17 results on '"Kristian Frank"'
Search Results
2. Numerical and experimental characterization of radiofrequency ablation in perfused kidneys.
- Author
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Kristian Frank, Herbert Lindenborn, and Dirk Dahlhaus
- Published
- 2012
- Full Text
- View/download PDF
3. Thermal performance analysis of GaN nanowire and fin-shaped power transistors based on self-consistent electrothermal simulations
- Author
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Kristian Frank, Muhammad Fahlesa Fatahilah, Friedhard Römer, Hutomo Suryo Wasisto, Feng Yu, Klaas Strempel, Bernd Witzigmann, Andreas Waag, and Hamed Kamrani
- Subjects
Materials science ,Nanowire ,02 engineering and technology ,Substrate (electronics) ,01 natural sciences ,law.invention ,Fin (extended surface) ,law ,0103 physical sciences ,Thermal ,Power semiconductor device ,Electrical and Electronic Engineering ,Safety, Risk, Reliability and Quality ,Sheet resistance ,010302 applied physics ,business.industry ,Transistor ,021001 nanoscience & nanotechnology ,Condensed Matter Physics ,Atomic and Molecular Physics, and Optics ,Surfaces, Coatings and Films ,Electronic, Optical and Magnetic Materials ,Heat flux ,Optoelectronics ,0210 nano-technology ,business - Abstract
We present self-consistent electrothermal simulations of the GaN nanowire-based field-effect transistor (NWFET) and vertical fin field-effect transistor (FinFET) by taking into account all major heat flux paths. Simulation results of a NWFET validated by experimental data are compared to the results of a vertical FinFET designed with same sizes that ensures a fair comparison of their thermal performance. It is found that the peak temperature in the NWFET is close to the uppermost contact, which facilitates heat removal from top. As a result, NWFETs have the potential to achieve a higher power density at a temperature limit compared with the FinFETs, especially when the heat removal from the top contact is eased. The impact of the thermal surface resistance of the top contact and substrate thinning on the thermal performance of these two vertical structures is also investigated.
- Published
- 2018
4. Performance analysis and simulation of vertical gallium nitride nanowire transistors
- Author
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Andreas Waag, Kristian Frank, Feng Yu, Bernd Witzigmann, Klaas Strempel, Hans Werner Schumacher, Friedhard Römer, Hutomo Suryo Wasisto, and Muhammad Fahlesa Fatahilah
- Subjects
Materials science ,Transconductance ,Nanowire ,Gallium nitride ,02 engineering and technology ,01 natural sciences ,law.invention ,Condensed Matter::Materials Science ,chemistry.chemical_compound ,law ,Saturation current ,0103 physical sciences ,Materials Chemistry ,Electrical and Electronic Engineering ,Saturation (magnetic) ,010302 applied physics ,business.industry ,Transistor ,Doping ,Condensed Matter::Mesoscopic Systems and Quantum Hall Effect ,021001 nanoscience & nanotechnology ,Condensed Matter Physics ,Electronic, Optical and Magnetic Materials ,Threshold voltage ,chemistry ,Optoelectronics ,0210 nano-technology ,business - Abstract
Gallium nitride (GaN) nanowire transistors are analyzed using hydrodynamic simulation. Both p-body and n-body devices are compared in terms of threshold voltage, saturation behavior and transconductance. The calculations are calibrated using experimental data. The threshold voltage can be tuned from enhancement to depletion mode with wire doping. Surface states cause a shift of threshold voltage and saturation current. The saturation current depends on the gate design, with a composite gate acting as field plate in the p-body device.
- Published
- 2018
5. On the structural properties of MgS-rich II–VI-based microcavities
- Author
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Andreas Rosenauer, Sebastian Klembt, Kristian Frank, Gang Qian, Carsten Kruse, Detlef Hommel, and Thorsten Klein
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Diffraction ,Materials science ,Field (physics) ,business.industry ,Stacking ,Condensed Matter Physics ,Molecular physics ,Dark field microscopy ,Inorganic Chemistry ,Transmission electron microscopy ,Metastability ,Microscopy ,Materials Chemistry ,Optoelectronics ,business ,Molecular beam epitaxy - Abstract
II–VI-based microcavities grown by molecular beam epitaxy designed for emission in the blue spectral region have been comprehensively characterized by transmission electron microscopy in bright field, dark field and scanning mode. These complex structures contain a high amount of MgS, which is correlated with the occurrence of a certain kind of defect. Typical V-shaped stacking faults caused by the metastability of zinc-blende and rocksalt MgS along different (111) glideplanes have been identified. Optimized growth conditions have been found where this defect type can be suppressed to a large extend. Results obtained by x-ray diffraction measurements are consistent with the microscopy findings.
