Adsorption of PTCDA on Terraces and at Steps Sites of the KCl(100) Surface

Scanning tunneling microscopy at low temperature and low molecular coverage (∼1%) was combined with density-functional theory calculations to investigate the adsorption of isolated perylene-3,4,9,10-tetracarboxyl acid dianhydride (PTCDA) molecules on KCl(100) surfaces. Experimentally, we used epitaxial thin KCl(100) films on Ag(100) of about 3 layers in thickness. After deposition at 100 K, the PTCDA molecules are statistically distributed on the terraces with an azimuthal orientation of the long axis along the polar ⟨110⟩ orientation. After annealing at about 150 to 200 K the molecules are exclusively found at step-edge sites. Thereby, several configurations are observed, the most typical being a site where the PTCDA molecules protrude into the step edge, forming vacancies at the step edge. The total-energy calculations predict this step-edge adsorption site to be energetically favorable compared to the adsorption on terraces. The corrugation of the calculated potential energy surface is below 1 eV, with...