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Title: Organic/inorganic hybrid materials for solution-processed photonic applications
Author: Strang, Andrew
ISNI:       0000 0004 5994 2115
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2016
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Solution-processable high refractive index materials are of immense interest for the production of optoelectronic devices, offering the prospect of high throughput, low cost and large area fabrication. In this thesis the optical characterisation of a relatively new type of optical material: the organic/inorganic molecular hybrid is presented. This molecular hybrid uses amorphous titanium oxide hydrates to cross-link polyvinyl alcohol (PVA) and in doing so enables the production of highly transparent thin film coatings with inorganic loadings approaching 100\%. The refractive index of the hybrid material is determined by probing Fabry-Perot oscillations in the transmittance spectra of thin films. This simultaneously demonstrates the high indices, low optical loss and excellent film quality possible for the hybrid. The refractive index is found to be tunable between 1.5 and 1.8 by adjusting the relative content of titanium. However, a simple thermal annealing process is shown to allow indices above 2, without compromising the transparency of the material. The index increase on thermal annealing is explained as the result of the large contraction that also occurs. The hybrid material is then used to fabricate several planar photonic structures. The first is a distributed Bragg reflector, in which the hybrid is deposited sequentially with a commercially available low index fluorinated polymer. The resulting structures have a high reflectance band that is controlled simply by the thicknesses of the high and low index layers. The DBRs demonstrate not only the excellent control of refractive index for the hybrid but also the ability to control layer thickness on the nanometre scale using dip coating. Infrared DBRs that are highly transparent in the visible are also fabricated as a possible application of the hybrid material system for heat management of glass clad buildings. Anti-reflective coatings are also demonstrated that remove 87\% of the reflectance from glass. This is achieved using a symmetrical bi-layer design, again dip cast from solution. Finally, all solution-processed optical microcavities are produced using the hybrid material. First, a simple defect is inserted into a DBR stack to induce a peak in transmittance inside the stop-band. This is then extended to include an emissive material: a perylene derivative, that is coupled to the cavity mode in order to alter its emission characteristics. This, to the best knowledge of the author, is the first demonstration of coupling in a solution-processed optical microcavity.
Supervisor: Stavrinou, Paul ; Stingelin, Natalie ; Bradley, Donal Sponsor: Engineering and Physcial Sciences Research Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral