arXiv subjects

Anna Regoutz

Publications and source records attributed to Anna Regoutz.

At least 19 recordsLinked to original sources

arXiv2026

Lifetime effects and satellites in the photoelectron spectrum of platinum metal

This work presents a comprehensive investigation of the electronic structure and many-body photoemission effects in metallic platinum using reflection high-energy electron energy-loss spec- troscopy (RHEELS), soft X-ray photoelectron spectroscopy (SXPS), and hard X-ray photoelectron spectroscopy (HAXPES), supported by ab initio calculations. Shallow and deep core state spectra enable the systematic characterisation of intrinsic line-shape asymmetries and satellite structures. Correlation of photoelectron satellites with RHEELS loss features allows the assignment of inter- band transitions, surface and bulk plasmons, plasmonic overtones, and semi-core ionisation losses across the Pt spectrum. Several previously unresolved satellite features and spin-orbit splittings are identified and discussed. Comparison of experimental valence band spectra with orbital-projected densities of states calculated using ab initio density functional theory (DFT) and G0W0 approaches, with and without spin-orbit coupling, demonstrates the critical role of relativistic effects in reproducing the Pt valence electronic structure. Together, these results establish a unified, internally consistent spectroscopic reference for metallic platinum, providing a robust framework for interpreting photoelectron spectra of Pt-containing catalysts, electronic materials, and related 5d transition metal systems.

cond-mat.mtrl-sci↗

arXiv2026

A Comprehensive Experimental and Theoretical Spectroscopic Study of Proteinogenic Amino Acids

Amino acids are essential building blocks of life, yet our understanding of their chemistry and electronic structure in the solid state remains limited. This is particularly important because amino acids in the solid state are relevant to biological and pharmaceutical processes. X-ray photoelectron spectroscopy provides a powerful experimental probe of chemical states and occupied electronic structure; however, most spectroscopy studies of amino acids focus on gas-phase species or surface adsorbates, while crystalline amino acids remain underexplored, largely because of experimental challenges associated with radiation damage. Additionally, the spectra are often complex and difficult to interpret, motivating a combined experimental-theoretical approach. This study combines X-ray photoelectron spectroscopy and density functional theory calculations to systematically investigate the core, semi-core, and valence states of 20 proteinogenic amino acids as well as selenomethionine which can be incorporated during protein synthesis and deliver the essential nutrient, Se, required by humans. Calculated relative core binding energies show excellent agreement with experiment and enable reliable assignments. Projections of the density of states provide insight into the influence of local coordination and extended crystal structure, yielding a systematic understanding of the electronic structure and bonding in solid-state AAs. The insights gained from this study enhance the understanding of crystalline amino acids and validate the robustness of an integrated experiment--theory framework.

physics.chem-ph↗

arXiv2026

Stabilisation of NV centres in diamond nanopillars at low temperature

Degradation of near surface nitrogen vacancy (NV) centers in diamond under optical illumination has restricted their deployment in applications such as scanning NV magnetomety, particularly under harsh environment such as low temperatures and vacuum. Previously, alumina passivation of planar diamond samples has been shown to reduce the degradation of near surface ensemble NV centers in vacuum. Here, we expand this study to incorporate photonic nanostructures by analyzing the single photon emission characteristics of NV centers embedded in an array of alumina-coated diamond nanopillars in high vacuum and low temperature (6K, high vacuum) environments under non-resonant (522 nm) laser exposure. We find that, in contrast to the oxygen-terminated diamond nanopillars, NV centers in the alumina-coated nanopillars demonstrate negligible change in the single photon purity and brightness over the course of laser exposure in vacuum. At low temperature, NV centers under alumina termination demonstrate stable single photon emission, whereas under oxygen termination the single photon purity degrades under high intensity laser exposure. Alumina surface passivation is therefore shown as a viable path toward the realization of robust NV-diamond based nanoscale sensing under non-ambient atmospheric environments, including using diamond scanning probes.

