Browsing by Subject "Al2O3"
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Item Open Access Finding an optimum surface chemistry for [Formula] systems as NOx storage materials(Bilkent University, 2010) Şentürk, Göksu SedaTitania promoted NOx storage materials in the form of BaO/TiO2/Al2O3 were synthesized via two different sol-gel preparation techniques, with varying surface compositions and morphologies [1, 2]. The influence of the TiO2 units on the NOx storage component (8 - 20 wt. % BaO), the nature of the crystallographic phases, thermal stabilities and the dispersion of the surface oxide/nitrate domains were investigated. The structural characterization of the synthesized NOx storage materials were analyzed by means of BET surface area analysis, X-ray diffraction (XRD), ex-situ Raman spectroscopy, scanning electron microscopy (SEM), energy dispersive X- ray (EDX) and transmission electron microscopy (TEM). Comparative analysis of the results showed that the TiO2/Al2O3 support material derived by the co-precipitation of the corresponding hydroxides via the sol-gel technique, exhibited distinctively more homogenous distribution of TiO2 domains. The functionality/performance of these materials upon NOx and SOx adsorptions were monitored by temperature programmed desorption (TPD) and insitu Fourier transform infrared (FTIR) spectroscopy. An improved Ba surface dispersion was observed for the BaO/TiO2/Al2O3 materials synthesized via the coprecipitation of alkoxide precursors which was found to originate mostly from the increased fraction of accessible TiO2/TiOx sites on the surface. These TiO2/TiOx sites functioned as strong anchoring sites for surface BaO domains and were tailored to enhance surface dispersion of BaO. The relative stability of the NOx species adsorbed on the BaO/TiO2/Al2O3 system was found to increase in the following order: NO+ /N2O3 on alumina << nitrates on alumina < surface nitrates on BaO < bridged/bidentate nitrates on large/isolated TiO2 clusters < bulk nitrates on BaO on alumina surface and bridged/bidentate nitrates on TiO2 crystallites homogenously distributed on the surface < bulk nitrates on the BaO sites located on the TiO2 domains. The detailed study of the interaction of SOx with BaO/TiO2/Al2O3 ternary oxide materials showed that titania (TiO2) was a promising candidate for improving the sulfur tolerance on these type of surfaces. Adsorption of SOx on both pure Al2O3 and TiO2 showed that Al2O3 formed strongly bound SOx species, that were thermally stable up to T > 1073 K. SOx adsorption directly altered stability of the nitrate species on the Ti/Al (Protocol 1, Protocol 2) samples. SOx uptake properties of the BaO/TiO2/Al2O3 materials were found to be strongly influenced by the morphology of the TiO2/TiOx domains and the BaO loadings (8/20 wt% BaO). Consequently, the presence of titania domains was seen to decrease the SOx desorption temperatures and enhance the sulfur-tolerance of these materials by destabilizing the accumulated sulfate species. SOx exposure on the synthesized materials led to a significant decrease in the NOx adsorption capacities. The results obtained from FT-IR spectra showed that the sulfur deposition on the NOx storage materials promoted by TiItem Open Access Nanometer-thick ınsertion layer for the effective passivation of surface traps and ımproved edge acuity for AlGaN/GaN HEMTs(Institute of Electrical and Electronics Engineers, 2023-09-30) Odabaşı, Oğuz; Ghobadi, Amir; Ghobadi, Türkan Gamze Ulusoy; Güneysu, Efkan; Urfalı, Emirhan; Yağlıoğlu, Gül; Bütün, Bayram; Özbay, EkmelIn AlGaN/GaN high electron mobility transistors (HEMTs), the existence of long lifetime surface traps can cause several adverse effects, including threshold voltage ( Vth ) instability and current collapse. Therefore, understanding the nature and lifetime of these traps is crucial to provide effective passivation. In this work, the nature of these traps is scrutinized by combining femtosecond transient optical and multiple structural analyses. Later, using a nanometer-thick Al2O3 insertion layer, these traps are effectively passivated. In order to observe the effect of the proposed passivation on device performance, HEMT devices were fabricated. As a result of this passivation, better edge acuity in ohmic contacts and protection of the surface of the epitaxy were achieved. The lag performance of the HEMT devices was significantly improved. It was found that the drain lag was reduced from 37.1% (for the standard