Transport In Molecular Devices Research Articles

Modulating Quantum Interference Between Destructive and Constructive States in Double N‐Substituted Single Molecule Junctions

AbstractQuantum interference (QI) plays a crucial role in determining the charge transport in molecular devices. In this work, the efficient modulation of QI in meta‐phenylene ethylene oligomer molecular devices by a double N‐substitution strategy is demonstrated. By altering the positions of two N atoms in the central ring with respect to the connecting sites, the molecular conductance can be tuned by more than one order of magnitude. Theoretical analysis, including magic ratio theory, orbital rule, and transmission simulations, reveals how the two N atoms synergistically modulate the molecule conductance between destructive QI and constructive QI states. Remarkably, addition of a second N atom does not simple reinforce the effect of the first; in contrast, it may completely cancel the effect of the first. Understanding the complex electronic interplay between the two N atoms in double N‐substituted molecules paves a path toward utilization of heterocyclic aromatic hydrocarbons in molecular electronics.

Transition between Nonresonant and Resonant Charge Transport in Molecular Junctions.

Efficient long-range charge transport is required for high-performance molecular electronic devices. Resonant transport is thought to occur in single molecule junctions when molecular frontier orbital energy levels align with electrode Fermi levels, thereby enabling efficient transport without molecular or environmental relaxation. Despite recent progress, we lack a systematic understanding of the transition between nonresonant and resonant transport for molecular junctions with different chemical compositions. In this work, we show that molecular junctions undergo a reversible transition from nonresonant tunneling to resonant transport as a function of applied bias. Transient bias-switching experiments show that the nonresonant to resonant transition is reversible with the applied bias. We determine a general quantitative relationship that describes the transition voltage as a function of the molecular frontier orbital energies and electrode Fermi levels. Overall, this work highlights the importance of frontier orbital energy alignment in achieving efficient charge transport in molecular devices.

Systematic Modulation of Charge Transport in Molecular Devices through Facile Control of Molecule-Electrode Coupling Using a Double Self-Assembled Monolayer Nanowire Junction.

We report a novel solid-state molecular device structure based on double self-assembled monolayers (D-SAM) incorporated into the suspended nanowire architecture to form a "Au|SAM-1||SAM-2|Au" junction. Using commercially available thiol molecules that are devoid of synthetic difficulty, we constructed a "Au|S-(CH2)6-ferrocene||SAM-2|Au" junction with various lengths and chemical structures of SAM-2 to tune the coupling between the ferrocene conductive molecular orbital and electrode of the junction. Combining low noise and a wide temperature range measurement, we demonstrated systematically modulated conduction depending on the length and chemical nature of SAM-2. Meanwhile, the transport mechanism transition from tunneling to hopping and the intermediate state accompanied by the current fluctuation due to the coexistence of the hopping and tunneling transport channels were observed. Considering the versatility of this solid-state D-SAM in modulating the electrode-molecule interface and electroactive groups, this strategy thus provides a novel facile strategy for tailorable nanoscale charge transport studies and functional molecular devices.

2D g-C3N4 monolayer for amino acids sequencing

An analog of graphene, graphitic carbon nitride (g-C3N4), is a promising metal-free conjugated polymer, owing to its excellent performance in biosensing and photocatalysis. We have demonstrated the adsorption of twenty-five amino acids (AA) employing DFT-D3 correction method of Grimme's dispersion and the non-equilibrium Green's function (NEGF) for describing the coherent transport in molecular devices coupled with adsorption energies, substrate-adsorbate distances, the density of states, charge transfer mechanism, molecular dynamics, work function, and bonding patterns. We have also depicted the current-voltage (I–V) characteristics where the curves of current vs. bias voltage (I–Vb) display a distinct response for each AA. Furthermore, we have illustrated the anti-bacterial mechanism of g-C3N4 utilizing bioinformatics study and compared it with DFT studies. We found evidence of a difference in transport, electronic as well as molecular mechanisms reinforcing the possibility of g-C3N4 applications based on sensors for AA sequencing of proteins, water-disinfection technique, and microbial control.

Understanding resonant charge transport through weakly coupled single-molecule junctions

Off-resonant charge transport through molecular junctions has been extensively studied since the advent of single-molecule electronics and is now well understood within the framework of the non-interacting Landauer approach. Conversely, gaining a qualitative and quantitative understanding of the resonant transport regime has proven more elusive. Here, we study resonant charge transport through graphene-based zinc-porphyrin junctions. We experimentally demonstrate an inadequacy of non-interacting Landauer theory as well as the conventional single-mode Franck–Condon model. Instead, we model overall charge transport as a sequence of non-adiabatic electron transfers, with rates depending on both outer and inner-sphere vibrational interactions. We show that the transport properties of our molecular junctions are determined by a combination of electron–electron and electron-vibrational coupling, and are sensitive to interactions with the wider local environment. Furthermore, we assess the importance of nuclear tunnelling and examine the suitability of semi-classical Marcus theory as a description of charge transport in molecular devices.