- Published
- 2013
6. Maximizing Activity and Stability by Turning Gold Catalysis Upside Down: Oxide Particles on Nanoporous Gold
- Author
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Lutz Mädler, Kristian Frank, Jürgen Biener, Björn Neumann, Andreas Rosenauer, Monika M. Biener, Marcus Bäumer, Arne Wittstock, and Andre Wichmann
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Inorganic Chemistry ,Nanopore ,chemistry.chemical_compound ,Materials science ,chemistry ,Nanoporous ,Organic Chemistry ,Oxide ,Nanotechnology ,Physical and Theoretical Chemistry ,Catalysis - Published
- 2013
7. CO oxidation on nanoporous gold: A combined TPD and XPS study of active catalysts
- Author
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Sarah Röhe, Arne Wittstock, Kristian Frank, Volkmar Zielasek, Andreas Schaefer, Marcus Bäumer, and Andreas Rosenauer
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Nanoporous ,Chemistry ,Inorganic chemistry ,chemistry.chemical_element ,Surfaces and Interfaces ,Condensed Matter Physics ,Redox ,Oxygen ,Surfaces, Coatings and Films ,Catalysis ,Adsorption ,X-ray photoelectron spectroscopy ,Desorption ,Materials Chemistry ,Leaching (metallurgy) - Abstract
Disks of nanoporous gold (np-Au), produced by leaching of silver from AgAu alloy and prepared as active catalysts for CO oxidation in a continuous-flow reactor, were investigated in detail by x-ray photoelectron spectroscopy and temperature-programmed desorption spectroscopy in ultra-high vacuum. Np-Au exhibits several oxygen species on and in the surface: Chemisorbed oxygen (O act ), probably generated at residual silver sites at the surface, is readily available after np-Au preparation and consumed by CO oxidation. It can be replenished on activated np-Au by exposure to O 2 . In addition, strongly bound oxygen, probably at subsurface sites, is present as a major species and not consumed by CO oxidation. Pronounced CO desorption at temperatures above 200 K observed after exposing np-Au to CO at 105 K indicates an additional, more stable type of CO binding sites on np-Au as compared to pure gold. Only CO at these binding sites is consumed by oxidation reaction with O act . It is proposed that the presence of strongly bound subsurface oxygen stabilizes CO adsorption on np-Au, thereby being as crucial for the observed catalytic activity of np-Au as residual silver.
- Published
- 2013
8. Oxygen-Controlled Photoconductivity in ZnO Nanowires Functionalized with Colloidal CdSe Quantum Dots
- Author
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Apurba Dev, Dongchao Hou, Tobias Voss, Andreas Rosenauer, and Kristian Frank
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Materials science ,Condensed Matter::Other ,business.industry ,Photoconductivity ,Nanowire ,Physics::Optics ,Nanotechnology ,Electron ,Condensed Matter::Mesoscopic Systems and Quantum Hall Effect ,Surfaces, Coatings and Films ,Electronic, Optical and Magnetic Materials ,Photoexcitation ,Condensed Matter::Materials Science ,Colloid ,General Energy ,Quantum dot ,Desorption ,Optoelectronics ,Physical and Theoretical Chemistry ,business ,Quantum tunnelling - Abstract
ZnO nanowire arrays were functionalized with colloidal CdSe quantum dots stabilized by 3-mercaptopropionic acid to form hybrid devices. The photoconductivity of the nanowire/quantum-dot devices was studied under selective photoexcitation of the quantum dots, and it was found that the dynamics strongly depend on the gas environment. Desorption of surface oxygen from both the ZnO nanowires and the CdSe quantum dots, activated by electron tunnelling between the nanowires and the quantum dots, is found to be the dominating process that determines the dynamics of the photoconductivity in the hybrid nanowire/quantum-dot devices.
- Published
- 2012
9. Optical Properties of CdTe QDs Formed Using Zn Induced Reorganization
- Author
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Andrzej Golnik, D. Hommel, Tomasz Kazimierczuk, J. Kobak, Andreas Rosenauer, Wojciech Pacuski, J. A. Gaj, Kristian Frank, C. Kruse, and Tomasz Jakubczyk
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Condensed Matter::Materials Science ,Photoluminescence ,Materials science ,Condensed Matter::Other ,Quantum dot ,Exciton ,General Physics and Astronomy ,Condensed Matter::Mesoscopic Systems and Quantum Hall Effect ,Molecular physics ,Biexciton ,Order of magnitude ,Cadmium telluride photovoltaics ,Line (formation) - Abstract
In this paper we present optical studies of CdTe quantum dots formed using Zn-induced reorganization. The pattern of quantum dot photoluminescence lines is found to be similar to typical results reported for quantum dots grown with other techniques, although the positively charged exciton line is relatively more pronounced. Also the energy spacing between biexciton and exciton lines is found to be larger than in typical results. Zn-induced reorganization results in quantum dots density higher by an order of magnitude than in Te-induced quantum dots.
