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G. Kastlunger, F. Schwarz, E. Lörtscher, R. Stadler:
"Charge Transport in redox-based molecular switches";
Poster: 13th European Conference on Molecular Electronics, Strassburg, Frankreich; 01.09.2015 - 05.09.2015.

Besides their applications as wires or rectifiers 1, the employment of single molecules as conductance switching building blocks is one fundamental operation required for electronic applications, e.g. in logic or memory applications in ultimately-scaled nanoelectronic circuits. Organometallic compounds with embodied redox-active centers seem to be a promising class of molecules as they form well- defined on and off states corresponding to their respective redox states triggerable solely by voltage in a two-terminal junction.
In our contribution, we focus on a description of electron transport through various transition metal complexes on the basis of density functional theory (DFT), where special emphasis is put on the interpretation of mechanically controlled break junction experiments (MCBJ).
As a first challenge a dinuclear Fe unit, {Fe}-C4-{Fe} with five different end groups was studied2. A voltage-induced conductance switching is found only in case of weak coupling (-N=C=S) at low temperature (T < 150 K) while the strong hybridization of metal states and molecular orbitals for the strong coupling case of a direct C-Au bond prohibits intrinsic redox degrees of freedom from operating. An excellent agreement between the theoretically predicted and experimentally measured conductances of the five {Fe}-C4-{Fe} complexes was achieved and a reasonable explanation for the switching properties has been found3.
Following these findings the influence of the metal center and its corresponding interactions with the ligands in mononuclear Fe, Mo and Ru complexes coupled covalently via a thiol anchor to Au electrodes was investigated. Although voltage-induced hysteresis is detected experimentally for all compounds the magnitude of the conductance change upon switching strongly depends on the atom type of the metal center. While the hysteresis is rather continuous for all compounds-, the Mo-bis(σ-arylacetylide) complex reveals additionally an abrupt switching for weak coupling achieved by pulling the junction. The reason for the distinctly different behaviour has been identified in the paramagnetic groundstate of the Mo-complex, which leads to a highly localized molecular eigenstate energetically close to the electrode's Fermi Level. Due to the weak coupling between the localized state and the metal electrode an electron can hop onto the metal center of the molecule and stay there with a finite life time. As a consequence an oxidized state is created in the junction, whose I/V properties differ notably from the ground state. With the source drain bias the complex can be oxidized and reduced leading to an on/off switching of unprecedented high ratios exceeding 1000.
In order to describe the two processes involved in creating hysteresis, namely coherent transport, responsible for the conductance, and electron hopping, which is the mechanism of the redox reaction, a bias-dependent switching formalism based on a combination of NEGF-DFT and Marcus theory has been developed, where the two redox states are treated explicitly 4.
Both experimental and theoretical results demonstrate the high potential of redox-active metalorganic molecular building-blocks for memory applications in ultimately-scaled devices with their origins rationalized in terms of their distinctive charging properties, which in this particular case crucially depend on the spin splitting in their respective ground state..

Projektleitung Robert Stadler:
Elektrochemische Interferenz