Electronic transport in a graphene single layer: application in amino acid sensing.

We modeled a type of field-effect transistor device based on graphene for the recognition of amino acids with a potential application in the building of a protein sequencer. The theoretical model used was a combination of density functional theory (DFT) with the non-equilibrium Green's function (NEGF) in order to describe the coherent transport in molecular devices. First, we studied the physisorption of each amino acid on a graphene sheet and we reported the adsorption energy, the adsorption distances, the equilibrium configuration and the charge transfer of ten amino acids that can be considered as representative of all of the amino acids: histidine (His), alanine (Ala), aspartic acid (Asp), tyrosine (Tyr), arginine (Arg), glutamic acid (Glu), glycine (Gly), phenylalanine (Phe), proline (Pro) and lysine (Lys). As a result, significant differences were found in the density of states (DOS) after adsorption and there was a change in the semi-metallic character of the graphene due to the lysine and arginine interactions. Furthermore, we noticed changes in the electrical characteristics of the devices, as the amino acids adsorbed onto the surface of the graphene. The curves of current vs. bias voltage (I-Vb) display a distinct response for each amino acid, i.e. the I-Vb curves produce a characteristic footprint for each amino acid. We identified a possible rectification mechanism related to the voltage profile asymmetry, where the amino acids can control the transport characteristics in the device, i.e. Lys and Phe amino acids physisorbed on graphene act as a molecular diode, where electrons can easily flow in one direction and decrease in the other. This may be promising for the prospect of biosensors: graphene could be used as an amino acid detector.

Electronic transport induced by edge modification of graphene electrodes in single molecular device

The aim was to study the electronic transport of molecular devices constructed by salicylidene aniline molecule and graphene electrodes using the density functional theory and non-equilibrium Green’s function. The calculated results demonstrate a switching behavior in two photoisomerizations. In addition, the modification of –OH on the edge of graphene nanoribbons induces the variety of the frontier molecular orbitals and the increase of current, but the decrease of switching ratio. The modification of F on the edge of graphene nanoribbons improves the switching ratio. We explain the negative differential resistance effect found in the device by transmission spectra and the evolution of local density of states at the bias. The results indicate that different modifications on the edge of graphene nanoribbons influence the electronic transport. This method can improve the application of the device in future molecular circuits.

OLIFE: Tight Binding Code for Transmission Coefficient Calculation

A new and human friendly transport calculation code has been developed. It requires a simple tight binding Hamiltonian as the only input file and uses a convenient graphical user interface to control calculations. The effect of magnetic field on junction has also been included. Furthermore the transmission coefficient can be calculated between any two points on the scatterer which ensures high flexibility to check the system. Therefore Olife can highly be recommended as an essential tool for pretesting studying and teaching electron transport in molecular devices that saves a lot of time and effort.

Spin-filtering and giant magnetoresistance effects in polyacetylene-based molecular devices

Using the non-equilibrium Green's function formalism in combination with density functional theory, we performed ab initio calculations of spin-dependent electron transport in molecular devices consisting of a polyacetylene (CnHn+1) chain vertically attached to a carbon chain sandwiched between two semi-infinite zigzag-edged graphene nanoribbon electrodes. Spin-charge transport in the device could be modulated to different magnetic configurations by an external magnetic field. The results showed that single spin conduction could be obtained. Specifically, the proposed CnHn+1 devices exhibited several interesting effects, including (dual) spin filtering, spin negative differential resistance, odd–even oscillation, and magnetoresistance (MR). Marked spin polarization with a filtering efficiency of up to 100% over a large bias range was found, and the highest MR ratio for the CnHn+1 junctions reached 4.6 × 104. In addition, the physical mechanisms for these phenomena were also revealed.

On the separability of the extended molecule: Constructing the best localized molecular orbitals for an organic molecule bridging two model electrodes.

