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G. Kastlunger, R. Stadler:
"Theory of charge transport through single redox-active transition metal complexes";
Poster: 15th European Conference on Solid State Chemistry, Universitaet Wien; 23.08.2015 - 26.08.2015.

The vision of molecular electronics is to employ small ensembles or even single molecules as active, functional building blocks in electronic circuits [1]. Besides current rectification, conductance switching is one fundamental operation required for various electronic applications, e.g. logic or memory. Most switching mechanisms are based on either conformational changes or the charging of the molecule in the junction. Organometallic compounds [2] with embodied redox-active centers are a promising class of molecules as functional electronic building-blocks since they can form stable on and off states corresponding to their respective redox states.
In our contribution we focus on a description of the electron transport through transition metal complexes on the basis of density functional theory (DFT), where a special emphasis is put on the interpretation of experimental results of our close collaborators at IBM Zürich.
For the interpretation of their mechanically controlled break junction experiments a dinuclear Fe unit, {Fe}-C4-{Fe} with five different end groups was studied [3]. A voltage-induced conductance switching is found 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 the same intrinsic redox degrees of freedom from operating. An excellent agreement between the theoretically predicted and experimentally measured conductances of the five Fe2-complexes has been achieved and a reasonable explanation for the switching properties has been found [4].
Following these findings the influence of the metal center and their corresponding interactions with the ligands in mononuclear Fe, Mo and Ru complexes coupled covalently via a thiol anchor to Au electrodes was investigated systematically. Although voltage-induced switching 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 switching is rather continuous for Fe and Ru, the Mo bis(σ-arylacetylide) complex reveals an abrupt hysteretic switching resulting in on/off conductance ratios of up to 400. The reason for the 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 small 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.
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 dependant switching formalism based on a combination of NEGF-DFT and Marcus theory has been developed, where the two redox states are treated explicitly [5,6].
The results could demonstrate the high potential of redox-active molecular building-blocks for memory applications in ultimately-scaled devices with their origins rationalized by a proper theoretical description.

References
[1] M. A. Ratner and A. Aviram, Chem. Phys. Lett. 29, 277 (1974).
[2] B. Kim et al., Phys. Chem. C, 111, 7521 (2007).
[3] F. Lissel et al, J. Am. Chem, 35, 4051 (2013).
[4] F. Schwarz, G. Kastlunger, et al., Nano Lett., 14, 5932 (2014).
[5] F. Schwarz, G. Kastlunger, et al., Submitted (2015).
[6] G. Kastlunger and R. Stadler, Phys. Rev. B 88, 035418 (2013)

Projektleitung Robert Stadler:
Elektrochemische Interferenz