Molecular transport junctions: vibrational effects

Abstract

Transport of electrons in a single molecule junction is the simplest problem in the generalsubject area of molecular electronics. In the past few years, this area has beenextended to probe beyond the simple tunnelling associated with large energy gapsbetween electrode Fermi level and molecular levels, to deal with smaller gaps, withnear-resonance tunnelling and, particularly, with effects due to interaction ofelectronic and vibrational degrees of freedom. This overview is devoted to thetheoretical and computational approaches that have been taken to understandingtransport in molecular junctions when these vibronic interactions are involved.After a short experimental overview, and discussion of different test beds andmeasurements, we define a particular microscopic model Hamiltonian. That overallHamiltonian can be used to discuss all of the phenomena dealt with subsequently. Theseinclude transition from coherent to incoherent transport as electron/vibration interactionincreases in strength, inelastic electron tunnelling spectroscopy and its interpretation andmeasurement, affects of interelectronic repulsion treated at the Hubbard level, noise inmolecular transport junctions, non-linear conductance phenomena, heating and heatconduction in molecular transport junctions and current-induced chemical reactions. Ineach of these areas, we use the same simple model Hamiltonian to analyse energetics anddynamics.While this overview does not attempt survey the literature exhaustively, it does provideappropriate references to the current literature (both experimental and theoretical). Wealso attempt to point out directions in which further research is required to answer cardinalquestions concerning the behaviour and understanding of vibrational effects in moleculartransport junctions.