PTCDA on Cu(111) partially covered with NaCl

Abstract

The organic molecule 3,4,9,10-perylene-tetracarboxylic dianhydride (PTCDA) was studiedby means of scanning tunneling microscopy (STM) on thin insulating NaCl films grown ona Cu(111) single crystal. The deposition of approximately two monolayers (ML) of sodiumchloride onto a Cu(111) substrate at a sample temperature of about 350 K causes a ratherrough growth of (100)-oriented NaCl islands up to a local height of 4 ML. For submonolayercoverages (0.1 and 0.4 ML) of PTCDA on a Cu(111) surface partly covered with NaCl,two different rod structures of PTCDA were found on the copper surface, whichare in contrast to previously published data for PTCDA on Cu(111) showing aherringbone-like arrangement. These findings can be explained by the formation of aNax–PTCDA complex. On NaCl covered areas, single PTCDA molecules adsorb atvacancies of [010] and [001] oriented steps of the NaCl(100) islands. In this case, theelectrostatic forces between the polar step edges and the PTCDA molecules aredominant. The terraces of the alkali halide surface are free of PTCDA molecules.