Tessarek, Christian
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Tessarek, Christian
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Tessarek, Christian
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Item-typ:Veröffentlichung, Rare-Earth-Doped Y4Al2O9 Nanoparticles for Stable Light-Converting Phosphors(American Chemical Society, 2019-12-24); ; ; ; The ternary mixed metal oxides with high specific surface area are industrially important for sensors, catalysis, energy storage, and optoelectronics. However, synthesis of such metal oxides is always challenging, especially when multicomponent mixtures occur due to very narrow formation temperature windows. Flame spray pyrolysis is one of the best known techniques which enable formation of pure phases optimizing the temperature profile via control over the fuel/oxidizer ratio. Here, Y4Al2O9 (monoclinic phase of yttrium aluminum oxide, also known as YAM) and Y4–xEuxAl2O9 (x = 0.05–1.0) were strategically synthesized with specific precursor–solvent chemistry. While the hydrated yttrium nitrate with 28.2% water was unsuitable for the formation of crystalline Y4Al2O9 particles, the use of organic precursor–solvent combinations (Y/Al 2:1) resulted in 16 nm phase pure, highly crystalline Y4Al2O9 particles. All the materials were characterized by using X-ray diffraction with Rietveld refinement, Raman spectroscopy, and transmission electron microscopy. To develop a stable light-converting phosphor, the Y4Al2O9 host was doped with Eu to investigate the photoluminescence properties of Y4–xEuxAl2O9 (x = 0.05–1.0). The results indicated increased photoluminescence intensity with increasing Eu3+ concentration up to x = 0.5, i.e., Y3.5Eu0.5Al2O9, and a subsequent drop or decrease in intensity for x ≥ 0.7. Hence, Y3.5Eu0.5Al2O9 is proposed for a potential light-converting phosphor.Wissenschaftlicher ArtikelBand:3Heft:116 11 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Excitons Bound to Defect States in Two-Dimensional (2D) MoS2(IEEE, 2021-04-29); ; ; ; In this work, the effect of atomic defects created by gallium ion irradiation on the optical properties of single-layer molybdenum disulfide is studied by means of micro-photoluminescence measurements. The induced defects give rise to an additional emission band located at about 170 meV below the free exciton. The micro-photoluminescence intensity of this defect-related emission band is found to be proportional to the defect density. The large spectral width suggests the presence of binding sites with different binding energies available for excitons that remain optically active up to 230 K.Wissenschaftlicher ArtikelBand:2039 31 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Phasenseparation von dünnen InGaN-Schichten in der metallorganischen Gasphasenepitaxie - Realisierung und Anwendung von InGaN Quantenpunkten(2011-07-06); ; ; This work is devoted to the understanding and realization of the InGaN quantum dot formation process. The growth is performed on an n-doped GaN layer in a metal organic vapor phase epitaxy reactor. Evidence has been found that spinodal and binodal decomposition are driving a separation process of a thin InGaN layer into two phases with different InN concentrations. The region with low InN content is forming the quantum dots on the surface of a GaN layer. The spinodal phase diagram has been calculated for the case of a strained InGaN layer on GaN. The accordance between theoretical and experimental results is shown. For device application it is necessary to cap these structures with a p-doped GaN layer. The influence of a GaN capping on the phase separated InGaN structures is discussed. Finally, the applications of InGaN quantum dots in LEDs, laser structures and microcavities is presented.Dissertation335 212 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Controlled Laser-Thinning of MoS2 Nanolayers and Transformation to Amorphous MoOx for 2D Monolayer Fabrication(American chemical society, 2020-07-20); ; ; ; Laser-thinning of 2D materials such as MoS2 is a promising approach for a local reduction of the number of multilayers down to a monolayer. For a precise control of the thinning process real-time monitoring is required. In this work, short-wavelength lasers emitting at 325 or 406 nm respectively are used for laser-thinning and simultaneous Raman or photoluminescence spectroscopy of MoS2. The time evolution of the Raman and photoluminescence bands during the process shows a layer-by-layer thinning of MoS2 and a transformation into amorphous MoOx in an oxygen-containing atmosphere. In addition to the E2g1 and A1g Raman modes, the E1g, B2g1, and second-order modes are analyzed by using the 325 nm laser for excitation to achieve a more accurate determination of the number of layers. As a promising alternative, photoluminescence spectroscopy is used to monitor the thinning progress by analysis of the emission energy and intensity of the direct as well as the indirect band gap transition. Atomic force microscopy measurements show an increased total height of the laser-treated region after thinning of MoS2 due to the presence of transformed MoOx. Local micropatterning of a bilayer is also demonstrated by laser-thinning down to a monolayer at selected positions. The results show a new monitoring approach for controlled fabrication of 2D monolayers.Wissenschaftlicher ArtikelBand:3Heft:842 39 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Spatially resolved luminescence properties of non- and semi-polar InGaN quantum wells on GaN microrods(IOP Publishing, 2018-08-03); ; ; ; Spatially resolved emission properties of InGaN/GaN quantum wells on the facets of microrods are analyzed by means of photoluminescence and cathodoluminescence measurements. We observe strongly localized emissions of the non- and semi-polar InGaN quantum wells from the top part of the microrods, suggesting optical emitters of high efficiency. The quantum wells are characterized by transmission electron microscopy measurements with respect to their indium composition and thickness. Those wells oriented in non- and semi-polar directions possess high In concentrations as well as low internal polarization fields which makes these directions excellent candidates for InGaN quantum well emitters. These investigations show that the applied microrod growth concept is an effective method to realize high quality InGaN quantum wells on GaN facets oriented in non- and semi-polar directions which makes optoelectronic emitters in the green gap region feasible.Wissenschaftlicher ArtikelBand:5127 21 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Biotic and Abiotic Interactions in Freshwater Mesocosms Determine Fate and Toxicity of CuO Nanoparticles(American Chemical Society, 2023-08-10); ; ; ; Transformation, dissolution, and sorption of copper oxide nanoparticles (CuO-NP) play an important role in freshwater ecosystems. We present the first mesocosm experiment on the fate of CuO-NP and the dynamics of the zooplankton community over a period of 12 months. Increasingly low (0.08-0.28 mg Cu L) and high (0.99-2.99 mg Cu L) concentrations of CuO-NP and CuSO (0.10-0.34 mg Cu L) were tested in a multiple dosing scenario. At the high applied concentration (CuO-NP_H) CuO-NP aggregated and sank onto the sediment layer, where we recovered 63% of Cu applied. For the low concentration (CuO-NP_L) only 41% of applied copper could be recovered in the sediment. In the water column, the percentage of initially applied Cu recovered was on average 3-fold higher for CuO-NP_L than for CuO-NP_H. Zooplankton abundance was substantially compromised in the treatments CuSO ( < 0.001) and CuO-NP_L ( < 0.001). Community analysis indicated that Cladocera were most affected (b = -0.49), followed by Nematocera (b = -0.32). The abundance of Cladocera over time and of Dixidae in summer was significantly reduced in the treatment CuO-NP_L ( < 0.001; < 0.05) compared to the Control. Our results indicate a higher potential for negative impacts on the freshwater community when lower concentrations of CuO-NP (<0.1 mg Cu L) enter the ecosystem.Wissenschaftlicher ArtikelBand:57Heft:3335 33