Understanding the effect of Mn²⁺ on Yb³⁺/Er³⁺ co-doped NaYF₄ upconversion and obtaining the optimal combination of these tridoping

In this work, we investigated in detail the upconversion properties of several types of nanoparticles, including NaYF₄:5%Yb³⁺/30%Mn²⁺, NaYF₄:40%Mn²⁺/x%Yb³⁺ (x% = 1, 5, 10, 20, 30, and 40), NaYF₄:2%Er³⁺/x%Mn²⁺ (x% = 20, 30, 40, 50, 60, and 70), NaYF₄:40%Mn²⁺/x%Er³⁺ (x% = 1, 2, 5, and 10), and NaYF₄:40%Mn²⁺/1%Yb³⁺/x%Er³⁺ (x% = 0, 2, 5, and 10). We studied their upconversion emission under 980 nm excitation in both pulsed and continuous wave modes at different synthesis temperatures. The nanoparticles were characterized using transmission electron microscopy (TEM), X-ray diffraction (XRD), and photoluminescence (PL) spectroscopy. The doping of Yb³⁺ and Mn²⁺ ions resulted in the nanoparticles assuming cubic and hexagonal crystal structures. The emission intensity increased (106.4 (a.u.*10³) to 334.4(a.u.*10³)) with increasing synthesis temperature from 120 to 140 °C, while a sharp decrease was observed when the synthesis temperature was increased to 200 °C. The gradual decrease in peak intensity with increasing Mn²⁺ concentration from 20 to 70% was attributed to energy transfer from Mn²⁺ to Yb³⁺. In NaYF₄:Mn²⁺/Yb³⁺/Er³⁺ UCNPs, increasing the Er³⁺ concentration from 0 to 10% led to the disappearance of the blue, orange, and green emission bands. The intense upconversion luminescence pattern with high spatial resolution indicates excellent potential for applications in displays, biological sensors, photodetectors, and solar energy converters.