Er3+/Yb3+ co-doped nanocrystal Y2O3 powders with different sizes were prepared by using a chemical auto-combustion reaction. X-ray diffraction (XRD), transmission electron microscopy (TEM), and scanning electron microscopy (SEM) were used to characterize the crystal structure and morphology of the samples. Fourier transform infrared (FT-IR) spectra showed that the surfaces of smaller particles absorbed a larger amount of NO−3, CO2−3 and OH− groups. These chemical groups caused an increase of the population ratios of the 4I13/2 level to the 4I11/2 level when the particle size decreased, thus resulting in a change of the red-green upconversion emission intensity ratio under 980 nm excitation. The 1.5 m emission intensity also changed with the particle size. These results suggest that surface status is an important factor influencing the luminescent properties of nanosized materials and that surface modification is needed in order to obtain the satisfied materials.
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