No abstract

A flatband representing a highly degenerate and dispersionless manifold state of electrons may offer unique opportunities for the emergence of exotic quantum phases. To date, definitive experimental demonstrations of flatbands remain to be accomplished in realistic materials. Here, we present the first experimental observation of a striking flatband near the Fermi level in the layered Fe_{3}Sn_{2} crystal consisting of two Fe kagome lattices separated by a Sn spacing layer. The band flatness is attributed to the local destructive interferences of Bloch wave functions within the kagome lattices, as confirmed through theoretical calculations and modelings. We also establish high-temperature ferromagnetic ordering in the system and interpret the observed collective phenomenon as a consequence of the synergetic effect of electron correlation and the peculiar lattice geometry. Specifically, local spin moments formed by intramolecular exchange interaction are ferromagnetically coupled through a unique network of the hexagonal units in the kagome lattice. Our findings have important implications to exploit emergent flat-band physics in special lattice geometries.

The activity and accessibility of MoS 2 edge sites are critical to deliver high hydrogen evolution reaction (HER) efficiency. Here, a porous carbon network confining ultrasmall N-doped MoS 2 nanocrystals (N-MoS 2 /CN) is fabricated by a selftemplating strategy, which realizes synergistically structural and electronic modulations of MoS 2 edges. Experiments and density functional theory calculations demonstrate that the N dopants could activate MoS 2 edges for HER, while the porous carbon network could deliver high accessibility of the active sites from N-MoS 2 nanocrystals. Consequently, N-MoS 2 /CN possesses superior HER activity with an overpotential of 114 mV at 10 mA cm −2 and excellent stability over 10 h, delivering one of best MoS 2based HER electrocatalysts. Moreover, this study opens a new venue for optimizing materials with enhanced accessible catalytic sites for energy-related applications.

Controllable nucleation sites play a key role in the selective growth of heterostructures. Here, we are the first to report a one-pot strategy to realize the confined and selective growth of large MoS2/WS2 lateral and vertical heterostructures. A hydroxide-assisted process is introduced to control the nucleation sites, thereby realizing the optional formation of lateral and vertical heterostructures. Time-of-flight secondary ion mass spectrometry verifies the critical role of hydroxide groups toward the controllable growth of these heterostructures. The size of the as-grown MoS2/WS2 lateral heterostructures can be as large as 1 mm, which is the largest lateral size reported thus far. The obtained MoS2/WS2 heterostructures have a high carrier mobility of ∼58 cm2 V–1 s–1, and the maximum on/off current ratio is >108. This approach provides not only a pathway for the selective growth of large MoS2/WS2 lateral and vertical heterostructures but also a fundamental understanding of surface chemistry for controlling the selective growth of transition-metal dichalcogenide heterostructures.

Two-dimensional transition metal dichalcogenides represent an emerging class of layered materials exhibiting various intriguing properties, and integration of such materials for potential device applications will necessarily invoke further reduction of their dimensionality. Using first-principles approaches, here we investigate the structural, electronic, and magnetic properties along the two different edges of zigzag MX (M = Mo, W; X = S, Se) nanoribbons. Along the M edges, we reveal a previously unrecognized but energetically strongly preferred (2 × 1) reconstruction pattern, which is universally operative for all the four systems (and possibly more), characterized by an elegant self-passivation mechanism through place exchanges of the outmost X and M edge atoms. In contrast, the X edges undergo a much milder (2 × 1) or (3 × 1) reconstruction for MoX or WX, respectively. These contrasting structural preferences of the edges can be exploited for controlled fabrication of properly tailored transition metal dichalcogenide nanoribbons under nonequilibrium growth conditions. We further use the zigzag MoX nanoribbons to demonstrate that the Mo and X edges possess distinctly different electronic and magnetic properties, which are significant for catalytic and spintronic applications.

