peaks at the eigenvalues of the spin singlet (−3J/4) and triplet (J/4) states, and that, different transmission properties appear at the mid-gap of the two eigenvalues: the spin-flip process shows a moderate decrease, whereas the no-flip process an abrupt drop. Dividing the s-d Hamiltonian into two submatrices and referring to the molecular orbital concept for the coherent electron transport, we found that the moderate decrease in the spin-flip process at the mid-gap is the result of a coherent-and-cooperative contribution from the singlet and triplet states of the conduction and localized electron spins, and that, the abrupt drop in the no-flip process at the mid-gap is mainly caused by the coherent cancellation from the singlet and triplet states. The molecular orbital concept available for the electron transport including spin-flip scattering processes is described in Green’s function method, in analogy to the one derived for the spinless electron transport.

Time-dependent density functional study of transport in molecular junctions

Electron transport in nanostructures is calculated and compared using a time-independent and a time-dependent first-principles framework. The time-independent approach uses the nonequilibrium Green's-function technique to calculate the current, while the time-dependent method extracts the current from the time propagated wave function. The approaches have been tested using gold-benzene-dithiolate-gold and gold-bipyridine-gold molecular junctions. The reasons for the differences in the current-voltage curves predicted by the two methods are discussed.

Application of a semiclassical model for the second-quantized many-electron Hamiltonian to nonequilibrium quantum transport: The resonant level model

A semiclassical approach is developed for nonequilibrium quantum transport in molecular junctions. Following the early work of Miller and White [J. Chem. Phys. 84, 5059 (1986)], the many-electron Hamiltonian in second quantization is mapped onto a classical model that preserves the fermionic character of electrons. The resulting classical electronic Hamiltonian allows for real-time molecular dynamics simulations of the many-body problem from an uncorrelated initial state to the steady state. Comparisons with exact results generated for the resonant level model reveal that a semiclassical treatment of transport provides a quantitative description of the dynamics at all relevant timescales for a wide range of bias and gate potentials, and for different temperatures. The approach opens a door to treating nontrivial quantum transport problems that remain far from the reach of fully quantum methodologies.

Anion Stabilization in Electrostatic Environments

Excess charge stabilization of molecules in metallic environments is of particular importance for fields such as molecular electronics and surface chemistry. We study the energetics of benzene and its anion between two metallic plates. We observe that orientational effects are important at small interplate separation. This leads to benzene oriented perpendicular to the gates being more stable than the parallel case due to induced dipole effects. We find that the benzene anion, known for being unstable in the gas phase, is stabilized by the plates at zero bias and an interplate distance of 21 Å. We also observe the effect of benzene under a voltage bias generated by the plates; under a negative bias, the anion becomes destabilized. We use the electron localization function to analyze the changes in electron density due to the bias. These findings suggest that image effects such as those present in nanoscale devices are able to stabilize excess charge and should be important to consider when modeling molecular transport junctions and charge-transfer effects.

Graphical prediction of quantum interference-induced transmission nodes in functionalized organic molecules

Quantum interference (QI) in molecular transport junctions can lead to dramatic reductions of the electron transmission at certain energies. In a recent work [Markussen et al., Nano Lett., 2010, 10, 4260] we showed how the presence of such transmission nodes near the Fermi energy can be predicted solely from the structure of a conjugated molecule when the energies of the atomic p(z) orbitals do not vary too much. Here we relax the assumption of equal on-site energies and generalize the graphical scheme to molecules containing different atomic species. We use this diagrammatic scheme together with tight-binding and density functional theory calculations to investigate QI in linear molecular chains and aromatic molecules with different side groups. For the molecular chains we find a linear relation between the position of the transmission nodes and the side group π orbital energy. In contrast, the transmission functions of functionalized aromatic molecules generally display a rather complex nodal structure due to the interplay between molecular topology and the energy of the side group orbital.

Nonequilibrium transport in molecular junctions with strong electron-phonon interactions

We present a combined theoretical approach to study the nonequilibrium transport properties of nanoscale systems coupled to metallic electrodes and exhibiting strong electron-phonon interactions. We use the Keldysh Green function formalism to generalize beyond linear theory in the applied voltage an equation of motion method and an interpolative self-energy approximation previously developed in equilibrium. We analyze the specific characteristics of inelastic transport appearing in the intensity versus voltage curves and in the conductance, providing qualitative criteria for the sign of the step-like features in the conductance. Excellent overall agreement between both approaches is found for a wide range of parameters.

