Nanoscale temperature measurements
Common macroscopic thermometers such as IR cameras do not have sufficiently high spatial resolution to map microscopic temperature gradients. Similarly, nanoscale temperature gradients remain inaccessible to microscopic thermometry methods using far-field optics or electrothermal methods.We are developing new thermometry techniques with ultrahigh spatial resolution to probe heat transfer at the nanoscale. We will use the scanning transmission electron microscopes at Rice’s state-of-the-art Electron Microscopy Center to demonstrate temperature mapping in the STEM, with the ultimate goal of achieving <10 nm spatial resolution thermal maps. These capabilities will offer new insight into heat transport across interfaces, and could enable improved hotspot measurements in electronic devices.
Publications: Wehmeyer et al, Appl. Phys. Lett. (2018)
Switchable and Nonlinear Thermal Devices
New thermal diodes, regulators, and switches can overcome the inherent limitations of traditional thermal resistors. Each of these new thermal components has a signature functionality: Thermal diodes rectify heat currents, thermal regulators maintain a desired temperature, and thermal switches actively control the heat transfer. We are researching new mechanisms to achieve tunable and nonlinear thermal performance, and demonstrating applications of these devices in solid-state refrigeration cycles.
Review article: Wehmeyer et al, Appl. Phys. Rev. (2017)
Sub-continuum transport simulations
Since classic continuum models such as Fourier’s law break down at the nanoscale, we develop new numerical simulations to model thermal transport in complicated nanostructures. For example, we use a ray tracing technique to track phonons as they scatter off the surfaces of a nanostructure, and use this information to calculate the thermal conductivity. Quantifying the size effects on the thermal conductivity allows us to test different models for phonon transport against experiments, and to design nanostructures to have a desired thermal performance.
Anisotropic thermal transport in nanostructures
Thermal transport in anisotropic materials can be counterintuitive; for example, the heat flux adiabats are no longer necessarily parallel to the temperature isotherms in anisotropic crystals such as black phosphorus. We use analytical modeling to understand heat transfer in anisotropic nanostructures by solving the governing Boltzmann Transport Equation. These analytical solutions simplify the analysis of heat transfer at the nanoscale by showing that a classic Onsager reciprocity relation for the thermal conductivity also extends to thin films.
Publications: Wehmeyer et. al., Phys. Rev. B (2018)