The electrostatic properties of a surface significantly influence the electronic and structural characteristics of oxide thin films. To unravel the underlying mechanisms, this study delves into the atomic structure and electrostatic nature of a Pr_0.8Sr_0.2NiO_2+x (0 < x < 1) film using advanced microscopy techniques. In a fully oxidized sample, a polar distortion coupled with octahedral rotations was observed. Interestingly, a more pronounced polar distortion was found in a partially reduced sample, extending approximately three unit cells from the surface. 4D-STEM directly visualized the local atomic electric field surrounding Ni atoms near the surface, revealing distinct valence variations confirmed by EELS. The strong surface reconstruction in the reduced sample is closely linked to the formation of oxygen vacancies during topochemical reduction. These findings offer valuable insights into the understanding and evolution of surface polarity at the atomic level. By combining STEM-ABF, 4D-STEM, and EELS, this study demonstrates the potential to simultaneously probe local structural and charge information, providing a foundation for comprehending polarity in nickelates at the atomic scale. Furthermore, these results inspire the engineering of polarity at the atomic level in functional materials, opening up avenues for manipulating surface properties through techniques like applying bias voltages or controlling oxygen vacancy distribution.

Details can be found at C. Yang, R. Pons, W. Sigle, H. Wang, E. Benckiser, G. Logvenov, B. Keimer, P. A van Aken. Direct observation of strong surface reconstruction in partially reduced nickelate films. Nat. Commun. 2024, 15, 378. https://doi.org/10.1038/s41467-023-44616-x.