Abstract:

We synthesized Tm³⁺-doped NaYF₄ microcrystals with various length-to-diameter ratios by using a facile hydrothermal method assisted with sodium citrate from precursor solutions with various pH values. The β-NaYF₄:Tm³⁺ samples were characterized by using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), Fourier-transform infrared (FT-IR) spectroscopy, and photoluminescence spectroscopy. XRD and SEM show that as the pH of the precursor solutions increased, the morphology of the microcrystals changed from long rods to short microprisms to microplates. We also investigated the luminescence properties of the β-NaYF₄:Tm³⁺ hexagonal microdisks and microrods. By selectively exciting the NaYF₄:Tm³⁺ microcrystals with a 656-nm pulsed laser at a pulse duration of 10 ns, they exhibited a strong single-band down-conversion emission at 800 nm. We systematically studied how the excitation wavelength, temperature, and length-to-diameter ratio of the particles affected the luminescence intensity of their near-infrared (NIR) single-band emission. As the length-to-diameter ratio of the NaYF₄:Tm³⁺ microcrystals increased, their luminescence intensity strengthened. Exploring the reason for this luminescence enhancement, we propose a mechanism based on vacancy defects.

Key words: NaYF₄:Tm³⁺, Selective excitation, Down-conversion luminescence, Length to diameter ratio of particles, Single-band luminescence emission