Metamaterial Design and elaborative approach for efficient selective solar absorber

The thesis is focused on developing spectral selective coatings (SSC) composed of multilayer
cermets and periodic array of resonating omega structures, turning them to behave
like metamaterials, while showing high thermal stability up to1000°C. The developed SSC
is intended to be used for the concentrated solar power (CSP) applications. With the aim
of achieving highest possible absorbance in the visible region of the spectrum and highest
reflectance in the infrared region of the spectrum. The thesis highlights the numerical
design, the synthesis and optical characterization of the SSC of approximately 500 nm
thickness. A bottom-up approach was adopted for the preparation of a stack with alternate
layers, consisting of a distribution of Titanium Nitride (TiN) nanoparticles with a layer
of Aluminum Nitride (AlN) on top. The TiN nanoparticles, laid on a Silicon substrate
by wet chemical method, are coated with conforming layer of AlN, via Plasma-enhanced
Atomic Layer Deposition (PE-ALD). The control of the morphology at the nanoscale
is fundamental for tuning the optical behaviour of the material. For this reason, two
composites were prepared. One starting with TiN dispersion made with dry TiN powder
and deionized water, and the other with ready-made TiN dispersion. Nano-structured
metamaterial based absorbers have many benefits over conventional absorbers, such as
miniaturisation, adaptability and frequency tuning. Dealing with the current challenges of
producing the new metamaterial based absorber with optimal nanostructure design along
with its synthesis within current nano-technological limits, we were capable of turning the
cermets into metamaterial. A periodic array of metallic omega structures was patterned
on top of both the composites I and II, by using e-beam lithography technique.
Parameters, such as the size of TiN nanoparticles, the thickness of AlN thin film and
the dimensions of the omega structure were all revealed by the numerical simulations,
performed using Wave-Optics module in COMSOL Multiphysics. The work showcased
clearly compares the two kinds of composites, using scanning electron microscope, X-ray
photoelectron spectroscopy(XPS) and electrical conductivity measurement. The improvement
in the optical performance of the SSC after the inclusion of metallic omega structures
in the uppermost layer of the two composites has been thoroughly investigated for light
absorption boosting. In addition, the optical performance of the two prepared composites
and the metamaterial is used as a means of validating the computational model.