Authors: Albert Tarancón | Nini Pryds
Source: Advanced Materials Interfaces (article, 2019-08-01)
Field: Materials Science — Electronic, Optical and Magnetic Materials

Functional Oxide Thin Films for Advanced Energy and Information Technology Complex oxide thin films are finding applications in a wide range of energy and information technologies, including electron and ion conductors, photovoltaics, thermoelectrics, dielectrics, and resistive switching. Irrespective of whether single thin films or more sophisticated multilayered systems are employed, each of these technologies requires a thorough knowledge of the oxide physicochemical properties, its interactions with the environment and the role of defect/interface/boundary effects that might stem between film and substrate, layers with different compositions or even simply from the grain boundaries. Although it is depending on the technology area, the key properties might be different while many of the materials optimization strategies may be common. Therefore, we believe that information sharing between diverse and even complementary researchers' communities can be extremely beneficial for a better understanding of the crucial aspects that need to be addressed to improve these functional oxide materials properties. The overall aim of this collection of reviewer's papers is sparking extensive discussions of the advances that materials physics and chemistry can make in thin film applications. Broad context Functional oxides are the most fascinating materials forming a variety of different degrees of structural complexity, which continues to drive much research in chemistry, physics, and materials science. Oxide heterostructures and thin films have been studied and used for many years due to the electron–electron correlations which play a large role in fascinating phenomena, leading to new yet largely unexplored physics and chemistry. These oxides include a wide variety of structures such as: wurtzite, corundum, zircon, fluorite, spinel, garnet, perovskite and many others. In this family of materials, oxide thin films materials containing transition metal cations with partially occupied d orbitals exhibit a variety of extraordinary properties such as high-temperature superconductivity, colossal magnetoresistance, ionic conductivity, thermoelectricity, ferromagnetism, ferroelectricity, and multiferroicity just to name a few. In particular, transition-metal oxides with perovskite-type structure, ABO3, are a very important class of functional oxide that exhibit a wide range of stoichiometries. In some cases, a sub-stoichiometry on the metal or oxygen site is accommodated by the formation of vacancy defects. Such as in ideal cubic perovskite, which consists of a three-dimensional network of corner-sharing BO6 octahedra where all B–O bonds are of the same length but can be slightly distorted leading to a wide variety of new electric, ionic and magnetic properties. Understanding the relationships between the structural, chemical and physical properties can lead to a broad range of novel functional thin films oxide materials and their device application as shown in this collection of Reviews (Figure 1). Current Status and Future Challenges The current Special Issue includes topical Review contributions from leading research groups in the field of functional oxides. The issue compiles new insights into the control and characterization of interfaces at unprecedented levels of precision while gathering experimental results on recently discovered interface phenomena which represent the foundational underpinnings of emerging disciplines that will necessarily bring radically new families of solid state energy and information devices in the near future. As discussed in several Reviews of this Special Issue, atomically engineered oxide interfaces represent a unique and precise way for a complete understanding of a wide variety of emerging phenomena as these are extremely sensitive at the atomic level. Being able to control the microstructure at the atomic level represents one of the major challenges of the field with different implications in model interfaces such as epitaxial heterostructures grown on single crystals or highly variable grain boundaries naturally present in nanocrystalline materials. In this special issue, reviews addressing the key aspects of heterostructures growth at different levels of control (including free surfaces of the employed substrates) are combined with strategies on how to tune grain boundary in nanostructured polycrystalline materials. On the fundamental level, article number 1900479 focuses on the necessity of growing high quality wide bandgap semiconductors to keep their unique high mobility close to room-temperature, which makes them promising candidates to foster future all-perovskite devices. The paper shows the tremendous effect of defective interfaces on the electronic mobility of alkaline-earth stannates and the current progress for facing the grand challenge of growing defect-free high-phase purity epitaxial films on cheap available substrates. Similarly, the impact of interfaces in controlling spin currents for developing more efficient spintronic-based devices article is reviewed in article number 1900475 while article number 1900547 clearly details the progress achieved in the last decades on monolayer-by-monolayer design and growth of perovskite oxides to engineer oxide interfaces with radically different electronic properties and enhanced redox performance surfaces. An entirely different challenge is faced when controlling naturally existing interfaces such as grain boundaries due to an intrinsic variability which is typically present in polycrystalline materials. In the case of nanocrystalline domains, this control is especially relevant since the material