Synthesis of Upconversion β-NaYF₄:Nd/Yb/Er Particles with Enhanced Luminescent Intensity through Control of Morphology and Phase.

Hexagonal NaYF₄:Nd/Yb/Er microcrystals and nanocrystals with well-defined morphologies and sizes have been synthesized via a hydrothermal route. The rational control of initial reaction conditions can not only result in upconversion (UC) micro and nanocrystals with varying morphologies, but also can produce enhanced and tailored upconversion emissions from the Yb/Er ion pairs sensitized by the Nd ions. The increase of reaction time converts the phase of NaYF₄:Nd/Yb/Er particles from the cubic to the hexagonal structure. The added amount of oleic acid plays a critical role in the shape evolution of the final products due to their preferential attachment to some crystal planes. The adjustment of the molar ratio of F/Ln can range the morphologies of the β-NaYF₄:Nd/Yb/Er microcrystals from spheres to nanorods. When excited by 808 nm infrared laser, β-NaYF₄:Nd/Yb/Er microplates exhibit a much stronger UC emission intensity than particles with other morphologies. This phase- and morphology-dependent UC emission holds promise for applications in photonic devices and biological studies.