Weakly Trapped, Charged, and Free Excitons in Single-Layer MoS2 in the Presence of Defects, Strain, and Charged Impurities

TL;DR

This study investigates how defects, strain, and impurities affect excitons in single-layer MoS2, revealing a defect-bound state unaffected by doping or strain, and identifying substitutional nitrogen atoms as defects.

Contribution

It disentangles intrinsic and extrinsic effects on excitonic properties in MoS2, using combined spectroscopic, microscopic, and theoretical methods to identify defect types and their influence.

Findings

Discovery of a defect-bound exciton state with 20 meV binding energy.

Identification of substitutional nitrogen atoms at sulfur sites as defects.

Demonstration that certain defect states are insensitive to strain and doping.

Abstract

Few- and single-layer MoS2 host substantial densities of defects. They are thought to influence the doping level, the crystal structure, and the binding of electron-hole pairs. We disentangle the concomitant spectroscopic expression of all three effects and identify to what extent they are intrinsic to the material or extrinsic to it, i.e., related to its local environment. We do so by using different sources of MoS2 -- a natural one and one prepared at high pressure and high temperature -- and different substrates bringing varying amounts of charged impurities and by separating the contributions of internal strain and doping in Raman spectra. Photoluminescence unveils various optically active excitonic complexes. We discover a defect-bound state having a low binding energy of 20 meV that does not appear sensitive to strain and doping, unlike charged excitons. Conversely, the defect does not significantly dope or strain MoS2. Scanning tunneling microscopy and density functional theory simulations point to substitutional atoms, presumably individual nitrogen atoms at the sulfur site. Our work shows the way to a systematic understanding of the effect of external and internal fields on the optical properties of two-dimensional materials.