LaPO4:Yb3+ core and LaPO4:Yb 3+/GdPO4 core/shell nanoparticles (NPs) are prepared from lanthanide chlorides and orthophosphoric acid at 200 C using diphenyl ether as a solvent with tributyl phosphate and trihexyl amine as coordinating molecules. Near infrared (NIR) emission is observed at 976 nm under 940 nm NIR excitation for both NPs, where these excitation and emission occur through transitions between Stark splitting levels of 2F7/2 and 2F5/2 of Yb3+. High resolution TEM observation reveals that LaPO4:Yb3+/GdPO4 core/shell consists of a single crystalline domain and hence the GdPO4 shell epitaxially grows on LaPO4:Yb3+ core. We investigate the change in NIR emission intensity and shell thickness with reaction time at 200 C.
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