Uniform colloidal spheres for RE3BO6 (RE = Eu-Yb,Y) and excitation-dependent luminescence of Y3BO6:Eu3+ red phosphor

Phosphor particles of spherical shape and uniform size are desired for high-definition displays to improve the resolution and the overall luminescent performance. However, the synthesis of RE₃BO₆ spherical particles is a considerable challenge in materials science. Here, uniform spheres of RE₃BO₆ (RE = Eu–Yb, Y) have been converted from their colloidal precursor spheres synthesized via homogeneous precipitation. The amorphous precursor spheres are solid particles with decreased boron going from the surfaces to the cores. Smaller particles were observed at decreased ionic radius from Eu³⁺ to Ho³⁺ (including Y³⁺), but particles with nearly unvaried sizes were observed by further decreasing the ionic radius from Ho³⁺ to Yb³⁺. They crystallized in monoclinic RE₃BO₆ at 900°C, with maintaining the spherical shape of precursors. However, the crystal growth and the densification toward the particle surfaces resulted in the formation of hollow spheres for smaller particles and core-shell structured spheres for larger particles. The parameters, a, b, and c, increase nearly monotonically with increasing the radius of rare earth ions. The uniform spheres of Y₃BO₆:Eu³⁺ exhibited a typical red emission at ~613 nm (⁵D₀ → ⁷F₂ electric dipole transition of Eu³⁺), with an intensity ratio I(⁵D₀ → ⁷F₂)/I(⁵D₀ → ⁷F₁) of ~3.5. The luminescence behavior of Y₃BO₆:Eu³⁺ phosphor is dependent on the excitation wavelength, which is closely related to the Eu³⁺ ions at different coordination sites. Driven by a 460-nm blue-LED chip, the Y₃BO₆:Eu³⁺ spheres exhibited a red emission with the CIE coordinates of (~0.65, ~0.35), indicating that they are an excellent red-emitting phosphor candidate for application in white-LEDs.