Considerations for extracting moir\'e-level strain from dark field intensities in transmission electron microscopy

TL;DR

This paper analyzes and improves Bragg interferometry (BI) in 4D-STEM for extracting moiré-scale strain, enhancing spatial resolution and expanding the range of twisted structures that can be studied.

Contribution

The paper identifies limitations in current BI techniques and proposes methods to improve spatial resolution and applicability to a broader range of twisted bilayer structures.

Findings

Enhanced spatial resolution in BI imaging.

Broader applicability to various twisted bilayer structures.

Identification of computational and acquisition limitations.

Abstract

Bragg interferometry (BI) is an imaging technique based on four-dimensional scanning electron microscopy (4D-STEM) wherein the intensities of select overlapping Bragg disks are fit or more qualitatively analyzed in the context of simple trigonometric equations to determine local stacking order. In 4D-STEM based approaches, the collection of full diffraction patterns at each real-space position of the scanning probe allows the use of precise virtual apertures much smaller and more variable in shape than those used in conventional dark field imaging, such that even buried interfaces marginally twisted from other layers can be targeted. A coarse-grained form of dark field ptychography, BI uses simple physically derived fitting functions to extract the average structure within the illumination region and is therefore viable over large fields of view. BI has shown a particular advantage for selectively investigating the interlayer stacking and associated moir\'e reconstruction of bilayer interfaces within complex multi-layered structures. This has enabled investigation of reconstruction and substrate effects in bilayers through encapsulating hexagonal boron nitride and of select bilayer interfaces within trilayer stacks. However, the technique can be improved to provide a greater spatial resolution and probe a wider range of twisted structures, for which current limitations on acquisition parameters can lead to large illumination regions and the computationally involved post-processing can fail. Here we analyze these limitations and the computational processing in greater depth, presenting a few methods for improvement over previous works, discussing potential areas for further expansion, and illustrating the current capabilities of this approach for extracting moir\'e-scale strain.