Physicists Observe Sub-1-nm² Ferroelectric Domains in Zirconia Thin Films

A research team from Washington University in St. Louis, the University of Nebraska–Lincoln, and partner institutions observed ferroelectric domains with cross-sectional areas below 1 nm² in zirconia (ZrO₂) thin films, according to a study published in Advanced Materials wiley.comAdvanced Materialswiley.comOpen source ↗. The findings confirm that polar regions can maintain stable polarization down to irreducible crystallographic dimensions.

Fluorite-structured ferroelectrics such as ZrO₂ and HfO₂ possess flat phonon bands that compress polar domains laterally to a half-unit-cell width of 0.27 nm. Using electron microscopy and atomic-scale modeling, the researchers identified discrete vertical dipole configurations bounded by charged 180° head-to-head (HH) and tail-to-tail (TT) domain walls. These charged interfaces compress the domains vertically to a thickness of 1 to 2.75 nm, producing in-plane domain footprints under 1 nm².

Charged Domain Wall Dynamics

The direct observations revealed that HH and TT walls form because of flat longitudinal optical polar bands. Electrostatic stabilization of the HH boundaries occurs through bound-charge compensation provided by interstitial oxygen atoms, which act as native structural defects. Computational analyses showed that these charged boundaries are electrically conducting.

Low Energy Barriers for Nanoelectronics

Calculations indicated that these domain boundaries have ultralow propagation barriers wiley.comAdvanced Materialswiley.comOpen source ↗. The HH walls exhibited an energy barrier of 1.6 meV, making them significantly more mobile than the TT walls, which registered a barrier of 22.3 meV under an applied electric field. The researchers noted that controlling such highly mobile, conducting interfaces offers a path toward low-voltage nanoelectronic devices and dense nonvolatile memory architectures.