X-ray photoelectron spectroscopy (XPS) and low energy ion scattering (LEIS) are important methods for identifying and quantifying the elements at surfaces. XPS largely deals with zero-loss peaks produced by unattenuated photoemission, and the focus of LEIS is similarly surface peaks resulting from direct scattering from surface atoms. However, the backgrounds of these spectra also contain information. They can help reveal the structures and distributions of the atoms and layers at surfaces. Here, we show abnormalities in XPS and LEIS backgrounds that are not accompanied by zero-loss photoemission or surface peaks. These features are due to buried atoms that are far enough from the surface that they cannot produce zero loss or surface peaks but close enough to affect spectral backgrounds. This Insight Note was written to alert the reader to the existence and usefulness of these backgrounds. Spectra with background abnormalities were poorly modeled by multiple linear regression, that is, as linear combinations of the pure-material spectra. However, XPS backgrounds with anomalies can be well modeled using the QUASES software, even when zero-loss peaks are not apparent. With QUASES, it is also possible to correct the substrate spectrum for distortions caused by inelastic electron scattering. When unusual baseline features appear in XPS or LEIS spectra, it can be helpful to use a complementary technique that probes at a greater depth to confirm the presence and identity of the atoms causing the anomaly. The effects in this work were demonstrated via a lighter coating of SiO2 on a heavier stainless-steel substrate.

Analysis of the spectrum of inelastically scattered electrons in XPS has been used for decades to determine the structure of materials on the nanoscale. Using the higher photon energies available in HAXPES, the method was recently shown to determine structures rather accurately with a more than 100 nm probing depth. In this paper, these HAXPES applications are briefly reviewed. Only two parameters are required for the analysis, namely, the inelastic mean free path and the cross section for inelastic electron scattering. We also discuss in detail how these parameters are best selected.

This paper reviews a procedure that allows for extracting primary photoelectron or Auger electron emissions from homogeneous isotropic samples. It is based on a quantitative dielectric description of the energy losses of swift electrons travelling nearby surfaces in presence of stationary positive charges. The theory behind the modeling of the electron energy losses, implemented in a freely available QUEELS-XPS software package, takes into account intrinsic and extrinsic effects affecting the electron transport. The procedure allows for interpretation of shake-up and multiplet structures on a quantitative basis. We outline the basic theory behind it and illustrate its capabilities with several case examples. Thus, we report on the angular dependence of the intrinsic and extrinsic Al 2s photoelectron emission from aluminum, the shake-up structure of the Ag 3d, Cu 2p, and Ce 3d photoelectron emission from silver, CuO and CeO2, respectively, and the quantification of the two-hole final states contributing to the L3M45M45 Auger electron emission of copper. These examples illustrate the procedure, that can be applied to any homogeneous isotropic material.

The article mentioned in the title of this comment paper reports on an investigation of the organic binder presence and distribution on stone wool fibres with surface sensitive techniques (X-ray photoelectron spectroscopy (XPS), QUASES XPS modelling, time-of-flight secondary ion mass spectrometry (ToF-SIMS) mapping) and attempts to correlate the results with fibre performance in in vitro acellular biosolubility tests. However, the study has assumptions, hypothesis and results that do not take into account the recognised science and regulations on biopersistence of stone wool fibres, limitations of the utilized surface sensitive techniques and modelling approach and it contains a contradiction with biosolubility experiments. In this comment article, we discuss these points, propose improved QUASES XPS modelling and present recent ToF-SIMS mapping results that reflect biosolubility behaviour of the stone wool fibres.

We present the user‐friendly and freely available software package QUEELS (QUantitative analysis of Electron Energy Losses at Surfaces) that allows to calculate effective inelastic scattering cross sections within the dielectric response description, for swift electrons travelling nearby surfaces in several environments. We briefly describe the underlying theoretical models and illustrate its use to evaluate the distribution of energy losses taking place in electron spectroscopies like transmission electron energy loss spectroscopy (TEELS), X‐ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES) and reflection electron energy loss spectroscopy (REELS), which are widely used for material analysis. This includes the intrinsic excitations due to the core hole in XPS and AES.

