Dependence of fluorescence lifetimes of Y2O3:Eu3+ nanoparticles on the surrounding medium

The radiative lifetime, ${\ensuremath{\tau}}_{R},$ of the ${}^{5}{D}_{0}$ metastable excited state of ${\mathrm{Eu}}^{3+}$ ions in nanocrystalline monoclinic ${\mathrm{Y}}_{2}{\mathrm{O}}_{3}$ samples is about four times longer than that in the micron size powder of the same material. The ${\mathrm{Eu}}^{3+}$ radiative lifetime was measured in nanocrystals surrounded with air as well as those immersed in different liquids. It is shown that the radiative lifetime changes with the index of refraction of the immersion medium and provides a unique test of the standard formula relating ${\ensuremath{\tau}}_{R}$ and the oscillator strength. The magnitude of the effect is determined by the ``filling factor'' (the fraction of the sample volume occupied by nanocrystals) which can therefore be determined.