Aperture contrast in thick amorphous specimens using scanning transmission electron microscopy.

Area of Science:

• Materials Science
• Microscopy
• Physics

Background:

• Contrast in scanning transmission electron microscopy (STEM) is limited in thick amorphous specimens.
• Electron scattering properties influence image formation and resolution.

Purpose of the Study:

• To determine the optimum collector aperture angle for maximizing contrast in STEM imaging of thick amorphous specimens.
• To provide a method for calculating this optimum angle based on specimen thickness and collection angle.

Main Methods:

• Theoretical calculations of transmitted electron current as a function of specimen thickness and collection angle.
• Experimental verification using biological sections up to 2 microns thick at various beam voltages (50-200 kV).
• Utilizing wide-angle convergent beam diffraction patterns to measure effective aperture angles and a line modulation mode for quantitative contrast measurements.

Main Results:

• Optimum collector aperture angles are typically several times the half-width of the elastic scattering distribution (often >= 10(-1) rad).
• Experimental results with biological sections confirmed theoretical predictions.
• Established a quantitative method for measuring image contrast and comparing electron beam penetrations.

Conclusions:

• An optimum collector aperture angle is critical for achieving high contrast in STEM imaging of thick amorphous samples.
• The study provides a calculable and experimentally verified method for determining this optimum angle.
• This approach enhances image quality and offers a tool for analyzing electron beam-specimen interactions.