CORRUGATION REVERSAL IN SCANNING TUNNELING MICROSCOPE IMAGES OF ORGANIC-MOLECULES

Submonolayer coverages of perylene-tetracarboxylic-dianhydride on Ag(1 10) were investigated with scanning tunneling microscopy (STM) at 295 and 50 K. The molecules form highly ordered rhombic islands which at 2 95 K coexist with molecules diffusing on the substrate terraces. At 50 K the molecular islands appear 1.4 Angstrom higher than the Ag substr ate in STM images, independent of the tunneling voltages and currents. At 295 K this apparent height is observed only at high tunneling resi stances. Lowering of the resistance decreases the height difference un til finally the contrast between islands and the surrounding substrate is reversed. At the same time, vacancies within islands show no contr ast reversal. We explain the contrast reversal by trapping of molecule s below the tip apex. [S0163-1829(98)00707-3].