Browsing by Subject "thin films"

Now showing 1 - 4 of 4

Open Access
Experimental and computational investigation of zinc oxide based surface acoustic wave devices
(2014) Özgöztaşı, Elif
Piezoelectric materials are used in different types of transducers such as microphones, accelerometers, speakers, hydrophones, pressure sensors etc. Compared to traditional bulk piezoelectric crystals, thin film piezoelectric materials are promising to realize integrated devices with CMOS technology. Among thin film materials, zinc oxide (ZnO) is attractive due to the giant piezoelectric effect when doped with vanadium. In this study, we investigate the deposition of thin film ZnO and V-doped ZnO films. Materials characterization of ZnO thin films is performed. We also investigate surface acoustic wave (SAW) devices based on ZnO thin films. SAW devices are formed by a pair of interdigitated transducers (IDTs), input and output IDTs. IDTs are fabricated onto the piezoelectric thin film. Applied oscillating electric field from input IDT creates surface acoustic waves in the piezoelectric thin film and these acoustic waves are converted back into an electrical signal at the output IDT. SAW devices based on ZnO and V-doped ZnO films were designed and fabricated. Frequency response of SAW devices is measured. In addition, finite element simulations of SAW devices are shown to be in agreement with measurement results. We discuss resonance frequency and insertion loss of SAW devices.
Open Access
Formation of Ge nanocrystals with CW laser irradiation of Siox:Ge thin films
(2015) Gümüş, Melike
Germanium and silicon are the materials which have effective absorption in the visible and near infrared region of electromagnetic spectrum; therefore they are preferred for optoelectronic device and solar cell applications. Si and Ge are the material of choice when it comes to solar cell applications due to their being low cost, widely available and inert. They have indirect bandgap and the absorption coefficient of indirect bandgap materials is lower than direct ones. It is known that decreasing dimensions of materials to nanometric sizes cause transition from indirect bandgap to direct bandgap behavior along with increasing band gap. Therefore decreasing their dimensions both a shift of the band gap toward the blue as well as an increase in absorption can be achieved. In this work, thin films of SiOx:Ge were fabricated with different germanium concentrations and annealed with CW Ar+ laser operating at 488 nm that resulted in formation of Ge nanocrystals in the SiOx matrix. Composition analysis of as grown samples were done by Rutherford Backscattering Spectroscopy, optical properties were determined by ellipsometry. Nanocrystal formation within laser irradiated samples was confirmed by Raman spectroscopy. Data were also collected about crystal formation by scanning surface texture with stylus surface profilometer. As a result of all the analysis, it was shown that crystal formation depends on germanium concentration in the SiOx matrix and laser irradiation power density
Open Access
Molten salt assisted self-assembly (MASA) : synthesis, characterization and solar cell performances of mesoporous silica-CdSe and titania-CdSe thin films
(2015-05) Han, Ahmet Selim
A series of solutions of a salt ([Cd(H2O)4](NO3)2), a polymerizing agent (Si(OCH3)4, TMOS or Ti(OC4H9), TTBO), two surfactants (cetyltrimethylammonium bromide, CTAB and 10-lauryl ether, C12H25(OCH2CH2)10OH, C12EO10) have been prepared and used as the molten salt assisted self-assembly precursors by only changing the Cd(II)/surfactant mole ratios for the preparation of mesoporous films. The thin films were prepared in two steps: in the first step, the titania particles (titania particles are typically 20-25 nm (P25)), dispersed in ethanol, have been spin coated over various substrates and annealed at 450oC and in the second step, the clear solutions given above (salt, CTAB, C12E10, silica or titania source and ethanol) were drop casted or spin coated over the titania (P25) films and calcined at 450oC. Slow calcination of the films (with an increment of 1 oC/min), starting from the melting point of the salt (around 65oC) to 450 oC has produced the mesoporous silica/titania-cadmium oxide (CdO) thin films denoted as meso-CdO-SiO2-P25 and meso-CdTiO3-P25. The clear solutions were also used to make the thin films without P25 and denoted as meso-CdTiO3. The films were then exposed to a H2Se atmosphere at 100oC for 30 min, and the samples were denoted as meso-CdSe-SiO2-P25, meso-CdSe-TiO2-P25 and meso-CdSe-TiO2. The silica sample was further treated with a dilute HF solution (etching process) that results silica free meso-CdSe-P25. The characterization of the materials produced in this thesis was made by using XRD, FT-IR spectroscopy, UV-VIS, Raman, EDX and Solar Measurement techniques. Silica samples have greater amount of CdSe than titania samples according to the Raman and EDX data which implies that silica samples are more reactive. Also in both silica and titania, the samples with a Cd/surfactant mole ratio of 6 have the greatest amount of CdSe. Fluorine doped SnO2 (FTO) has been used as a transparent conductive substrate for the preparation of the anode electrode for the solar measurements. According to the solar measurements, the silica and titania samples on P25 show greater efficiency than the titania samples without P25 and generally have similar efficiencies (the efficiency is the ratio of the electrical output of a solar cell to the incident energy in the form of sunlight). The most efficient samples are generally the samples prepared using Cd/surfactant mole ratios equal to 6 and 8. In order to increase the efficiency, 3rd row transition metal cations such as Mn(II), Fe(III), Cu(II) and Co(II) are doped into above samples. The efficiency has been increased by 10 % in the silica and 30 % in the titania samples upon doping with 15% Mn(II). The solar cell characteristics of the electrodes were tested using the following parameters; by changing the Cd(II)/surfactant mole ratio (many samples were prepared and used for this purpose), by changing the aging of the electrolyte (Na2S/S8), by using multiple coating the MASA solution in the preparation stage of the electrodes, by doping the samples with various transition metal ions, and finally by modifying the surface of the electrode by coating with ZnS or CTAB. Each of these parameters has an effect on the cell performance. The effect of each parameter is monitored by measuring the I-V characteristics of the cells and found out that the best results were obtained from the electrodes made using 6 or 8 Cd/surfactant mole ratio in MASA system, freshly prepared Na2S/S8 electrolyte, coating of the MASA solution twice on electrode, doping Mn(II) cations with 0.15 Mn(II)/Cd(II) ratio and using CTAB for coating anode electrode.
Open Access
Template-free synthesis of organically modified silica mesoporous thin films for TNT sensing
(American Chemical Society, 2010) Yildirim, A.; Budunoglu, H.; Deniz, H.; Güler, Mustafa O.; Bayındır, Mehmet
In this paper, we present a facile, template-free sol−gel method to produce fluorescent and highly mesoporous organically modified silica (ORMOSIL) thin films for vapor phase sensing of TNT. An alkyltrifunctional, methyltrimethoxysilane MTMS precursor was used to impart hydrophobic behavior to gel network in order to form the spring back effect. In this way, porous films (up to 74% porosity) are obtained at ambient conditions. Fluorescent molecules are physically encapsulated in the ORMOSIL network during gelation. Fluorescence of the films was found to be stable even after 3 months, proving the successful fixing of the dye into the ORMOSIL network. The functional ORMOSIL thin films exhibited high fluorescence quenching upon exposition to TNT and DNT vapor. Fluorescence quenching responses of the films are thickness-dependent and higher fluorescence quenching efficiency was observed for the thinnest film (8.6% in 10 s). The prepared mesoporous ORMOSIL thin films have great potential in new sensor and catalysis applications.