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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.

Two-dimensional (2D) V–V binary materials are gaining extensive attention because of high carrier mobility, largely tunable optical exciton properties, and high sensitivity to optical excitation. However, the luminescence efficiency and the radiation lifetime of black phosphorus are limited because of the high anisotropy feature of excitons. In addition, the effect of intrinsic dipole on the exciton behavior in these materials has yet to be explored. In the present work, the excitonic properties of α-, β-, γ-, and δ-phase 2D V–V nitrogen–phosphorus binary allotropes are explored using the Bethe–Salpeter equations combined with G0W0approximation systematically. The nitrogen substitution leads to the significantly modified electronic properties, including the increased oxidation activation energy from 0.03 to 0.40 eV, the enlarged band gap and direct-to-indirect band structure. It also leads to the reduced anisotropy but the opposite excitonic behavior due to the different introduction of intrinsic dipole between the α and β phases, including the brightness, radiation lifetime, effective mass, and real-space distribution. Because of high stability, long lifetime, and highly localized exciton states, the V–V binary materials with various phases have wide potentials for multifunctional and integrated applications.

Using first-principles calculations, we first reported that Janus group III chalcogenide monolayers have giant out-of-plane second harmonic generation susceptibility, and it is even 1–2 orders of magnitude larger than that of the in-plane components. In the Ga2SSe monolayer, the χzxx(2)/χxxy(2)and χzzz(2)/χxxy(2)are up to 125:1 and 86:1, respectively. The large intraband\interband velocity transition matrix and charge transfer are responsible for the strong second harmonic response of Janus group III chalcogenide monolayers. The present work provides a general protocol to design two-dimensional nonlinear optical materials with strong out-plane response for applications in optoelectronic and nonlinear optical devices.

Two-dimensional (2D) van der Waals (vdW) heterobilayers have attracted great attention owing to their unique interlayer topology. Using first-principles calculations, we theoretically investigate vdW heterobilayers composed of a Janus gallium chalcogenide monolayer Ga2XY (X, Y = S, Se, Te) and a ferromagnetic chromium bromide monolayer (CrBr3) to disclose the intrinsic coupling of out-of-plane asymmetric 2D ferroelectricity and ferromagnetism. The Ga2STe/CrBr3and Ga2SeTe/CrBr3vdW heterobilayers exhibit metallic properties, while the Ga2SSe/CrBr3vdW heterobilayer shows semiconducting properties with type-II band alignment. Band deformation and splitting are greatly governed by coupled electric and magnetic dipoles with different alignments. The coupling strength between 2D ferroelectricity and ferromagnetism is demonstrated by the binding energy, charge transfer, and potential energy difference. Interlayer coupling regulated by dipole inversion leads to the diversity of partial charge density. Such heterobilayers integrated with 2D ferroelectricity and ferromagnetism are beneficial for the application in multifunctional optoelectronics and spintronics.