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Investigating the 2D Self-Assembly of Photo-sensitive Molecules on Semiconductor and Insulating Surfaces

Final Report Summary - PHOTOSURF (Investigating the 2D Self-Assembly of Photo-sensitive Molecules on Semiconductor and Insulating Surfaces)

The aims of PHOTOSURF were to investigate the self-assembly of photoactive molecules, on surfaces using scanning tunnelling microscopy (STM). One achievement has been the study of DNA base functionalised molecules. Selective hydrogen bonding interactions between DNA bases was used to template self-assembly of complex 2D molecular networks. As part of collaboration with chemists at the University of Nottingham (Prof Neil Champness) and the University of Liverpool (Dr Anna Slater) this work was published in Chemical Science (DOI: 10.1039/C4SC03531C) and is now an ongoing project to construct a library of DNA base functionalised molecules for the 2D self-assembly of increasingly complex 2D molecular architectures.

Another aim of PHOTOSURF was to study 2D molecular self-assembly on semiconductors under ambient conditions. Our work focussed on the industrially relevant substrate TiO2. Used in a range of applications, including in dye sensitised solar cells, TiO2 has been widely studied by STM under vacuum. However, few studies exist under ambient conditions. Through collaboration with Prof Geoff Thornton at UCL the stability of TiO2 under various ambient environments (e.g. inert gases, water and electrolyte solutions) was investigated. In addition, the adsorption of photoactive molecules such as porphyrins was investigated. This work resulted in a paper that is currently in preparation and a further grant application to study the electrochemistry of individual dye molecules on TiO2. The work on imaging in electrolyte solutions has also led to a joint project with Prof David Williams and Prof Juliet Gerrard at the University of Auckland using STM to image protein self-assembly in buffer solutions. This work resulted in a recently accepted publication in Biointerphases.

PHOTOSURF also investigated two-dimensional covalent-organic frameworks (2D-COFs) consisting of porphyrins. This work included scientists at UCL (Dr Rob Palgrave, Dr Tracey Clarke) and at external institutions (Dr Hugo Bronstein, Cambridge; Dr Anna Slater, Liverpool). The growth of monolayer and bilayer 2D-COFs linked via imine bonds was studied using STM and X-ray photoelectron spectroscopy (XPS). This work was published in Chemical Communications (DOI: 10.1039/C6CC03895F) with two further publications in preparation. Key results show that XPS can characterise the chemical nature of the bonds formed between porphyrin molecules and that these bonds can be converted following 2D-COF growth to secondary amines, thereby improving the 2D-COF stability.

PHOTOSURF has advanced the state of the art in 2D self-assembly of photo-active molecules providing new insights into how molecular structure and growth conditions influence the chemical nature of 2D layers. These results will underpin future projects in the fellow’s group and the wider scientific community. The award of the career integration grant (CiG) allowed the fellow to integrate with the 2D materials research community and to forge important new links both nationally and internationally. This progress allowed the fellow to: 1) cement a permanent position at UCL; 2) build his research group (3 PhD students, 1 graduated 2 current; 2 MSc students); 3) win additional funding (EPSRC First Grant (EP/N021789/1); and 4) gain recognition via invitations to scientific conferences (British Crystallographic Association Spring Meeting 2017, EMN Supramolecular Materials Meeting 2017).