752 research outputs found
Mono- and Bilayer WS2 Light-Emitting Transistors
We have realized ambipolar ionic liquid gated field-effect transistors based
on WS2 mono- and bilayers, and investigated their opto-electronic response. A
thorough characterization of the transport properties demonstrates the high
quality of these devices for both electron and hole accumulation, which enables
the quantitative determination of the band gap ({\Delta}1L = 2.14 eV for
monolayers and {\Delta}2L = 1.82 eV for bilayers). It also enables the
operation of the transistors in the ambipolar injection regime with electrons
and holes injected simultaneously at the two opposite contacts of the devices
in which we observe light emission from the FET channel. A quantitative
analysis of the spectral properties of the emitted light, together with a
comparison with the band gap values obtained from transport, show the internal
consistency of our results and allow a quantitative estimate of the excitonic
binding energies to be made. Our results demonstrate the power of ionic liquid
gating in combination with nanoelectronic systems, as well as the compatibility
of this technique with optical measurements on semiconducting transition metal
dichalcogenides. These findings further open the way to the investigation of
the optical properties of these systems in a carrier density range much broader
than that explored until now.Comment: 22 pages, 6 figures, Nano Letters (2014
Scanning photocurrent microscopy reveals electron-hole asymmetry in ionic liquid-gated WS2 transistors
We perform scanning photocurrent microscopy on WS2 ionic liquid-gated field
effect transistors exhibiting high-quality ambipolar transport. By properly
biasing the gate electrode we can invert the sign of the photocurrent showing
that the minority photocarriers are either electrons or holes. Both in the
electron- and the hole-doping regimes the photocurrent decays exponentially as
a function of the distance between the illumination spot and the nearest
contact, in agreement with a two-terminal Schottky-barrier device model. This
allows us to compare the value and the doping dependence of the diffusion
length of the minority electrons and holes on a same sample. Interestingly, the
diffusion length of the minority carriers is several times larger in the hole
accumulation regime than in the electron accumulation regime, pointing out an
electron-hole asymmetry in WS2
Microscopic Origin of the Valley Hall Effect in Transition Metal Dichalcogenides Revealed by Wavelength Dependent Mapping
The band structure of many semiconducting monolayer transition metal
dichalcogenides (TMDs) possesses two degenerate valleys, with equal and
opposite Berry curvature. It has been predicted that, when illuminated with
circularly polarized light, interband transitions generate an unbalanced
non-equilibrium population of electrons and holes in these valleys, resulting
in a finite Hall voltage at zero magnetic field when a current flows through
the system. This is the so-called valley Hall effect that has recently been
observed experimentally. Here, we show that this effect is mediated by
photo-generated neutral excitons and charged trions, and not by inter-band
transitions generating independent electrons and holes. We further demonstrate
an experimental strategy, based on wavelength dependent spatial mapping of the
Hall voltage, which allows the exciton and trion contributions to the valley
Hall effect to be discriminated in the measurement. These results represent a
significant step forward in our understanding of the microscopic origin of
photo-induced valley Hall effect in semiconducting transition metal
dichalcogenides, and demonstrate experimentally that composite quasi-particles,
such as trions, can also possess a finite Berry curvature.Comment: accepted for publication in Nano Letter
Predicting biological invasions in marine habitats through eco-physiological mechanistic models: a case study with the bivalve Brachidontes pharaonis
Aim We used a coupled biophysical ecology (BE)-physiological mechanistic
modelling approach based on the Dynamic Energy Budget theory (DEB,
Dynamic energy budget theory for metabolic organisation, 2010, Cambridge
University Press, Cambridge; DEB) to generate spatially explicit predictions of physiological performance (maximal size and reproductive output) for the invasive mussel, Brachidontes pharaonis.
Location We examined 26 sites throughout the central Mediterranean Sea.
Methods We ran models under subtidal and intertidal conditions; hourly
weather and water temperature data were obtained from the Italian Buoy
Network, and monthly CHL-a data were obtained from satellite imagery.
Results Mechanistic analysis of the B. pharaonis fundamental niche shows that
subtidal sites in the Central Mediterranean are generally suitable for this invasive bivalve but that intertidal habitats appear to serve as genetic sinks.
Main conclusions A BE-DEB approach enabled an assessment of how the
physical environment affects the potential distribution of B. pharaonis. Combined with models of larval dispersal, this approach can provide estimates of the likelihood that an invasive species will become established
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Methylation of the Brain Derived Neurotrophic Factor Gene in Alcohol Use Disorder
Abstract
Background: Alcohol use disorder (AUD) is a significant health problem in the U.S. Specifically, AUD is associated with memory problems and other cognitive defects. DNA methylation of CpG sites and other epigenetic factors have been found to play an important role in the development of Alcohol Use Disorders (AUD). Because of this, it was hypothesized that individuals with AUD would show differential methylation in neurotrophic factors, specifically, the BDNF gene, as compared to controls.
