The concentration dependent up-conversion luminescence of Ho³⁺ and Yb³⁺ co-doped β-NaYF₄

The concentration dependent up-conversion luminescence of 0.1; 0.2; 0.5; 1; 1.5; 2; 3; 4 and 5% Ho³⁺ and 20% Yb³⁺ co-doped β-NaYF₄ colloidal nanocrystals have been studied. The obtained nanocrystals were homogenous in size and shape, as well as the crystallographic structure was pure. The absorption spectra, upconversion lifetimes, up-conversion emission and Quantum Yield power dependence have been measured to estimate the optimum doping concentration of Ho³⁺ ions. The experimental results were supported by the calculations based on Judd-Ofelt theory, in order to quantify the oscillator strengths in absorption and luminescence, excited-state radiative lifetimes, energy-transfer probabilities, and to estimate the quantum efficiencies of this interesting material. We have observed evident trend in rise- and luminescence decay profiles with rising Ho³⁺ activator concentration, and the 1–2% Ho³⁺ ions doping was found to be the most efficient nanophosphors. - Highlights: • The 2% Ho³⁺ doping was found to be the most efficient for upconversion in NaYF₄:20%Yb NPs, with QY reaching over 0.2%. • The experimental results were coherent and successfully quantified using Judd-Ofelt theory. • Green to red emission intensity ratio strongly depends on the concentration of Ho³⁺ ions and excitation intensity.