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. ► Ultrathin FePc and (t-Bu) 4ZnPc monolayers as buffer layers. ► The surface charge spilled out into the vacuum vanishes for larger distances. ► The buffer layer decouples free molecules electronically from the metal surface. ► Intrinsic electronic structure of free molecule can be preserved on buffer layer. ► We directly observed detailed electronic structures of the molecule by STM.