Hard X-ray Photoelectron Spectroscopy Research Articles

Estimation of the band alignment of metal/AlScN interfaces by hard X-ray photoelectron spectroscopy

This paper describes the band alignment of metal/AlSc(O)N measured using hard X-ray photoelectron spectroscopy. The band offset was determined by using the difference in binding energy from the core level to the upper edge of the VB, and the bandgap was determined from the energy loss spectrum of photoelectrons. The introduction of oxygen into AlScN to make AlScON decreases both the conduction band and VB offsets. The change in barrier height of the metal/AlScN structure is very small relative to the difference in the work function of the metal. This suggests that Fermi-level pinning occurs at the metal/AlScN interface.

HAXPES reference spectra of In, Sn, and ITO with Cr Kα excitation

Hard x-ray photoelectron spectroscopy using monochromatic Cr Kα radiation (5414.8 eV) has been used to acquire XPS and Auger data on sputtered In, Sn, and ITO samples. Survey data, high-resolution scans of all observed photoelectron peaks, and high-resolution scans of Auger lines are presented herein.

Electrodeposition of Cu with Amino Acids toward Electrocatalytic Enhancement of CO2 Reduction Reaction

This study investigates the effect of five amino acids on the electrodeposition of Cu to enhance its electrocatalytic performance in CO2 reduction reactions (CO2RR). The amino acids significantly influenced the deposition potential, crystallite size, and surface morphology of the electrodeposited Cu. Electrodeposited Cu with amino acids exhibit significantly smaller crystallites and higher particle density on carbon paper relative to amino acid-free samples. The integration of amino acids into the electrodeposited Cu was confirmed via high-angle annular dark-field scanning transmission electron microscopy, energy dispersive X-ray spectroscopy and hard X-ray photoelectron spectroscopy. All electrodeposited Cu exhibits a higher faradaic efficiency (FE) in the electrochemical reduction of CO2 to CH4 relative to Cu foil (24.2%), regardless of the presence or absence (55.0%) of amino acids when the electrolysis was conducted at −1.27 V vs RHE. Electrodeposited Cu with L-histidine, containing an imidazole group, demonstrates a higher FE of CH4 (67.6%) and effectively suppressed the hydrogen evolution reaction, highlighting the crucial role of amino acid functional groups, particularly imidazole, in augmenting the electrochemical conversion of CO2 to CH4. The study demonstrates the critical influence of specific functional groups in amino acids on the catalytic efficiency of electrodeposited Cu in CO2RR electrocatalysis applications.

Investigation of annealing temperature dependent sub-cycling behavior for HfZrOx-based ferroelectric capacitor

Ferroelectric HfO2 thin film has been widely explored due to its superior characteristics, such as high switching speed, scalability, and long data retention. However, it still faces challenges in achieving good stability due to the wake-up and split-up effects. In this study, the sub-cycling behavior of Hf0.5Zr0.5O2-based ferroelectric capacitors (FeCaps) with various annealing temperatures is investigated. Our results suggest that the FeCaps with higher annealing temperatures demonstrate an increased resistance to the split-up effect and exhibit less distorted hysteresis loops compared to their lower-temperature counterparts. Symmetrical sub-cycling reveals pronounced current split-up and diminished switching current peaks in the FeCaps with lower annealing temperatures, whereas those annealed at higher temperatures show minimal current split-up and enhanced performance. Asymmetrical sub-cycling shows that lower annealing temperatures cause local domain pinning, while higher temperatures result in imprint-like behavior. Synchrotron-based extended x-ray absorption fine structure and hard x-ray photoelectron spectroscopy analyses reveal the potential of nitrogen doping in HfZrOx under high-temperature annealing processes, forming the Hf–N species to mitigate the amount of charged oxygen vacancy (VO2+) in the interfacial region. This study elucidates the relationship between VO2+ distribution and the split-up effect during sub-cycling, providing critical insights for enhancing the sub-cycling performance and stability of HfO2-based devices.

Structure-Related Electronic and Magnetic Properties in Ultrathin Epitaxial NixFe3-xO4 Films on MgO(001).

