The physical properties and the potential applications of a material are defined exclusively by the way in which the matter packs together and how discrete portions of this matter form on a macroscopic level. For example, the mineral quartz is a commonly used material for optical experiments. The way in which silicon and oxygen atoms pack allow a fairly uniform transmission of light from 1400nm to 200nm and our ability to grow large single-crystals, allow for applications such as cuvettes and optical windows. Issues arise when developing materials if they possess the required physical properties but cannot be grown in the right morphology or they have the required morphology but need their properties tuned. Quartz would not be as useful, if it only grew as highly anisotropic, crystalline needles. This thesis investigates the control of crystal packing (polymorphism) of molecular crystals, specifically the polyaromatic hydrocarbon coronene, using magnetic fields applied during the crystal growing process. A previously unknown polymorph of coronene was observed and fully characterised. The polymorph is accessible when grown from solution under a field between 0.9 T to 1.0 T. When investigating why this selectivity may be occurring, it was found that not only is the molecular conformation of the new polymorph the most energetically favourable via simulation, but the new polymorph is also accessible at cryogenic temperatures. Previous data reported on low-temperature coronene is readdressed with the knowledge of the new polymorph. Further to polymorphism, the morphology of a complex oxide material, specifically the hightemperature superconductor yttrium barium copper oxide (YBCO), is addressed in attempts to grow nanowires by mimicking the mechanism of micro-crucible growth without the use of a carbonaceous bio-template. Nanowires of various lengths and widths were grown using different amounts of a sodium-based flux. The nanowires were identified as a range of materials (Y2BaCuO5 (Y211), YBa2Cu3O7¡x (Y123), Ba2CuO) based on the temperature and sodium content. This method was easily translated to other, more complex, metal-oxide systems and promises to be a general route to the formation of inorganic nanowires.
|Date of Award||19 Jun 2018|
- The University of Bristol
|Supervisor||Simon R Hall (Supervisor) & Chris Bell (Supervisor)|