The influence of Mg doping on the nucleation of self-induced GaN nanowires

GaN nanowires were grown without any catalyst by plasma-assisted molecular beam epitaxy. Under supply of Mg, nanowire nucleation is faster, the areal density of wires increases to a higher value, and nanowire coalescence is more pronounced than without Mg. During nanowire nucleation the Ga desorption was monitored insitu by line-of-sight quadrupolemass spectrometry for various substrate temperatures. Nucleation energies of 4.0±0.3 eV and 3.2±0.3 eV without and with Mg supply were deduced, respectively. This effect has to be taken into account for the fabrication of nanowire devices and could be employed to tune the NW areal density

PRAS40 suppresses atherogenesis through inhibition of mTORC1-dependent pro-inflammatory signaling in endothelial cells

Endothelial pro-inflammatory activation plays a pivotal role in atherosclerosis, and many pro-inflammatory and atherogenic signals converge upon mechanistic target of rapamycin (mTOR). Inhibitors of mTOR complex 1 (mTORC1) reduced atherosclerosis in preclinical studies, but side effects including insulin resistance and dyslipidemia limit their clinical use in this context. Therefore, we investigated PRAS40, a cell type-specific endogenous modulator of mTORC1, as alternative target. Indeed, we previously found PRAS40 gene therapy to improve metabolic profile; however, its function in endothelial cells and its role in atherosclerosis remain unknown. Here we show that PRAS40 negatively regulates endothelial mTORC1 and pro-inflammatory signaling. Knockdown of PRAS40 in endothelial cells promoted TNFα-induced mTORC1 signaling, proliferation, upregulation of inflammatory markers and monocyte recruitment. In contrast, PRAS40-overexpression blocked mTORC1 and all measures of pro-inflammatory signaling. These effects were mimicked by pharmacological mTORC1-inhibition with torin1. In an in vivo model of atherogenic remodeling, mice with induced endothelium-specific PRAS40 deficiency showed enhanced endothelial pro-inflammatory activation as well as increased neointimal hyperplasia and atherosclerotic lesion formation. These data indicate that PRAS40 suppresses atherosclerosis via inhibition of endothelial mTORC1-mediated pro-inflammatory signaling. In conjunction with its favourable effects on metabolic homeostasis, this renders PRAS40 a potential target for the treatment of atherosclerosis

Protection Mechanism against Photocorrosion of GaN Photoanodes Provided by NiO Thin Layers

The photoelectrochemical properties of n-type Ga-polar GaN photoelectrodes covered with NiO layers of different thicknesses in the range 0–20 nm are investigated for aqueous solution. To obtain layers of well-defined thickness and high crystal quality, NiO is grown by plasma-assisted molecular-beam epitaxy. Stability tests reveal that the NiO layers suppress photocorrosion. With increasing NiO thickness, the onset of the photocurrent is shifted to more positive voltages and the photocurrent is reduced, especially for low bias potentials, indicating that hole transfer to the electrolyte interface is hindered by thicker NiO layers. Furthermore, cathodic transient spikes are observed under intermittent illumination, which hints at surface recombination processes. These results are inconsistent with the common explanation of the protection mechanism that the band alignment of GaN/NiO enables efficient hole-injection, thus preventing hole accumulation at the GaN surface that would lead to anodic photocorrosion. Interestingly, the morphology of the etch pits as well as further experiments involving the photodeposition of Ag indicate that photocorrosion of GaN photoanodes is related to reductive processes at threading dislocations. Therefore, it is concluded that the NiO layers block the transfer of photogenerated electrons from GaN to the electrolyte interface, which prevents the cathodic photocorrosion. © 2020 The Authors. Solar RRL published by Wiley-VCH Gmb

Metal - Insulator transition driven by vacancy ordering in GeSbTe phase change materials

Phase Change Materials (PCMs) are unique compounds employed in non-volatile random access memory thanks to the rapid and reversible transformation between the amorphous and crystalline state that display large differences in electrical and optical properties. In addition to the amorphous-to-crystalline transition, experimental results on polycrystalline GeSbTe alloys (GST) films evidenced a Metal-Insulator Transition (MIT) attributed to disorder in the crystalline phase. Here we report on a fundamental advance in the fabrication of GST with out-of-plane stacking of ordered vacancy layers by means of three distinct methods: Molecular Beam Epitaxy, thermal annealing and application of femtosecond laser pulses. We assess the degree of vacancy ordering and explicitly correlate it with the MIT. We further tune the ordering in a controlled fashion attaining a large range of resistivity. Employing ordered GST might allow the realization of cells with larger programming windows

Metal - Insulator transition driven by vacancy ordering in GeSbTe phase change materials

