Theoretical study of aromatic single-molecule junctions with DFT-based methods

Resumen

In this thesis, we preset a theoretical study of electron transport properties through single-molecule junctions relevant for the development of the next-generation of electronic devices. We have followed an organic line of research, addressing in fist place the well-known problem of the HOMO-LUMO gap underestimation in DFT-NEGF calculations, which is caused by the self-energy interaction error in the xc-functional and the lack of description of image charge effects. To this end, we propose an efficient and robust implementation of the DFT+S approach which corrects these shortcomings in periodic systems, validating its accuracy by comparison with experimental results. Then, we provide a useful method to estimate conductance trends in p-conjugated molecules weakly coupled to gold electrodes based on the study of the HOMO by means of the extended-Hückel model. Our study shows that diamino acenes generally have better conductance than dimethylthio acenes and di-(4-methylthio)phenyl acenes, and, that conductance values can differ significantly by simply changing the acene size or the relative position of the anchoring groups. Finally, we have investigated the effect of having an electrode with a moiré pattern by adding a monolayer of graphene on top of Ru(0001) by epitaxial growth. We compare the effect of using 4-aminobenzonitrile and aniline as single-molecule junctions, showing physical and chemical interactions with graphene, respectively. This study shows that the zero-bias conductance for 4- aminobenzonitrile is mainly influenced by the tip electrode and the adsorption site, while for aniline, the conductance is controlled by the interaction with both surfaces exploiting middle hybrid states. We have also investigated the properties of these devices under a non-equilibrium regime, finding that the current flow varies depending on the site and type of adsorption. For 4-aminobenzonitrile (physical adsorption), it increases when applying positive bias and the opposite is observed in aniline (chemical adsorption). Throughout this thesis, we intent to show the potential of these systems by presenting methods and techniques that could be useful when designing and optimizing molecular electronic devices.