physics.app-ph↗

arXiv2025

Simulation of Radiation Damage on [M(COD)Cl]$_2$ using Density Functional Theory

Theoretical calculations of materials have in recent years shown promise in facilitating the analysis of convoluted experimental data. This is particularly invaluable in complex systems or for materials subject to certain environmental conditions, such as those exposed to X-ray radiation during routine characterisation. Despite the clear benefit in this use case to shed further light on intermolecular damage processes, the use of theory to study radiation damage of samples is still not commonplace, with very few studies in existing literature. In this paper, we demonstrate the potential of density functional theory for modelling the electronic structure of two industrially important organometallic systems of the formula [M(COD)Cl]$_2$ where M=Ir/Rh and COD=1,5-cyclooctadiene, which are subject to X-ray irradiation via X-ray Diffraction and X-ray Photoelectron Spectroscopy. Our approach allows calculated spectra to be compared directly to experimental data, in this case, the X-ray photoelectron valence band spectra, enabling the valuable correlation of individual atomic states to the electronic structure.

cond-mat.mtrl-sci↗

arXiv2025

Investigating Discontinuous X-ray Irradiation as a Damage Mitigation Strategy for [M(COD)Cl]$_2$ Catalysts

With the advent of ever more intense and focused X-ray sources, including in laboratories, at synchrotrons, and at X-ray free electron lasers, radiation-induced sample change and damage are becoming increasingly challenging. Therefore, the exploration of possible mitigation strategies is crucial to continue to allow the collection of robust and repeatable data. One mitigation approach is the introduction of short, X-ray-free ``dark'' periods. However, it is unclear whether this strategy minimises damage or, in actuality, promotes it through a phenomenon called ``dark progression'', i.e. the increase or progression of radiation damage that occurs after the X-ray beam is turned off. This work discusses the influence of introducing dark periods and their duration on the radiation-induced changes in two model small-molecule catalysts, [Ir(COD)Cl]$_2$ and [Rh(COD)Cl]$_2$, exposed to X-ray radiation in powder diffraction (PXRD) and photoelectron spectroscopy (XPS) experiments. This provides, for the first time, insights into how damage progresses under varying radiation regimes and allows the distinction between the processes that affect the unit cell itself, the individual molecular units, and the respective atomic chemical environments. Furthermore, it provides the basis for informed decision-making in the design of future experiments where the need to minimise radiation-induced damage is crucial.

physics.chem-ph↗

arXiv2024

Enabling two-dimensional electron gas with high room-temperature electron mobility exceeding 100 cm$^2$/Vs at a perovskite oxide interface

In perovskite oxide heterostructures, bulk functional properties coexist with emergent physical phenomena at epitaxial interfaces. Notably, charge transfer at the interface between two insulating oxide layers can lead to the formation of a two-dimensional electron gas (2DEG) with possible applications in, e.g., high-electronmobility transistors and ferroelectric field-effect transistors. So far, the realization of oxide 2DEGs is, however, largely limited to the interface between the single-crystal substrate and epitaxial film, preventing their deliberate placement inside a larger device architecture. Additionally, the substrate-limited quality of perovskite oxide interfaces hampers room-temperature 2DEG performance due to notoriously low electron mobility. In this work, we demonstrate the controlled creation of an interfacial 2DEG at the epitaxial interface between perovskite oxides BaSnO$_3$ and LaInO$_3$ with enhanced room-temperature electron mobilities up to 119 cm$^2$/Vs - the highest room-temperature value reported so far for a perovskite oxide 2DEG. Using a combination of state-of-the-art deposition modes during oxide molecular beam epitaxy, our approach opens up another degree of freedom in optimization and $in$-$situ$ control of the interface between two epitaxial oxide layers away from the substrate interface. We thus expect our approach to apply to the general class of perovskite oxide 2DEG systems and to enable their improved compatibility with novel device concepts and integration across materials platforms.

cond-mat.mes-hall↗

arXiv2024

Atomic multiplet and charge-transfer screening effects in 1$s$ and 2$p$ core-level X-ray photoelectron spectra of early 3$d$ transition-metal oxides

