A bona fide two-dimensional percolation model: an insight into the optimum photoactivator concentration in La2/3-x Eu x Ta2O7 nanosheets.

La-Eu solid solution nanosheets La2/3-x Eu x Ta2O7 have been synthesized, and their photoluminescence properties have been investigated. La2/3-x Eu x Ta2O7 nanosheets were prepared from layered perovskite compounds Li2La2/3-x Eu x Ta2O7 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 TaO6 octahedra tilting for the La- and Eu-rich phases. La2/3-x Eu x Ta2O7 nanosheets exhibit red emission, characteristic of the f-f transitions in Eu3+ 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 La2/3-x Eu x Ta2O7 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 La2/3-x Eu x Ta2O7 nanosheets are excellent models for validating the theory of optimum photoactivator concentration in the truly two-dimensional photoactivator matrix.