Strain relaxation and epitaxial relationship of perylene overlayer on Ag(110).

We present a room temperature STM study of perylene self-assembly on Ag(110) beyond the monolayer coverage regime. Coupling of the perylene aromatic boards yields π-π bonded stacks. The perylene stacks self-assemble into a continuous three-dimensional epitaxial overlayer of (3 × 5) symmetry. The self-assembly is driven by thermodynamic balance established under coupling of the intra- and intermolecular interactions and the molecule-substrate interaction all accommodating the short-range thermal motion of the constituent molecules. The balance bestows to the overlayer the unique ability to accommodate the underlying substrate morphology and to spread over the surface steps as a single structure preserving its lateral order and keeping epitaxial relationship with every surface terrace. The complete epitaxy is driven by (i) anchoring of half of the perylene stacks into specific adsorption sites on each terrace, (ii) interlacing of the perylene stacks across the steps within the entire H-bonded network, and (iii) relaxation of the overlayer strain via enhancement of the overlayer-specific vibrational modes and short-range thermal motion of the constituent molecules. This complete epitaxy phenomenon is described via (i) structural and statistical analysis of the molecularly resolved STM topographies, (ii) monitoring of the short-range molecular displacements under the strain relaxation, (iii) highlighting of specific intra-molecular and inter-molecular vibration modes through detailed analysis of HREELS spectra, and (iv) parametrization of the intermolecular interaction via pair potential calculation.