The ferromagnetic semiconducting materials with tetrahedral coordination structure have lower crystal field repulsion energy and variable structure, which would be beneficial to achieve high Curie temperature and multiferroics. Based on density functional theory calculations, the monolayer VXCl (X=Te, Se, S) and VMSe2 (M=Al, Ga, In) with tetrahedral coordination structure are predicted to be ferromagnetic semiconductors with high Tc and large magnetocrystalline anisotropy. With the monolayer BiCrSe3 (Tc above 400K) being treated as the representative sample of octahedral materials, we reveal the disadvantages of two different coordination structures in 2D condition, namely tetrahedral and octahedral coordination, and find that modulation of bond angles is effective and feasible to enhance the magnetic exchange of tetrahedral materials. Moreover, the two series of predicted materials have favorable piezoelectric properties. Our work paves a feasible route for finding new low-dimensional ferromagnetic materials with excellent properties.

Based on first-principles calculations, we systematically explored the electronic structures of the ZrS 2 /MoTe 2 heterostructure. The results show that Rashba splitting and type-III band alignment coexist in this heterostructure system. The presence of Rashba spin splitting makes this system of interest for spin-field-effect transistor applications. The effects of biaxial strain and an applied electric field on the electronic structure of a heterostructure were also explored. In the strain range of −2 to 6%, the system keeps the structural stability, and the electronic structure maintains the type-III band alignment. In particular, a crucial change from a type-III to a type-II band alignment occurred under the negative electric field of −0.4 to −0.6 V/Å, which can be beneficial to design multi-purpose devices. The current work predicts that the ZrS 2 /MoTe 2 heterostructure is an excellent candidate for the realization of multiple band arrangement conversion and tunnel-field-effect transistors, which deserves further experimental research.

Two-dimensional (2D) transition metal trihalide (TMX3, X = Cl, Br, I) family has attracted considerable attention in recent years due to the realizations of CrCl3, CrBr3, and CrI3 monolayers. Up to now the main focus of the theoretically predicted TMX3 monolayers has been on the Chern insulator states, which can realize the quantum anomalous Hall effect. Here, using first-principles calculations, we theoretically demonstrate that the stable OsCl3 monolayer has a ferromagnetic ground state and a spinpolarized Dirac point without spin-orbit coupling (SOC), which disappears in the band structure of Janus OsBr1.5Cl1.5 monolayer. We find that OsCl3 exhibits in-plane magnetization when SOC is included. By manipulating the magnetization direction along the C2 symmetry axis of the OsCl3 structure, a gapless half-Dirac semimetal state with SOC can be achieved, which is different from the gapped Chern insulator state.Both semimetal states of OsCl3 monolayer without and with SOC exhibit a linear half-Dirac point (twofold degenerate) with high Fermi velocities. The achievement of the 2D semimetal state with SOC is expected to be found in other TMX3 monolayers, and we confirm it in TiCl3 monolayer. This provides a different perspective to study the band structure with SOC of the 2D TMX3 family.

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The electronic structures and Rashba effect of two-dimensional polar tetragonal perovskites TlSnX3 (X=Cl, Br, I) are investigated by first-principles density functional theory, and intrinsic Rashba effect are found around the...