Electron transport in molecular junctions

This thesis addresses the electron transport in molecular junctions, focusing on the energy level alignment and correlation effects. Various levels of theory have been applied to study the structural and electronic effects in different molecular junctions, starting from the single particle density functional theory (DFT) description over the semi-empirical DFT+∑, to the sophisticated fully self-consistent GW approach. We find that in order to obtain a quantitative description of the conductance and the thermopower, it is necessary to go beyond the single particle description.
The effect of side groups on the benzene-diamine (BDA) molecule has furthermore been studied and it is found that the correct energy level alignment for the BDA molecule in Au contacts is only captured by the GW approach. Consequently, the GW approach provides an accurate description for the conductance change resulting from the side groups. The failure of the DFT based description is due to the strong energy level pinning when the BDA molecule is in contact with Au contacts.
The effect of contact geometries on the conductance and the thermopower has also been addressed. It is found that both GW and the DFT+∑ with a certain image charge position are in quantitative agreement with the experiments, while pure DFT is not. This is the consequence of the accurate energy level alignment, where the DFT+∑ method corrects the self-interaction error in the standard DFT functional and uses a static image charge model to include the image charge effect on the energy level renormalization.
Additionally, the gating of the 4,4’-bipyridine (44BP) molecule contacted to either Ni or Au electrodes has been investigated. Here it is found that the gating mechanism is conceptually different between two cases. In the case of Ni contacts where the lowest unoccupied molecular level (LUMO) of the 44BP molecule hybridizes strongly with Ni 3d orbitals, the gating is auxiliary by the so-called spinterface.
Finally, the correlation effect of the image charge beyond the energy level renormalization has been studied. It is shown that the finite response time of the electrodes to form image charge can suppress the conductance by a factor of 2. This correlation effect is only captured in the GW approach.