Determination of the depth distribution of complex nanostructures by X‐ray photoelectron spectroscopy (XPS) inelastic background analysis may be complicated if the sample materials have widely different inelastic scattering cross‐sections. It was recently demonstrated that this may be solved by using a mixture of cross‐sections. This permits retrieval of depth distributions of complex stacks and deeply buried layers with a typical 5% accuracy. This requires however that the cross‐sections of the individual sample materials are known which is often not the case and this can complicate practical use for routine analysis. In this paper, we explore to what extent a suitable two‐ or three‐parameter cross‐section can be defined independent of prior knowledge of the cross‐sections involved but simply defined by fitting the cross‐section parameters to the spectrum being analyzed. This paper presents a theoretical study following our recent paper that explored how to make the best choice of inelastic mean free path and inelastic scattering cross‐section for the inelastic background analysis with the Quases‐Tougaard software. It was previously shown that a rough analysis of the inelastic background could give a good idea of the depth distribution. Here, we demonstrate with model spectra from buried layers created with Quases‐Tougaard Generate software that a rather accurate analysis can be performed for very different cases with an average ~5% error. This analysis is easy to apply as it only needs the two‐ or three‐parameter cross‐sections generated with the Quases‐Tougaard software. This study is aimed to improve routine analysis of the inelastic background of XPS and hard X‐ray photoelectron spectroscopy (HAXPES) spectra.

In X‐ray excited photoelectron emission (XPS), the shape and intensity of photoelectron peaks are strongly affected by extrinsic excitations due to electron transport out of the surface. It is also influenced by intrinsic excitations due to the sudden creation of the static core hole. In order to approximately determine the primary excitation spectrum of the considered transition corrected for both extrinsic and intrinsic excitations, we developed in a previous work [E. Gnacadja, N. Pauly, S. Tougaard, Surf. Interface Anal. 52 (2020) 413] a universal analytical expression for the energy loss cross section including extrinsic and intrinsic excitations. We apply the present universal cross section to test to what extent these primary excitations spectra can be used for XPS quantification based on peak area ratios. The procedure is applied to the study of three sets of polycrystalline alloys (Cu0.75Au0.25, Cu0.50Au0.50, and Cu0.25Au0.75) and to three metal oxides (HfO2, ZrO2, and Cu2O). We show that although the individual peaks are very different from those obtained with the classical universal Tougaard cross section, the determined quantitative compositions are equivalent (but not better). This implies that the relative contribution from intrinsic excitations is roughly the same for all peaks for a given sample and they therefore cancel out when peak area ratios are considered.

The methodology presented within this work is a result of years of interactions between many junior and senior X-ray Photoelectron Spectroscopy (XPS) users operating within the CasaXPS spectral processing and interpretation program framework. In particular, discussions arising from a series of workshops have been a significant source for developing the overall XPS data processing concept and are the motivation for creating this work. These workshops organized by the Institut des Matériaux Jean Rouxel (IMN), Nantes gather both experienced and novice users of XPS for a week of discourse in conceptual experiment design and the resulting data processing. However, the framework constructed and utilized within these workshops encouraged the dissemination of knowledge beyond XPS data analysis and emphasized the importance of a multi-disciplinary collaborative approach to surface analysis problem-solving. The material presented here embodies data treatment originating from data made available to the first CNRS Thematic Workshop presented at Roscoff 2013. The methodology described here has evolved over the subsequent workshops in 2016 and 2019 and currently represents the philosophy used in CasaXPS spectral data processing paradigm.

The inelastic background of hard x-ray photoelectron spectroscopy data is analyzed to paint a depth-resolved picture of the CdS/Cu(In,Ga)Se2 (CdS/CIGSe) layer structure. The CdS/CIGSe interface is the central component in next-generation chalcopyrite thin-film photovoltaic devices. By analyzing both, the (unscattered) core-level peaks and the inelastic background, and by varying the excitation photon energy from 2.1 up to 14 keV, we can derive photoemission information over a broad range of electron kinetic energies and, hence, sampling depths. With this complementary information, the CdS film thickness of a CdS/CIGSe interface can be accurately determined as a function of the CdS deposition time. For the thinner CdS films, the film thickness can be shown to vary laterally. Furthermore, small amounts of Se and process-related Rb can be detected in a thin (∼2 nm) surface layer of all investigated CdS films.

