Direct observation of dynamic energy transport and high thermal conductance across solid–molecule junctions

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. Here, we report dynamic energy transport processes across a molecular junction 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 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 independence but with a length-dependent retention time. We find an ultrahigh thermal conductivity for the methylene lattice and a high interfacial thermal conductance at early times under the condition of impulsive heating. In contrast, these values become less significant to the thermalization and recovery of the whole assembly system at longer times. We anticipate that this time- and spatially-resolved experimental approach will enable future quantitative assessment of interfacial heat transport across solid–molecule junctions.