1,089 research outputs found
Zn3As2 Nanowires and nanoplatelets: highly efficient infrared emission and photodetection by an earth abundant material
The development of earth abundant materials for optoelectronics and photovoltaics promises improvements in sustainability and scalability. Recent studies have further demonstrated enhanced material efficiency through the superior light management of novel nanoscale geometries such as the nanowire. Here we show that an industry standard epitaxy technique can be used to fabricate high quality II-V nanowires (1D) and nanoplatelets (2D) of the earth abundant semiconductor Zn3As2. We go on to establish the optoelectronic potential of this material by demonstrating efficient photoemission and detection at 1.0 eV, an energy which is significant to the fields of both photovoltaics and optical telecommunications. Through dynamical spectroscopy this superior performance is found to arise from a low rate of surface recombination combined with a high rate of radiative recombination. These results introduce nanostructured Zn3As2 as a high quality optoelectronic material ready for device exploration.T.B., P.C., Y.G., H.H.T., and C.J. acknowledge the Australian
Research Council. T.B., P.C., Y.G., H.H.T., and C.J. thank the
Australian National Fabrication Facility for access to the growth
and microscopy facilities and Centre for Advanced Microscopy
and Australian Microscopy and Microanalysis Research Facility
for access to microscopy facilities used in this work. Y.W., B.B.,
H.E.J., and L.M.S. acknowledge the financial support of the
National Science Foundation through grants DMR-1105362,
1105121, and ECCS-1100489
Ultrafast Photoinduced Band Splitting and Carrier Dynamics in Chiral Tellurium Nanosheets
Trigonal tellurium (Te) is a chiral semiconductor that lacks both mirror and
inversion symmetries, resulting in complex band structures with Weyl crossings
and unique spin textures. Detailed time-resolved polarized reflectance
spectroscopy is used to investigate its band structure and carrier dynamics.
The polarized transient spectra reveal optical transitions between the
uppermost spin-split H4 and H5 and the degenerate H6 valence bands (VB) and the
lowest degenerate H6 conduction band (CB) as well as a higher energy transition
at the L-point. Surprisingly, the degeneracy of the H6 CB (a proposed Weyl
node) is lifted and the spin-split VB gap is reduced upon photoexcitation
before relaxing to equilibrium as the carriers decay. Using ab initio density
functional theory (DFT) calculations we conclude that the dynamic band
structure is caused by a photoinduced shear strain in the Te film that breaks
the screw symmetry of the crystal. The band-edge anisotropy is also reflected
in the hot carrier decay rate, which is a factor of two slower along c-axis
than perpendicular to it. The majority of photoexcited carriers near the
band-edge are seen to recombine within 30 ps while higher lying transitions
observed near 1.2 eV appear to have substantially longer lifetimes, potentially
due to contributions of intervalley processes in the recombination rate. These
new findings shed light on the strong correlation between photoinduced carriers
and electronic structure in anisotropic crystals, which opens a potential
pathway for designing novel Te-based devices that take advantage of the
topological structures as well as strong spin-related properties.Comment: 42 pages, 13 figure
Nasopharyngeal pneumococcal density is associated with viral activity but not with use of improved stoves among young Andean children
Indoor smoke exposure is common in developing countries and may influence nasopharyngeal (NP) pneumococcal colonization density and risk of acute respiratory illness. We compared colonization density among Andean children living in households previously enrolled in a randomized controlled trial of a home intervention package including improved stoves to reduce smoke, kitchen sinks, and water disinfection.; We enrolled 260 children aged <3 years and made weekly household visits to assess for acute respiratory illness (ARI) and collect nasal swabs for respiratory virus polymerase chain reaction (PCR) testing during ARI. At monthly intervals, NP swabs were collected to determine pneumococcal colonization density through quantitative lytA PCR. We used linear quantile mixed-effects models to compare median log-transformed colonization densities among children in households randomized to the control (n = 129) versus intervention (n = 131) in sequential time points, accounting for random effects of multiple="multiple" samples from individual children. Other covariates included age, sex, month, antibiotic exposure, and timing of sample collection relative to ARI with and without viral detection.; Age and sociodemographic characteristics were similar between groups. Although no differences were observed in densities between groups, colonization density varied significantly over time in both groups, with highest densities coinciding with spring months. Time during and after virus-associated ARI was also associated with higher pneumococcal colonization density than time remote from ARIs.; A home intervention package, including improved stoves, was not associated with changes in pneumococcal densities in young Andean children. However, increasing pneumococcal density was observed with spring season and viral-associated ARIs
Growth, Structural and Optical Properties of High Quality GaAs Nanowires for Optoelectronics
