Experimental and theoretical analysis of H-bonded supramolecular assemblies of PTCDA molecules

Using a systematic method based on considering all possible hydrogen bond connections between molecules and subsequent density-functional theory (DFT) calculations, we investigated planar superstructures that the perylene-3,4,9,10-tetracarboxylic-3,4,9,10-dianhydride (PTCDA) molecules can form in one and two dimensions. Structures studied are mostly based on two molecule unit cells and all assemble in flat periodic arrays. We show that 42 different monolayer structures are possible, which can be split into eight families of distinct structures. A single representative of every family was selected and relaxed using DFT. We find square, herringbone and brick wall phases (among others) which were already observed on various substrates. Using scanning tunneling microscopy in ultrahigh vacuum, we also observed herringbone and square phases after sublimation of PTCDA molecules on the Au(111) surface at room temperature, the square phase being observed for the first time on this substrate. The square phase appears as a thin stripe separating two herringbone domains and provides a perfect structural matching for them. A similar structural formation serving as a domain wall between two other phases has been recently reported on the same surface formed by melamine molecules [ F. Silly et al. J. Phys. Chem. C 112 11476 (2008)]. Our theoretical analysis helps to account for these and other observed complex structures.