- Published
- 2011
10. Coherently embedded Ag nanostructures in Si: 3D imaging and their application to SERS
- Author
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R. R. Juluri, Parlapalli V. Satyam, Arnab Ghosh, Andreas Rosenauer, Anjan Bhukta, Knut Müller, Ashutosh Rath, D. Narayana Rao, Marco Schowalter, Kristian Frank, R. Sathyavathi, Florian F. Krause, and Tim Grieb
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Multidisciplinary ,Materials science ,Nanostructure ,Silicon ,Annealing (metallurgy) ,Oxide ,chemistry.chemical_element ,Nanotechnology ,Surface-enhanced Raman spectroscopy ,Article ,chemistry.chemical_compound ,chemistry ,Physical vapor deposition ,Scanning transmission electron microscopy ,ddc:000 ,Thin film - Abstract
Surface enhanced Raman spectroscopy (SERS) has been established as a powerful tool to detect very low-concentration bio-molecules. One of the challenging problems is to have reliable and robust SERS substrate. Here, we report on a simple method to grow coherently embedded (endotaxial) silver nanostructures in silicon substrates, analyze their three-dimensional shape by scanning transmission electron microscopy tomography and demonstrate their use as a highly reproducible and stable substrate for SERS measurements. Bi-layers consisting of Ag and GeOx thin films were grown on native oxide covered silicon substrate using a physical vapor deposition method. Followed by annealing at 800°C under ambient conditions, this resulted in the formation of endotaxial Ag nanostructures of specific shape depending upon the substrate orientation. These structures are utilized for detection of Crystal Violet molecules of 5 × 10(-10) M concentrations. These are expected to be one of the highly robust, reusable and novel substrates for single molecule detection.
- Published
- 2013
11. The role of surface functionalization of colloidal alumina particles on their controlled interactions with viruses
- Author
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Andreas Rosenauer, Artur Fink, Kaibo Li, Beate Piel, Kristian Frank, Andreas Dotzauer, Julia Wehling, Laura Treccani, Fabian Meder, Susan Koeppen, and Kurosch Rezwan
- Subjects
Materials science ,Surface Properties ,viruses ,Static Electricity ,Biophysics ,Bioengineering ,02 engineering and technology ,Conjugated system ,Cell Line ,Biomaterials ,03 medical and health sciences ,chemistry.chemical_compound ,Colloid ,Electrokinetic phenomena ,Adsorption ,Capsid ,Microscopy, Electron, Transmission ,Aluminum Oxide ,Organic chemistry ,Animals ,Colloids ,030304 developmental biology ,Levivirus ,0303 health sciences ,Temperature ,021001 nanoscience & nanotechnology ,3. Good health ,Steam ,Sulfonate ,chemistry ,Chemical engineering ,Mechanics of Materials ,Ceramics and Composites ,Surface modification ,Particle ,0210 nano-technology ,Hydrophobic and Hydrophilic Interactions ,Bacteriophage phi X 174 - Abstract
Materials that interact in a controlled manner with viruses attract increasing interest in biotechnology, medicine, and environmental technology. Here, we show that virus–material interactions can be guided by intrinsic material surface chemistries, introduced by tailored surface functionalizations. For this purpose, colloidal alumina particles are surface functionalized with amino, carboxyl, phosphate, chloropropyl, and sulfonate groups in different surface concentrations and characterized in terms of elemental composition, electrokinetic, hydrophobic properties, and morphology. The interaction of the functionalized particles with hepatitis A virus and phages MS2 and PhiX174 is assessed by virus titer reduction after incubation with particles, activity of viruses conjugated to particles, and imaged by electron microscopy. Type and surface density of particle functional groups control the virus titer reduction between 0 and 99.999% (5 log values). For instance, high sulfonate surface concentrations (4.7 groups/nm 2 ) inhibit attractive virus–material interactions and lead to complete virus recovery. Low sulfonate surface concentrations (1.2 groups/nm 2 ), native alumina, and chloropropyl-functionalized particles induce strong virus-particle adsorption. The virus conformation and capsid amino acid composition further influence the virus–material interaction. Fundamental interrelations between material properties, virus properties, and the complex virus–material interaction are discussed and a versatile pool of surface functionalization strategies controlling virus–material interactions is presented.
