Periodic Reporting for period 1 - BETTERXPS (Tackling the Peak Assignment Problem in X-ray Photoelectron Spectroscopy with First Principles Calculations)
Berichtszeitraum: 2024-01-01 bis 2025-12-31
Scientific progress is strongly linked to developments in methods of characterization - to control something, we need to understand it, and to understand something, we need to be able to probe it. For example, it would be hard to imagine modern biology without optical microscopy, nanotechnology without electron microscopy, synthetic organic chemistry without nuclear magnetic resonance (NMR) spectrometry, or astronomy without telescopes. In a similar manner, surface science is reliant on techniques that allow us to probe the chemical compositions of surfaces. In order to understand phenomena like corrosion and degradation, processes like heterogeneous catalysis, or the operation of various functional surfaces, e.g. antimicrobial surface coatings or gas sensors, we need to be able to determine the structures of surfaces at the atomic level. In other words, we need to be able to study the chemical compositions of surfaces.
X-ray Photoelectron Spectroscopy (XPS) is one of the most commonly used analytical techniques in experimental surface science. In XPS, the energy that is required to remove a core electron from a particular atom is measured. Since that energy depends on the chemical environment of the atom, an XPS spectrum contains valuable information about surface chemistry. However, the interpretation of XPS spectra is challenging. Often, a detailed spectrum of a complex surface can be acquired, but it can be of little value if the origin of the detected spectral features is not understood. Such problems in the analysis of XPS spectra are widespread, and commonly discussed in the scientific literature.
The aim of this project is to develop novel computational methods for guiding the analysis of XPS spectra and to test them in real-world applications, by bringing together theoreticians with research groups involved in experimental surface science. Our ultimate goal is to advance surface science by making XPS a better and more reliable tool for determining the chemical environments of surface atoms.
X-ray Photoelectron Spectroscopy (XPS) is one of the most commonly used analytical techniques in experimental surface science. In XPS, the energy that is required to remove a core electron from a particular atom is measured. Since that energy depends on the chemical environment of the atom, an XPS spectrum contains valuable information about surface chemistry. However, the interpretation of XPS spectra is challenging. Often, a detailed spectrum of a complex surface can be acquired, but it can be of little value if the origin of the detected spectral features is not understood. Such problems in the analysis of XPS spectra are widespread, and commonly discussed in the scientific literature.
The aim of this project is to develop novel computational methods for guiding the analysis of XPS spectra and to test them in real-world applications, by bringing together theoreticians with research groups involved in experimental surface science. Our ultimate goal is to advance surface science by making XPS a better and more reliable tool for determining the chemical environments of surface atoms.
The most important quantity measured in XPS is the core electron binding energy. Therefore, to analyze XPS spectra, reliable, accurate, and easy to use methods for calculating core electron binding energies are required. In the BETTERXPS project, work has been carried out to develop routines for calculating core electron binding energies in the electronic structure program package FHI-aims. In particular, the internal routines for Δ-Self-Consistent-Field (ΔSCF) calculations, referenced by the keywords deltascf_projector and deltascf_basis have been updated, to permit more efficient calculations of periodic systems. In addition, new routines for the automatic generation of basis sets for ΔSCF calculations have been implemented, and the tutorials and the manual have been updated to cover the new functionalities.
The performance of the ΔSCF method for calculating core electron binding energies has also been tested for a series of medium sized molecules, in a case study based on recent experimental measurements carried out at MAX IV Laboratory. In addition, in a case study of adsorbates on catalyst surfaces, single crystals of NiSe were prepared, and characterized using X-ray diffraction and XPS. Finally, in a case study of layered materials, spectra of MAX phases and MXenes have been recorded.
The performance of the ΔSCF method for calculating core electron binding energies has also been tested for a series of medium sized molecules, in a case study based on recent experimental measurements carried out at MAX IV Laboratory. In addition, in a case study of adsorbates on catalyst surfaces, single crystals of NiSe were prepared, and characterized using X-ray diffraction and XPS. Finally, in a case study of layered materials, spectra of MAX phases and MXenes have been recorded.
The improvements to the ΔSCF functionality in FHI-aims address two of main challenges in practical ΔSCF calculations: robustness, and applicability to periodic systems. One section of the new FHI-aims roadmap article (preprint available at arXiv:2505.00125) is dedicated to the current status of this implementation. The implementation in FHI-aims now enables scalable all-electron ΔSCF calculations of systems containing up to a thousand atoms: to the best of our knowledge, such functionality is not currently available in any other codes. However, further testing and validation is still required to better understand the strengths and limitations of the new implementation.
The study of medium sized molecules indicated that the good accuracy of the ΔSCF method observed in earlier studies of very small molecules also holds up when larger systems are considered. This is important for the general utility of the approach, as most practical XPS studies are concerned with chemically complex systems.
The study of medium sized molecules indicated that the good accuracy of the ΔSCF method observed in earlier studies of very small molecules also holds up when larger systems are considered. This is important for the general utility of the approach, as most practical XPS studies are concerned with chemically complex systems.