From Point Defects to Defect Complexes

Most of the materials have dopants to create functionalities. However, due to the high demands on material functionality in cutting-edge devices, the conventional concept of point defects is no longer sufficient. Therefore, attempts to break through the limitations of point defects by doping different elements to create defect complexes have begun to progress. This section begins with examples of point defects and their role in a wide range of materials, including semiconductors, superconductors, catalysts, scintillators, metals, and glasses. Then, the limitations of the point defects and their solutions using defect complexes are presented with examples of semiconductor and glass. In the latter part of this section, examples of defect complexes are presented, divided into dopant-vacancy pairs and complex defects with sub-nanometer scales. These confer novel functionalities to materials. For example, nitrogen vacancy (NV) centers in diamond have been considered as a promising for quantum bits, and Mn4CaO5 clusters in photosystem II protein play an important role in photosynthesis. Finally, I introduce characterization techniques and theoretical methods to correctly understand the structures and properties of the defect complexes.