Sub‐nm 2 Ferroelectric Domains via Charged 180° Walls in ZrO 2
Nashrah Afroze, Hamoon Fahrvandi, Guodong Ren, Pawan Kumar, Christopher Nelson, Sarah Lombardo, Mengkun Tian, Ping‐Che Lee, Jiayi Chen, Manifa Noor, Kisung Chae, Sanghyun Kang, Prasanna Venkat Ravindran, Matthew Bergschneider, Gwan Yeong Jung, Pravan Omprakash, Gardy K. Ligonde, Nujhat Tasneem, Dina Triyoso, Steven Consiglio, Kanda Tapily, Robert Clark, Gert Leusink, Jayakanth Ravichandran, Shimeng Yu, Andrew R Lupini, Andrew C Kummel, Kyeongjae Cho, Duk‐Hyun Choe, Nazanin Bassiri‐Gharb, Josh Kacher, Rohan Mishra, Jun Hee Lee, Asif KhanABSTRACT
Flat phonon bands in fluorite ferroelectrics ( or ) shrink polar domains laterally to an irreducible half‐unit‐cell width (0.27 nm) within which the vertical arrangement of dipoles is expected to remain uniform. Here, we report on the direct observation of nonuniform and nearly discrete vertical arrangements of dipoles in thin films consisting of closely spaced head‐to‐head (HH) and tail‐to‐tail (TT) charged 180 walls, each exhibiting a distinct bulk‐like structure. These charged domain walls (CDWs) further compress the irreducibly narrow, laterally stacked domains vertically to a thickness of 1–2.75 nm, yielding in‐plane domains with sub‐ footprints—among the smallest ever reported for any ferroelectric material. The HH and TT walls form due to their flat longitudinal optical (LO) polar bands, and the former are electrostatically stabilized by bound‐charge compensation via interstitial oxygen atoms, which act as natural structural defects at the HH walls. Moreover, these walls are predicted to be conducting and to exhibit ultralow propagation barriers, with HH walls (1.6 meV) being far more mobile than TT walls (22.3 meV), indicating strong potential for low‐voltage, domain‐wall‐based nanoelectronics.