Hard X-ray Photoelectron Spectroscopy (HAXPES) provides minimally destructive depth profiling into the bulk, extending the photoelectron sampling depth. Detection of deeply buried layers beyond the elastic limit is enabled through inelastic background analysis. To test the robustness of this technique, we present results on a thin (18 nm) layer of metal-organic complex buried up to 200 nm beneath organic material. Overlayers with thicknesses 25-140 nm were measured using photon energies ranging 6-10 keV at the 109 end station at Diamond Light Source, and a new fixed energy Ga K alpha (9.25 keV) laboratory-based HAXPES spectrometer was also used to measure samples with overlayers up to 200 nm thick. The sampling depth was varied: at Diamond Light Source by changing the photon energy, and in the lab system by performing angle-resolved measurements. For all the different overlayers and sampling depths, inelastic background modelling consistently provided thicknesses which agreed, within reasonable error, with the ellipsometric thickness. Relative sensitivity factors were calculated, and these factors consistently provided reasonable agreement with the expected nominal stoichiometry, suggesting the calculation method can be extended to any element. These results demonstrate the potential for the characterisation of deeply buried layers using synchrotron and laboratory-based HAXPES.

X‐ray photoemission spectroscopy (XPS) measurements in near‐ambient pressure (NAP) conditions result in a signal loss of the primary spectrum as a result of inelastic scattering of photoelectrons in the gas phase. The inelastic scattering of the primary electrons gives rise to a secondary signal that can result in additional and often unwanted features in the measured spectrum. In the present work, we derive equations that can be used to model the resulting signal and provide equations that can be used to simulate or remove the inelastic scattering signal from measured spectra. We demonstrate this process for photoemission spectra of a wide range of kinetic energies, measured from Au, Ag, and Cu, in a variety of gases (N2, He, H2, and O2). The work is supplemented with an open‐source software in which the algorithms described here have been implemented and can be used to remove the gas phase inelastic scattering signal.

This guide is intended for both the novice in x-ray photoelectron spectroscopy (XPS) as well as users with some experience. XPS is one of the most widely used methods to characterize surface nanostructured samples, and XPS is now also commonly accessible to most material scientists through XPS facility centers. It is, therefore, increasingly used as a routine analysis technique to complement other techniques. This has led to an increase in the number of users who may not have a full understanding of the details of XPS and consequently must rely on the report provided by the XPS center. The purpose of this practical guide on the aspects of quantitative XPS is first to put the reader in a position to be able to understand and judge the meaning and possible errors in atomic concentrations based on analysis of peak intensities, which is the standard way quantitative XPS is reported. We discuss different ways to improve the analysis. This is attained by giving the reader a good understanding of how the intensities in the peak and the background of inelastically scattered electrons are linked together through the depth distribution of atoms. We then explore how this can be applied to greatly increase the capabilities of XPS to more accurately determine the composition and structure of surfaces on the nanoscale. This is possible at different levels of sophistication. We first see how a visual inspection of the XPS survey spectrum can be applied to get a quick rough indication of the structure. Next, we go through other more quantitative methods that are being used. The practical application of these techniques is illustrated by several examples.

Near-ambient pressure XPS (NAP-XPS) is a less traditional form of XPS that allows samples to be analyzed at relatively high pressures, i.e., at 2500 Pa or greater. With NAP-XPS, one can analyze moderately volatile liquids, biological samples, porous materials, and/or polymeric materials that outgas significantly. In this submission, the authors show C 1s, O 1s, and survey NAP-XPS spectra from 1,4-polymyrcene. The C 1s and O 1s envelopes are fit with Gaussian–Lorentzian product, asymmetric Lorentzian, and Gaussian–Lorentzian sum functions. Water vapor and argon are used to control sample charging, and the corresponding signals from the gases are present in the survey spectra. The effect of background gas pressure on photoelectron attenuation is illustrated with a sample of polytetrafluoroethylene.

