ORDERED COLLOIDAL-MASK APPLICATIONS IN LIGHT MANAGEMENT

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2021-7-28
SEYEDPOUR ESMAEILZAD, SEYEDEHNASIM
Dielectric colloidal micro-/nanospheres (CMNS) are promising candidates for different applications. In this thesis, colloidal spheres are either used for light management in photonic devices such as solar cells or surface enhanced Raman scattering (SERS) substrates. CMNS arrays can direct the broad incident radiation into a set of tighter foci, at which light intensity becomes considerably concentrated, enabling higher conversion efficiency. Furthermore, the CMNS arrays acting as an effective medium on the device surface can reduce reflection and facilitate improved forward scattering and hence the utilization of light within the device. Therefore, uniform arrays of CMNS located on top of the photonic devices can behave as antireflection coatings or as micro-lenses which can be regarded as a surface distributed concentrator within the framework of concentrated photovoltaics (CPV). Fabrication of such a lighttrapping structure is low-cost and less complicated than common alternatives such as vacuum evaporated multilayer antireflection coatings. In this thesis, experimental demonstration and computational support of the optimal size and shape of such CMNS arrays are illustrated. Besides, thin metallic film coated dielectric nanospheres are demonstrated to have a high potential for fabrication of cost-effective SERS substrates. In addition to the morphological advantages that nanospheres offer for attaining a high density of hotspots, possessing shape adjustability by uncomplicated thermal treatment make them an attractive platform for tuneable SERS substrates. Furthermore, when combined with oblique angle metal deposition technique, adjustable gaps at a high density and adjustable shape of metal films, such as silver (Ag) films, can be achieved on nanospheres. Applying small changes in deposition angle can provide means for fine adjustment of the SERS enhancement factor (EF), resulting in EF up to 108 measured using crystal violet dye molecule as a Raman analyte. This practice paves the way for the fabrication of high EF SERS substrates at a reasonable cost using a monolayer of self-organized nanosphere pattern. An ultra-thin Ag film coated at 5o tilt is shown to be an excellent substitute for a film deposited at 0o with double the thickness. There is a strong agreement between the experimental results and finite element method based simulations exhibiting expected field enhancements up to 109 at a tilt angle of 5o. In summary, the demonstration of several ordered colloidal mask applications in light management for photonic devices is aimed at this thesis.

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Citation Formats
S. SEYEDPOUR ESMAEILZAD, “ORDERED COLLOIDAL-MASK APPLICATIONS IN LIGHT MANAGEMENT,” Ph.D. - Doctoral Program, Middle East Technical University, 2021.