Interfacial Defects Dictated In Situ Fabrication of Yolk-Shell Upconversion Nanoparticles by Electron-Beam Irradiation.

Homogeneous core-shell structured nanoparticles (NPs) are prevailingly designed to accommodate lanthanide emitters, and such an epitaxial shell deposited on core NP is generally believed to help eliminate surface traps or defects on the as-prepared core. However, upon electron-beam irradiation to core-shell-shell NaLuF4 :Gd/Yb/Er@NaLuF4 :Nd/Yb@NaLuF4 upconversion NPs (UCNPs), it is revealed that interfacial defects actually exist at the core-shell and shell-shell interfaces, even with a higher density than the bulk-phase defects in inner core. Because of such higher density of interfacial defects, the kinetic energies transferred from energetic electrons to atoms may trigger the faster Na/F atom ejections and outward atom migrations in the coating layers than in the inner core of NPs, which ultimately results in the in situ formation of novel yolk-shell UCNPs. These findings provide new insights into interfacial defects in homogeneous core-shell structured NaLnF4 NPs, and pave the way toward utilizing the interactions of high-energy particles with materials for in situ fabrication of novel nanostructures.