Improved optical activation of ion-implanted Zn acceptors in GaN by annealing under N2 overpressure

We investigated the properties of ion-implanted GaN:Zn annealed under various conditions using photoluminescence (PL) and high resolution x-ray diffraction (HRXRD). Epitaxial GaN/sapphire of high optical quality was ion-implanted with a 1013 cm−2 dose of Zn+ ions at 200 keV. The sample was capped with 200 Å of SiNx and then diced into numerous pieces which were annealed under varied conditions in an attempt to optically activate the Zn. Annealing was performed in a tube furnace under flowing N2, an atmospheric pressure MOCVD reactor under flowing NH3 or N2, and under an N2 overpressure of 190 atm. The observed improvement in the optical quality of GaN:Zn annealed under N2 overpressure yields further insights into the trade-off between defect annealing and N loss from the GaN crysta

Efficient optical activation of ion-implanted Zn acceptors in GaN by annealing under 10 kbar N2 overpressure

We continue our investigations into the optical activation of Zn-implanted GaN annealed under ever higher N2 overpressure. The samples studied were epitaxial GaN/sapphire layers of good optical quality which were implanted with a 1013 cm−2 dose of Zn+ ions at 200 keV, diced into equivalent pieces and annealed under 10 kbar of N2. The N2 overpressure permitted annealing at temperatures up to 1250°C for 1 hr without GaN decomposition. The blue Zn-related photoluminescence (PL) signal rises sharply with increasing anneal temperature. The Zn-related PL intensity in the implanted sample annealed at 1250°C exceeded that of the epitaxially doped GaN:Zn standard proving that high temperature annealing of GaN under kbar N2 overpressure can effectively remove implantation damage and efficiently activate implanted dopants in GaN. We propose a lateral LED device which could be fabricated using ion implanted dopants activated by high temperature annealing at high pressur

Insight from the draft genome of Dietzia cinnamea P4 reveals mechanisms of survival in complex tropical soil habitats and biotechnology potential

The draft genome of Dietzia cinnamea strain P4 was determined using pyrosequencing. In total, 428 supercontigs were obtained and analyzed. We here describe and interpret the main features of the draft genome. The genome contained a total of 3,555,295 bp, arranged in a single replicon with an average G+C percentage of 70.9%. It revealed the presence of complete pathways for basically all central metabolic routes. Also present were complete sets of genes for the glyoxalate and reductive carboxylate cycles. Autotrophic growth was suggested to occur by the presence of genes for aerobic CO oxidation, formate/formaldehyde oxidation, the reverse tricarboxylic acid cycle and the 3-hydropropionate cycle for CO2 fixation. Secondary metabolism was evidenced by the presence of genes for the biosynthesis of terpene compounds, frenolicin, nanaomycin and avilamycin A antibiotics. Furthermore, a probable role in azinomycin B synthesis, an important product with antitumor activity, was indicated. The complete alk operon for the degradation of n-alkanes was found to be present, as were clusters of genes for biphenyl ring dihydroxylation. This study brings new insights in the genetics and physiology of D. cinnamea P4, which is useful in biotechnology and bioremediation

The effect of ISM absorption on stellar activity measurements and its relevance for exoplanet studies

Past ultraviolet and optical observations of stars hosting close-in Jupiter-mass planets have shown that some of these stars present an anomalously low chromospheric activity, significantly below the basal level. For the hot Jupiter planet host WASP-13, observations have shown that the apparent lack of activity is possibly caused by absorption from the intervening interstellar medium (ISM). Inspired by this result, we study the effect of ISM absorption on activity measurements (S and log R'HK indices) for main-sequence late-type stars. To this end, we employ synthetic stellar photospheric spectra combined with varying amounts of chromospheric emission and ISM absorption. We present the effect of ISM absorption on activity measurements by varying several instrumental (spectral resolution), stellar (projected rotational velocity, effective temperature, and chromospheric emission flux), and ISM parameters (relative velocity between stellar and ISM Ca II lines, broadening b-parameter, and Ca II column density). We find that for relative velocities between the stellar and ISM lines smaller than 30–40 km s−1 and for ISM Ca II column densities log NCaII ⪆ 12, the ISM absorption has a significant influence on activity measurements. Direct measurements and three dimensional maps of the Galactic ISM absorption indicate that an ISM Ca II column density of log NCaII = 12 is typically reached by a distance of about 100 pc along most sight lines. In particular, for a Sun-like star lying at a distance greater than 100 pc, we expect a depression (bias) in the log R'HK value larger than 0.05–0.1 dex, about the same size as the typical measurement and calibration uncertainties on this parameter. This work shows that the bias introduced by ISM absorption must always be considered when measuring activity for stars lying beyond 100 pc. We also consider the effect of multiple ISM absorption components. We discuss the relevance of this result for exoplanet studies and revise the latest results on stellar activity versus planet surface gravity correlation. We finally describe methods with which it would be possible to account for ISM absorption in activity measurements and provide a code to roughly estimate the magnitude of the bias. Correcting for the ISM absorption bias may allow one to identify the origin of the anomaly in the activity measured for some planet-hosting stars

Contact-Based Assembly of Nano-scale Structures: Synthesis of Mechanics and Tools of Nanomanipulation

