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

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

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

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

Salmonella in pigs and pork and their antimicrobial resistance - 10 years of surveillance in Germany

Salmonella from pigs and pork have been submitted to the National Reference Laboratory for the Analysis and Testing of Zoonoses (Salmonella) (NRL Salm) for a number of years. This study retrospectively analyses the data of Salmonella and their antimicrobial resistance generated between 2000 and 2009. A total of 4163 isolates from pigs and 1839 isolates from pork submitted to the NRL Salm were serotyped and tested for antimicrobial resistance using the broth microdilution method. Minimum inhibitory concentrations (MIC) were evaluated using epidemiological cut-off values as provided by EUCAST (www.eucast.org) at the time of interpretation (2010). The majority of isolates from pigs and pork belonged to three serovars: S. Typhimurium (66 and 52%), monophasic variant of S. Typhimurium (1,4,[5],12:i:-) (11 and 10%) and S. Derby (7 and 10%). In both origins the number of S. Typhimurium decreased by roughly 50% while monophasic variant of S. Typhimurium increased from zero to 32 and 26%, respectively. The proportion of S. Derby varied between 5 and 12% in both origins