Direct Observation of Energy Transport Dynamics and High Thermal Conductance across Single Solid-Molecule Junctions

TL;DR

This study uses ultrafast electron diffraction to directly observe energy transfer dynamics at the atomic level across solid-molecule interfaces, revealing high thermal conductance and ballistic heat transfer in molecular junctions.

Contribution

It provides the first direct atomic-level observation of interfacial energy transport and quantifies thermal conductance across single solid-molecule junctions.

Findings

High interfacial thermal conductance (~300 MW/m^2/K) observed.

Methylene lattice exhibits chain-length insensitivity in dynamics.

Evidence supports ballistic intrachain heat transfer.

Abstract

Interfaces play a crucial role in energy transport at the nanoscale. However, direct experimental observations of interfacial thermal conductance across molecular junctions have remained challenging due to the high spatiotemporal resolution required for probing. Here, we report dynamic energy transport processes across multi-component molecular junctions observed at the atomic level by employing reflection ultrafast electron diffraction. A clear temporal sequence of energy transfer is revealed at early times following photoexcitation of Au(111) surfaces chemically bonded with self-assembled monolayers (SAMs) of alkanethiols: from the gold surface layer (SL) to the head groups of a SAM and then to the CH2-CH2 methylene lattice. Remarkably, the structural dynamics of the gold SL differ significantly from those of clean gold. Furthermore, the methylene lattice dynamics exhibit chain-length insensitivity but with a length-dependent retention time to reach full thermalization. Quantitatively, we find an agreement in the increased out-of-plane atomic motions between the surface gold atoms and the SAM methylene units, signifying the nature of motion-based coupling for interfacial energy transport. High interfacial thermal conductance (~300 MW/m^2/K) and high thermal conductivity for the methylene lattice (~60 W/m/K) are obtained under the condition of impulsive heating, which provide strong support for previous room-temperature theoretical predictions and also the ballistic nature of intrachain heat transfer. This time- and spatially-resolved experimental approach will enable future quantitative assessment of interfacial energy transport across solid-molecule junctions.