Electron Transport In Molecular Junctions Research Articles

A DFT Study on the Relationship Between Molecular Structure and Electron Transport in Molecular Junctions

A DFT Study on the Relationship Between Molecular Structure and Electron Transport in Molecular Junctions

Nonadiabatic vibronic effects in single-molecule junctions: A theoretical study using the hierarchical equations of motion approach

The interaction between electronic and vibrational degrees of freedom is an important mechanism in nonequilibrium charge transport through molecular nanojunctions. While adiabatic polaron-type coupling has been studied in great detail, new transport phenomena arise for nonadiabatic coupling scenarios corresponding to a breakdown of the Born-Oppenheimer approximation. Employing the numerically exact hierarchical equations of motion approach, we analyze the effect of nonadiabatic electronic-vibrational coupling on electron transport in molecular junctions considering a series of models with increasing complexity. The results reveal a significant influence of nonadiabatic coupling on the transport characteristics and a variety of interesting effects, including negative differential conductance. The underlying mechanisms are analyzed in detail.

Investigating electronic properties and electron transport in molecular junctions of new model CoNiO2 doped with Li and Na via extended tight-binding based on DFT computational method

In this study, Li and Na atoms were doped in CoNiO2 transition metal to determine the band structure and density of states (DOS) by using density functional theory (DFT). The extended tight binding DFT (GFN1-xTB) was adopted to investigate the compounds for figuring out the geometry optimization process. The transmission spectra and electric current passing through metal-molecular junctions were calculated by using nonequilibrium Green’s-function (NEGF) method. The results obtained after DFT calculations indicated that while Li-doped CoNiO2 exhibited a semiconductor behaviour, Na-doped CoNiO2 exhibited a metallic behaviour.

Reduced-Order Modeling Approach for Electron Transport in Molecular Junctions.

To describe nonequilibrium transport processes in a quantum device with infinite baths, we propose to formulate the problems as a reduced-order problem. Starting with the Liouville-von Neumann equation for the density-matrix, the reduced-order technique yields a finite system with open boundary conditions. We show that with appropriate choices of subspaces, the reduced model can be obtained systematically from the Petrov-Galerkin projection. The self-energy associated with the bath emerges naturally. The results from the numerical experiments indicate that the reduced models are able to capture both the transient and steady states.

Giant Conductance Enhancement of Intramolecular Circuits through Interchannel Gating

Summary For neutral intramolecular circuits with two constitutionally identical branches, a maximum 4-fold increase in total conductance can be obtained according to constructive quantum interference (CQI). For charged intramolecular circuits, however, the strong electrostatic interactions entangle the quantum states of these two parallel pathways, thus introducing complicated transport behavior that warrants experimental investigation of the intramolecular circuit rules. Here, we report that a tetracationic cyclophane with parallel channels exhibits a 50-fold conductance enhancement compared with that of a single-channel control, an observation that supplements intramolecular circuit law in systems with strong Coulombic interactions. Flicker noise measurements and theoretical calculations show that strong electrostatic interactions between charged parallel channels—serving as the chemical gate to promote the effective conductance of each channel—and CQI boosts the total conductance of the two-channel circuit. The molecular design presented herein constitutes a proof-of-principle approach to charged intramolecular circuits that are desirable for quantum circuits and devices.

Novel electron-deficient oligo(phenyleneethynylene) derivatives for molecular electronics

ABSTRACTThis work reports synthesis and characterizations of two new electron-poor “oligo(phenyleneethynylene) (OPE) type” molecular wires for fundamental studies of electron transport in molecular junctions. These OPE derivatives display three aromatic rings functionalized (i) with NO2 (OPN) or fluorine (OPF) groups on the central aryl core and (ii) with the requisite protected thiolate anchoring groups on the lateral rings at both ends. We show that the moderately effective Sonogashira couplings can give access to such rare electrodeficient molecules but are unfortunately associated with significant side reactions. We detail the choice of adequate reaction conditions to allow the recovery of suitable amounts of compounds bearing several strongly electron-withdrawing substituents on their central ring for further study of the physical properties.

The DFT-NEGF scrutiny of doped fullerene junctions

Using the smallest non-classical fullerene, we investigate the impact of doping at the molecule-electrode interface on the electron transport of molecular junctions. This is accomplished by employing the density functional theory combined with the non-equilibrium Green's function. We contemplate different electronic parameters, namely, density of states, transmission coefficient, energy levels, molecular orbitals, conduction gaps, electron density, and their charge transfer. The relevance of these physical parameters is obtained to calculate their electrical parameters, current, and conductance, computed from Landauer-Büttiker formalism. The molecule-electrode coupling is influenced by the nature of doping atoms and affects the junction devices in a unique course. A particular aftermath is noticed in Au-C18O2-Au device with highest ballistic transport despite the electro-negative nature of oxygen atoms. Moreover, an interesting feature is observed in Au-C18Be2-Au device with double-barrier transmission resonance and corresponding oscillating conductance. Graphical abstract The doped C20 fullerene in molecular and device mode.

