SCANNING TUNNELING MICROSCOPE INVESTIGATIONS OF LEAD-PHTHALOCYANINE ON MOS2

Monolayers of the nonplanar organic molecule lead-phthalocyanine (PbPc ) on MoS2 have been prepared by organic molecular beam epitaxy and inv estigated with video scanning tunneling microscopy. Three different cr ystallographic phases have been observed. Two of them, the close packe d and the rowlike phase, have already been observed in an earlier stud y of the planar Cu-Pc. With PbPc we have now observed an additional ph ase, where three close packed rows alternate with one or two isolated single rows. In contrast to Cu-Pc, in submolecularly resolved images t he PbPc appear in two different states, with either a dark or a bright center. This is attributed to the nonplanar molecular geometry, which allows the formation of two different adsorption geometries, the Pb a bove or below the molecular plane. In video sequences the transition f rom one state into the other could be observed. (C) 1996 American Vacu um Society.