Abstract
The red emission through upconversion (UC) upon 980 nm excitation based on Er3+/Yb3+ 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 Er3+ intermediate state 4I13/2 to the 4F9/2 level by energy transfer from Yb3+ is studied for three popular hosts (β-NaYF4, Ba5Gd8Zn4O21, and Y2O3) that have different phonon energies. Their emission efficiencies of the red emitting state are calculated, and the radiative lifetime of the 4F9/2 level in Ba5Gd8Zn4O21 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-NaYF4 is 89 an...
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