Controllable valley magnetic response in phase-transformed tungsten diselenide

Achieving valley pseudospin with large polarization is crucial in the implementation of quantum information applications.Transition metal dichalcogenides(TMDC)with different phase structures provide an ideal platform for valley modulation.The valley splitting has been achieved in hybrid phase WSe_(2),while its valley polarization remains unstudied.Magnetic field controllable valley polarization is explored in WSe_(2)with coexistence of H and T phases by an all-optical route.A record high valley polarization of 58.3%is acquired with a 19.9%T phase concentration under a 4-T magnetic field and nonresonant excitation.The enhanced valley polarization is dependent on the phase component and shows various increasing slopes,owing to the synergy between the T phase WSe_(2)and the magnetic field.The magnetic field controlled local magnetic momentums are revealed as the mechanism for the large valley polarization in H∕T-WSe_(2).This speculation is also verified by theoretical simulations of the nonequilibrium spin density.These results display a considerable valley magnetic response in phase-engineered TMDC and provide a large-scale scheme for valley polarization applications.

Direct synthesis of moirésuperlattice through chemical vapor deposition growth of monolayer WS_(2)on plasma-treated HOPG

Vertical van der Waals(vdW)heterostructures composed of two-dimensional(2D)layered materials have recently attracted substantial interests due to their unique properties.However,the direct synthesis of moirésuperlattice remains a great challenge due to the difficulties in heterogeneous nucleation on smooth vdW surfaces.Here,we report a controllable chemical vapor deposition growth of complete monolayer WS_(2)on highly ordered pyrolytic graphite(HOPG)substrates through the plasma pretreatment.The results show that the morphologies of the grown WS_(2)have a strong dependence on the plasma parameters,including gas composition,source power,and treatment time.It is found that the surface C–C bonds are broken in the plasma pretreated HOPG,and the formed small clusters can act as the nucleation sites for the subsequent growth of WS_(2).Moreover,the height of clusters dominates the growth mode of WS_(2)islands.A transition from a 2D mode to three-dimensional(3D)growth mode occurs when the height is higher than the interlayer spacing of the heterostructure.Besides,diverse moirésuperlattices with different twist angles for WS_(2)/HOPG heterostructures are observed,and the formation mechanism is further analyzed by firstprinciples calculations.