Session V17: Focus Session: Electron, Ion, and Exciton Transport in Nanostructures - Junctions

Effects of electron-phonon coupling in the Kondo regime of a two-orbital single-molecule junction

Single-molecule junctions (SMJs) are electronic devices formed by a molecule bridging the gap between two metallic contacts. Despite their apparent simplicity, such systems have attracted much attention for the rich variety of experimentally accessible physics that they display. The spatial confinement of electrons in molecules can lead to collective phenomena such as Coulomb blockade and the Kondo effect, as well as to strong coupling of electrons to molecular vibrations. We explore the interesting interplay of electron-electron and electron-phonon interactions in a model of an SMJ in which the central molecule has two active orbitals. The nonperturbative numerical renormalization group method is used to treat the many-body Kondo physics and electron-phonon coupling on equal footing. Electron-phonon coupling renormalizes the energies and Coulomb interactions of the molecular orbitals. The effects are most pronounced in cases where both molecular orbitals lie close to the Fermi energy of the contacts. Here, a sufficiently strong phonon-assisted inter-orbital tunneling can suppress the Kondo effect and cause a crossover to a phonon-dominated regime having very different electrical transport properties.   

Rectification in Symmetric Conjugated Molecules with Asymmetric Linkers

Demonstrating single-molecule rectification is an important step towards the realization of molecule-based electronic devices. Most molecules put forward as potential rectifiers employ asymmetric molecular backbones. In contrast, we show that we can create rectifying junctions by designing asymmetry only into the linker groups used to bond the molecule to metal electrodes. Our molecules consist of a conjugated backbone terminated with methylsulfide on one end and methyl-trimethyltin on the other. These molecules couple to Au electrodes through an Au-SMe donor acceptor bond, which serves as the electronically weak link, and a Au-C covalent bond, which is created in-situ after the SnMe$_{3}$ cleaves off [1]. We create thousands of molecular junctions using a modified STM setup in a solution of molecules, measure their current-voltage (IV) characteristics and create averaged IV curves. We find that asymmetrically terminated molecules show non-linear IV curves with significant rectification, while molecules terminated symmetrically with either SMe or SnMe$_{3}$ do not show substantial rectification. We also find that the rectification direction is dependent on molecular orientation in the junction. [1] Chen, W., et al., J. Am. Chem. Soc., 2011. 133(43): p. 17160-17163   

Simultaneous Determination of Conductance and Thermopower of Single Molecule Junctions

We present a study of concurrent determination of conductance ($G)$ and thermopower ($S)$ of single-molecule junctions via direct measurement of electrical and thermoelectric currents. The junctions are created using the STM-based break-junction technique where a cold Au tip is repeatedly brought in and out of contact with a hot Au-on-mica substrate in an environment of the target molecule. We explore several amine-Au and pyridine-Au linked molecules that are predicted to conduct through either the highest occupied molecular orbital (HOMO) or the lowest unoccupied molecular orbital (LUMO), respectively. We find that the Seebeck coefficient is negative for pyridine-Au linked LUMO-conducting junctions and positive for amine-Au linked HOMO-conducting junctions. From histograms of thousands of junctions, we use the most probable Seebeck coefficient to determine a power factor, \textit{GS}$^{2}$, for each junction studied, and find that \textit{GS}$^{2 }$generally increases with $G$.   

Theory of Solvent-Mediated Environmental Effects on Molecular-Scale Transport

Single-molecule junctions, formed with well-defined and robust metal-molecule contacts, can provide an ideal model system to study mechanisms of charge transport at the molecular scale. However, the presence of solvent is often unavoidable, and recent experiments have shown that the junction conductance can be altered by a factor of two depending on the solvent present [1]. It has been proposed that the binding of the solvent to the gold electrodes changes their local work function, which in turn alters the conductance of the junction in a predictable manner. Here, we use a first-principles scattering-state approach, based on self-energy corrected density functional theory, to explore the transmission and conductance of bipyridine- and diaminestilbene-Au molecular junctions in the presence of solvent molecules, using an analytical model to compare with experimental results. We acknowledge DOE for support, and NERSC for computational resources.   

