Sensitization of Eu³⁺ Luminescence in Eu:YPO₄ Nanocrystals

Abstract

Eu:YPO 4 · x H 2 O ( x = 0.5–1) nanocrystals were synthesized by a liquid–solid–solution (LSS) solvothermal method and dispersed in chloroform. In order to sensitize the emission from the Eu 3+ ions, 2-thenoyltrifluoroacetone (HTTFA) was used to replace a significant fraction of the oleate capping ligand on the as-prepared Eu:YPO 4 · x H 2 O ( x = 0.5–1) nanocrystals. During the ligand exchange, HTTFA reacts with oleate, forming oleic acid and 2-thenoyltrifluoroacetonate, TTFA. The negatively charged TTFA then displaces the neutral oleic acid ligand from the surface of the nanoparticles. The resulting surface-modified samples were less dispersible in chloroform than were the as-prepared, oleate-capped nanoparticles but were easily dispersed in pyridine, forming very clear mixtures. The resulting surface-modified nanoparticles were characterized by X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, transmission electron microscopy (TEM), UV–vis absorption spectroscopy, photoluminescence (PL) spectroscopy, and time-resolved luminescence spectroscopy. XRD analysis indicates that the samples are crystalline with a hexagonal phase. The oleate-capped Eu:YPO 4 · x H 2 O ( x = 0.5–1) nanocrystals have a zeolite structure with a porous surface. The morphology and quality of the nanoparticles remained unchanged upon ligand exchange. The FTIR spectrum of the surface-modified (TTFA-sensitized) Eu:YPO 4 · x H 2 O ( x = 0.5–1) nanocrystals shows signals for both 2-thenoyltrifluoroacetate and oleate. Using UV–vis absorbance and elemental analysis, it is estimated that approximately half of the native oleate capping ligands are replaced with TTFA. Colloidal dispersions in pyridine show characteristic emission of Eu 3+ 5 D 0 → 7 F J ( J = 0–4) when excited at the TTFA absorbance band at 350 nm. Ligand excitation at 350 nm results in an enhancement of external quantum efficiency of Eu 3+ emission of up to 4700× relative to direct Eu 3+ excitation at 464 nm. The ability to sensitize emission from these nanocrystals greatly increases their potential for application in display and lighting fields.