Electrostatic dielectric capacitors with ultrahigh power densities are sought after for advanced electronic and electrical systems owing to their ultrafast charge-discharge capability. However, low energy density resulting from low breakdown strength and suppressed polarization still remains a daunting challenge for practical applications. We propose a microstructural strategy with dendritic nanopolar (DNP) regions self-assembled into an insulator, which simultaneously enhances breakdown strength and high-field polarizability and minimizes energy loss and thus markedly improves energy storage performance and stability. For illustration, in this study, we achieved a high energy density of 215.8 joules per cubic centimeter with an efficiency of 80.7% at a high electric field of 7.4 megavolts per centimeter in a DNP structure–designed PbZr _0.53 Ti _0.47 O ₃ -MgO film. The proposed strategy is generally applicable for development of high-performance dielectric microcapacitors.

The grain boundary (GB) plays a critical role in a material’s properties and device performance. Therefore, the characterization of a GB’s atomic structure and electrostatic characteristics is a matter of great importance for materials science. Here, we report on the atomic structure and electrostatic analysis of a GB in a SrTiO₃ bicrystal by four-dimensional scanning transmission electron microscopy (4D-STEM). We demonstrate that the Σ5 GB is Ti-rich and poor in Sr. We investigate possible effects on the variation in the atomic electrostatic field, including oxygen vacancies, Ti-valence change, and accumulation of cations. A negative charge resulting from a space-charge zone in SrTiO₃ compensates a positive charge accumulated at the GB, which is in agreement with the double-Schottky-barrier model. It demonstrates the feasibility of characterizing the electrostatic properties at the nanometer scale by 4D-STEM, which provides comprehensive insights to understanding the GB structure and its concomitant effects on the electrostatic properties.

The NdNiO₃ (NNO) system has attracted a considerable amount of attention owing to the discovery of superconductivity in Nd_0.8Sr_0.2NiO₂. In rare-earth nickelates, Ruddlesden–Popper (RP) faults play a significant role in functional properties, motivating our exploration of its microstructural characteristics and the electronic structure. Here, we employed aberration-corrected scanning transmission electron microscopy and spectroscopy to study a NdNiO₃ film grown by layer-by-layer molecular beam epitaxy (MBE). We found RP faults with multiple configurations in high-angle annular dark-field images. Elemental intermixing occurs at the SrTiO₃–NdNiO₃ interface and in the RP fault regions. Quantitative analysis of the variation in lattice constants indicates that large strains exist around the substrate–film interface. We demonstrate that the Ni valence change around RP faults is related to a strain and structure variation. This work provides insights into the microstructure and electronic-structure modifications around RP faults in nickelates.

The polarity of a surface can affect the electronic and structural properties of oxide thin films through electrostatic effects. Understanding the mechanism behind these effects requires knowledge of the atomic structure and electrostatic characteristics at the surface. In this study, we use annular bright-field imaging to investigate the surface structure of a Pr_0.8Sr_0.2NiO_2+x (0 < x < 1) film. We observe a polar distortion coupled with octahedral rotations in a fully oxidized Pr_0.8Sr_0.2NiO₃ sample, and a stronger polar distortion in a partially reduced sample. Its spatial depth extent is about three unit cells from the surface. Additionally, we use four-dimensional scanning transmission electron microscopy (4D-STEM) to directly image the local atomic electric field surrounding Ni atoms near the surface and discover distinct valence variations of Ni atoms, which are confirmed by atomic-resolution electron energy-loss spectroscopy (EELS). Our results suggest that the strong surface reconstruction in the reduced sample is closely related to the formation of oxygen vacancies from topochemical reduction. These findings provide insights into the understanding and evolution of surface polarity at the atomic level.

Two-dimensional (2D) van der Waals (vdW) materials offer rich tuning opportunities generated by different stacking configurations or by introducing intercalants into the vdW gaps. Current knowledge of the interplay between stacking polytypes and intercalation often relies on macroscopically averaged probes, which fail to pinpoint the exact atomic position and chemical state of the intercalants in real space. Here, by using atomic-resolution electron energy-loss spectroscopy in a scanning transmission electron microscope, we visualize a stacking-selective self-intercalation phenomenon in thin films of the transition-metal dichalcogenide (TMDC) Nb_1+xSe₂. We observe robust contrasts between 180°-stacked layers with large amounts of Nb intercalants inside their vdW gaps and 0°-stacked layers with little detectable intercalants inside their vdW gaps, coexisting on the atomic scale. First-principles calculations suggest that the films lie at the boundary of a phase transition from 0° to 180° stacking when the intercalant concentration x exceeds ~0.25, which we could attain in our films due to specific kinetic pathways. Our results offer not only renewed mechanistic insights into stacking and intercalation, but also open up prospects for engineering the functionality of TMDCs via stacking-selective self-intercalation.