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Based on the density functional theory calculations, we report the stability, electron structure, and potential applications in single-layer (SL) NiX2(X = S, Se, and Te) that show high thermal, dynamical, and mechanical stability and possess ultralow isotropic Young’s modulus. Moreover, these NiS2, NiSe2, and NiTe2monolayers have high energy storage capacities of 874, 496, and 342 mA h g–1and low diffusion energy barriers of 0.23/0.24/0.27 and 0.12/0.13/0.12 eV when used as anodes in lithium-ion batteries and sodium-ion batteries, respectively. SL NiS2and NiSe2show semiconducting traits in their freestanding crystals. Specifically, the 2D NiTe2monolayer is determined to be an intrinsic superconductor with a transition temperature (Tc) of ∼2.24 K. The Tccan be improved to 5.48 K when subjected to an in-plane compressive strain of 6%. These results enrich the family of transition metal-based 2D materials with multifunctionality and facilitate further experimental efforts toward next-generation nanodevices.

The breaking of the out-of-plane symmetry makes a two-dimensional (2D) Janus monolayer a new platform to explore the coupling between ferroelectricity and ferromagnetism. Using density functional theory in combination with Monte Carlo simulations, we report a novel phase-switchable 2D multiferroic material VInSe3with large intrinsic out-of-plane spontaneous electric polarization and a high Curie temperature (Tc). The structural transition energy barrier between the two phases is determined to be 0.4 eV, indicating the switchability of the electric polarizations and the potential ferroelectricity. Carrier doping can boost the Curie temperature above room temperature, attributing to the enhanced magnetic exchange interaction. A transition from the ferromagnetic (FM) state to the antiferromagnetic (AFM) state can be induced by carrier doping in octahedra-VInSe3, while FM coupling is well-preserved in tetrahedron-VInSe3, which can be regulated to be either an XY or Ising magnet at an appropriate carrier concentration. These findings not only enrich the family of high-Tclow-dimensional monolayers but also offer a new direction for the design and multifunctional application of multiferroic materials.