Structure and Non-Equilibrium Heat-Transfer of a Physisorbed Molecular Layer on Graphene

TL;DR

This study investigates the structure and non-equilibrium heat transfer of a physisorbed molecular layer on graphene, revealing its arrangement, stability, and thermal boundary resistance through advanced microscopy and ultrafast diffraction techniques.

Contribution

It provides detailed structural characterization and measures the thermal boundary resistance of a molecular layer on graphene using time-resolved diffraction methods.

Findings

Molecular layer forms an oblique, non-commensurate structure with six domains.

Layer is weakly physisorbed and desorbs at room temperature.

Thermal boundary resistance between the layer and graphene is approximately 2×10⁻⁸ K·m²/W.

Abstract

The structure of a physisorbed sub-monolayer of 1,2-bis(4-pyridyl)ethylene (bpe) on epitaxial graphene is investigated by Low-Energy Electron Diffraction and Scanning Tunneling Microscopy. Additionally, non-equilibrium heat-transfer between bpe and the surface is studied by Ultrafast Low-Energy Electron Diffraction. Bpe arranges in an oblique unit cell which is not commensurate with the substrate. Six different rotational and/or mirror domains, in which the molecular unit cell is rotated by 28{\pm}0.1{\deg} with respect to the graphene surface, are identified. The molecules are weakly physisorbed, as evidenced by the fact that they readily desorb at room temperature. At liquid nitrogen temperature, however, the layers are stable and time-resolved experiments can be performed. The temperature changes of the molecules and the surface can be measured independently through the Debye-Waller factor of their individual diffraction features. Thus, the heat flow between bpe and the surface can be monitored on a picosecond timescale. The time-resolved measurements, in combination with model simulations, show the existence of three relevant thermal barriers between the different layers. The thermal boundary resistance between the molecular layer and graphene was found to be 2{\pm}1{\cdot}10-8 K m2 W-1.