Browsing by Subject "Elevated temperature"
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Item Open Access Direct evidence for the instability and deactivation of mixed-oxide systems: influence of surface segregation and subsurface diffusion(2011) Emmez, E.; Vovk, E. I.; Bukhtiyarov V. I.; Ozensoy, E.In the current contribution, we provide a direct demonstration of the thermally induced surface structural transformations of an alkaline-earth oxide/transition metal oxide interface that is detrimental to the essential catalytic functionality of such mixed-oxide systems toward particular reactants. The BaOx/TiO2/Pt(111) surface was chosen as a model interfacial system where the enrichment of the surface elemental composition with Ti atoms and the facile diffusion of Ba atoms into the underlying TiO2 matrix within 523 873 K leads to the formation of perovskite type surface species (BaTiO3/Ba2TiO4/BaxTiyOz). At elevated temperatures (T > 973 K), excessive surface segregation of Ti atoms results in an exclusively TiO2/TiOx-terminated surface which is almost free of Ba species. Although the freshly prepared BaOx/TiO2/Pt(111) surface can strongly adsorb ubiquitous catalytic adsorbates such as NO2 and CO2, a thermally deactivated surface at T > 973 K practically loses all of its NO2/CO2 adsorption capacity due to the deficiency of surface BaOx domains.Item Open Access Variable temperature-scanning Hall probe microscopy with GaN/AlGaN two-dimensional electron gas (2DEG) micro Hall sensors in 4.2-425 K range using novel quartz tuning fork AFM feedback(IEEE, 2008) Akram, Rizwan; Dede, Münir; Oral, AhmetIn this paper, we present the fabrication and variable temperature (VT) operation of Hall sensors, based on GaN/AlGaN heterostructure with a two-dimensional electron gas (2DEG) as an active layer, integrated with quartz tuning fork (QTF) in atomic force-guided (AFM) scanning Hall probe microscopy (SHPM). Physical strength and a wide bandgap of GaN/AlGaN heterostructure makes it a better choice to be used for SHPM at elevated temperatures, compared to other compound semiconductors (AlGaAs/GaAs and InSb), which are unstable due to their narrower bandgap and physical degradation at high temperatures. GaN/AlGaN micro Hall probes were produced using optical lithography and reactive ion etching. The active area, Hall coefficient, carrier concentration, and series resistance of the Hall sensors were ∼1 × 1 μm, 10 mΩ/G at 4.2 K, 6.3 × 10 12 cm -2 and 12 kΩ at room temperature and 7 mΩ/G, 8.9 × 10 12 cm -2 and 24 kΩ at 400 K, respectively. A novel method of AFM feedback using QTF has been adopted. This method provides an advantage over scanning tunneling-guided feedback, which limits the operation of SHPM the conductive samples and failure of feedback due to high leakage currents at high temperatures. Simultaneous scans of magnetic and topographic data at various pressures (from atmospheric pressure to high vacuum) from 4. to 425K will be presented for different samples to illustrate the capability of GaN/AlGaN Hall sensors in VT-SHPM.