The past two decades have shown that the exploration of properties on the nanoscale can lead to substantially new insights regarding fundamental issues, but also to novel technological perspectives. Simultaneously it became so fashionable to decorate activities with the prefix 'nano' that it has become devalued through overuse. Regardless of fashion and prejudice, this article shows that the crystallizing field of 'nanoionics' bears the conceptual and technological potential that justifies comparison with the well-acknowledged area of nanoelectronics. Demonstrating this potential implies both emphasizing the indispensability of electrochemical devices that rely on ion transport and complement the world of electronics, and working out the drastic impact of interfaces and size effects on mass transfer, transport and storage. The benefits for technology are expected to lie essentially in the field of room-temperature devices, and in particular in artificial self-sustaining structures to which both nanoelectronics and nanoionics might contribute synergistically.

/pdf/nanoionics-ion-transport-and-electrochemical-storage-in-2fy7wmukp8.pdf

Nanoionics: ion transport and electrochemical storage in confined systems.

Ionic conduction in space charge regions

Direct observations using scanning transmission electron microscopy (STEM) of the grain-boundary chemistry of selectively doped SrTiO3 and BaTiO3 show the predominant solute segregation in both systems to be that of acceptors (negative effective charge). Appreciable donor segregation is not observed even at lattice concentrations as high as 10 mol%. Donor and acceptor codoped materials show segregation of the acceptor only. The results are consistent with a grain-boundary space-charge distribution consisting of a positive boundary and negative space charge. All grain boundaries examined also show an excess of Ti relative to the A-site cations, suggesting that the positive boundary charge is at least partially accommodated by an excess of Ti ions. The sign and magnitude of the electrostatic potential appear to be remarkably insensitive to changes in lattice defect structure with solute doping. Grain-boundary chemistry appears dominated by space-charge segregation, in contrast with the predictions of recent atomistic simulations which neglect the space-charge potential.

Grain‐Boundary Chemistry of Barium Titanate and Strontium Titanate: I, High‐Temperature Equilibrium Space Charge

This review represents the authors' view of the evolution of solid-state ionics over approximately the past 100 years. A brief history, introducing milestones of the development of this discipline, is followed by a short summary of the theory of ionic conduction in the bulk and the more recently developed theory of ionic conduction at interfaces. The central part of the article gives examples of ionic-conducting materials systems with structures ranging from one- to three-dimensional disorder. Important experimental techniques for analyzing ionic conduction, including alternating-current impedance spectroscopy, direct-current coulometry, and direct-current current-voltage measurements with blocking electrodes, are also summarized. The main technological applications, that is, batteries, solid-oxide fuel cells, electrochemical sensors, electrochromic windows, and oxygen-separation membranes, are reviewed. Finally, new concepts in solid-state ionics are presented, including the investigation of new materials (such as nanostructured phases), the study of boundaries (for example, using microelectrodes), the development of computational techniques, and the connections with other classes of materials (notably magnetic and semiconducting materials).

Solid‐State Ionics: Roots, Status, and Future Prospects

The current status and future prospects of non-contact atomic force microscopy (nc-AFM) and Kelvin probe force microscopy (KPFM) for studying insulating surfaces and thin insulating films in high resolution are discussed. The rapid development of these techniques and their use in combination with other scanning probe microscopy methods over the last few years has made them increasingly relevant for studying, controlling, and functionalizing the surfaces of many key materials. After introducing the instruments and the basic terminology associated with them, state-of-the-art experimental and theoretical studies of insulating surfaces and thin films are discussed, with specific focus on defects, atomic and molecular adsorbates, doping, and metallic nanoclusters. The latest achievements in atomic site-specific force spectroscopy and the identification of defects by crystal doping, work function, and surface charge imaging are reviewed and recent progress being made in high-resolution imaging in air and liquids is detailed. Finally, some of the key challenges for the future development of the considered fields are identified.

R.B. Poeppel

Papers

Origin of equilibrium space charge potentials in ionic crystals