Figure 4

Energy level diagram and upconversion mechanism for the Yb³⁺ and Er³⁺ co- doped UCNPs system upon 980 nm laser excitation. Three major emission peaks centered at 520 nm, 540 nm, and 656 nm are observed in the range of UV to visible light spectrum, where two green emissions at 520 nm and540 nm are assigned to the ²H_11/2⁴I_15/2 and ⁴S_3/2⁴I_15/2 transition, respectively; and the red emission at 656 nm is assigned to the ⁴F_9/2⁴I_15/2 transition. The UCPL of Er³⁺ and Yb³⁺ ions co-doped nanoparticles excited by a 980 nm laser emit yellowish green light, which is a combination of green and red colour emissions.