Thermodynamics of Nanosystems with a Special View to Charge Carriers

Basic ingredients of interfacial thermodynamics are recapitulated with a special eye on the nanometer-size regime. Questions are then briefly tackled that arise if, in heterogeneous systems, the constituent phases shrink to atomistic dimensions. Particularly helpful in this context are thermodynamic approaches, in which the introduction of interfacial tension is avoided. While the first part addresses ground structure quantities, the second part deals with questions of size and confinement effects on entropy and energy of ionic and electronic defects. These defects represent the respective excitations within this ground structure. The article emphasizes the similarities between ions and electrons manifested in the statistics rather than elaborating on the discrepancies that are primarily reflected by different densities of states and mobilities. It is, therefore, not the intention of the article to address aspects of nanoelectronics that rely on quantum transport for which many reviews are available. Nonetheless all these discussed aspects are directly relevant for both nanoionics and nanoelectronics.

Nanoionics: ion transport and electrochemical storage in confined systems

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.

Mesoscopic fast ion conduction in nanometre-scale planar heterostructures

Ion conduction is of prime importance for solid-state reactions in ionic systems, and for devices such as high-temperature batteries and fuel cells, chemical filters and sensors. Ionic conductivity in solid electrolytes can be improved by dissolving appropriate impurities into the structure or by introducing interfaces that cause the redistribution of ions in the space-charge regions. Heterojunctions in two-phase systems should be particularly efficient at improving ionic conduction, and a qualitatively different conductivity behaviour is expected when interface spacing is comparable to or smaller than the width of the space-charge regions in comparatively large crystals. Here we report the preparation, by molecular-beam epitaxy, of defined heterolayered films composed of CaF2 and BaF2 that exhibit ionic conductivity (parallel to the interfaces) increasing proportionally with interface density--for interfacial spacing greater than 50 nanometres. The results are in excellent agreement with semi-infinite space-charge calculations, assuming a redistribution of fluoride ions at the interfaces. If the spacing is reduced further, the boundary zones overlap and the predicted mesoscopic size effect is observed. At this point, the single layers lose their individuality and an artificial ionically conducting material with anomalous transport properties is generated. Our results should lead to fundamental insight into ionic contact processes and to tailored ionic conductors of potential relevance for medium-temperature applications.