Thermal transport of molecular junctions in the pair tunneling regime

Charge and heat transport through a single molecule tunnel-coupled to external normal electrodes have been studied. The molecule with sufficiently strong interaction between lectrons and vibrational internal degrees of freedom can be characterized by the negative effective charging energy U$<0$. Such a molecule has been considered and modeled by the Anderson Hamiltonian. The electrical conductance, thermopower and thermal conductance of the system have been calculated as a function of gate voltage in the weak coupling limit within the rate equation approach. In the linear regime the analytic formulae for the transport coefficients in the pair dominated tunneling are presented. The effects found in the nonlinear transport include {\it inter alia} the rectification of the heat current. The sense of forward (reverse) direction, however, depends on the tuning parameter and can be controlled by the gate voltage. We also discuss the quantitation of the thermal conductance and the departures from the Wiedemann-Franz law.

Coherent Tunneling Transport in Molecular Junctions

Using saturated alkyl chain series with a dithiol anchor group, we systematically examined the intrinsic charge transport of single-molecule junctions in an electromigrated nanogap electrode. The saturated alkyl molecular system constitutes an important control series in molecular transport experiments to corroborate valid molecular junctions, because molecular energy levels remain nearly unchanged with molecular length, and the transport mechanism has been unambiguously established. Inelastic electron tunneling spectroscopy, temperature-variable current−voltage measurement, length-dependent conductance measurement, and transition voltage spectroscopy all validate the observation of intrinsic molecular properties in the electromigrated nanogap junctions.

Electronic transport in biphenyl single-molecule junctions with carbon nanotubes electrodes: The role of molecular conformation and chirality

We investigate, by means of ab initio calculations, electronic transport in molecular junctions composed of a biphenyl molecule attached to metallic carbon nanotubes. We find that the conductance is proportional to ${\text{cos}}^{2}\text{ }\ensuremath{\theta}$, with $\ensuremath{\theta}$ the angle between phenyl rings, when the Fermi level of the contacts lies within the frontier molecular orbitals energy gap. This result, which agrees with experiments in biphenyl junctions with nonorganic contacts, suggests that the ${\text{cos}}^{2}\text{ }\ensuremath{\theta}$ law has a more general applicability, irrespective of the nature of the electrodes. We calculate the geometrical degree of chirality of the junction, which only depends on the atomic positions, and demonstrate that it is not only proportional to ${\text{cos}}^{2}\text{ }\ensuremath{\theta}$ but also is strongly correlated with the current through the system. These results indicate that molecular conformation plays the preponderant role in determining transport properties of biphenyl-carbon nanotubes molecular junctions.

First-Principles Study for Detection of Inelastic Electron Transport in Molecular Junctions by Internal Substitution

The idea of doping and chemical substitutions for bridge molecules is one of promising techniques to control the transport properties and engineering of molecular devices. In the present article, we extend this concept and propose the use of internal substitutions to detect the mechanism of ballistic and inelastic transport through molecular junctions. By performing first-principles transport calculations for several internally substituted systems, we show systematic pathway analysis of tunneling electrons and vibronic states as well as electron−phonon couplings on bridge molecules. The correlation of inelastic electron tunneling spectroscopy (IETS) and Raman spectroscopy is also discussed.

Theoretical calculations of electron transport in molecular junctions: Inflection behavior in Fowler-Nordheim plot and its origin

We investigated the origin of an inflection behavior appearing in Fowler-Nordheim (F-N) plot of current-voltage characteristics for molecular junctions using two different levels of calculation methods: nonequilibrium Green's-function technique combined with the density-functional theory and tight-binding approximation. Although the inflection has so far been interpreted from the naive model that the charge transport mechanism transits from a direct to the F-N tunneling, our results indicated that the inflection does not necessarily mean the transition between the two regimes. We found from the close examination of the relation between the behavior of the F-N curve and the transmission function that the inflection takes place when the molecular level responsible for electric currents approaches to the edge of the bias window. While our interpretation for the inflection drastically differ from the conventional model, the F-N plots obtained from our calculations showed closely similar behavior as those from the recent experiments.

Dipole tuning of charge transport in molecular junctions

Molecules with different magnitude and direction of dipole moments are sandwiched between mercury (Hg) and p(+)-Si to form Hg-molecules-(p(+)) Si junctions. The importance of the dipole moment of molecules in controlling the symmetry of current-voltage curves is shown by carrying out charge transport measurements at these molecular junctions. Junction parameters are obtained by considering charge transport across the junction as a combination of tunneling and Schottky emission. An interesting effect due to dipole reversal resulting in rectification of junction curves in the opposite quadrants of the current-voltage axis is observed and analyzed. The junction curves also exhibit a different degree of rectification with increasing magnitude of the dipole moment of the sandwiched molecules.

