Apparent self-heating of individual upconverting nanoparticle thermometers

TLDR

Individual luminescent nanoparticles enable sub‑diffraction thermometry, but self‑heating from high excitation intensities has been largely uninvestigated because models predict negligible heating. We systematically varied the nanoparticle’s thermal environment—substrate conductivity, contact resistance, and particle size—to reconcile the discrepancy between model predictions and experimental observations. We found that both ratiometric and lifetime thermometry of individual NaYF4:Yb3+,Er3+ nanoparticles show an apparent temperature rise exceeding 50 K with increasing excitation intensity, yet this effect persists regardless of substrate conductivity, contact resistance, or particle size, indicating it is an artifact caused by enhanced radiative and non‑radiative relaxation from higher‑lying Er3+ levels, with important implications for single‑particle thermometry.

Abstract

Individual luminescent nanoparticles enable thermometry with sub-diffraction limited spatial resolution, but potential self-heating effects from high single-particle excitation intensities remain largely uninvestigated because thermal models predict negligible self-heating. Here, we report that the common "ratiometric" thermometry signal of individual NaYF4:Yb3+,Er3+ nanoparticles unexpectedly increases with excitation intensity, implying a temperature rise over 50 K if interpreted as thermal. Luminescence lifetime thermometry, which we demonstrate for the first time using individual NaYF4:Yb3+,Er3+ nanoparticles, indicates a similar temperature rise. To resolve this apparent contradiction between model and experiment, we systematically vary the nanoparticle's thermal environment: the substrate thermal conductivity, nanoparticle-substrate contact resistance, and nanoparticle size. The apparent self-heating remains unchanged, demonstrating that this effect is an artifact, not a real temperature rise. Using rate equation modeling, we show that this artifact results from increased radiative and non-radiative relaxation from higher-lying Er3+ energy levels. This study has important implications for single-particle thermometry.