Size dependence of the upconverted luminescence of Y2O2S: Er3+/Yb3+ microspheres for use in ratiometric thermometry

The size of lanthanide upconversion nanocrystals has a considerable impact on their luminous characteristics, but the underlying mechanism is yet unknown. Six typical samples were chosen for systematic size effect experiments after we manufactured monodisperse Y 2 O 2 S: Er 3+ /Yb 3+ microspheres. Our results establish surface defects induced by specific surface area as the fundamental mechanism governing nanoscale structure-property correlations, showing that phosphor particle size can be directionally controlled to maximize both luminescent performance and temperature-sensing capabilities. We have identified the mechanism behind size-dependent optical properties: larger particles alter the emission color and significantly enhance the luminescence intensity. Temperature-sensing behavior also exhibits strong size dependence: absolute sensitivity (S a ) correlates negatively with size and positively with temperature at the thermally coupled level (TCL); at the non-thermally coupled level (NTCL), S a correlates negatively with both size and temperature. It's interesting to note that the S a value for the smallest sample is 17 times higher at TCLs, reaching 0.0866 K -1 . Our findings provide a clear fundamental understanding of size effects in nanoscale lanthanide upconversion luminescence and offer important insights for nanoscale temperature sensing.