Coulomb repulsion effect in two-electron nonadiabatic tunneling through a one-level redox molecule.

We investigated Coulomb repulsion effects in nonadiabatic (diabatic) two-electron tunneling through a redox molecule with a single electronic level in a symmetric electrochemical contact under ambient conditions, i.e., room temperature and condensed matter environment. The electrochemical contact is representative of electrochemical scanning tunneling microscopy or a pair of electrochemical nanoscale electrodes. The two-electron transfer molecular system also represents redox molecules with three electrochemically accessible oxidation states, rather than only two states such as comprehensively studied. It is shown that depending on the effective Coulomb repulsion energy, the current/overpotential relation at fixed bias voltage shows two narrow ( approximately k(B)T) peaks in the limit of strong electron-phonon coupling to the solvent environment. The system also displays current/bias voltage rectification. The differential conductance/bias voltage correlation can have up to four peaks even for a single-level redox molecule. The peak position, height, and width are determined by the oxidized and reduced states of both the ionization and affinity levels of the molecule and depend crucially on the Debye screening of the electric field in the tunneling gap.

A theory of redox-mediated electron tunneling through an electrochemical two-center contact.

The dependences of the tunnel current on the overpotential and bias voltage for a symmetric electrochemical contact involving two redox groups are calculated. The main physical situations involving various combinations of the strengths of the electronic coupling of the redox groups with each other and with the electrodes are considered in detail. The treatment is more rigorous and complete as compared with previous work. In particular, totally adiabatic transitions are discussed and the potential distribution in the tunnel gap is taken into account. It is shown that the system under consideration manifests negative differential resistance and rectification. A new effect is predicted in the current/overpotential dependence, namely the appearance of two maxima. The experimental data of (Tran et al 2006 Faraday Discuss. 131 197) are addressed. It is concluded that they are compatible with the assumption on a strong screening of the electric potential within the tunnel gap.

Negative differential resistance and switching behavior of redox-mediated tunnel contact.

Theoretical description of various properties of redox-mediated tunnel contacts is presented. The dependences of the current on the overpotential and bias voltage under the sweeping voltammetry conditions are addressed. The effect of switching between two redox states on the shape of current/voltage characteristics is discussed. The shot noise and telegraph noise of the bridged contacts involving redox group are considered. Functional properties of the contact as a means for the information processing are discussed.

Electric double layer effect on observable characteristics of the tunnel current through a bridged electrochemical contact.

Scanning tunneling microscopy and electrical conductivity of redox molecules in conducting media (aqueous or other media) acquire increasing importance both as novel single-molecule science and with a view on molecular scale functional elements. Such configurations require full and independent electrochemical potential control of both electrodes involved. We provide here a general formalism for the electric current through a redox group in an electrochemical tunnel contact. The formalism applies broadly in the limits of both weak and strong coupling of the redox group with the enclosing metal electrodes. Simple approximate expressions better suited for experimental data analysis are also derived. Particular attention is given to the effects of the Debye screening of the electric potential in the narrow tunneling gap based on the limit of the linearized Poisson-Boltzmann equation. The current/overpotential relation shows a maximum at a position which depends on the ionic strength. It is shown, in particular, that the dependence of the maximum position on the bias voltage may be nonmonotonous. Approximate expressions for the limiting value of the slope of the current/overpotential dependence and the width of the maximum on the bias voltage are also given and found to depend strongly on both the Debye screening and the position of the redox group in the tunnel gap, with diagnostic value in experimental data analysis.

Effect of low-level alpha-radiation on bioluminescent assay systems of various complexity.

This study addresses the effects of low-level alpha-radiation on bioluminescent assay systems of different levels of organization: in vivo and in vitro. Three bioluminescent assay systems are used: intact bacteria, lyophilized bacteria, and bioluminescent system of coupled enzyme reactions. Solutions of 241Am(NO3)3 are used as a source of alpha-radiation. It has been shown that activation processes predominate in all the three bioluminescent assay systems subjected to short-term exposure (20-55 h) and inhibition processes in the systems subjected to longer-term exposure to radiation. It has been found that these effects are caused by the radiation component of 241Am3+ impact. The intensity of the 241Am3+ effect on the bioluminescent assay systems has been shown to depend on the 241Am3+ concentration, level of organization and integrity of the bioluminescent assay system. The bioluminescent assay systems in vivo have been found to be highly sensitive to 241Am3+ (up to 10(-17) M).

Mechanism of electrochemical charge transport in individual transition metal complexes.

