Monolayers of MoS$_{2}$ on Ag(111) as decoupling layers for organic molecules: resolution of electronic and vibronic states of TCNQ

TL;DR

This study demonstrates that a monolayer of MoS$_{2}$ on Ag(111) acts as an effective decoupling layer, enabling detailed resolution of electronic and vibronic states of TCNQ molecules by reducing substrate interactions.

Contribution

The paper shows that MoS$_{2}$ monolayers on Ag(111) can serve as decoupling layers, allowing access to molecular electronic and vibronic states that are otherwise obscured by substrate effects.

Findings

MoS$_{2}$ monolayer creates a semiconducting gap on Ag(111).

Narrowed LUMO of TCNQ observed on MoS$_{2}$/Ag(111).

Vibronic features identified via Franck-Condon analysis.

Abstract

The electronic structure of molecules on metal surfaces is largely determined by hybridization and screening by the substrate electrons. As a result, the energy levels are significantly broadened and molecular properties, such as vibrations are hidden within the spectral lineshapes. Insertion of thin decoupling layers reduces the linewidths and may give access to the resolution of electronic and vibronic states of an almost isolated molecule. Here, we use scanning tunneling microscopy and spectroscopy to show that a single layer of MoS$_{2}$ on Ag(111) provides a semiconducting band gap that may prevent molecular states from strong interactions with the metal substrate. We show that the lowest unoccupied molecular orbital (LUMO) of tetra-cyano-quino-dimethane (TCNQ) molecules is significantly narrower than on the bare substrate and that it is accompanied by a characteristic satellite structure. Employing simple calculations within the Franck-Condon model, we reveal their vibronic origin and identify the modes with strong electron-phonon coupling.