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

High-quality dispersible nanocrystals of REPO 4 (RE = La, Eu, Tb, Y, Ho), LaPO 4:Ce,Tb, and REPO 4: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 H 3 PO 4 as a precipitator in the LAER, we obtained REPO 4 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 LaPO 4:Ce,Tb and REPO 4: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 I 610 / I 590 determined by the symmetries of the Eu 3+ ions in the host lattice are observed to vary with crystallite size for the differently shaped REPO 4: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.