2020同步年報

Physics and Materials Science 011 A fter the discovery of graphene, monolayer MoS 2 —a layered van der Waals semiconducting transition- metal dichalcogenide (TMD)—has emerged as another two- dimensional (2D) material prototype, which can be ob- tained by ex-situ exfoliation or in-situ chemical vapor depo- sition (CVD). Bulk MoS 2 has an indirect bandgap, which becomes a direct bandgap when its thickness decreases to a monolayer. Moreover, because of the strong spin-orbit coupling and the absence of inversion symmetry in the monolayer regime, spin splitting arises at the boundaries of the surface Brillouin zone, specifically, at points K and - K , to conserve the time-reversal symmetry. Such a unique band structure provides a possibility to encode information through the material valley pseudospin. Valley-based elec- tronics is described as valleytronics, a name inspired after another famous field, spintronics. Building Spintronic Devices with Functional Heterojunctions Magnetic anisotropy is a material preference that involves magnetization aligned along a specific direction and provides a basis for spintronic devices. Clarifying the ferromagnetic behavior in 2D materials can provide a knowledge that is required to build functional heterojunctions in the future. MoS 2 can serve also as a spacer in a spin-valve device to exploit its semiconducting nature and its stable spin polar- ization in the out-of-plane direction. A MoS 2 -based het- erostructure is encouraging, but the large discrepancy in magnetoresistance between measurement and prediction indicates that we have yet to identify all factors relevant to the spin-dependent transport in TMD-based spin valves. A direct investigation of the fundamental magnetic proper- ties of ferromagnetic (FM)–TMD heterojunctions is believed to be informative but remains scattered. An experimental study of a Fe/MoS 2 heterojunction found, however, that deposited Fe aggregates into nanoparticles with no sign of magnetic coupling to MoS 2 . Co/MoS 2 was suggested to be different, based on a prediction of calculations from first principles, because the energetically favored Co-S bonding at the Co/MoS 2 interface would lead to a spin imbalance on the MoS 2 side. Fig. 1 : XMCD. (a) Schematic diagram describing the principle of XMCD. (b) Schematic diagram illustrating the experimental setup. (c) XMCD image of co- balt (9 ML) on monolayer MoS 2 . The inset shows the direction of incident light. (d) The corresponding m-XAS of regions A and B, the positions of which are marked in (c), are shown. The bottom spectrum illustrates the asymmetric nature of spectra A and B, that proves the grey-scale contrast in (c) to be a consequence of the XMCD effect. [Reproduced from Ref. 1]