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Self Assembled Monolayer Tunnel Junctions Engineering with Eutectic Gallium Indium Tips

Final Report Summary - SAM-TUNEGAIN (Self Assembled Monolayer Tunnel Junctions Engineering with Eutectic Gallium Indium Tips)

The objective of this project is to gain control on the mechanism of transport properties of Self Assembled Monolayer (SAM)-based molecular tunnel junctions. The methodology developed by the Whitesides group over the last few years has allowed to increase the reliability of molecular tunnel junctions, allowing to measure their tunnel transport rates with much more accuracy than it had been possible until now, demonstrating, for example, how SAMs of alkylthiolates on Ag or Au with odd or even number of atoms show different conductivities. The Whitesides group has also demonstrated the efficient rectification of Fc-terminated SAMs and suggested a mechanism to explain this result. In this project, we propose to rationally engineer the orbital energies of SAMs terminal groups in order to obtain predictable tunneling and/or hopping behaviors. The good development of this project will require the fellow to identify the target molecules by Ab Initio calculations, to engage in organic synthesis to prepare them, and to use a wide range of material science techniques to prepare, characterize and analyze the transport properties of their SAMs. The knowledge acquired by the fellow in Prof. Whitesides group in Harvard will then be transferred to the use of tunnel junctions bearing molecular magnetic end-groups, and potentially allowing to design and prepare highly magneto-resistive junctions. The outcome of this work would on one hand improve the fundamental knowledge on molecular tunnel junctions, potentially allowing to use them for molecular electronics, and could provide highly functional molecular spintronics devices by fine tuning of their spin injection barriers.
The outgoing phase of this project involves multiple aspects that range from molecular modelling (M1) to identifying promising compounds that are then synthesized (M2) and whose physical properties are ultimately established via formation of self-assembled monolayers and measurement of the transport properties of their tunnel junctions (M3). This design loop is then iterated to gain control and knowledge over the physical properties of the systems used in this study.

During the outgoing phase we have thus developed a methodology to simulate accurately the tunneling barrier heights for a wide variety of organic molecules and applied this methodology to rationalizing the experimental results obtained by the lab and to predict the behavior of some molecular rectifiers that were then synthesized and probed experimentally. This methodology was adapted to the accurate prediction of current densities across SAMs of various non-rectifying junctions, and to rationalize the behavior of some systems that behaved unexpectedly.

An additional benefit from the outgoing phase was the involvement of the fellow to a wide variety of projects ranging from non-conventional nanofabrication of plasmonic materials to the biochemistry of low-cost immunoassays.

Upon reintegration in the European Research Area, the fellow has applied the know-how acquired in the Whiteside´s lab (on molecular junctions, nanofabrications and plasmonics) to study surface supported molecular magnetic materials. This is complementing the knowledge present in the return host and allows the fellow to develop his own lines of research at the crossroads of molecular magnetism and molecular electronics. In particular, in the recent months we have developped an EGaIn platform in Bordeaux that now allows to perform measurements of charge transport across monolayers or thin films of molecular materials. We are currently using it to study bistable magnetic molecules and to characterize and drive the properties of ferroelectric thin films.