How does tuning the van der Waals bonding strength affect adsorbate structure?

Organic molecular thin-films are employed for manufacturing a wide variety of electronic devices, including memory devices and transistors. A precise description of the atomic-scale interactions in aromatic carbon systems is of paramount importance for the design of organic thin-films and carbon-based nanomaterials. Here we investigate the binding and structure of pyrazine on graphite with neutron diffraction and spin-echo measurements. Diffraction data of the ordered phase of deuterated pyrazine, (C₄D₄N₂), adsorbed on the graphite (0001) basal plane surface are compared to scattering simulations and complemented by van der Waals corrected density functional theory calculations. The lattice constant of pyrazine on graphite is found to be (6.06 ± 0.02) Å. Compared to benzene (C₆D₆) adsorption on graphite, the pyrazine overlayer appears to be much more thermodynamically stable, up to 320 K, and continues in layer-by-layer growth. Both findings suggest a direct correlation between the intensity of van der Waals bonding and the stability of the self-assembled overlayer because the nitrogen atoms in the six-membered ring of pyrazine increase the van der Waals bonding in comparison to benzene, which only contains carbon atoms.