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 (E_F). 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.