We present a comparative analysis of 1$s$ and 2$p$ core-level hard X-ray photoelectron spectroscopy (HAXPES) spectra in metallic VO$_2$ and CrO$_2$. Even though the V 1$s$ and 2$p$ spectra in VO$_2$ display similar line shapes except the absence or presence of a spin-orbit coupling splitting, the Cr 1$s$ and 2$p$ spectra exhibit distinct main-line shapes. The experimental HAXPES spectra are analyzed by the Anderson impurity model based on the density functional theory + dynamical mean-field theory and a conventional MO$_6$ cluster model. We elucidate the complex interplay between formation of the intra-atomic multiplet and charge transfer effect on the chemical bonding followed by the 1$s$ and 2$p$ core electron excitations. We demonstrate the advantage of the 1$s$ excitations to the routinely-employed 2$p$ excitations for distinguishing between metal-ligand and metal-metal charge transfer contributions in early 3$d$ transition-metal oxides.

cond-mat.mtrl-sci↗

arXiv2023

Stability of near surface nitrogen vacancy centers using dielectric surface passivation

We study the photo-physical stability of ensemble near-surface nitrogen vacancy (NV) centers in diamond under vacuum and air. The optically detected magnetic resonance contrast of the NV centers was measured following exposure to laser illumination, showing opposing trends in air compared to vacuum (increasing by up to 9% and dropping by up to 25%, respectively). Characterization using Raman and X-ray photoelectron spectroscopies suggests a surface reconstruction: In air, atmospheric oxygen adsorption on surface leads to an increase in NV- fraction, whereas in vacuum, net oxygen desorption increases the NV0 fraction. NV charge state switching is confirmed by photoluminescence spectroscopy. Deposition of ~ 2 nm alumina (Al2O3) over the diamond surface was shown to stabilize the NV charge state under illumination in either environment, attributed to a more stable surface electronegativity. The use of alumina coating on diamond is therefore a promising approach to improve the resilience of NV sensors.

physics.app-ph↗

arXiv2023

Revealing the bonding nature and electronic structure of early transition metal dihydrides

Hydrogen as a fuel plays a crucial role in driving the transition to net zero greenhouse gas emissions. To realise its potential, obtaining a means of efficient storage is paramount. One solution is using metal hydrides, owing to their good thermodynamical absorption properties and effective hydrogen storage. Although metal hydrides appear simple compared to many other energy materials, understanding the electronic structure and chemical environment of hydrogen within them remains a key challenge. This work presents a new analytical pathway to explore these aspects in technologically relevant systems using Hard X-ray Photoelectron Spectroscopy (HAXPES) on thin films of two prototypical metal dihydrides: YH$_{2-δ}$ and TiH$_{2-δ}$. By taking advantage of the tunability of synchrotron radiation, a non-destructive depth profile of the chemical states is obtained using core level spectra. Combining experimental valence band spectra collected at varying photon energies with theoretical insights from density functional theory (DFT) calculations, a description of the bonding nature and the role of d versus sp contributions to states near the Fermi energy are provided. Moreover, a reliable determination of the enthalpy of formation is proposed by using experimental values of the energy position of metal s band features close to the Fermi energy in the HAXPES valence band spectra.

cond-mat.mtrl-sci↗

arXiv2023

Capturing the dynamics of Ti diffusion across Ti$_x$W$_{1-x}$/Cu heterostructures using X-ray photoelectron spectroscopy

