Domain-dependent strain and stacking in two-dimensional van der Waals ferroelectrics
Chuqiao Shi, Nannan Mao, Kena Zhang, Tianyi Zhang, Ming‐Hui Chiu, Kenna Ashen, Bo Wang, Xiuyu Tang, Galio Guo, Shiming Lei, Long‐Qing Chen, Ye Cao, Xiaofeng Qian, Jing Kong, Yimo Han
Abstract
Van der Waals (vdW) ferroelectrics have attracted significant attention for their potential in next-generation nano-electronics. Two-dimensional (2D) group-IV monochalcogenides have emerged as a promising candidate due to their strong room temperature in-plane polarization down to a monolayer limit. However, their polarization is strongly coupled with the lattice strain and stacking orders, which impact their electronic properties. Here, we utilize four-dimensional scanning transmission electron microscopy (4D-STEM) to simultaneously probe the in-plane strain and out-of-plane stacking in vdW SnSe. Specifically, we observe large lattice strain up to 4% with a gradient across ~50 nm to compensate lattice mismatch at domain walls, mitigating defects initiation. Additionally, we discover the unusual ferroelectric-to-antiferroelectric domain walls stabilized by vdW force and may lead to anisotropic nonlinear optical responses. Our findings provide a comprehensive understanding of in-plane and out-of-plane structures affecting domain properties in vdW SnSe, laying the foundation for domain wall engineering in vdW ferroelectrics.