Eu<sub>0.56</sub>Ta<sub>2</sub>O<sub>7</sub>:  A New Nanosheet Phosphor with the High Intrananosheet Site Photoactivator Concentration

High photoactivator concentration nanosheets Eu<sub>0.56</sub>Ta<sub>2</sub>O<sub>7</sub> have been prepared by the soft chemical exfoliation reaction of the layered perovskite Li<sub>2</sub><sub>-</sub><i><sub>x</sub></i>H<i><sub>x</sub></i>Eu<sub>0.56</sub>Ta<sub>2</sub>O<sub>7</sub> with a tetrabutylammonium hydroxide aqueous solution. In-plane X-ray diffraction and TEM and AFM observation results indicate that the Eu<sub>0.56</sub>Ta<sub>2</sub>O<sub>7</sub> nanosheet is crystalline and retains the layered perovskite structural feature of the bulk precursors. The most intense emission is observed from the <sup>5</sup>D<sub>0</sub> → <sup>7</sup>F<sub>2</sub> transition of Eu<sup>3+</sup>. The intensity of this emission by the host excitation exceeds 18 times that by the direct Eu<sup>3+</sup> excitation.