Interdiffusion phenomena between adjacent materials are highly prevalent in semiconductor device architectures and can present a major reliability challenge for the industry. To fully capture and better understand these phenomena, experimental approaches must go beyond static and post-mortem studies to include in-situ and in-operando setups. Here, soft and hard X-ray photoelectron spectroscopy (SXPS and HAXPES) is used to monitor diffusion in real-time across a proxy device. The device consists of a Si/SiO\textsubscript{2}/Ti$_x$W$_{1-x}$(300 nm)/Cu(25 nm) thin film material stack, with the Ti$_x$W$_{1-x}$ film acting as a diffusion barrier between Si and Cu. The monitoring of diffusion is achieved through the continuous collection of spectra whilst in-situ annealing to 673 K. Ti within the TiW is found to be highly mobile during annealing, diffusing out of the barrier and accumulating at the Cu surface. Increasing the Ti concentration within the Ti$_x$W$_{1-x}$ film increases the quantity of accumulated Ti, and Ti is first detected at the Cu surface at temperatures as low as 550 K. Surprisingly, at low Ti concentrations ($x$ = 0.054), W is also mobile and diffuses alongside Ti. These results provide crucial evidence for the importance of diffusion barrier composition on their efficacy during device application, delivering insights into the mechanisms underlying their effectiveness and limitations.

cond-mat.mtrl-sci↗

arXiv2023

Satellites in the Ti~1$s$ core level spectra of SrTiO$_3$ and TiO$_2$

Satellites in core level spectra of photoelectron spectroscopy (PES) can provide crucial information on the electronic structure and chemical bonding in materials, particular in transition metal oxides. This paper explores satellites of the Ti 1$s$ and 2$p$ core level spectra of SrTiO$_3$ and TiO$_2$. Conventionally, soft x-ray PES (SXPS) probes the Ti 2$p$ core level; however, it is not ideal to fully capture satellite features due to its inherent spin-orbit-splitting (SOS). Here, hard x-ray PES(HAXPES) provides access to the Ti 1$s$ spectrum instead, which allows us to study intrinsic charge responses upon core-hole creation without the complication from SOS and with favorable intrinsic linewidths. The experimental spectra are theoretically analyzed by two impurity models, including an Anderson impurity model (AIM) built on local density approximation (LDA) and dynamical mean-field theory (DMFT), and a conventional TiO$_6$ cluster model. The theoretical results emphasize the importance of explicit inclusion of higher-order Ti-O charge-transfer processes beyond the nearest-neighboring Ti-O bond to simulate the core level spectra of SrTiO$_3$ and TiO$_2$. The AIM approach with continuous bath orbitals provided by LDA+DMFT represents the experimental spectra well. Crucially, with the aid of the LDA+DMFT method, this paper provides a robust prescription of how to use the computationally cheap cluster model in fitting analyses of core level spectra.

cond-mat.mtrl-sci↗

arXiv2022

Variability in X-ray induced effects in [Rh(COD)Cl]2 with changing experimental parameters

X-ray characterisation methods have undoubtedly enabled cutting-edge advances in all aspects of materials research. Despite the enormous breadth of information that can be extracted from these techniques, the challenge of radiation-induced sample change and damage remains prevalent. This is largely due to the emergence of modern, high-intensity X-ray source technologies and growing potential to carry out more complex, longer duration in-situ or in-operando studies. The tunability of synchrotron beamlines enables the routine application of photon energy-dependent experiments. This work explores the structural stability of [Rh(COD)Cl]2, a widely used catalyst and precursor in the chemical industry, across a range of beamline parameters that target X-ray energies of 8 keV, 15 keV, 18 keV and 25 keV, on a powder X-ray diffraction synchrotron beamline at room temperature. Structural changes are discussed with respect to absorbed X-ray dose at each experimental setting associated with the respective photon energy. In addition, the X-ray radiation hardness of the catalyst is discussed, by utilising the diffraction data at the different energies to determine a dose limit, which is often considered in protein crystallography and typically overlooked in small molecule crystallography. This work not only gives fundamental insight into how damage manifests in this organometallic catalyst, but will encourage careful consideration of experimental X-ray parameters before conducting diffraction on similar radiation-sensitive organometallic materials.

