Title

Defects in Epitaxial MSe2 (M= Mo, W) - a Scanning Tunneling Microscopy Study

Speaker

Prof. Maohai Xie (谢茂海)

Department of Physics, University of Hong Kong, Hong Kong　　　　　　　　　　　 　

Time

2：00pm, December 7, 2015

Place

Room 9004 at the HFNL building

Professor Maohai Xie received his Ph.D. from Imperial College, University of London in 1994, M.S. from Institute of Semiconductors, CAS in 1988, and B.S from Tianjin University in 1985. Now he is a Professor in Department of Physics, University of Hong Kong. His research interests include atomic kinetics on surfaces, surface properties of materials, molecular-beam epitaxy, scanning probe microscopy and spectroscopy. He has published more than 150 peer reviewed papers and given more than 20 invited talks or colloquiaat international research institutions and conferences.

Abstract

Ultrathin films of transition metal dichalcogenides (TMDs) are of great current research interests for their promises in two-dimensional electronics, optoelectronics, spin and valley electronics. In this talk, I shall describe our latest efforts of molecular-beam epitaxy (MBE) of ultrathin layers of MoSe2 and WSe2. By scanning tunneling microscopy and spectroscopy (STM/S), we reveal both line and point defects in the as-grown films. Dense networks of inversion domain boundaries are seen to persist in as-grown MoSe2, giving rise to midgap states 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. A point 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, a point defect in monolayer WSe2 induces quasi-particle interference, which evidences spin-conserving inter-Q valley scattering. The results affirm spin-valley coupling and large spin-splitting at the Q valleys in monolayer WSe2 and the inefficient spin-flipping scattering implies long valley and spin lifetime, a key figure of merit for valley-spintronic applications.