Tailoring IV Characteristics and Rectification in Single-Molecule Junctions

Asymmetry in the current-voltage characteristics, or current rectification, of nanoscale junctions is a critical property for many optoelectronic and energy conversion applications using nanostructured materials. Here, we compute the conductance, IV characteristics, and bias-dependent rectification of a class of molecular junctions, consisting of donor-acceptor molecules in contact with Au electrodes, using quantitative first-principles calculations [1]. We relate the rectification to the identities of the donor and acceptor moieties through the junction energy level alignment and dipole moments and find, surprisingly, that a large asymmetry in the contact coupling leads to weak rectification. We explain our results with an analytic coherent tunneling model, and suggest concrete strategies for obtaining high rectification in experimentally-achievable systems. [1] Darancet et al. , submitted (2011) .   

Electronic transport properties of functional single molecule junctions

We report experimental studies of single molecule conductance using two techniques: statistical measurements with repeatedly-formed breakjunctions in ambient conditions and low-temperature measurements with electromigrated breakjunctions. In each case, we chose molecules with specific functions. With statistical measurements, we measured the nonconducting open and conducting closed forms of dithienylethene, a photochromic (optically switchable) molecule. These molecules were synthesized with pyridine endgroups to achieve relatively well-defined and stable contacts to Au electrodes. For the closed isomer, we find a conductance of $(3.3 \pm 0.5)$ G$_{0}$, while that of the open isomer is below the noise floor of our measurement. We can therefore set a lower limit of 30 for the on/off ratio of this molecule. We are currently investigating the use of electromigrated graphene nano-constrictions and breakjunctions for spin-polarized single molecule conductance measurements and plan to present initial results for this technique.   

Bias-Dependent Noise Measurements in Individual Electromigrated Nanoscale Junctions

Shot noise provides insight into the correlated motion of electrons in nanostructures. Previous measurements have examined shot noise in mechanically controlled break junctions (MCBJs),looking at a large ensemble of junction configurations at room temperature. Electromigrated, lithographically created Au structures at liquid nitrogen and helium temperatures allow for noise measurements of individual junction configurations. High frequency excess noise is amplified by a RF amplifier chain and measured in combination with lock-in techniques simultaneously with the dc conductance. Noise measurements across bias and temperature are compared to previous experiments preformed with MCBJs at room temperature, with an emphasis on the dependence of the noise on bias conditions. We find that measured noise in individual junctions can exhibit nonlinearities with bias as well as asymmetries as a function of the polarity of bias current. Individual junctions can also switch stochastically between different atomic configurations with nearly identical conductances but strikingly different noise properties. We discuss these observations in the context of ``traditional'' shot noise, bias-dependent noise processes, and 1/f contributions to the measured noise.   

Shot noise measurements as a function of bias in STM-style gold junctions

Shot noise measurements provide more detailed information regarding conductance channels than transport alone. Because of nearly fully transmitted modes, shot noise in nanoscale junctions is suppressed strongly near certain conductance values. In this experiment, we use a gold tip in a STM-style geometry to make nanoscale junctions, which function as the noise source when under bias. Peaks of conductance histograms and related mean square, radio frequency shot noise are successfully measured simultaneously at room temperature, at a series of voltage biases. Those peaks and related shot noise suppressions appear near integer multiples of the conductance quantum G0, especially the first three. We are able to measure shot noise at biases as low as tens of millivolts, and make use of even lower biases to estimate the systematical background existing in our measurements. Combined with a radio frequency reflection measurement, we convert the measured signal to current noise across the junction. The relevant noise processes and their evolution with bias across the junctions will be discussed.   

Testing the charge transport mechanism in molecule-nanoparticle networks by molecular exchange

Nanoparticles molecular networks have recently emerged as useful toolbox for molecular electronics studies. Nanoparticles bridging the size gap between molecules and macroscopic electrical interconnects make possible the realization of large self-assemblies of particles interlinked by molecules, with unique advantage of high reproducibility and robustness. This results from averaging over ensembles of molecules and make possible applications ranging from electrical, optical, mechanical, to spintronics devices. Electronic properties of well-organized two-dimensional networks, where Coulomb blockade regime is expected to be predominant, make these materials of high interest as model systems. We present temperature-dependent transport properties of networks bridging high aspect ratio trenches, making possible measurements over a wide temperature range. We study how the charge transport is modified when reversible molecular exchange is performed, tailoring the intrinsic conductance of the molecule. Quantitative agreement with experiments is obtained using a model clarifying the role played by the intrinsic conduction properties of the molecules and the geometry of the network.   

