The structure of thin 3,4,9,10-perylenetetracarboxylic-dianhydride (PTCDA) films on Cu(110) was studied by scanning tunnelling microscopy (STM) from submonolayer to monolayer coverage. While no long-range ordering was found after deposition at room temperature, the formation of a well-defined superstructure is observed after thermal annealing. It appears that the formation of the superstructure is driven by the interaction between the oxygen atoms of the PTCDA and the copper atoms of the substrate. While the distance between the molecules fits well to the atomic lattice of the Cu(110) surface along the  direction, the mismatch along the  direction leads to a periodic buckling normal to the surface accompanied by a restructuring of the substrate.
- MOLECULAR SEMICONDUCTOR