367 research outputs found
Modeling Changes in Measured Conductance of Thin Boron Carbide Semiconducting Films Under Irradiation
Semiconducting, p-type, amorphous partially dehydrogenated boron carbide films (a-B10C2+x:Hy) were deposited utilizing plasma enhanced chemical vapor deposition (PECVD) onto n-type silicon thus creating a heterojunction diode. A model was developed for the conductance of the device as a function of perturbation frequency (��) that incorporates changes of the electrical properties for both the a-B10C2+x:Hy film and the silicon substrate when irradiated. The virgin model has 3 independent variables (R1, C1, R3), and 1 dependent variable (��). Samples were then irradiated with 200 keV He+ ions, and the conductance model was matched to the measured data. It was found that initial irradiation (0.1 displacements per atom (dpa) equivalent) resulted in a decrease in the parallel junction resistance parameter from 6032 Ω to 2705 Ω. Further irradiation drastically increased the parallel junction resistance parameter to 39000 Ω (0.2 dpa equivalent), 77440 Ω (0.3 dpa equivalent), and 190000 Ω (0.5 dpa equivalent). It is believed that the initial irradiation causes type inversion of the silicon substrate changing the original junction from a p-n to a p-p+ with a much lower barrier height leading to a lower junction resistance component between the a-B10C2+x:Hy and irradiated silicon. Additionally, it was found that after irradiation, a second parallel resistor and capacitor component is required for the model, introducing 2 additional independent variables (R2, C2). This is interpreted as the junction between the irradiated and virgin silicon near ion end of range
Radiation tolerance of nanocrystalline ceramics: insights from Yttria Stabilized Zirconia.
Materials for applications in hostile environments, such as nuclear reactors or radioactive waste immobilization, require extremely high resistance to radiation damage, such as resistance to amorphization or volume swelling. Nanocrystalline materials have been reported to present exceptionally high radiation-tolerance to amorphization. In principle, grain boundaries that are prevalent in nanomaterials could act as sinks for point-defects, enhancing defect recombination. In this paper we present evidence for this mechanism in nanograined Yttria Stabilized Zirconia (YSZ), associated with the observation that the concentration of defects after irradiation using heavy ions (Kr(+), 400 keV) is inversely proportional to the grain size. HAADF images suggest the short migration distances in nanograined YSZ allow radiation induced interstitials to reach the grain boundaries on the irradiation time scale, leaving behind only vacancy clusters distributed within the grain. Because of the relatively low temperature of the irradiations and the fact that interstitials diffuse thermally more slowly than vacancies, this result indicates that the interstitials must reach the boundaries directly in the collision cascade, consistent with previous simulation results. Concomitant radiation-induced grain growth was observed which, as a consequence of the non-uniform implantation, caused cracking of the nano-samples induced by local stresses at the irradiated/non-irradiated interfaces
Disks around massive young stellar objects: are they common?
We present K-band polarimetric images of several massive young stellar
objects at resolutions 0.1-0.5 arcsec. The polarization vectors around
these sources are nearly centro-symmetric, indicating they are dominating the
illumination of each field. Three out of the four sources show elongated
low-polarization structures passing through the centers, suggesting the
presence of polarization disks. These structures and their surrounding
reflection nebulae make up bipolar outflow/disk systems, supporting the
collapse/accretion scenario as their low-mass siblings. In particular, S140
IRS1 show well defined outflow cavity walls and a polarization disk which
matches the direction of previously observed equatorial disk wind, thus
confirming the polarization disk is actually the circumstellar disk. To date, a
dozen massive protostellar objects show evidence for the existence of disks;
our work add additional samples around MYSOs equivalent to early B-type stars.Comment: 9 pages, including 2 figures, 1 table, to appear on ApJ
Mechanical modulation of single-electron tunneling through molecular-assembled metallic nanoparticles
We present a microscopic study of single-electron tunneling in nanomechanical
double-barrier tunneling junctions formed using a vibrating scanning nanoprobe
and a metallic nanoparticle connected to a metallic substrate through a
molecular bridge. We analyze the motion of single electrons on and off the
nanoparticle through the tunneling current, the displacement current and the
charging-induced electrostatic force on the vibrating nanoprobe. We demonstrate
the mechanical single-electron turnstile effect by applying the theory to a
gold nanoparticle connected to the gold substrate through alkane dithiol
molecular bridge and probed by a vibrating platinum tip.Comment: Accepted by Phys. Rev.
