638 research outputs found
Origin of the Mosaicity in Graphene Grown on Cu(111)
We use low-energy electron microscopy to investigate how graphene grows on
Cu(111). Graphene islands first nucleate at substrate defects such as step
bunches and impurities. A considerable fraction of these islands can be
rotationally misaligned with the substrate, generating grain boundaries upon
interisland impingement. New rotational boundaries are also generated as
graphene grows across substrate step bunches. Thus, rougher substrates lead to
higher degrees of mosaicity than do flatter substrates. Increasing the growth
temperature improves crystallographic alignment. We demonstrate that graphene
growth on Cu(111) is surface diffusion limited by comparing simulations of the
time evolution of island shapes with experiments. Islands are dendritic with
distinct lobes, but unlike the polycrystalline, four-lobed islands observed on
(100)-textured Cu foils, each island can be a single crystal. Thus, epitaxial
graphene on smooth, clean Cu(111) has fewer structural defects than it does on
Cu(100).Comment: Article revised following reviewer comment
Sampling Local Fungal Diversity in an Undergraduate Laboratory using DNA Barcoding
Traditional methods for fungal species identification require diagnostic morphological characters and are often limited by the availability of fresh fruiting bodies and local identification resources. DNA barcoding offers an additional method of species identification and is rapidly developing as a critical tool in fungal taxonomy. As an exercise in an undergraduate biology course, we identified 9 specimens collected from the Hendrix College campus in Conway, Arkansas, USA to the genus or species level using morphology. We report that DNA barcoding targeting the internal transcribed spacer (ITS) region supported several of our taxonomic determinations and we were able to contribute 5 ITS sequences to GenBank that were supported by vouchered collection information. We suggest that small-scale barcoding projects are possible and that they have value for documenting fungal diversity
Extraordinary epitaxial alignment of graphene islands on Au(111)
Pristine, single-crystalline graphene displays a unique collection of
remarkable electronic properties that arise from its two-dimensional, honeycomb
structure. Using in-situ low-energy electron microscopy, we show that when
deposited on the (111) surface of Au carbon forms such a structure. The
resulting monolayer, epitaxial film is formed by the coalescence of dendritic
graphene islands that nucleate at a high density. Over 95% of these islands can
be identically aligned with respect to each other and to the Au substrate.
Remarkably, the dominant island orientation is not the better lattice-matched
30^{\circ} rotated orientation but instead one in which the graphene [01] and
Au [011] in-plane directions are parallel. The epitaxial graphene film is only
weakly coupled to the Au surface, which maintains its reconstruction under the
slightly p-type doped graphene. The linear electronic dispersion characteristic
of free-standing graphene is retained regardless of orientation. That a weakly
interacting, non-lattice matched substrate is able to lock graphene into a
particular orientation is surprising. This ability, however, makes Au(111) a
promising substrate for the growth of single crystalline graphene films
Oxidation resistance of graphene-coated Cu and Cu/Ni alloy
The ability to protect refined metals from reactive environments is vital to
many industrial and academic applications. Current solutions, however,
typically introduce several negative effects, including increased thickness and
changes in the metal physical properties. In this paper, we demonstrate for the
first time the ability of graphene films grown by chemical vapor deposition to
protect the surface of the metallic growth substrates of Cu and Cu/Ni alloy
from air oxidation. SEM, Raman spectroscopy, and XPS studies show that the
metal surface is well protected from oxidation even after heating at 200
\degree C in air for up to 4 hours. Our work further shows that graphene
provides effective resistance against hydrogen peroxide. This protection method
offers significant advantages and can be used on any metal that catalyzes
graphene growth
First narrow-band search for continuous gravitational waves from known pulsars in advanced detector data
Spinning neutron stars asymmetric with respect to their rotation axis are potential sources of
continuous gravitational waves for ground-based interferometric detectors. In the case of known pulsars a
fully coherent search, based on matched filtering, which uses the position and rotational parameters
obtained from electromagnetic observations, can be carried out. Matched filtering maximizes the signalto-
noise (SNR) ratio, but a large sensitivity loss is expected in case of even a very small mismatch
between the assumed and the true signal parameters. For this reason, narrow-band analysis methods have
been developed, allowing a fully coherent search for gravitational waves from known pulsars over a
fraction of a hertz and several spin-down values. In this paper we describe a narrow-band search of
11 pulsars using data from Advanced LIGO’s first observing run. Although we have found several initial
outliers, further studies show no significant evidence for the presence of a gravitational wave signal.
Finally, we have placed upper limits on the signal strain amplitude lower than the spin-down limit for 5 of
the 11 targets over the bands searched; in the case of J1813-1749 the spin-down limit has been beaten for
the first time. For an additional 3 targets, the median upper limit across the search bands is below the
spin-down limit. This is the most sensitive narrow-band search for continuous gravitational waves carried
out so far
The resolved stellar populations in the LEGUS galaxies1.
