42 research outputs found
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 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.Comment: This document is the unedited Author's version of a Submitted Work
that was subsequently accepted for publication in Nano Letters (2019),
copyright (C) American Chemical Society after peer review. To access the
final edited and published work see
https://doi.org/10.1021/acs.nanolett.9b01521, the supporting information is
available (free of charge) under the same lin
Recommended from our members
One year of Raman lidar observations of free-tropospheric aerosol layers over South Africa
Raman lidar data obtained over a 1 year period has been analysed in relation to aerosol layers in the free troposphere over the Highveld in South Africa. In total, 375 layers were observed above the boundary layer during the period 30 January 2010 to 31 January 2011. The seasonal behaviour of aerosol layer geometrical characteristics, as well as intensive and extensive optical properties were studied. The highest centre heights of free-tropospheric layers were observed during the South African spring (2520 ± 970 m a.g.l., also elsewhere). The geometrical layer depth was found to be maximum during spring, while it did not show any significant difference for the rest of the seasons. The variability of the analysed intensive and extensive optical properties was high during all seasons. Layers were observed at a mean centre height of 2100 ± 1000 m with an average lidar ratio of 67 ± 25 sr (mean value with 1 standard deviation) at 355 nm and a mean extinction-related Ångström exponent of 1.9 ± 0.8 between 355 and 532 nm during the period under study. Except for the intensive biomass burning period from August to October, the lidar ratios and Ångström exponents are within the range of previous observations for urban/industrial aerosols. During Southern Hemispheric spring, the biomass burning activity is clearly reflected in the optical properties of the observed free-tropospheric layers. Specifically, lidar ratios at 355 nm were 89 ± 21, 57 ± 20, 59 ± 22 and 65 ± 23 sr during spring (September–November), summer (December–February), autumn (March–May) and winter (June–August), respectively. The extinction-related Ångström exponents between 355 and 532 nm measured during spring, summer, autumn and winter were 1.8 ± 0.6, 2.4 ± 0.9, 1.8 ± 0.9 and 1.8 ± 0.6, respectively. The mean columnar aerosol optical depth (AOD) obtained from lidar measurements was found to be 0.46 ± 0.35 at 355 nm and 0.25 ± 0.2 at 532 nm. The contribution of free-tropospheric aerosols on the AOD had a wide range of values with a mean contribution of 46%
An overview of the first decade of PollyNET : an emerging network of automated Raman-polarization lidars for continuous aerosol profiling
© Author(s) 2016. This work is distributed under the Creative Commons Attribution 3.0 LicenseA global vertically resolved aerosol data set covering more than 10 years of observations at more than 20 measurement sites distributed from 63° N to 52° S and 72° W to 124° E has been achieved within the Raman and polarization lidar network PollyNET. This network consists of portable, remote-controlled multiwavelength-polarization-Raman lidars (Polly) for automated and continuous 24/7 observations of clouds and aerosols. PollyNET is an independent, voluntary, and scientific network. All Polly lidars feature a standardized instrument design with different capabilities ranging from single wavelength to multiwavelength systems, and now apply unified calibration, quality control, and data analysis. The observations are processed in near-real time without manual intervention, and are presented online at http://polly.tropos.de/. The paper gives an overview of the observations on four continents and two research vessels obtained with eight Polly systems. The specific aerosol types at these locations (mineral dust, smoke, dust-smoke and other dusty mixtures, urban haze, and volcanic ash) are identified by their Ångström exponent, lidar ratio, and depolarization ratio. The vertical aerosol distribution at the PollyNET locations is discussed on the basis of more than 55 000 automatically retrieved 30 min particle backscatter coefficient profiles at 532 nm as this operating wavelength is available for all Polly lidar systems. A seasonal analysis of measurements at selected sites revealed typical and extraordinary aerosol conditions as well as seasonal differences. These studies show the potential of PollyNET to support the establishment of a global aerosol climatology that covers the entire troposphere.Peer reviewe
One year of Raman lidar observations of free-tropospheric aerosol layers over South Africa
Raman lidar data obtained over a 1 year period has been analysed in
relation to aerosol layers in the free troposphere over the Highveld in
South Africa. In total, 375 layers were observed above the boundary layer
during the period 30 January 2010 to 31 January 2011. The seasonal behaviour
of aerosol layer geometrical characteristics, as well as intensive and
extensive optical properties were studied. The highest centre heights of
free-tropospheric layers were observed during the South African spring (2520 ± 970 m a.g.l., also elsewhere). The geometrical layer depth was found
to be maximum during spring, while it did not show any significant
difference for the rest of the seasons. The variability of the analysed
intensive and extensive optical properties was high during all seasons.
