Exploring the Potential Use of Metal-Supported Endofullerenes and Exofullerenes as Multistate Switches for Molecular Electronics

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Silicon is the base material for current electronic components such as transistors and switches. With progress in solution processing techniques for electronics integration (e.g. nanolithography), every year is accompanied with a reduction in size of individual components. Moore's law predicts that the electronic components will soon reach scaling limits in the miniaturization process. Hence, research into alternatives to silicon-based components is vital to sustain our ability to devise better performing and cost-effective electronic devices that afford greater flexibility and reduced energy consumption.The field of molecular electronics aims at advancing the miniaturization of electronic devices, by exploiting single molecules to perform the function of individual components [1]. A molecular switch is defined as a molecule that displays stability in two or more states (e.g. "on" and "off" involving conductance, conformation etc.) and upon application of a controlled external perturbation, electric or otherwise, undergoes a reversible change such that the molecule is altered. Previous work has shown multi-state molecular switches with up to four and six distinct states [2,3]. Our research group in St Andrews has recently reported on a multi-state single molecule switch using the endohedral fullerene Li@C60 that displays 14 molecular states which can be statistically accessed [4]. This was achieved using low temperature scanning tunnelling microscopy and spectroscopy. In this thesis, we propose to further explore the use of metal-supported fullerenes as multistate switches for molecular electronics. Amongst the topics we want to address: (1) The exact excitation mechanism operating in Li@C60 multi-switching is unknown. We believe that it relies on resonant tunnelling via the superatom molecular orbitals of the fullerene cage as a means of Li activation [4]. We will explore strategies to prove this or otherwise. (2) Do other endofullerene species display multi-state switching? (3) Exofullerenes adsorbed on adequate supports have been poorly investigated to date. We will study, by means of STM, a few selected systems synthesized by our organic chemistry collaborators. (4) Can photons (UV and near-VIS) be used as excitation source to induce Li switching in Li@C60? We will couple our STM setup with a UV source to investigate the potential use of exofullerenes towards novel nano-opto-electronic devices.The present thesis work is part of a multidisciplinary collaboration involving theoretical and experimental studies related to the development of multistate molecular switches based on endohedral fullerenes. Our collaborators are: (1) Prof. Eleanor Campbell (Edinburgh) for gas-phase spectroscopy measurements, (2) Dr Andreas Stasch (St Andrews) for the organic synthesis of selected exofullerenes, and (3) Dr Amy Khoo (A*STAR Singapore) for high-level DFT-based calculations. [1] Aviram, A.; Ratner, M. A. Chemical Physics Letters 1974, 29, 277-283.[2] Auwaarter, W.; Seufert, K.; Bischoff, F.; Ecija, D.; Vijayaraghavan, S.; Joshi, S.; Klappenberger, F.; Samudrala, N.; Barth, J. V. Nature Nanotechnology 2012, 7, 41-46.[3] Huang, T.; Zhao, J.; Feng, M.; Popov, A.A.; Yang, S.; Dunsch, L.; Petek, H. Nano Letters 2011, 11, 5327-5332.[4] Chandler, H.J.; Stefanou, M.; Campbell, E.E.B.; Schaub, R. Nature Communications 2019, 10, 2283.