Global ETD Search

1	One-atom-thick crystals as a novel class of proton conducting materials Lozada Hidalgo, Marcelo January 2015 (has links) Graphene, a one-atom-thick sheet of carbon atoms, is impermeable to all atoms and molecules; the same can be expected for other 2D crystals like hexagonal boron nitride (hBN). In this work we show that monolayers of graphene and hBN are highly permeable to thermal protons. As a reference, we show that monolayers of molybdenum disulphide as well as bilayers of graphene and tetralayers of hBN are not. Moreover, we show that water plays a crucial role in the transport mechanism. Because of the zero point energy of vibration in the oxygen-hydrogen bonds in water, protons face energy barriers smaller than previously predicted by theory. The effect, revealed by substituting hydrogen for deuterium, also shows that protons and deuterons transport at different rates across the membranes; establishing them as membranes with subatomic selectivity. Beyond the purely scientific implications, our results establish monolayers of graphene and hBN as a promising new class of proton conducting materials with potential applications in fuel cells, hydrogen purification and isotope enrichment technologies. 500
2	Graphene-hybrid devices for spintronics Sambricio Garcia, Jose Luis January 2017 (has links) This thesis explores the use of 2D materials (graphene and hBN) for spintronics. Interest on these materials in spintronics arose from theoretical predictions of high spin filtering in out-of-plane transport through graphene and hBN sandwiched by ferromagnets. Similarly, 5-layer graphene was forecast to be a perfect spin filter. In the case of in-plane spin transport, graphene was expected to be an excellent material due to its low spin-orbit coupling and low number of defects. Although there already exist experimental works that attempted to explore the aforementioned predictions, they have failed so far to comply with the expected results. Earlier experimental works in graphene and hBN out-of-plane spin transport achieved low spin filtering on the order of a few percent; while spin relaxation parameters in graphene for in-plane spin transport remained one or two orders of magnitude below the predicted values. In the case of vertical devices, the failure to meet the theoretical expectations was attributed to the oxidation of the ferromagnets and the lack of an epitaxial interface between the later and the graphene or hBN. Similarly, the exact mechanisms that lead to high spin relaxation for in-plane spin transport in graphene are not completely understood, in part due to the low-quality of the explored devices. In this thesis we analyze new architectures and procedures that allowed us to fabricate ultraclean and oxidation-free interfaces between ferromagnets and graphene or hBN. In these devices we encountered negative and reversible magnetoresistance, that could not be explained with the previous theoretical models. We propose a new model based on a thorough characterization of the devices and well-known properties of graphene that were not taken into account in the previous model. We also employed a novel type of contact to graphene (1D-contacts) and applied it for the first time to achieve spin-injection in graphene. The main advantage of this type of contact is the full encapsulation of graphene with hBN, which leads to high quality graphene spintronic devices. 500
3	Structural Studies of Boron Nitride Compounds Under Extreme Conditions Sterling, Spencer 27 October 2021 (has links) This document will present the work done on BN under high pressure conditions, both at room temperatures and at high temperatures under laser heating conditions. These experiments are performed to identify possible phase transitions within the BN system and characterize the materials present under the given conditions using a mixture of X-ray diffraction and Raman and infrared spectroscopies are employed. A review of the background and motivations for studies of BN under extreme conditions, as well as the techniques employed, is given as an introduction. A phase transition from hexagonal boron nitride (hBN) to wurtzite boron nitride (wBN) is observed beginning at 9 GPa and room temperature, with coexistence of the two phases until 14 GPa for hydrostatic conditions and to above 20 GPa for non-hydrostatic conditions. This transition is partially reversible below 2 GPa. The formed