Identifying substitutional oxygen as a prolific point defect in monolayer transition metal dichalcogenides

Abstract

Chalcogen vacancies are generally considered to be the most common point defects in transition metal dichalcogenide (TMD) semiconductors because of their low formation energy in vacuum and their frequent observation in transmission electron microscopy studies. Consequently, unexpected optical, transport, and catalytic properties in 2D-TMDs have been attributed to in-gap states associated with chalcogen vacancies, even in the absence of direct experimental evidence. Here, we combine low-temperature non-contact atomic force microscopy, scanning tunneling microscopy and spectroscopy, and state-of-the-art ab initio density functional theory and GW calculations to determine both the atomic structure and electronic properties of an abundant chalcogen-site point defect common to MoSe 2 and WS 2 monolayers grown by molecular beam epitaxy and chemical vapor deposition, respectively. Surprisingly, we observe no in-gap states. Our results strongly suggest that the common chalcogen defects in the described 2D-TMD semiconductors, measured in vacuum environment after gentle annealing, are oxygen substitutional defects, rather than vacancies.

Intrinsic and Extrinsic Defect-Related Excitons in TMDCs

“…Instead, this observation indicates that the defect-related peak has at least partially extrinsic character. We speculate that defect-related excitons interact with oxygen molecules on the surface of MoS2 through passivation of a sulfur vacancy by an oxygen molecule, known to eliminate the midgap states accessible for excitons 35 . The increase of ND after annealing and its subsequent drop after functionalization is consistent with removing and then depositing molecules.…”

“…Third, are defect-related excitons of an extrinsic or intrinsic origin? Previously, the D peak has been ascribed separately to intrinsic structural defects 28,30,31 in TMDCs or to extrinsic impurities on TMDC surface [32][33][34][35] . The final and the most interesting question is whether the D peak can be used to gauge chemical functionalization of TMDCs.…”

Defect Engineering of Photocatalysts for Solar Energy Conversion

“…To analyze the electronic structure of individual defects, noncontact atomic force microscopy (AFM) and scanning tunneling microscopy (STM) have been utilized to check the influence of the point defect confined in MoSe 2 monolayers upon the electronic structures . For instance, the hexagonal lattice of bright features was indexed to the outer chalcogen atoms by CO‐tip noncontact‐AFM, confirming the identification of the lattice sites of MoSe 2 . Moreover, three bright protrusions on hydroxylated TiO 2 surface were observed by STM, indicating the visualization of OVs as point defects, adsorbed H 2 O (ad‐H 2 O) molecules, and OH groups and thus proving a promising way for the visualization investigation of the defects (Figure e).…”

“…To analyze the electronic structure of individual defects, noncontact atomic force microscopy (AFM) and scanning tunneling microscopy (STM) have been utilized to check the influence of the point defect confined in MoSe 2 monolayers upon the electronic structures . For instance, the hexagonal lattice of bright features was indexed to the outer chalcogen atoms by CO‐tip noncontact‐AFM, confirming the identification of the lattice sites of MoSe 2 .…”

Atomistic T -matrix theory of disordered two-dimensional materials: Bound states, spectral properties, quasiparticle scattering, and transport

“…We start by discussing the impact of defects on the DOS, and in particular the defect-induced in-gap states observed in various STM/STS experiments. [10][11][12][13][14][15] Figure 7 shows the DOS for disordered MoS 2 (top) and WSe 2 (bottom) with different types of defects. The dashed vertical lines mark the position of the valence and conduction band edges (black) as well as the Fermi energy (E F ; red dashed line).…”