Structure and vibrational properties of the PTCDA/Ag(111) interface: Bilayer vs. monolayer

TL;DR

This study investigates the structural and vibrational characteristics of PTCDA layers on Ag(111), comparing monolayer and bilayer configurations using IR spectroscopy and DFT, revealing insights into interfacial interactions and spectral features.

Contribution

It provides a detailed comparison of monolayer and bilayer PTCDA on Ag(111), highlighting the challenges in DFT modeling of physisorbed layers and clarifying IR spectral contributions.

Findings

DFT accurately describes monolayer geometry and IR spectrum.

Bilayer IR spectra include contributions from both contact and physisorbed layers.

Removing certain vibrational peaks aligns calculations with experimental data.

Abstract

The structural and vibrational properties of metal-organic interfaces have been examined by means of infrared (IR) absorption spectroscopy and density functional theory (DFT) with an approach accounting for long-range dispersive interactions. We focus on a comparative study of the PTCDA monolayer and bilayer on Ag(111). The equilibrium geometry at the molecule-metal interface and the infrared spectrum of the chemisorbed monolayer of PTCDA on Ag(111) are well described by the computations. In the bilayer structure, the presence of a physisorbed adlayer on top of PTCDA/Ag(111) presents a challenge for DFT. As previously described for other systems, the polarization of the substrate is not captured correctly and results in too low energies of frontier molecular orbitals. This results in an apparent contribution from the vibrations of second-layer PTCDA to the IR spectrum from interfacial dynamical charge transfer processes. After removing these peaks with artificially strong intensity, calculated and experimental data show good agreement and the IR spectrum can be described as the sum of the spectra of the PTCDA/Ag(111) contact layer and a physisorbed PTCDA monolayer on top.