Nanoscale doping and mixing of binary oxide thin films: concepts, properties and applications

Oxide thin films are nowadays recognized as essential ingredients in many areas of modern technology. While binary oxides already exhibit a rich variety of structural, electronic, magnetic, and chemical properties, doping or mixing them with foreign cations allows further engineering to meet specific application requests. In this review, we first outline the fabrication and characterization techniques used to study mixed oxide thin films, both from an experimental and a theoretical viewpoint. We then provide a detailed overview of recent advances in the field, drawing on both experimental findings and theoretical insights. This section which is organized according to the dominant cation in doped films, focuses on systems with distinct thin film characteristics compared to the bulk, such as thickness-dependent properties and surface/interface effects. The subsequent section then explores the fundamental principles that govern cation mixing, addressing both solid solutions and ordered compounds within a unified framework. While these principles are relatively well understood in bulk materials, additional complexity exists in thin films due to finite size effects, as well as the interaction with the surrounding atmosphere (notably oxygen) and/or the substrate which supports the thin films. In several cases, these factors are shown to dramatically alter the mixing behavior, affecting the phase diagrams, the stability and composition of ordered phases, among others. The final section adopts a transversal perspective, and summarizes the benefits of cation mixing in oxide thin films for engineering their structural, electronic, magnetic, and reactivity properties, and expanding their potential for a use in a broad range of technological applications.