Layers were observed at a mean centre height of 2100 ± 1000 m with an
average lidar ratio of 67 ± 25 sr (mean value with 1 standard
deviation) at 355 nm and a mean extinction-related Ångström exponent
of 1.9 ± 0.8 between 355 and 532 nm during the period under study.
Except for the intensive biomass burning period from August to October, the
lidar ratios and Ångström exponents are within the range of previous
observations for urban/industrial aerosols. During Southern Hemispheric
spring, the biomass burning activity is clearly reflected in the optical
properties of the observed free-tropospheric layers. Specifically, lidar
ratios at 355 nm were 89 ± 21, 57 ± 20, 59 ± 22
and 65 ± 23 sr during spring (September–November), summer (December–February), autumn (March–May) and winter (June–August),
respectively. The extinction-related Ångström exponents between 355
and 532 nm measured during spring, summer, autumn and winter were 1.8 ± 0.6, 2.4 ± 0.9, 1.8 ± 0.9 and 1.8 ± 0.6,
respectively. The mean columnar aerosol optical depth (AOD) obtained from
lidar measurements was found to be 0.46 ± 0.35 at 355 nm and 0.25 ± 0.2 at 532 nm. The contribution of free-tropospheric aerosols on the
AOD had a wide range of values with a mean contribution of 46%
MRI Pattern Recognition in Multiple Sclerosis Normal-Appearing Brain Areas
Objective Here, we use pattern-classification to investigate diagnostic information for multiple sclerosis (MS; relapsingremitting type) in lesioned areas, areas of normalappearing grey matter (NAGM), and normal-appearing white matter (NAWM) as measured by standard MR techniques. Methods A lesion mapping was carried out by an experienced neurologist for Turbo Inversion Recovery Magnitude (TIRM) images of individual subjects. Combining this mapping with templates from a neuroanatomic atlas, the TIRM images were segmented into three areas of homogenous tissue types (Lesions, NAGM, and NAWM) after spatial standardization. For each area, a linear Support Vector Machine algorithm was used in multiple local classification analyses to determine the diagnostic accuracy in separating MS patients from healthy controls based on voxel tissue intensity patterns extracted from small spherical subregions of these larger areas. To control for covariates, we also excluded group-specific biases in deformation fields as a potential source of information. Results Among regions containing lesions a posterior parietal WM area was maximally informative about the clinical status (96% accuracy, p<10−13). Cerebellar regions were maximally informative among NAGM areas (84% accuracy, p<10−7). A posterior brain region was maximally informative among NAWM areas (91% accuracy, p<10−10). Interpretation We identified regions indicating MS in lesioned, but also NAGM, and NAWM areas. This complements the current perception that standard MR techniques mainly capture macroscopic tissue variations due to focal lesion processes. Compared to current diagnostic guidelines for MS that define areas of diagnostic information with moderate spatial specificity, we identified hotspots of MS associated tissue alterations with high specificity defined on a millimeter scale
Haze in Pluto's atmosphere: Results from SOFIA and ground-based observations of the 2015 June 29 Pluto occultation
On UT 29 June 2015, the occultation by Pluto of a bright star (r′ = 11.9) was observed from the Stratospheric Observatory for Infrared Astronomy (SOFIA) and several ground-based stations in New Zealand and Australia. Pre-event astrometry allowed for an in-flight update to the SOFIA team with the result that SOFIA was deep within the central flash zone (~22 km from center). Analysis of the combined data leads to the result that Pluto's middle atmosphere is essentially unchanged from 2011 and 2013 (Person et al. 2013; Bosh et al. 2015); there has been no significant expansion or contraction of the atmosphere. Additionally, our multi-wavelength observations allow us to conclude that a haze component in the atmosphere is required to reproduce the light curves obtained. This haze scenario has implications for understanding the photochemistry of Pluto's atmosphere