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

Area of Science:

• Materials Science
• Condensed Matter Physics
• Nanotechnology

Background:

• Precise control of impurity concentrations is crucial for semiconductor device performance.
• Two-dimensional semiconductors (TDSs), like transition metal dichalcogenides (TMDs), are highly sensitive to defects due to their atomic thinness.
• Technological advancement in TMDs requires understanding and mitigating detrimental defects while enabling functional impurity incorporation.

Purpose of the Study:

• To comprehensively identify and characterize point defects in monolayer tungsten disulfide (WS2) grown via chemical vapor deposition.
• To assess the electronic properties and impact of these defects on the material's behavior.
• To provide insights for targeted defect engineering and doping strategies in TMDs.

Main Methods:

• Utilized a combination of experimental techniques: scanning tunneling microscopy/spectroscopy (STM/S), CO-tip noncontact atomic force microscopy (nc-AFM), and Kelvin probe force spectroscopy (KPFS).
• Employed theoretical calculations: density functional theory (DFT) and tight-binding (TB) models.
• Directly measured local strain associated with defects.

Main Results:

• Identified four substitutional defects: CrW, MoW (at W sites), OS (at S sites, top/bottom), and two types of negatively charged defects (CDs).
• Characterized the electronic fingerprints, confirming defect assignments and revealing in-gap states; CrW defects create deep unoccupied states influenced by spin-orbit splitting and strain.
• Observed significant band bending and work function increase (up to 1.1 V) due to abundant negatively charged defects, and hybrid states in CrW dimers.

Conclusions:

• The electronic properties of WS2 are significantly influenced by specific point defects, with CrW, OS, and CDs exhibiting distinct electronic signatures.
• Charge localization, spin-orbit coupling, and strain are critical factors in the formation of deep defect states in WS2.
• This detailed understanding of defect behavior provides a foundation for future defect engineering and doping strategies in TMDs for tailored electronic applications.