High quality multi-functional NaErF4nanocrystals: structure-controlled synthesis, phase-induced multi-color emissions and tunable magnetic properties

Abstract

In this paper, optical/magnetic multi-functional NaErF₄ nanocrystals with high quality, different phases and shapes were synthesized by a simple hydrothermal method using oleic acid as the capping agent. The structure, upconversion luminescence and magnetic properties were characterized by various techniques. The transmission electron microscopy results reveal that the nanocrystals are of high quality and can be self-assembled into a two-dimensional ordered structure. Moreover, the crystal phase and shape can be readily controlled by adjusting the reaction temperature and F⁻ content. The results reveal that high temperature can favor the formation of a hexagonal phase structure and promote the phase transformation from the cubic to hexagonal phase. The phase transformation mechanism based on the free energy theory was discussed in detail. In addition, the F⁻ content plays a critical role in determining the morphology of the final products. A high F⁻ content is beneficial for the formation of the one-dimensional rod-like shape. Interestingly, phase-induced upconversion luminescence color tuning from red to green was observed. All of the as-prepared NaErF₄ nanocrystals possess paramagnetic properties at room temperature and the magnetizations of the nanocrystals with spherical-like cubic, cubic, and rod-like shapes were measured as 1.69 emu g⁻¹, 2.53 emu g⁻¹and 2.29 emu g⁻¹ at 20 kOe, respectively, which are larger than most previously reported Gd-based nanocrystals. In addition, T₂-weighted magnetic resonance imaging based on these NaErF₄ nanocrystals was demonstrated for the first time. Therefore, these tunable upconversion fluorescent NaErF₄ nanocrystals with excellent paramagnetic properties can be used as promising dual-modal nanoprobes for optical bioimaging and magnetic resonance imaging, and may have potential applications in bioseparation.