Point Defects and Localized Excitons in 2D WSe₂.

Area of Science:

• Materials Science
• Condensed Matter Physics
• Nanotechnology

Background:

• Point defects in 2D materials are crucial for applications, but their identification remains challenging.
• Previous studies suggested tungsten (W) vacancies as the dominant defect in 2D WSe2, contrary to theoretical predictions favoring chalcogen vacancies.
• Single-photon emission (SPE) in 2D WSe2 has been experimentally observed, with point defects hypothesized to be essential for exciton localization.

Purpose of the Study:

• To accurately identify the predominant point defects in CVD-grown 2D WSe2.
• To investigate the role of these defects in the optical properties, specifically single-photon emission (SPE).
• To determine which specific point defects are responsible for the observed SPE in 2D WSe2.

Main Methods:

• First-principles calculations to model defect structures and properties.
• Scanning tunneling microscopy (STM) and scanning transmission electron microscopy (STEM) for experimental defect characterization.
• GW-Bethe-Salpeter equation calculations to predict optical properties and exciton localization.

Main Results:

• Tungsten (W) vacancies were found to be absent in CVD-grown 2D WSe2.
• Oxygen-passivated selenium (Se) vacancies (OSe) and oxygen interstitials (Oins) were identified as the prevalent defects.
• Only oxygen interstitials (Oins) were predicted to cause localized excitons within the energy range of previously observed SPE.

Conclusions:

• Oxygen interstitials (Oins) are the likely source of single-photon emission (SPE) in 2D WSe2.
• The findings challenge previous assumptions about W vacancies being the dominant defect.
• This research provides insights for realizing SPE in related 2D materials and guides future experimental searches for SPE in 2D WSe2.