The electrostatic potential profile along a biased molecular wire: A model quantum-mechanical calculation

Helical Molecule as an Efficient Rectifier: Effects of Molecular Conformation and Transverse Electric Field

The phenomenon of charge current rectification is critically investigated using a single stranded helical molecule in presence of transverse electric field. Two different helical molecules, DNA and protein, are taken into account to explore the specific roles of molecular conformation on rectification, which have not been addressed so far to the best of our concern. Sandwiching the molecular system within source and drain electrodes, we compute charge currents for two bias polarities and the degree of current rectification based on non-equilibrium Green's function formalism within a tight-binding framework. At non-zero electric field, site energies of the molecule are modulated in a cosine form, similar to the well known Aubry-André-Harper relation, resulting an atypical and fragmented energy band spectrum. The appearance of non-uniform site energies plays the central role for generating different currents in two bias polarities, and thus, the current rectification. We find that a high degree of current rectification can be established using the helical system and it becomes more effective for the protein molecule than the DNA one. At the end, the rectification operation considering a more general helical structure is discussed to make the present communication a self-contained one. Our proposition may provide a new route of getting controlled current rectification using similar kind of biological molecules and other tailor made helical geometries.

Enhanced current rectification in graphene nanoribbons: effects of geometries and orientations of nanopores

We discuss the possibility of getting rectification operation in graphene nanoribbon (GNR). For a system to be a rectifier, it must be physically asymmetric and we induce the asymmetry in GNR by introducing nanopores. The rectification properties are discussed for differently structured nanopores. We find that shape and orientation of the nanopores are critical and sensitive to the degree of current rectification. As the choice of Fermi energy is crucial for obtaining significant current rectification, explicit dependence of Fermi energy on the degree of current rectification is also studied for a particular shape of the nanopore. Finally, the role of nanopore size and different spatial distributions of the electrostatic potential profile across the GNR are explored. The stability of the nanopores is also discussed with a possible solution. Given the simplicity of the proposed method and promising results, the present proposition may lead to a new route of getting current rectification in different kinds of materials where nanopores can be formed selectively.

Efficient current rectification in driven acenes

We examine the current–voltage (I–V) characteristics of different polyacenes, such as anthracene, tetracene, pentacene, etc., under the influence of an arbitrarily polarized light.

Magnetoresistance of nanoscale molecular devices based on Aharonov-Bohm interferometry

Control of conductance in molecular junctions is of key importance in the growing field of molecular electronics. The current in these junctions is often controlled by an electric gate designed to shift conductance peaks into the low bias regime. Magnetic fields, on the other hand, have rarely been used due to the small magnetic flux captured by molecular conductors (an exception is the Kondo effect in single-molecule transistors). This is in contrast to a related field, electronic transport through mesoscopic devices, where considerable activity with magnetic fields has led to a rich description of transport. The scarcity of experimental activity is due to the belief that significant magnetic response is obtained only when the magnetic flux is of the order of the quantum flux, while attaining such a flux for molecular and nanoscale devices requires unrealistic magnetic fields. Here we review recent theoretical work regarding the essential physical requirements necessary for the construction of nanometer-scale magnetoresistance devices based on an Aharonov-Bohm molecular interferometer. We show that control of the conductance properties using small fractions of a magnetic flux can be achieved by carefully adjusting the lifetime of the conducting electrons through a pre-selected single state that is well separated from other states due to quantum confinement effects. Using a simple analytical model and more elaborate atomistic calculations we demonstrate that magnetic fields which give rise to a magnetic flux comparable to 10(-3) of the quantum flux can be used to switch a class of different molecular and nanometer rings, ranging from quantum corrals, carbon nanotubes and even a molecular ring composed of polyconjugated aromatic materials. The unique characteristics of the magnetic field as a gate is further discussed and demonstrated in two different directions. First, a three-terminal molecular router devices that can function as a parallel logic gate, processing two logic operations simultaneously, is presented. Second, the role of inelastic effects arising from electron-phonon couplings on the magnetoresistance properties is analyzed. We show that a remarkable difference between electric and magnetic gating is also revealed when inelastic effects become significant. The inelastic broadening of response curves to electric gates is replaced by a narrowing of magnetoconductance peaks, thereby enhancing the sensitivity of the device.

Binding at molecule/gold transport interfaces. V. Comparison of different metals and molecular bridges

The geometric and electronic structural properties of symmetric and asymmetric metal cluster-molecule-cluster' complexes have been explored. The metals include Au, Ag, Pd, and Al, and both benzenedithiol and the three isometric forms of dicyanobenzene are included as bridging molecules. Calculated properties such as cluster-molecule interface geometry, electronic state, degree of metal --> molecule charge transfer, metal-molecule mixing in the highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) energy region, the HOMO-LUMO gap, cluster --> cluster' charge transfer as a function of external field strength and direction, and the form of the potential profile across such complexes have been examined. Attempts are made to correlate charge transport with the characteristics of the cluster-complex systems. Indications of rectification in complexes that are asymmetric in the molecule, clusters, and molecule-cluster interfaces are discussed. The results obtained here are only suggestive because of the limitations of the cluster-complex model as it relates to charge transport.

NEGF-HF method in molecular junction property calculations

Electron-electron interaction is an essential issue in predicting the properties of a molecular conduction junction. An accurate treatment requires taking proper account of the potential distribution across the junction as well as for the change in electronic structure under the external voltage drop. Another important point is the necessity to treat molecules as open quantum systems. In this paper we address these issues within the framework of the non-equilibrium Green's function formalism at the Hartree-Fock level (NEGF-HF), which permits a self-consistent treatment of the problem. We apply the method to study the experimentally observed asymmetry in the I-V curves with respect to polarity of voltage bias in Hg-Au junctions containing bilayers of alkanethiols of various chain lengths. The origin of the effect is suggested to be the asymmetric behavior of the character of the highest occupied molecular orbital (HOMO) of the junction at opposite biases, which leads to different effective barriers for electron transfer across the junction at opposite signs of the voltage drop. The calculated potential profile shows the capacitor-like nature for the junction with the weak link.

Thermally Activated Conduction in Molecular Junctions

We report temperature dependence measurements on the conductance of individual molecular wires. The results show for the first time in a molecular junction the theoretically predicted transition from coherent superexchange tunneling conductance to an activated hopping mechanism as temperature is increased.

Ab initiostudy of the alternating current impedance of a molecular junction

The small-bias conductance of the C6 molecule, stretched between two metallic leads, is studied using time-dependent density functional theory within the adiabatic local density approximation. The leads are modeled by jellium slabs, the electronic density and the current density are described on a grid, whereas the core electrons and the highly oscillating valence orbitals are approximated using standard norm-conserving pseudopotentials. The jellium leads are supplemented by a complex absorbing potential that serves to absorb charge reaching the edge of the electrodes and hence mimic irreversible flow into the macroscopic metal. The system is rapidly exposed to a ramp potential directed along the C6 axis, which gives rise to the onset of charge and current oscillations. As time progresses, a fast redistribution of the molecular charge is observed, which translates into a direct current response. Accompanying the dc signal, alternating current fluctuations of charge and currents within the molecule and the metallic leads are observed. These form the complex impedance of the molecule and are especially strong at the plasmon frequency of the leads and the lowest excitation peak of C6. We study the molecular conductance in two limits: the strong coupling limit, where the edge atoms of the chain are submerged in the jellium and the weak coupling case, where the carbon atoms and the leads do not overlap spatially.