Color tunability of the upconversion emission in Er–Yb doped the wide band gap nanophosphors ZrO2 and Y2O3

Recently an increasing interest in the investigation of nanocrystalline wide band gap oxides for both fundamental research and potential applications in upconversion phosphors, fluorescent labels for sensitive detection of biomolecules, white light generation, and two-photon confocal-microscope imaging. Er3+/Yb3+ doped Y2O3 and ZrO2 present favorable physical properties, such as a wide band gap between 5.6 and 7.0 eV, and ease of synthesis in the nanometer regime. In particular, the intensity of the visible rare earth luminescence depends on dopant concentration, crystalline phase and size of the nanocrystals. As a result, it is possible to control the emission color between red and green by adjusting the Er and Yb dopant concentrations. In this work, based on a stationary solution of the rate equations that govern the luminescence emission, we derive a simple model that allows the prediction of the overall color emission depending on the Yb and Er concentration. In addition, our model allows the facile quantification of the upconversion mechanisms responsible of the visible color emission in these nanophosphors under 970 nm excitation.