Recent increases in terrestrial carbon uptake at little cost to the water cycle
Quantifying the responses of the coupled carbon and water cycles to current global warming and rising atmospheric CO2 concentration is crucial for predicting and adapting to climate changes. Here we show that terrestrial carbon uptake (i.e. gross primary production) increased significantly from 1982 to 2011 using a combination of ground-based and remotely sensed land and atmospheric observations. Importantly, we find that the terrestrial carbon uptake increase is not accompanied by a proportional increase in water use (i.e. evapotranspiration) but is largely (about 90%) driven by increased carbon uptake per unit of water use, i.e. water use efficiency. The increased water use efficiency is positively related to rising CO2 concentration and increased canopy leaf area index, and negatively influenced by increased vapour pressure deficits. Our findings suggest that rising atmospheric CO2 concentration has caused a shift in terrestrial water economics of carbon uptake
Heterologous Expression and Biochemical Characterisation of Fourteen Esterases from Helicoverpa armigera
Esterases have recurrently been implicated in insecticide resistance in Helicoverpa armigera but little is known about the underlying molecular mechanisms. We used a baculovirus system to express 14 of 30 full-length esterase genes so far identified from midgut cDNA libraries of this species. All 14 produced esterase isozymes after native PAGE and the isozymes for seven of them migrated to two regions of the gel previously associated with both organophosphate and pyrethroid resistance in various strains. Thirteen of the enzymes obtained in sufficient yield for further analysis all showed tight binding to organophosphates and low but measurable organophosphate hydrolase activity. However there was no clear difference in activity between the isozymes from regions associated with resistance and those from elsewhere in the zymogram, or between eight of the isozymes from a phylogenetic clade previously associated with resistance in proteomic and quantitative rtPCR experiments and five others not so associated. By contrast, the enzymes differed markedly in their activities against nine pyrethroid isomers and the enzymes with highest activity for the most insecticidal isomers were from regions of the gel and, in some cases, the phylogeny that had previously been associated with pyrethroid resistance. Phospholipase treatment confirmed predictions from sequence analysis that three of the isozymes were GPI anchored. This unusual feature among carboxylesterases has previously been suggested to underpin an association that some authors have noted between esterases and resistance to the Cry1Ac toxin from Bacillus thuringiensis. However these three isozymes did not migrate to the zymogram region previously associated with Cry1Ac resistance.This study was supported by an Australian Postgraduate Award and Top-up Scholarship from the Cotton Catchment Cummunities CRC to Claire
Farnsworth and the China Scholarship Council to Yongqiang Li. The funders had no role in study design, data collection and analysis, decision to publish, or
preparation of the manuscript
Fermi-level alignment at metal-carbon nanotube interfaces: application to scanning tunneling spectroscopy
At any metal-carbon nanotube interface there is charge transfer and the
induced interfacial field determines the position of the carbon nanotube band
structure relative to the metal Fermi-level. In the case of a single-wall
carbon nanotube (SWNT) supported on a gold substrate, we show that the charge
transfers induce a local electrostatic potential perturbation which gives rise
to the observed Fermi-level shift in scanning tunneling spectroscopy (STS)
measurements. We also discuss the relevance of this study to recent experiments
on carbon nanotube transistors and argue that the Fermi-level alignment will be
different for carbon nanotube transistors with low resistance and high
resistance contacts.Comment: 4 pages, 3 ps figures, minor corrections, accepted by Phys. Rev. Let
The sky brightness and transparency in i-band at Dome A, Antarctica
The i-band observing conditions at Dome A on the Antarctic plateau have been
investigated using data acquired during 2008 with the Chinese Small Telescope
ARray. The sky brightness, variations in atmospheric transparency, cloud cover,
and the presence of aurorae are obtained from these images. The median sky
brightness of moonless clear nights is 20.5 mag arcsec^{-2} in the SDSS
band at the South Celestial Pole (which includes a contribution of about 0.06
mag from diffuse Galactic light). The median over all Moon phases in the
Antarctic winter is about 19.8 mag arcsec^{-2}. There were no thick clouds in
2008. We model contributions of the Sun and the Moon to the sky background to
obtain the relationship between the sky brightness and transparency. Aurorae
are identified by comparing the observed sky brightness to the sky brightness
expected from this model. About 2% of the images are affected by relatively
strong aurorae.Comment: There are 1 Latex file and 14 figures accepted by A
Spontaneous Liquid Crystal and Ferromagnetic Ordering of Colloidal Magnetic Nanoplates
Ferrofluids are familiar as colloidal suspensions of ferromagnetic
nanoparticles in aqueous or organic solvents. The dispersed particles are
randomly oriented but their moments become aligned if a magnetic field is
applied, producing a variety of exotic and useful magneto-mechanical effects. A
longstanding interest and challenge has been to make such suspensions
macroscopically ferromagnetic, that is having uniform magnetic alignment in
absence of a field. Here we report a fluid suspension of magnetic nanoplates
which spontaneously aligns into an equilibrium nematic liquid crystal phase
that is also macroscopically ferromagnetic. Its zero-field magnetization
produces distinctive magnetic self-interaction effects, including liquid
crystal textures of fluid block domains arranged in closed flux loops, and
makes this phase highly sensitive, with it dramatically changing shape even in
the Earth's magnetic field
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