We used electrochemical scanning tunneling microscopy (STM) and spectroscopy (STS) to elucidate the mechanism of electron transport through individual pyridyl-based Os complexes. Our tunneling data obtained by two-dimensional electrochemical STS and STM imaging lead us to the conclusion that electron transport occurs by thermally activated hopping. The conductance enhancement around the redox potential of the complex, which is reminiscent of switching and transistor characterics in electronics, is reflected both in the STM imaging contrast and directly in the tunneling current. The latter shows a biphasic distance dependence, in line with a two-step electron hopping process. Under conditions where the substrate/molecule electron transfer (ET) step is dominant in determining the overall tunneling current, we determined the conductance of an individual Os complex to be 9 nS (Vbias = 0.1 V). We use theoretical approaches to connect the single-molecule conductance with electrochemical kinetics data obtained from monolayer experiments. While the latter leave some controversy regarding the degree of electronic coupling, our results suggest that electron transport occurs in the adiabatic limit of strong electronic coupling. Remarkably, and in contrast to established ET theory, the redox-mediated tunneling current remains strongly distance dependent due to the electronic coupling, even in the adiabatic limit. We exploit this feature and apply it to electrochemical single-molecule conductance data. In this way, we attempt to paint a unified picture of electrochemical charge transport at the single-molecule and monolayer levels.

In situ superexchange electron transfer through a single molecule: a rectifying effect.

An increasingly comprehensive body of literature is being devoted to single-molecule bridge-mediated electronic nanojunctions, prompted by their prospective applications in molecular electronics and single-molecule analysis. These junctions may operate in gas phase or electrolyte solution (in situ). For biomolecules, the latter is much closer to their native environment. Convenient target molecules are aromatic molecules, peptides, oligonucleotides, transition metal complexes, and, broadly, molecules with repetitive units, for which the conducting orbitals are energetically well below electronic levels of the solvent. A key feature for these junctions is rectification in the current-voltage relation. A common view is that asymmetric molecules or asymmetric links to the electrodes are needed to acquire rectification. However, as we show here, this requirement could be different in situ, where a structurally symmetric system can provide rectification because of the Debye screening of the electric field in the nanogap if the screening length is smaller than the bridge length. The Galvani potentials of each electrode can be varied independently and lead to a transistor effect. We explore this behavior for the superexchange mechanism of electron transport, appropriate for a wide class of molecules. We also include the effect of conformational fluctuations on the lowest unoccupied molecular orbital (LUMO) energy levels; that gives rise to non-Arrhenius temperature dependence of the conductance, affected by the molecule length. Our study offers an analytical formula for the current-voltage characteristics that demonstrates all these features. A detailed physical interpretation of the results is given with a discussion of reported experimental data.

Effect of internal conversion of vibrational quanta in electron tunneling: a scheme for direct experimental observation and current/voltage relationships.

Electron tunneling through bridge molecular groups with a strong coupling to a local quantum vibrational mode is considered. A scheme is suggested for direct experimental observation of the effect of internal conversion of vibrational quanta in inelastic electron tunneling. The effect consists of excitation and re-absorption of vibrational quanta in bridge by tunneling electrons. The tunnel current produced by the absorption of vibrational quanta by the same or another electron can in principle be detected in an experimental setup a scheme of which is suggested. Current/voltage dependences have general spectroscopic features. Possible effects that can take place in the case of additional strong interaction with classical vibrational modes are discussed.

Quantum mechanical model for the dissociative adsorption of diatomic molecules on metal surfaces.

A quantum mechanical nonadiabatic theory of dissociative adsorption of diatomic molecules X2 on metal surface is presented. The following reaction coordinates are used to construct crossing diabatic potential energy surfaces (PES): the distance y between the atoms of the X2 molecule, the distance x of the X2 molecular axis from the surface, the set of coordinates describing possible displacements of metal atoms under adsorption. Expression for the rate constant is derived using the model potentials describing vibrations along these coordinates. The calculated dependency of the rate constant W on the reaction heat DeltaE is compared with that in classical approximation. It is shown that quantum effects lead to a weaker dependence of W on DeltaE as compared to that for classical one.

Electron correlation effects in the adiabatic charge transfer reactions at the metal/polar liquid interface.

New simple expressions for average number of electrons in the valence orbital of a reacting ion and the charge susceptibility are obtained that allow one to calculate adiabatic free energy surfaces (AFES) and corresponding kinetic regime diagrams (KRD) for adiabatic processes of electron transfer from the ion, located in a polar liquid, to a metal within the framework of the exactly solvable (in the limit T-->0) model of the metal with the infinitely wide conduction band. This model represents one of limiting cases of the Anderson model that may be applied to s-p metals. Unlike previous studies of the adiabatic reactions in the model of the metal with the infinitely wide conduction band, the present work takes into account the electron-electron correlation effects in an exact manner. General results are illustrated with KRD which determine the regions of the physical parameters of the system corresponding to various types of electron transfer processes. AFES are calculated for some typical parameters sets. The exact AFES are compared with those calculated within the Hartree-Fock approximation. It is shown that the correlation effects are of importance and results not only in a considerable decrease of the activation free energy but also to qualitatively different shapes of AFES in some regions of the system parameters.