Tailoring Charge-Transfer at Metal-Organic Interfaces Using Designer Shockley Surface States.

Metal-organic interfaces determine critical processes in organic electronic devices. The frontier molecular orbitals (highest occupied and lowest unoccupied molecular orbital, HOMO and LUMO) are crucial in determining charge-injection and charge-collection processes into and from the organic semiconductor films. Here we show that we are able to tune the interfacial electronic structure of a strongly interacting interfacial system formed by adsorption of the electron acceptor 1,4,5,8,9,11-hexaazatriphenylenehexacarbonitrile (HATCN, C18N12) on Ag thin films on Cu(111). The thickness-dependent Shockley surface state emerging on this layered metallic system couples to the LUMO, which allows precise control over the energetic position and filling of the charge-transfer interface state relative to the Fermi level (EF). Our ability to tune the interfacial electronic structure while maintaining the structure of the molecular film represents an important step toward designing organic semiconductor interfaces.