Browsing by Subject "thin film"

Now showing 1 - 4 of 4

Open Access
Formation of pyrene excimers in mesoporous organically modified silica thin films for visual detection of nitroaromatic explosives
(2013) Beyazkılıç, Pınar
Pyrene is a polycyclic aromatic hydrocarbon compound. Pyrene has been extensively applied as probing and sensing molecule because of excimer fluorescence which is formed upon interaction of two pyrene molecules in close proximity. In this thesis, we prepared porous thin films with bright pyrene excimer fluorescence and demonstrated their application in visual and rapid detection of nitroaromatic explosive vapors. The fluorescent films were obtained by physically encapsulating the pyrene molecules in the mesoporous organically modified silica (ormosil) networks which were synthesized via a facile template-free sol-gel method. Formation and stability of pyrene excimers were investigated in both porous and nonporous ormosil thin films. Excimer emission was found to be significantly brighter and more stable in porous films compared to nonporous films. The excellent stability of the pyrene excimers in the porous films is due to the nanoscale confinement of pyrene molecules in the porous ormosil network. We studied the nitroaromatic explosive sensing performances of the pyrene doped porous films. Films exhibited a rapid and visible fluorescence quenching when they were exposed to TNT vapor. Fluorescence quenching efficiency of an approximately 100 nm thick porous film was calculated to be 55.6% after exposure to TNT vapor for 30 seconds revealing a rapid sensing behavior. Fluorescence quenching of the films can be easily observed under UV light enabling naked-eye detection of nitroaromatic explosives. A selective quenching was observed in the excimer emission against vapors of nitroaromatic molecules; trinitrotoluene (TNT), dinitrotoluene (DNT) and nitrobenzene (NB) among various aromatic and nonaromatic compounds. Furthermore, quenched excimer emission of the films can be recovered by simply washing the films with water. It is shown that the films can be reused for at least five times after washing. To this respect, pyrene doped ormosil thin films can be presented as facile materials for nitroaromatic explosive sensing applications.
Open Access
Formation of silicon nanocrystals by laser processing of silicon rich oxides
(2012) Gündoğdu, Sinan
Silicon nanocrystals are well known to exhibit strong luminescence in the visible. Extension of this into a nanocrystal network would be beneficial for many applications. In the light of recent advances on exciton-plasmon interactions and photovoltaic cells, there is renewed interest in the use of nanostructures. Due to quantum confinement, silicon nanoclusters with increased band gaps, are promising for down conversion light and enhanced emission on plasmonic surfaces. Conventional techniques utilize high-temperature processing to obtain the Si-SiO2 phase separation which uses high thermal budget, not suitable for localized applications not compatible with glass substrates or thin-film stacked structures. An alternative approach capable of avoiding high temperature processing is laser irradiation of substochiometric amorphous silicon oxides. In this work, continuous-wave laser processing of Si-rich oxide thin films with varying Si content were performed in order to obtain Si nanocrystals embedded in silica. The role of composition, dwell times and power densities were investigated for Si-SiO2 phase separation. We present cw laser processing of PECVD grown and sputtered SiOx films. XPS, RBS and ERDA techniques were used for the stoichiometry analysis of different composition as grown samples and their optical properties were determined through ellipsometry analysis. Processing was performed with an Ar+ laser at 488 nm. The structural changes due to processing were investigated by Raman and photoluminescence spectroscopy. It has been shown that silicon nanocrystals formation depends both on precursor gas composition (hydrogen-diluted SiH4 and N2O or CO2 gases) and on laser power density. PECVD grown hydrogenated SiOx films were compared with sputtered films with and without hydrogen to identify the role of hydrogen for phase separation.
Open Access
Organically modified silica based nanomaterials for functional surfaces
(2012) Budunoğlu, Hülya
Organically modified silicas (ormosils) are unique materials due to their combined properties achieved from organics and inorganics. Ormosils contain at least one non-hydrolysable organic groups which results in a decrease of rigid Si-O-Si bonds, introducing a flexible character. Therefore, ormosils exhibit both flexibility of organics and atmospheric stability of inorganics. Organic group determines the functionalities of ormosils, thus their properties can be adjusted by choice of appropriate organic modification. Ormosils can be easily prepared in mild conditions of sol-gel technique, and can be applied on different surfaces by low cost and simple techniques. In this thesis, we prepared superhydrophobic-superhydrophilic, antireflectiveantifogging, anticorrosion and antiicing (ice retarding) functional surfaces using organically modified silica and its nano-composites in thin film form. Methyltrimethoxysilane (MTMS) is used in the synthesis of all films due to its intrinsically hydrophobic nature. This monomer is found to enable porous film formation without any modifications at ambient temperature and pressure. Superhydrophobic ormosil aerogel films with water contact angles reaching 179.9 and porosity of 86 % have been prepared using phase separated colloidal suspensions of MTMS, which exhibited flexibility, thermal stability and superhydrophilic transition after annealing at 600 C. Antireflective films with high mechanical stability are prepared from co-condensation of MTMS with tetraethylorthosilicate monomer, which exhibited transmission as high as 99.6 % with flexibility and transition to antifogging after annealing at 600 C. Anticorrosion films for glass surfaces have been prepared by encapsulation of ZnO and ZrO2 nanoparticles to yield nano-composites of porous and nonporous ormosil films, which resulted in four times less corrosion compared to bare glass and acts as a barrier layer for corrosion of glass substrates against alkaline corrosion. In formation of antiicing coatings various combinations of ormosil films mentioned are used and correlation between contact angle, stability of contact angle against cooling, surface roughness and freezing times are investigated. Compared to bare glass, freezing times are increased two order of magnitudes.
Open Access
Plasmonic nanoparticles by laser dewetting of thin metallic films
(2013) Sarıtaş, Seval
In this work, formation of metal nanoparticles via laser induced dewetting and their plasmonic properties have been investigated. The effects of metal film, substrate type, laser power density and dwell time on dewetting phenomenon were analyzed. Silver and gold thin films were fabricated with thermal evaporation on various substrates. Next, they were characterized by the ellipsometry, UV-VIS spectroscopy and atomic force microscopy (AFM) as the characteristic of the thin film affects dewetting. Samples were then processed by a cw argon laser. Varying the dwell time and power density, Ag and Au nanoparticles with different morphology were obtained. At the final stages of dewetting, nanoparticles attained spherical shapes. Particle size distribution and length scale analysis were performed using the images obtained from scanning electron microscope (SEM). Using these results, relations between the average particle size and film thickness, as well as the relation between length scale and film thickness were obtained to verify the occurrence of dewetting. Substrate and film type were observed to affect the particle morphology and particle size. Moreover, plasmonic resonance effect of Ag and Au nanoparticles were observed via the optical absorbance measurements. Multilayered metallic nanoparticles and embedded nanoparticles were fabricated and were found to display plasmonic properties.