Application of XPS taken under ambient pressure conditions has become widespread in recent years. The distortion of the spectrum caused by scattering effects as the photoelectrons travel through the gas to the spectrometer causes a problem for interpretation of the spectra. In this paper we propose a method to correct for this. We use the QUASES software for which the cross section for inelastic electron scattering is an essential parameter. To determine this, we analyze a spectrum from a pure gold foil taken under 5 mbar N-2 gas pressure and compare this to a spectrum of the gold foil taken under vacuum conditions and construct a cross section which accounts perfectly for the distortions caused by the gas. The procedure was then validated by first applying this same cross section to correct spectra taken under N-2 gas pressures in the range 0.85 - 5.0 mbar. The effective thickness of the gas was the only adjustable parameter and the corrected spectrum was found to be in perfect agreement with the undistorted gold reference spectrum for all gas pressures. Next these determined effective thicknesses were found to be perfectly linearly dependent on the gas pressure. These two tests prove the validity of the method.

Optical properties of polycrystalline molybdenum are determined from ultraviolet up to extreme ultraviolet by reflection electron energy loss spectroscopy (REELS). Calculations are performed within the dielectric response theory by means of the quantitative analysis of electron energy losses at surfaces QUEELS-ϵ(k,ω)-REELS software [Surf. Interface Anal.36, 824 (2004)SIANDQ0142-242110.1002/sia.1774] that allows the simulation of inelastic scattering cross sections, using a parametric energy loss function describing the optical response of the material. From this energy loss function, the real and imaginary parts of the dielectric function, the refractive index, and the extinction coefficient are deduced and compared with previously published results.

The high importance of X‐ray photoelectron spectroscopy (XPS) in surface analysis is well established. In XPS, the shape of the measured peaks is affected by two classes of energy loss: extrinsic losses because of the transport of photoelectrons in the matter and intrinsic losses because of the sudden creation of the static core hole. In order to perform a quantitative, comprehensive determination of the zero‐energy loss spectrum, a systematic and physically meaningful background subtraction method must be used. In this paper, we propose a universal analytical expression to model the energy loss cross section of the emitted photoelectrons for transition metals and their oxides. The proposed expression is a generalization of the well‐known Tougaard's universal inelastic scattering cross section to also account for the intrinsic losses. We demonstrate the use of this to determine the primary excitation spectra of several transition metals and their oxides, and we compare the results with a more accurate calculation based on the dielectric response model for XPS.

Matthew R. Linford1, Vincent S. Smentkowski2*, John T. Grant3, C. Richard Brundle4, Peter M.A. Sherwood5, Mark C. Biesinger6, Jeff Terry7, Kateryna Artyushkova8, Alberto Herrera-Gómez9, Sven Tougaard10, William Skinner11, Jean-Jacques Pireaux12, Christopher F. McConville13, Christopher D. Easton14, Thomas R. Gengenbach14, George H. Major1, Paul Dietrich15, Andreas Thissen15, Mark Engelhard16, Cedric J. Powell17, Karen J. Gaskell18 and Donald R. Baer16 Department of Chemistry and Biochemistry, Brigham Young University, Provo, UT 84602, USA; General Electric Research, Niskayuna, NY 12309, USA; Surface Analysis Consultant, Clearwater, FL 33767, USA; C.R. Brundle & Associates, Soquel, CA 95073, USA; University of Washington, Box 351700, Seattle, WA 98195, USA; Surface Science Western, University of Western Ontario, London, Ontario N6G 0J3, Canada; Department of Physics, Illinois Institute of Technology, Chicago, IL 60616, USA; Physical Electronics, Chanhassen, MN 55317, USA; CINVESTAV – Unidad Queretaro, Real de Juriquilla 76230, Mexico; Department of Physics, University of Southern Denmark, Odense 5230, Denmark; Future Industries Institute, University of South Australia, Mawson Lakes, SA 5095, Australia; University of Namur, Namur Institute of Structured Matter, B-5000 Namur, Belgium; College of Science, RMIT University, Melbourne, VIC 3001, Australia; CSIRO Manufacturing, Ian Wark Laboratories, Clayton, VIC 3168, Australia; SPECS Surface Nano Analysis GmbH, 13355 Berlin, Germany; Pacific Northwest National Laboratory, Richland, WA 99354, USA; National Institute of Standards and Technology, Gaithersburg, MD 20899, USA and University of Maryland, College Park, MD 20742, USA

