Dipoles, conjugation and molecular electronics

This thesis is aimed to expand the understanding of the properties of large-area molecular junctions comprising SAMs with EGaIn as the top electrode. The question whether the molecules or the electrodes dominate tunneling charge transport is important and ubiquitous in Molecular Electronics. A way to manipulate the transition voltage and, subsequently, answer the aforementioned question via synthetic manipulation of molecular dipoles is shown in chapter 1. Next, we, for the first time, relate the intrinsic molecular dipole moment of a SAM to asymmetry in the current/voltage characteristic. We tested the statistical significance of the effect and ascribed it to the collective action of embedded dipoles arising from pyrimidyl groups that are arranged parallel or antiparallel to the transport direction. Additional calculations reveal, that the bias-induced (de)localization of the frontier states that mitigate transport is the cause of the observed asymmetry. Then we describe reversible in-place switching of molecular tunneling junctions comprising SAMs between rectifying (diode) and non-rectifying (resistor) states. The switching process is affected by the protonation state of densely-packed carboxylic acid groups at the interface between the top-contact and the monolayer. Finally, we perform a systematic study of the influence of the electronic structure of oligo(p-phenylene ethynylene)s on charge transport in SAM-based EGaIn junctions. Out of many variables we found that electronic coupling to the bottom electrode has the strongest effect on the charge transport properties.