The smallest possible nanocrystals of semiionic oxides

General bonding principles are used to predict the structure of individual nanocrystals in nanocrystalline materials with semiionic bonding. The relationship between the general principles and actual nanocrystal structures is demonstrated using titanium dioxide in the anatase form. The proposed nanocrystals simultaneously fulfill strict criteria of stoichiometry, high coordination, and balanced charge distribution. The smallest such nanocrystals are remarkably simple, e.g., consisting of less than 100 atoms in anatase. According to computer simulations, these nanocrystals show strong quantum size effects, while other clusters of similar size instead show typical defect characteristics.