Self-assembled monolayers (SAM) form an attractive research platform due to their chemical tuneability for surface modifications and wide-ranging technological opportunities including molecular electronics, sensing, heterogeneous catalysis, as well as biophysical and medicinal applications. Structures and heat transport dynamics of SAMs are particularly important as they play critical roles in the interfacial behavior at the nanoscale. However, even with the existing reports using time-resolved sum-frequency generation or two-dimensional infrared vibrational echo spectroscopy, time-averaged scanning thermal microscopy, and theoretical molecular dynamics simulations, the understanding of ultrafast structural dynamics and the energy transport mechanism at the gold–sulfur interface and along the molecular chains of a SAM is still far from complete, especially when the external stimulus is applied on the same side of a SAM. In this presentation, we discuss the method of ultrafast electron diffraction to directly probe transient structural changes induced in n-alkanethiol SAMs on single-crystalline Au(111). Following an impulse heating of Au(111) using a femtosecond laser pulse, energy transport from the substrate to n-alkanethiol SAMs is obtained by monitoring the electron diffraction intensity as a function of delay time to extract the temperature evolution. Different parts of the gold–SAM system may be accessed at different depths by changing the electron grazing incidence angle, which allows a cross-examination of the dynamical responses. We find that the structural dynamics of SAMs exhibits a chain-length dependence and the thermal conductivity is derived based on a thermal conduction model. Judging from the time constants and the molecular motions involved, the importance of a ballistic heat transport mechanism and a potential bottleneck effect at the gold–sulfur head groups is questioned. We conclude with an outlook of future studies to further understand energy transport in nanofilms with different assembly structures.

Structural and energy transport dynamics in n-alkanethiol self-assembled monolayers on Au(111) revealed by ultrafast electron diffraction

ACS Spring 2022

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