SiNx passivated design) to 10.4% for the modified HEMT design. In operating this transistor as a power amplifier, nearly no change in the quiescent bias point was observed after consecutive load–pull measurements, which shows the stability of the fabricated device.Item Open Access NOx reduction on a transition metal-free γ-Al2O3 catalyst using dimethylether (DME)(2008) Ozensoy, E.; Herling, D.; Szanyi, J.NO2 and dimethylether (DME) adsorption as well as DME and NO2 co-adsorption on a transition metal-free γ-alumina catalyst were investigated via in-situ transmission Fourier transform infrared spectroscopy (in-situ FTIR), residual gas analysis (RGA) and temperature programmed desorption (TPD) techniques. NO2 adsorption at room temperature leads to the formation of surface nitrates and nitrites. DME adsorption on the alumina surface at 300 K leads to molecularly adsorbed DME, molecularly adsorbed methanol and surface methoxides. Upon heating the DME-exposed alumina to 500-600 K the surface is dominated by methoxide groups. At higher temperatures methoxide groups are converted into formates. At T > 510 K, formate decomposition takes place to form H2O(g) and CO(g). DME and NO2 co-adsorption at 423 K does not indicate a significant reaction between DME and NO2. However, in similar experiments at 573 K, fast reaction occurs and the methoxides present at 573 K before the NO2 adsorption are converted into formates, simultaneously with the formation of isocyanates. Under these conditions, NCO can further be hydrolyzed into isocyanic acid or ammonia with the help of water which is generated during the formate formation, decomposition and/or NCO formation steps. © 2008 Elsevier B.V. All rights reserved.Item Open Access Passivation of type II InAs / GaSb superlattice photodetectors with atomic layer deposited Al2O3(SPIE, 2012) Salihoğlu, Ömer; Muti, Abdullah; Kutluer, K.; Tansel, T.; Turan, R.; Kocabaş, Coşkun; Aydınlı, AtillaWe have achieved significant improvement in the electrical performance of the InAs/GaSb midwave infrared photodetector (MWIR) by using atomic layer deposited (ALD) aluminium oxide (Al2O3) as a passivation layer. Plasma free and low operation temperature with uniform coating of ALD technique leads to a conformal and defect free coverage on the side walls. This conformal coverage of rough surfaces also satisfies dangling bonds more efficiently while eliminating metal oxides in a self cleaning process of the Al2O3 layer. Al2O3 passivated and unpassivated diodes were compared for their electrical and optical performances. For passivated diodes the dark current density was improved by an order of magnitude at 77 K. The zero bias responsivity and detectivity was 1.33 A/W and 1.9 x 1013 Jones, respectively at 4 µm and 77 K. Quantum efficiency (QE) was determined as %41 for these detectors.Item Open Access TiO2-Al2O3 binary mixed oxide surfaces for photocatalytic NOx abatement(Elsevier, 2014) Soylu, A. M.; Polat, M.; Erdogan, D. A.; Say, Z.; Yıldırım, C.; Birer, Ö.; Ozensoy, E.TiO2-Al2O3 binary oxide surfaces were utilized in order to develop an alternative photocatalytic NOx abatement approach, where TiO2 sites were used for ambient photocatalytic oxidation of NO with O2 and alumina sites were exploited for NOx storage. Chemical, crystallographic and electronic structure of the TiO2-Al2O3 binary oxide surfaces were characterized (via BET surface area measurements, XRD, Raman spectroscopy and DR-UV-Vis Spectroscopy) as a function of the TiO2 loading in the mixture as well as the calcination temperature used in the synthesis protocol. 0.5 Ti/Al-900 photocatalyst showed remarkable photocatalytic NOx oxidation and storage performance, which was found to be much superior to that of a Degussa P25 industrial benchmark photocatalyst (i.e. 160% higher NOx storage and 55% lower NO2(g) release to the atmosphere). Our results indicate that the onset of the photocatalytic NOx abatement activity is concomitant to the switch between amorphous to a crystalline phase with an electronic band gap within 3.05-3.10 eV; where the most active photocatalyst revealed predominantly rutile phase together and anatase as the minority phase.Item Open Access Utilization of reducible mixed metal oxides as promoters for the enhancement of sulfur regeneration in nsr catalysts(Bilkent University, 2016-07) Samast, Zehra AybegümPt functionalized binary, ternary, and quaternary oxides (e.g. Pt/BaO/CeO2/ZrO2/Al2O3) were synthesized by wetness impregnation method and characterized by