Effects of electrolyte gating on single-molecule electronic transport
Theoretical Chemistry: Electronic Structure, Dynamics, Simulation
Final Report Abstract
It was the goal of the project to develop an atomistic theory for adiabatic three-terminal transport through single molecules immersed in electrolytes, which was to be applied to current experiments in the field. In spite of the drastic funding reduction with respect to the original research proposal (by more than 50%), the work done within this project led to a series of interesting results, which made the object of eleven peer-reviewed publications. This work required a combination of complementary methods, including from molecular simulation, transport theories (with emphasis on molecular gating), ab initio quantum chemical approaches and multimode phonon analysis in molecules of interest for the single-molecule electronic transport under electrolyte gating. To get insight into the problem of gating in molecular electronic devices, we have first considered the molecular orbital (MO) gating in vacuo. We have focused our attention on a recent experiment wherein electrostatically-gated single-molecular transistors have been fabricated and characterized in great detail. Revealing a linear dependence of the MO energy of the gate potential based on the experimental data represents the central issue for demonstrating the realization of the gating effect. Our analysis showed that the situation is more intricate than initially claimed by other authors. In our studies, we solved this issue by pointing out serious drawbacks of Simmons-based approaches as used in previous publications. In another series of works, we have attempted to better understand the Newns-Anderson model, which often represents the framework of existing theoretical studies on the molecular transport in solvents. These works have demonstrated that, in spite of its simplicity, this model is able to quantitative reproduce full I −V curves up to the highest biases, which real molecular junctions can withstand. Furthermore, we have shown that the strong stochastic fluctuations occurring in molecular junctions can be quantitatively described within this framework. As planned, particular attention in this project was paid to the bipyridine unit. Our works have demonstrated a highly nontrivial intramolecular reorganization, which traces back to the most salient structural feature of this molecule, namely the twist angle between the two pyridine rings. For the same molecule, we showed that the solvent acts as a selective passive gate potential, which shifts the HOMO and LUMO energies in opposite directions. We studied layer-by-layer assemblies of robust molecular wires in cooperation with experimental groups in Germany and Italy, addressing the key issue how to integrate the molecular wires with Si-based electronics materials and processing conditions. One major obstacle towards fabrication of large-area molecular junctions is the deposition of the top electrode on ultra-thin organic layers, as present day techniques often lead to damage and electrical shorts in the fragile molecular layers. Here we demonstrated that FeII-terpyridine based molecular wires, organized in dense and sturdy films, enable the fabrication of robust, large area crossbar junctions using a conventional top electrode evaporation process. Further, we investigated electrolytically fabricated and gated atomic copper and silver point contacts. These contacts, which are fabricated and measured in the group of Prof. Schimmel at KIT show interesting, material dependent conductance levels and vary substantially regarding the processing and conductance properties. Here we studied copper and reference silver junctions and performed conductance simulations based on the Landauer formalism for junction geometries with varying number of atomic point contacts.
Publications
- “Ambipolar transition voltage spectroscopy: Analytical results and experimental agreement”, Phys. Rev. B 85, 035442 (2012)
Bâldea, I.
(See online at https://doi.org/10.1103/PhysRevB.85.035442) - “Evidence on single-molecule transport in electrostatically-gated molecular transistors”, Phys. Lett. A, 376, 1472 (2012)
Bâldea, I. and Köppel, H.
(See online at https://doi.org/10.1016/j.physleta.2012.03.021) - “Interpretation of stochastic events in single-molecule measurements of conductance and transition voltage spectroscopy”, J. Am. Chem. Soc. 134, 7958 (2012)
Bâldea, I.
(See online at https://doi.org/10.1021/ja302248h) - “Transition voltage spectroscopy in vacuum break junction: the standard tunneling barrier model and beyond”, Phys. Stat. Solidi (b), 249, 1791 (2012)
Bâldea, I. and Köppel, H.
(See online at https://doi.org/10.1002/pssb.201248034) - “(4, 4’)-Bipyridine in vacuo and in solvents: A quantum chemical study of a prototypical floppy molecule from a molecular transport perspective”, Phys. Chem. Chem. Phys. 15, 1151 (2013)
Bâldea, I., Köppel, H., and Wenzel, W.
(See online at https://doi.org/10.1039/c2cp43627b) - “Ultrarobust Thin-Film Devices from Self-Assembled Metal-Terpyridine Oligomers”, Advanced Materials 28, 3473 (2016)
Karipidou, Z., Branchi, B., Sarpasan, M., Knorr, N., Rodin, V., Friederich, P., Neumann, T., Meded, V., Rosselli, S., Nelles, G., Wenzel, W., Rampi, M.A. and von Wrochem, F.
(See online at https://doi.org/10.1002/adma.201504847)