cond-mat.mtrl-sci↗

arXiv2022

Probing Disorder in 2CzPN using Core and Valence States

Molecules which exhibit thermally activated delayed fluorescence (TADF) show great promise for use in efficient, environmentally-friendly OLEDs, and thus the design of new TADF emitters is an active area of research. However, when used in devices, they are typically in the form of disordered thin films, where both the external molecular environment and thermally-induced internal variations in parameters such as the torsion angle can strongly influence their electronic structure. In this work, we use density functional theory and X-ray photoelectron spectroscopy to investigate the impact of disorder on both core and valence states in the TADF emitter 2CzPN. By simulating gas phase molecules displaying varying levels of disorder, we assess the relative sensitivity of the different states to factors such as varying torsion angle. The theoretical results for both core and valence states show good agreement with experiment, thereby also highlighting the advantages of our approach for interpreting experimental spectra of large aromatic molecules, which are too complex to interpret based solely on experimental data.

physics.chem-ph↗

arXiv2022

Tackling Disorder in $γ$-Ga$_2$O$_3$

Ga$_2$O$_3$ and its polymorphs are attracting increasing attention. The rich structural space of polymorphic oxide systems such as Ga$_2$O$_3$ offers potential for electronic structure engineering, which is of particular interest for a range of applications, such as power electronics. $γ$-Ga$_2$O$_3$ presents a particular challenge across synthesis, characterisation, and theory due to its inherent disorder and resulting complex structure -- electronic structure relationship. Here, density functional theory is used in combination with a machine learning approach to screen nearly one million potential structures, thereby developing a robust atomistic model of the $γ$-phase. Theoretical results are compared with surface and bulk sensitive soft and hard X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, spectroscopic ellipsometry, and photoluminescence excitation spectroscopy experiments representative of the occupied and unoccupied states of $γ$-Ga$_2$O$_3$. The first onset of strong absorption at room temperature is found at 5.1 eV from spectroscopic ellipsometry, which agrees well with the excitation maximum at 5.17 eV obtained by PLE spectroscopy, where the latter shifts to 5.33 eV at 5 K. This work presents a leap forward in the treatment of complex, disordered oxides and is a crucial step towards exploring how their electronic structure can be understood in terms of local coordination and overall structure.

cond-mat.mtrl-sci↗

arXiv2022

Evaluation of the Thermal Stability of TiW/Cu Heterojunctions Using a Combined SXPS and HAXPES Approach

Power semiconductor device architectures require the inclusion of a diffusion barrier to suppress, or at best prevent the interdiffusion between the copper metallisation interconnects and the surrounding silicon substructure. The binary pseudo-alloy of titanium-tungsten (TiW), with $>$70~at.\% W, is a well established copper diffusion barrier but is prone to degradation via the out-diffusion of titanium when exposed to high temperatures ($\geq$400$^\circ$C). Here, the thermal stability of physical vapour deposited (PVD) TiW/Cu bilayer thin films in Si/SiO\textsubscript{2}(50~nm)/TiW(300~nm)/Cu(25~nm) stacks were characterised in response to annealing at 400$^\circ$C for 0.5~h and 5~h, using a combination of soft and hard X-ray photoelectron spectroscopy (SXPS and HAXPES) and transmission electron microscopy (TEM). Results show that annealing promoted the segregation of titanium out of the TiW and interdiffusion into the copper metallisation. Titanium was shown be driven toward the free copper surface, accumulating there and forming a titanium oxide overlayer upon exposure to air. Annealing for longer timescales promoted a greater out-diffusion of titanium and a thicker oxide layer to grow on the copper surface. However, interface measurements suggest that the diffusion is not significant enough to compromise the barrier integrity and the TiW/Cu interface remains stable even after 5~h of annealing.

