White Emission by Frequency Up-Conversion in Yb³⁺-Ho³⁺-Tm³⁺ Triply Doped Hexagonal NaYF₄ Nanorods

Abstract

Hexagonal Yb3+-Ho3+-Tm3+ triply doped NaYF4 nanorods were synthesized via a hydrothermal method using oleic acid as a stabilizing agent. White up-conversion (UC) luminescence consisting of the blue UC radiations at 450 and 475 nm corresponding to the 1D2 → 3F4 and 1G4 → 3H6 transitions of Tm3+ ion, green at 545 nm to the 5S2 /5F4 → 5I8 transition of Ho3+ ions and red at 695 nm to the 3F3 → 3H6 of Tm3+ ion and at 650 nm including both the 5F5 → 5I8 transition of the Ho3+ ion and the 3F2 → 3H6 transition of Tm3+ ion, was observed in the as-prepared nanorods under the excitation of a 980 nm diode laser. The calculated color coordinates display that with the increase of pump power densities, the tendency of white color output turns toward the blue region, revealing that white UC light can be fine-tuned in a wide range of pump power densities. On the basis of spectral and pump power dependence analyses, the UC mechanisms for the fine-tuned white output were discussed in detail.