Spectral probing of surface luminescence of cubic Lu₂O ₃: Eu³⁺ nanocrystals synthesized by hydrothermal approach

Cubic Lu₂O₃:Eu³⁺ nanorods, nanosheets, and nanoparticles are synthesized by a hydrothermal approach by adjusting the pH values of the precursor solutions. The unique luminescence properties of the nanocrystals are presented by the appearances of a long tail at: the long-wavelength side of the charge transfer band (CTB), a novel ⁷F₀-⁵D₀ broad line, and the enhanced 624 nm emission line. The observed phenomena are more obvious with decreasing sample size in the order of nanorods > nanosheets > nanoparticles. Based on the experimental results of the thermal diffusion processes of Eu ³⁺ from, the surface to the inside of Lu₂O₃ particles, we conclude that the unique luminescence properties of the nanocrystals originate from the surface Eu³⁺. By the spectral decomposition, we obtain the excitation spectra of the interior Eu³⁺ and the surface Eu³⁺ of nanocrystals. In comparison with the bulk sample, the CTB of the interior Eu³⁺ presents an obvious blue shift due to the size confinement effects, while that of the surface Eu³⁺ shows a prominent red shift due to the distorted surface environments. The shorter lifetimes of the surface Eu³⁺ compared to that of the interior Eu³⁺ further indicate the distorted local environments on the surface of the nanocrystals. The shifts and the broadening of the CTB in Eu³⁺-doped nanocrystals can be well understood by the interior Eu³⁺ and the surface Eu³⁺.