- Published
- 2013
- Full Text
- View/download PDF
12. Numerical and experimental characterization of radiofrequency ablation in perfused kidneys
- Author
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Dirk Dahlhaus, Kristian Frank, and Herbert Lindenborn
- Subjects
Kidney ,Materials science ,Radiofrequency ablation ,Radio Waves ,Experimental validation ,Models, Biological ,law.invention ,medicine.anatomical_structure ,In vivo ,law ,medicine.artery ,medicine ,Catheter Ablation ,In vivo measurements ,Humans ,Renal artery ,Perfusion ,Ex vivo ,Biomedical engineering - Abstract
We develop a three-dimensional finite element model in order to predict the resulting temperature distribution of a radiofrequency ablation (RFA) treatment in human kidneys. Here, a strong cooling effect results from a high degree of blood perfusion, which is modeled via two different approaches. The influence of big blood vessels for treatments close to renal hilus is modeled by including a cylindrical cooling tube based on the renal artery (or vein) in the kidney model. The influence of the perfusion of small arterioles and capillaries is represented by Pennes' approach in the bioheat equation. The experimental validation is performed by an in vivo RFA treatment on porcine kidney. Prior to the in vivo measurements several ex vivo experiments on fresh kidneys are carried out as a plausibility check for the model. During the treatments temperature profiles are measured using thermocouples which are radially arranged around the RFA applicator trocar. The evaluated data for each sensor show a deviation between 0.01 and 12 % from the simulation results. The approach serves for the design of a preplanning tool for RFA treatment in the future.
- Published
- 2013
13. Investigation of a Nanoporous Gold / TiO2 Catalyst by Electron Microscopy and Tomography
- Author
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Marcus Bäumer, Arne Wittstock, Andre Wichmann, Andreas Rosenauer, Lutz Mädler, and Kristian Frank
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Materials science ,Ion beam ,Nanoporous ,Transmission electron microscopy ,Scanning transmission electron microscopy ,Surface modification ,Nanoparticle ,Energy filtered transmission electron microscopy ,Nanotechnology ,Focused ion beam - Abstract
Nanoporous gold is a material with many possible applications e.g. in catalysts, sensors and electrode materials. We studied the functionalization of the nanoporous gold with TiO2 particles. Aiming at the low temperature oxidation of CO, the nanoporous gold can be coated with TiO2 in order to enhance catalytic activity. Structure and distribution of the TiO2 on the gold surface are important structural features, which were investigated by transmission electron microscopy. The preparation of the porous gold was tested with focused ion beam - preparation, conventional Ar+ ion beam preparation of nanoporous gold embedded in epoxy and ultramicrotome preparation of nanoporous gold embedded in epoxy. Considering the beam damage on the structure and the contamination of the surface, ultramicrotome preparation turned out to be the best solution. It was shown, that the gold ligaments are abundantly covered by approximately 5 nm TiO2 particles. The determination of the largest lattice fringe distance in high resolution mode revealed that the crystalline nanoparticles consist of the anatase phase. The spatial Ti distribution was measured with energy filtered transmission electron microscopy. Scanning transmission electron microscopy tomography was applied to reconstruct the three-dimensional structure of the gold coated with TiO2 particles.
- Published
- 2013
14. Photoconductivity of ZnO Nanowires Decorated with CdSe Quantum Dots
- Author
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Dongchao Hou, Tobias Voss, Andreas Rosenauer, Kristian Frank, and Apurba Dev
- Subjects
Colloid ,Electron transfer ,Nanostructure ,Materials science ,business.industry ,Quantum dot ,Photoconductivity ,Desorption ,Zno nanowires ,Nanowire ,Optoelectronics ,business - Abstract
A hybrid assembly was built using ZnO nanowire (NW) arrays and colloidal CdSe quantum dots (QDs) stabilized by 3-mercaptopropionic acid (MPA). The QDs were chemically linked to the nanowires through the bonds formed between the outgoing carboxyl groups of the QD stabilizers and the zinc ions on the nanowire surface. An efficient clustering attachment of the QDs was achieved via partial removal of the stabilizers of the QDs. The photoconductivity of the NW/QD assembly was investigated by selective excitation of the CdSe QDs. Oxygen desorption from the nanowire surface enhances the photoconductivity and a model involving electron transfer between the QDs and the nanowires is proposed to explain the experimental results.
