High resolution scanning-tunneling-microscopy imaging of individual molecular orbitals by eliminating the effect of surface charge

Ultrathin Iron(II) Phthalocyanine (FePc) monolayers have been employed to decouple individual FePc molecules electronically from the metallic substrate, which allows the intrinsic electronic structure of the free molecule to be preserved and imaged by means of low-temperature scanning tunneling microscopy (STM). High-resolution images reveal the standard "cross" structure for molecules adsorbed at the metal surface, but a detailed electronic structure, corresponding to the highest occupied molecular state (HOMO), for molecules adsorbed at the buffer layer. Different STM images of the molecules in the first and second layer are referred to different interactions between molecules and substrate. This interpretation is verified by control experiments on a second molecule, tetra-tert-butyl zinc Phthalocyanine ((t-Bu) ₄ZnPc). We therefore conclude that this effect is generic and can be used to investigate a molecule's electronic structure in detail.