Quantum phase transitions arising from competing electron-electron and electron-phonon interactions in a two-orbital single-molecule junction

Electron-electron and electron-phonon interactions both play important roles in determining the transport properties of nanostructures such as single-molecule junctions. We use the numerical renormalization group to study a molecule with two active electronic orbitals connecting a pair of metallic leads. We focus on quantum phase transitions (QPTs) that can be accessed by varying couplings to a local vibrational mode, particularly the strength of phonon-assisted tunneling between the two molecular orbitals. One type of QPT arises in situations where, in the absence of electron-phonon interactions, one of the molecular orbitals manifests the many-body Kondo effect with its characteristic zero-bias anomaly in the electrical conductance through the junction. At a critical coupling, the system undergoes a first-order QPT to a low-conductance phase in which the electron-phonon interaction overwhelms the strong bare electron-electron repulsion and Kondo physics is completely destroyed. A second type of first-order QPT is found in cases where there is also a Holstein coupling of local phonons to the molecular charge. We will explain the conditions that give rise to QPTs, as opposed to crossovers, between different ground states of this system.   

Motion and Photon Emission of Single Molecules in Space-Time

We have visualized tunneling electron induced motion of single Zn-Etioporphyrin molecules adsorbed on the thin oxide film grown on NiAl(110) surface using scanning tunneling microscopy (STM). When tunneling electrons are injected resonantly to an unoccupied molecular orbital, nearly bistable switching in tunneling current occurs and the molecule starts exhibiting vibronic progression in its photon emission spectra. The switching behavior was spatially mapped by recording time traces on individual pixels of an STM image. We reconstruct the motion using the spatial distribution of amplitude, frequency, and on-time of the switching, and interpret it as the planar hindered rotation shuttling between two different adsorption configurations. The angle of rotation is close to 45 degrees, and the on-time reveals the nature of local potential barrier. Due to the two fold symmetry of the molecules under interrogation, the conductance switching shows different polarities. The electronic excitation of the molecule leads to vibronic transition in which the molecule emits photons and vibrates inside the local potential wells. Considering significant corrugation of oxide surfaces, rotation mediated by quantum tunneling of ethyl groups of the molecule will be discussed.   

Atomic-scale motor driven by the current-induced forces

From first-principles approaches, we investigate the current-induced forces in an asymmetric molecular junction using Hellmann-Feynman type theorem in the framework of density functional theory in scattering approaches. We observe that it is possible to construct atomic-scale systems where the current-induced forces can be used to rotate the atoms. As an example, we consider a junction formed by the benzene molecule which directly connected to the Pt electrodes, where the benzene molecule is highly tilted. The highly tilted benzene molecule causes the streamline flow of the current to curve considerably to one side of the benzene ring. This could cause a net torque due to the unbalanced current-induced forces, which tend to rotate the benzene molecule in a manner similar to a stream of water rotates a waterwheel. Thus, the highly asymmetric single molecule junctions offer the atomic-scale systems to explore the possibility of nano-motors driven by non-equilibrium electron transport. The authors thank National Science Council (Taiwan) for support under Grant NSC 100-2112-M-012-MY3   

Capacitance variations of a Nanocapacitor in the Field Emission Regime

The electronic transport properties and mechanical forces between two metallic electrodes separated by a nanometer-sized vacuum gap have been studied using a Scanning Tunnelling Microscope combined with a Tuning Fork Force sensor. When applying a voltage difference to the electrodes above their work function energy, the Field Emission regime can be acceded at which Field Emission Resonances take place. Under these circumstances a decrease of the capacitance has been found to occur showing a new mechanism of capacitor leaking in the quantum regime.   

Probing Water Structures in Nanopores via Tunneling

We study the effects of volumetric constraints on the structure and electronic transport properties of distilled water in a synthetic nanopore. Combining classical molecular dynamics simulations with the Landauer approach to scattering theory as originally done in the context of DNA sequencing [1], we develop a relationship between the electronic current and the structure the water assumes in the confining pore-electrode system. Prior research in the field shows a tendency for the tunneling current through water to fluctuate due to local cavities in the water's structure. We show a shift in the tunneling current's dependence on pore diameter at the transition from exclusion of water to a monolayer. Furthermore, we argue that the current with respect to pore diameter does not follow a simple exponential curve at this transition as one would expect from tunneling. This research develops our understanding of water as a complex medium and describes fundamental physics of aqueous solutions. \\[4pt] [1] J. Lagerqvist, M. Zwolak, and M. Di Ventra, \textit{Fast DNA sequencing via transverse electronic transport}, Nano Lett. \textbf{6}, 779 (2006).