Long-term stability and tree-ring oxidation of WSe₂ using phase-contrast AFM

Area of Science:

• Materials Science
• Surface Science
• Nanotechnology

Background:

• Tungsten diselenide (WSe2) is a layered material with potential applications.
• Understanding the long-term stability of WSe2 in ambient conditions is crucial for its practical use.
• Oxidative defects can impact the performance and longevity of layered materials.

Purpose of the Study:

• To investigate the long-term evolution of oxidative defects in WSe2 under ambient conditions.
• To establish a correlation between surface features, degradation patterns, and environmental factors.
• To provide insights into the degradation mechanisms and protective strategies for layered materials.

Main Methods:

• Utilized atomic force microscopy (AFM), specifically phase-imaging, for defect analysis.
• Employed topographic thickness measurements to determine flake layer arrangements.
• Corroborated findings with cross-sectional transmission electron microscopy (TEM).

Main Results:

• Phase-imaging AFM effectively identified exposed and covered step-edges in WSe2.
• Exposed step-edges showed degradation over time, while covered ones remained stable.
• Degradation patterns exhibited a "tree-ring" like structure correlated with sun exposure, with darkness slowing the process.
• Similar degradation was observed in MoTe2 and MoS2.

Conclusions:

• Covered step-edges in WSe2 offer long-term stability in ambient conditions.
• Environmental factors, particularly light exposure, accelerate the degradation of exposed WSe2.
• This study provides a comprehensive dataset on WSe2 degradation and highlights methods for controlling it.