From Defects to Photoluminescence in h-BN 2D and 0D Nanostructures

In the present Account, we report the recent progress of our research group on experimental and theoretical studies of defects in 2D and 0D hexagonal boron nitride. The studies of the effect of defects in boron-based structures have been also extended to boron oxide glasses. Engineering the different types of defects in h-BN is paramount because many functional properties of the material depend on them. This is particularly true for h-BN nanomaterials because of the main role played by surfaces. An important finding is that the formation of defects is directly dependent on the synthesis route; bottom-up or top-down syntheses generate different types of defects whose origins are generally connected to vacancies, dangling bonding, and substitutional oxygen impurities. We have focused our attention, in particular, on the correlation between defects and photoluminescence. The first part of this Account is dedicated to a general overview of defects that form in h-BN systems. In the second and third parts, we report on the rise of fluorescence in different types of h-BN nanostructures, in particular nanoflakes and BN dots. h-BN nanoflakes become fluorescent due to the presence of substitutional oxygen in the structure. The emission depends on the thermal processing of the material. A postsynthesis thermal treatment, because it induces the condensation of oxygen-related bonds that at the origin of fluorescence, changes the photoluminescence according to the degree of condensation of the structure. In the case of BN, the defects in dots and 0D nanostructures are discussed as a function of their preparation route. The analysis of defects in h-BN dots shows that not only vacancies and impurities can contribute to emission but also structural defects such as Stone-Wales. Understanding the origin of such defects and correlating them with specific optical properties is of the utmost importance because comprehending such phenomena could also guide the fabrication of new boron oxide emissive materials. In the last example, we show that the formation of defects, such as dangling bondings and vacancies, is the basis of a surprising phosphorescence at room temperature in boron oxide materials. We have observed, in particular, that the rise of boric acid phosphorescence after heat treatment is related to the presence of defects. The afterglow results from a trapping and detrapping process, which delays recombination at the active optical centers. The formation of near UV and blue optical transitions in absorption is revealed by a time-dependent density functional analysis of defective BOH molecules and clusters. In thermally processed boric acid samples, these defects cause photoluminescence.