Near-Infrared-to-Near-Infrared Excited-State Absorption in LaPO4:Nd3+ Nanoparticles for Luminescent Nanothermometry

Luminescent thermometry (LT) is a technique that enables contactless temperature determination based on temperature dependent luminescence of phosphors. Among different LTs that have been described in the literature so far, the luminescence intensity ratio (LIR) based thermometers have shown the highest application potential. Nevertheless, to determine accurate temperature, ratiometric method encounters technical restrictions related to the need for spectral separation of temperature dependent emission bands. An alternative ratiometric approach can exploit the intensity of a single emission band being excited in two ways related to ground state absorption (GSA) and excited state absorption (ESA). In this work, this approach, i.e., luminescent thermometry involving ESA process in LaPO4:Nd3+ nanocrystals, was demonstrated. Thermal energy delivered to the system was responsible for partial population of Nd3+: I-4(11/2) level, which enabled non-GSA-absorption of 1060 nm excitation line and resulted in appearance of strongly temperature dependent emission band at 890 nm. The further temperature increase favored population of higher laying levels, resulting in observation of 810 and 750 nm emission. On the other hand, the intensity of the emission band at 890 nm being excited in a resonant GSA way via the 808 nm line was strong and barely dependent on temperature, thus serving as a reference. Therefore, three luminescence intensity ratio (LIRi) equations were defined to determine temperature in a contactless way. The subsequent LIRs were calculated as the ratios of emission intensities at 890, 810, and 750 nm being excited in a non-GSA-resonant ESA-resonant way normalized to the band at 890 nm excited with 808 nm line (through GSA). The highest relative sensitivities were unprecedentedly high and reached S-1 = 7.19%/degrees C at 30 degrees C, S-2 = 3.04%/degrees C at 100 degrees C, and S-3 = 4.35%/degrees C at 180 degrees C for the subsequent LIRi ratios.