In the molecular electronics field it is highly desirable to engineer the structure of molecules to achieve specific functions. In particular, diode (or rectification) behaviour in single molecules is an attractive device function. Here we study charge transport through symmetric tetraphenyl and non-symmetric diblock dipyrimidinyldiphenyl molecules covalently bound to two electrodes. The orientation of the diblock is controlled through a selective deprotection strategy, and a method in which the electrode-electrode distance is modulated unambiguously determines the current-voltage characteristics of the single-molecule device. The diblock molecule exhibits pronounced rectification behaviour compared with its homologous symmetric block, with current flowing from the dipyrimidinyl to the diphenyl moieties. This behaviour is interpreted in terms of localization of the wave function of the hole ground state at one end of the diblock under the applied field. At large forward current, the molecular diode becomes unstable and quantum point contacts between the electrodes form.

We investigated a mechanism of rectification in di-block oligomer diode molecules that have recently been synthesized and showed a pronounced asymmetry in the measured I-V spectrum. The observed rectification effect is due to the resonant nature of electron transfer in the system and localization properties of bound state wave functions of resonant states of the tunneling electron interacting with asymmetric molecule in an electric field. The asymmetry of the tunneling wave function is enhanced or weakened depending on the polarity of applied bias. The conceptually new theoretical approach, the Green's function theory of sub-barrier scattering, is able to provide a physically transparent explanation of this rectification effect based on the concept of the bound state spectrum of a tunneling electron. The theory predicts the characteristic features of the I-V spectrum in qualitative agreement with experiment.In their pioneering paper [1] Aviram and Ratner proposed the idea of a molecular rectifier that contains donor and acceptor π-conjugated segments separated by an insulating σ-bonded segment of molecular wire. Several molecular rectifying systems have been synthesized in the past decade using Langmuir-Blodgett molecular assembly [2][3][4][5][6]. Attempts to provide experimental proof of molecular rectification were complicated by difficulty in establishing reproducible electrical contacts between metallic electrodes and a single molecule which resulted in uncontrollable interface rectification effects [7].Recently, a new class of diode molecules has been synthesized based on di-block oligomer molecules [8,9]. These molecules consisting of thiophene and thiazole structural units, have shown a pronounced rectification effect as a result of built-in chemical asymmetry. Importantly, it was unambiguously shown that the rectification effect is an intrinsic property of di-block oligomer molecules, and not due to the molecule-electrode interfacial interactions. In addition, by synthesizing diode molecules with different terminal thiol groups, it has become possible to assemble the diode molecules between gold electrodes with pre-defined rectification direction [10].In this letter we explain the mechanism of rectification in di-block oligomer diode molecules. We demonstrate that the observed asymmetry of current-voltage characteristics is the result of the resonant character of electron transport in molecular diodes and spatial asymmetry of the wave-function of a tunneling electron interacting with asymmetric molecule in an applied electric field. The asymmetry of the tunneling wave function is enhanced or weakened depending on the polarity of applied bias.

The temperature dependence of tunneling transition of the atomic particle in solids is studied near absolute zero. The different mechanisms of the temperature dependence are considered. They are the medium reorganization, the potential barrier parameters modulation, and the under-barrier friction The nonadiabatic effects are also considered. The rate constant K is described by formula ln K=ln K0+C4T4+C5T5+C6T6+C8T8 at low temperatures. It was conducted through the comparison of theory with the experimental data from the article of Kumada et al. [Chem. Phys. Lett. 261, 463 (1996)]. It turned out that good agreement takes place if one takes into account the quantum properties of the hydrogen crystal with the assumption of the dominated role of medium reorganization.

A theory for sensor response to a reducing hydrogen gas is presented for semiconductor tin dioxide (SnO 2 ) nanoparticle thin films. Pre-existing oxygen vacancies in SnO 2 act as electron donors to the conduction band. Oxygen atoms appearing upon dissociation of atmospheric oxygen at the surface of nanoparticles serve as electron traps, thus, decreasing the concentration of conduction electrons. Sensor response is caused by an increase in the film conductivity upon the addition of the reducing analyte gas H 2 , which reacts with atomic oxygen at the surface of SnO 2 nanoparticles to form water molecules in the gas phase, and is then followed by the transfer of electrons back into the conduction band. The theoretical description of sensor response takes into account the kinetics of surface chemical reactions that both control the concentration of electrons within the conduction band, and the physics of electron transport in nanostructured SnO 2 . The theory, which couples the electronic response with the microstructure of the film and the chemical environment, predicts sensor sensitivity as a function of temperature, hydrogen pressure, and average size of SnO 2 nanoparticles in agreement with experiment.