Conductance Of Molecular Junctions Research Articles

Robust conductance of dumbbell molecular junctions with fullerene anchoring groups

The conductance of a molecular wire connected to metallic electrodes is known to be sensitive to the atomic structure of the molecule-metal contact. This contact is to a large extent determined by the anchoring group linking the molecular wire to the metal. It has been found experimentally that a dumbbell construction with C(60) molecules acting as anchors yields more well-defined conductances as compared to the widely used thiol anchoring groups. Here, we use density functional theory to investigate the electronic properties of this dumbbell construction. The conductance is found to be stable against variations in the detailed bonding geometry and in good agreement with the experimental value of G=3×10(-4) G(0). Electron tunneling across the molecular bridge occurs via the lowest unoccupied orbitals of C(60) which are pinned close to the Fermi energy due to partial charge transfer. Our findings support the original motivation to achieve conductance values more stable towards changes in the structure of the molecule-metal contact leading to larger reproducibility in experiments.

Carbon tips as electrodes for single-molecule junctions

We study electron transport through single-molecule junctions formed by an octanethiol molecule bonded with the thiol anchoring group to a gold electrode and the opposing methyl endgroup to a carbon tip. Using the scanning tunneling microscope based break junction technique, we measure the electrical conductance of such molecular junctions. We observe the presence of well-defined conductance plateaus during the stretching of the molecular bridge, which is the signature of the formation of a molecular junction.

Charge-transfer contribution to surface-enhanced Raman scattering in a molecular junction: Time-dependent correlations

Raman spectroscopy of molecular junctions is a promising diagnostic and control tool. We present a model for charge-transfer contribution to surface-enhanced Raman spectroscopy, generalizing previous considerations to strong laser pulses of arbitrary time dependence. The approach paves a way to realistic simulations of Raman spectroscopy experiments in molecular junctions. We demonstrate that the optical response of molecular conduction junctions is correlated with the electron transport. Feynman diagrams responsible for such similarity are analyzed, and a possible explanation for observed (anti)correlated behavior of Stokes signal and conductance is proposed.

Raman Scattering and Electronic Heating in Molecular Conduction Junctions

Surface-enhanced Raman scattering (SERS) was recently used to monitor nonequilibrium properties of molecular conduction junctions. Ward et al. (Nat. Nanotechnol. 2011, 6, 33) have used such measurements to estimate heating of the molecular vibrations (indicated by the ratio between Stokes and anti-Stokes Raman peaks) as well as the electronic metal substrate (inferred from the corresponding components of the Raman continuum). The latter observation suggests, contrary to standard assumptions, significant heating of the metal contacts. Here, we discuss this observation by advancing a theory of the electronic Raman scattering background in biased current carrying molecular junctions and using it to estimate the electronic heating, as seen in the Raman signal. We reach the unexpected conclusion that while heating of the electronic background in Raman scattering from biased molecular junctions is indeed observed, this does not necessarily imply an appreciable deviation from thermal equilibrium in the electronic distributions in the leads.

Nonlinear Charge Transport in Redox Molecular Junctions: A Marcus Perspective

Redox molecular junctions are molecular conduction junctions that involve more than one oxidation state of the molecular bridge. This property is derived from the ability of the molecule to transiently localize transmitting electrons, implying relatively weak molecule-leads coupling and, in many cases, the validity of the Marcus theory of electron transfer. Here we study the implications of this property on the nonlinear transport properties of such junctions. We obtain an analytical solution of the integral equations that describe molecular conduction in the Marcus kinetic regime and use it in different physical limits to predict some important features of nonlinear transport in metal-molecule-metal junctions. In particular, conduction, rectification, and negative differential resistance can be obtained in different regimes of interplay between two different conduction channels associated with different localization properties of the excess molecular charge, without specific assumptions about the electronic structure of the molecular bridge. The predicted behaviors show temperature dependences typically observed in the experiment. The validity of the proposed model and ways to test its predictions and implement the implied control strategies are discussed.

