Investigating the formation of novel on-surface synthesised porphyrin-functionalised graphene nanoribbon nanostructures

Abstract

The vast field of on-surface synthesis encompasses elements of physics, chemistry and surface science which are involved in the formation of self-assembled, covalently bonded molecular materials with tuneable properties based upon the structure of the initial precursor molecule selected. Combined with the vast array of possible functionalisation available to porphyrin molecules, the use of the on-surface synthesis (OSS) reaction has a fantastic potential to form covalently bonded nanostructures with a select set of properties that has still not yet been fully realised. Most prior experiments involving the OSS reaction were focused on the formation of atomically precise graphene nanoribbons (GNRs) which exhibit a diverse array of structures and novel properties depending on the initial precursor molecule selected, such as the band gap dependence on ribbon width and electric and magnetic properties which are edge structure dependant. This thesis extends prior work via the use of porphyrin-based precursors with unique functionalisation to grow both 1D and 2D nanostructures structures with a wide array of connectivity’s between adjacent units with the future goal of integrating such nanostructures into electronic devices such as field effect transistors (FETs) for the detection of Volatile organic compounds (VOC’s). The work presented within this thesis is primarily concerned with the spectroscopic and microscopic investigations into the OSS reaction of five distinct porphyrin precursor molecules grown in-situ on stepped and flat gold surfaces and on the in-place transfer of 1D porphyrin chains and 7-AGNRs from a gold substrate onto an insulating SiO2/Si substrate, which is a necessary first step for their potential future integration into FET based devices. Synchrotron radiation-based X-ray photoemission spectroscopy (XPS) and near-edge x-ray absorption fine structure spectroscopy (NEXAFS) performed at the BESSY II, MAX IV and Diamond synchrotron facilities along with scanning tunnelling microscopy (STM) performed in both DCU and at the MAX IV synchrotron facility constitute the main techniques that were employed for the study of each precursor molecule, from the initial deposition up until the formation of the final 1D chain or 2D network OSS product. Synchrotron based XPS measurements provided a detailed description into the chemical changes experienced by each precursor molecule as it progressed throughout the OSS reaction into their final form, by monitoring how the Br3d, C1s and N1s spectral envelope developed as a function of temperature. For select precursor molecules, the interactions experienced between each other and with the underlying gold surface were probed via comparative XPS experiments at different coverages and using alternate gold crystal structures. Synchrotron based NEXAFS measurements yielded information on the geometry of each precursor molecule on the underlying gold surface and how it evolved as a function of temperature throughout the OSS reaction, this was done by monitoring changes within the C-K edge, N-K edge, and Ni-L edge NEAXFS transitions at temperatures of interest throughout. Monitoring of the C-K edge NEXAFS before and after an in-place transfer was used determine the effectiveness of such a transfer for both 1D porphyrin chains and 7-AGNRs from a gold onto a SiO2/Si substrate. The results presented within this thesis provide a detailed description of the chemical and geometrical changes experienced by each precursor molecule. A fixed stoichiometry based XPS model in combination with a NEXAFS model, using free base and metal base tetra phenyl porphyrins (TPP’s) for state determination works well in describing how each respective OSS reaction evolves with temperature. The in-place transfer of both DBBA derived 7-AGNRs and Ni-(DBrP-DP)P 1D porphyrin chains from a gold onto a SiO2/Si substrate were deemed successful via a comparative C-K edge NEXAFS study. The use of high symmetry molecular analogues for DFT based calculations, worked quite well for modelling the shake up transition gap within the C1s XPS of Ni-(DBrP-DP)P precursors throughout the OSS reaction but were not as successful in modelling the shake up transition gap throughout the OSS reaction for the Ni-(DBr-DP)P precursor molecules.