NaYF4:Eu2+ Microcrystals: Synthesis and Intense Blue Luminescence

In this work, NaYF 4:Eu 2+ microcrystals with an intense blue luminescence were successfully fabricated via a solution-based route. During the sample preparation, oleic acid and cetyltrimethylammonuim bromide were used as the surfactants to tune the morphology, while citric acid was taken as a ligand to stabilize the β-phase NaYF 4 and to completely reduce Eu 3+ to Eu 2+ for activation of blue luminescence. As confirmed by transmission electron microscopy and scanning electron microscopy measurements, all as-prepared samples crystallized in hexagonal rods or tubes, depending on the types of surfactants used. The rod samples had a diameter of about 1.7 µm and a length in the range of 2.6−3 µm, while the tube samples showed an external diameter in the range of 0.8−1 µm and a length in the range of 2.5−3.5 µm with an internal diameter of about 0.5 µm. Both rods and tubes were capped with surfactants, which enabled them to stably disperse in glycol to form transparent solutions. This work seems to be the first example of the successful preparation of stable dispersion of micron luminescence solids. These transparent solutions showed an intense blue luminescent emission of Eu 2+ with a quantum yield of about 14%. Finally, the mechanism for the fabrication of the rods and tubes as well as the Eu 3+ reduction was discussed.