Chemical Fabrication of Heterometallic Nanogaps for Molecular Transport Junctions

We report a simple and reproducible method for fabricating heterometallic nanogaps, which are made of two different metal nanorods separated by a nanometer-sized gap. The method is based upon on-wire lithography, which is a chemically enabled technique used to synthesize a wide variety of nanowire-based structures (e.g., nanogaps and disk arrays). This method can be used to fabricate pairs of metallic electrodes, which exhibit distinct work functions and are separated by gaps as small as 2 nm. Furthermore, we demonstrate that a symmetric thiol-terminated molecule can be assembled into such heterometallic nanogaps to form molecular transport junctions (MTJs) that exhibit molecular diode behavior. Theoretical calculations demonstrate that the coupling strength between gold and sulfur (Au-S) is 2.5 times stronger than that of Pt-S. In addition, the structures form Raman hot spots in the gap, allowing the spectroscopic characterization of the molecules that make up the MTJs.

Effect of Noise on DNA Sequencing via Transverse Electronic Transport

Previous theoretical studies have shown that measuring the transverse current across DNA strands while they translocate through a nanopore or channel may provide a statistically distinguishable signature of the DNA bases, and may thus allow for rapid DNA sequencing. However, fluctuations of the environment, such as ionic and DNA motion, introduce important scattering processes that may affect the viability of this approach to sequencing. To understand this issue, we have analyzed a simple model that captures the role of this complex environment in electronic dephasing and its ability to remove charge carriers from current-carrying states. We find that these effects do not strongly influence the current distributions due to the off-resonant nature of tunneling through the nucleotides—a result we expect to be a common feature of transport in molecular junctions. In particular, only large scattering strengths, as compared to the energetic gap between the molecular states and the Fermi level, significantly alter the form of the current distributions. Since this gap itself is quite large, the current distributions remain protected from this type of noise, further supporting the possibility of using transverse electronic transport measurements for DNA sequencing.

Effects of dephasing in molecular transport junctions using atomistic first principles

We report a theoretical investigation of dephasing effects to quantum transport properties of molecular junctions. The quantum transport analysis is done by density functional theory carried out within the nonequilibrium Green's function framework, and the dephasing effect is modeled within the B\uttiker-probe approach. We observe two distinct behaviors in the three systems we studied: either an increase or a decrease in electronic conduction with dephasing. For a 1,4-benzenedithiol molecule and an atomic gold chain, where the conducting molecular levels are located away from the Fermi level, conduction is seen to increase due to reduced destructive interference resulting from the B\uttiker probe. On the other hand, for a very thin Al nanowire we find that backscattering dominates over the phase-randomization and the current decreases with dephasing. The resistance follows Ohm's law while the resistivity scales linearly with the scattering rate. Finally, a comparison between the B\uttiker-probe model and a more microscopic dephasing model shows nearly identical transport characteristics. From a computational point of view, the B\uttiker-probe model has an order of magnitude speed up.

Spectroscopic Tracking of Molecular Transport Junctions Generated by Using Click Chemistry

AbstractMit einem Klick die Lücke gefüllt: Mithilfe der Klick‐Chemie wird ein Moleküldraht, der eine durch „On‐Wire“‐Lithographie erzeugte Nanolücke überbrückt, in situ modular aufgebaut (siehe Bild). Der Draht entsteht in hoher Ausbeute, und es können unterschiedliche Moleküle getestet werden; die Triazolgruppe bleibt auch im Moleküldraht konjugiert. Der Aufbau des Drahtes wurde Raman‐spektroskopisch verfolgt.magnified image

Cover Picture: Spectroscopic Tracking of Molecular Transport Junctions Generated by Using Click Chemistry (Angew. Chem. Int. Ed. 28/2009)

AbstractThe in situ generation of molecules that bridge nanogaps generated by on‐wire lithography is achieved by using click chemistry. C. A. Mirkin, M. A. Ratner, and co‐workers show in their Communication on page 5178 ff. that these molecular junctions give high yields, and they describe how this strategy can be generalized to incorporate diverse molecular architectures within the nanogaps. Tracking of the molecular assembly process within the nanogaps is allowed by their strong electromagnetic field.magnified image

Spectroscopic tracking of molecular transport junctions generated by using click chemistry.

Click to fill the gap: The in situ modular fabrication of molecular transport junctions in nanogaps generated by on-wire lithography is achieved by using click chemistry (see picture). The formation of molecular junctions proceeds in high yields and can be used to test different molecules; the triazole group also maintains conjugation in the molecular wires. Raman spectroscopy is used to characterize the molecular assembly processes.