Methods: Precuneus brain tissue from 49 Alcohol Use Disorder cases and 47 controls were obtained from the New South Wales and Victorian Tissue Resource Centre. DNA from each sample was extracted, and methylation levels were analyzed using Infinium HumanMethylation450 BeadChip. 450,000 CpG sites were interrogated between AUD cases and controls; however, only 97 CpG sites in the Brain-derived neurotrophic factor (BDNF) gene were analyzed.
Results: Of the 97 CpG sites in the BDNF gene, only 4 displayed significant methylation differences between AUD cases and controls with the employment of a Bonferroni p-value correction. To obtain more accurate results of overall methylation, CpG sites were grouped according to gene loci (Promoter, 5’UTR, first exon, gene body, and 3’UTR). Only the gene body of the BDNF gene approached significance (p=0.093) for a difference in methylation between AUD cases and controls with AUD cases being slightly hypomethylated.
Conclusion: No BDNF gene regions were found to be differentially methylated in the AUD group. This could be due to the small and homogenous sample. However, one CpG region in the gene body approached statistical significance. Overall, gene body methylation is important for splicing kinetics. This may lead to different BDNF mRNA transcripts being produced between AUD cases and controls which can affect the half-life of BDNF mRNA. Future research is needed to examine this possibility in a larger study
Dynamic Energy Budget model parameter estimation for the bivalve Mytilus californianus: Application of the covariation method
Dynamic Energy Budget (DEB) models serve as a powerful tool for describing the flow of energy through organ-
ismsfrom assimilation offoodtoutilization for maintenance,growth andreproduction.The DEB theoryhas been
successfully applied to several bivalve species to compare bioenergetic and physiological strategies for the utili-
zation of energy. In particular, mussels within the
Mytilus edulis
complex (M. edulis,M. galloprovincialis
, and M. trossulus) have been the focus of many studies due to their economic and ecological importance, and their
worldwide distribution. However, DEB parameter values have never been estimated for Mytilus californianus
,a species that is an ecological dominant on rocky intertidal shores on the west coast of North America and
which likely varies considerably from mussels in the M. edulis complex in its physiology. We estimated a set of
DEB parameters for M. californianus
using the covariation method estimation procedure and compared these
to parameter values from other bivalve species. Model parameters were used to compare sensitivity to environ-
mental variability among species, as a
first examination of how strategies for physiologically contending with
environmental change by M. californianus
may differ from those of other bivalves. Results suggest that based
on the parameter set obtained,
M. californianus has favorable energetic strategies enabling it to contend with a range of environmental conditions. For instance, the allocation fraction of reserve to soma (
\u3ba ) is among the highest of any bivalves, which is consistent with the observation that this species can survive over a wide
range of environmental conditions, including prolonged periods of starvatio
Extreme Water Velocities: Topographical Amplification of Wave-Induced Flow in the Surf Zone of Rocky Shores
Water velocities as high as 25 m s-1 have been recorded in the surf zone of wave-swept rocky shores-velocities more than twice the phase speed of the breaking waves with which they are associated. How can water travel twice as fast as the waveform that initially induces its velocity? We explore the possibility that the interaction of a wave with the local topography of the shore can greatly amplify the water velocities imposed on intertidal plants and animals. Experiments in a laboratory wave tank show that interactions between bores refracted by a prowlike beach can produce jets in which the velocity is nearly twice the bore\u27s phase speed. This velocity can be further amplified by a factor of 1.3-1.6 if the jet strikes a vertical wall. This type of topographically induced amplification of water velocity could result in substantial spatial variation in wave-induced hydrodynamic forces and might thereby help to explain the patchwork nature of disturbance that is characteristic of intertidal communities
Intravenous tPA therapy does not worsen acute intracerebral hemorrhage in mice
Tissue plasminogen activator (tPA) is the only FDA-approved treatment for reperfusing ischemic strokes. But widespread use of tPA is still limited by fears of inadvertently administering tPA in patients with intracerebral hemorrhage (ICH). Surprisingly, however, the assumption that tPA will worsen ICH has never been biologically tested. Here, we assessed the effects of tPA in two models of ICH. In a mouse model of collagenase-induced ICH, hemorrhage volumes and neurological deficits after 24 hrs were similar in saline controls and tPA-treated mice, whereas heparin-treated mice had 3-fold larger hematomas. In a model of laser-induced vessel rupture, tPA also did not worsen hemorrhage volumes, while heparin did. tPA is known to worsen neurovascular injury by amplifying matrix metalloproteinases during cerebral ischemia. In contrast, tPA did not upregulate matrix metalloproteinases in our mouse ICH models. In summary, our experimental data do not support the assumption that intravenous tPA has a deleterious effect in acute ICH. However, due to potential species differences and the inability of models to fully capture the dynamics of human ICH, caution is warranted when considering the implications of these findings for human therapy
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