Energy transport dynamics in different nanostructures are important for both fundamental understanding and practical use of heat management at the nanoscale. It was reported that even with the same layered solid materials, their thermal conductivity may strongly depend on the degree of stacking disorder in the crystal structures. In this presentation, we present an entirely different system using reflection ultrafast electron diffraction for direct probing of structural dynamics. Solid-supported methanol assemblies can form both two-dimensional (2D) and three-dimensional (3D) orders depending on the annealing temperature and time;<sup>[1]</sup> the former layered structure may resemble those of 2D and van der Waals (vdW) solids but with much weaker in-plane intermolecular hydrogen bonds. Our results show that the energy transport dynamics in these two molecular assembly structures are drastically different. Following an energy impulse in an InAs(111) supporting substrate, the energy transfer dynamics of the 2D-layered structure is found consistent with a thermal diffusion mechanism, as prominent delays are observed in the response of molecular motions even with a small thickness increase. However, much faster energy transport is observed in the 3D-ordered structure, suggesting a ballistic type of propagation at work. The distinct behavior may have a direct connection to the efficiency of vibrational coupling between vdW-interacted methanol layers.

Order-determined energy transport dynamics in solid-supported interfacial methanol revealed by ultrafast electron diffraction

ACS Spring Meeting 2021

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