Improved Upconversion Luminescence Properties of Gd₂O₃:Er³⁺/Gd₂O₃:Yb³⁺ Core–Shell Nanorods

The Gd₂O₃:Er³⁺/Gd₂O₃:Yb³⁺ core–shell nanorods (NRs) as well as Er³⁺ and Yb³⁺ homogeneously codoped Gd₂O₃ NRs were synthesized by the hydrothermal method. The average diameters of the NRs are ∼20 nm and the lengths are 150–200 nm. The thickness of the Gd₂O₃:Yb³⁺ shells on the Gd₂O₃:Er³⁺ cores are ∼5 nm. The upconversion luminescence (UCL) properties of the core–shell NRs have been studied in contrast to homogeneously codoped NRs under 978-nm laser diode excitation. Green emissions of ²H_11/2, ⁴S_3/2–⁴I_15/2, and red emissions of ⁴F_9/2–⁴I_15/2 were observed. Relative intensity of the red (⁴F_9/2–⁴I_15/2) to the green (⁴S_3/2/²H_11/2–⁴I_15/2) and the intensity ratio (R_HS) of ²H_11/2–⁴I_15/2 to ²H_11/2–⁴I_15/2 decrease obviously in the core–shell NRs compared to the NRs. The power-dependence of UCL intensity and the thermal effect caused by 978-nm diode laser irradiation was studied. The result indicates that the cross-relaxation effect and the thermal effect in the core–shell structure is effectively depressed.