Concentration-Dependent Near-Infrared to Visible Upconversion in Nanocrystalline and Bulk Y₂O₃:Er³⁺

We investigated the upconversion properties of nanocrystalline and bulk Y 2 O 3:Er 3+ as a function of the erbium concentration (1, 2, 5, 10, 25, and 35 mol %). Following excitation with 980 nm, upconverted emission is observed from the 2 H 11/2, 4 S 3/2, and 4 F 9/2 excited states to the 4 I 15/2 ground state centered at 525, 550, and 660 nm, respectively. As the dopant concentration is increased, the upconverted luminescence revealed not only an overall increase in intensity but also an enhancement of the red ( 4 F 9/2 → 4 I 15/2 ) emission with respect to the green ( 2 H 11/2, 4 S 3/2 → 4 I 15/2 ) emission. A cross-relaxation process is involved in populating the 4 F 9/2 state, which bypasses the green-emitting states. Blue upconversion, observed in bulk Y 2 O 3:Er 3+ only, also showed a concentration dependence. The population of the 2 P 3/2 state was achieved through a three-step phonon-assisted energy-transfer process.