Single-Nanoparticle Luminescence Thermometry
As device length scales trend downward, poor thermal dissipation increasingly leads to nanoscale hotspots that limit overall performance. To address this challenge, developing nanoscale thermometry tools is key. One option is to utilize the temperature-dependent luminescence of an individual nanoparticle. When an isolated nanoparticle is excited using a diffraction-limited laser beam, the temperature-dependent emission comes only from the nanoparticle, thereby circumventing the optical diffraction limit. This approach allows for single-point temperature measurements with spatial resolution governed by the nanoparticle size. We have used individual NaYF4:Yb3+,Er3+ upconverting nanoparticles with characteristic dimensions 50 nm and smaller to perform such measurements. We are currently investigating other luminescent materials that are promising nanothermometry candidates.
Identifying Thermal vs. Non-Thermal Effects on Luminescent Signals
Individual upconverting nanoparticles require high excitation intensities, but the possibility of single-particle self-heating has received limited attention because even conservative thermal estimates predict negligible self-heating. Unexpectedly, we observed that the commonly used “ratiometric” thermometry signal of individual 50 x 50 x 50 nm3 NaYF4:Yb3+,Er3+ nanoparticles increases with excitation intensity, implying a temperature rise greater than 50 K if interpreted as thermal. Luminescence lifetime thermometry, which we demonstrated 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 varied the nanoparticle’s thermal environment by changing the substrate thermal conductivity, the nanoparticle-substrate contact resistance via the application of several coatings, and the nanoparticle size. The apparent self-heating remains unchanged in all cases, and we demonstrate that this effect is an artifact rather than a true temperature rise. Instead, rate equation modeling shows that this artifact results from increased radiative and non-radiative relaxation from higher-lying Er3+ energy levels.
High-Temperature Thermal Metrology
High-temperature operation can be advantageous for technologies such as incandescent lighting and thermoelectric energy conversion, where higher operation temperatures increase overall efficiency. However, high operating temperatures also pose challenges for conventional thermal metrology. For example, radiation contributions become important at high temperatures due to their T4 scaling, which also introduces a nonlinearity that complicates heat transfer models. Similarly, at higher temperatures thermal radiation shifts towards visible wavelengths and increases in magnitude, rendering detectors optimized for infrared emission from lower-temperature samples unsuitable. In collaboration with Prof. Liangbing Hu’s group at the University of Maryland, we combined image processing with a numerically solved, nonlinear heat transfer model model to extract the thermal conductivity of reduced graphene oxide materials synthesized by our collaborators that operate at temperatures up to 3,000 K.
See also: T. Li, A.D. Pickel et al., “Thermoelectric properties and performance of flexible reduced graphene oxide films up to 3000 K,” Nature Energy 3, 148-156 (2018). News & Views: Bring on the Heat