Characterizing Molecule-Metal Surface Chemistry with Ab-Initio Simulation of X-ray Absorption and Photoemission Spectra_2

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Characterizing Molecule-Metal Surface

Chemistry with Ab-Initio Simulation of X-ray

Absorption and Photoemission Spectra

Samuel J. Hall,†,‡Benedikt P. Klein,†,¶and Reinhard J. Maurer∗,†

†Department of Chemistry, University of Warwick, Gibbet Hill Road, Coventry, CV4 7AL, United

Kingdom

‡MAS Centre of Doctoral Training, Senate House, University of Warwick, Gibbet Hill Road,

Coventry, CV4 7AL, United Kingdom

¶Diamond Light Source, Harwell Science and Innovation Campus, Didcot, OX11 0DE, United

Kingdom

E-mail: r.maurer@warwick.ac.uk

Abstract

X-ray photoemission and x-ray absorption spectroscopy are important techniques to char-

acterize chemical bonding at surfaces and are often used to identify the strength and nature

of adsorbate-substrate interactions. In this study, we judge the ability of x-ray spectroscopic

techniques to identify different regimes of chemical bonding at metal-organic interfaces. To

achieve this, we sample different interaction strength regimes in a comprehensive and system-

atic way by comparing two topological isomers, azulene and naphthalene, adsorbed on three

metal substrates with varying reactivity, namely the (111) facets of Ag, Cu, and Pt. Using

density functional theory, we simulate core-level binding energies and x-ray absorption spec-

tra of the molecular carbon species. The simulated spectra reveal three distinct characteristics

based on the molecule-speciﬁc spectral features which we attribute to types of surface chemical

arXiv:2210.02187v2 [cond-mat.mtrl-sci] 8 Feb 2023

bonding with varying strength. We ﬁnd that weak physisorption only leads to minor changes

compared to the gas-phase spectra, weak chemisorption leads to charge transfer and signiﬁcant

spectral changes, while strong chemisorption leads to a loss of the molecule-speciﬁc features

in the spectra. The classiﬁcation we provide is aimed at assisting interpretation of experimental

x-ray spectra for complex metal-organic interfaces.

Introduction

The strength and nature of interactions at hybrid organic-inorganic interfaces inﬂuence the charge

transport across the interface. This in turn controls the performance of organic electronic devices,

e.g. organic light-emitting diodes1,2 or organic ﬁeld effect transistors.3,4 To gain insight into the

fundamental mechanisms of the interaction at the interface, model systems consisting of organic

molecules adsorbed on single-crystal metal surfaces are often studied using surface science tech-

niques.5Here, X-ray core-level spectroscopies such as X-ray photoelectron spectroscopy (XPS)

and near edge X-ray absorption ﬁne structure (NEXAFS) spectroscopy represent effective tools to

characterize the structure and electronic structure of the investigated model systems.6,7

However, the interpretation of the experimental spectra can be highly challenging. Often a large

number of unoccupied states contribute to the spectra and overlap signiﬁcantly. This complicates

the assessment of how core levels of different atoms (in XPS and NEXAFS) and different valence

states (in NEXAFS) contribute to the measured spectra. Furthermore, without any further atomic-

level information on the adsorption structure and electronic properties of the interface, the spectra

cannot be connected to important quantities that relate to the nature of the molecule-surface bond,

such as the adsorption energy and height, and more conceptual quantities such as the magnitude

of charge transfer, and the hybridization between the electronic states originating from surface and

molecule, respectively.

First-principles core-level spectroscopy simulations support the interpretation of experimental

spectra and are able to disentangle spectra into individual transitions between core and valence

states of the system. The methodology in this work is based on core-level constrained density

functional theory (DFT) which has been applied before to similar problems and was shown to pro-

vide a robust approach for core-level spectroscopy simulations of 1s states in organic molecules.8,9

Application of this method enabled a detailed understanding of the adsorption geometry, chemical

bonding and electronic structure.10–12

The interaction between a molecule and a metal surface is dependent on the electronic and

geometric structure of both participants. On the side of the metal, the reactivity of the substrate can

be modiﬁed by changing its elemental composition while maintaining the same crystal structure

and surface orientation. Noble and coinage metal surfaces with a (111) surface orientation are

commonly used as model substrates for fundamental studies. For our work, we chose the three

metal substrates Ag(111), Cu(111), and Pt(111). Within these, the reactivity of the metal surface

increases from Ag(111) to Cu(111) to Pt(111), as can be directly inferred from the d-band model

of surface chemical bonding.13,14

On the side of the organic molecule, a wide range of options to tune reactivity exist. The

structural variety of organic molecules is almost inﬁnite and minor structural changes can lead to

large changes in reactivity. Here, we chose two simple aromatic hydrocarbons, azulene (Az) and

naphthalene (Nt). These two molecules are an isomeric pair of bicyclic aromatic hydrocarbons and

only differ by the topology of their aromatic system. Naphthalene consists of an alternant 6-6 ring

structure and azulene of a nonalternant 5-7 ring structure (see insets in Fig. 1a).15 This topological

difference between azulene and naphthalene has a large inﬂuence on the molecular properties.

