Spin-boson description of electron transmission through a molecular wire

An expression for the bridge-mediated electron transfer rate in dye-sensitized solar cells

We have derived an expression for the rate of electron transfer between a semiconductor and a redox centre connected to the semiconductor via a molecular bridge. This model is particularly useful to study the charge recombination (CR) process in dye-sensitized solar cells, where the dye is often connected to the semiconductor by a conjugated bridge. This formalism, designed to be coupled with density functional theory electronic structure calculations, can be used to explore the effect of changing the bridge on the rate of interfacial electron transfer. As an example, we have evaluated the CR rate for a series of systems that differ in the bridge length.

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.

Charge transmission through a molecular wire: The role of terminal sites for the current-voltage behavior

The current-voltage and the conductance-voltage characteristics are analyzed for a particular type of molecular wire embedded between two electrodes. The wire is characterized by internal molecular units where the lowest occupied molecular orbital (LUMO) levels are positioned much above the Fermi energy of the electrodes, as well as above the LUMO levels of the terminal wire units. The latter act as specific intermediate donor and acceptor sites which in turn control the current formation via the superexchange and sequential electron transfer mechanisms. According to the chosen wire structure, intramolecular multiphonon processes may block the superexchange component of the interelectrode current, resulting in a negative differential resistance of the molecular wire. A pronounced current rectification appears if (i) the superexchange component dominates the electron transfer between the terminal sites and if (ii) the multiphonon suppression of distant superexchange charge hopping events between those sites is nonsymmetric.

Inelastic effects in Aharonov-Bohm molecular interferometers

Inelastic effects arising from electron-phonon coupling in molecular Aharonov-Bohm (AB) interferometers are studied using the nonequilibrium Green's function method. Results for the magnetoconductance are compared for different values of the electron-phonon coupling strength. At low-bias voltages, the coupling to the phonons does not change the lifetime and leads mainly to scattering phase shifts of the conducting electrons. As a result of these dephasing processes, the magnetoconductance of the molecular AB interferometer becomes more sensitive to the threading magnetic flux as the electron-phonon coupling is increased, opposite to the behavior of an electric gate.

Vibrational inelastic scattering effects in molecular electronics

We describe how to treat the interaction of traveling electrons with localized vibrational modes in nanojunctions. We present a multichannel scattering technique, which can be applied to calculate the transport properties for realistic systems, and we show how it is related to other methods that are useful in particular cases. We apply our technique to describe recent experiments on the conductance through molecular junctions.

Inelastic electron tunneling spectroscopy in molecular junctions: Peaks and dips

We study inelastic electron tunneling through a molecular junction using the nonequilibrium Green's function formalism. The effect of the mutual influence between the phonon and the electron subsystems on the electron tunneling process is considered within a general self-consistent scheme. Results of this calculation are compared to those obtained from the simpler Born approximation and the simplest perturbation theory approaches, and some shortcomings of the latter are pointed out. The self-consistent calculation allows also for evaluating other related quantities such as the power loss during electron conduction. Regarding the inelastic spectrum, two types of inelastic contributions are discussed. Features associated with real and virtual energy transfer to phonons are usually observed in the second derivative of the current I with respect to the voltage Phi when plotted against Phi. Signatures of resonant tunneling driven by an intermediate molecular ion appear as peaks in the first derivative dI/dPhi and may show phonon sidebands. The dependence of the observed vibrationally induced lineshapes on the junction characteristics, and the linewidth associated with these features are also discussed.

Bridge mediated two-electron transfer reactions: On the influence of intersite Coulomb interactions

Donor-acceptor two-electron transfer (TET) mediated by a linear molecular bridge is described theoretically. The particular case is considered where the TET takes place in the presence of a strong electronic intersite coupling within the bridge and against the background of fast vibrational relaxation processes. For such a situation the coarse-grained description of bridge-assisted electron transfer in molecular systems can be utilized [Petrov et al., J. Phys. Chem. B 106, 3092 (2002)]. In the present case it leads to kinetic equations and rate expression for TET reactions. Our recent treatment of completely nonadiabtic TET reactions [Petrov et al., J. Chem. Phys. 120, 4441 (2004)] including a reduction to single-exponential kinetics (with overall transfer rate K(TET)) is generalized here to the case of strong intrabridge coupling and the presence of intersite Coulomb interactions. The dependence of K(TET) on the bridge length which is determined by a separate stepwise and concerted contribution is discussed in detail. It is found that the intersite Coulomb interaction favors the TET if the donor and the acceptor are uncharged in their completely reduced states (with two excess electrons present).

Vibrational effects in laser-driven molecular wires

The influence of an electron-vibrational coupling on the laser control of electron transport through a molecular wire that is attached to several electronic leads is investigated. These molecular vibrational modes induce an effective electron-electron interaction. In the regime where the wire electrons couple weakly to both the external leads and the vibrational modes, we derive within a Hartree-Fock approximation a nonlinear set of quantum kinetic equations. The quantum kinetic theory is then used to evaluate the laser driven, time-averaged electron current through the wire-leads contacts. This formalism is applied to two archetypical situations in the presence of electron-vibrational effects, namely, (i) the generation of a ratchet or pump current in a symmetrical molecule by a harmonic mixing field and (ii) the laser switching of the current through the molecule.