Final-State Simulations of Core-Level Binding Energies at Metal-Organic Hybrid Interfaces: Artifacts Caused by Spurious Collective Electrostatic Effects.

Thomas C Taucher,Oliver T Hofmann,Egbert Zojer

doi:10.1021/acsomega.0c03209

Abstract

Core-level energies are frequently calculated to explain the X-ray photoelectron spectra of metal-organic hybrid interfaces. The current paper describes how such simulations can be flawed when modeling interfaces between physisorbed organic molecules and metals. The problem occurs when applying periodic boundary conditions to correctly describe extended interfaces and simultaneously considering core hole excitations in the framework of a final-state approach to account for screening effects. Since the core hole is generated in every unit cell, an artificial dipole layer is formed. In this work, we study methane on an Al(100) surface as a deliberately chosen model system for hybrid interfaces to evaluate the impact of this computational artifact. We show that changing the supercell size leads to artificial shifts in the calculated core-level energies that can be well beyond 1 eV for small cells. The same applies to atoms at comparably large distances from the substrate, encountered, for example, in extended, upright-standing adsorbate molecules. We also argue that the calculated work function change due to a core-level excitation can serve as an indication for the occurrence of such an artifact and discuss possible remedies for the problem.

Highlights

In addition to the chemical environment, core-level binding energies are influenced by the local electrostatic potential at the position of the excited atom.[3−6] This effect is related to observations for ionic crystals that the differences in Madelung energies between the bulk and the surface can amount to ∼1 eV.[7]
Electrostatic shifts are of particular relevance for interfaces in cases where large dipoles occur. This is very common for hybrid organic−inorganic interfaces, where interfacial potential shifts are typically associated with collective electrostatic effects.[4,5,8−12] in this context, electrostatically triggered core-level shifts on the order of 1 eV have been observed for polar self-assembled monolayers (SAMs) adsorbed on metal substrates.[3,4,13−15] For such systems, the electrostatic shifts can be straightforwardly rationalized by the periodic arrangement of polar entities at the interface
The superposition of their fields causes a step in the electrostatic energy that changes the sample work function and the energetic positions of core levels relative to the Fermi level of the substrate, as described in detail in ref 5

Summary

Introduction

X-ray photoelectron spectroscopy (XPS), known as electron spectroscopy for chemical analyses (ESCA), is a widely used technique to analyze the chemical structure of surfaces and interfaces, providing qualitative and quantitative information about the chemical neighborhood of specific atoms.[1,2] In addition to the chemical environment, core-level binding energies are influenced by the local electrostatic potential at the position of the excited atom.[3−6] This effect is related to observations for ionic crystals that the differences in Madelung energies between the bulk and the surface can amount to ∼1 eV.[7] Electrostatic shifts are of particular relevance for interfaces in cases where large dipoles occur. The interpretation of experimentally acquired spectra frequently relies on first-principle simulations.[2,16] There is a broad range of different approaches to simulate XP spectra.[2,17,18]

Methods

Results

Conclusion