Arcidiacono, Tecla; (2021) Fabrication and characterization of direct and inverse opals for the manipulation of spontaneous emission in conjugated polymers. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

In this work, I investigated and improved a technique, known as vertical method, for growing synthetic opals by means of colloidal self-assembly of polymeric microparticles. This enables an unprecedentedly easy tuning of the opal photonic bands and thus enhanced control of the radiative properties of conjugated polymers after their infiltration into such photonic structures. The first part of the thesis is dedicated to direct opals defined as a close-packed array of spherical particles. In particular, I present a study of the optical response of polystyrene opals across an extended energy range (0.8 < a/λ < 1.8, where a is the lattice parameter and λ is the wavelength of light), which I achieved by adopting the aforementioned vertical method technique. Furthermore, in addition to discussing the conventional photonic stop band (PSB), I highlight the high-energy photonic bands associated to the Van Hove singularities which demonstrate the fcc arrangement of the microparticles signalling the high-quality standard of fabricated opals. The second part of the work is focused on inverse opals, namely a close-packed array of spherical voids. This is formed by co-growing a sacrificial opal template with a material of high refractive index (i.e. silica) and the removal of the spheres by calcination. Typically, the central frequency of the PSB is tuned by changing the sphere diameters of the opal template. In contrast, when keeping the sphere diameters fixed, I found that I could blue-shift the PSB by ~100 nm by varying the SiO2/colloidal ratio. To verify the performance of the photonic crystals obtained, I studied the spontaneous emission of luminescent organic polymers (i.e. PDPV, and the conjugated polyrotaxane PDV10Li β-CD) after their infiltration into the opals. I achieved, respectively for direct and inverse opals, a strong reshaping of the photoluminescence and a modification on the value of the radiative constant.

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