Charge transport measurements in single-molecule junctions give fundamental insight into electronic interactions at the molecular scale. The formation of stable junctions, however, is still a major challenge, because lithography processes commonly used in microelectronics become unreliable at length-scales below 10 nm. Therefore, measurements of single-molecule contacts have to rely on statistical information, evaluating a large number of contacts and extracting common quantities. In this presentation, we will present measurements of charge transport through single molecules using the mechanically controllable break junction (MCBJ) technique, which allows measurement of molecular contacts with long-time stability. Therefore, two measurement principles can be applied, measuring conductance while changing the distance between the metal electrodes, and measuring current-voltage characteristics at a fixed distance. Both measurement methods are evaluated statistically using a large number of single-molecule contacts which are formed by repeatedly opening and closing the break junctions.
Here we show how we compare the data originating from our measurements with the so-called single-level-model [1], which assumes that only one molecular level is participating in charge transport through the junction. Using this model, we can extract energy level and electronic coupling to the leads of this level. Evaluation of a large number of curves enables us to reveal changes of these quantities once the molecular structure is modified. We will show measurements on Salen molecules including various metal ions, which influence the coupling parameter [2]. In addition, we show that we can follow the different positions of a single molecule on a molecular contact during opening [3]. These measurements show that we can achieve a very good control and understanding of single-molecule contacts experimentally.
[1] L. A. Zotti, T. Kirchner, J.-C. Cuevas, F. Pauly, T. Huhn, E. Scheer, and A. Erbe,Revealing the Role of Anchoring Groups in the Electrical Conduction Through Single-Molecule Junctions, Small 6, 1529 (2010).
[2] F. Kilibarda et al., Single-Molecule Doping: Conductance Changed By Transition Metal Centers in Salen Molecules, Advanced Electronic Materials 7, 2100252 (2021).
[3] M. Lokamani et al., Stretch Evolution of Electronic Coupling of the Thiophenyl Anchoring Group with Gold in Mechanically Controllable Break Junctions, J. Phys. Chem. Lett. 14, 5709 (2023).