Modulation of phosphor luminescence performance by high concentration self-sensitization of Er and Ho–Yb ion co-doping under 1550 nm excitation

Abstract

In this paper, upconversion fluoride phosphors NaY_{1−x−y−z}F₄:Er_x³⁺, Ho_y³⁺, Yb_z³⁺ were synthesized by the low-temperature combustion method. And the optimal ratio of lanthanide ion doping in the matrix lattice was determined by the control variable method. First of all, the experimental results show that some Er³⁺-2 ions may be present in the samples doped with a high concentration of Er³⁺ ions, and the energy carried by the 1550 nm photons is absorbed by them and transferred to the remaining Er³⁺-1 ions in the form of sensitizers, which leads to saturation excitation of these Er³⁺-1 ions. Thus, the upconversion luminescence intensity of the Er³⁺ ion-doped samples was dramatically increased. Secondly, it was found that the singly doped Ho³⁺ ion samples also showed significant absorption of 1550 nm photons. In contrast, doping Yb³⁺ ions in samples singly doped with Ho³⁺ ions produces the opposite effect of Er³⁺ ions. The upconversion luminescence intensity of the Ho³⁺ ion-doped samples is significantly quenched. According to the above experimental phenomena, when a small amount of Ho³⁺ ions are doped into the matrix lattice of the sample doped with a high concentration of Er³⁺ ions, firstly, these Ho³⁺ ions can act as transient energy transition centers in the lattice. Secondly, they can also play the role of another self-absorption activation center in the matrix lattice. The upconversion luminescence performance of the Er³⁺–Ho³⁺ ion co-doped samples is significantly enhanced, so the characteristic emissions of Er³⁺ and Ho³⁺ ions are highly overlapped in the visible region. A small amount of Yb³⁺ ions continue to be doped into the Er³⁺–Ho³⁺ ion co-doped system, due to the significant quenching effect of the Yb³⁺ ions on the luminescence of the Ho³⁺ ions. The Yb³⁺ ions mainly play the role of reverse energy transfer centers between the Er³⁺–Yb³⁺ ions in the crystal lattice. This results in the upconversion luminescence intensity of the triple-doped samples being enhanced significantly by increasing the utilization of the system for the 1550 nm photons. In this paper, the phase composition and morphology of the phosphors were studied by an X-ray diffractometer and scanning electron microscope. The upconversion luminescence mechanism of Er³⁺–Ho³⁺–Yb³⁺ ion triple-doped samples under 1550 nm excitation and the sensitization interactions between the ions were systematically investigated by upconversion emission spectra and fluorescence lifetime. This work provides a new idea for the design of high-color purity upconversion luminescent phosphors under 1550 nm excitation, and the prepared phosphors can be applied in the field of display lighting.