X-ray sources with a photon energy higher than 2140 eV are increasingly being used for routine x-ray photoelectron spectroscopy (XPS) on laboratory-based instruments. This analytical approach is termed “HAXPES” (hard x-ray photoelectron spectroscopy). This article provides an overview of the current and potential future uses of laboratory-based HAXPES in comparison to routine XPS performed using Al Kα and Mg Kα x-ray sources. The standardization of XPS has occurred over 30 years and many of the procedures and reference works are specific to the use of Al Kα and Mg Kα x-ray sources. In this article, we discuss the translation of standard XPS practices to HAXPES, indicate useful resources for HAXPES users, and highlight areas where there is a need for improved information and guidance.

X-ray photoelectron spectroscopy (XPS) is one of the most common techniques used to analyze the surface composition of catalysts and support materials used in polymer electrolyte membrane (PEM) fuel cells and electrolyzers, providing important insights for further improvement of their properties. Characterization of catalyst layers (CLs) is more challenging, which can be at least partially attributed to the instability of ionomer materials such as Nafion during measurements. This work explores the stability of Nafion during XPS measurements, illuminating and addressing Nafion degradation concerns. The extent of Nafion damage as a function of XPS instrumentation, measurement conditions, and sample properties was evaluated across multiple instruments. Results revealed that significant Nafion damage to the ion-conducting sulfonic acid species (>50% loss in sulfur signal) may occur in a relatively short time frame (tens of minutes) depending on the exact nature of the sample and XPS instrument. This motivated the development and validation of a multipoint XPS data acquisition protocol that minimizes Nafion damage, resulting in reliable data acquisition by avoiding significant artifacts from Nafion instability. The developed protocol was then used to analyze both thin film ionomer samples and Pt/C-based CLs. Comparison of PEM fuel cell CLs to Nafion thin films revealed several changes in Nafion spectral features attributed to charge transfer due to interaction with conductive catalyst and support species. This study provides a method to reliably characterize ionomer-containing samples, facilitating fundamental studies of the catalyst-ionomer interface and more applied investigations of structure-processing-performance correlations in PEM fuel cell and electrolyzer CLs.

Monochromatic Cr Kα radiation (5414.8 eV) was used to acquire high-energy photoelectron spectroscopy (HAXPES) data on pure Mo and bulk MoSe2 compound with special attention paid to binding energy scale correction and quantification. The reported spectra include a survey scan and high-resolution Mo 2s, Mo 2p1/2, Mo 2p3/2, Mo 3d, Mo 3p1/2, Mo 3p3/2, Mo 3s, Mo 4s, Mo4p, Se2s, Se 2p1/2, Se 2p3/2, Se 3s, Se 3p1/2, Se 3p3/2, and Se 3d core-levels. The data will be useful as reference core-level spectra for HAXPES studies on molybdenum and its compounds.

Energy storage (batteries, capacitors) and conversion (solar cells) systems are increasing demand for vehicles, mobile electronics, and other areas as alternatives to fossil fuels.These systems are complex and multi-component with many materials and interfaces that are difficult to handle and characterize. To optimize materials properties and improve battery lifetimes, there is a pressing need for physicochemical characterization approaches with air-free handling capabilities, high spatial resolution, the ability to analyze buried interfaces without damaging the chemistry, and the need to bridge the gap by in situ and operando surface analysis. Developments in X-ray Photoelectron Spectrometers (XPS) open up new capabilities to address these challenges. In this talk, I will discuss how we can overcome obstacles in analyzing battery materials using: Inert environment transfer vessel: This allows for handling battery materials in an air-free environment, which is essential for preventing oxidation and other degradation reactions. Microprobe X-ray source with <5 µm spatial resolution: This provides high spatial resolution for analyzing small areas of interest, such as SEI, dendrites and other features. Hard X-ray source and cluster ion gun source: These allow for the analysis of buried interfaces not accessible by conventional soft X-ray XPS. In situ energy gap measurements: These allow for the study of the changes in the electronic structure of battery materials as they are cycled, which can provide insights into the mechanisms of charge storage and discharge. Operando XPS experiments: These allow for the study of the changes in the surface structure of battery materials as they are operating, which can provide insights into the factors that affect their performance. Figure 1

Monochromatic Cr Kα radiation (5414.8 eV) was used to acquire high-energy photoelectron spectroscopy data on pure W and bulk WSe2 compound. The reported spectra include a survey scan and high-resolution W 3p1/2, W 3p3/2, W 3d3/2, W 3d5/2, W 4s, W 4p1/2, W 4p3/2, W 4d, W 4f, Se 2s, Se 2p1/2, Se 2p3/2, Se 3s, and Se 3p core-levels. The data will be useful as reference core-level spectra for HAXPES studies on tungsten and its compounds.

