Solvothermal synthesis and tailored upconversion emission of monodisperse ultrasmall face-centered cubic Sr2YF7 nanocrystals

Monodisperse Sr2YF7 nanocrystals with the size of sub-10 nm were synthesized via a solvothermal method by using oleate as a capping ligand. X-ray diffraction and transmission electron microscopy assays reveal that the as-synthesized Sr2YF7 nanocrystals are face-centered cubic structure with the cell parameter a = 5.704 Å. Intense upconversion luminescence can be observed in Er3+/Yb3+ and Tm3+/Yb3+ codoped Sr2YF7 nanocrystals under the excitation of a 980 nm laser and near-white-color emission can be obtained in Er3+/Tm3+/Yb3+ tri-doped Sr2YF7 nanocrystals. Energy transfer (ET) from Yb3+ to Er3+ and Tm3+ is mainly responsible to pump Er3+ and Tm3+ from the ground state to higher states. In Er3+/Tm3+/Yb3+ tri-doped Sr2YF7 nanocrystals, ET from Tm3+ to Er3+ leads average three-photon process, which renders the 523 nm emission.

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► Monodisperse ultrasmall Sr2YF7 nanocrystals (sub-10 nm) were synthesized.
► Sr2YF7 nanocrystals are of fcc structure with the cell parameter a = 5.704 Å.
► Intense upconversion can be obtained in Ln3+ doped Sr2YF7 nanocrystals.
► ET from Tm3+ to Er3+ leads three-photon process, which renders the 523 nm emission.