Layer-selective epitaxial self-assembly of porphyrins on ultrathin insulators

Copper-Octaethyl Porphyrin self-assembly has been studied on NaCl islands, 1-3 monolayers thick, grown on metal substrates. Extended ordered molecular monolayers are observed for the first time on ultrathin insulator films. The assembly occurs in hierarchical fashion, starting on the metal substrate, then followed by assembly on the first and second NaCl layers, clearly demonstrating a decrease in adsorption energy for increasing insulator layer thickness. The underlying mechanisms are discussed on the basis of molecule-substrate interactions. Voltage-dependent STM images reveal differences of the electronic structure for molecules adsorbed on metal and NaCl/metal areas.