Artificial Nanoparticle-Polymer Superlattices
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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.