Optical-telecommunication-band fluorescence properties of Er3+-doped YAG nanocrystals synthesized by glycothermal method

Er3+-doped YAG (Er:YAG) nanocrystals were prepared by a glycothermal synthesis, and the effect of heat-treatment on the size of the nanocrystals and their fluorescence properties in optical-telecommunication-band were examined. The crystallite size of the as-prepared nanocrystals was around 32 nm, which was evaluated from the broadening of X-ray diffraction peaks. When the heat-treatment temperature was 600 °C, the crystallite size was almost independent of the heat-treating time, while it increased with time at 800 and 1000 °C. The emission intensity of the 4I13/2 → 4I15/2 transition and the lifetime of the 4I13/2 level increased with increasing heat-treatment temperature and time. The decrease in the organic residues due to the combustion during the heat treatment at 600 °C and the decrease in the specific surface area induced by the heat treatment at 800 and 1000 °C are responsible for the observed phenomena, i.e., the increase in the emission intensity and lifetime. Quantum efficiencies of Er3+ ions in the nanocrystals were also evaluated based on an assumption that the radiative decay rates of the nanocrystals are equal to that of the bulk crystal. The relationship between the quantum efficiency and the specific surface area of the nanocrystals were discussed with a simple model, which led to the conclusion that Er3+ ions in the surface region with thickness about 10 nm do not work well as luminescence centers due to the many defects. The Er:YAG nanocrystals prepared by the glycothermal method had emission linewidth broader than the polycrystalline bulk crystal prepared by a solid-state reaction.