Towards semiconductor junctions based on discotic liquid crystals and templated inorganic oxides

• Main Author: Rungsirisakun, Ratana
• Published: University of Leeds 2010
• Subjects: 541.377
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.658559

This thesis is concerned with the investigation of organogelating triphenylene derivatives as templates for the sol-gel deposition of metal oxides including Ti02 and WO3. The goal was to make p-n junction devices, hybrid Ti02/triphenylene or WO3/triphenylene. Triphenylene acts as a p~type semiconductor, and the metal oxide acts as an n-type semiconductor. The photoelectrochemical properties of some prototype devices 0 were investigated. This thesis is structured in eight chapters. Chapter one reviews sol-gel chemistry which is the best route to synthesise nanostructured inorganic oxides, the use of organic templates in the sol-gel process, and the experimental techniques commonly used to characterise structured inorganic oxides. Chapter two reviews the electrochemistry of organic semiconductors, including triphenylene compounds, the electrochemistry of Ti02 and WO), and the goals of this project. In Chapter three, the gelating and silica templating properties of a well-known triphenylene, TP6E02M are investigated. TP6E02M forms gels in aqueous solution at high concentration. For the first time, cyclic voltammetry is used to characterise the solution and gel states of this compound. TP6E02M is shown to be a poor template for silica deposition. In Chapter four, the organogelating and silica templating properties of Aida-type triphenylenes (with alkyl-ethyleneoxy side chains) are investigated. These include a novel compound, TP6C 16E03M. It is shown that none of these triphenylene compounds formed gels in any of the solvents investigated. The composite silica films prepared according to Aida's method seem, however to contain fibrils. In Chapter five, the organogelating and silica templating properties of Ansell-type triphenylene (with amino acid side chains) are investigated. It is shown for the first time that organogels are formed by TP6AcAOH in DMSO, TP6AcAOH in EtOH/H20, TP6AcAOH In MeOH/H20, TP6AcAOEt in EtOH, and TP6AcDEA in MeOH. The self-assembly of TP6AcAOH is pH-switchable. All of these gels are characterised using different techniques. The gel fibres probably consist of multiple aggregated stacks of triphenylene. The silica templated with TP6AcAOEt in ethanol using water as catalyst consisted partially of rod-like nanoparticles. In Chapter six, the sol-gel synthesis of templated WO) is investigated. It is shown that Pluronic F127 can act as a template in the sol-gel synthesis of WO), affecting the morphology of the obtained product, when WCI6 is used as the reactant. We obtained aggregates of short, rod-like nanoparticles with 0.1 g F127, and aggregated spherical particles with 0.3 g F 127. In Chapter seven, hybrid triphenylene-WO) and triphenyleneTi02 devices are constructed and their photochemical properties are investigated. Neither type of devices seems to behave as a p-n junction but this may be because the partically reduced, conducting form of WO) is present, while Ti02 may be present in the wrong crystalline form. In Chapter eight, future possibilities for composite triphenylene/metal oxide devices are considered.