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Current rectification, switching, polarons, and defects in molecular electronic devices

Devices for nano- and molecular size electronics are currently a focus of research aimed at an efficient current rectification and switching. A few generic molecular scale devices are reviewed here on the basis of first-principles and model approaches. Current rectification by (ballistic) molecular quantum dots can produce the rectification ratio ~100. Current switching due to conformational changes in the molecules is slow, on the order of a few kHz. Fast switching (~1THz) may be achieved, at least in principle, in a degenerate molecular quantum dot with strong coupling of electrons with vibrational excitations. We show that the mean-field approach fails to properly describe intrinsic molecular switching and present an exact solution to the problem. Defects in molecular films result in spurious peaks in conductance, apparent negative differential resistance, and may also lead to unusual temperature and bias dependence of current. The observed switching in many cases is_extrinsic_, caused by changes in molecule-electrode geometry, molecule reconfiguration, metallic filament formation through, and/or changing amount of disorder in a molecular film. We give experimental examples of telegraph "switching" and "hot spot" formation in the molecular films.