Resolution of Intramolecular Dipoles and a Push-Back Effect of Individual Molecules on a Metal Surface

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 ethyldiaminodicyanoquinone molecules, which exhibit a large dipole moment in the gas phase, on a Au(111) surface. Employing a combination of scanning tunneling microscopy and noncontact 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.

Variations of Vibronic States in Densely Packed Structures of Molecules with Intramolecular Dipoles

“…A similar arrangement has been reported for the same molecules in direct contact to an Au(111) surface. [ 28 ] A Cl‐functionalized tip (see Methods) enhances the asymmetric appearance of the individual molecules (Figure 1b) and evidences the alternating antiparallel orientation with respect to the nearest neighbors. While the STM image is not conclusive on the orientation of the individual molecules, frequency‐shift () images of the same area allow for an unambiguous identification of the molecular arrangement (Figure 1f).…”

“…We note that the same molecules in direct contact to Au(111) also revealed the presence of an intramolecular dipole, which was shown to be enhanced as compared to the gas‐phase molecule by charge transfer from the substrate. [ 28 ]…”

“…The resonance structure (onset at ) is much sharper than observed for the same molecules in direct contact to the Au(111) substrate. [ 28 ] The smaller linewidth indicates longer excited state lifetimes and less hybridization with the substrate. These observations are in agreement with earlier studies of organic molecules on MoS 2 /Au(111), which pointed out the decoupling efficiency of the monolayer MoS 2 .…”

“…[ 27 ] Surprisingly, this dipole moment was found to be enlarged when the molecule is in contact to a metal substrate due to charge transfer. [ 28 ] The interaction with the substrate prohibited the resolution of vibronic states. Hence, we now use a monolayer of MoS 2 for decoupling the molecules from the metal substrate.…”