cond-mat.mtrl-sci↗

arXiv2021

Lifetime effects and satellites in the photoelectron spectrum of tungsten metal

Tungsten is an important and versatile transition metal and has a firm place at the heart of many technologies. A popular experimental technique for the characterisation of tungsten and tungsten-based compounds is X-ray photoelectron spectroscopy (XPS), which enables the assessment of chemical states and electronic structure through the collection of core level and valence band spectra. However, in the case of metallic tungsten, open questions remain regarding the origin, nature, and position of satellite features that are prominent in the photoelectron spectrum. These satellites are a fingerprint of the electronic structure of the material and have not been thoroughly investigated, at times leading to their misinterpretation. The present work combines high-resolution soft and hard X-ray photoelectron spectroscopy (SXPS and HAXPES) with reflection electron energy loss spectroscopy (REELS) and a multi-tiered ab-initio theoretical approach, including density functional theory (DFT) and many-body perturbation theory (G0W0 and GW+C), to disentangle the complex set of experimentally observed satellite features attributed to the generation of plasmons and interband transitions. This combined experiment-theory strategy is able to uncover previously undocumented satellite features, improving our understanding of their direct relationship to tungsten's electronic structure. Furthermore, it lays the groundwork for future studies into tungsten based mixed-metal systems and holds promise for the re-assessment of the photoelectron spectra of other transition and post-transition metals, where similar questions regarding satellite features remain.

cond-mat.mtrl-sci↗

arXiv2021

Thermal and Oxidation Stability of Ti$_x$W$_{1-x}$ Diffusion Barriers Investigated by Soft and Hard X-ray Photoelectron Spectroscopy

The binary alloy of titanium-tungsten (TiW) is an established diffusion barrier in high-power semiconductor devices, owing to its ability to suppress the diffusion of copper from the metallisation scheme into the surrounding silicon substructure. However, little is known about the response of TiW to high temperature events or its behaviour when exposed to air. Here, a combined soft and hard X-ray photoelectron spectroscopy (XPS) characterisation approach is used to study the influence of post-deposition annealing and titanium concentration on the oxidation behaviour of a 300~nm-thick TiW film. The combination of both XPS techniques allows for the assessment of the chemical state and elemental composition across the surface and bulk of the TiW layer. The findings show that in response to high-temperature annealing, titanium segregates out of the mixed metal system and upwardly migrates, accumulating at the TiW/air interface. Titanium shows remarkably rapid diffusion under relatively short annealing timescales and the extent of titanium surface enrichment is increased through longer annealing periods or by increasing the precursor titanium concentration. Surface titanium enrichment enhances the extent of oxidation both at the surface and in the bulk of the alloy due to the strong gettering ability of titanium. Quantification of the soft X-ray photoelectron spectra highlights the formation of three tungsten oxidation environments, attributed to WO$_2$, WO$_3$ and a WO$_3$ oxide coordinated with a titanium environment. This combinatorial characterisation approach provides valuable insights into the thermal and oxidation stability of TiW alloys from two depth perspectives, aiding the development of future device technologies.

cond-mat.mtrl-sci↗

arXiv2020

A Combined Density Functional Theory and X-ray Photoelectron Spectroscopy Study of the Aromatic Amino Acids

Amino acids are essential to all life. However, our understanding of some aspects of their intrinsic structure, molecular chemistry, and electronic structure is still limited. In particular the nature of amino acids in their crystalline form, often essential to biological and medical processes, faces a lack of knowledge both from experimental and theoretical approaches. An important experimental technique that has provided a multitude of crucial insights into the chemistry and electronic structure of materials is X-ray photoelectron spectroscopy. Whilst the interpretation of spectra of simple bulk inorganic materials is often routine, interpreting core level spectra of complex molecular systems is complicated to impossible without the help of theory. We have previously demonstrated the ability of density functional theory to calculate binding energies of simple amino acids, using $Δ$SCF implemented in a systematic basis set for both gas phase (multiwavelets) and solid state (plane waves) calculations. In this study, we use the same approach to successfully predict and rationalise the experimental core level spectra of phenylalanine (Phe), tyrosine (Tyr), tryptophan (Trp), and histidine (His) and gain an in-depth understanding of their chemistry and electronic structure within the broader context of more than 20 related molecular systems. The insights gained from this study provide significant information on the nature of the aromatic amino acids and their conjugated side chains.

cond-mat.mtrl-sci↗