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Browsing Fizik by Author "Açıkgöz, Sabriye."
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Item Fluorescence dynamics of dye-dye and dye-nanostructures interactions(Thesis (Ph.D.)-Bogazici University. Institute for Graduate Studies in Science and Engineering, 2011., 2011.) Açıkgöz, Sabriye.; İnci, Naci.The decay dynamics of (Boradiazaindacene) BODIPY dye molecules embedded in Porous silicon (PSi) nanostructures produced by electrochemical anodization of p-type silicon wafers in an HF solution are investigated using time-resolved lifetime measurements. Illumination light and resistivity effects on the growth mechanism of the PSi are studied. Coherence is observed to be the foundation of regularity in obtaining conical shapes and the pillar size is almost linearly proportional to the illumination wavelength. Moreover, high resistivity of silicon wafer considerably changes the surface topography of the PSi and silicon nanospheres are obtained instead of nanopillars. The decay rate of the BODIPY embedded in the vicinity of various size pillar tips is affected due to different apex angles of the conical nature. Interaction between BODIPY and nanospheres is explained using confocal FLIM technique. It is observed that there is an efficient energy transfer mechanism between BODIPY and PSi. Energy transfer efficiency strongly depends on the thickness of the silicon dioxide layer covering the PSi. As oxidation increases, energy transfer rate decreases. This change in energy transfer rate seems to obey NSET mechanism, which allows us to obtain a three dimensional topographic map of the developed oxide layer on the rough and complicated surface of a PSi nanostructure. Finally, interaction between BODIPY dye molecules, which are covalently bound to a PEG based hydrogel is also studied. Since the hydrogel structure is capable of absorbing a large amount of water, without dissolving and losing its shape, upon swelling, the distance between the BODIPY dyes is controllably changed; it is observed that the fuorescence lifetime of BODIPY increases. The decay dynamics of the BODIPY dye molecules confined within a hydrogel cluster obeys FRET rather than self (or contact) quenching.Item Plastic fiber optic humidity and gas sensor(Thesis (M.S.)-Bogazici University. Institute for Graduate Studies in Science and Engineering, 2007., 2007.) Açıkgöz, Sabriye.; İnci, Naci.In this thesis, a simple and low cost plastic optical fiber (POF) sensor with a humidity sensing range from 13 % to 95 % relative humidity (RH) is described. The sensing mechanism of POF humidity sensor is based on the alterations of refractive index of a polymeric thin film which is coated directly onto a polished segment of plastic optical fiber as cladding layer. When the refractive index of the cladding layer of POF changes, due to the relative humidity, fiber output voltage changes significantly. Gammaisocyanatopropyltriethoxysilane end-capped polyethylene glycol (PEG-Si), which is a highly hydrophilic polymer, is used as the coated polymer film. At low humidity levels, PEG-Si has a semicrystalline form; therefore, its refractive index slowly decreases with respect to increase in the amount of absorbed water molecules. As a result, the fiber output voltage decreases linearly up to 80 % RH; then, it shows a turning point and fiber output voltage starts to increase. Since PEG-Si melts from the semicrystalline form to a gel form, its refractive index shows a sudden decrease. To overcome this instability problem, PEG-Si polymer layer is enriched with hydrogen. The turning point in the hydrogenated PEG-Si shifts to higher values of relative humidity and the sensor output gives a linear and repeatable response to the humidity. The response of PEG-Si to various gases such as acetone, methanol, hexane, benzene, and toluene is also studied. It is observed that when acetone, methanol, and hexane vapors are increased, the fiber output voltage increased; whereas toluene and benzene decreased the output voltage due to their ring structure and their high refractive index.