Enhanced upconversion in Dy³⁺, Yb³⁺ co-doped Gd₂O₃ monodisperse nanocrystals

Dy³⁺/Yb³⁺ co-doped Gd₂O₃ monodispersed nanocrystals have been synthesized using the sodium dodecyl benzene sulfonate (SDBS) as the surfactant. The SDBS surfactant plays a critical role in the forming of monodisperse Gd₂O₃ structures with sphere-like morphologies. The growth mechanism of the nanocrystals was proposed in detail. The samples annealed at 400 °C for 4 h exhibit good crystallinity and narrow size distribution with average size of 6–9 nm. Green and red emission of Dy³⁺ related to ⁴G_11/2–⁶H_11/2 and ⁴G_11/2–⁶H_9/2 transition was observed under the laser excitation at 980 nm. An involvement of three photons has been observed for these emissions. The energy distribution of upconversion spectra depended on doping concentration and the intensity changed with the increasing of Dy³⁺ concentration. The mechanisms of upconversion by multi-photon absorption and energy transfer processes transitions were interpreted and explained.