Hexagonal sodium yttrium fluoride mesocrystals: One-pot hydrothermal synthesis, formation mechanism and multicolor up-/down-converted luminescence for anti-counterfeiting and fingerprint detection

Currently, a precise architecture manipulation of mesocrystals with controllable morphology and tunable size remains a huge challenge and is highly desired for their practical applications. Here, we develop a facile one-pot strategy for the direct synthesis of uniform and monodispersed beta-NaYF4 mesocrystals with cantaloupe-like shape and adjustable dimension. To investigate the formative mechanism of beta-NaYF4 superstructures in depth, the evolution processes for the crystal structure and morphology of beta-NaYF4 mesocrystals under different experimental conditions have been systematically researched and discussed. A series of experimental results demonstrate that intrinsic crystallographic structure and external factors govern the crystal growth process of hexagonal-phased NaYF4. The processes of dissolution-renucleation, anisotropic growth, subsequent oriented attachment and self-assembly are believed to be responsible for the formation of cantaloupe-shaped beta-NaYF4 mesocrystals. Moreover, the luminescence properties of beta-NaYF4:RE3+ (RE = Ce, Tb, Eu, Yb, Tm and Ho) mesocrystals are also studied and efficient multicolor upconverting and downshifting emissions can be easily realized. As a proof of concept experiment, the as-synthesized beta-NaYF4:RE3+ phosphors are employed to demonstrate their promising applications in anti-counterfeiting and fingerprint identification.