Carrier Trapping by Oxygen Impurities in Molybdenum Diselenide

TL;DR

This study investigates how oxygen impurities trap photo-excited carriers in MoSe2, revealing defect states responsible for trapping through ultrafast spectroscopy and first-principles calculations, aiding defect engineering in transition metal dichalcogenides.

Contribution

It demonstrates the creation and characterization of oxygen defect states in MoSe2 and their role in carrier trapping using combined experimental and theoretical methods.

Findings

Oxygen impurities increase carrier trapping in MoSe2.

Transient absorption signals correlate with oxygen defect density.

Density functional theory shows oxygen at Mo vacancies creates mid-gap states.

Abstract

Understanding defect effect on carrier dynamics is essential for both fundamental physics and potential applications of transition metal dichalcogenides. Here, the phenomenon of oxygen impurities trapping photo-excited carriers has been studied with ultrafast pump-probe spectroscopy. Oxygen impurities are intentionally created in exfoliated multilayer MoSe2 with Ar+ plasma irradiation and air exposure. After plasma treatment, the signal of transient absorption first increases and then decreases, which is a signature of defect capturing carriers. With larger density of oxygen defects, the trapping effect becomes more prominent. The trapping defect densities are estimated from the transient absorption signal, and its increasing trend in the longer-irradiated sample agrees with the results from X-ray photoelectron spectroscopy. First principle calculations with density functional theory reveal that oxygen atoms occupying Mo vacancies create mid-gap defect states, which are responsible for the carrier trapping. Our findings shed light on the important role of oxygen defects as carrier trappers in transition metal dichalcogenides, and facilitates defect engineering in relevant material and device applications.