Humidity-controlled rectification switching in ruthenium-complex molecular junctions.

Although molecular rectifiers were proposed over four decades ago 1,2 , until recently reported rectification ratios (RR) were rather moderate 2-11 (RR ~ 101). This ceiling was convincingly broken using a eutectic GaIn top contact 12 to probe molecular monolayers of coupled ferrocene groups (RR ~ 105), as well as using scanning tunnelling microscopy-break junctions 13-16 and mechanically controlled break junctions 17 to probe single molecules (RR ~ 102-103). Here, we demonstrate a device based on a molecular monolayer in which the RR can be switched by more than three orders of magnitude (between RR ~ 100 and RR ≥ 103) in response to humidity. As the relative humidity is toggled between 5% and 60%, the current-voltage (I-V) characteristics of a monolayer of di-nuclear Ru-complex molecules reversibly change from symmetric to strongly asymmetric (diode-like). Key to this behaviour is the presence of two localized molecular orbitals in series, which are nearly degenerate in dry circumstances but become misaligned under high humidity conditions, due to the displacement of counter ions (PF6-). This asymmetric gating of the two relevant localized molecular orbital levels results in humidity-controlled diode-like behaviour.

A gate-tunable single-molecule diode.

In the pursuit of down-sizing electronic components, the ultimate limit is the use of single molecules as functional devices. The first theoretical proposal of such a device, predicted more than four decades ago, is the seminal Aviram-Ratner rectifier that exploits the orbital structure of the molecule. The experimental realization of single-molecule rectifiers, however, has proven to be challenging. In this work, we report on the experimental realization of a gate-tunable single-molecule rectifier with rectification ratios as high as 600. The rectification mechanism arises from the molecular structure and relies on the presence of two conjugated sites that are weakly coupled through a saturated linker. The observed gate dependence not only demonstrates tunability of the rectification ratio, it also shows that the proposed rectification mechanism based on the orbital structure is operative in the molecule.

Stretching-Induced Conductance Increase in a Spin-Crossover Molecule.

We investigate transport through mechanically triggered single-molecule switches that are based on the coordination sphere-dependent spin state of Fe(II)-species. In these molecules, in certain junction configurations the relative arrangement of two terpyridine ligands within homoleptic Fe(II)-complexes can be mechanically controlled. Mechanical pulling may thus distort the Fe(II) coordination sphere and eventually modify their spin state. Using the movable nanoelectrodes in a mechanically controlled break-junction at low temperature, current-voltage measurements at cryogenic temperatures support the hypothesized switching mechanism based on the spin-crossover behavior. A large fraction of molecular junctions formed with the spin-crossover-active Fe(II)-complex displays a conductance increase for increasing electrode separation and this increase can reach 1-2 orders of magnitude. Theoretical calculations predict a stretching-induced spin transition in the Fe(II)-complex and a larger transmission for the high-spin configuration.

Charge transport across insulating self-assembled monolayers: non-equilibrium approaches and modeling to relate current and molecular structure.

This paper examines charge transport by tunneling across a series of electrically insulating molecules with the structure HS(CH2)4CONH(CH2)2R) in the form of self-assembled monolayers (SAMs), supported on silver. The molecules examined were studied experimentally by Yoon et al. (Angew. Chem. Int. Ed. 2012, 51, 4658-4661), using junctions of the structure AgS(CH2)4CONH(CH2)2R//Ga2O3/EGaIn. The tail group R had approximately the same length for all molecules, but a range of different structures. Changing the R entity over the range of different structures (aliphatic to aromatic) does not influence the conductance significantly. To rationalize this surprising result, we investigate transport through these SAMs theoretically, using both full quantum methods and a generic, independent-electron tight-binding toy model. We find that the highest occupied molecular orbital, which is largely responsible for the transport in these molecules, is always strongly localized on the thiol group. The relative insensitivity of the current density to the structure of the R group is due to a combination of the couplings between the carbon chains and the transmission inside the tail. Changing from saturated to conjugated tail groups increases the latter but decreases the former. This work indicates that significant control over SAMs largely composed of nominally insulating groups may be possible when tail groups are used that are significantly larger than those used in the experiments of Yoon et al.1.

