Fermi Level Engineering

The Fermi energy is the key quantity for comparing charge compensation mechanisms. It provides a unique basis for predicting properties of oxide electroceramics and is essential for the adjustment and improvement of manifold material properties.

The Idea

The charges introduced by doping can be compensated by different mechanisms, which result in a wide variation of material properties enabling a plethora of different applications.
The charges introduced by doping can be compensated by different mechanisms, which result in a wide variation of material properties enabling a plethora of different applications.

The main strategy to adjust properties of functional ceramics is the subtle and controlled variation of the chemical composition, for example by substitution. While isovalent substitution or the formation of solid solutions is electrically neutral, heterovalent substitution (doping) introduces charged defects. Doping can result in different properties, which is primarily determined by how the charges, which are introduced by thedopant species, are compensated in the material. In semiconductors, dopants are mostly electronically compensated by electrons and electron-holes (holes), which are highly mobile electronic charge carriers. Doping in oxides is often ionically compensated by introduction of ionic defects, such as vacancies or interstitials, which can result in ion conducting materials to be used in fuel cells, batteries or membranes. The compensation by ionic defects is also used to explain the variation of properties of soft (donor) or hard (acceptor) doped piezoelectrics. (Hyper-)doping, where the solubility limit is exceeded, can result in the segregation of species, either to grain boundaries, surfaces and heterointerfaces or to the formation of secondary phases. Such effects are, for example, intentionally utilized in ZnO varistor ceramics or BaTiO3 conductors (PTCR: positive temperature coefficient of resistors). Furthermore, charge compensation can occur via valence changes of other constituents, being the origin of unique magnetic properties as the basis for memory and sensor applications. The variety of charge compensation mechanisms, resulting material properties, and applications based on the different compensation mechanisms are summarized in the figure on the right.

The collaborative research center FLAIR will explore Fermi level engineering as a new avenue towards the design of oxide electroceramics by using the Fermi energy, which describes the occupancy of electronic states by electrons. In doing so, the influence of the Fermi energy in the bulk, at surfaces, grain boundaries, heterointerfaces, and their related space-charge regions will directly be exploited to predict the prevailing charge compensation mechanism and the resulting material properties for a given composition and structure. The inherent relation between the Fermi energy and phase stability will further be applied to derive novel synthesis routes and to control microstructure evolution.

The projects (A01-B08) of the CRC FLAIR address the research questions with the concept of Fermi level engineering in two research areas (A & B) for three example applications of oxide electroceramics. The projects’ research covers five fundamental research topics and is supplemented by five supporting projects (Z01-Z05).
The projects (A01-B08) of the CRC FLAIR address the research questions with the concept of Fermi level engineering in two research areas (A & B) for three example applications of oxide electroceramics. The projects’ research covers five fundamental research topics and is supplemented by five supporting projects (Z01-Z05).

The 20 projects of the CRC are connected with each other on three organization levels (see figure on the right): five fundamental research topics, two research areas, and three application clusters. The structure chosen provides a strong thematic coherency within and in between the projects. The goal of the CRC FLAIR is to implement Fermi level engineering as the tool to predict material properties for given composition and processing parameters. To reach this goal, one has to start at a fundamental level, i.e., to understand how the Fermi energy is connected to the properties of defects (1), segregation (2), materials synthesis (3), spacecharge regions (4), and microstructure (5). These five Fundamental Research Topics constitute the backbone of Fermi level engineering along which the scientific discussions within the CRC are focused and which need to be controlled for tailoring material properties.

Being aware that this requires experimental variability to be addressed, it is convenient to separate conceptually between the role of the Fermi energy in homogeneous (bulk) materials and that at interfaces as this will substantially reduce the complexity. The separation is the basis for the structure of the CRC, in which the 15 research projects are assigned to one of the two Research Areas. On the one hand, projects A01–A07 in research area A, Materials and Defects, are focusing on the role of the Fermi energy in homogeneous materials by comparing defect properties of materials with systematically varied cation and anion composition. On the other hand, projects B01–B08 in research area B, Fermi Level Engineering at Interfaces, are addressing all aspects related to the variation of the Fermi energy at the boundaries of crystalline grains, which are surfaces, grain boundaries, and heterointerfaces. The two research areas are complemented by the five projects of area Z, Supporting Structures.

The understanding of the fundamental research topics and the results obtained in the two research areas and project area Z will be combined in the Application Clusters where Fermi level engineering is applied to advance material properties and synthesis: Membranes, Catalysts, and Piezoelectrics and Dielectrics.

List of Projects

The collaborative research center FLAIR is organized in two research areas on Materials and Defects (Area A) as well as Interfaces (Area B). In addition, there is an area Z for supporting structures. On this page, you find an overview of the individual projects of these different areas.

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