Giant contrast reversal in scanning tunnelling microscopy of zincporphyrin monolayers on graphite

Zincporphyrin trimer molecules deposited on highly oriented pyrolitic graphite have been imaged using scanning tunnelling microscopy. At bias voltages below 1.5 V, we observed giant contrast reversal where the adsorbed molecules appear more than 3 nm lower than the graphite substrate! The contrast reversal is caused by a combined effect of molecular attachment to the tip and the voltage-dependence of the molecular conductivity.