Fast Fisher-Lee approach for conductance calculations on BTB-based molecular junctions: effects of isomerization and electrode coupling

In this work, we have implemented the Fisher-Lee formalism to couple non-equilibrium Green functions with tight-binding Density Functional to tackle large molecular systems. This method is used to determine the decay constant of a set of oligomers based on seven different monomers taken from the literature in the non-resonant tunneling regime. Results show good agreement with experimental measurements. The approach is then applied to explore the conformational pattern effect as well as the asymmetry and the strength of coupling with the electrode of bisthienylbenzene oligomers sandwiched between gold (Au),titanium (Ti)and graphene (G)) electrodes The results indicate that conformational patterns have low impact on the conductance, since the delocalization of {\pi}-electrons exhibits similar behavior for all the conformations explored. The calculated attenuation factor is found to be comparable with the strength of contact coupling (Ti > Au = G). Electronic analysis of metal-molecule interactions reveals the ionic nature of Ti-C bonds, through the emergence of a local dipole contributing to the work function variation of 0.35 eV. In addition, the Ti d-orbitals are found to be strongly coupled with the lowest unoccupied orbital (LUMO) of BTB, thus facilitating charge transfer from Ti to the molecule, at the origin of this strong interfacial dipole. However, the Au-C bond is found to be similar to the C-C bond, with pure covalent character. The results confirm the hole transport mechanism observed experimentally in the cases of Au-(BTB)n-Au and Au-(BTB)n-Ti, and predict possible combined mechanism of both hole and electron transport in the case of Ti(BTB)n-Ti.