Based on a quantum chemical valence formalism that allows the rigorous construction of best-localized molecular orbitals on specific parts of an extended system, we examined the separability of individual components of model systems relevant to the description of electron transport in molecular devices. We started by examining how to construct the maximally localized electronic density at the tip of a realistic model of a gold electrode. By varying the number of gold atoms included in the local region where to project the total electronic density, we quantitatively assess how many molecular orbitals are entirely localized in that region. We then considered a 1,4-benzene-di-thiol molecule connected to two model gold electrodes and examined how to localize the electronic density of the total system in the extended molecule, a fractional entity comprising the organic molecule plus an increasing number of the closest metal atoms. We were able to identify in a rigorous manner the existence of three physically different electronic populations, each one corresponding to a distinct set of molecular orbitals. First, there are those entirely localized in the extended molecule, then there is a second group of those completely distributed in the gold atoms external to that region, and, finally, there are those delocalized over the entire system. This latter group can be associated to the shared electronic population between the extended molecule and the rest of the system. We suggest that the treatment here presented could be useful in the theoretical analysis of the electronic transport in nanodevices whenever the use of localized molecular states are required by the physics of the specific problem, such as in cases of weak coupling and super-exchange limits.

Current-conserving and gauge-invariant quantum ac transport theory in the presence of phonon

Using the nonequilibrium Green’s function (NEGF) approach, we develop a microscopic ac transport theory in the presence of electron-phonon interaction. Taking into account the self-consistent Coulomb interaction, the displacement current is included. This ensures that our theory satisfies the current-conserving and gauge-invariant conditions. Importantly, the inclusion of self-consistent Coulomb interaction naturally connects the NEGF formalism to the density functional theory (DFT). This allows us to calculate the self-consistent Hamiltonian using DFT within the NEGF framework, which paves the way for first principles ac transport calculation of nanoelectronic devices in the presence of electron-phonon interaction. It is known that the inelastic electron tunneling spectroscopy (IETS) is a powerful tool in studying the inelastic dc quantum transport in molecular devices. The basic idea of IETS is to obtain the information of vibrational spectrum of molecular devices by measuring the second derivative of the dc current with respect to the bias voltage. In the ac transport, we find that the phonon spectrum and electron-phonon coupling strength can be obtained from the second derivative of the admittance with respect to the frequency which is the working principle of the inelastic electron admittance spectroscopy (IEAS). Hence we propose to use IEAS to probe the effect of the phonon in ac transport. As an example, dynamic conductance of a quantum dot is discussed in detail and the concept of IEAS is demonstrated.

Electronic Transport Properties of the Azobenzene-Based Optical Molecular Switch with Different Substituents

By applying nonequilibrium Green’s function formalism combined first-principles density functional theory, we investigate the electronic transport properties of the azobenzene -based optical molecular switch with different substituents. Theoretical results show that the donor/acceptor substituent plays an important role in the electronic transport of molecular devices. The switching performance can be improved to some extent through suitable donor and acceptor substituents.

Molecular orbital view of the electronic coupling between two metal nanoparticles

The electronic coupling between metal nanoparticles is responsible for intriguing new phenomena observed when the particles are near touching contact, which is exemplified by recent investigations of nanoparticle dimers. However, little is known about the role of the molecular orbitals of the nanoparticle dimers. The expectation is that the physics and chemistry of the system must be reflected in the orbitals that control the bonding at touching contact. This expectation is borne out in the present investigation in which we present a comprehensive theoretical study based on density-functional theory of the electronic coupling between two silver nanoparticles. We explain our findings by studying the molecular orbitals of the dimers as a function of the separation and relative orientation between the nanoparticles. We show that as the nanoparticles approach each other a bond-forming step takes place, and that the strength of the hybridization is a key element to determine various properties of the system. We find that the relative orientation between the nanoparticles plays an important role in determining the strength of the coupling which can be visualized by the spatial distribution of the highest occupied molecular orbitals. Moreover, the strength of the coupling will in turn determine the ease ofmore » their transition to the nonlinear dielectric-response regime. This effect allows for the tunability of the electronic coupling and magnetic moment of the dimer. Our findings are essential for understanding and tailoring desired physical and chemical properties of closely aggregated nanoparticles relevant for applications such as surface-enhanced Raman scattering and quantum transport in molecular devices.« less

Exploring the limits of the self-consistent Born approximation for inelastic electronic transport

The nonequilibrium Green's function formalism is today the standard computational method for describing elastic transport in molecular devices. This can be extended to include inelastic scattering by the so-called self-consistent Born approximation (SCBA), where the interaction of the electrons with the vibrations of the molecule is assumed to be weak and it is treated perturbatively. The validity of such assumption and therefore of the SCBA is difficult to establish with certainty. In this work we explore the limitations of the SCBA by using a simple tight-binding model with the electron-phonon coupling strength $\ensuremath{\alpha}$ chosen as a free parameter. As model devices we consider Au monatomic chains and a ${\text{H}}_{2}$ molecule sandwiched between Pt electrodes. In both cases, our self-consistent calculations demonstrate a breakdown of the SCBA for large $\ensuremath{\alpha}$ and we identify a weak and a strong-coupling regime. For weak coupling our SCBA results compare closely with those obtained with exact scattering theory. However in the strong-coupling regime large deviations are found. In particular we demonstrate that there is a critical coupling strength, characteristic of the materials system, beyond which multiple self-consistent solutions can be found depending on the initial conditions in the simulation. These are entirely due to the large contribution of the Hartree self-energy and completely disappear when this is neglected. We attribute this feature to the breakdown of the perturbative expansion leading to the SCBA.

