High-entropy oxides (HEO) with entropic stabilization and compositional flexibility have great potential application in batteries and catalysis. In this work, HEO thin films were synthesized by pulsed laser deposition (PLD) from a rock-salt (Co0.2Ni0.2Cu0.2Mg0.2Zn0.2)O ceramic target. The films exhibited the target’s crystal structure, were chemically homogeneous, and possessed a three-dimensional (3D) island morphology with connected randomly shaped nanopores. The effects of varying PLD laser fluence on crystal structure and morphology were explored systematically. Increasing fluence facilitates film crystallization at low substrate temperature (300 °C) and increases film thickness (60–140 nm). The lateral size of columnar grains, islands (19 nm to 35 nm in average size), and nanopores (9.3 nm to 20 nm in average size) increased with increasing fluence (3.4 to 7.0 J/cm2), explained by increased kinetic energy of adatoms and competition between deposition and diffusion. Additionally, increasing fluence reduces the number of undesirable droplets observed on the film surface. The nanoporous HEO films can potentially serve as electrochemical reaction interfaces with tunable surface area and excellent phase stability. Graphical abstract

Interfaces such as grain boundaries (GBs) and heterointerfaces (HIs) are known to play a crucial role in structure-property relationships of polycrystalline materials. While several methods have been used to characterize such interfaces, advanced transmission electron microscopy (TEM) and scanning TEM (STEM) techniques have proven to be uniquely powerful tools, enabling quantification of atomic structure, electronic structure, chemistry, order/disorder, and point defect distributions below the atomic scale. This review focuses on recent progress in characterization of polycrystalline oxide interfaces using S/TEM techniques including imaging, analytical spectroscopies such as energy dispersive X-ray spectroscopy (EDXS) and electron energy-loss spectroscopy (EELS) and scanning diffraction methods such as precession electron nano diffraction (PEND) and 4D-STEM. First, a brief introduction to interfaces, GBs, HIs, and relevant techniques is given. Then, experimental studies which directly correlate GB/HI S/TEM characterization with measured properties of polycrystalline oxides are presented to both strengthen our understanding of these interfaces, and to demonstrate the instrumental capabilities available in the S/TEM. Finally, existing challenges and future development opportunities are discussed. In summary, this article is prepared as a guide for scientists and engineers interested in learning about, and/or using advanced S/TEM techniques to characterize interfaces in polycrystalline materials, particularly ceramic oxides.

Transmission electron microscopy (TEM), and its counterpart, scanning TEM (STEM), are powerful materials characterization tools capable of probing crystal structure, composition, charge distribution, electronic structure, and bonding down to the atomic scale. Recent (S)TEM instrumentation developments such as electron beam aberration-correction as well as faster and more efficient signal detection systems have given rise to new and more powerful experimental methods, some of which (e.g., 4D-STEM, spectrum-imaging, in situ/operando (S)TEM)) facilitate the capture of high-dimensional datasets that contain spatially-resolved structural, spectroscopic, time- and/or stimulus-dependent information across the sub-angstrom to several micrometer length scale. Thus, through the variety of analysis methods available in the modern (S)TEM and its continual development towards high-dimensional data capture, it is well-suited to the challenge of characterizing isometric mixed-metal oxides such as pyrochlores, fluorites, and other complex oxides that reside on a continuum of chemical and spatial ordering. In this review, we present a suite of imaging and diffraction (S)TEM techniques that are uniquely suited to probe the many types, length-scales, and degrees of disorder in complex oxides, with a focus on disorder common to pyrochlores, fluorites and the expansive library of intermediate structures they may adopt. The application of these techniques to various complex oxides will be reviewed to demonstrate their capabilities and limitations in resolving the continuum of structural and chemical ordering in these systems.

This article was updated to correct errors in References 17 and 20 introduced during the production process.

Precipitation of metal nanoparticles from a host oxide matrix under a reducing environment, also called "exsolution", is an in-situ synthesis strategy to obtain well dispersed catalyst nanoparticles and introduce percolating channels on/inside functional oxide thin films [1][2][3]. Exsolved nanoparticles in thin films are commonly fabricated in multiple steps, first by host oxide film preparation and then by reduction of the host film by heating in a reducing atmosphere such as hydrogen [4][5][6][7]. In this work, exsolved nanoparticles were successfully prepared in polycrystalline perovskite complex concentrated oxide thin films in one step during synthesis using pulsed laser deposition (PLD) and without further reduction [8]. This one-step method for exsolved oxide film fabrication can effectively reduce processing time and cost as well as maintain high density of exsolved metal nanoparticles. Additionally, tuning PLD growth parameters can modify morphology and structure of the exsolved phase from metal nanostructures to core-shell nanoparticles. Oxygen vacancies in the host perovskite film induce exsolution, and result in the formation of secondary metal, alloy, or metal-oxide phases. Crystallographic misfit strain between the nanostructures and oxide matrix provides additional driving force for mass transport and the formation of unexpected complex multi-element nanostructures, as well as a percolating channel network [9]. More, grain boundaries between nanocolumnar grains serve as oxygen vacancy sinks, providing reactive sites which localize exsolution and contribute to high exsolution density. The effects of PLD growth parameters on structure and composition of the exsolved oxide films were directly and clearly observed with spherical-aberration-corrected scanning transmission electron microscopy (STEM) imaging with energy dispersive X-ray spectroscopy (EDS) and electron energy-loss spectroscopy (EELS). With this one-step method for exsolved functional oxide thin film fabrication, multiple exsolved phases with various composition and structure can be constructed with advanced functionality for catalysis and/or charge transport [10].