How Substitutional Point Defects in Two-Dimensional WS₂ Induce Charge Localization, Spin–Orbit Splitting, and Strain

Abstract

Control of impurity concentrations in semiconducting materials is essential to device technology. Because of their intrinsic confinement, the properties of two-dimensional semiconductors such as transition metal dichalcogenides (TMDs) are more sensitive to defects than traditional bulk materials. The technological adoption of TMDs is dependent on the mitigation of deleterious defects and guided incorporation of functional foreign atoms. The first step toward impurity control is the identification of defects and assessment of their electronic properties. Here, we present a comprehensive study of point defects in monolayer tungsten disulfide (WS 2 ) grown by chemical vapor deposition using scanning tunneling microscopy/spectroscopy, CO-tip noncontact atomic force microscopy, Kelvin probe force spectroscopy, density functional theory, and tight-binding calculations. We observe four different substitutional defects: chromium (Cr W ) and molybdenum (Mo W ) at a tungsten site, oxygen at sulfur sites in both top and bottom layers (O S top/ bottom), and two negatively charged defects (CD type I and CD type II). Their electronic fingerprints unambiguously corroborate the defect assignment and reveal the presence or absence of in-gap defect states. Cr W forms three deep unoccupied defect states, two of which arise from spin− orbit splitting. The formation of such localized trap states for Cr W differs from the Mo W case and can be explained by their different d shell energetics and local strain, which we directly measured. Utilizing a tight-binding model the electronic spectra of the isolectronic substitutions O S and Cr W are mimicked in the limit of a zero hopping term and infinite on-site energy at a S and W site, respectively. The abundant CDs are negatively charged, which leads to a significant band bending around the defect and a local increase of the contact potential difference. In addition, CD-rich domains larger than 100 nm are observed, causing a work function increase of 1.1 V. While most defects are electronically isolated, we also observed hybrid states formed between Cr W dimers. The important role of charge localization, spin−orbit coupling, and strain for the formation of deep defect states observed at substitutional defects in WS 2 as reported here will guide future efforts of targeted defect engineering and doping of TMDs.

In Situ Oxygen Doping of Monolayer MoS2 for Novel Electronics

“…The uniformly distributed hexagonal halo patterns in Figure 3b are assigned to O S substitutional sites in the MoS 2 crystal lattice, in agreement with previous reports. [15,16,18] Importantly, two different halo patterns having obvious contrasts than the background MoS 2 lattice are visualized. Our simulated STM images based on density-functional theory (DFT) calculations (details in the Experimental Section) show that these two features, i.e., the darker/brighter, correspond to substitutional O atom in the top/ bottom S layer (O S-top /O S-bottom ), as illustrated in the suggested atomic structures and simulated STM images in Figure 3d,e.…”

Resonant and bound states of charged defects in two-dimensional semiconductors

“…1(b)], which we refer to as "CDs" and have identified in the figure with red dashed circles. [18] They are among the most abundant point defects in our samples but their density varies considerably between different CVD preparations. Similar STM contrast has been observed in CVD-grown MoS 2 [19], MOCVD-and MBE-grown WSe 2 [20,21], MBE-grown MoSe 2 [21] and natural bulk MoS 2 (0001) [22].…”

“…Non-contact atomic force microscopic (nc-AFM) imaging with a carbon monoxide (CO) functionalized tip [23] indicates that the charged defect is a chalcogen substitution [18]. The chemical origin of the CDs is yet unknown.…”

“…The chemical origin of the CDs is yet unknown. Possible candidates include a CH or N substitution at the S site [18]. These substitutions introduce unoccupied defect states close the valence band edge of WS 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).…”