title: A bona fide two-dimensional percolation model: an insight into the optimum photoactivator concentration in La2/3-xEuxTa2O7nanosheets.
abstract: La-Eu solid solution nanosheets La2/3-xEuxTa2O7have been synthesized, and their photoluminescence properties have been investigated. La2/3-xEuxTa2O7nanosheets were prepared from layered perovskite compounds Li2La2/3-xEuxTa2O7as 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 rangesx≤ 0.3 andx≥ 0.4. This discontinuity is attributed to the difference in degree of TaO6octahedra tilting for the La- and Eu-rich phases. La2/3-xEuxTa2O7nanosheets 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-xEuxTa2O7nanosheets increases as the experimentally determined Eu contents increase up tox=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-xEuxTa2O7nanosheets are excellent models for validating the theory of optimum photoactivator concentration in the truly two-dimensional photoactivator matrix.
authors.affiliation: International Center for Materials Nanoarchitectonics, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan; Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan.
authors.forename: Tadashi C
authors.initials: TC
authors.lastname: Ozawa
authors.affiliation: Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan; Collaborative Innovation Center for Nanotech FIBER, Shinshu University, 3-15-1 Tokida, Ueda, Nagano 386-8567, Japan.
authors.forename: Katsutoshi
authors.initials: K
authors.lastname: Fukuda
authors.affiliation: International Center for Materials Nanoarchitectonics, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan; Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan.
authors.forename: Yasuo
authors.initials: Y
authors.lastname: Ebina
authors.affiliation: Materials Analysis Station, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan.
authors.forename: Kosuke
authors.initials: K
authors.lastname: Kosuda
authors.affiliation: Materials Analysis Station, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan.
authors.forename: Akira
authors.initials: A
authors.lastname: Sato
authors.affiliation: Quantum Beam Center, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan.
authors.forename: Yuichi
authors.initials: Y
authors.lastname: Michiue
authors.affiliation: Transmission Electron Microscopy Cluster, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan.
authors.forename: Keiji
authors.initials: K
authors.lastname: Kurashima
authors.affiliation: International Center for Materials Nanoarchitectonics, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan; Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan.
authors.forename: Takayoshi
authors.initials: T
authors.lastname: Sasaki
keywords: charge transfer; lanthanide; layered oxide; phosphor; rare earth