Absence of Quantum-Confined Stark Effect in GaN Quantum Disks Embedded in (Al,Ga)N Nanowires Grown by Molecular Beam Epitaxy.

Several of the key issues of planar (Al,Ga)N-based deep-ultraviolet light-emitting diodes could potentially be overcome by utilizing nanowire heterostructures, exhibiting high structural perfection, and improved light extraction. Here, we study the spontaneous emission of GaN/(Al,Ga)N nanowire ensembles grown on Si(111) by plasma-assisted molecular beam epitaxy. The nanowires contain single GaN quantum disks embedded in long (Al,Ga)N nanowire segments essential for efficient light extraction. These quantum disks are found to exhibit intense light emission at unexpectedly high energies, namely, significantly above the GaN bandgap, and almost independent of the disk thickness. An in-depth investigation of the actual structure and composition of the nanowires reveals a spontaneously formed Al gradient both along and across the nanowire, resulting in a complex core/shell structure with an Al-deficient core and an Al-rich shell with continuously varying Al content along the entire length of the (Al,Ga)N segment. This compositional change along the nanowire growth axis induces a polarization doping of the shell that results in a degenerate electron gas in the disk, thus screening the built-in electric fields. The high carrier density not only results in the unexpectedly high transition energies but also in radiative lifetimes depending only weakly on temperature, leading to a comparatively high internal quantum efficiency of the GaN quantum disks up to room temperature.

Effect of surface roughness, chemical composition, and native oxide crystallinity on the orientation of self-assembled GaN nanowires on Ti foils.

We report on plasma-assisted molecular beam epitaxial growth of almost randomly oriented, uniformly tilted, and vertically aligned self-assembled GaN nanowires (NWs), respectively, on different types of polycrystalline Ti foils. The NW orientation with respect to the substrate normal, which is affected by an in situ treatment of the foil surface before NW growth, depends on the crystallinity of the native oxide. Direct growth on the as-received foils results in the formation of ensembles of nearly randomly oriented NWs due to the strong roughening of the surface induced by chemical reactions between the impinging elements and Ti. Surface nitridation preceding the NW growth is found to reduce this roughening by transformation of the uppermost layers into TiN and TiO x N y species. These compounds are more stable against chemical reactions and facilitate the growth of uniformly oriented GaN NW ensembles on the surface of the individual grains of the polycrystalline Ti foils. If an amorphous oxide layer is present at the foil surface, vertically oriented NWs are obtained all across the substrate because this layer blocks the transfering of the epitaxial information from the underlying grains. The control of NW orientation and the understanding behind the achievement of vertically oriented NWs obtained in this study represent an important step towards the realization of GaN NW-based bendable devices on polycrystalline metal foils.

Altered basal ganglia functional connectivity in multiple sclerosis patients with fatigue.

BACKGROUND: Fatigue is one of the most frequent and disabling symptoms in multiple sclerosis, but its pathophysiological mechanisms are poorly understood. It is in particular unclear whether and how fatigue relates to structural and functional brain changes. OBJECTIVE: We aimed to analyse the association of fatigue severity with basal ganglia functional connectivity, basal ganglia volumes, white matter integrity and grey matter density. METHODS: In 44 patients with relapsing-remitting multiple sclerosis and 20 age- and gender-matched healthy controls, resting-state fMRI, diffusion tensor imaging and voxel-based morphometry was performed. RESULTS: In comparison with healthy controls, patients showed alteration of grey matter density, white matter integrity, basal ganglia volumes and basal ganglia functional connectivity. No association of fatigue severity with grey matter density, white matter integrity and basal ganglia volumes was observed within patients. In contrast, fatigue severity was negatively correlated with functional connectivity of basal ganglia nuclei with medial prefrontal cortex, precuneus and posterior cingulate cortex in patients. Furthermore, fatigue severity was positively correlated with functional connectivity between caudate nucleus and motor cortex. CONCLUSION: Fatigue is associated with distinct alterations of basal ganglia functional connectivity independent of overall disability. The pattern of connectivity changes suggests that disruption of motor and non-motor basal ganglia functions, including motivation and reward processing, contributes to fatigue pathophysiology in multiple sclerosis.

