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Title: The on-surface self-assembly, reactions and electronic properties of PXX and binol derivatives
Author: Lawrence, James
Awarding Body: University of Warwick
Current Institution: University of Warwick
Date of Award: 2018
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Peri-xanthenoxanthene (PXX) is an electron donor molecule that has been previously used as an active component in organic thin film transistors (OTFTs), exhibiting desirable properties such as high levels of stability, efficient charge injection and high mobility. Previous studies of PXX and related molecules have shown that small changes to its structure enable the adjustment (in energy) of its frontier orbitals, enabling precise tweaking of its energetic alignment with other materials in devices. In this thesis, PXX and related molecules are studied at metallic interfaces via scanning tunnelling microscopy (STM), scanning tunnelling spectroscopy (STS) and x-ray photoelectron spectroscopy (XPS) in idealised conditions. In particular, on-surface reactions are used to form extended nanoribbons of PXX and related molecules, and the dependence of the donor/acceptor strength on the length of these structures is examined via STS. The on-surface reactions of a related molecule, binol, were studied on Cu(111) and Au(111) via STM and XPS with the aim of forming PXX in an idealised analogous reaction to solution phase synthetic methods that involve metal catalysts. Binol was found to form metal-organic structures on Cu(111) with annealing, with disordered covalent structures forming at higher temperatures. Au(111) did not catalyse on-surface reactions of binol, with the addition of a co-deposited metal (Fe) required to initiate the formation of metal-organic structures and covalent products at higher temperatures. PXX was found to adsorb onto metallic surfaces, with indications of molecule-metal decoupling on Au(111), such as the high levels of mobility exhibited and the clear signals in STM imaging that resemble its gas phase molecular orbitals. The addition of bromine atoms to its structure allowed for the formation of halogen bonded networks and enabled the on-surface synthesis of extended PXX nanoribbons via metal-catalysed on-surface Ullmann and dehydrogenation reactions. STS measurements on the PXX nanoribbons give a strong indication of an increased donor strength (higher HOMO energy) with increased length. An acceptor analogue of brominated PXX, Vat Orange 3 (VO3), was also studied on Au(111). After the formation of halogen bonded networks upon its deposition, annealing was found to yield extended nanoribbon structures that exhibited increased acceptor strength with increased length. Finally, on-surface donor-acceptor (DA) type structures involving PXX were examined. Its deposition with a common acceptor molecule, tetracyanoquinodimethane (TCNQ) was found to lead to the formation of mixed assemblies that showed signs of increased levels of surface-mediated charge transfer to TCNQ. Similar results were found when instead using PXX nanoribbons, with any differences ascribed to the change in the stabilities of the mixed assemblies due to changes in intermolecular interactions. Covalently bonded DA nanoribbon structures, formed from PXX and VO3, were also examined via STM/STS, and the tweaking of the frontier energy levels and band gaps via co-polymerisation was demonstrated.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: QD Chemistry