Unravelling the effect of sulfur vacancies on the electronic structure of MoS2 crystal

Molybdenum disulfide (MoS2) is one of transition metal dichalcogenides (TMDCs) two-dimensional layered semiconductor with great potentials in electronics, optoelectronics, and spintronic devices. The sulfur vacancies in MoS2 are the most prevalent defects. However, the effect of sulfur vacancies on the electronic structure of MoS2 is still in dispute. Here we experimentally and theoretically investigated the effect of sulfur vacancies in MoS2. The vacancies were intentionally introduced by the thermal annealing of MoS2 crystals in a vacuum environment. Angle-resolved photoemission spectroscopy (ARPES) was used directly to observe the electronic structure of MoS2 single crystals. The experimental result distinctly revealed the appearance of an occupied defect state just above the valence band maximum (VBM) and the upward shift of VBM after creating sulfur vacancies. In addition, density functional theory (DFT) calculations also confirmed the existence of occupied defect state close to VBM as well as two deep unoccupied states induced by sulfur vacancies. Our results provide evidence to contradict that sulfur vacancies are the origin of n-type behavour in MoS2. This work provides a rational strategy for tuning the electronic structures of MoS2.