Efficient bifunctional electrocatalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are required for metal air batteries, to replace costly metals, such as Pt and Ir/Ru based compounds, which are typically used as benchmarks for ORR and OER, respectively. Isolated single atomic sites coordinated with nitrogen on carbon supports (M‐N‐C) have promising performance for replacement of precious metal catalysts. However, most of monometallic M‐N‐C catalysts demonstrate unsatisfactory bifunctional performance. Herein, a facile way of preparing bimetallic Fe and Co sites entrapped in nitrogen‐doped hollow carbon nanospheres (Fe,Co‐SA/CS) is explored, drawing on the unique structure and pore characteristics of Zeolitic imidazole frameworks and molecular size of Ferrocene, an Fe containing species. Fe,Co‐SA/CS showed an ORR onset potential and half wave potential of 0.96 and 0.86 V, respectively. For OER, (Fe,Co)‐SA/CS attained its anodic current density of 10 mA cm–2 at an overpotential of 360 mV. Interestingly, the oxygen electrode activity (ΔE) for (Fe,Co)‐SA/CS and commercial Pt/C‐RuO2 is calculated to be 0.73 V, exhibiting the bifunctional catalytic activity of (Fe,Co)‐SA/CS. (Fe,Co)‐SA/CS evidenced desirable specific capacity and cyclic stability than Pt/C‐RuO2 mixture when utilized as an air cathode in a homemade Zinc‐air battery.

We are reporting for the first time the pH responsiveness of liquid crystal (LC) microdroplets decorated with an amphiphilic block copolymer of PAA-b-LCP. We successfully demonstrated the adsorption of block copolymer on LC droplets by fluorescence microscopy and pH response to the radial-to-bipolar orientational change of the LC droplets by changing pH from 12 to 2 through the polarized optical microscope (POM). We believe that our results may pave the way for the generation of monodisperse droplets decorated by various amphiphilic block copolymers which respond to several kinds of the external stimuli. These developments may be important for potential applications of the LC droplets in sensing and encapsulation fields.

with thermodynamic oxygen to water conversion potential of 1.23 V (vs reversible hydrogen electrode (RHE)). [1] Especially so in recent years, platinum (Pt)-based catalysts are widely utilized as the state of the art catalyst for ORR. [2] However, they are known to suffer from ineluctable concerns with regards to scarcity, extreme cost, poor stability in long term usage and poisoning effects from methanol crossover. [3,4] The potential of rechargeable metal-air batteries (metal = Zn, Li, Al, Mg, etc.) in future energy applications can be simply revealed from its high theoretical energy density values, in the range of 1086-11 140 Wh kg -1 . [5,6] Metal anodes, such as Zn, are earth abundant, cheap, safe for handling, and environmentally benign, favoring suitability of metal-air batteries for domestic and industrial applications. [7,8] Despite the intensive research being undertaken in the field of Zn-air batteries, major degradation issues remain with regards to the following: a) degradation of cathode materials-catalyst, carbon support and binders and b) morphological degradation of Zn anode during repeated cycles. [9] Thus, in order to realize and leverage on the full potential of metal air batteries, the primary task would be to develop highly efficient and stable air cathodes that favor oxygen chemistry, mainly the ORR.Earth abundant metal-based catalysts, like metal oxides, [10] perovskites, [11] spinel type, [12] MXenes , [13,14] and mixed metal oxides [15,16] have been investigated as ORR catalysts. In addition, heteroatom-doped carbon structures containing metallic elements, like cobalt, nickel, and iron, have emerged as promising candidates to replace Pt. Despite intensive research in metal pnictogens and chalcogens over the years, there is a lack in exploration of metal-metalloids, such as amorphous metal borides for the ORR. Transition metal borides show superior oxygen evolution reaction (OER) performance in alkaline solution compared with noble metal catalysts, metal oxides, and metal alloy counterparts. [17] In contrast, transition metal borides undergo severe oxidation, limiting activities that may be derived and consequently, their ORR performance. [18] To date, only J. Ma et al [19] and K. Elumeeva et al [20] reported the usage of CoB as an ORR electrocatalyst, but the performance obtained was not satisfying with those the state-of-the-art catalysts. CoB nanosheets are expected to be highly beneficial for electrochemical applications (for example, ORR) because 1) the synthesis of metal borides by chemical reduction is fast and facile compared with the time consuming synthesis of many other Compositional and structural engineering of metal-metalloid materials can boost their electrocatalytic performance. Herein, a highly efficient and stable electrocatalytic system for the oxygen reduction reaction is obtained by creating heterointerfaces between N-doped carbon and cobalt boride nanosheets. Furthermore, a detailed investigation on the effect of annealing temperature as well as the amount of carbon ...