Large negative differential conductance in single-molecule break junctions.

Molecular electronics aims at exploiting the internal structure and electronic orbitals of molecules to construct functional building blocks. To date, however, the overwhelming majority of experimentally realized single-molecule junctions can be described as single quantum dots, where transport is mainly determined by the alignment of the molecular orbital levels with respect to the Fermi energies of the electrodes and the electronic coupling with those electrodes. Particularly appealing exceptions include molecules in which two moieties are twisted with respect to each other and molecules in which quantum interference effects are possible. Here, we report the experimental observation of pronounced negative differential conductance in the current-voltage characteristics of a single molecule in break junctions. The molecule of interest consists of two conjugated arms, connected by a non-conjugated segment, resulting in two coupled sites. A voltage applied across the molecule pulls the energy of the sites apart, suppressing resonant transport through the molecule and causing the current to decrease. A generic theoretical model based on a two-site molecular orbital structure captures the experimental findings well, as confirmed by density functional theory with non-equilibrium Green's functions calculations that include the effect of the bias. Our results point towards a conductance mechanism mediated by the intrinsic molecular orbitals alignment of the molecule.

Large tunable image-charge effects in single-molecule junctions.

Metal/organic interfaces critically determine the characteristics of molecular electronic devices, because they influence the arrangement of the orbital levels that participate in charge transport. Studies on self-assembled monolayers show molecule-dependent energy-level shifts as well as transport-gap renormalization, two effects that suggest that electric-field polarization in the metal substrate induced by the formation of image charges plays a key role in the alignment of the molecular energy levels with respect to the metal's Fermi energy. Here, we provide direct experimental evidence for an electrode-induced gap renormalization in single-molecule junctions. We study charge transport through single porphyrin-type molecules using electrically gateable break junctions. In this set-up, the position of the occupied and unoccupied molecular energy levels can be followed in situ under simultaneous mechanical control. When increasing the electrode separation by just a few ångströms, we observe a substantial increase in the transport gap and level shifts as high as several hundreds of meV. Analysis of this large and tunable gap renormalization based on atomic charges obtained from density functional theory confirms and clarifies the dominant role of image-charge effects in single-molecule junctions.

An all-electric single-molecule motor.

Many types of molecular motors have been proposed and synthesized in recent years, displaying different kinds of motion, and fueled by different driving forces such as light, heat, or chemical reactions. We propose a new type of molecular motor based on electric field actuation and electric current detection of the rotational motion of a molecular dipole embedded in a three-terminal single-molecule device. The key aspect of this all-electronic design is the conjugated backbone of the molecule, which simultaneously provides the potential landscape of the rotor orientation and a real-time measure of that orientation through the modulation of the conductivity. Using quantum chemistry calculations, we show that this approach provides full control over the speed and continuity of motion, thereby combining electrical and mechanical control at the molecular level over a wide range of temperatures. Moreover, chemistry can be used to change all key parameters of the device, enabling a variety of new experiments on molecular motors.

Vibrational excitations in weakly coupled single-molecule junctions: a computational analysis.

In bulk systems, molecules are routinely identified by their vibrational spectrum using Raman or infrared spectroscopy. In recent years, vibrational excitation lines have been observed in low-temperature conductance measurements on single-molecule junctions, and they can provide a similar means of identification. We present a method to efficiently calculate these excitation lines in weakly coupled, gateable single-molecule junctions, using a combination of ab initio density functional theory and rate equations. Our method takes transitions from excited to excited vibrational state into account by evaluating the Franck-Condon factors for an arbitrary number of vibrational quanta and is therefore able to predict qualitatively different behavior from calculations limited to transitions from ground state to excited vibrational state. We find that the vibrational spectrum is sensitive to the molecular contact geometry and the charge state, and that it is generally necessary to take more than one vibrational quantum into account. Quantitative comparison to previously reported measurements on pi-conjugated molecules reveals that our method is able to characterize the vibrational excitations and can be used to identify single molecules in a junction. The method is computationally feasible on commodity hardware.