Lanthanide-doped Na _xScF _{3+ x} nanocrystals: Crystal structure evolution and multicolor tuning

Rare-earth-based nanomaterials have recently drawn considerable attention because of their unique energy upconversion (UC) capabilities. However, studies of Sc ³⁺-based nanomaterials are still absent. Herein we report the synthesis and fine control of Na _xScF _3+x nanocrystals by tuning of the ratio of oleic acid (OA, polar surfactant) to 1-octadecene (OD, nonpolar solvent). When the OA:OD ratio was increased from low (3:17) to high (3:7), the nanocrystals changed from pure monoclinic phase (Na ₃ScF ₆) to pure hexagonal phase (NaScF ₄) via a transition stage at an intermediate OA:OD ratio (3:9) where a mixture of nanocrystals in monoclinic and hexagonal phases was obtained and the coexistence of the two phases inside individual nanocrystals was also observed. More significantly, because of the small radius of Sc ³⁺, Na _xScF _3+x:Yb/Er nanocrystals show different UC emission from that of NaYF ₄:Yb/Er nanocrystals, which broadens the applications of rare-earth-based nanomaterials ranging from optical communications to disease diagnosis.