Electrical detection of spin state switching in electromigrated nanogap devices

Gee, Alex

Physics
January 2020

Thesis or dissertation


Rights
© 2020 Alex Gee. All rights reserved. No part of this publication may be reproduced without the written permission of the copyright holder.
Abstract

Spin crossover is an effect shown in some transition metal complexes where the spin state of the molecule undergoes a transition from a low spin to a high spin state via the application of light, pressure or a change in temperature. This behaviour makes these complexes an attractive candidate to form electronic molecular-scale switches as the electrical resistance of the compound differs between the two spin states. Although the spin crossover effect is commonly studied in its bulk form, the integration of a single molecule into a solid-state device while maintaining the magnetic bi-stability is highly desirable, but remains challenging. This is not only due to difficulties in capturing a single molecule between electrodes and making electrical connections but it is also due to the strong coupling effects imparted on the molecule by the high-density metallic states of the electrodes that can prevent the spin transition from occurring.

In recent years there have been many attempts at studying spin crossover complexes at a single molecule level. Many of these have used scanning tunneling microscopy or break junction techniques. While these studies have highlighted the unique and promising electronic properties of these compounds, these techniques are unsuitable for real world devices. This thesis demonstrates a means to make electrical contact to single or small numbers of molecules between gold electrodes fabricated using a bilayer nanoimprint lithography and a feedback controlled electromigration method. This method, enabling high throughput and low-cost fabrication is potentially suitable for scaling to large area planar devices and as such may be used for commercially producing molecular devices.

To validate the quality of the nanogaps, devices containing self-assembled monolayers of benzenethiol were first studied. The shape and magnitude of I-V curves measured on nanogap devices containing the benzenethiol monolayers are in good agreement with previously published work using similar molecules in mechanically controlled break junctions. The resulting I-V characteristics were analyzed using the single level resonant tunneling model as well as transition voltage spectroscopy and are consistent with transport through molecular junctions in which the benzenethiol molecules are 𝜋- stacked. These highly conducting molecular junctions may have potential uses for “soft” coupling to sensitive target molecules.

Following validation of the molecular nanojunction fabrication and measurement process, the experimental work shifted to studying electronic transport through spin crossover complexes with a focus on Schiff-base compounds that are specifically tailored for surface deposition. In the case of measurements made on the bulk compound, a sharp spin transition centered at a temperature around 80 K was observed, while a shift to lower temperatures was found for thin films of the complex. In contrast, nanojunction devices containing single molecules displayed very different behaviour, with distinct and reproducible telegraphic-like switching between two resistance states when cooled below 160 K. These two states are attributed to the two different spin states of the complex. The presence of these two resistive states indicates that the spin crossover is preserved at the single molecule level and that a spin-state dependent tunneling process is taking place. Interestingly, in some cases a multi-level switching behaviour is detected with four possible conductance states. This behaviour is attributed to the presence of two spin crossover molecules in the nanogap.

Publisher
Department of Physics and Mathematics, The University of Hull
Supervisor
Kemp, Neil
Sponsor (Organisation)
University of Hull; European Cooperation in the Field of Scientific and Technical Research (Organization)
Qualification level
Doctoral
Qualification name
PhD
Language
English
Extent
4 MB
Identifier
hull:17873
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