Operando electron microscopy investigation of
polar domain dynamics in twisted 2D materials
Control of interlayer stacking angle in two-dimensional (2-D) van der Waals (vdW) heterostructure enables one to engineer the crystal symmetry to imprint novel functionality[1]. By stacking two layers of transition metal dichalcogenides (TMD) with designed twist angle, one can break the inversion symmetry and thereby develop vertical electric polarization. The direction of the electric polarization can be switched electrically, suggesting that the twisted bilayer TMD can host ferroelectricity. Such ferroelectricity reported in twisted bilayer vdW system is distinguished from conventional ferroelectrics in that the lateral sliding of the constituent layers induces vertical electric polarizations. Here we employ operando transmission electron microscopy (TEM) to investigate the domain dynamics in 2-D vdW ferroelectrics. Operando TEM technique enables one to examine the structural change in the environment that mimics the electrical device operating condition. We find the domain dynamics in response to vertical electric fields is governed by the consecutive domain wall pinning-depinning process as noted by Barkhausen noises in the polarization hysteresis loop[2]. Moreover, exploiting stroboscopic operando TEM on the vdW ferroelectrics, we directly measured the domain wall velocity of 300 mm/s which is found to be limited by various disorders present in the specimens[2]. Aberration corrected scanning transmission electron microscopy analysis identifies the microstructural origin for the domain wall pinning, providing structural insight on how to improve the switching speed of vdW ferroelectrics.
[1] H. Yoo et al., Atomic and electronic reconstruction at the van der Waals interface in twisted bilayer graphene, Nat. Mater. 18, 448 (2019)
[2] K. Ko et al., Operando electron microscopy investigation of polar domain dynamics in twisted van der Waals homobilayers, Nat. Mater. 22, 992 (2023)