ULISSE - Using Electrostatic Interactions to Control Supramolecular Self-Assembly at Surfaces

Lead Research Organisation: University of Warwick
Department Name: Chemistry

Abstract

A major promise made by Nanotechnology Research to contemporary society is that next-generation molecular-scale devices will be faster, more versatile and more energy efficient than the ones based on current technology. Organic molecules are among the best candidate bricks for future nanoscale device fabrication. Their chemical structure can be easily modified, suggesting that carefully designed molecules could in principle assemble spontaneously into any desired structure, with no need of top down intervention. The optimisation by design of the chemical linkage between molecules has been, correspondently, an extensively explored concept in the molecular self-assembly field, albeit in some way limited in its scope by the short-range character of the bonding. Very recent research suggests that controlling the state of charge of organic molecules may provide a further direct handle to determine the assembly features on larger scales, through the contribution of long-range electrostatic interactions. However, this possibility is still virtually unexplored, in spite of its potential impact on nanofabrication. The electronic properties of molecular organic materials are, meanwhile, also attracting a massive, and ever growing, interest. Indeed, Organic Electronics is currently a booming field, with novel light-emission and light-energy conversion applications of strategic importance for society being investigated and early devices being produced. Once more, some crucial properties of these devices are determined by processes occurring at the nanometre scale and involve electrostatic interactions. Charge transfer between a metal surface and a layer of organic molecules deposited on it is known, e.g., to control the electric conduction properties of a metal-organic contact. However, no consensus has been reached yet on how to model such metal-organic interfaces properly, and device design is often a trial-and-error process.The present project will study charge transfer processes between organic molecules and metallic substrates, and their connection with self-assembly. Using both theory and experiment, we will investigate if these processes can be predicted and controlled by appropriate choices of molecules, substrates and coverages. The work will be useful for both understanding fundamental principles of molecular self-organisation and for unravelling the fundamental mechanisms that govern energy level alignment at metal-organic interfaces. Crucially, the whole will be much more than the sum of the parts. Namely, starting from charge transfer and pursuing the self-assembly route, we will determine if long-range forces between charged molecules can drive the spontaneous formation of novel classes of supramolecular structures. This would represent a novel tool for predicting and controlling the assembly. Conversely, starting from the observed assembly and pursuing the Organic Electronic route, we will investigate if specific molecular linkage patterns can reveal the occurrence of charge transfer. This would provide a novel route to precious information on the electronic properties of metal-organic interfaces. Electronic structure calculations, photoemission spectroscopy experiments, molecular dynamics simulations, and scanning tunnelling microscopy imaging will be used throughout the investigation. This will link two strategic fields of research which can greatly benefit from each other, namely nanofabrication by supramolecular assembly and molecular electronics, for the first time in an integrated UK-based project.

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