Probing Heat Transport and Energy Conversion at the Atomic and Single Molecule Scale

Abstract

The study of heat transport and energy conversion constitutes an important subject in modern physics and engineering research. The widely-applied Fourier’s and Planck’s laws have proven to be very useful to describe heat transport via conduction and radiation at the macroscale. However, at the nanoscale, recent studies have highlighted the breakdown of these laws and important deviations from macroscale physical pictures. Understanding these emerging properties of energy transport and conversion at the nanoscale is expected to be crucial to developing a variety of technologies ranging from nanoelectronics, photonics, to thermoelectrics and photovoltaics. Unfortunately, in contrast to extensive studies of optical and electronic properties, thermal properties of materials and devices ranging in size from the atomic and single-molecule scale to the realm of a few nanometers, have remained largely unexplored due to the challenges in performing experiments with desired resolution.

My research aims to overcome these challenges by developing a series of experimental techniques and leveraging them to systematically answer a number of long-standing open questions including, i) How is heat conducted in atomic-sized objects such as single-atom junctions and single molecules? ii) How is heat converted to electrical energy at the atomic and molecular level via thermoelectric effects? Vice versa, can we observe the conversion of electrical to heating or cooling energy at these limits? iii) How is heat transferred radiatively in nanoscale gaps between surfaces?

To study heat transport in atomic-sized junction, we custom-fabricated extremely sensitive scanning thermal microscopy probes with picowatt resolution. These probes enabled us to perform measurements of thermal transport in metallic wires only single- or few-atom wide. These measurements enable the first-ever observation of quantized thermal transport at room temperature, i.e., measured thermal conductance is at values corresponding to the multiple integers of universal thermal conductance quantum. We were also able to validate the Wiedemann-Franz law all the way down to the single-atom limit with high accuracy. Furthermore, applying these probes, we performed experiments to quantify heat transport in single-molecule junctions to study how phonons transport at the molecular limit. Using alkanedithiol molecules of varying length as a prototypical system, we found that phonon transport in single-molecule junctions is ballistic in nature, nearly independent on the molecular length.

Moreover, we developed a unified platform with which multiple transport properties including energy dissipation, electrical conductance, and thermoelectric effects, can be measured for various molecular structures. By leveraging this platform, we have demonstrated molecular-scale refrigeration based on the Peltier effect. We found that by altering the structure of a molecule by a few atoms the cooling characteristics could be dramatically modulated, indicating the intimate relationship between the energy dissipation and the electron transmission in single-molecule junctions. Moreover, with STM-based scanning thermal probes, we investigated radiative heat transfer at the atomic scale, i.e. in gaps between two surfaces approaching a few nanometers down to ~2 Ångströms. In this regime, classical Planck’s law breaks down, giving rise to a dramatically enhanced radiative heat flux. We measured heat flux and found that thermal radiation in nanometer gaps indeed increase, via near-field electromagnetic effects, by several orders of magnitude. Our observations agreed well with the predictions of fluctuational electrodynamics. The presented techniques set the stage for future explorations of thermal and electrical properties of a broad range of atomic and molecular materials, low-dimensional structures, and nanoscale devices.