2020同步年報

012 ACTIVITY REPORT 2020 To solve the contradiction between experiment and the prediction of calculation from first principles, Der-Hsin Wei (NSRRC), Yann-Wen Lan (National Taiwan Normal Universi- ty) and their teams investigated the magnetic domain con- figuration and chemical states of Co/MoS 2 with a photo- emission electron microscope (PEEM) end station located at TLS 05B2 . An ultrathin film of Co was deposited onto flakes of SiO 2 -supported monolayer MoS 2 . The spatial distribution of the photo-emitted electrons under the X-ray magnet- ic-circular-dichroism (XMCD) effect was resolved with the PEEM, allowing the observation of magnetic domains in the Co layer. The authors extracted two micro-area spectra in the photoemission image according to their intensity vari- ation as a function of photon energy. Wei and Lan found that the recorded spectra showed a typical XMCD signa- ture-opposite enhancement at Co L 3 and L 2 resonances in Fig. 1(c) that has indeed a magnetic origin. Their next task was to examine the relevance of the MoS 2 crystalline structure for the domain formation. With careful control of the degree of crystallinity in monolayer MoS 2 , they adjusted the lateral dimensions of the magnetic domains from tens of micrometers to sub-micrometers. Because CVD-grown triangular MoS 2 flakes are typically treated as a single crystalline grain, and as the edge of the grain is in either an armchair or a zig-zag configuration, they found that well defined crystallinity in the MoS 2 layer would not only promote the magnetization alignment in a Co layer but also affect how the domains are divided. In summary, Wei and Lan studied the magnetic domain configuration and chemical states of Co/MoS 2 . They con- firmed that an ultrathin Co film deposited on monolayer MoS 2 can form ferromagnetic domains of micrometer size. Furthermore, the magnetization and the boundaries of domains have preferred directions or paths that are parallel to either the zig-zag or the armchair directions of the MoS 2 crystal structure. According to the evidence from X-ray photoelectron spectra of charge donation at Co/MoS 2 , they suggested that the orbital hybridization at the interface is what distinguishes the magnetic properties of Co/MoS 2 here and Fe/MoS 2 reported earlier. Their work clarifies the puzzle existing from previous experiments and calculations, and provides micro-magnetic and micro-spectral evidence that consolidates the knowledge required to build func- tional heterojunctions based on 2D materials. (Reported by Cheng-Maw Cheng) This report features the work of Der-Hsin Wei, Yann-Wen Lan and their collaborators published in Nanoscale Horiz. 5 , 1058 (2020). TLS 05B2 EPU – PEEM • XPS • Materials Science, Condensed-matter Physics Reference 1. C.-I Lu, C.-H. Huang, K.-H. Ou Yang, K. B. Simbulan, K.-S. Li, F. Li, J. Qi, M. Jugovac, I. Cojocariu, V. Feyer, C. Tusche, M.- T. Lin, T.-H. Chuang, Y.-W. Lan, D.-H. Wei, Nanoscale Horiz. 5 , 1058 (2020).