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

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.PHDMechanical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/147709/1/longji_1.pd

Tuning light emission crossovers in atomic-scale aluminum plasmonic tunnel junctions

Atomic sized plasmonic tunnel junctions are of fundamental interest, with great promise as the smallest on-chip light sources in various optoelectronic applications. Several mechanisms of light emission in electrically driven plasmonic tunnel junctions have been proposed, from single-electron or higher order multi-electron inelastic tunneling to recombination from a steady-state population of hot carriers. By progressively altering the tunneling conductance of an aluminum junction, we tune the dominant light emission mechanism through these possibilities for the first time, finding quantitative agreement with theory in each regime. Improved plasmonic resonances in the energy range of interest increase photon yields by two orders of magnitude. These results demonstrate that the dominant emission mechanism is set by a combination of tunneling rate, hot carrier relaxation timescales, and junction plasmonic properties.Comment: 25 pages, 5 figures, plus 19 pages supporting information with 10 figure

Thermal conductance of single-molecule junctions

Single-molecule junctions have been extensively used to probe properties as diverse as electrical conduction, light emission, thermoelectric energy conversion, quantum interference, heat dissipation and electronic noise at atomic and molecular scales. However, a key quantity of current interest???the thermal conductance of single-molecule junctions???has not yet been directly experimentally determined, owing to the challenge of detecting minute heat currents at the picowatt level. Here we show that picowatt-resolution scanning probes previously developed to study the thermal conductance of single-metal-atom junctions, when used in conjunction with a time-averaging measurement scheme to increase the signal-to-noise ratio, also allow quantification of the much lower thermal conductance of single-molecule junctions. Our experiments on prototypical Au???alkanedithiol???Au junctions containing two to ten carbon atoms confirm that thermal conductance is to a first approximation independent of molecular length, consistent with detailed ab initio simulations. We anticipate that our approach will enable systematic exploration of thermal transport in many other one-dimensional systems, such as short molecules and polymer chains, for which computational predictions of thermal conductance have remained experimentally inaccessible

Quantized thermal transport in single-atom junctions

Thermal transport in individual atomic junctions and chains is of great fundamental interest because of the distinctive quantum effects expected to arise in them. By using novel, custom-fabricated, picowatt-resolution calorimetric scanning probes, we measured the thermal conductance of gold and platinum metallic wires down to single-atom junctions. Our work reveals that the thermal conductance of gold single-atom junctions is quantized at room temperature and shows that the Wiedemann-Franz law relating thermal and electrical conductance is satisfied even in single-atom contacts. Furthermore, we quantitatively explain our experimental results within the Landauer framework for quantum thermal transport. The experimental techniques reported here will enable thermal transport studies in atomic and molecular chains, which will be key to investigating numerous fundamental issues that thus far have remained experimentally inaccessible.publishe

Influence of Quantum Interference on the Thermoelectric Properties of Molecular Junctions

Molecular junctions offer unique opportunities for controlling charge transport on the atomic scale and for studying energy conversion. For example, quantum interference effects in molecular junctions have been proposed as an avenue for highly efficient thermoelectric power conversion at room temperature. Toward this goal, we investigated the effect of quantum interference on the thermoelectric properties of molecular junctions. Specifically, we employed oligo(phenylene ethynylene) (OPE) derivatives with a para-connected central phenyl ring (para-OPE3) and meta-connected central ring (meta-OPE3), which both covalently bind to gold via sulfur anchoring atoms located at their ends. In agreement with predictions from ab initio modeling, our experiments on both single molecules and monolayers show that meta-OPE3 junctions, which are expected to exhibit destructive interference effects, yield a higher thermopower (with ∼20 μV/K) compared with para-OPE3 (with ∼10 μV/K). Our results show that quantum interference effects can indeed be employed to enhance the thermoelectric properties of molecular junctions