Resolution of intramolecular dipoles and push-back effect of individual molecules on a metal surface

TL;DR

This study investigates how large dipolar molecules behave on gold surfaces, revealing that their dipole moments are retained or enhanced and that they induce a local push-back effect, altering the surface's electronic properties.

Contribution

It provides direct experimental and theoretical evidence of the persistent and increased molecular dipoles and the push-back effect at the single-molecule level on a metal surface.

Findings

Large dipole moments persist in molecules on Au(111)

Dipole moments are increased compared to gas phase

Local contact potential is decreased by molecular adsorption

Abstract

Molecules consisting of a donor and an acceptor moiety can exhibit large intrinsic dipole moments. Upon deposition on a metal surface, the dipole may be effectively screened and the charge distribution altered due to hybridization with substrate electronic states. Here, we deposit Ethyl-Diaminodicyanoquinone molecules, which exhibit a large dipole moment in gas phase, on a Au(111) surface. Employing a combination of scanning tunneling microscopy and non-contact atomic force microscopy, we find that a significant dipole moment persists in the flat-lying molecules. Density-functional theory calculations reveal that the dipole moment is even increased on the metal substrate as compared to the gas phase. We also show that the local contact potential across the molecular islands is decreased by several tens of meV with respect to the bare metal. We explain this by the induced charge-density redistribution due to the adsorbed molecules, which confine the substrate's wavefunction at the interface. Our local measurements provide direct evidence of this so-called push-back or cushion effect at the scale of individual molecules.