Acoustically driven photon antibunching in nanowires.

The oscillating piezoelectric field of a surface acoustic wave (SAW) is employed to transport photoexcited carriers, as well as to spatially control exciton recombination in GaAs-based nanowires (NWs) on a subns time scale. The experiments are carried out in core-shell NWs transferred to a SAW delay line on a LiNbO(3) crystal. Carriers generated in the NW by a focused laser spot are acoustically transferred to a second location, leading to the remote emission of subns light pulses synchronized with the SAW phase. The dynamics of the carrier transport, investigated using spatially and time-resolved photoluminescence, is well-reproduced by computer simulations. The high-frequency contactless manipulation of carriers by SAWs opens new perspectives for applications of NWs in opto-electronic devices operating at gigahertz frequencies. The potential of this approach is demonstrated by the realization of a high-frequency source of antibunched photons based on the acoustic transport of electrons and holes in (In,Ga)As NWs.

Correlation between In content and emission wavelength of In(x)Ga(1-x)N/GaN nanowire heterostructures.

GaN nanowire ensembles with axial In(x)Ga(1-x)N multi-quantum-wells (MQWs) were grown by molecular beam epitaxy. In a series of samples we varied the In content in the MQWs from almost zero to around 20%. Within the nanowire ensemble, the MQWs fluctuate strongly in composition and size. Statistical information about the composition was obtained from x-ray diffraction and Raman spectroscopy. Photoluminescence at room temperature was obtained in the range of 2.2 to 2.5 eV, depending on In content. Contrary to planar MQWs, the intensity increases with increasing In content. We compare the observed emission energies with transition energies obtained from a one-dimensional model, and conclude that several mechanisms for carrier localization affect the luminescence of these three-dimensional structures.

Macro- and micro-strain in GaN nanowires on Si(111).

We analyze the strain state of GaN nanowire ensembles by x-ray diffraction. The nanowires are grown by molecular beam epitaxy on a Si(111) substrate in a self-organized manner. On a macroscopic scale, the nanowires are found to be free of strain. However, coalescence of the nanowires results in micro-strain with a magnitude from ± (0.015)% to ± (0.03)%. This micro-strain contributes to the linewidth observed in low-temperature photoluminescence spectra.

[Studies on the effects of intravenous administration of glucose, fructose, invertose and sorbitol on various blood constituents of blood plasma (monosaccharides, insulin, lactate, pyruvate and free fatty acids as well as glutamate-oxaloacetate transaminase) in the horse]. / Untersuchungen über den Einfluss der intravenösen Verabreichung von Lösungen der Glukose, der Fruktose, des Invertzuckers und des Sorbits auf verschiedene Bestandteile des Blutes bzw. Blutplasmas (Monosaccharide, Insulin, Laktat, Pyruvat, freie Fettsäuren und Glutamat-Oxalazetat-Transaminase) vom Pferd.

Horses were examined for the behaviour of various blood constituents prior to and following infusions of solutions of glucose, fructose, invertose, and sorbitol. Infusion of 0.5 g/kg live weight glucose to six horses was followed by half-life variation between eleven and 23 minutes. Subsequent infusion of invertose to the same animals usually caused prolongation of glucose half-life. Half-life values were between 17 and 33 minutes for fructose and between 21 and 80 minutes for glucose. Infusion of 0.5 g/kg live weight fructose to two horses was followed by half-life values between 17 and 18 minutes, while the half-life values of sugar alcohol were 16, 16, 27, and 29 minutes in four horses who had received sorbitol. Sugar or sorbitol infusion was not followed by substantive change of lactate and pyruvate concentrations in the blood or free fatty acids in the blood plasma or GOT activity. The rise of insulin in the blood plasma was differentiated. Invertose and sorbitol solutions, consequently, can be recommended for application to horses.