Phase selective synthesis of quantum cutting nanophosphors and the observation of a spontaneous room temperature phase transition

Abstract

Oxygen-free Eu³⁺-doped NaGdF₄ nanocrystals with high quantum cutting efficiency are accessible at low temperatures (room temperature to 80 °C) using task-specific ionic liquids (ILs) as structure directing agents and only water as solvent. Selective tuning of the shape, morphology and, most importantly, the crystal phase of the host lattice is achieved by changing the alkyl side length, the H-bonding capabilities and the concentration of 1-alkyl-3-methylimidazolium bromide ILs, [C_nmim]Br. When using [C₂mim]Br, hexagonal NaGdF₄ nanoparticles are obtained. In the case of methylimidazolium bromides with longer pendant alkyl chains such as butyl (C₄), octyl (C₈) or decyl (C₁₀), extremely small nanoparticles of the cubic polymorph form, which then convert even at room temperature (RT) to the thermodynamically favored hexagonal modification. To the best of our knowledge, this kind of spontaneous phase transition is not yet reported. The hexagonal nanomaterial shows a substantial quantum cutting efficiency (154%) whilst in the cubic material, the effect is negligible (107%). The easy yet highly phase selective green synthesis of the materials promises large scale industrial application in environmentally benign energy efficient lighting.