Sub-Unit Cell Spatial Resolution of Phase and Polarization Mapping in Scanning Electron Diffraction

TL;DR

This paper demonstrates enhanced spatial resolution in 4DSTEM diffraction analysis, enabling polarization and phase mapping at the sub-unit cell level in BaTiO3, revealing local structural fluctuations within nanoregions.

Contribution

The study introduces a method to improve spatial resolution in 4DSTEM by masking overlapping diffraction regions, allowing unit cell level mapping of polarization and phases.

Findings

Achieved mapping at the size of single projected unit cells.

Observed local fluctuations within nanoregions of BaTiO3.

Revealed different structural characters at various length scales.

Abstract

Diffraction analysis in four dimensional scanning transmission electron microscopy now enables the mapping of local structures including symmetry, strain, and polarization of materials. However, measuring the distribution of these configurations at the unit cell level remains a challenge because most analysis methods require the diffraction disks to be separated, limiting the electron probe sizes to be larger than a unit cell. Here we show improved spatial resolution in mapping the polarization displacement and phases of BaTiO3 sampled at a rate equivalent to the size of the projected unit cells using 4DSTEM. This improvement in spatial resolution is accomplished by masking out the overlapping regions in partially overlapped CBED patterns. By reducing the probe size to the order of single projected unit cells in size, the fluctuations of symmetry and structure inside the rhombohedral symmetry nanoregions inside of tetragonal domains can be observed. With the mapping performed at the unit cell resolution the nano-sized rhombohedral domains break down with local fluctuation, showing different character at different length scales.