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.

Effects of coupling to vibrational modes on the ac conductance of molecular junctions

We theoretically examine the effect of the coupling of the transport electrons to a vibrational mode of the molecule on the ac linear-response conductance of molecular junctions. Representing the molecule by a single electronic state, we find that at very low temperatures the frequency-dependent conductance is mainly enhanced (suppressed) by the electron-vibration interaction when the chemical potential is below (above) the energy of that state. The vertex corrections of the electron-vibration interaction induce an additional peak structure in the conductance, which can be observed by tuning the tunnel couplings with the leads.

Effect of phonons on the ac conductance of molecular junctions

We theoretically examine the effect of a single phonon mode on the structure of the frequency dependence of the ac conductance of molecular junctions, in the linear response regime. The conductance is enhanced (suppressed) by the electron-phonon interaction when the chemical potential is below (above) the energy of the electronic state on the molecule.PACS numbers: 71.38.-k, 73.21.La, 73.23.-b

An electrochemical jump-to-contact STM-break junction approach to construct single molecular junctions with different metallic electrodes

In this communication, we demonstrate an approach for conductance measurement of single molecular junctions with different metallic electrodes based on STM-break junction (STM-BJ) technique with electrochemical strategy. Conductance of molecular junctions formed with succinic acid using Cu, Ag and Au as metal electrodes has been systemically studied, the values being 18.6, 13.2 and 5.6 nS for Cu, Ag and Au electrodes, respectively. The observed decrease in conductance indicates the weakening of electronic coupling efficiency at the electrode–molecule contacts in the order of Cu > Ag > Au, which should be taken into account in evaluating the molecular conductance in the junctions.

Two-state conductance in single Zn porphyrin molecular junctions

Conductance measurements were taken by forming single molecule junctions between a scanning tunneling microscope tip and a gold substrate. We observed the existence of a two-state conductance in porphyrin molecules ligating a zinc atom. Peaks in the conductance histograms showed molecules changed from a high conductance state to a low conductance state. This effect was not observed for porphyrin molecules without a ligating atom. We discuss how this phenomenon may be attributed to conformational changes in the molecule.

Cyclic Conductance Switching in Networks of Redox-Active Molecular Junctions

Redox-active dithiolated tetrathiafulvalene derivatives (TTFdT) were inserted in two-dimensional nanoparticle arrays to build interlinked networks of molecular junctions. Upon oxidation of the TTFdT to the dication state, we observed a conductance increase of the networks by up to 1 order of magnitude. Successive oxidation and reduction cycles demonstrated a clear switching behavior of the molecular junction conductance. These results show the potential of interlinked nanoparticle arrays as chemical sensors.

Controlled Stability of Molecular Junctions

Using single molecules as the building blocks of electronic devices is the ultimate goal of molecular electronics. However, measuring, understanding and manipulating the transport of charge through molecules attached to nanosize electrodes remains a difficult task. It is thus important to fabricate molecular junctions exhibiting reproducible and stable electrical response, and evaluate their performance. [1,2] While chemists are able to synthesize molecules with amazing electronic, optical, magnetic, thermoelectric or electromechanical properties and with the potential to provide electronic devices with novel functionality, the integration into single molecular devices remains the main problem of the field “molecular electronics”. It has been demonstrated that the electronic transport through a molecule depends not only on its intrinsic properties but also on specif ic details of the contacts and the local environment. [3] Here we report a new strategy to monitor the conductance of such molecular junctions enabling to gain further insight into the detailed nature of the conductance of single molecules. Due to the stability and reproducibility of our junctions, our method provides a new perspective on studies of electronic transport on the nanometer scale.

