A bona fide two-dimensional percolation model: an insight into the optimum photoactivator concentration in La2/3-xEuxTa2O7 nanosheets

La-Eu solid solution nanosheets La 2/3−x Eu x Ta 2 O 7 have been synthesized, and their photoluminescence properties have been investigated. La 2/3−x Eu x Ta 2 O 7 nanosheets were prepared from layered perovskite compounds Li 2 La 2/3−x Eu x Ta 2 O 7 as the precursors by soft chemical exfoliation reactions. Both the precursors and the exfoliated nanosheets exhibit a decrease in intralayer lattice parameters as the Eu contents increase. However, there is a discontinuity in this trend between the nominal Eu content ranges x≤ 0.3 and x ≥ 0.4. This discontinuity is attributed to the difference in degree of TaO 6 octahedra tilting for the La- and Eu-rich phases. La 2/3−x Eu x Ta 2 O 7 nanosheets exhibit red emission, characteristic of the f-f transitions in Eu 3+ photoactivators. The photoluminescence emission can be obtained from both host and direct photoactivator excitation. However, photoluminescence emission through host excitation is much more dominant than that through direct photoactivator excitation, and this behavior is consistent with that of all the other rare-earth photoactivated nanosheets reported previously. The absolute photoluminescence quantum efficiency of the La 2/3−x Eu x Ta 2 O 7 nanosheets increases as the experimentally determined Eu contents increase up to x=0.45 and decrease above it. This result is in good agreement with the optimum photoactivator concentration expected from the percolation theory. These solid solution La 2/3−x Eu x Ta 2 O 7 nanosheets are excellent models for validating the theory of optimum photoactivator concentration in the truly two-dimensional photoactivator matrix.