Orderly Aligned and Highly Luminescent Monodisperse Rare-Earth Orthophosphate Nanocrystals Synthesized by a Limited Anion-Exchange Reaction

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Abstract

High-quality dispersible nanocrystals of REPO4 (RE = La, Eu, Tb, Y, Ho), LaPO4:Ce,Tb, and REPO4:Eu (RE = La,Y) with diverse shapes (nanopolyhedra, quasinanorods, nanorods, and nanowires) were synthesized at temperatures in the range of 180−260 °C in long alkyl-chain solvents (organic acids and amines), via a limited anion-exchange reaction (LAER). Using H3PO4 as a precipitator in the LAER, we obtained REPO4 nanocrystals from the precipitation reaction significantly retarded by the strong bonding interactions among the rare earth cations and organic acid ligands. The shape of the nanocrystals can be manipulated to 0D polyhedra, 1D rods, and 1D wires by changing the composition of the solvent and the alkyl-chain length of the capping ligands. The LaPO4:Ce,Tb and REPO4:Eu (RE = La,Y) nanocrystals show intensive green and red emissions under UV excitation, respectively, with quantum yields higher than 40%. Interestingly, the charge-transfer band decided by the RE−O bond distance and the ratio of the I610/I590 determined by the symmetries of the Eu3+ ions in the host lattice are observed to vary with crystallite size for the differently shaped REPO4:Eu nanocrystals. The highly luminescent nanocrystals can be orderly aligned to form various 2D and 3D nanoarrays on the copper grids by tuning the concentration of the nanocrystal dispersion in cyclohexane, showing their potential in constructing new self-assembled luminescence devices. Moreover, the demonstrated LAER strategy is extendable to the synthesis of many other rare earth salt nanocrystals, such as vanadates and fluorides.

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