Structural transitions of perylene and coronene on silver and gold surfaces: A molecular-beam epitaxy LEED study

The growth of perylene and coronene up to a monolayer is investigated continuously by low-energy electron diffraction (LEED) on the metal surfaces Ag(110), Au(110) $1\times 2,$ and Au(111) $22\times \sqrt{3}.$ Both molecules show [restricted on Au(110) $1\times 2]$ the evolution from isotropic disordered structure in the submonolayer regime to a highly (substrate-dependent) ordered monolayer. Two-dimensional gas, fluid, and crystalline phases can be distinguished. On the rough Au(110) $1\times 2$ surface, a periodic structure in the [001] direction can be observed, while in the $\left[11¯0\right]$ direction, diffraction patterns arise from diffuse LEED intensity. Just before the monolayer is complete, structural transitions between highly ordered structures occur in all presented adsorbates. Leaving and reaching of commensurate structures show “stick-slip” behavior. The lateral ordering process of these molecules allows epitaxial growth without domain walls because crystallization does not start from islands in the submonolayer regime.