STM INVESTIGATIONS OF NTCDA ON WEAKLY INTERACTING SUBSTRATES

Monolayers of the organic molecule naphthalene-1,4,5,8-tatracarboxylic -dianhydride (NTCDA) on graphite and MoS2 have been prepared by organi c molecular beam epitaxy. Two different phases have been observed: one phase where the molecules lie flat on the surface and form a coincide nt superstructure, and another bulk-like phase where the molecules for m stacks with the molecular plane perpendicular to the substrate. The submolecular image contrast of hat-lying molecules is in agreement wit h the expectation derived from former results on the larger molecule P TCDA, indicating the same underlying contrast mechanism. STS measureme nts show a strong rectifying behaviour of the molecular layer. The nor malized differential conductance shows two maxima which correspond to optical absorption measurements.