Submonolayer coverages of perylene-tetracarboxylic-dianhydride on Ag(110) were investigated with scanning tunneling microscopy (STM) at 295 and 50 K. The molecules form highly ordered rhombic islands which at 295 K coexist with molecules diffusing on the substrate terraces. At 50 K the molecular islands appear 1.4 Å higher than the Ag substrate in STM images, independent of the tunneling voltages and currents. At 295 K this apparent height is observed only at high tunneling resistances. Lowering of the resistance decreases the height difference until finally the contrast between islands and the surrounding substrate is reversed. At the same time, vacancies within islands show no contrast reversal. We explain the contrast reversal by trapping of molecules below the tip apex.

• Received 14 July 1997

DOI:https://doi.org/10.1103/PhysRevB.57.4081