Abstract
Hexagonal boron nitride (h-BN) is a key ingredient for various 2D van der Waals heterostructure devices, but the exact role of h-BN encapsulation in relation to the internal defects of 2D semiconductors remains unclear. Here, it is reported that h-BN encapsulation greatly removes the defect-related gap states by stabilizing the chemisorbed oxygen molecules onto the defects of monolayer WS(2) crystals. Electron energy loss spectroscopy (EELS) combined with theoretical analysis clearly confirms that the oxygen molecules are chemisorbed onto the defects of WS(2) crystals and are fixated by h-BN encapsulation, with excluding a possibility of oxygen molecules trapped in bubbles or wrinkles formed at the interface between WS(2) and h-BN. Optical spectroscopic studies show that h-BN encapsulation prevents the desorption of oxygen molecules over various excitation and ambient conditions, resulting in a greatly lowered and stabilized free electron density in monolayer WS(2) crystals. This suppresses the exciton annihilation processes by two orders of magnitude compared to that of bare WS(2). Furthermore, the valley polarization becomes robust against the various excitation and ambient conditions in the h-BN encapsulated WS(2) crystals.