Effect of Li⁺ ions on enhancement of near-infrared upconversion emission in Y₂O₃:Tm³⁺/Yb³⁺ nanocrystals

Near-infrared (NIR) to NIR upconversion emission is investigated in Tm ³⁺/Yb³⁺/Li⁺ triply doped Y₂O ₃ nanocrystals. Li⁺ ions doped in Y₂O ₃:Tm³⁺/Yb³⁺ nanocrystals can greatly enhance the NIR upconversion emission intensity of Tm³⁺ ions. The abnormal shift of the (222) diffraction peak position determined from x-ray diffraction measurements is discussed, by introducing Li⁺ ions in the Y ₂O₃:Tm³⁺/Yb³⁺ nanocrystals. The cause of the enhancement is the modification of the local symmetry induced by the Li⁺ ions, which increases the intra-4f transitions of Tm ³⁺ ion. Li⁺ ions doped in Y₂O ₃:Tm³⁺/Yb³⁺ nanocrystals also can reduce the OH groups, dissociate the Yb³⁺ and Tm³⁺ ion clusters, and create the oxygen vacancies, which are the other reasons for enhancing the upconversion emission intensities. This material may be promising for in vitro and in vivo bioimaging probes.