Electron transport in doped fullerene molecular junctions

The effect of doping on the electron transport of molecular junctions is analyzed in this paper. The doped fullerene molecules are stringed to two semi-infinite gold electrodes and analyzed at equilibrium and nonequilibrium conditions of these device configurations. The contemplation is done using nonequilibrium Green’s function (NEGF)-density functional theory (DFT) to evaluate its density of states (DOS), transmission coefficient, molecular orbitals, electron density, charge transfer, current, and conductance. We conclude from the elucidated results that Au–C[Formula: see text]Li4–Au and Au–C[Formula: see text]Ne4–Au devices behave as an ordinary p–n junction diode and a Zener diode, respectively. Moreover, these doped fullerene molecules do not lose their metallic nature when sandwiched between the pair of gold electrodes.

Fluctuating-bias controlled electron transport in molecular junctions

We consider the problem of transport through a multiterminal molecular junction in the presence of a stochastic bias, which can also be used to describe transport through fluctuating molecular energy levels. To describe these effects, we first make a simple extension of our previous work [Phys. Rev. B 91, 125433 (2015)] to show that the problem of tunneling through noisy energy levels can be mapped onto the problem of a noisy driving bias, which appears in the Kadanoff-Baym equations for this system in an analogous manner to the driving term in the Langevin equation for a classical circuit. This formalism uses the nonequilibrium Green's function method to obtain analytically closed formulas for transport quantities within the wide-band limit approximation for an arbitrary time-dependent bias and it is automatically partition free. We obtain exact closed formulas for both the colored and white noise-averaged current at all times. In the long-time limit, these formulas possess a Landauer-B\uttiker--type structure which enables the extraction of an effective transmission coefficient for the transport. Expanding the Fermi function into a series of simple poles, we find an exact formal relation between the parameters which characterize the bias fluctuations and the poles of the Fermi function. This enables us to describe the effect of the temperature and the strength of the fluctuations on the averaged current which we interpret as a quantum analog to the classical fluctuation-dissipation theorem. We use these results to convincingly refute some recent results on the multistability of the current through a fluctuating level, simultaneously verifying that our formalism satisfies some well-known theorems on the asymptotic current. Finally, we present numerical results for the current through a molecular chain which demonstrate a transition from nonlinear to linear $I\text{\ensuremath{-}}V$ characteristics as the strength of fluctuations is increased, as well as a stochastic resonance effect in the conductance of this system.

Electron transport in molecular junctions with graphene as protecting layer

We present ab initio transport calculations for molecular junctions that include graphene as a protecting layer between a single molecule and gold electrodes. This vertical setup has recently gained significant interest in experiment for the design of particularly stable and reproducible devices. We observe that the signals from the molecule in the electronic transmission are overlayed by the signatures of the graphene sheet, thus raising the need for a reinterpretation of the transmission. On the other hand, we see that our results are stable with respect to various defects in the graphene. For weakly physiosorbed molecules, no signs of interaction with the graphene are evident, so the transport properties are determined by offresonant tunnelling between the gold leads across an extended structure that includes the molecule itself and the additional graphene layer. Compared with pure gold electrodes, calculated conductances are about one order of magnitude lower due to the increased tunnelling distance. Relative differences upon changing the end group and the length of the molecule on the other hand, are similar.

A quasi-classical mapping approach to vibrationally coupled electron transport in molecular junctions

We develop a classical mapping approach suitable to describe vibrationally coupled charge transport in molecular junctions based on the Cartesian mapping for many-electron systems [B. Li and W. H. Miller, J. Chem. Phys. 137, 154107 (2012)]. To properly describe vibrational quantum effects in the transport characteristics, we introduce a simple transformation rewriting the Hamiltonian in terms of occupation numbers and use a binning function to facilitate quantization. The approach provides accurate results for the nonequilibrium Holstein model for a range of bias voltages, vibrational frequencies, and temperatures. It also captures the hallmarks of vibrational quantum effects apparent in step-like structure in the current-voltage characteristics at low temperatures as well as the phenomenon of Franck-Condon blockade.