Most real core-shell nanoparticle (CSNP) samples deviate from an ideal core-shell structure potentially having significant impact on the particle properties. An ideal structure displays a spherical core fully encapsulated by a shell of homogeneous thickness, and all particles in the sample exhibit the same shell thickness. Therefore, analytical techniques are required that can identify and characterize such deviations. This study demonstrates that by analysis of the inelastic background in X-ray photoelectron spectroscopy (XPS) survey spectra, the following types of deviations can be identified and quantified: the nonuniformity of the shell thickness within a nanoparticle sample and the incomplete encapsulation of the cores by the shell material. Furthermore, CSNP shell thicknesses and relative coverages can be obtained. These results allow for a quick and straightforward comparison between several batches of a specific CSNP, different coating approaches, and so forth. The presented XPS methodology requires a submonolayer distribution of CSNPs on a substrate. Poly(tetrafluoroethylene)-poly(methyl methacrylate) and poly(tetrafluoroethylene)-polystyrene polymer CSNPs serve as model systems to demonstrate the applicability of the approach.

Core-shell nanoparticles (CSNPs) have become indispensable in various industrial applications. However, their real internal structure usually deviates from an ideal core-shell structure. To control how the particles perform with regard to their specific applications, characterization techniques are required that can distinguish an ideal from a nonideal morphology. In this work, we investigated poly(tetrafluoroethylene)-poly(methyl methacrylate) (PTFE-PMMA) and poly(tetrafluoroethylene)-polystyrene (PTFE-PS) polymer CSNPs with a constant core diameter (45 nm) but varying shell thicknesses (4-50 nm). As confirmed by transmission scanning electron microscopy (T-SEM), the shell completely covers the core for the PTFE-PMMA nanoparticles, while the encapsulation of the core by the shell material is incomplete for the PTFE-PS nanoparticles. X-ray photoelectron spectroscopy (XPS) was applied to determine the shell thickness of the nanoparticles. The software SESSA v2.0 was used to analyze the intensities of the elastic peaks, and the QUASES software package was employed to evaluate the shape of the inelastic background in the XPS survey spectra. For the first time, nanoparticle shell thicknesses are presented, which are exclusively based on the analysis of the XPS inelastic background. Furthermore, principal component analysis (PCA)-assisted time-of-flight secondary-ion mass spectrometry (ToF-SIMS) of the PTFE-PS nanoparticle sample set revealed a systematic variation among the samples and, thus, confirmed the incomplete encapsulation of the core by the shell material. As opposed to that, no variation is observed in the PCA score plots of the PTFE-PMMA nanoparticle sample set. Consequently, the complete coverage of the core by the shell material is proved by ToF-SIMS with a certainty that cannot be achieved by XPS and T-SEM.

The approach of inelastic background analysis was previously demonstrated to be a useful tool for retrieving the depth distribution of buried layers with an accuracy, which is better than 5% even for some complex samples. This paper presents a study that attempt at rationalizing the approach by exploring how to make the best choice of the inelastic mean free path and the inelastic scattering cross section, which are the two main input parameters needed in the analysis. To this end, spectra from buried layers were created with Quases‐Generate software. The layers consisted of Si 1s recorded at 6099 eV and Au 4d recorded at 1150 eV kinetic energy buried under overlayers of Si, Au, Al, polymer, or Ta. Spectra from samples with a wide range of buried layer thickness and overlayer thickness were created. Subsequently, these spectra were analyzed with Quases‐Analyze software and for each case the analysis was done with different combinations of the input parameters. Among these, the best choice for all cases was to use an effective IMFP and effective inelastic scattering cross section with relative weights being half the thickness of the buried layer and the full thickness of the overlayer. This general formula together with a new version of the software makes the inelastic background analysis of buried layers faster and easier to apply even for nonspecialists.