Real-time monitoring of the internal temperature of a battery with an optical fiber sensor based on β-NaYF₄:Er³⁺/Yb³⁺@NaYF₄ nanoparticles.

Near-infrared (NIR) to NIR upconversion emission is investigated in Tm³⁺/Yb³⁺/Li⁺ triply doped Y₂O₃ nanocrystals. Li⁺ ions doped in Y₂O₃: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 Y₂O₃: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 Tm³⁺ ion. Li⁺ ions doped in Y₂O₃: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.