Application of quantum chemistry to nanotechnology: electron and spin transport in molecular devices

Rapid progress of nanotechnology requires developing novel theoretical methods to explain complicated experimental results and predict new functions of nanodevices. Thus, for the last decade, one of the challenging works of quantum chemistry is to understand the electron and spin transport phenomena in molecular devices. This critical review provides an extensive survey of on-going research and its current status in molecular electronics with the focus on theoretical applications to diverse types of devices along with a brief introduction of theoretical methods and its practical implementation scheme. The topics cover diverse molecular devices such as molecular wires, rectifiers, field effect transistors, electrical and optical switching devices, nanosensors, spin-valve devices, negative differential resistance devices and inelastic electron tunnelling spectroscopy. The limitations of the presented method and the possible approaches to overcome the limitations are addressed (183 references).

Orbital Views of the Electron Transport in Molecular Devices

Extended pi-conjugated molecules are interesting materials that have been studied theoretically and experimentally with applications to conducting nanowire, memory, and diode in mind. Chemical understanding of electron transport properties in molecular junctions, in which two electrodes have weak contact with a pi-conjugated molecule, is presented in terms of the orbital concept. The phase and amplitude of the HOMO and LUMO of pi-conjugated molecules determine essential properties of the electron transport in them. The derived rule allows us to predict single molecules' essential transport properties, which significantly depend on the type of connection between a molecule and electrodes. Qualitative predictions based on frontier orbital analysis about the site-dependent electron transport in naphthalene, phenanthrene, and anthracene are compared with density functional theory calculations for the molecular junctions of their dithiolate derivatives, in which two gold electrodes have strong contact with a molecule through two Au-S bonds.

Effects of end group functionalization and level alignment on electron transport in molecular devices

The effect of metal-molecule coupling on electron transport is examined in the prototypical case of alkane chains sandwiched between gold contacts and bridged by either amine or thiol groups. The results show that end group functionalization plays a crucial role in controlling electron transport, and that the symmetries and spatial extent of orbitals near the Fermi level control the conductivity rather than the strength of the bonding. For amine/Au and thiol/Au junctions, a crossover in conductivity with increasing bias is predicted.

Effects of Relative Orientation of Molecules on ElectronTransport in Molecular Devices

Effects of relative orientation of the molecules on electron transportin molecular devices are studied by the non-equilibrium Green functionmethod based on density functional theory. In particular, two moleculardevices with planar Au7 and Ag3 clusters sandwichedbetween the Al(100) electrodes are studied. In each device, two typicalconfigurations with the clusters parallel and vertical to theelectrodes are considered. It is found that the relative orientationaffects the transport properties of these two devices completelydifferently. In the Al(100)–Au7–Al(100) device, theconductance and the current of the parallel configuration are muchlarger than those in the vertical configuration, while in theAl(100)–Ag3–Al(100) device, an opposite conclusion isobtained.

A generalized quantum chemical approach for elastic and inelastic electron transports in molecular electronics devices

A generalized quantum chemical approach for electron transport in molecular devices is developed. It allows one to treat devices where the metal electrodes and the molecule are either chemically or physically bonded on equal footing. An extension to include the vibration motions of the molecule has also been implemented which has produced the inelastic electron-tunneling spectroscopy of molecular electronics devices with unprecedented accuracy. Important information about the structure of the molecule and of metal-molecule contacts that are not accessible in the experiment are revealed. The calculated current-voltage (I-V) characteristics of different molecular devices, including benzene-1,4-dithiolate, octanemonothiolate [H(CH2)8S], and octanedithiolate [S(CH2)8S] bonded to gold electrodes, are in very good agreement with experimental measurements.

Electronic transport in molecular devices: the role of coherent and incoherent electron–phonon scattering

The Green function for the coupled electron–phonon system is derived from a quantized version of the SSH Hamiltonian and is solved exactly for relevant modes of vibration. This Green function is used to compute the elastic and inelastic current components thought a di-thio-phenylene molecule in between Au contacts.