Molten salt synthesis of β-NaYF₄: Yb³⁺, Ln³⁺ (Ln = Er, Tm, and Ho) micro/nanocrystals with controllable morphology and multicolor upconversion luminescence

Well-defined β-NaYF₄:Yb³⁺, Ln³⁺ (Ln = Er, Tm, Ho) micro/nanocrystals were successfully synthesized for the first time by a facile mass production molten salt method without using any surfactant. X-ray diffraction, scanning electron microscopy, transmission electron microscopy, high-resolution transmission electron microscopy, and photoluminescence spectra were used to characterize the products. The results show that with increasing reaction time, the phase of NaYF₄ transforms from cubic to hexagonal, while the morphology changes from nanoparticles to solid microsheets then to porous microsheets. By changing the dopant species (Ln³⁺) in β-NaYF₄, multicolor (green, blue, yellow, and white) upconversion (UC) emissions can be realized under 980 nm laser diode excitation. The UC mechanisms in co-doped and tri-doped β-NaYF₄ samples were analyzed in detail based on the emission spectra. The merit of multicolor emissions in the visible region endows this kind of material with potential applications in the field of light displays, lasers, and optoelectronic devices.