becomes interface-dominated essentially behaving as the interface itself. Nano-engineering this type of interface-dominated oxides is the focus of article number 1900462. This Review dedicates special attention to the fabrication of ad hoc interface-dominated mixed ionic-electronic conductor materials able to control electrical and electrochemical properties by directly growing vertically aligned nanostructures or tuning the cation local non-stoichiometry at the grain boundary level. These two novel strategies represent exciting examples of how interface nano-engineering can be directly implemented in real devices. This type of grain boundary control can be also of great interest for thermoelectric materials as discussed in article number 1900222. This Review presents the high interest in engineering interfaces to develop next-generation thermoelectric materials after discussing how the convenient phonon scattering occurring at grain boundaries degrades the promising electronic properties of high power factor thermoelectric materials such as STO. In parallel to the highly precise fabrication techniques, advanced characterization methods at the nanoscale have been developed in the last decades to improve our understanding. These new techniques achieved unprecedented levels of accuracy at the atomic scale being able to precisely study relevant phenomena occurring at interfaces such as cationic inter-diffusions, crystallographic texturing or accumulation of defects and impurities due to chemical or steric strain. Very interestingly, non-destructive optical techniques are emerging techniques offering detailed information of thin films even when these are embedded in devices or while in operation. In this sense, article number 1900547 presents resonant X-ray reflectometry as a technique sensitive to elemental concentrations and magnetic structures with sub-angstrom depth resolution while article number 1900496 describes the analysis of the optical absorption to study the defect chemistry and kinetics in mixed ionic-electronic conductor oxides at high temperatures under controlled atmospheres. More focused on surface analysis, article number 1900580 reviews the application of surface sensitive techniques such as surface diffraction and ion scattering spectroscopy by direct probing electrodes in electrochemical cells, which offers exceptional information of the surface chemistry and structure evolution under operation. Besides a constant progress in designing, controlling and characterizing these oxide thin films and interfaces, novel breakthrough concepts and enhanced architectures continuously emerge pushing forward the frontiers of knowledge and the state of the art of the technology. This enhancement ranges from trivial increase of the active area for high footprint capacity three-dimensional all-solid state thin film Li-ion batteries, presented in article number 1900805, to intrinsically improved energy conversion efficiencies reached by electrode heterostructures in solar water-splitting cells, discussed in article number 1900299. Heterostructuring is also considered a promising strategy for the practical implementation of environmentally-friendly solid state refrigeration devices based on caloric effects, i.e., the changes of temperature upon the application of external field such magnetic, electric or strain. The use of tunable interfacial strain in heterostructures can enlarges the caloric effects while allowing the combination of synergistic multi-caloric effects, as extensively reviewed in article number 1900291. Other new and exciting device concepts are proposed and discussed in article number 1900480. This stimulating contribution to this Special Issue is focused on recently discovered 2D electron gases occurring in conductive oxide interfaces and their implementation in novel field effect devices like transistors or sensors. A more sophisticated control of such conductive oxide interfaces could even give rise to still undreamed devices based on, for instance, new phenomena such as the long-range non-coulombic electron–electron interaction suggested for LaAlO3/SrTiO3 interfaces in article number 1900301. Beyond these (and other) original and captivating devices, one can envisage the combined implementation of complementary designs in totally new concepts that would revolutionize well established fields such as energy or computing. Among other synergistic combinations, the integration of miniaturized solid state energy devices able to smartly harvest and store energy was recently proposed by the editors of this collection giving rise to a new family of adaptive energy systems called “Harvestorers”. Similarly, the comprehensive and fascinating Progress Report number 1900471 suggests on the integration of photovoltaics together with neuromorphic technologies envisioning the goal of a new era of photo-neuromorphic systems enabling self-powered artificial intelligence. All in all, we hope that this Special Issue will serve as a source of inspiration for researchers of different fields in their continuous search of novel interfacial phenomena and their practical implementation into highly performing solid state based thin films oxide devices that, alone or synergistically combined in new game changer concepts, will contribute to the progress of our society. The HARVESTORE project has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No 824072. Nanotechnology ; Thin film ; Materials science ; Context (archaeology) ; Engineering physics ; Photovoltaics ; Heterojunction ; Oxide ; Perovskite (structure) ; Optoelectronics ; Physics ; Photovoltaic system ; Engineering ; Chemical engineering Magnetic and transport properties of perovskites and related materials ; Electronic and Structural Properties of Oxides ; Advanced Condensed Matter Physics