Electron beam processing and spectroscopic characterisation of 2D materials and novel nanostructures
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Interest in the inorganic two-dimensional (2D) materials black phosphorus (BP) and hexagonal boron nitride (hBN) has exponentially risen due to the distinct advantages they possess over their bulk counterparts. Excellent and unique mechanical, electronic and optical properties have recently been discovered which can potentially span a large range of conceptually new applications such as high speed flexible electronics, sub-diffractional nanophotonic devices, strain engineered variable sensors and robust quantum information processing systems. To take advantage of these unique properties a greater understanding of the underlying chemical mechanisms involved in the manipulation/modification of BP and hBN is required. In this project, the nanofabrication tool focused electron beam induced processing (FEBIP) is used to do this all within a scanning electron microscope (SEM). FEBIP is a nanofabrication technique in which electrons are used to decompose surface adsorbed precursor molecules, typically under a constant partial pressure of a precursor gas. A highly focused beam can be used to deposit or etch nanoscale structures on a solid substrate. This technique differentiates itself from classical growth methods through other unique aspects, which include; growth of fully realised three-dimensional (3D) nanostructures, selective surface termination, defect generation and real-time imaging of chemical reaction fronts. The aim of this project was to elucidate experimentally the fundamental processes that govern the chemical reactions occurring during FEBIP. The first study outlined in this thesis involves the creation of an automated scanning program to reliably manipulate the electron beam for accurate data acquisition and pattern formation. To showcase these capabilities, 3D Pt nanostructures such as high aspect ratio pillars and helices fabricated with Pt(PF₃)₄-mediated electron beam induced deposition. Post-growth annealing in a water vapour environment was found to improve Pt deposit purity by volatilising phosphorus contaminants in the form of phosphoric acid. Annealing in H₂O under optimized conditions, yielded platinum that is pure within the detection limit of wavelength dispersive x-ray spectroscopy. Following from the information obtained through purification of phosphorus contaminants, attention was focused on the simulation of degradation of the highly unstable few-layer BP via electron beam irradiation performed in-situ. The real time imaging capabilities allowed for rapid stabilisation techniques to be found via controlled gas mixing and temperature dependent studies. The degradation pathway was found to proceed through the creation of phosphoric acid in a H₂O environment and is shown to be dependent on temperature. A low temperature heat cycle was then formulated to remove intercalated water and oxygen species to cease the degradation of few-layer BP for up to four weeks per heat cycle. Next, chemical dry etching of hexagonal Boron Nitride (hBN) was performed in a water vapour environment to create nanostructure geometries such as high resolution patterns, nanoribbons, and particles with high fidelity. Steps are also taken toward deterministic generation of defects and single photon emitters in hBN. The product of the electron induced dissociation of hBN at the surface of the material results in the production of nitrogen and boron radicals which then react with H₂O to produce boric and nitric acids. These then can be used as etch precursors for other materials such as silver nanowires. This two step etching process was then reimagined using an in-situ delocalised plasma and electron beam irradiation to widen the scope of etch-able materials such as silver, using a gas phase etch process. These results broaden the scope of material selection available in FEBIP. The exploitation of the unique aspects demonstrated with this technique in this project increases the applicability and versatility of FEBIP as a prominent tool for nanofabrication of 2D materials and complex 3D nanostructures. Furthermore, the results presented here constitute’ the first step towards integration of few-layer BP and hBN into high-performance optoelectronic devices, quantum information systems and various environmental sensor applications.
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