Transition Voltage: Coverage Dependance and Impact of Anchoring Groups

When characterizing molecular and monolayer junctions one of the key-parameters used is the so-called transition voltage. Unfortunately, its relation to the intrinsic electronic properties of the junction is still poorly understood, although Baldea, Frisbie and co-workers have recently obtained an excellent correlation between the energetic positions of the electronic transport channels extracted from a simple one-state model and the positions of the peaks in the density of states obtained via photoelectron spectroscopy measurements.
This raises the question, to what extent a model building on a single, discrete energy level can be sufficient to represent the complex electronic structure of the junction. We investigated the applicability of the aforementioned model especially in view of the vastly different coupling strength between substrates and molecules upon changing the docking chemistry.
For this, we simulated the electronic structure of a wide range of molecular junctions using density functional theory based methods and calculated the current-voltage characteristics by employing the Landauer-Büttiger formalism. Finally, from these characteristics we fitted the energetic position of the above-mentioned discrete “transport” level. Its properties are then compared to the actual electronic structure of the junction.