Calculation of Electronic Lifetimes in Molecular Junctions

We study the electronic lifetimes in molecular junctions using the Keldysh nonequilibrium Green function (NEGF) model . It is found that two characteristic time scales which govern the electron tunneling phenomena. One is the electronic lifetime in which electronic energy in the molecular electronic state dissipates into the right/left electrode or vibrational mode , the other is the residence time due to virtual excitation of electrons between the two electrode s and the molecular electronic state . And using a set of the parameters which characterize the molecular junction , we show that the molecular vibrational frequency, the position of the electronic state of the molecul e , the strength of electron - vibration coupling (via the electron - phonon interaction ) and the distance between the molecule and the electrode s (via the molecule - electrode coupling ) represent substantial contributions that affect the electronic lifetime in molecular junctions , as result all are represent important parameters to determine the nature and the mechanisms of electron transport in molecular junctions .

Tuning the electron transport of molecular junctions by chemically functionalizing anchoring groups: First-principles study

In this first-principles study, we present density-functional calculations of the electronic structures and electron transport properties of organic molecular junctions with several anchoring groups containing atoms with different electronegativities, i.e., benzenediboronate (BDB), benzenedicarboxylate (BDC), and dinitrobenzene (DNB) molecular junctions sandwiched between two Cu(110) electrodes. The electronic-structure calculations exhibit a significant difference in the density of states not only at the anchoring groups but also at the aromatic rings of the molecular junctions, suggesting that the electron transport is specific for each system. Our transport calculations show that the BDB and DNB molecular junctions have finite electron transmissions at the zero-bias limit while the BDC molecular junction has a negligible electron transmission. Moreover, for the BDB and DNB systems, the electron transmission channels around the Fermi energy reveal fingerprint features, which provide specific functionalities for the molecular junctions. Therefore, our theoretical results demonstrate the possibility to precisely tune the electron transport properties of molecular junctions by engineering the anchoring groups at the single-atom level.

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.

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.

Asymmetric electron transport realized by decoupling between molecule and electrode

We studied the contact coupling effect on the asymmetric electron transport in molecular junctions by the first-principles density functional theory incorporating with the non-equilibrium Green's function method. To realize the decoupling, a rigid saturated ring is inserted into the metallic electrode and conjugated molecular bridge (linear oligo phenylene ethynylene and cyclic porphine). As a tunneling barrier, the saturated ring reduces the conductance by 2-3 orders of magnitude. However, the electronic decoupling greatly improves the asymmetric electron transport. In the case of the linear system, the favorite direction of electron transport is from the strong coupling end to the weak coupling one with a rectification ratio of 5 at 2.0 V. In addition, the rectification performance is sensitive to the molecular proportion of the molecular wire length and the tunneling barrier width. When the same barrier is applied, shortening the length of conjugated part can reduce rectification performance. The mechanism of rectification is analyzed by means of the potential drop, the spatial distribution of the molecular orbitals and the transmission spectra.

Theoretical Study of Long-Range Electron Transport in Molecular Junctions

A study of long-range electron transport through a series of molecular junctions comprised of π-conjugated molecules is presented. A theoretical model is built by using a quantum chemistry based Landauer theory combined with Green’s function formalism and the D’Amato−Pastawski model to describe the electronic conduction in the presence of incoherent scattering effects. Effects of incoherent transport on the total conductance under the nonbiased, stationary, and nondissipative conditions are investigated. Calculation results with the Newns−Anderson approximation show that the distance dependence of electron conduction through molecular wires smoothly interpolates the transition of the dominant transporting mechanism from coherent tunneling with the exponential decay in short distance to incoherent tunneling associated with sequential hopping with weak distance dependence.

Molecular modeling of inelastic electron transport in molecular junctions

A quantum chemical approach for the modeling of inelastic electron tunneling spectroscopyof molecular junctions based on scattering theory is presented. Within a harmonicapproximation, the proposed method allows us to calculate the electron–vibration couplingstrength analytically, which makes it applicable to many different systems. The calculatedinelastic electron transport spectra are often in very good agreement with theirexperimental counterparts, allowing the revelation of detailed information about molecularconformations inside the junction, molecule–metal contact structures, and intermolecularinteraction that is largely inaccessible experimentally.

Electron transport in molecular junctions

Building an electronic device using individual molecules is one of the ultimate goals in nanotechnology. To achieve this it will be necessary to measure, control and understand electron transport through molecules attached to electrodes. Substantial progress has been made over the past decade and we present here an overview of some of the recent advances. Topics covered include molecular wires, two-terminal switches and diodes, three-terminal transistor-like devices and hybrid devices that use various different signals (light, magnetic fields, and chemical and mechanical signals) to control electron transport in molecules. We also discuss further issues, including molecule-electrode contacts, local heating- and current-induced instabilities, stochastic fluctuations and the development of characterization tools.