The transfer of heat in nanostructures works differently than it does in the large objects we see and use in everyday life. In nanostructures, both classical and quantum mechanical effects are important. In solids, for example, phonons carry heat energy away from its source as they scatter and diffuse through a material. However, when the heat source is very tiny (with nanoscale dimensions), the carriers travel rapidly together for a distance roughly the size of the heat source before scattering and diffusing. This process can create localized hot spots, or nonlocal heating, in a nanomaterial.
Usually the flow of heat is determined by diffusion and driven by a difference in temperature, or thermal gradient. However, in nanoscale devices, the scale of the device can be smaller than the distance over which the heat carriers (phonons) scatter, and therefore heat flow is reduced. In this regime, it is not even possible to define a temperature. By heating an array of nanowires with a femtosecond laser and measuring how fast heat dissipates into two different materials, the Kapteyn/Murnane group was able to observe and accurately measure ballistic thermal transport from a tiny nanoscale hotspot for the first time. These measurements could have a significant impact on heat management and reliability in future nanoscale devices.
The group’s growing understanding of nanoscale heat transport will aid in the design of nanosystems to provide better insulation or to recover heat or electric energy. The new understanding is also vital in nanoelectronics and other systems where the management of heat transport has already become a bottleneck to making smaller devices. For instance, when devices are much smaller than about 50 nm, nonlocal heat transport can lead to a “heat-sink” bottleneck, which can interfere with device operation.
The design of reliable nanoelectronic devices in the future will require co-engineering of both electron and phonon transport. Accomplishing this is predicated on a good understanding of nanoscale energy. Advanced dense magnetic storage devices will also require a precise understanding of heat transport at the nanoscale because they use heat energy to manipulate magnetic states.
Significant progress in understanding heat transport at the nanoscale has occurred during the past 20 years. However, many fundamental aspects of energy flow in nanostructures are still not well understood. One reason is that there have been very few experimental tools for probing nano systems measuring < 30 nm on fast timescales of picoseconds to femtoseconds. And, the basic models describing heat transport are still under development.
However, things may soon change. Ultrafast coherent beams produced by high harmonic generation (HHG) (invented by the Kapteyn/Murnane group) can directly measure energy flow at the nanoscale. Early work has already shown that ballistic heat transport decreases significantly as compared with Fourier law predictions.