We investigate how growth parameters may be chosen to obtain high quality GaAs nanowires suitable for optoelectronic device applications. Growth temperature and precursor flows have a significant effect on the morphology, crystallographic quality, intrinsic doping and optical properties of the resulting nanowires. Significantly, we find that low growth temperature and high arsine flow rate improve nanowire optical properties, reduce carbon impurity incorporation and drastically reduce planar crystallographic defects. Additionally, cladding the GaAs nanowire cores in an AlGaAs shell enhances emission efficiency. These high quality nanowires should create new opportunities for optoelectronic devices
Norovirus infections in young children in Lusaka Province, Zambia: clinical characteristics and molecular epidemiology
Abstract Background The burden, clinical features, and molecular epidemiology of norovirus infection in young children in southern Africa are not well defined. Methods Using data from a health facility-based surveillance study of children <5 years in Lusaka Province, Zambia presenting with diarrhea, we assessed the burden of norovirus infection. A convenience sample of 454 stool specimens was tested for norovirus using reverse-transcriptase polymerase chain reaction (RT-PCR). RT-PCR positive samples underwent additional nucleotide sequencing for genogroup and genotype identification. Clinical features and severity of diarrheal illnesses were compared between norovirus-positive and -negative subjects using Chi-squared and t-tests. Results Norovirus was detected in 52/454 (11.5%) specimens tested. Abdominal pain, fever, and vomiting were the most common presenting features in norovirus-associated illnesses. However, there were no significant differences in the clinical features of norovirus-positive compared to norovirus-negative illnesses. Of 43 isolates that were available for sequencing, 31 (72.1%) were genogroup II (GII) and 12 (27.9%) were genogroup I (GI). The distribution of genotypes was diverse. Conclusions Noroviruses were detected in approximately 10% of young children with diarrhea in the Lusaka Province of Zambia, with GII representing the majority of infections. These findings support the role of norovirus in symptomatic diarrhea disease in Africa. Further studies are needed to confirm these observations and to evaluate prevention strategies
Use of advanced recombinant lines to study the impact and potential of mutations affecting starch synthesis in barley
The effects on barley starch and grain properties of four starch synthesis mutations were studied during the introgression of the mutations from diverse backgrounds into an elite variety. The lys5f (ADPglucose transporter), wax (granule-bound starch synthase), isa1 (debranching enzyme isoamylase 1) and sex6 (starch synthase IIa) mutations were introgressed into NFC Tipple to give mutant and wild-type BC(2)F(4) families with different genomic contributions of the donor parent. Comparison of starch and grain properties between the donor parents, the BC(2)F(4) families and NFC Tipple allowed the effects of the mutations to be distinguished from genetic background effects. The wax and sex6 mutations had marked effects on starch properties regardless of genetic background. The sex6 mutation conditioned low grain weight and starch content, but the wax mutation did not. The lys5 mutation conditioned low grain weight and starch content, but exceptionally high β-glucan contents. The isa1 mutation promotes synthesis of soluble α-glucan (phytoglycogen). Its introgression into NFC Tipple increased grain weight and total α-glucan content relative to the donor parent, but reduced the ratio of phytoglycogen to starch. This study shows that introgression of mutations into a common, commercial background provides new insights that could not be gained from the donor parent
ROPS: A New Search for Habitable Earths in the Southern Sky
We present the first results from our Red Optical Planet Survey (ROPS) to
search for low mass planets orbiting late type dwarfs (M5.5V - M9V) in their
habitable zones (HZ). Our observations, with the red arm of the MIKE
spectrograph (0.5 - 0.9 microns) at the 6.5 m Magellan Clay telescope at Las
Campanas Observatory indicate that >= 92 per cent of the flux lies beyond 0.7
microns. We use a novel approach that is essentially a hybrid of the
simultaneous iodine and ThAr methods for determining precision radial
velocities. We apply least squares deconvolution to obtain a single high S/N
ratio stellar line for each spectrum and cross correlate against the
simultaneously observed telluric line profile, which we derive in the same way.
Utilising the 0.62 - 0.90 micron region, we have achieved an r.m.s. precision
of 10 m/s for an M5.5V spectral type star with spectral S/N ~160 on 5 minute
timescales. By M8V spectral type, a precision of ~30 m/s at S/N = 25 is
suggested, although more observations are needed. An assessment of our errors
and scatter in the radial velocity points hints at the presence of stellar
radial velocity variations. Of our sample of 7 stars, 2 show radial velocity
signals at 6-sigma and 10-sigma of the cross correlation uncertainties. If the
signals are planetary in origin, our findings are consistent with estimates of
Neptune mass planets that predict a frequency of 13 - 27 per cent for early M
dwarfs.Our current analysis indicates the we can achieve a sensitivity that is
equivalent to the amplitude induced by a 6 M_Earth planet orbiting in the
habitable zone. Based on simulations, we estimate that <10 M_Earth habitable
zone planets will be detected in a new stellar mass regime, with <=20 epochs of
observations.Comment: MNRAS accepted: 14 pages, 8 figures, 3 table
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