Electronic Band Structure Study of Millimeter-Sized Mono-Layer WSe2 using Angle-Resolved Photoemission Spectroscopy

The transition metal dichalcogenide (TMDC) semiconductors MX2 (M = Mo, W; X = S, Se) exhibit unique properties in the mono- or few-layer forms. The monolayer MX2 crystals of TMDC have also showed great potential in the next-generation electronics, optoelectronics and spintronics. Of particular interest is the monolayer WSe2 which has the largest spin-splitting in the valence band at the Brillouin zone corner among all the MX2 semiconductors. We will present electronic structure study on pristine and alkali metal-doped WSe2 monolayer by high resolution angle-resolved photoemission spectroscopy (ARPES). By utilizing an improved exfoliation method, we have prepared milimeter-sized mono-layer WSe2 with a high quality. We found experimental evidence of the direct gap transition in monolayer WSe2 as predicted by density functional theory. The observed direct gap and spin splitting of the upper valence band at K point is ~ 1.1 eV and 500 meV, respectively. We also found some additional and interesting electronic state features which are to be discussed within the frameworks of quantum well state and monolayer WSe2-substrate interaction. Our study may pave a way for engineering band structure and tuning electronic properties in mono-layer TMDC semiconductor.