Clarke, Michael Peter
(2025)
Topographical and chemical characterisation of temperature controlled on-surface reactions.
PhD thesis, University of Nottingham.
Abstract
On-surface synthesis, the process of producing covalently bonded structures from molecular building blocks confined on a surface, is a research theme worthy of investigation as it offers a route towards the fabrication of nanoscale two-dimensional devices. Alongside the potential applications within the burgeoning field of nanoelectronics, confining materials to surfaces allows the use of a variety of surface analysis techniques. Scanning probe microscopy, for example, can reveal with unrivalled detail mechanistic information about the progress of reactions in general. Developing our understanding of these processes can allow for the formation of more precise and complex structures ‘on-surface’ and may pave the way for developing new methodologies within solution phase synthesis.
In this thesis, a variety of molecular species, confined to two-dimensions by supporting substrates, are investigated, and the self-assembled structures formed following deposition, and subsequent temperature-induced reaction steps, are characterised. Particular emphasis is placed on Ullmann-type coupling reactions, a versatile and oft-utilised route to covalent bonding on surfaces. The work described within this thesis takes place predominantly under ultra-high vacuum (UHV) conditions, utilising a variety of surface-sensitive techniques: primarily scanning tunnelling microscopy (STM), but also X-ray spectroscopy techniques such as X-ray photoelectron spectroscopy (XPS), near-edge X-ray absorption spectroscopy (NEXAFS) and X-ray standing wave (XSW) analysis. Each of the chapters describing the experimental results aims to develop our understanding of the mechanisms underlying the behaviours of molecules in surfaces and particularly on the kinetic properties of reactions, in order to better facilitate more efficient and selective synthetic pathways.
The species covered in this thesis span a broad range, and demonstrate the breadth of utility for this type of ‘surface science’ based approach. Firstly, the formation of a polymer based upon a diketopyrrolopyrrole (DPP) is studied. This monomer unit possesses aryl-halide groups to facilitate on- surface covalent coupling and is functionalised with alkyl chains which drive the self-assembly of both the monomer material prior to reaction and the domains of polymeric material following on-surface synthesis. The self-assembled structure of closepacked domains of the monomer units, and the ordered polymers, are investigated and characterised using STM and XPS.
Secondly, two groups of larger molecular species are investigated: a series of porphyrin-based nanorings, and porphyrin-doped polymer chains, which serve as a precursor to a target porphyrin-graphene nanoribbon. Both are formed via novel in-solution synthesis, and require vital on- surface characterisation to accompany the in-solution chemistry and to confirm the successful synthesis of these materials. Both are investigated via STM, with morphological characterisation of the nanorings via this technique crucial to understanding the cyclic structure and flexibility of the molecules. For the graphitic nanoribbon species, our focus is the inclusion of porphyrin species within graphene nanoribbons to create porphyrinfused graphene nanoribbons (PGNRs). A combination of scanning tunnelling microscopy (STM) and photoelectron spectroscopy (PES) techniques are used to characterise the novel porphyrin-fused graphene nanoribbon. This nanoribbon is formed on-surface from a linear polymer consisting of regularly spaced Ni-porphyrin units linked by sections of aryl rings which fuse together during the reaction to form graphitic regions between neighbouring Ni-porphyrin units.
Lastly, a temperature-programmed PES study of brominated tetraphenyl porphyrin is conducted, with a novel Arrhenius analysis opening a pathway to empirical measurement of kinetic properties of reactions. This is enabled via temperature-controlled continuous XPS measurement, and the reaction is also characterised by stepwise STM, NEXAFS and XPS. A comparison is made between the properties of the Ullmann-type coupling reaction on both Au(111) and Cu(111) surfaces.
The work described in this thesis develops atop the existing literature for the stepwise analysis of surface-confined covalent reactions, and demonstrates the flexibility of surface-confined techniques such as STM for their unrivalled molecular resolution. The combination of STM with X-ray spectroscopic techniques underpins much of the work and displays the strength of combined topographic and chemical characterisation when analysing the evolution of a system, and demonstrates novel ways in which these techniques can be used to further our understanding of on-surface synthesis.
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