Manipulation phonon energy for improved thermometric sensitivity of only-core nanoparticles

Abstract

Luminescent nano thermometry using upconversion nanoparticles (UCNPs) has gained attention in recent years due to its potential advantages over conventional methods. One major challenge in practical applications of nanothermometers is achieving high sensitivity. While efforts have been made to improve the sensitivity of individual core nanoparticles, there are few reports on incorporating phonon energy and local crystal field distortions to improve the sensitivity of only-core upconversion nanoparticles. In this study, we propose a new approach to improve the sensitivity of only-core UCNPs thermometers through phonon tuning and self-photogenic heating. We use the ratio of the intensity of two bands (centered at 520 nm and 540 nm) based on dopant Er3+ as a referenced signal for optical sensing of temperature. We use a host material NaYF4 doped with K+ to produce local crystal field changes at the interface, modifying the phonon energy mode. As the temperature increases, the vibrational mode cannot propagate smoothly along the crystal axis throughout the crystal, disrupting the direction of lattice heat conduction and producing a significant local thermal effect on the nanoscale. This leads to a significant increase in sensitivity, and we achieve an enhanced thermal sensitivity more than twice that of conventional core-only nanoparticles.