Construction of new useful nanoscopic structures and mechanisms—such as nano-electromechanical systems (NEMS)—requires advanced nano-fabrication tools and techniques. Emerging NEMS devices potentially involve complex, asymmetric, three-dimensional arrangements of nano-scale elements, which are beyond the capabilities of currently available "top-down" and "bottom-up" manufacturing methods. Mechanical assembly of nano-scale objects via "contact-based" manipulation has the potential to fill the void between these currently available methods and as such may prove fundamental in the advancement of nanomanufacturing. Fabrication of advanced nanoscopic structures and mechanism by means of mechanical manipulation is currently limited due to an insufficient understanding of nano-scale multibody systems. This includes uncertainty regarding the response of individual nano-scale bodies and multi-link chains to externally applied forces. Expanding the understanding of these behaviors may enable the design and operation of advanced nanomanipulation tools as well as the assemblies they produce. Toward these ends, this work investigates the kinematics and mechanics of nanoscopic multibody systems. This is accomplished in three parts: through the construction of a novel optical based nanomanipulation system; the experimental study of multi-walled carbon nanotubes (MWCNTs), pairs of tubes in contact, and multi-pair chains; and the integration of the observed mechanical behavior into a multibody system model. Here, the nanomanipulation system demonstrates the ability to use optical visualization in the positioning of free-standing structures with a minimum dimension as low as 100 nm under ambient conditions. This system is used to examine both the structural properties of multi-walled carbon nanotubes (MWCNTs) and the mechanics of the 'joint' formed between pairs when assembled into chains. Specific attention is given to this nano-scale joint regarding its formation due to natural adhesion, its kinematic and mechanic characteristics, and the alteration of these characteristics through the use of localized heat treatment. Finally, the observations are used to propose a joint model capturing both the elastic and inelastic behavior of the adhesive contact for use in dynamic multibody formulations. Through these contributes, this work offers tools and mechanical theory for use in advancing the field of contact nanomanipulation

Gruppe-III-Nitride, Verbindungshalbleiter mit grossem Bandabstand. Teilvorhaben: Schichtherstellung mittels Gasquellen-MBE Abschlussbericht

A Gas-Source MBE system was modified for expitaxial growth of nitride based semiconductors and LED structures. Therefore, we developed an 'on surface cracking (OSC)' technique, for the dissociation of ammonia on the growing GaN surface, using experimental data and theoretical descriptions. Using OSC for epitaxial growth on GaN substrates we achieved world record excitonic linewidths being as narrow as 0.5 meV. At 4.2 K free A and B excitons are clearly visible, above 50 K the free exciton C emerges. Thus, the quality of the layers is superior to the best data reported from Japan. Epitaxial growth on sapphire also yielded good material qualities. Undoped GaN layers with a thickness of 2 #mu#m reveal free carrier concentrations of n=2 x 10"1"7 cm"-"3 with mobilities of #mu#=230 cm"2/Vs. Photoluminescence linewidths are 5 meV (4.2 K), XRD reveal linewidths below 450'' for the (0002)-Reflex. AlGaN layers contain up to 35% Al; GaN quantum structures embedded in AlGaN barriers show clear quantum size effects. InGaN layer up to 15% In content have been grown, however, a further optimization is necessary. N-doping of GaN using Si is straight forward, p-doping, however, appeared to be challenging. Eventually, p-doping was achieved using either elemental Mg or, for the first time in MBE, the gaseous precursor Methyl-Cp2Mg. In both cases a thermal activation of the Mg is not necessary. A full LED technology has been developed for processing and fabrication of blue light emitting diodes. Mask layout, dry etching, contacts as well as housing technology allow to manufacture ultraviolet (UV) and blue LEDs. High grade LEDs, realized from MBE and MOVPE epitaxial layers, are characterized using their electrical and optical properties and underwent first life-time tests. Using a modified vertical structure we achieved homotype UV LEDs with linewidths as narrow as 12 nm. AlGaN/GaN DH LEDs yielded a further reduction to 8 nm linewidth. Those homotype UV LEDs and AlGaN/GaN DH LEDs are spectrally narrow light sources with many applications as pump sources in white LEDs and in spectroscopic applications for the UV range. (orig.)SIGLEAvailable from TIB Hannover: F99B816+a / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekBundesministerium fuer Bildung, Wissenschaft, Forschung und Technologie, Bonn (Germany)DEGerman

N-acetylaspartate catabolism determines cytosolic acetyl-CoA levels and histone acetylation in brown adipocytes

Histone acetylation depends on the abundance of nucleo-cytoplasmic acetyl-CoA. Here, we present a novel route for cytoplasmic acetyl-CoA production in brown adipocytes. N-acetylaspartate (NAA) is a highly abundant brain metabolite catabolized by aspartoacylase yielding aspartate and acetate. The latter can be further used for acetyl-CoA production. Prior to this work, the presence of NAA has not been described in adipocytes. Here, we show that accumulation of NAA decreases the brown adipocyte phenotype. We increased intracellular NAA concentrations in brown adipocytes via media supplementation or knock-down of aspartoacylase and measured reduced lipolysis, thermogenic gene expression, and oxygen consumption. Combinations of approaches to increase intracellular NAA levels showed additive effects on lipolysis and gene repression, nearly abolishing the expression of Ucp1, Cidea, Prdm16, and Ppara. Transcriptome analyses of aspartoacylase knock-down cells indicate deficiencies in acetyl-CoA and lipid metabolism. Concordantly, cytoplasmic acetyl-CoA levels and global histone H3 acetylation were decreased. Further, activating histone marks (H3K27ac and H3K9ac) in promoters/enhancers of brown marker genes showed reduced acetylation status. Taken together, we present a novel route for cytoplasmic acetyl-CoA production in brown adipocytes. Thereby, we mechanistically connect the NAA pathway to the epigenomic regulation of gene expression, modulating the phenotype of brown adipocytes