STM studies of thin PTCDA films on Ag/Si(111)-√3x√3

Thin films of the large planar organic molecule 3,4,9,10-perylene tetracarboxylic dianhydride on the Ag/Si(111)-root 3 x root 3 and surface have been studied by scanning tunneling microscopy. The coverage was between 0.3 and 10 monolayers. In submonolayers the molecules form two different phases with a commensurate relationship to the substrate: a herringbone phase and a square phase. Adsorption sites are determined for the square phase. Additional herringbone phases were also observed with other unit cell size or lower order in relation to the substrate. The molecules lie flat on the surface and form a full monolayer before multilayers are developed. Both the square and herringbone phases have been observed for the second layer. With more than 3 layers, islands were built up layer by layer in agreement with the alpha bulk phase. On top of the islands there was always a herringbone structure.