Measurement of voltage-dependent electronic transport across amine-linkedsingle-molecular-wire junctions

We measure the conductance and current–voltage characteristics of two amine-terminated molecularwires— 4,4′-diaminostilbene and bis-(4-aminophenyl)acetylene—by breaking Au point contacts in amolecular solution at room temperature. Histograms compiled from thousands ofmeasurements show a slight increase in the molecular junction conductance (I/V) as the bias is increased to nearly 450 mV. Comparatively, similar conductance measurementsmade with 1,6-diaminohexane, a saturated molecule, demonstrate almost no bias dependence.We also present a new technique to measure a statistically defined current–voltage (I–V) curve. Application to all three molecules shows that4,4′-diaminostilbene exhibits the largest increase in differential conductance as afunction of applied bias. This indicates that the predominant transport channel for4,4′-diaminostilbene (the highest occupied molecular orbital) is closer to the Fermi level of themetal than that of the other molecules, consistent with the trends observed in themolecular ionization potential. We find that junctions constructed with the conjugatedmolecules show greater noise in individual junctions and less structural stability, onaverage, at biases greater than 450 mV. In contrast, junctions formed with the alkane cansustain a bias of up to 900 mV. This significantly affects the statistically averagedI–V characteristic measured for the conjugated molecules at higher bias.

Cooling mechanisms in molecular conduction junctions

While heating of a current carrying Ohmic conductors is an obvious consequence of the diffusive nature of the conduction in such systems, current-induced cooling has been recently reported in some molecular conduction junctions. In this paper, we demonstrate by simple models the possibility of cooling molecular junctions under applied bias, and discuss several mechanisms for such an effect. Our model is characterized by single electron tunneling between electrodes represented by free electron reservoirs through a system characterized by its electron levels, nuclear vibrations and their structures. We consider cooling mechanisms resulting from (a) cooling of one electrode surface by tunneling-induced depletion of high-energy electrons; (b) cooling by coherent sub resonance electronic transport analogous to atomic laser-induced cooling and (c) the incoherent analog of process (b)---cooling by driven activated transport. The non-equilibrium Green function formulation of junction transport is used in the first two cases, while a master equation approach is applied in the analysis of the third.

Tuning the conductance of molecular junctions: Transparent versus tunneling regimes

We present a theoretical study of the transport characteristics of molecular junctions, where first-row diatomic molecules are attached to (001) gold and platinum electrodes. We find that the conductance of all of these junctions is on the order of the conductance quantum unit ${G}_{0}$, spelling out that they belong to the transparent regime. We further find that the transmission coefficients show wide plateaus as a function of the energy, instead of the usual sharp resonances that signal the molecular levels in the tunneling regime. We use Caroli's model to show that this is a rather generic property of the transparent regime of a junction, which is driven by a strong effective coupling between the delocalized molecular levels and the conduction channels at the electrodes. We analyze the transmission coefficients and chemical bonding of gold/benzene and gold/benzene-dithiolate junctions to understand why the latter show large resistances while the former are highly conductive.

Chiral electron transport: Scattering through helical potentials

We present a model for the transmission of spin-polarized electrons through oriented chiral molecules, where the chiral structure is represented by a helix. The scattering potential contains a confining term and a spin-orbit contribution that is responsible for the spin-dependent scattering of electrons by the molecular target. The differential scattering cross section is calculated for right- and left-handed helices and for arbitrary electron spin polarizations. We apply our model to explain chiral effects in the intensity of photoemitted polarized electrons transmitted through thin organic layers. These are molecular interfaces that exhibit spin-selective scattering with surprisingly large asymmetry factors as well as a number of remarkable magnetic properties. In our model, differences in intensity are generated by the preferential transmission of electron beams whose polarization is oriented in the same direction as the sense of advance of the helix. This model can be easily extended to the Landauer regime of conductance where conductance is due to elastic scattering, so that we can consider the conductance of chiral molecular junctions.

