Atomic and Electronic Structure of Strongly Charged Domain Walls in van der Waals {\alpha}-In$_2$Se$_3$
Here, we use atomic resolution scanning transmission electron microscopy (STEM) and first principles calculations to study the atomic and electronic structure of strongly charged domain walls in $\alpha$-In$_2$Se$_3$. STEM imaging and density functional theory (DFT) show that head-to-head (HH) domain walls contain a layer of nonpolar $\beta$-In$_2$Se$_3$, whereas tail-to-tail (TT) domain walls are atomically abrupt. We apply 4D STEM and multislice electron ptychography to map ferroelectric domains in 2D and 3D, showing that nearly $180^\circ$ domain walls exhibit complex, curved 3D structures that differ from ideal $180^\circ$ structures. Band structure calculations show localized conducting states within a $\sim$ 1 nm thick layer at both HH and TT domain walls, such as a midgap state at the $\beta$ layer of the HH domain wall. These properties make strongly charged domain walls in $\alpha$-In$_2$Se$_3$ excellent candidates for realizing 2D electron or hole gases and domain wall engineering in van der Waals ferroelectrics.