报告题目：Line and point defects in MBE-grown ultrathin MSe2 (M = Mo, W) films
报 告 人：Maohai Xie,Physics Department, The University of Hong Kong, Pokfulam Road, Hong Kong
报告时间：2015年7月13日10：30
报告地点：理科楼三楼报告厅（C302）
报告摘要：Ultrathin films of transition metal dichalcogenides (TMDs) are attracting intensive research interests lately for their promises in 2D electronics, optoelectronics, spin and valley electronics. The midgap states at the edges of clustered TMDs are pivotal for catalytic applications of the TMDs, such as hydrodesulfurization reaction. In this talk, I shall describe our latest efforts of molecular-beam epitaxy (MBE) of ultrathin films of MSe2 (M = Mo or W) and surface characterizations by scanning tunneling microscopy (STM). We reveal not only the growth characteristics as well as intrinsic properties of MSe2 monolayer and bilayer, but also the line and point defects in as-grown films. Firstly, dense networks of inversion domain boundary (DB) defects are seen to persist in as-grown MoSe2 giving rise to midgap states being modulated by the moiré potential and the quantum size effect. The formation of such defects is closely dependent on the MBE conditions, allowing one to achieve a varying density of the DB defects for different application purposes. Second, a donor defect and its ionization by the STM tip induced electric field are evidenced, providing important hint on the dielectric properties of the material. Finally, point defects have induced quasi-particle interference patterns in monolayer WSe2 involving the Q-valleys in the conduction band due to the spin-conserving scattering processes, while spin-flipping intervalley scattering is absent. The latter affirms spin-valley coupling and large spin-splitting at the Q valleys in monolayer WSe2. Inefficient spin-flipping scattering implies long valley and spin lifetime, which is a key figure of merit for valley-spintronic applications.
Reference:
“Molecular-beam epitaxy of monolayer and bilayer WSe2: a scanning tunneling microscopy / spectroscopy study and deduction of exciton binding energy”, H.J. Liu, L. Jiao, L. Xie, F. Yang, J.L. Chen, W.K. Ho, C.L. Gao, J.F. Jia, X.D. Cui, M.H. Xie, 2D Mater. 2, 034004 (2015)
2. “Line and point defects in MoSe2 bilayer studied by scanning tunneling microscopy and spectroscopy”, H.J. Liu, H. Zheng, F. Yang, L. Jiao, J.L. Chen, W.K. Ho, C.L. Gao, J.F. Jia, M.H. Xie, ACS Nano. 9, 6619 (2015)
3. “Molecular-beam epitaxy of monolayer MoSe2: growth characteristics and domain boundary formation”, L. Jiao, H.J. Liu, J.L. Chen, Y. Yi, W.G. Chen, Y. Cai, J.N. Wang, X.Q. Dai, N. Wang, W.K. Ho, M.H. Xie, New J. Phys. 17, 053023 (2015)
4. “Dense network of one-dimensional midgap metallic modes in monolayer MoSe2 and their spatial undulations” H.J. Liu, L. Jiao, F. Yang, Y. Cai, X. Wu, W.K. Ho, C.L. Gao, J.F. Jia, N. Wang, H. Fan, W. Yao, M.H. Xie, Phys. Rev. Lett. 113, 066105 (2014)
