Nanocrystallization in Metal Nitrides and Oxynitrides

Oxynitrides are attracting attention as new inorganic compounds with potential applications such as high-temperature ceramics, phosphors for light-emitting diodes, photocatalysts, and electronics. In addition, new materials are being developed based on the occurrence of nanocrystallization in transition metal nitrides and oxynitrides. Crystallization of transition metal nitride (TMN) nanoparticles from amorphous TM-SiON oxynitrides has recently been reported. TMN nanoparticles have been proposed to apply for localized surface plasmon resonance (LSPR) and have the potential to exhibit performance superior to those of gold and silver nanoparticles. Their refractory nature also makes them suitable for developing new technologies, including optoelectronics, plasmonics, and metamaterials. When embedded in a silica matrix, TMN plasmonic nanoparticles are more compatible with biomedical and complementary metal-oxide semiconductor (CMOS) applications. In addition, TM oxynitrides with a perovskite structure have found applications as relaxor-type dielectric materials, as well as photocatalysts for water splitting. Polar nanoregions (PNRs) in noncentrosymmetric nanocrystallites that precipitate from the oxynitride melt are important for developing superior relaxor-type ferroelectrics. However, to control the formation of PNRs and thus improve the relaxor behavior, it is crucial to determine their local crystal structure. This perspective describes recent advantages with regard to two types of nanocrystallization in metal nitrides and oxynitrides, forming either discrete TMN nanoparticles embedded in a silica matrix or local PNRs in TM oxynitride perovskites, and discusses their future prospects.