Abstract: Upconversion (UC) enables the conversion of lower energy to higher energy photons and has gained interest for the application in solar cells and nanocrystal biolabels. Er3+-doped Gd2O2S is a highly promising UC material for applications. A record UC quantum yield of 12% was recently measured in Gd2O2S doped with 10% Er3+ upon monochromatic excitation into the 4I13/2 state at 1510 nm for moderate excitation densities (700 W m-2). In this work, the focus is on the dynamics of the infrared (4I13/2, ~1500 nm) to near-infrared (4I11/2, ~1000 nm) UC luminescence in Er3+-doped Gd2O2S. On the basis of luminescence spectra (emission and excitation), UC emission decay curves, and rate equation modeling, it is shown that energy transfer upconversion (ETU) is the mechanism responsible for the 4I11/2 UC luminescence, and the ETU parameter for Gd2O2S:10%Er3+ is determined to be WETU = 6.3 X 10-19 cm3 s-1. The UC dynamics depend on the Er3+ concentration and, similar to triplet?triplet annihilation UC in organic materials, on the excitation power. The results highlight the subtle balance between desired energy transfer processes leading to ETU and undesired energy migration to quenching sites. The overall UC efficiency is dependent not only on the material and composition but also on the synthesis process.

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