Defect Passivation of 2D Semiconductors by Fixating Chemisorbed Oxygen Molecules via h‐BN Encapsulations

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.