Room temperature synthesis of β-NaGdF4: RE3+ (RE= Eu, Er) nanocrystallites and their luminescence

A room temperature synthesis was developed for phase pure β-NaGdF4 nanocrystallites as well as 5, 10, and 20% Eu3+ or 5% Er3+ doped material. Rare earth acetates and NaCl react in a 1:2 M ratio with a variable excess of NH4F in ethylene glycol within 24 hours. Since the thermodynamic stability of the hexagonal phase decreases along the lanthanide series, a larger excess of NH4F was required for the synthesis of luminescent samples doped with the smaller Er3+ ions than for Eu3+ doped or pure β-NaGdF4. The materials were characterized by powder X-ray diffraction, electron microscopy, and luminescence spectroscopy. The Eu3+-doped samples show 5D0→7FJ and 5D1→7FJ luminescence after Eu3+ excitation at 394 nm or Gd3+ excitation at 273 nm and 308 nm. The ratio of 5D1 vs. 5D0 luminescence is influenced by the excitation wavelength and the Eu3+ concentration. The Er3+-doped samples show green and red upconversion luminescence, respectively, from the 2H11/2+4S3/2→4I15/2 and 4F9/2→4I15/2 transitions after 970 nm excitation.