Abstract
AbstractThe 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, nonequilibrium 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 ± 0.1° 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 is found to be 2 ± 1 × 10−8 K m2 W−1.
Highlights
The properties of ultra-thin molecular layers on surfaces are governed by the delicate interplay between adsorbate-adsorbate and adsorbate-surface interactions.[1]
The substrate consists of three distinct parts: 6H-Silicon Carbide (SiC), a Graphitic Buffer Layer (BL) and Few-Layer Graphene (FLG), as shown in Figure 1 B
High-resolution Scanning Tunneling Microscopy (STM) images of the graphene surface show a shortrange hexagonal structure corresponding to the graphene lattice
Summary
The properties of ultra-thin molecular layers on surfaces are governed by the delicate interplay between adsorbate-adsorbate and adsorbate-surface interactions.[1]. Used techniques which satisfy both requirements include Scanning Tunneling Microscopy (STM) and Low-Energy Electron Diffraction (LEED).[4]. Many elementary processes, such as vibrational relaxation, molecular motion, and bond formation/breaking, occur on femto- to nanosecond timescales.[5] experimental techniques with high surface sensitivity and high temporal resolution are desirable. Specific ultrafast (non-)linear spectroscopic methods are able to track changes at surfaces,[6] but the development of techniques that can resolve structural details to a high degree has proven to be exceedingly challenging. Time-resolved variants of Reflection High-Energy Electron Diffraction[7] and STM[8] are available. Ultrafast LEED (ULEED) has recently been developed in our group[9,10] to enable the investigation of dynamical phenomena on surfaces with a temporal resolution down to a picosecond.[11]
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