wBN has a high concentration of defects. For recovered samples, defects couple with the 532.18 nm excitation laser producing a heating effect, observed as a Raman downshift with increasing laser power. The bulk modulus B0 and pressure derivative of the bulk modulus B0′ of hBN are estimated to be 30.6 ± 0.5 GPa and 8.7 ± 0.7, respectively. The bulk modulus of wBN is estimated to be 392 ± 5 GPa, leading to a Vickers hardness of 68 ± 1 GPa. Extra diffraction lines are observed for hBN samples loaded with N2, indicating a potential new structure arising from a reaction of N2 with hBN, but Raman spectroscopy fails to corroborate this ﬁnding. The crystallinity of the hBN samples and the choice of pressure transmitting medium are shown to have little to no effect on the estimated physical properties of hBN. Laser heating is performed on hBN with various sample assemblies. The effectiveness of different assemblies is discussed. NaCl is used as a pressure and temperature gauge local to the X-ray probe to contrast the stationary ruby pressure gauge and the non-local black body temperature measurement. A large contrast between the two temperature measurements yields doubt that the intended temperatures of around 2000 K are produced in the sample. Observation of the proposed high-pressure high-temperature transition to body-centered tetragonal BN or intercalated BN cannot be conﬁrmed, likely due to insufﬁcient heating. The prospects for studying Li-BN intercalation compounds under extreme conditions is discussed. An initial experiment on the system studied with X-ray diffraction is unable to conﬁrm heating of the material nor the presence of intercalation compounds. Pressure Temperature Crystallography Extreme Conditions hBN Diamond Anvil Cell XRD FTIR Raman Spectroscopy Wurtzite NaCl Intercalation Materials Synchrotron Transition Defects Heating
4	Příprava a charakterizace atomárně tenkých vrstev / Fabrication and characterization of atomically thin layers Tesař, Jan January 2020 (has links) Tato práce se zabývá oblastí dvourozměrných materiálů, jejich přípravou a analýzou. Pravděpodobně nejznámějším zástupcem dvourozměrných materiálů je grafen. Tento 2D allotrop uhlíku, někdy nazývaný „otec 2D materiálů“, v sobě spojuje neobyčejnou kombinaci elektrických, tepelných a mechanických vlastností. Grafen získal mnoho pozornosti a byl také připraven mnoha metodami. Jedna z těchto metod však stále vyniká nad ostatními kvalitou produkovaného grafenu. Mechanická exfoliace je ve srovnání s jinými technikami velmi jednoduchá, takto připravený grafen je však nejkvalitnější. Práce je také zaměřena na optimalizaci procesu tvorby heterostruktur složených z vrstev grafenu a hBN. Dle prezentovaného postupu bylo připraveno několik van der Waalsových heterostruktur, které byly analyzovány Ramanovskou spektroskopií, mikroskopií atomových sil a nízkoenergiovou elektronovou mikroskopií. Měření pohyblivosti nosičů náboje bylo provedeno v GFET uspořádání. Získané hodnoty pohyblivosti prokázaly vynikající transportní vlastnosti exfoliovaného grafenu v porovnání s grafenem připraveným jinými metodami. V práci popsaný proces přípravy je tedy vhodný pro výrobu kvalitních heterostruktur.
5	Boron Nitride Catalysts for Methanol Oxidation Hazel, Justin Andrew 26 July 2022 (has links) No description available. Chemical Engineering boron nitride hBN h-BN methanol oxidation vanadium catalysis catalyst catalysts methanol active site heterogeneous catalysis reactor reaction support redox
6	Applications of Complex Network Dynamics in Ultrafast Electronics Charlot, Noeloikeau Falconer 08 September 2022 (has links) No description available. Electromagnetism Engineering Experiments High Temperature Physics Information Science Low Temperature Physics Materials Science Medical Imaging Mathematics Quantum Physics Solid State Physics Physics Scientific Imaging Technology Theoretical Physics Systems Design Nanotechnology Information Systems Electrical Engineering Condensed Matter Physics Applied Mathematics Information Technology Particle Physics Computer Science Computer Engineering Electromagnetics Boolean Network Field Programmable Gate Array Ultrafast Electronics Chaos Computing Dynamical Systems Time to Digital Converter Measurement Fractal Basin Physically Unclonable Function Physical Unclonable Function Hybrid Boolean Network Waveform Capture Device PUF FPGA TDC WCD HBN HBN-PUF

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