Probing Radiative Thermal Transport at the Nanoscale.

Abstract

Thermal radiative emission from a hot to a cold surface plays an important role in many applications, including energy conversion, thermal management, lithography, data storage, and thermal microscopy. While thermal radiation at length scales larger than the dominant wavelength is well understood in terms of Planck’s law and the Stefan-Boltzmann law, near-field thermal radiation is not. With constantly advancing micro- and nanofabrication techniques and ever smaller devices a substantial need for a better and more reliable understanding of the fundamental physics governing nanoscale radiative heat transfer has arisen. Unfortunately, and in stark contrast to the abundance of theoretical and numerical work, there have only been limited experimental efforts and achievements. The central challenge in the field is to accurately and unambiguously characterize radiative heat transport between well-defined surfaces across nanometer distances.

The key scientific and technological questions that I have experimentally addressed during my doctoral study include: How does radiative heat transfer between an emitter and a receiver depend on their spatial separation (gap size), and does the radiative heat flux increase by over five orders of magnitude as the gap size is reduced to a few nanometers, as theoretically predicted? Can polar dielectric and metallic thin films support substantial near-field heat flow enhancement? For single-digit nanometer gaps, is the widely used theoretical framework of fluctuational electrodynamics (still) applicable? To address these challenging questions in gap sizes as small as tens of nanometers, we developed a nanopositioning platform to precisely control the gap between a microfabricated emitter device and a suspended receiver/calorimeter device which enables simultaneous measurement of the radiative heat flow across the gap. Further, we employed an atomic force microscope (AFM) in conjunction with stiff custom-fabricated scanning thermal microscopy (SThM) probes to explore the extreme near-field characterized by gaps of a few nanometers. In both approaches, high vacuum, vibration isolation and temperature control are implemented for accurate thermal measurements and for maintaining a stable gap. Finally, we performed state-of-the-art fluctuational electrodynamics-based calculations and analysis to compare theoretical predictions with experimental observations.