Raman Scattering from Nonequilibrium Molecular Conduction Junctions

Raman scattering is a potentially important probe of structure, dynamics, and thermal properties of single-molecule conduction junctions. We combine a nonequilibrium Green's function description of the junction with a generalized scattering theory of the Raman process, which provides the first theoretical description of Raman scattering from such systems. The voltage dependence of the Raman flux shows a characteristic behavior at the conductance threshold resulting from (a) partial populations in the ground and excited molecular levels that give rise to two scattering pathways as well as interference between them and (b) junction heating that affects the Raman intensities. Comparing "effective temperatures" obtained from Raman scattering and heat balance serves to establish the integrity of this concept for nonequilibrium junctions.

A theoretical view of unimolecular rectification

The concept of single molecule rectifiers proposed in a theoretical work by Aviram andRatner in 1974 was the starting point of the now vibrant field of molecular electronics. Inthe meantime, a built-in asymmetry in the conductance of molecular junctions has beenreported at the experimental level. In this contribution, we present a theoreticalcomparison of three different types of unimolecular rectifiers: (i) systems where the donorand acceptor parts of the molecules are taken from charge-transfer salt components;(ii) zwitterionic systems and (iii) tour wires with nitro substituents. We conduct an analysisof the rectification mechanism in these three different types of asymmetric moleculeson the basis of parameterized quantum chemical models as well as with a fullnon-equilibrium Green’s function–density functional theory (NEGF-DFT) treatmentof the current–voltage characteristics of the respective metal–molecule–metaljunctions. We put a particular emphasis on the prediction of rectification ratios(RRs), which are crucial for the assessment of the technological usefulness of singlemolecule junctions as diodes. We also compare our results with values reportedin the literature for other types of molecular rectification, where the essentialasymmetry is not induced by the structure of the molecule alone but either bya difference in the electronic coupling of the molecule to the two electrodes orby attaching alkyl chains of different lengths to the central molecular moiety.

Conductance values of alkanedithiol molecular junctions

We study the electrical conductance of octanedithiol molecular junctions using a mechanically controllable break-junction setup. The stability of the system allows control of whether the electrodes get into contact before each new molecular junction formation or not (contact and non-contact modes). We find three characteristic conductance values for octanedithiol. Well-defined peaks in the conductance histograms at multiples of 1.2×10−5 G0 suggest that this value corresponds to the conductance of a single molecular junction conductance. Reproducible features are also observed at 4.5×10−5 and 2.3×10−4 G0. The first value has the strongest statistical weight, whereas the second is only observed in the non-contact mode. We propose that these two values reflect the formation of several molecular junctions in parallel between the electrodes.

Role of the virtual orbitals and HOMO-LUMO gap in mean-field approximations to the conductance of molecular junctions

We investigate the role of virtual orbitals on the molecular conductance at the static mean-field level of approximation in the nonequilibrium Green's functions (NEGFs) formalism, within a model system with wide-band leads. We use a projector operator formalism to define corrections to the molecular Hamiltonian that solely act on its virtual-orbital subspace, leaving ground-state properties invariant. In its most trivial limit, these corrections reduce to the scissor operator. More generally, we also propose a parameterless correction aimed toward improved fundamental energy gap estimates in the context of local self-energy approximations. We demonstrate how within any mean-field description dramatical differences in conductance values can be generated with the application of the scissor operators. For our model system, we show how the dramatical difference in conductance values obtained from the restricted Hartree--Fock and Hartree--Fermi--Amaldi approximations can be related to the different HOMO-LUMO gaps of each method and to a lesser extent differences in the molecular orbitals. Finally, we comment on the impact of different coupling-strength regimes and illustrate how the ${E}_{g}$-related differences in the two mean-field descriptions gradually lose their impact on the conductance with increasing molecular coupling to the leads. Our analysis points out some implications of the pragmatic use of the Kohn--Sham (KS) potentials in the NEGF-KS theory, and how to modify the scheme toward better physical consistency so that the resulting Hamiltonian is more physical and the position of the transmission resonances is improved.