X-ray Diffraction (XRD), Brunauer–Emmett–Teller (BET) surface area analysis, in-situ Fourier Transform Infrared (FTIR), and temperature programmed desorption (TPD) techniques. Effect of the synthesis sequence on the NOx storage capacity was investigated by synthesizing subsequently impregnated and co-impregnated ternary oxides. Influence of BaO loading on NOx uptake of quaternary oxides was examined by utilizing two different BaO loadings namely; 8 wt% and 20 wt% on co-impregnated ternary oxide, Pt10-10CeZrAl. Co-presence of CeO2-ZrO2 oxide domains leads to an increase in NOx storage. As BaO loading increases in quaternary oxides, thermal stabilities of nitrates and nitrites increase due to the formation of bulk/ionic nitrates. Although BaO impregnation on co-impregnated ternary oxides leads to a decrease in specific surface area (SSA) values due to sintering, NOx adsorption on BaO-functionalized quaternary oxides was found to be higher than the BaO deficient ternary oxides. Upon sulfur poisoning, formation of strongly bound bulk/ionic sulfate/sulfite functional groups on BaO containing catalysts result in a need for higher temperatures for complete sulfur regeneration. Comparison of the CeO2-ZrO2 promoted systems with that of the Pt/ 20 wt% Ba/Al2O3 conventional NOx Storage Reduction (NSR) catalyst suggests that ceria-zirconia promotion enhances the sulfur tolerance. In conclusion, in this study a new NSR catalyst namely, Pt20Ba10-10CeZrAl, which is promoted with reducible mixed metal oxides, was synthesized and characterized. This novel NSR catalyst formulation revealed favorable sulfur resistance with minor sacrifice in NOx storage ability.Item Open Access ZnO, TiO² and exotic materials for low temperature thin film electronic devices(Bilkent University, 2012) Oruç, Feyza BozkurtThe metal-oxide-semiconductor field-effect transistor (MOSFET) technology is the core of integrated circuit industry. Nearly all electronic devices around us contain transistors for various purposes like electronic switches, amplifiers or sensors. As the need for more complex and miniature circuits has arisen, scaling down transistor sizes become the top priority. As Moore’s law indicates, number of transistors on integrated circuits doubles every two years but in future fabrication challenges and limitations like quantum effects seen in small devices will block further miniaturization. New growth techniques are required for depositing conformal, high quality films -like high-k dielectrics instead of SiO2- with atomic thickness control to reduce possible problems. Atomic layer deposition techniques are developed to meet these requirements. The field of thin film transistors (TFT), which is a subset of MOSFET’s have first started to be used in flat panel displays but now they are used in various fields, since their functional properties make them powerful candidates for sensor applications. ALD technology is important also for TFT applications since its low temperature growth mechanism allows fabricating TFT’s on various substrates like flexible and/or transparent ones. With ALD technique, transistors can be built even on cloths which makes the dream of e-suits real. In this thesis, thin film transistors are designed and fabricated using atomic layer deposition technique both for channel and dielectric layer growth. Design and fabrication steps of the TFT devices are realized in a cleanroom environment. The fabricated TFT’s are mainly characterized by measuring their current-voltage relations. A parameter analyzer with a probe station is used for such measurements. ALD grown ZnO TFT’s and the effect of growth temperature on performance characteristics are examined. High performance devices having very high Ion/Ioff ratios are fabricated at a temperature low as 80°C. ALD grown TiO2 TFT’s are also fabricated and effects of annealing temperature on device performance are analyzed. This study is, to the best of our knowledge, the first demonstration of TiO2 TFT’s grown by a thermal-ALD system. GaN and pentacene TFT’s are also fabricated and showed promising results. Pentacene TFTs have a special importance since it is a p-type organic semiconductor which gives us the opportunity to work on hybrid organic-inorganic structures. In conclusion, TFT devices based on ALD grown channel and/or dielectric layers show very encouraging results in terms of low cost, low temperature fabrication opportunities and freedom of using any substrate that can handle ALD processing temperature.