Browsing by Author "Kauppinen, Christoffer, Dr., Aalto University, Finland"

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Grass-like Aluminum Oxide: Fabrication and Applications
(2021) Isakov, Kirill
School of Electrical Engineering | Doctoral dissertation (article-based)
This thesis presents grass-like alumina (GLA) – a new nanoporous coating with atomiclayer deposition-based fabrication methodology. GLA inherits valuable features of atomic layer deposition: scalability to coating hundreds of components in parallel and ability to uniformly coat substrates with extremely complicated topographies. Unique features of GLA are gradient refractive index, and high roughness and surface area.Fabrication methods for GLA are developed in this work and presented in detail. Several applications of GLA are introduced and divided into two main areas: antireflective coatings and nanoscale surface area enhancement method. Gradient refractive index of GLA spanning from 1.6 to 1 makes it a perfect match for reducing reflection from glass and plastic substrates. Transmission for doubleside coated glass achieves remarkable 99.3% average value over entire visible region of light spectrum at normal incidence. The positive effect is even more pronounced at higher incidence angle – GLA-coated glass transmits more light at 65° incidence than uncoated glass at normal incidence. Such performance makes GLA an excellent broadband and omnidirectional anti-reflective coating. Due to the fact that it is also anon-line-of-sight coating which can be applied in a batch process, GLA is a new strong contestant in the field of anti-reflective coatings. Rough surface of GLA enabled upgrading the developed anti-reflective coating with superhydrophobic properties. This was achieved by adding a thin top layer of a low surface energy material – fluoropolymer. Water contact angles of over 170° were achieved demonstrating excellent superhydrophobic behavior without any loss in antireflective properties. Superhydrophobic properties are known to enable self-cleaning capabilities, which are very valuable for coating exposed to atmospheric effects. Enhancement of surface area of a functional material by a GLA substructure has been first demonstrated with improving absorption of black silicon-NbN complex in the infrared region of light spectrum. The same principle was used to develop a high surface area electrode composed of GLA sandwiched between two thin TiN layers.The electrode has shown to be highly conductive despite containing dielectric alumina, and was able to increase capacitance of a porous silicon supercapacitor by up to 4 times. The whole electrode is a nanoscale structure, which allows its combination with existing 3D structures utilized in supercapacitors, solid capacitors and batteries.