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Two-dimensional (2D) metallic transition metal dichalcogenides (MTMDCs) are emerging as an appealing class of materials for a wide range of research topics, including electronics, spintronics, and energy-related fields, in view of their unique physical and chemical properties. Notably, the controlled synthesis of such promising materials is a prerequisite. In this Perspective, we review the up-to-date advances regarding the syntheses and physical properties of 2D MTMDCs, as well as their multifunctional applications. First, a variety of synthetic strategies of 2D MTMDCs, such as chemical exfoliation, chemical vapor transport, atomic layer deposition, molecular beam epitaxy, and chemical vapor deposition, are systematically summarized. Second, the fundamental physical properties of 2D MTMDCs are introduced, including charge density wave order, ferromagnetism, and superconductivity. Third, the versatile applications of 2D MTMDCs in electronic devices and energy-related fields are discussed. Finally, the challenges about the exploration of controlled syntheses, interesting physical issues, and multifunctional applications are highlighted, and future directions are also proposed. We believe that this Perspective is comprehensive and insightful for the controllable syntheses, physical property characterizations, and application exploitations of 2D MTMDCs.

Bi 2 Se 3 thin films were electrochemically deposited on Ti and indium tin oxide-coated glass substrates, respectively, at room temperature, using Bi(NO 3 ) 3 ·5H 2 O and SeO 2 as starting materials in diluted HNO 3 solution. A conventional three-electrode cell was used with a platinum sheet as a counter electrode, and a saturated calomel electrode was used as a reference electrode. The films were annealed in argon atmosphere. The influence of cold isostatic pressing before annealing on the microstructure and thermoelectric properties of the films was investigated. X-ray diffraction analysis indicates that the film grown on the indium tin oxide-coated glass substrate is pure rhombohedral Bi 2 Se 3 , and the film grown on the Ti substrate consists of both rhombohedral and orthorhombic Bi 2 Se 3 .

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We report the mechanism of hydrogel formation in dilute aqueous solutions (>15 mg/mL) by 2 nm metal−organic cages (MOCs). Experiments and all-atom simulations confirm that with the addition of small electrolytes, the MOCs self-assemble into 2D nanosheets via counterion-mediated attraction because of their unique molecular structure and charge distribution as well as σ−π interactions. The stiff nanosheets are difficult to bend into 3-D hollow, spherical blackberry type structures, as observed in many other macroion systems. Instead, they stay in solution and their very large excluded volumes lead to gelation at low (∼1.5 wt %) MOC concentrations, with additional help from hydrophobic and partial π−π interactions similar to the gelation of graphene oxides.