Semiconductor interface investigation via analytical characterization represents a major focus for Failure Analysis and Process Characterization Teams. Investigation can be performed via destructive techniques through depth profiling until reaching the interface while collecting matrix and impurity distributions as a function of depth and also via nondestructive techniques which preserves information about the interface. Interface characterization involves the deployment of Failure Analysis resources, employing advance imaging and materials characterization techniques like TEM (transmission electron microscopy), SEM (Scanning Electron Microscopy), DSIMS (dynamic secondary mass spectrometry), TOFSIMS (time of flight secondary ion mass spectrometry), Auger, XRF (X-Ray Fluorescence), and XPS (X-ray Photoelectron Spectroscopy). As first exampled in the case study detailed in this paper, increased aluminum resistance was caused by oxidation of aluminum metal while in a cooling chamber prior to barrier layer deposition due to chamber lid atmospheric leaks. High resistivity interconnect issues can be related to interface quality and diminish product performance up to complete failure. Typical methods like cross section analysis via TEM and SEM did not reveal any abnormality. This prompted further investigation where innovative depth profiling methods of DSIMS / TOFSIMS to the encapsulated interface was used, highlighting the presence of oxidized aluminum metal. In another example, analysis was performed nondestructively utilizing a newly developed High Energy HAXPS mode, allowing for the successful characterization of interfaces and high electron mobility transistors where layer interaction is vital.

Journal Article Analysis of Thin Films and Buried Interfaces by Soft and Hard X-ray Photoemission Get access Kateryna Artyushkova, Kateryna Artyushkova Physical Electronics Inc, Chanhassen, MN, United States Corresponding author: kartyushkova@phi.com Search for other works by this author on: Oxford Academic Google Scholar Jennifer Mann, Jennifer Mann Physical Electronics Inc, Chanhassen, MN, United States Search for other works by this author on: Oxford Academic Google Scholar Sarah Zaccarine Sarah Zaccarine Physical Electronics Inc, Chanhassen, MN, United States Search for other works by this author on: Oxford Academic Google Scholar Microscopy and Microanalysis, Volume 29, Issue Supplement_1, 1 August 2023, Page 768, https://doi.org/10.1093/micmic/ozad067.379 Published: 22 July 2023

Monochromatic Cr Kα radiation (5414.8 eV) was used to acquire high-energy photoelectron spectroscopy data on pure Al. The reported spectra include a survey scan and high-resolution Al 1s, Al 2s, Al 2p, and O 1s core-levels. The data will be useful as a comparison for the study in this field.

Si3N4 thin film grown by low-pressure chemical vapor deposition was measured by high-energy photoelectron spectroscopy using monochromatic Cr Kα (5414.8 eV) radiation. A survey scan and high-resolution spectra of Si 1s, Si 2s, Si 2p, and N 1s are reported.

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A thick alumina sample was analyzed by hard x-ray photoelectron spectroscopy by means of a lab-scale spectrometer equipped with a Cr Kα (5414.8 eV) excitation source. The reported spectra include a survey scan as well as Al 1s, Al 2s, Al 2p, O 1s, and C 1s core-level spectra.

