Steering interfacial molecular structures by substrate-molecule charge transfer

Abstract

Intermixed self-assembled monolayers (SAMs) provide a powerful approach for tuning inorganic-organic interfaces, but their local structure and energetics often remain elusive. Here, an intermixed stripe phase of fullerene $C_{60}$ and Zn(II)-5,10,15,20-tetrakis(4-aminophenyl)porphyrin (Zn-TAPP) on a Ag(111) surface is identified, establishing a direct link between molecular assembly and charge transfer. Scanning tunnelling spectroscopy (STS) and Kelvin probe force spectroscopy (KPFS) have been used to map electronic states and local contact potential differences (LCPD). Compared to close-packed $C_{60}$ islands, the stripe phase exhibits a larger interfacial dipole and increased occupation of previously unoccupied $C_{60}$ states, consistent with enhanced charge transfer from Ag(111) to $C_{60}$. Analysis indicates that larger $C_{60}$ spacing in the stripe geometry reduces Coulomb repulsion, stabilizing the intermixed phase and enhancing charge transfer with respect to close-packed domains. These results provide direct evidence of charged $C_{60}$ molecules in stripe structures and guidelines for engineering intermixed SAMs with tailored interfacial electronic properties.