Characterization of Coulomb Interactions in Electron Transport through a Single Hetero-Helicene Molecular Junction Using Scanning Tunneling Microscopy

TL;DR

This study investigates Coulomb interactions affecting electron transport in a chiral hetero-helicene molecule using STM, revealing a significant energy gap discrepancy caused by electron-electron interactions during tunneling.

Contribution

It provides the first detailed analysis of Coulomb interactions in electron transport through a hetero-helicene molecule using combined STM measurements and first principles calculations.

Findings

Measured a 3.2 eV transport energy gap larger than theoretical predictions.

Demonstrated Coulomb interactions cause energy discrepancy during tunneling.

Highlighted the importance of Coulomb effects in molecular junctions.

Abstract

Characterization of the structural and electron transport properties of single chiral molecules provides critical insights into the interplay between their electronic structure and electrochemical environments, providing broader implications given the significance of molecular chirality in chiroptical applications and pharmaceutical sciences. Here, we examined the topographic and electronic features of a recently developed chiral molecule, B,N-embedded double hetero[7]helicene, at the edge of Cu(100) supported NaCl thin film with scanning tunneling microscopy and spectroscopy. An electron transport energy gap of 3.2 eV is measured, which is significantly larger than the energy difference between the highest occupied and the lowest unoccupied molecular orbitals given by theoretical calculations or optical measurements. Through first principles calculations, we demonstrated that this energy discrepancy results from the Coulomb interaction between the tunneling electron and the molecule's electrons. This occurs in electron transport processes when the molecule is well decoupled from the electrodes by the insulating decoupling layers, leading to a temporary ionization of the molecule during electron tunneling. Beyond revealing properties concerning a specific molecule, our findings underscore the key role of Coulomb interactions in modulating electron transport in molecular junctions, providing insights into the interpretation of scanning tunneling spectroscopy features of molecules decoupled by insulating layers.