Far-Field Optical Nanothermometry via Luminescent Nanomaterials

Andrea Pickel, Department of Mechanical Engineering, University of Rochester

Hopeman 224

From transistors to LEDs, as device length scales trend downward, poor thermal dissipation increasingly leads to nanoscale hotspots that limit performance. To address this challenge, nanoscale thermometry tools must be developed. Conventional far-field optical methods like thermoreflectance provide a convenient non-contact approach, but these techniques are fundamentally diffraction limited. To circumvent the optical diffraction limit, we employ the temperature-dependent luminescence of lanthanide-doped upconverting nanoparticles to achieve sub-50 nm single-point temperature measurements. Single-particle measurements typically require excitation intensities orders of magnitude higher than nanoparticle ensembles, but the potential for single-particle self-heating has received limited attention because even highly conservative thermal estimates predict negligible self-heating. Unexpectedly, we observe an increase in the common “ratiometric” thermometry signal of individual NaYF₄:Yb³⁺,Er³⁺ nanoparticles corresponding to a temperature rise over 50 K if interpreted as thermal. To resolve this apparent conflict between model and experiment, we systematically vary the substrate thermal conductivity, nanoparticle-substrate contact resistance, and nanoparticle size. We experimentally demonstrate that this effect is an artifact, not a true temperature rise. Instead, rate equation modeling shows that this artifact is due to increased radiative and non-radiative relaxation from higher-lying Er³⁺ energy levels. These results have important implications for the calibrations required for accurate single-particle thermometry.