Thompson, RJ; (2014) Topographical, chemical and optical studies of single crystal rubrene. Doctoral thesis , UCL (University College London).

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Abstract

This thesis presents a study of material characterisation methods and techniques, which can determine the effects of surface and structural defects on the properties of single crystal rubrene. A method of cleaving is devised to produce surfaces free from previous exposure to the ambient. This method reproducibly provides large terraces intersected by straight, well orientated step edges. Atomic force microscopy studies show the cleaved surfaces undergo environmentally dependent molecular reorganization. This results in the formation of nanoscale ‘beads’ at molecular step edges and narrow ‘fingers’, one molecule high. The beads show an insulating behaviour with increased conduction at the surrounding surface. The methodology of applying time of flight secondary ion mass spectroscopy to rubrene crystals is developed to study the chemical composition both at and below the surface. This shows a uniform oxide (C42H28O) covering the surface with an increased concentration of peroxide (C42H28O2) located at crystallographic defects. To investigate the effect of surface and defects on exciton dynamics in rubrene a confocal photoluminescence (PL) arrangement is designed and built. An extended PL distribution is imaged providing evidence of exciton diffusion within the material. This diffusion is seen to increase within the bulk with a suppression of emission at 603nm. Defects are seen to affect PL with emission of the 650nm PL band having a greater contribution in the presence of defects. This emission is also spatially displaced from the maximum intensity of the other bands. These results imply the existence of a defect mediated recombination pathway. These studies show that environmental reactions readily occur at the location of crystallographic defects and step edges. This is of importance to the operation of rubrene electrical devices. This work provides a set of techniques and developed methodologies which enable the characterisation of technologically important processes on rubrene. These should extend to other organic single crystals such as pentacene and tetracene.

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