Aberrations‐Corrected Ratiometric Parameters and PLQYs for Upconversion Thermometry in Hexagonal NaYF 4 :Yb 3+ ,Er 3+ and NaYF 4 :Yb 3+ ,Tm 3+ in Micro and Nanoscale Phosphors Using Novel Integrating Sphere Setup

ABSTRACT Photoluminescent materials have attracted a great attention due to their potential in remote temperature sensing, where conventional thermometers are not suitable, for instance in biological and medical applications. One of the popular techniques involves ratiometric thermometry, where the luminescence intensity ratio (LIR) between two thermally coupled transitions of a lanthanide ion is used for temperature readouts. However, aberrations in the optical collection system modify the LIR from setup to setup, preventing to have a standard to be shared from lab to lab. In this work, the thermometric parameters for upconverting (UC) lanthanide‐doped NaYF 4 phosphors across a range of sizes (micron‐sized powders and nanoparticles) are obtained. More importantly, their absolute photoluminescence quantum yields (PLQY) are determined at different temperatures in a novel setup using a modified integrating sphere with a temperature‐controlled sample holder. Despite integrating sphere set‐up allowing for correction of aberrations arising from collection optical system, emission self‐absorption has a major effect on LIR.