Reversed surface corrugation in STM images on Au(111) by field-induced lateral motion of adsorbed molecules

Submonolayer coverages of the organic molecule 1-nitronaphthalene adsorbed on the reconstructed Au(111) surface were investigated by scanning tunneling microscopy at 50 and 10 K. At 50 K, upon switching the polarity of the tunneling voltage a peculiar change in the image contrast is observed. At negative sample–tip bias voltage the dislocation lines of the Au(111) surface reconstruction are imaged as protrusions and molecular aggregates appear 0.15 nm higher than the substrate. At reversed bias, the interior of the fcc and hcp domains exhibits the same height as the molecular clusters, while the domain walls of the surface reconstruction appear as depressions with a strongly increased corrugation amplitude. At 10 K, for the same tunneling parameters, no such contrast reversal is observed. Controlled manipulation experiments at 10 K demonstrate that contrast reversal at 50 K is due to long-range attractive tip–molecule interactions which assemble small molecular aggregates below the tip apex.