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Understanding defect effect on carrier dynamics is essential for bothfundamental physics and potential applications of transition metaldichalcogenides. Here, the phenomenon of oxygen impurities trappingphoto-excited carriers has been studied with ultrafast pump-probe spectroscopy.Oxygen impurities are intentionally created in exfoliated multilayer MoSe2 withAr+ plasma irradiation and air exposure. After plasma treatment, the signal oftransient absorption first increases and then decreases, which is a signatureof defect capturing carriers. With larger density of oxygen defects, thetrapping effect becomes more prominent. The trapping defect densities areestimated from the transient absorption signal, and its increasing trend in thelonger-irradiated sample agrees with the results from X-ray photoelectronspectroscopy. First principle calculations with density functional theoryreveal that oxygen atoms occupying Mo vacancies create mid-gap defect states,which are responsible for the carrier trapping. Our findings shed light on theimportant role of oxygen defects as carrier trappers in transition metaldichalcogenides, and facilitates defect engineering in relevant material anddevice applications.

Carrier Trapping by Oxygen Impurities in Molybdenum Diselenide