Thermal transport through molecular monolayers in plasmonic nanogaps.

Thermal transport through molecules has been exceptionally difficult to measure robustly. This is despite many intriguing theoretical predictions such as thermal diodes or gated thermal conduction, with consequent useful applications. Here, we present a significant development in time-resolved photothermal measurements using the extreme plasmonic enhancement of light inside molecular nanogaps to follow thermal transport on the nanoscale. We exploit free-standing sheets of close-packed nanoparticles, which are locally heated using mid-infrared pulses, and track their cooling by lateral transport on microsecond timescales. The rate of thermal relaxation is found to be dependent on nanogap composition, measured for a series of thiolated aromatic molecules. Observed trends in thermal conductivity with molecular length and contact strength are broadly reproduced by non-equilibrium molecular dynamics simulations. However, we find additional consideration of molecular interactions and dynamic disorder are crucial in fully understanding our experimental findings.