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The importance of controlling both the charge carrier density and the band gap of a semiconductor cannot be overstated, as it opens the doors to a wide range of applications, including, e.g., highly-tunable transistors, photodetectors, and lasers. Bernal-stacked bilayer graphene is a unique van-der-Waals material that allows tuning the band gap by an out-of-plane electric field. Although the first evidence of the tunable gap was already found ten years ago, it took until recent to fabricate sufficiently clean heterostructures where the electrically induced gap could be used to fully suppress transport or confine charge carriers. Here, we present a detailed study of the tunable band gap in gated bilayer graphene characterized by temperature-activated transport and finite-bias spectroscopy measurements. The latter method allows comparing different gate materials and device technologies, which directly affects the disorder potential in bilayer graphene. We show that graphite-gated bilayer graphene exhibits extremely low disorder and as good as no subgap states resulting in ultraclean tunable band gaps up to 120 meV. The size of the band gaps are in good agreement with theory and allow complete current suppression making a wide range of semiconductor applications possible., 21 pages, 10 figures

We report on ballistic transport over more than 28 ��m in graphene grown by chemical vapor deposition (CVD) that is fully encapsulated in hexagonal boron nitride. The structures are fabricated by an advanced dry van-der-Waals transfer method and exhibit carrier mobilities of up to three million cm$^2$/(Vs). The ballistic nature of charge transport is probed by measuring the bend resistance in cross- and square-shaped devices. Temperature dependent measurements furthermore prove that ballistic transport is maintained exceeding 1 ��m up to 200 K., 13 pages, 3 figures

In this thesis single- (SLG) and bi-layer (BLG) graphene on hexagonal boron nitride (hBN) are investigated by means of Raman spectroscopy and transport experiments. The hBN-graphene heterostructures are fabricated by a state-of-the-art transfer process of exfoliated graphene on exfoliated hBN. The high quality structures feature low residual charge carrier densities of n∗ ≈ 5×10$^{10}$ cm$^{−2}$ and high carrier mobilities of μ ≈ 70,000 cm$^2$/Vs. Thermal annealing experiments show that these superior sample characteristics can even be maintained after substantial heat treatments. A subsequent experiment investigates the scattering mechanism limiting the carrier mobility in SLG. By correlating Raman spectroscopy to transport measurements we find new evidence that strain variations are the scattering source for charge carriers in SLG. The limits of carrier mobility are further investigated in case of BLG. A thorough analysis of the weak localization effect leads to the conclusion that intravalley scattering originating from nanometer-scale strain variations is the most likely source for the mobility limiting scattering events. To obtain direct evidence for this hypothesis we further investigate the relation of n∗ to μ for a large number of samplesand observe the predicted dependence for strain variations as the main scattering source. In consequence, we conclude that strain fluctuations are the most probable source for carrier scattering in both SLG and BLG.In a following set of experiments the opening of the band gap in BLG in dependence of the applied perpendicular electric field is investigated. The investigation particularly focuses on the origin of the sub gap conductance which leads to a maximum resistance on the order of ten megaohm. Temperature dependent transport and bias spectroscopy measurements show that this non-vanishing conductance is mediated by variable range hopping through a low number of hopping states. By developing a simple model we can unambiguously rule out long range potential disorder as the origin of the hopping states. Further measurements of a ”corbino-like” device suggest the presence of AB/BA stacking domain walls in the bulk of the investigated BLG. A final experiment investigates the influence of the rough edges in graphene nanostructures i.e. quantum dots (QDs). The obtained results show that edge roughness is the dominant source of disorder in QDs on hBN with diameters below 100 nm.

The work at hand deals with the mass transfer through mixed conducting high temperature membranes with the material composition Ba0.5Sr0.5Co0.8Fe0.2O3-d (BSCF), which are used for the oxygen separation from air. Thereby the analysis considers all relevant mass transport resistances and shows their influence on the oxygen mass transfer across the membrane. For this purpose, experimental studies on the O2-permeation rate and CFD-simulations of the test facilities are made and the mass transfer models are validated by an inverse parameter fitting. Thereby the CFD simulation gives detailed information about the conditions near the membrane surface, which are crucial for the oxygen transport across the membrane. By experimental measurement methods these information are normally not accessible. This approach makes it possible to determine the intrinsic material constants CWagner and KWagner of the Wagner-equation and the characteristic membrane thickness LC(T). Further possible concentration polarization effects in the gas phases and the mass transport resistances (bulk transport in the membrane material, surface exchange and the gas phase transport) can be determined. The work at hand shows, that the oxygen mass transfer through the BSCF membranes at temperatures above 800°C is limited only slightly by the surface exchange processes. Due to that an increase of the O2-permeation rate will be possible by further decreasing the membrane thickness. Furthermore there exist significant concentration polarization effects, which have an effect on the mass transport across the membrane. Here the feed and permeate gas phase resistances together can account for up to 40% of the total mass transfer resistance across the membrane. Further the influence of the flue gas components CO2, SO2 and evaporated chromium from steel on the oxygen mass transfer has been studied. Here the perovskite materials BSCF and Sr0.5Ca0.5Mn0.8Fe0.2O3-d(SCMF) and the perovskite related material La2NiO4+d have been studied experimentally and evaluated concerning their applicability in the OXYCOAL-AC power plant process. The results show that BSCF membranes have the highest oxygen permeation rate, but in contact with CO2-rich gas they show degradation effects. These lead to a decreasing of the O2-permeation rate. SCMF and La2NiO4+d show a better stability against CO2, but they are also not applicable, if they are exposed to 360 ppm SO2. Further it has been determined in long term tests that both SCMF and BSCF react with evaporated CrO3 from steel, which leads to a slow decrease of the O2-permeation rate.

BACKGROUND AND PURPOSE: Chronic alcohol abuse may cause neuropsychological disorders and result in brain atrophy. The purpose of this study was to evaluate the metabolic, morphologic, and functional cerebral changes in the early stage of abstinence from chronic alcoholism. METHODS: Seventeen alcohol-dependent patients underwent MR imaging and MR spectroscopy on days 1 through 3 and days 36 through 39 of abstinence. In addition, psychological performance measures testing intelligence, concentration, attention, and memory were applied. Neuropsychological data were correlated with spectroscopic and volumetric results by using a Pearson's product moment correlation. The same measurements were also performed in 12 healthy, age-matched control subjects. Peak integral values for N-acetylaspartate (NAA) and choline (Cho) were referred to the peak integral value of creatine (Cr) as the internal reference. RESULTS: NAA/Cr was decreased in the patients in both the frontal lobes and cerebellum immediately after cessation of drinking (days 1 through 3). After 36 to 39 days of abstinence, NAA/Cr had significantly increased in the patients and corresponded to performance on psychological tests. The Cho/Cr ratio was decreased in the cerebellum during early abstinence but was recovered on days 36 through 39. The patients had enlarged CSF spaces 1 to 3 days after detoxification, which decreased during sobriety. The extent of brain atrophy did not correspond to performance on psychological performance tests. CONCLUSION: Regression of brain atrophy and metabolic recovery occurs at an early stage after abstinence from chronic alcohol abuse. MR spectroscopy findings return to normal metabolic levels within weeks after detoxification. The recovery of NAA/Cr is associated with improved performance on neuropsychological tests.

Science advances 1(6), e1500222-e1500222 (2015). doi:10.1126/sciadv.1500222, Published by American Association for the Advancement of Science, Washington, DC [u.a.]