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

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

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

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

Long-term, high frequency in situ measurements of intertidal mussel bed temperatures using biomimetic sensors

At a proximal level, the physiological impacts of global climate change on ectothermic organisms are manifest as changes in body temperatures. Especially for plants and animals exposed to direct solar radiation, body temperatures can be substantially different from air temperatures. We deployed biomimetic sensors that approximate the thermal characteristics of intertidal mussels at 71 sites worldwide, from 1998-present. Loggers recorded temperatures at 10-30 min intervals nearly continuously at multiple intertidal elevations. Comparisons against direct measurements of mussel tissue temperature indicated errors of similar to 2.0-2.5 degrees C, during daily fluctuations that often exceeded 15 degrees-20 degrees C. Geographic patterns in thermal stress based on biomimetic logger measurements were generally far more complex than anticipated based only on 'habitat-level' measurements of air or sea surface temperature. This unique data set provides an opportunity to link physiological measurements with spatially-and temporally-explicit field observations of body temperature

Racial/ethnic differences in hepatic steatosis in a population‐based cohort of post‐menopausal women: the Michigan Study of Women's Health Across the Nation

Aims The prevalence of hepatic steatosis may differ between post‐menopausal African‐American women and non‐Hispanic white women and by sex hormone binding globulin level. We examined prevalence of hepatic steatosis by race/ethnicity and associations with sex hormone binding globulin. Methods Participants included post‐menopausal women who underwent hepatic ultrasound ( n  = 345) at the Michigan site of the Study of Women's Health Across the Nation, a population‐based study. We examined hepatic steatosis prevalence by race/ethnicity and used logistic regression models to calculate the odds of hepatic steatosis with race/ethnicity and sex hormone binding globulin, after adjustment for age, alcohol use, waist circumference, high density lipoprotein cholesterol, triglycerides, systolic blood pressure and use of medications reported to lower intrahepatic fat. Results Fewer African‐American women than non‐Hispanic white women had hepatic steatosis (23 vs. 36%, P  = 0.01). African‐American women had lower triglyceride and low‐density lipoprotein cholesterol levels, but higher blood pressure and follicle‐stimulating hormone levels ( P  < 0.05). In the optimal‐fitting multivariable models, women in the highest tertile of sex hormone binding globulin (60.2–220.3 nmol/l) had a lower odds of hepatic steatosis (odds ratio 0.43, 95% CI 0.20–0.93) compared with women in the lowest tertile of sex hormone binding globulin (10.5–40.3 nmol/l). There was an interaction between race/ethnicity and medication use whereby non‐Hispanic white women using medications had three times higher odds of hepatic steatosis compared with African‐American women not using medications (odds ratio 3.36, 95%  CI 1.07–10.58). Interactions between race/ethnicity and other variables, including sex hormone levels, were not significant. Conclusions Hepatic steatosis on ultrasound may be more common in post‐menopausal non‐Hispanic white women than African‐American women and was associated with lower levels of sex hormone binding globulin. What's new? Although hepatic steatosis is common in post‐menopausal women, previous studies have not examined risk factors in this population, particularly sex steroids and sex hormone binding globulin. We report that increased sex hormone binding globulin, the primary binding protein of sex hormones and a risk factor for diabetes, was strongly associated with decreased odds of hepatic steatosis in both race/ethnicities.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/101792/1/dme12225.pd