- Published
- 2012
15. Monolithic ZnTe-based pillar microcavities containing CdTe quantum dots
- Author
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Detlef Hommel, Matthias Florian, Tomasz Jakubczyk, Wojciech Pacuski, Carsten Kruse, Andreas Rosenauer, J. A. Gaj, Kristian Frank, Frank Jahnke, J. Kobak, and Marco Schowalter
- Subjects
Materials science ,business.industry ,High-refractive-index polymer ,Mechanical Engineering ,Superlattice ,Physics::Optics ,Bioengineering ,General Chemistry ,Substrate (electronics) ,Epitaxy ,Focused ion beam ,Cadmium telluride photovoltaics ,Condensed Matter::Materials Science ,Mechanics of Materials ,Quantum dot ,Scanning transmission electron microscopy ,Optoelectronics ,General Materials Science ,Electrical and Electronic Engineering ,business - Abstract
Micropillars of different diameters have been prepared by focused ion beam milling out of a planar ZnTe-based cavity. The monolithic epitaxial structure, deposited on a GaAs substrate, contains CdTe quantum dots embedded in a ZnTe λ-cavity delimited by two distributed Bragg reflectors (DBRs). The high refractive index material of the DBR structure is ZnTe, while for the low index material a short-period triple MgTe/ZnTe/MgSe superlattice is used. The CdTe quantum dots are formed by a novel Zn-induced formation process and are investigated by scanning transmission electron microscopy. Micro-photoluminescence measurements show discrete optical modes for the pillars, in good agreement with calculations based on a vectorial transfer matrix method. The measured quality factor reaches a value of 3100.
- Published
- 2011
16. Towards optical hyperdoping of binary oxide semiconductors
- Author
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K. Sebald, Apurba Dev, Tobias Voss, Andreas Rosenauer, A. Schneider, and Kristian Frank
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Materials science ,business.industry ,Doping ,Wide-bandgap semiconductor ,General Physics and Astronomy ,chemistry.chemical_element ,Laser ,Fluence ,Amorphous solid ,law.invention ,Antimony ,chemistry ,law ,Femtosecond ,Optoelectronics ,Crystallite ,business - Abstract
Surface structuring with ultrashort laser pulses is of high interest as a scalable doping technique as well as for surface nanostructuring applications. By depositing a layer of antimony before the irradiation of ZnO, we were able to incorporate a large quantity of Sb atoms into the single crystalline region of the laser modified surface for potential p-type doping. We have studied the incorporation of antimony and the material properties of laser-induced periodic surface structures (LIPSS) on c-plane ZnO upon femtosecond laser processing at two different peak fluences. We observe high spatial frequency LIPSS with structure periods from 200–370 nm and low spatial frequency LIPSS with periods of 600–700 nm. At a fluence of 0.8 J/cm2, close the ablation threshold of ZnO, the LIPSS are single crystalline except for a few nanometers of amorphous material. At a peak laser fluence of 3.1 J/cm2, they consist of polycrystalline and single crystalline ZnO areas. However, the polycrystalline part dominates with a thickness of about 500 nm.
- Published
- 2013
17. Investigation of diffusion in AlAs/GaAs distributed Bragg reflectors using HAADF STEM imaging
- Author
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Armando Rastelli, Kristian Frank, Marco Schowalter, Oliver G. Schmidt, Hong Seok Lee, Tomi Leinonen, Robert Imlau, Andreas Rosenauer, M Tewes, Mircea Guina, and M. Tavast
- Subjects
History ,Materials science ,business.industry ,Concentration ratio ,Dark field microscopy ,Computer Science Applications ,Education ,law.invention ,Optics ,Transmission electron microscopy ,law ,Microscopy ,Scanning transmission electron microscopy ,Cathode ray ,Electron microscope ,business ,Dimensionless quantity - Abstract
In this contribution we have studied the diffusion of Al in AlAs/GaAs distributed Bragg-reflectors using the high angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) intensity. The measured intensity is normalized to the intensity of the incoming electron beam using a detector scan. The normalized intensity can be directly compared with a set of frozen lattice simulations yielding specimen thickness in regions with known composition or concentration in regions with known thickness. The thickness was evaluated both from GaAs and AlAs regions yielding that the specimen was about 15 nm thinner in AlAs regions due to oxidation. For the concentration evaluation the thickness was derived from GaAs regions and concentrations up to 1.2 were found due to the overestimated thickness. Concentration profiles were scaled down to 1.0 and fitted to the solution of Fick's laws.
- Published
- 2011
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