Mind the Gap-Imaging Buried Interfaces in Twisted Oxide Moirés

The ability to tune electronic structure in twisted stacks of layered, two-dimensional (2D) materials has motivated the exploration of similar moiré physics with stacks of twisted oxide membranes. Due to the intrinsic three-dimensional nature of bonding in many oxides, achieving atomic-level coupling is significantly more challenging than in 2D materials. Although clean interfaces with atomic-level proximity have been demonstrated in bulk ceramic bicrystals using high-temperature and high-pressure processing to facilitate atomic diffusion that flattens rough interfaces, such conditions are not readily accessible when bonding oxide membranes. This study shows how topographic mismatch from surface roughness of the membranes restricts atomic-scale proximity at the interface to isolated patches even after contaminants and amorphous interlayers are eliminated. The reduced ability of 2D materials to conform to a membrane's step-terrace topography also limits atomic-scale contact. In all these material systems, the interface morphology is best characterized using cross-sectional imaging and is necessary to corroborate investigations of interlayer coupling. When imaging the stacked membranes in projection, conventional through-focal imaging is found to be insensitive to the buried interface, whereas electron ptychography reliably resolves structural variations on the order of a nanometer. These findings highlight interface roughness as a key challenge for oxide twistronics.