Luminescent properties of rare earth ions in one-dimensional oxide nanowires.

Rare-earth doped one-dimensional oxide nanowires including LaPO4, La2O3, and Gd2O3 were synthesized by the hydrothermal method. Their luminescent properties including local environments, electronic transitions, energy transfer, and frequency up-conversion luminescence processes were systematically studied. In LaPO4:Eu and La2O3:Eu nanowires, different symmetry sites of Eu3+ ions were identified, which obviously differed from those of the corresponding micrometer-sized particles. This was attributed to crystal field degeneration in the fringe along the length axis. In LaPO4:Eu nanowires, the electronic transition rate of 5D1-sigmaJ7FJ increased approximately 2 times over that of the zero-dimensional nanoparticles and micrometer-sized particles, which was related to the variation of dipole field induced by shape anisotropy. Considering the nonradiative relaxations, meanwhile, the luminescent quantum efficiency for 5D1-sigmaJ7FJ transitions of Eu3+ in nanowires increased 100% over that in nanoparticles and 20% over that in micrometer particles. In Gd2O3:Eu3+, LaPO4:Ce3+, and LaPO4:Tb3+ nanowires and micrometer-sized particles, the electronic transition rate of rare earths had only a little variation. In LaPO4:Ce3+/Tb3+ nanowires, the energy transfer rate of Ce3+--> Tb3+ decreased 3 times compared to that in micrometer rods. Despite this, the brightness for the 5D4-7F5 green emissions of Tb3+ increased several times due to decreased energy transfer from the excited states higher than 5D4 to some defect levels. In Gd2O3:Er3+/Yb3+ nanocrystals, as the shape varied from nanopapers to nanowires, the relative intensity of up-conversion luminescence of 2H(11/2)/4S(3/2)-4I(15/2) and 4F(9/2)-4I(15/2) to the infrared down-conversion luminescence of 4I(13/2)-4I(15/2) increased remarkably, indicating efficient up-conversion luminescence. Our present results indicate that rare-earth-doped oxide nanowires is a type of new and efficient phosphors.