HIGHLY ORDERED STRUCTURES AND SUBMOLECULAR SCANNING-TUNNELING-MICROSCOPY CONTRAST OF PTCDA AND DM-PBDCI MONOLAYERS ON AG(111) AND AG(110)

Highly ordered monolayers of two perylene derivatives (perylene-3,4,9, 10-tetracarboxylic-3,4,9, 10-dianhydride (PTCDA) and N,N'-dimethylpery lene-3,4,9,10-bis(dicarboximide) (DM-PBDCI)) were prepared by vacuum s ublimation on Ag(111) and Ag(110) surfaces. Using scanning tunnelling microscopy (STM) and low energy electron diffraction, the nucleation, growth, step site interaction. and superstructures were investigated a nd compared. All adsorbate structures are commensurate, with the excep tion of DM-PBDCI on Ag(111). A similar, but not identical, submolecula r STM resolution was obtained for both molecules and on both surfaces. The local distribution shows ten dominant intensity maxima per molecu le and corresponds to that of the lowest unoccupied orbital. From thes e results we draw conclusions concerning the tunnelling mechanism, the adsorbate-substrate, and the adsorbate-adsorbate interactions, which are discussed in relation to the formerly published results on the ele ctronic structure of PTCDA/Ag(111). (C) 1998 Elsevier Science B.V. All rights reserved.