Scanning transmission electron microscopy under controlled low-pressure atmospheres

Area of Science:

• Materials Science
• Surface Science
• Electron Microscopy

Background:

• Transmission electron microscopy (TEM) requires vacuum to prevent electron-gas interactions.
• Residual gas molecules in TEM can interact with the electron beam, potentially altering sample structures.
• Typical TEM pressures (10^-7 mbar) allow for significant gas molecule interactions.

Purpose of the Study:

• To investigate the effect of controlled gas environments on graphene samples in a modified scanning TEM (STEM).
• To assess the potential for gas-assisted cleaning and etching of carbon nanomaterials under atomic resolution conditions.
• To understand the influence of residual gases on sample integrity during high-resolution imaging.

Main Methods:

• Utilized a modified Nion UltraSTEM 100 scanning TEM with base pressure of 2×10^-10 mbar.
• Introduced controlled amounts of gases (air, O₂, H₂O, H₂, N₂) into the objective area up to 10^-6 mbar.
• Maintained atomic resolution imaging conditions throughout the experiments.

Main Results:

• Air exposure effectively cleaned graphene contamination without damaging the lattice.
• Oxygen (O₂) and water vapor (H₂O) demonstrated etching effects on graphene.
• Oxygen exhibited nearly twice the etching efficiency of water, attributed to its molecular composition.
• Hydrogen (H₂) and nitrogen (N₂) environments showed no significant etching effect.
• Demonstrated that residual gas environments significantly influence TEM observations.

Conclusions:

• Controlled gas environments in STEM can be utilized for effective cleaning and chemical etching of carbon-based materials.
• Oxygen is a particularly efficient etchant for graphene under these conditions.
• The findings underscore the importance of managing residual gas composition in TEM for accurate material analysis and controlled modification.