Understanding the interplay of defects, oxygen, and strain in 2D materials for next-generation optoelectronics

Abstract

Two-dimensional transition metal dichalcogenides are leading materials for next-generation optoelectronics, but fundamental problems stand enroute to commercialization. These problems include, firstly, the widely debated defect- and strain-induced origins of intense low-energy broad luminescence peaks (L-peaks) observed at low temperatures. Secondly, the role of oxygen in tuning the properties via chemisorption and physisorption is intriguing but challenging to understand. Thirdly, our physical understanding of the benefits of hexagonal boron nitride (hBN) encapsulation is inadequate. Using a series of samples, we decouple the contributions of oxygen, defects, adsorbates, and strain on the optical properties of monolayer MoS2. The defect origin of the L-peak is confirmed by temperature- and power-dependent photoluminescence (PL) measurements, with a dramatic redshift of ∼130 meV for oxygen-assisted chemical vapour deposition (O-CVD) samples compared with exfoliated samples. Anomalously, the O-CVD samples show high A-exciton PL at room temperature (cf exfoliated), but reduced PL at low temperatures, attributed to the strain-induced direct-to-indirect bandgap crossover in low-defect O-CVD MoS2. These observations are consistent with our density functional theory calculations and are supported by Raman spectroscopy. In the exfoliated samples, the charged O adatoms are identified as thermodynamically favourable defects, and create in-gap states. The beneficial effect of encapsulation originates from the reduction of charged O adatoms and adsorbates. This experimental–theoretical study uncovers the type of defects in each sample, enables an understanding of the combined effect of defects, strain, and oxygen on the band structure, and enriches our understanding of the effects of encapsulation. This work proposes O-CVD as a method for creating high-quality materials for optoelectronics.