Thermally controlled Er3+-Ho3+ energy transfer in NaYGeO4 olivine phosphor for enhanced NIR-II luminescence and sensitive thermometry

Lanthanide NIR-II luminescence free from thermal quenching is attractive for various applications at elevated temperatures. Here, we present a thermal enhancement of Er 3+ NIR-II luminescence based on the synergy of phonon assisted Er 3+ -Er 3+ cross relaxations and controllable Er 3+ -Ho 3+ energy transfer. The lattice expansion of NaYGeO 4 olivine phosphor upon heating (298–823 K) suppresses the Er 3+ -Ho 3+ energy transfer and leads up to 46.1 times enhancement of Er 3+ NIR-II luminescence under 379 nm excitation. The NIR-II luminescence evolution as increasing temperature allows dual-mode ratiometric thermometry, i.e., luminescence intensity ratios (LIRs) based on Er 3+ 4 I 13/2 Stark sublevels and on Er 3+ 4 I 13/2 and Ho 3+ 5 I 6 levels. Obtained relative sensitivities above 0.46% K ‒1 over a broad temperature range of 298–673 K with a maximum of 2.32% K -1 suggest NaYGeO 4 :Er 3+ /Ho 3+ phosphor as a promising NIR-II luminescence thermometer. Moreover, this research offers an available perspective for designing thermal-enhanced NIR luminescence in ordinary thermal expansion materials.