Spin-Enhanced O-H Cleavage in Electrochemical Water Oxidation.

Herein we report the vital role of spin polarization in proton-transfer-mediated water oxidation over a magnetized catalyst. During the electrochemical oxygen evolution reaction (OER) over ferrimagnetic Fe3 O4 , the external magnetic field induced a remarkable increase in the OER current, however, this increment achieved in weakly alkaline pH (pH 9) was almost 20 times that under strongly alkaline conditions (pH 14). The results of the surface modification experiment and H/D kinetic isotope effect investigation confirm that, at weakly alkaline pH, during the nucleophilic attack of FeIV =O by molecular water, the magnetized Fe3 O4 catalyst polarizes the spin states of the nucleophilic attacking intermediates. The spin-enhanced singlet O-H cleavage and triplet O-O bonding occur synergistically, which promotes the O2 generation more significantly than the strongly alkaline case involving only spin-enhanced O-O bonding.

Heat transport in Rayleigh-Bénard convection with linear marginality.

Recent direct numerical simulations (DNS) and computations of exact steady solutions suggest that the heat transport in Rayleigh-Bénard convection (RBC) exhibits the classical [Formula: see text] scaling as the Rayleigh number [Formula: see text] with Prandtl number unity, consistent with Malkus-Howard's marginally stable boundary layer theory. Here, we construct conditional upper and lower bounds for heat transport in two-dimensional RBC subject to a physically motivated marginal linear-stability constraint. The upper estimate is derived using the Constantin-Doering-Hopf (CDH) variational framework for RBC with stress-free boundary conditions, while the lower estimate is developed for both stress-free and no-slip boundary conditions. The resulting optimization problems are solved numerically using a time-stepping algorithm. Our results indicate that the upper heat-flux estimate follows the same [Formula: see text] scaling as the rigorous CDH upper bound for the two-dimensional stress-free case, indicating that the linear-stability constraint fails to modify the boundary-layer thickness of the mean temperature profile. By contrast, the lower estimate successfully captures the [Formula: see text] scaling for both the stress-free and no-slip cases. These estimates are tested using marginally-stable equilibrium solutions obtained under the quasi-linear approximation, steady roll solutions and DNS data. This article is part of the theme issue 'Mathematical problems in physical fluid dynamics (part 1)'.

Designing Two-Dimensional Versatile Room-Temperature Ferromagnets via Assembling Large-Scale Magnetic Quantum Dots.

Two-dimensional (2D) ferromagnets possess astonishing potential in new-concept spintronics. However, most of the reported intrinsic 2D ferromagnets show a low Curie temperature far below room temperature. Here, we propose a series of 2D magnetic covalent and metal organic frameworks (COFs/MOFs) by assembling triangular zigzag graphene quantum dots (TZGDs) with various linkages, involving small-sized TZGDs, nonmetal atoms, magnetic metal atoms, and molecules. Upon first-principles calculations, we demonstrate 2D magnetic semiconductors with an enhanced Curie temperature of up to 472 K can be realized through the strong p(d)-p direct exchange interaction between TZGDs and linkages. Particularly, the TZGD size hardly affects the Curie temperature, whereas linkages can modulate the Curie temperature significantly. The TZGD size and linkages can regulate the electronic and magnetic properties of TZGD-based 2D ferromagnets. Our results confirm the possibility of designing 2D ferromagnets based on TZGDs and motivate the research of 2D ferromagnets on magnetic quantum dots and molecular magnets.

Observation of Gap Opening in 1T' Phase MoS2 Nanocrystals.

Two-dimensional (2D) transition metal dichalcogenides (TMDs) manifest in various polymorphs, which deliver different electronic properties; the most prominent among them include the semiconducting 2H phase and metallic 1T (or distorted 1T' phase) phase. Alkali metal intercalation or interface strain has been used to induce semiconductor-to-metal transition in a monolayer MoS2 sheet, leading to exotic quantum states or improved performance in catalysis. However, the direct growth of 1T or 1T' phase MoS2 is challenging due to its metastability. Here, we report MBE growth of isolated 1T' and 2H MoS2 nanocrystals on a Au substrate; these nanocrystals can be differentiated unambiguously by their electronic states using scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). By studying the initial stages of nucleation during molecular beam epitaxy (MBE) of MoS2, we could identify atomic clusters (30-50 atoms) with intralayer stacking corresponding to 1T' and 2H separately, which suggests a deterministic growth mechanism from initial nuclei. Furthermore, a topological insulator type behavior was observed for the 1T' MoS2 crystals, where an energy gap opening of 80 meV was measured by STS in the basal plane at 5 K, with the edge of the nanocrystals remaining metallic.

Mo-Terminated Edge Reconstructions in Nanoporous Molybdenum Disulfide Film.

The catalytic and magnetic properties of molybdenum disulfide (MoS2) are significantly enhanced by the presence of edge sites. One way to obtain a high density of edge sites in a two-dimensional (2D) film is by introducing porosity. However, the large-scale bottom-up synthesis of a porous 2D MoS2 film remains challenging and the correlation of growth conditions to the atomic structures of the edges is not well understood. Here, using molecular beam epitaxy, we prepare wafer-scale nanoporous MoS2 films under conditions of high Mo flux and study their catalytic and magnetic properties. Atomic-resolution electron microscopy imaging of the pores reveals two new types of reconstructed Mo-terminated edges, namely, a distorted 1T (DT) edge and the Mo-Klein edge. Nanoporous MoS2 films are magnetic up to 400 K, which is attributed to the presence of Mo-terminated edges with unpaired electrons, as confirmed by density functional theory calculation. The small hydrogen adsorption free energy at these Mo-terminated edges leads to excellent activity for the hydrogen evolution reaction.

