Layered antiferromagnetism is the spatial arrangement of ferromagnetic layers with antiferromagnetic interlayer coupling. The van der Waals magnet chromium triiodide (CrI3) has been shown to be a ...layered antiferromagnetic insulator in its few-layer form, opening up opportunities for various functionalities in electronic and optical devices. Here we report an emergent nonreciprocal second-order nonlinear optical effect in bilayer CrI3. The observed second-harmonic generation (SHG; a nonlinear optical process that converts two photons of the same frequency into one photon of twice the fundamental frequency) is several orders of magnitude larger than known magnetization-induced SHG and comparable to the SHG of the best (in terms of nonlinear susceptibility) two-dimensional nonlinear optical materials studied so far (for example, molybdenum disulfide). We show that although the parent lattice of bilayer CrI3 is centrosymmetric, and thus does not contribute to the SHG signal, the observed giant nonreciprocal SHG originates only from the layered antiferromagnetic order, which breaks both the spatial-inversion symmetry and the time-reversal symmetry. Furthermore, polarization-resolved measurements reveal underlying C2h crystallographic symmetry-and thus monoclinic stacking order-in bilayer CrI3, providing key structural information for the microscopic origin of layered antiferromagnetism. Our results indicate that SHG is a highly sensitive probe of subtle magnetic orders and open up possibilities for the use of two-dimensional magnets in nonlinear and nonreciprocal optical devices.
Abstract
Developing robust nonprecious electrocatalysts towards hydrogen/oxygen evolution reactions is crucial for widespread use of electrochemical water splitting in hydrogen production. Here, we ...report that intermetallic Co
3
Mo spontaneously separated from hierarchical nanoporous copper skeleton shows genuine potential as highly efficient electrocatalysts for alkaline hydrogen/oxygen evolution reactions in virtue of in-situ hydroxylation and electro-oxidation, respectively. The hydroxylated intermetallic Co
3
Mo has an optimal hydrogen-binding energy to facilitate adsorption/desorption of hydrogen intermediates for hydrogen molecules. Associated with high electron/ion transport of bicontinuous nanoporous skeleton, nanoporous copper supported Co
3
Mo electrodes exhibit impressive hydrogen evolution reaction catalysis, with negligible onset overpotential and low Tafel slope (~40 mV dec
−1
) in 1 M KOH, realizing current density of −400 mA cm
−2
at overpotential of as low as 96 mV. When coupled to its electro-oxidized derivative that mediates efficiently oxygen evolution reaction, their alkaline electrolyzer operates with a superior overall water-splitting output, outperforming the one assembled with noble-metal-based catalysts.
•Cold plate cooling of prismatic Li-ion battery under 5C discharging is modeled.•Temperature patterns on Li-ion battery with cold plate cooling are discussed.•Thermal responses of Li-ion battery ...under external shorting are simulated.•3D thermal modeling of 50 V Li-ion battery pack with 14 20 Ah cells is performed.•Multi-scale multi-domain modeling is demonstrated for large Li-ion battery pack.
In this work, three-dimensional (3D) thermal modeling of a single Li-ion battery cell and a 50 V Li-ion battery pack composed of 14 prismatic batteries is performed. Equivalent circuit model is used for subscale electrochemical modeling. The heat generated by electrical resistance and electrochemistry reactions is then solved in the cell domain which resolves the heat transfer in battery and battery pack. The modeling provides highly resolved temporal 3D insight into thermal and battery dynamics under fast discharging and abusive condition. It is found that with low coolant velocity, the cell temperature easily exceeds 40 °C, and temperature non-uniformity exceeds the limit value of 5 °C under 5C discharging condition; and under external shorting condition, temperature rises fast and reaches the 80 °C point quickly which can further trigger thermal runaway. On the other hand, with adequate coolant flow rate, the cell temperature and temperature gradients are effectively limited to tolerable level under both 5C discharging and external shorting conditions. Thus, coolant rate needs to be carefully designed to prevent high temperature and localized high temperature spot in Li-ion battery. The modeling method used in this study can be applied to general Li-ion battery and Li-ion battery pack designing.
