Visible upconversion in rare earth ion-doped Gd₂O₃ nanocrystals

Rare earth ions (Tm³⁺, Er³⁺, and Yb ³⁺)-doped cubic Gd₂O₃ nanocrystals were prepared by a simple sol-gel method. Raman and FT-IR spectra were measured to evaluate the vibrational feature of the samples. Under 980 nm laser excitation, blue (488 nm), green (564 nm), and red (661 nm) upconversion has been recorded in Gd₂O₃:Tm+Yb and Gd₂O₃:Er (Gd ₂O₃:Er+Yb), respectively. A great enhancement of red emission and diminishment of green emission of Er³⁺ in Gd ₂O₃Er+Yb have been observed. Laser power and doping concentration dependence of the upconverted emissions were studied to understand the upconversion mechanisms. Excited absorption and energy-transfer processes are discussed as the possible mechanisms for the visible emissions.