How much does surface polymorphism influence the work function of organic/metal interfaces?

TL;DR

This study investigates how surface polymorphism affects the work function of organic/metal interfaces by exploring millions of polymorphs through theoretical search, revealing significant variations influenced by molecular orientation and kinetic trapping.

Contribution

The paper introduces a comprehensive theoretical approach combining structure search, first-principles calculations, and machine learning to quantify work function variations due to surface polymorphism.

Findings

Kinetic trapping can cause work function differences of a few hundred meV.

Molecular reorientation can lead to changes of several eV in work function.

Thermodynamic equilibrium reduces work function spread but uncertainties remain at high temperatures.

Abstract

Molecules adsorbing on metal surfaces form a variety of different surface polymorphs. How strongly this polymorphism affects interface properties is a priori unknown. In this work we investigate how strongly the surface polymorphism influences the interface work functions for various metal/organic interfaces. To evaluate the whole bandwidth of possible polymorphs, we perform full theoretical structure search, probing millions of polymorph candidates. All of these candidates might be observed in reality, either by kinetic trapping or by thermodynamic occupation. Employing first-principles calculations and machine learning we predict and analyze the work function changes for those millions of candidates for three physically distinct model systems: the weakly interacting naphthalene on Cu(111), the strongly interacting anthraquinone on Ag(111), and tetracyanoethylene, which undergoes a re-orientation from lying to standing polymorphs on the Cu(111) surface. These thorough investigations indicate that kinetic trapping of flat lying molecules can lead to work function differences of a few hundred meV. If the molecules also reorientate, this can increase to a change of several eV. We further show that the spread in work function decreases when working in thermodynamic equilibrium, but thermally occupied phases still lead to an intrinsic uncertainty at elevated temperatures.