In this research, we demonstrate the large spin and valley splittings in Janus monolayer WSSe brought by the interaction with a magnetic substrate. The MnO(111) substrate plays the role of ferromagnet since the importance lies in the magnetism of the surface Mn atoms. The responses of both in-plane and out-of-plane spin splittings with the magnetic direction of the substrate are investigated. It is indicated that a large valley splitting of about 410 meV at the $K$ and $K^{\prime }$ points can be produced by the magnetic substrate, which is positively correlated with the vertical magnetic moments of the surface Mn atoms. In addition, a helical gap is opened and the Rashba spin-orbit coupling induced dispersion relation is also significantly modified under the action of proximity coupling with the substrate. Through the effective Hamiltonian method, the derived effective Zeeman fields from the $K/K^{\prime }$ point and the $\mathrm{\Gamma }$ point are greatly different, due to the combination with other factors, such as the difference of the $g$ factor, the existence of charge transfer, or electrostatic interaction with the substrate. The greatly manipulated asymmetric electronic structure and spin/valley splittings can be realized in the spin- and valley-polarized transmission, which are of some significance for the realization of spintronic and valleytronic devices using magnetic substrates and Janus two-dimensional materials.

Due to the breaking of the time reversal symmetry and spatial inversion symmetry, hexagonal ferrovalley materials have intrinsic large valley polarization. Model analysis shows that tuning the two different band gaps of valleys can realize phase transitions between ferrovalley semiconductors, half valley metals, and valley-polarized quantum anomalous Hall semiconductors. Through first-principle calculations, monolayer ferrovalley materials RuXY (X, Y = F, Cl, Br), which exhibit valley splitting at the top valence band and the bottom conduction band, are predicted to achieve this valley-dependent topological phase transition. Due to the different orbital proportions of d orbitals, the valley splitting at the top valence band is much greater than that at the bottom conduction band. Strain can regulate the interaction between orbitals, thus producing valley-dependent band inversion, leading to the quantum spin or valley Hall effect. The chiral edge states are demonstrated under appropriate biaxial strain. The topological phase transition is related to the inversion of the band structure and Berry curvatures at K and K′ valleys. These results have certain significance for the design of two-dimensional valley-dependent quantum materials and the application of valleytronic devices.

A local large-scale reversible phase transition of MoTe ₂ film was accomplished through the heat treatment.

Two-dimensional (2D) molybdenum telluride (MoTe₂) is attracting increasing attention for its potential applications in electronic, optoelectronic, photonic and catalytic fields, owing to the unique band structures of both stable 2H phase and 1T′ phase. However, the direct growth of high-quality atomically thin MoTe₂ with the controllable proportion of 2H and 1T′ phase seems hard due to easy phase transformation since the potential barrier between the two phases is extremely small. Herein, we report a strategy of the phase-controllable chemical vapor deposition (CVD) synthesis for few-layer (<3 layer) MoTe₂. Besides, a new understanding of the phase-controllable growth mechanism is presented based on a combination of experimental results and DFT calculations. The lattice distortion caused by Te vacancies or structural strain might make 1T′-MoTe₂ more stable. The conditions for 2H to 1T′ phase conversion are determined to be the following: Te monovacancies exceeding 4% or Te divacancies exceeding 8%, or lattice strain beyond 6%. In contrast, sufficient Te supply and appropriate tellurization velocity are essential to obtaining the prevailing 2H-MoTe₂. Our work provides a novel perspective on the preparation of 2D transition metal chalcogenides (TMDs) with the controllable proportion of 2H and 1T′ phase and paves the way to their subsequent potential application of these hybrid phases.