Transition metal-nitrogen-carbon (M-N-C) materials have been the focus of scientists' efforts to address the rising need for earth-abundant materials solutions for energy technology and decarbonization of the economy. They are viewed as one of the most promising candidates to replace platinum group metal (PGM) catalysts in the fuel cell and energy conversion fields, including the application of oxygen reduction reaction, carbon dioxide reduction reaction, and nitrogen reduction reaction. In the effort to improve M-N-C materials properties and achieve atomic dispersity of the transition metal in the carbonaceous matrix, a re-pyrolysis process has been proposed. This secondary heat treatment process of already obtained primary pyrolysis-derived M-N-C materials has been widely reported to substantially improve the electrochemical performance and operational stability of the catalysts. Here, we report a systematic investigation of this process used on samples of templated M-N-C catalysts to obtain state-of-the-art catalysts via in situ heating X-ray photoelectron spectroscopy (XPS), scanning transmission electron microscopy (STEM), energy-dispersive X-ray spectroscopy (EDS), electron energy loss spectroscopy (EELS), X-ray diffraction (XRD), and X-ray computed tomography (CT) characterization methods. It is found that the re-pyrolysis of M-N-C materials could result in the partial amorphization of the carbonaceous substrate. It causes the rearrangement and transformation of multitudinous N moieties, leading to optimization of their morphological display and association with atomically dispersed transition metal dopants. Ultimately, the re-pyrolysis results in an increase in uniformity of the active Fe-Nx sites distribution without the formation of nano-crystalline phases (metallic or carbide) and with overall preservation of the morphology of the carbonaceous framework achieved during the first formative pyrolysis step of the templated synthesis. These observations provide confirmation that empirically established re-pyrolysis is recommended to be used on all M-N-C materials despite the different synthesis routes to obtain a practical advanced catalytic material.

An Al2O3 sapphire sample was analyzed using high-resolution high-energy x-ray photoelectron spectroscopy (HAXPES). The HAXPES spectra of Al2O3 obtained using monochromatic Cr Kα radiation at 5414.8 eV include two survey scans (Al Kα and Cr Kα) and high-resolution spectra of Al 2p, Al 2s, Al 1s, and O 1s.

Interactions between a transition metal (oxide) catalyst and a support can tailor the number and nature of active sites, for instance in the methanol oxidation reaction. We here use ambient pressure X-ray photoelectron spectroscopy (AP-XPS) to identify and compare the surface adsorbates that form on amorphous metal oxide films that maximize such interactions. Considering Al(1-x)MxOy (M = Fe or Mn) films at a range of methanol : oxygen gas ratios and temperatures, we find that the redox-active transition metal site (characterized by methoxy formation) dominates dissociative methanol adsorption, while basic oxygen sites (characterized by carbonate formation) play a lesser role. Product detection, however, indicates complete oxidation to carbon dioxide and water with partial oxidation products (dimethyl ether) comprising a minor species. Comparing the intensity of methoxy and hydroxyl features at a fixed XPS chemical shift suggests methanol deprotonation during adsorption in oxygen rich conditions for high transition metal content. However, increasing methanol partial pressure and lower metal site density may promote oxygen vacancy formation and the dehydroxylation pathway, supported by a nominal reduction in the oxidation state of iron sites. These findings illustrate that AP-XPS and mass spectrometry together are powerful tools in understanding metal-support interactions, quantifying and probing the nature of catalytic active sites, and considering the link between electronic structure of materials and their catalytic activity.

Single-crystalline NaCl was analyzed using high-resolution high-energy x-ray photoelectron spectroscopy (HAXPES). The HAXPES spectra of NaCl obtained using monochromatic Cr Kα radiation at 5414.8 eV include two survey scans (Al Kα and Cr Kα) and high-resolution spectra of Na 1s, Na 2p, Na 2s, Cl 1s, Cl 2p, Cl 2s, and Cl KLL.

Silicon nitride (Si3N4) grown by metalorganic chemical vapor deposition on Si was analyzed using high-resolution high-energy x-ray photoelectron spectroscopy (HAXPES). The HAXPES spectra of Si3N4 obtained using monochromatic Cr Kα radiation at 5414.8 eV include two survey scans (Al Kα and Cr Kα) and high-resolution spectra of Si 2p, Si 2s, Si 1s, and N 1s.

Photoelectron spectroscopy is a characterization technique which plays a key role in device technology, a field requiring, very often, a reliable and reproducible analysis of buried, critical interfaces. The recent advent of laboratory hard X-ray spectrometers opens new perspectives toward routine studies of technologically-relevant samples for the qualification of processes and materials. In this review, the status of hard X-ray photoelectron spectroscopy (HAXPES) implemented with chromium Kα excitation (5.414 keV) and applied to technological research in nanoelectronics is presented. After an account of the role of synchrotron HAXPES and the specific effects to care about at the practical level, different aspects are developed, first for illustrating the benefits of the technique through specific application cases in the field of resistive memories and power transistors. Then, we provide a status update on quantification in HAXPES, both from core-level intensities and inelastic background analysis. Finally, we present preliminary results in a novel analytical field, operando HAXPES, where a prototypical device is operated in situ during the laboratory HAXPES experiment, opening up the possibility of unravelling the mechanisms occurring at buried interfaces and governing device operation.