Artificial Nanoparticle-Polymer Superlattices

Abstract

Hybrid superlattices (SLs) consisting of soft polymers with low refractive index polymers and hard oxide nanoparticles with high refractive are highly suitable materials for the application in 1D phononic and photonic crystals. In this theses the synthesis of titania and barium titanate nanospheres as building blocks for such SLs is described, including the influence of solvents and ligands on the particles synthesis. In the case of titania nanoparticles the morphology could be changed from spheres to cubes and the assembly of these nanocubes into mesocrystals is presented. In the case of barium titanate the addition of different amounts of ligand into the reaction mixture, to control the morphology, is investigated and first experiments on the assembly of ferroelectric barium titanate rods in an electric field are conducted. Further different SLs are built-up and characterized using scanning electron microscopy, UV-Vis- and Brillouin spectroscopy to monitor the morphology, to identify the photonic band gaps and to invest the phonon propagation respectively. In this work, the focus is set on the fabrication of the SLs using the facile method of spin coating. Special emphasis was put on the conditions needed to obtain highly homogenous multilayered SLs in the nanometer range from poly(methyl methacrylate) (PMMA) as soft low refractive index materials, and titania, barium titanate and silica nanoparticles as hard, high refractive index materials. Beside simple (AB)n structured SLs, with A being PMMA and B being oxide nanoparticles, more complex structured SLs are invested. For example a tandem SL built-up from two different (AB)n-type lattices and SLs with a higher periodicity like (ABAC)ntype SLs were built-up and characterized. Further the difficulties of spin coating a SL containing colloidal particles as a defect layer in an (AB)n-type SLs and the influence on the phononic band structure is discussed. Finally the substitution of PMMA through stimuli responsive poly(2-vinyl pyridine) (P2VP) was examined and preliminary experiment on the swelling behaviour of P2VP as a layer material showed the potential of P2VP to build water vapour responsive photonic crystals. The studies conducted in this work help to offer the fundamental knowledge about the propagation of photon and phonon within 1D SLs, focusing on the synthesis of building blocks and the fabrication of SLs. It is expected that this understanding will enormously contribute to the development of acoustic diodes and heat flow managing devices in the future.