Comparison of Experimental STEM Conditions for Fluctuation Electron Microscopy

Area of Science:

• Materials Science
• Electron Microscopy
• Condensed Matter Physics

Background:

• Variable-resolution fluctuation electron microscopy (VR-FEM) is a powerful technique for analyzing materials at the nanoscale.
• Understanding the impact of experimental parameters on VR-FEM data is crucial for accurate interpretation.
• Probe size variation is a key experimental condition affecting data quality.

Purpose of the Study:

• To investigate the influence of different probe generation methods on VR-FEM data.
• To compare the effects of semi-convergence angle variation versus defocus on normalized variance.
• To theoretically analyze beam coherence and its impact on VR-FEM measurements.

Main Methods:

• Acquisition of VR-FEM data on amorphous silicon and PdNiP under controlled electron doses.
• Generation of STEM probes with varying sizes using semi-convergence angle adjustments and defocus.
• Theoretical analysis of beam coherence using geometrical optics.
• Fitting experimental beam profiles to validate theoretical models.

Main Results:

• Defocusing resulted in a lower normalized variance compared to varying the semi-convergence angle.
• The trend of normalized variance with probe size differed significantly between the two methods.
• Theoretical analysis and experimental fits confirmed the link between probe coherence and illumination conditions.

Conclusions:

• Probe defocusing is a more effective method for controlling normalized variance in VR-FEM.
• Optimizing exposure time for nanobeam diffraction patterns can further enhance data quality.
• The study provides insights into optimizing VR-FEM experiments for reliable materials characterization.