Graphene with massless Dirac fermions can have exceptionally strong third-order optical nonlinearities. Yet reported values of nonlinear optical susceptibilities for third-harmonic generation (THG), ...four-wave mixing (FWM) and self-phase modulation vary over six orders of magnitude. Such variation likely arises from frequency-dependent resonance effects of different processes in graphene under different doping. Here, we report an experimental study of THG and FWM in graphene using gate tuning to adjust the doping level and vary the resonant condition. We find that THG and sum-frequency FWM are strongly enhanced in heavily doped graphene, while the difference-frequency FWM appears just the opposite. Difference-frequency FWM exhibited a novel divergence towards the degenerate case in undoped graphene, leading to a giant enhancement of the nonlinearity. The results are well supported by theory. Our full understanding of the diverse nonlinearity of graphene paves the way towards future design of graphene-based nonlinear optoelectronic devices.
Artificial structures made of stacked two-dimensional crystals have recently been the focus of intense research activity. As in twisted or stacked graphene layers, these structures can show unusual ...behaviours and new phenomena. Among the various layered compounds that can be exfoliated, transition-metal dichalcogenides exhibit interesting properties governed by their structural symmetry and interlayer coupling, which are highly susceptible to stacking. Here, we obtain-by folding exfoliated MoS2 monolayers-MoS2 bilayers with different stacking orders, as monitored by second harmonic generation and photoluminescence. Appropriate folding can break the inversion symmetry and suppress interlayer hopping, evoking strong valley and spin polarizations that are not achieved in natural MoS2 bilayers of Bernal stacking. It can also enlarge the indirect bandgap by more than 100meV through a decrease in the interlayer coupling. Our work provides an effective and versatile means to engineer transition-metal dichalcogenide materials with desirable electronic and optical properties.
For centrosymmetric materials such as monolayer graphene, no optical second-harmonic generation (SHG) is generally expected, because it is forbidden under the electric-dipole approximation. Yet we ...observe a strong, doping-induced SHG from graphene, with its highest strength comparable to the electric-dipole-allowed SHG in noncentrosymmetric 2D materials. This novel SHG has the nature of an electric-quadrupole response, arising from the effective breaking of inversion symmetry by optical dressing with an in-plane photon wave vector. More remarkably, the SHG is widely tuned by carrier doping or chemical potential, being sharply enhanced at Fermi-edge resonances but vanishing at the charge neutral point that manifests the electron-hole symmetry of massless Dirac fermions. This striking behavior in graphene, which should also arise in graphenelike Dirac materials, expands the scope of nonlinear optics and holds the promise of novel optoelectronic and photonic applications.
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•The motivation of this study is to shed light on PEM fuel cell with thin MEA.•3D non-isothermal two phase PEM fuel cell modeling established and performed.•Effects of ORR activity, O2 resistance, ...operating pressure and humidity are analyzed.•Thin membrane is easier hydrated under low humidity, but can suffer H2 crossover.•A 50 cm2 PEM fuel cell with thin MEA and counter-cross flow field is simulated.
Compared to conventional proton exchange membrane (PEM) fuel cell with thick membrane electrode assembly, today’s automotive PEM fuel cell adopts thin MEA with thin catalyst coated membrane (CCM). The objective of this modeling study is to shed light on PEM fuel cell with thin MEA under low humidity operation condition. The effects of cathode catalyst oxygen reduction reaction activity and local O2 transport resistance on automotive PEM fuel cell high current operation are elucidated. The local O2 transport resistance is found to have large impacts on limiting current density. The effects of cathode humidifier elimination, operating pressure, and stoichiometry ratio on PEM fuel cell performance are also discussed. The thin membrane in thin MEA has much lower ohmic resistance compared with thick membrane, and it is easy to be hydrated under low humidity condition. But H2 crossover is stronger for thin membrane and needs to be watched out when thinning the membrane. In addition, a 50 cm2 PEM fuel cell with counter-cross flow field is simulated for demonstration of medium scale PEM fuel cell modeling. Under 0.65 V and 50%/0% humidity operating conditions, the 50 cm2 PEM fuel cell is predicted to supply average current density of 2.51 A/cm2.
