First-principles study of electronic transport in organic molecular junctions

This thesis focuses on the theoretical description of coherent electronic transport in organic molecular junctions. The ab-initio theoretical methods and the theory of quantum transport in nanoscale systems are presented. The Landauer theory of transport formulated in terms of Green's function is analyzed by means of the embedding theory for a simplified model in which electrons are considered as moving in a one-dimensional modulated potential introduced to simulate resonant tunneling junctions. Following the introductory section, relevant systems of interest from both basic and technology points of view are investigated. The transport properties of two-dimensional graphene/graphene-nanoribbon (GNR) heterojunctions are shown to critically depend upon the geometrical features of the GNR. Diarylethene junctions with graphene electrodes are comprehensively analyzed, with emphasis on the photoswitching properties of the system. The use of graphene electrodes can improve the performance of such switching junctions as compared with the use of other substrates. A full characterization of a platinum/pyrazine bistable junction studied in a recent experiment is then established. The switching mechanism has been determined as a result of a molecule-lead configurational rearrangement. A final section is devoted to the description of a new methodology to calculate the elastic lifetimes of electronic states of adsorbates on surfaces. The method has been applied to dye molecules on TiO2 substrates, which are relevant for photovoltaics applications. The effects of modification of the spacers between the acceptor and donor part of the dyes are analyzed.