[Zurück]

G. Kastlunger, R. Stadler:
"Theory of charge transport through single redox-active transition metal complexes";
Vortrag: Vienna Young Scientist Symposium, TU Wien; 25.06.2015 - 26.06.2015.

Introduction
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 set on the interpretation of experimental results of our close collaborators at IBM Zürich.
Influence of the molecular anchor groups on conductance and intrinsic functionality
For the interpretation of their mechanically controlled break junction experiments a
dinuclear Fe unit, {Fe}-C4-{Fe}, with an extensive charge-delocalization over the entire
unsaturated organometallic backbone 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].
Redox-reaction based conductance switching
Following these findings the influence of the atom type of the metal center and their
corresponding interactions with the ligands in mononuclear Fe, Mo and Ru complexes
covalently and weakly coupled via -N=C=S 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 up to 400. The reason for the different behaviours has been
identified in the paramagnetic groundstate of the Mo-complex, which leads to a higly
localized molecular eigenstate energetically near the electrode 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. As consequence a metastable oxidized complex is created
in the junctions, whose I/V properties differ notably from the groundstate, and which can be
switched on and off by the source drain bias.
In order to describe the two competing processes, 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 NEGF-DFT and Marcus
theory has been developed, where the two redox states are threated explicitly [5].
Conclusions
The results could demonstrate the high potential of redox-active molecular building-blocks
for multi-level memory applications in ultimately-scaled devices if their origins can be
rationalized by a proper theoretical description.
Acknowledgements
The authors would like to thank the Austrian Science Fund FWF (project number P22548)
for their funding and are deeply indebted to the Vienna Scientific Cluster VSC, on whose
computing facilities all DFT calculations have been performed (project Nr. 70174). In
addition,GK is gratefully receiving a grant co-sponsored by the AustrianAcademy of
Science ÖAW, the Springer Verlag and the Austrian Chemical Society GÖCH.
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).

Projektleitung Robert Stadler:
Elektrochemische Interferenz