Blending with biodegradable poly (butylene succinate) (PBS) is an effective way for improving the performance of PLA. However, the property of the blends is not significantly improved due to poor ...compatibility between the two polymers. Herein, we propose a simple method to prepare biodegradable PLA/PBS blends successfully by UV-induced reactive extrusion. Multifunctional monomer trimethylolpropane tri-acrylate (TMPTA) is used as chain extender to react with PLA and PBS to generate graft copolymers at the PLA/PBS interface. Addition of glycidyl methacrylate (GMA) could further promote the graft reaction, thus increase the compatibility and the mechanical properties of the blends. The occurrence of the long chain branched reaction between the PLA and PBS is evidenced by the rheological results and Fourier-transform infrared spectroscopy analysis. Appropriate amount of GMA reduces the size of dispersed PBS phase and enhances the interfacial strength. The crystallization of PLA is effectively improved, as evidenced by the increased cool crystallization peak and apparent enhancement crystallinity. Ultimately, PLA/PBS blends compatibilized with GMA/TMPTA exhibit a remarkable improvement in elongation at break of 341%, notched impact strength and a preferable maintenance of 50 MPa in tensile strength.
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•The synergistic effect of GMA and TMPTA plays a good role in compatibilization.•Chain extension and crystallization of PLA/PBS blends are realized simultaneously.•This strategy can maintain good mechanical properties of composites.
Aiming at the construction of novel stimuli‐responsive fluorescent system with precisely tunable emissions, the typical 9,14‐diphenyl‐9,14‐dihydrodibenzoa, cphenazine (DPAC) luminogen with attractive ...vibration‐induced emission (VIE) behavior has been introduced into 2rotaxane as a stopper. Taking advantage of their unique dual stimuli‐responsiveness towards solvent and anion, the resultant 2rotaxanes reveal both tunable VIE and switchable circularly polarized luminescence (CPL). Attributed to the formation of mechanical bonds, DPAC‐functionalized 2rotaxanes display interesting VIE behaviors including white‐light emission upon the addition of viscous solvent, as evaluated in detail by femtosecond transient absorption (TA) spectra. In addition, ascribed to the regulation of chirality information transmission through anion‐induced motions of chiral wheel, the resolved chiral 2rotaxanes reveal unique switchable CPL upon the addition of anion, leading to significant increase in the dissymmetry factors (glum) values with excellent reversibility. Interestingly, upon doping the chiral 2rotaxanes in stretchable polymer, the blend films reveal remarkable emission change from white light to light blue with significant 6.5‐fold increase in glum values up to −0.035 under external tensile stresses. This work provides not only a new design strategy for developing molecular systems with fluorescent tunability but also a novel platform for the construction of smart chiral luminescent materials for practical use.
The dual stimuli‐responsiveness of novel 2rotaxanes endows them with both tunable vibration‐induced emission and switchable circularly polarized luminescence, providing a novel platform for the construction of smart chiral luminescent materials for practical use.
The Goddard microphysics was recently improved by adding a fourth ice class (frozen drops/hail). This new 4ICE scheme was developed and tested in the Goddard Cumulus Ensemble (GCE) model for an ...intense continental squall line and a moderate, less organized continental case. Simulated peak radar reflectivity profiles were improved in intensity and shape for both cases, as were the overall reflectivity probability distributions versus observations. In this study, the new Goddard 4ICE scheme is implemented into the regional‐scale NASA Unified‐Weather Research and Forecasting (NU‐WRF) model, modified and evaluated for the same intense squall line, which occurred during the Midlatitude Continental Convective Clouds Experiment (MC3E). NU‐WRF simulated radar reflectivities, total rainfall, propagation, and convective system structures using the 4ICE scheme modified herein agree as well as or significantly better with observations than the original 4ICE and two previous 3ICE (graupel or hail) versions of the Goddard microphysics. With the modified 4ICE, the bin microphysics‐based rain evaporation correction improves propagation and in conjunction with eliminating the unrealistic dry collection of ice/snow by hail can replicate the erect, narrow, and intense convective cores. Revisions to the ice supersaturation, ice number concentration formula, and snow size mapping, including a new snow breakup effect, allow the modified 4ICE to produce a stronger, better organized system, more snow, and mimic the strong aggregation signature in the radar distributions. NU‐WRF original 4ICE simulated radar reflectivity distributions are consistent with and generally superior to those using the GCE due to the less restrictive domain and lateral boundaries.
Key Points
New 4ICE microphysics scheme is implemented in a regional scale model
Radar reflectivities and rain rate intensities are sensitive to the microphysics scheme
The new 4ICE scheme produces radar structures superior to original 4ICE and 3ICE schemes
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