The precise control of edge geometry and crystal shape of monolayer MoS 2 is particular of importance for their applications in nanoelectronics and photo-electro catalysts. Here we reveal a crucial role of chemical potential in the determination of equilibrium shape (ES) and edge structure of monolayer MoS 2 by using density-functional theory calculations. Applying Wulff construction rule, our results demonstrate the shape evolution of monolayer MoS 2 flake from the dodecagonal shape, then to the hexagonal shape, to the triangular shape with the variation of chemical potential from the Mo-rich to the S-rich condition, and the edge structure of ES changes correspondingly from mixed zigzag/armchair edges to pure zigzag edges. This finding can be applied to explain extensive experimental observations about the morphology of MoS 2 domains. Meanwhile, the edge magnetism and electronic structures of monolayer MoS 2 domains are found to be dependent on their edge structure, which provides specific guidance for the magnetic modulation of monolayer MoS 2 and designing more effective MoS 2 -based catalysts.

Superior catalytic activity and high chemical stability of inexpensive electrocatalysts for the hydrogen evolution reaction (HER) are crucial to the large-scale production of hydrogen from water. The nonprecious two-dimensional MoSe materials emerge as a potential candidate, and the improvement of their catalytic activity depends on the optimization of active reaction sites at both the edges and the basal plane. Herein, the structural stability, electrocatalytic activity, and HER mechanisms on a series of MoSe catalytic structures including of point defects, holes, and edges have been explored by using first-principles calculations. Our calculated results demonstrate that thermodynamically stable defects (e.g., V, V, Se, and V) and edges (e.g., Mo-R and Se-R) in MoSe are very similar to the case of MoS, but their HER activity is higher than that of the corresponding structures in MoS, which is in good agreement with experimental observations. Furthermore, a Fermi-abundance model is proposed to explain the fundamental correlation between the HER activity of various MoSe catalysts and their intrinsic electronic structures, and this model is also applicable for assessing the HER activity of other types of catalysts, such as MoS and Pt. Moreover, two different HER mechanisms have been revealed in the MoSe catalytic structures: the Volmer-Tafel mechanism is preferred for the V and V structures, whereas the Volmer-Heyrovsky mechanism is more favorable for other MoSe catalytic structures. The present work suggests that MoSe with appropriate defects and edges is able to compete against the Pt-based catalysts and also opens a route to design highly active electrocatalysts for the HER.

Atomically thin Janus transition metal dichalcogenides (JTMDs) with an asymmetric structure have emerged as a new class of intriguing two-dimensional (2D) semiconductor materials. Using state-of-the-art density functional theory (DFT) calculations, we systematically investigate the structural, electronic, and optical properties of JTMD monolayers and heterostructures. Our calculated results indicate that the JTMD monolayers suffer from a bending strain but present high thermodynamic stability. All of them are semiconductors with a band-gap range from 1.37 to 1.96 eV. They possess pronounced optical absorption in the visible-light region and cover a large range of carrier mobilities from 28 to 606 cm2 V-1 s-1, indicating strong anisotropic characteristics. Significantly, some monolayer JTMDs (e.g., WSSe and WSeTe) exhibit superior mobilities than conventional TMD monolayers, such as MoS2. Moreover, the absolute band-edge positions of the JTMD monolayers are higher than the water redox potential, and most JTMD heterostructures have a type-II band alignment that contributes to the separation of carriers. Our work suggests that the 2D JTMD monolayers are promising for nanoelectronic, optoelectronic, and photocatalytic applications.

Two-dimensional (2D) layered MoS2 nanosheets possess great potential as anode materials for lithium ion batteries (LIBs), but they still suffer from poor cycling performance. Improving the cycling stability of electrode materials depends on a deep understanding of their dynamic structural evolution and reaction kinetics in the lithiation process. Herein, thermodynamic phase diagrams and the lithiation dynamics of MoS2-based nanostructures with the intercalation of lithium ions are studied by using first-principles calculations and ab initio molecular dynamics simulations. Our results demonstrate that the continuous intercalation of Li ions induces structural destruction of 2H phase MoS2 nanosheets in the discharge process that follows a layer-by-layer dissociation mechanism. Meanwhile, the intercalation of Li ions leads to a structural transition of MoS2 nanosheets from the 2H to the 1T phase due to the ultralow transition barriers (∼0.1 eV). We find that the phase transition can slow down the dissociation of MoS2 nanosheets during lithiation. The result can be applied to explain extensive experimental observation of the fast capacity fading of MoS2-based anode materials between the first and the subsequent discharges. To suppress the dissociation of MoS2 nanosheets in the lithiation process, we propose a strategy by constructing a sandwich-like graphene/MoS2/graphene structure that indicates high chemical stability, superior conductivity, and high Li-ion mobility in the charge/discharge process, implying the possibility to induce an improvement in the anode cycling performance. This work opens a new route to rational design layered transition-metal disulfide (TMD) anode materials for LIBs with superior cycling stability and electrochemical performance.

Flexible dielectric and electronic materials with high dielectric constant (k) and low loss are constantly pursued. Encapsulation of conductive fillers with insulating shells represents a promising approach, and has attracted substantial research efforts. However, progress is greatly impeded due to the lack of a fundamental understanding of the polarization mechanism. In this work, a series of core–shell polymer composites is studied, and the correlation between macroscopic dielectric properties (across entire composites) and microscopic polarization (around single fillers) is investigated. It is revealed that the polarization in polymer conductor composites is determined by electron transport across multiple neighboring conductive fillers—a domain‐type polarization. The formation of a core–shell filler structure affects the dielectric properties of tpolymer composites by essentially modifying the filler‐cluster size. Based on this understanding, a novel percolative composite is prepared with higher‐than‐normal filler concentration and optimized shell's electrical resistivity. The developed composite shows both high‐k due to enlarged cluster size and low loss due to restrained charge transport simultaneously, which cannot be achieved in traditional percolative composites or via simple core–shell filler design. The revealed polarization mechanism and the optimization strategy for core–shell fillers provide critical guidance and a new paradigm, for developing advanced polymer dielectrics with promising property sets.