The Legacy ExtraGalactic UV Survey (LEGUS) is a multiwavelength Cycle 21 Treasury program on the Hubble Space Telescope. It studied 50 nearby star-forming galaxies in 5 bands from the near-UV to the I-band, combining new Wide Field Camera 3 observations with archival Advanced Camera for Surveys data. LEGUS was designed to investigate how star formation occurs and develops on both small and large scales, and how it relates to the galactic environments. In this paper we present the photometric catalogs for all the apparently single stars identified in the 50 LEGUS galaxies. Photometric catalogs and mosaicked images for all filters are available for download. We present optical and near-UV color–magnitude diagrams for all the galaxies. For each galaxy we derived the distance from the tip of the red giant branch. We then used the NUV color–magnitude diagrams to identify stars more massive than 14 M ⊙, and compared their number with the number of massive stars expected from the GALEX FUV luminosity. Our analysis shows that the fraction of massive stars forming in star clusters and stellar associations is about constant with the star formation rate. This lack of a relation suggests that the timescale for evaporation of unbound structures is comparable or longer than 10 Myr. At low star formation rates this translates to an excess of mass in clustered environments as compared to model predictions of cluster evolution, suggesting that a significant fraction of stars form in unbound systems
A hybrid MBE-based growth method for large-area synthesis of stacked hexagonal boron nitride/graphene heterostructures
Van der Waals heterostructures combining hexagonal boron nitride (h-BN) and graphene offer many potential advantages, but remain difficult to produce as continuous films over large areas. In particular, the growth of h-BN on graphene has proven to be challenging due to the inertness of the graphene surface. Here we exploit a scalable molecular beam epitaxy based method to allow both the h-BN and graphene to form in a stacked heterostructure in the favorable growth environment provided by a Ni(111) substrate. This involves first saturating a Ni film on MgO(111) with C, growing h-BN on the exposed metal surface, and precipitating the C back to the h-BN/Ni interface to form graphene. The resulting laterally continuous heterostructure is composed of a top layer of few-layer thick h-BN on an intermediate few-layer thick graphene, lying on top of Ni/MgO(111). Examinations by synchrotron-based grazing incidence diffraction, X-ray photoemission spectroscopy, and UV-Raman spectroscopy reveal that while the h-BN is relaxed, the lattice constant of graphene is significantly reduced, likely due to nitrogen doping. These results illustrate a different pathway for the production of h-BN/graphene heterostructures, and open a new perspective for the large-area preparation of heterosystems combining graphene and other 2D or 3D materials
Association of IL-4RA single nucleotide polymorphisms, HLA-DR and HLA-DQ in children with Alternaria-sensitive moderate-severe asthma
<p>Abstract</p> <p>Background</p> <p>Asthma afflicts 6% to 8% of the United States population, and severe asthma represents approximately 10% of asthmatic patients. Several epidemiologic studies in the United States and Europe have linked <it>Alternaria </it>sensitivity to both persistence and severity of asthma. In order to begin to understand genetic risk factors underlying <it>Alternaria </it>sensitivity and asthma, in these studies we examined T cell responses to <it>Alternaria </it>antigens, HLA Class II restriction and HLA-DQ protection in children with severe asthma.</p> <p>Methods</p> <p>Sixty children with <it>Alternaria</it>-sensitive moderate-severe asthma were compared to 49 children with <it>Alternaria</it>-sensitive mild asthma. We examined HLA-DR and HLA-DQ frequencies in <it>Alternaria</it>-sensitive asthmatic by HLA typing. To determine ratios of Th1/Th2 <it>Alternaria</it>-specific T-cells, cultures were stimulated in media alone, <it>Alternaria alternata </it>extract and Alt a1. Sensitivity to IL-4 stimulation was measured by up-regulation of CD23 on B cells.</p> <p>Results</p> <p>Children with <it>Alternaria</it>-sensitive moderate-severe asthma trended to have increased sensitivities to <it>Cladosporium </it>(46% versus 35%), to <it>Aspergillus </it>(43% versus 28%), and significantly increased sensitivities to trees (78% versus 57%) and to weeds (68% versus 48%). The IL-4RA ile75val polymorphism was significantly increased in <it>Alternaria</it>-sensitive moderate-severe asthmatics, 83% (0.627 allele frequency) compared to <it>Alternaria</it>-sensitive mild asthmatics, 57% (0.388 allele frequency). This was associated with increased sensitivity to IL-4 stimulation measured by significantly increased IL-4 stimulated CD23 expression on CD19+ and CD86+CD19+ B cells of <it>Alternaria</it>-sensitive moderate-severe asthmatics. IL-5 and IL-13 synthesis was significantly increased in <it>Alternaria</it>-sensitive moderate-severe asthmatics compared to mild asthmatics to <it>Alternaria </it>extract and Alt a1 stimulation. The frequency of HLA-DQB1*03 allele was significantly decreased in <it>Alternaria</it>-sensitive moderate-severe asthmatics compared to mild asthmatics, 39% versus 63%, with significantly decreased allele frequency, 0.220 versus 0.398.</p> <p>Summary</p> <p>In children with <it>Alternaria</it>-sensitive moderate severe asthma, there was an increased Th2 response to <it>Alternaria </it>stimulation and increased sensitivity to IL-4 stimulation. This skewing towards a Th2 response was associated with an increased frequency of the IL-4RA ile75val polymorphism. In evaluating the HLA association, there was a decreased frequency of HLA-DQB1*03 in <it>Alternaria</it>-sensitive moderate severe asthmatic children consistent with previous studies suggest that HLA-DQB1*03 may be protective against the development of mold-sensitive severe asthma.</p
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