Session: Rising Stars of Mechanical Engineering Celebration & Showcase

Paper Number: 149840

149840 - Thermal Transport and Energy Conversion at Extreme Scales 

Introdution: Understanding and control of matter, energy and information at the nanoscale is one of the major hallmarks of modern physical and engineering sciences. Devices at the atomic and single-molecule scale represent the size limit to the miniaturization of any physical machine and have been studied to create unprecedented functionalities and new technologies in molecular and quantum electronics. In the context of thermal transport and thermal enegy conversion, directly probing heat transfer down to these extreme scales can elucidate the fundamental thermal transport and dissipation limits in these ultraminiaturized electronic and optical devices. A thorough understanding of the structure-property relationships of thermal transport and enegy conversion via single atoms and molecules or gaps of similar length scale is of particular importance to address the question that what are the ultimate limits to thermal conductivities of any materials and thermal energy conversion efficiency. Furthermore, extreme confinement of electrons and phonons and their interactions could lead to unique quantum charge and thermal transport properties, which could be leveraged as extra “tuning knobs” to enhance many different physical properties of materials that don't have classical analogy, creating new paradigms of quantum electronics, optoelectronics and thermoelectrics.

Contribution of current work: There have been a large number of theoretical studies have predicted a gamut of interesting thermal and thermoelectric effects in single molecule junctions. However, understanding of the general
principles underlying these prediction remains at the qualitative level. In fact, a key physical quantity—the thermal conductance of single-molecule junctions—has eluded direct experimental determination due to challenges in detecting extremely small picowatt-level heat currents that occur in such systems. Here, I will present my recent breakthrough of a novel type of custom fabricated scanning thermal microscopy (SThM) probes with single digit picowatt-resolution. This technique enabled the first-ever quantitative measurement of the thermal conductance of single-molecule junctions and investigate the length dependent thermal transport and the long pursued interference effect of phonon waves at room temperature. In conjuntion with the single molecule level thermal transport measurements, we have also developed molecular dynamics simulation methods which demonstrate higher accuracy and predictive power for room temperaure phonon transport study, which is not adequetly addressed by existing first principle simulation methods. The technique presented here represents a major breakthrough in the field of nanoscale energy transport that will enable further studies of thermal transport in many other 1D systems, short molecules and polymers, for which interesting thermal transport properties such phonon localization, divergent heat conduction, and thermal rectification effect that have been theoretically studied extensive over the past two decades but remain experimentally inaccessible.

Presenting Author: Longji Cui University of Colorado Boulder

Presenting Author Biography: Dr. Longji Cui is currently a faculty member in the Department of Mechanical Engineering and Materials Science and Engineering at the University of Colorado Boulder. He is affiliated with the Center of Experiments on Quantum Materials at CU Boulder. He received his PhD in Mechanical Engineering from the University of Michigan, Ann Arbor, and did his postdoc research in the Department of Physics and Astronomy and Smalley-Curl Institute at Rice University. His research interests include atomic, molecular, and quantum energy transport and conversion, high-resolution sensing technique, functional scanning probe microscopy, and thermal renewable energy technology.

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