Emergence of Droplets at the Nonequilibrium All-Aqueous Interface in a Vertical Hele-Shaw Cell.

The interfacial phenomena at liquid-liquid interfaces remain the subject of constant fascination in science and technology. Here, we show that fingers forming at the interface of nonequilibrium all-aqueous systems can spontaneously break into an array of droplets. The dynamic formation of droplets at the water-water (w/w) interface is observed when a less dense aqueous phase, for instance, the dextran solution, is placed on a denser aqueous phase, the polyethylene glycol solution, in a vertical Hele-Shaw cell. Because of the gradual diffusion of water from the upper phase into the lower phase, a dense layer appears at the nonequilibrium w/w interface. As a result, a periodic array of fingers emerge and sink. Remarkably, these fingers break up and an array of droplets are emitted from the interface. We characterize the wavelength of fingering by measuring the average distance between the dominant fingers. By varying the initial concentrations of the two nonequilibrium aqueous phases, we identify experimentally a phase diagram with a wide parameter space in which finger breaking occurs. Finally, plenty of droplets, spontaneously formed when one phase is continuously deposited onto another aqueous phase, further confirm the robustness of our experimental results. Our work suggests a simple yet efficient approach with a potential upscalability to generate all-aqueous droplets.

Fabry-Perot Cavity-Enhanced Optical Absorption in Ultrasensitive Tunable Photodiodes Based on Hybrid 2D Materials.

Monolayer two-dimensional (2D) transition metal dichalcogenides (TMDs) show interesting optical and electrical properties because of their direct bandgap. However, the low absorption of atomically thin TMDs limits their applications. Here, we report enhanced absorption and optoelectronic properties of monolayer molybdenum disulfide (MoS2) by using an asymmetric Fabry-Perot cavity. The cavity is based on a hybrid structure of MoS2/ hexagonal boron nitride (BN)/Au/SiO2 realized through layer-by-layer vertical stacking. Photoluminescence (PL) intensity of monolayer MoS2 is enhanced over 2 orders of magnitude. Theoretical calculations show that the strong absorption of MoS2 comes from photonic localization on the top of the microcavity at optimal BN spacer thickness. The n/n+ MoS2 homojunction photodiode incorporating this asymmetric Fabry-Perot cavity exhibits excellent current rectifying behavior with an ideality factor of 1 and an ultrasensitive and gate-tunable external photo gain and specific detectivity. Our work offers an effective method to achieve uniform enhanced light absorption by monolayer TMDs, which has promising applications for highly sensitive optoelectronic devices.

Molecular Beam Epitaxy of Highly Crystalline Monolayer Molybdenum Disulfide on Hexagonal Boron Nitride.

Atomically thin molybdenum disulfide (MoS2), a direct-band-gap semiconductor, is promising for applications in electronics and optoelectronics, but the scalable synthesis of highly crystalline film remains challenging. Here we report the successful epitaxial growth of a continuous, uniform, highly crystalline monolayer MoS2 film on hexagonal boron nitride (h-BN) by molecular beam epitaxy. Atomic force microscopy and electron microscopy studies reveal that MoS2 grown on h-BN primarily consists of two types of nucleation grains (0° aligned and 60° antialigned domains). By adopting a high growth temperature and ultralow precursor flux, the formation of 60° antialigned grains is largely suppressed. The resulting perfectly aligned grains merge seamlessly into a highly crystalline film. Large-scale monolayer MoS2 film can be grown on a 2 in. h-BN/sapphire wafer, for which surface morphology and Raman mapping confirm good spatial uniformity. Our study represents a significant step in the scalable synthesis of highly crystalline MoS2 films on atomically flat surfaces and paves the way to large-scale applications.

Chemical Stabilization of 1T' Phase Transition Metal Dichalcogenides with Giant Optical Kerr Nonlinearity.

The 2H-to-1T' phase transition in transition metal dichalcogenides (TMDs) has been exploited to phase-engineer TMDs for applications in which the metallicity of the 1T' phase is beneficial. However, phase-engineered 1T'-TMDs are metastable; thus, stabilization of the 1T' phase remains an important challenge to overcome before its properties can be exploited. Herein, we performed a systematic study of the 2H-to-1T' phase evolution by lithiation in ultrahigh vacuum. We discovered that by hydrogenating the intercalated Li to form lithium hydride (LiH), unprecedented long-term (>3 months) air stability of the 1T' phase can be achieved. Most importantly, this passivation method has wide applicability for other alkali metals and TMDs. Density functional theory calculations reveal that LiH is a good electron donor and stabilizes the 1T' phase against 2H conversion, aided by the formation of a greatly enhanced interlayer dipole-dipole interaction. Nonlinear optical studies reveal that air-stable 1T'-TMDs exhibit much stronger optical Kerr nonlinearity and higher optical transparency than the 2H phase, which is promising for nonlinear photonic applications.

Gap States at Low-Angle Grain Boundaries in Monolayer Tungsten Diselenide.

Two-dimensional (2D) transition metal dichalcogenides (TMDs) have revealed many novel properties of interest to future device applications. In particular, the presence of grain boundaries (GBs) can significantly influence the material properties of 2D TMDs. However, direct characterization of the electronic properties of the GB defects at the atomic scale remains extremely challenging. In this study, we employ scanning tunneling microscopy and spectroscopy to investigate the atomic and electronic structure of low-angle GBs of monolayer tungsten diselenide (WSe2) with misorientation angles of 3-6°. Butterfly features are observed along the GBs, with the periodicity depending on the misorientation angle. Density functional theory calculations show that these butterfly features correspond to gap states that arise in tetragonal dislocation cores and extend to distorted six-membered rings around the dislocation core. Understanding the nature of GB defects and their influence on transport and other device properties highlights the importance of defect engineering in future 2D device fabrication.