Analysis of HR-STEM theory for thin specimen

Area of Science:

• Materials Science
• Physics
• Electron Microscopy

Background:

• Scanning Transmission Electron Microscopy (STEM) is a powerful imaging technique.
• Traditional STEM imaging models often rely on approximations like the weak phase approximation (WPA) or linear contrast transfer functions (CTF), which have limitations.
• STEM imaging is inherently nonlinear, posing challenges for accurate modeling.

Purpose of the Study:

• To develop a compact, nonlinear mathematical model for STEM imaging (including bright field and dark field).
• To provide a description valid for thin samples without using the WPA or complex simulations.
• To clarify the underlying objects corresponding to different STEM imaging modes.

Main Methods:

• Derivation of a nonlinear mathematical model for STEM imaging.
• The model describes STEM images as cross-correlations of sample-independent and sample-dependent functions.
• Validation using simulated STEM images.

Main Results:

• The derived model accurately describes STEM imaging for thin samples without WPA.
• The model reveals that STEM images can be represented by two cross-correlation terms.
• These terms correspond to known imaging modes like WPA and annular dark field (ADF).
• A significant reduction in computation time (3 orders of magnitude) was achieved.

Conclusions:

• The developed nonlinear model offers a more accurate and efficient description of STEM imaging.
• Understanding the model's components clarifies the nature of different STEM techniques.
• This work provides a foundation for improved STEM image analysis and simulation.