Correlation between the Covalency and the Thermometric Properties of Yb3+/Er3+ Codoped Nanocrystalline Orthophosphates

Lanthanide doped NaYF₄ nanoparticles are most frequently studied hosts materials for numerous biomedical applications. Although efficient upconversion can be obtained in fluoride nanomaterials and good homogeneity of size and morphology is achieved, they are not very predestined for extensive material optimisation towards enhanced features and function. Here we study the impact of RE=Y, Lu, La and Gd ions within Yb³⁺/Er³⁺ co-doped nanocrystalline REPO₄ orthophosphates. The enhanced luminescent thermometry features were found in relation to the covalency of RE³⁺-O^2- bond, being modulated by these optically inactive rare earth ions substitutes. Up to 30% relative sensitivity enhancement was found (from c.a.3.0 to c.a.3.8 %/K at -150^oC) by purposefully increasing the covalence of the RE³⁺-O^2- bond. These studies form the basis for intentional optimisation thermally coupled based luminescent thermometers, such as Yb³⁺-Er³⁺ upconverting ratiometric thermometer.