Molecular Diffusion and Self-Assembly: Quantifying the Influence of Substrate hcp and fcc Atomic Stacking

Area of Science:

• Surface science
• Materials science
• Physical chemistry

Background:

• Molecular diffusion is crucial for surface-confined self-assembly and synthesis.
• Substrate topography significantly impacts molecular assembly, alignment, and reaction pathways.
• Understanding the interplay between topography and diffusion is key to controlling thermodynamic processes.

Purpose of the Study:

• To investigate the influence of substrate topography on molecular diffusion and self-assembly.
• To identify preferential adsorption sites for tetraphenylporphyrin (2H-TPP) on Au(111).
• To correlate molecular island nucleation and growth with variations in diffusion barriers.

Main Methods:

• Variable-temperature scanning tunneling microscopy (STM) was used to observe molecular diffusion.
• Arrhenius analysis was applied to quantify diffusion barriers on the Au(111) surface reconstruction.
• Characterization of molecular diffusion within different atomic arrangements (fcc and hcp regions) of the herringbone reconstruction.

Main Results:

• Tetraphenylporphyrin (2H-TPP) exhibits preferential adsorption sites on Au(111).
• Spatial variations in diffusion barriers, driven by the Au(111) 22× √3 herringbone reconstruction, were observed.
• Higher diffusion barriers were found within fcc regions compared to hcp regions.
• Island growth predominantly occurred within fcc regions due to these diffusion barrier differences.

Conclusions:

• The atomic arrangement of the Au(111) surface reconstruction directly influences molecular diffusion barriers.
• Preferential molecular self-assembly is governed by substrate-induced variations in diffusion, specifically favoring fcc regions.
• This study demonstrates a clear link between substrate structure, molecular diffusion, and the resulting molecular self-assembly patterns.