Solutions of azulene show a brilliant blue color and azulene has a substantial dipole moment while

naphthalene is colorless and possesses no dipole moment.16,17 The ability of the two molecules to

interact with metal surfaces is also strongly inﬂuenced by their different topologies.

A series of recent publications have produced a comprehensive picture of the surface bond of

naphthalene and azulene adsorbed onto Cu, Ag and Pt (111) surfaces.18–21 The thorough charac-

terization in the literature includes the eludication of the adsorption geometry, energetics and elec-

tronic properties by means of various experimental techniques such as near incidence X-ray stand-

ing wave (NIXSW),18,19 low-energy electron diffraction (LEED),18 XPS,18,19,21 NEXAFS,18,19,21

temperature programmed desorption (TPD),18,20 and single-crystal adsorption calorimetry (SCAC),21

all combined with DFT simulations. The two molecules and three surfaces represent six interface

models that are comprehensively characterized in terms of their structure and electronic proper-

ties, which makes them ideally suited for the investigation of how the nature of the respective

molecule-metal interaction is reﬂected in core-level spectroscopic signatures.

In this manuscript, we build on previously published work on the six molecule-metal inter-

face models18,19,21 and present a comprehensive comparative study of ﬁrst principles simula-

tions of C 1s XPS and C-K edge NEXAFS signatures of azulene and naphthalene adsorbed at

Cu(111), Ag(111), and Pt(111). We employ the Delta-Self-Consistent-Field (∆SCF)22–24 and

Delta-Ionization-Potential-Transition Potential (∆IP-TP)25–27 methods to characterize and analyze

the XP and NEXAFS spectra of these large periodic systems. Using these simulations, we iden-

tify three different molecule-metal surface bonding regimes: physisorption, weak chemisorption

(one-way charge transfer), and strong chemisorption (two-way charge transfer leading to molecule-

metal hybridization), with each regime showing characteristic signatures and changes in the re-

spective spectra compared to the gas-phase data. We expect our ﬁndings to be useful to interpret

experimental spectral changes for complex hybrid organic-inorganic thin ﬁlms.

Computational Details

The structural models used for the spectroscopic simulations in this study were taken from the

literature.18,19,21 In these publications, the structural optimization was performed using a combi-

nation of the PBE functional and the DFT-D3 van der Waals dispersion correction. The reported

structures were previously found to be in good agreement with experimental data, which has been

summarized in the SI in Figs. S1 and S2. Metal surfaces were modelled as 4-layer slabs with a

(2√3×2√3)-R30° unit cell containing 48 metal atoms in total. More details on the computational

settings employed can be found in the relevant literature references.18–21,28

All core-level calculations in this study are based on previously optimised structures and were

performed with the electronic structure software package CASTEP 18.1129 which utilizes periodic

boundary conditions (PBC). Default on-the-ﬂy generated ultrasoft pseudopotentials and the PBE

exchange correlation functional30 were used throughout. We employ a planewave (PW) cutoff

energy of 450 eV and a k-grid of 6 ×6×1 for all metal surfaces (1 ×1×1 for the gas-phase

calculations). These values provide a converged potential for carbon and for all three metal surfaces

investigated in this work. An electronic convergence criterion for the total density of at least

1×10−6eV/atom was employed. The inﬂuence of these parameters has been tested thoroughly in

a previous publication and shown to give well converged results.9

XPS simulations employed the ∆SCF method,22,23,31 calculating the core-electron binding en-

ergy (BE) from the difference in total energy of two singlepoint calculations, one being the ground-

state conﬁguration and the second a core-hole excited conﬁguration, where one electron is removed

from the 1s orbital. This method is implemented in CASTEP by modifying the pseudopotential

deﬁnition of the excited atom to include a full core-hole.9,32,33 Such an excited state calculation

is carried out for every individual carbon atom in the molecule in order to produce the full XP

spectrum.

NEXAFS simulations were performed using the ∆IP-TP method.9,27 The TP approach allows

for all transition energies from the 1s state of one atom into all possible unoccupied states to be

calculated in a single calculation. Modiﬁed pseudopotentials are used again but here include only

half a core-hole instead of a full core-hole. The ELNES module33–35 in CASTEP was used to sim-

ulate NEXAFS energies and transition dipole moments. This module performs a total energy SCF

calculation followed by a band structure calculation in order to converge the unoccupied states.

Inclusion of 800 unoccupied bands was enough to cover the spectral range for all systems. The

∆IP-TP extension of the TP method involves the shift of all transition energies (1s →unoccupied

states) belonging to each atom according to the XPS binding energies of its 1s electrons, which

were obtained by the ∆SCF calculation in the previous step. This ionization potential correction

aligns all individual core-level spectra to the same energy frame.

Post-processing of the data was carried out through the use of a dedicated tool, MolPDOS,36

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CharacterizingMolecule-MetalSurfaceChemistrywithAb-InitioSimulationofX-rayAbsorptionandPhotoemissionSpectraSamuelJ.Hall,,BenediktP.Klein,,¶andReinhardJ.Maurer,DepartmentofChemistry,UniversityofWarwick,GibbetHillRoad,Coventry,CV47AL,UnitedKingdomMASCentreofDoctoralTraining,SenateHouse,Universi...

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