Structural characterization of few-layer chromium trihalides using 4D-STEM

The recent discovery of long-range magnetic ordering in atomically thin chromium trihalides (CrX₃; X = Cl, Br, I) have attracted significant interest due to having the tremendous opportunities for exploring novel magnetic phenomena at the two-dimensional (2D) limit [1]. In this study, we characterize the crystal structure including various stacking sequences of few-layer chromium trichlorides using a four-dimensional scanning transmission electron microscopy (4D-STEM). 4D-STEM allows us to directly quantify local structure, deformation, and different stacking sequences [2]. In order to make a chromium trichloride TEM sample, we exfoliate bilayer or trilayer chromium trichloride flakes and encapsulate them with two few-layer graphene sheets inside the glovebox. The encapsulation is necessary because chromium trichlorides are sensitive to air and moisture. The 2D heterostructure is then transferred onto a quantifoil TEM substrate and studied under TEM. By using 4D-STEM, we are able to identify different stacking orders based on crystallographic symmetry in our bilayer and trilayer chromium trichloride samples. In CrX_3, the stacking sequence dictates the magnetic ordering [3]. Therefore, our study is a crucial step toward controlling the magnetic properties of few-layer chromium trichlorides.



References

1. 1. X. Cai et al., Nano Lett. 19, 3993-3998 (2019).
   
   2. B. H. Savitzky et al., Microscopy and Microanalysis 27, 712–743 (2021).
   
   3. M. Gibertini et al., J. Phys. D: Appl. Phys. 54, 064002 (2021).