Phonon Energy Dependent Energy Transfer Upconversion for the Red Emission in the Er³⁺/Yb³⁺ System

The red emission through upconversion (UC) upon 980 nm excitation based on Er 3+ /Yb 3+ combination is very attractive for bioimaging applications. The intensity of the red emission is observed to be strongly dependent on the host materials. However, the origin of the behavior and the quantitative dependence remain unclear. Here, the effectiveness of the second step UC excitation from the Er 3+ intermediate state 4 I 13/2 to the 4 F 9/2 level by energy transfer from Yb 3+ is studied for three popular hosts (β-NaYF 4, Ba 5 Gd 8 Zn 4 O 21, and Y 2 O 3 ) that have different phonon energies. Their emission efficiencies of the red emitting state are calculated, and the radiative lifetime of the 4 F 9/2 level in Ba 5 Gd 8 Zn 4 O 21 is reported for the first time. We present a spectroscopic method to evaluate the relative energy transfer coefficients for the three hosts and find the coefficient increases markedly with the increase of phonon energy, reflecting the nature of phonon-assisted energy transfer. The coefficient for β-NaYF 4 is 89 and 408 times smaller than that for the other two oxide hosts, well revealing the origin of the green emission governed UC in β-NaYF 4 and the red emission in the other two. Accordingly, a comprehensive analysis of the luminescence dynamical processes shows that selecting material with appropriate phonon energy is essential for both effective excitation and efficient emission of the red level.