Phase Change Materials (PCMs) are unique compounds employed in non-volatile random access memory thanks to the rapid and reversible transformation between the amorphous and crystalline state that display large differences in electrical and optical properties. In addition to the amorphousto-crystalline transition, experimental results on polycrystalline GeSbTe alloys (GST) films evidenced a Metal-Insulator Transition (MIT) attributed to disorder in the crystalline phase. Here we report on a fundamental advance in the fabrication of GST with out-of-plane stacking of ordered vacancy layers by means of three distinct methods: Molecular Beam Epitaxy, thermal annealing and application of femtosecond laser pulses. We assess the degree of vacancy ordering and explicitly correlate it with the MIT. We further tune the ordering in a controlled fashion attaining a large range of resistivity. Employing ordered GST might allow the realization of cells with larger programming windows

Ein mehrkanaliges Biosensormesssystem zur Überwachung der Nitrifikation in Abwasserreinigungsanlagen

Bei der biologischen Abwasserreinigung treten häufig Störungen der Nitrifikationsstufe (biochemische Oxidation von Ammonium über Nitrit zu Nitrat) auf, die durch Hemmstoffe sowie durch Stossbelastungen hoher Stickstofffrachten (N-BSB) verursacht werden. Dadurch gelangen erhöhte Mengen an sauerstoffzehrenden reduzierten Stickstoffverbindungen in die Oberflächengewässer. Dies ist besonders kritisch bei Vorflutern mit einem bereits niedrigem Sauerstoffgehalt, da durch diese reduzierten Stickstoffverbindungen die Konzentration an gelöstem Sauerstoff in Folge mikrobieller Oxidation stark absinken kann und somit die Biocönose im Gewässer nachhaltig gestört wird. Da es sich bei Nitrit und Ammoniak zudem um starke Fischgifte handelt, ist die Elimination dieser Stickstoffverbindungen aus dem Abwasser nicht zuletzt auch gesetzlich vorgeschrieben. Zur Erfassung von Störungen der Nitrifikationsstufe wurde deswegen in den vergangenen Jahren am ISWA ein Nitrifikanten-Einzelbiosensor entwickelt. Dabei wird über den Sauerstoffverbrauch des Immobilisates die bakterielle Stoffwechselaktivität überwacht, wobei dies ein Maß für das Vorhandensein von Hemmstoffen bzw. Nitrifikationssubstraten in einer Probe ist. Der Vorteil dieses Systems ist vor allem darin zu sehen, dass die damit durchgeführten Messungen sowohl schnell als auch reproduzierbar durchführbar sind. So ist mit diesem Geräteprototyp über die Durchführung von ca. 10 – 15 Einzelmessungen verschiedener Probenverdünnungen die Quantifizierung der Hemmwirkung von einer Probe pro Messtag möglich. Schlussfolgerung und Ausblick Die Untersuchungen zur Bestimmung der nitrifikationshemmenden Wirkung von Standardhemmstoffen, Abwasserproben und des N-BSB ergaben, dass die neu entwickelten Nitrifikanten-Biosensoren schnell und mit hoher Signalstabilität auf sich ändernde Hemmstoff- und Substratkonzentrationen reagieren und somit für die Klärwerksüberwachung prinzipiell einsetzbar sind

Polarized recombination of acoustically transported carriers in GaAs nanowires

The oscillating piezoelectric field of a surface acoustic wave (SAW) is employed to transport photoexcited electrons and holes in GaAs nanowires deposited on a SAW delay line on a LiNbO3 crystal. The carriers generated in the nanowire by a focused light spot are acoustically transferred to a second location where they recombine. We show that the recombination of the transported carriers occurs in a zinc blende section on top of the predominant wurtzite nanowire. This allows contactless control of the linear polarized emission by SAWs which is governed by the crystal structure. Additional polarization-resolved photoluminescence measurements were performed to investigate spin conservation during transport

Structural investigation of nanocrystalline graphene grown on (6√3×6√3) R30°-reconstructed SiC surfaces by molecular beam epitaxy

Growth of nanocrystalline graphene films on (6√3×6√3) R30°- reconstructed SiC surfaces was achieved by molecular beam epitaxy, enabling the investigation of quasi-homoepitaxial growth. The structural quality of the graphene films, which is investigated by Raman spectroscopy, increases with growth time. X-ray photoelectron spectroscopy proves that the SiC surface reconstruction persists throughout the growth process and that the synthesized films consist of sp2-bonded carbon. Interestingly, grazing incidence X-ray diffraction measurements show that the graphene domains possess one single in-plane orientation, are aligned to the substrate, and offer a noticeably contracted lattice parameter of 2.446 Å. We correlate this contraction with theoretically calculated reference values (all-electron density functional calculations based on the van der Waals corrected PBE functional) for the lattice parameter contraction induced in ideal, free-standing graphene sheets by: substrate-induced buckling, the edges of limited-size flakes, and typical point defects (monovacancies, divacancies, Stone-Wales defects)