Off-stoichiometric NixFe3-xO4 ultrathin films (x < 2.1) with varying Ni content x and thickness 16 (±2) nm were grown on MgO(001) by reactive molecular beam epitaxy. Synchrotron-based high-resolution X-ray diffraction measurements reveal vertical compressive strain for all films, resulting from a lateral pseudomorphic adaption of the film to the substrate lattice without any strain relaxation. Complete crystallinity with smooth interfaces and surfaces is obtained independent of the Ni content x. For x < 1 an expected successive conversion from Fe3O4 to NiFe2O4 is observed, whereas local transformation into NiO structures is observed for films with Ni content x > 1. However, angle-resolved hard X-ray photoelectron spectroscopy measurements indicate homogeneous cationic distributions without strictly separated phases independent of the Ni content, while X-ray absorption spectroscopy shows that also for x > 1, not all Fe2+ cations are substituted by Ni2+ cations. The ferrimagnetic behavior, as observed by superconducting quantum interference device magnetometry, is characterized by decreasing saturation magnetization due to the formation of antiferromagnetic NiO parts.

Interface defect formation for atomic layer deposition of SnO2 on metal halide perovskites

With the rapidly advancing perovskite solar cell (PSC) technology, dedicated interface engineering is critical for improving device stability. Atomic layer deposition (ALD) grown metal oxide films have drawn immense attention for the fabrication of stable PSC. Despite the advantages of ALD, the deposition of metal oxides directly on bare perovskite has so far not been achieved without damaging the perovskite layer underneath. In addition, the changes to the physicochemical and electronic properties at the perovskite interface upon exposure to the ALD precursors can alter the material and hence device functionality. Herein, we report on a synchrotron-based hard X-ray photoelectron spectroscopy (HAXPES) investigation of the interface between metal halide perovskite (MHP) absorber and ALD-SnO2 electron transport layer. We find clear evidence for the formation of new chemical species (nitrogen compound, lead dihalides) and an upward band bending in the MHP and downward band bending in the SnO2 towards the MHP/ALD-SnO2 interface. The upward bending at the interface forms an electron barrier layer of ∼400 meV, which is detrimental to the PSC performance. In addition, we assess the effectiveness of introducing a thin interlayer of the organic electron transport material Phenyl-C61-butyric acid methyl ester (PCBM) between MHP and ALD-SnO2 to mitigate the effects of ALD deposition.

Vacancy-Engineered Nickel Ferrite Forming-Free Low-Voltage Resistive Switches for Neuromorphic Circuits.

Innovations in resistive switching devices constitute a core objective for the development of ultralow-power computing devices. Forming-free resistive switching is a type of resistive switching that eliminates the need for an initial high voltage for the formation of conductive filaments and offers promising opportunities to overcome the limitations of traditional resistive switching devices. Here, we demonstrate mixed charge state oxygen vacancy-engineered electroforming-free resistive switching in NiFe2O4 (NFO) thin films, fabricated as asymmetric Ti/NFO/Pt heterostructures, for the first time. Using pulsed laser deposition in a controlled oxygen atmosphere, we tune the oxygen vacancies together with the cationic valence state in the nickel ferrite phase, with the latter directly affecting the charge state of the oxygen vacancies. The structural integrity and chemical composition of the films are confirmed by X-ray diffraction and hard X-ray photoelectron spectroscopy, respectively. Electrical transport studies reveal that resistive switching characteristics in the films can be significantly altered by tuning the amount and charge state of the oxygen vacancy concentration during the deposition of the films. The resistive switching mechanism is seen to depend upon the migration of both singly and doubly charged oxygen vacancies formed as a result of changes in the nickel valence state and the consequent formation/rupture of conducting filaments in the switching layer. This is supported by the existence of an optimum oxygen vacancy concentration for efficient low-voltage resistive switching, below or above which the switching process is inhibited. Along with the filamentary switching mechanism, the Ti top electrode also enhances the resistive switching performance due to interfacial effects. Time-resolved measurements on the devices display both long- and short-term potentiation in the optimized vacancy-engineered NFO resistive switches, ideal for solid-state synapses achieved in a single system. Our work on correlated oxide forming-free resistive switches holds significant potential for CMOS-compatible low-power, nonvolatile resistive memory and neuromorphic circuits.

Effects of plasma oxidation and plasma nitridation on chemical bonding state of AlScN evaluated by AR-HAXPES

The effect of plasma treatment on AlScN films is investigated using angle-resolved hard X-ray photoelectron spectroscopy (HAXPES). As a result, it was observed that the ratio of the oxide component to the nitride component in the AlScN film increased with oxygen plasma treatment. It was also observed that AlScN was less susceptible to oxidation after 1 min of pre-nitridation. Furthermore, the inhibition of oxidation was more pronounced for Sc atoms than for Al atoms. These suggest that the nitridation treatment reduced the number of nitrogen vacancies around Sc atoms. These facts mean that the increase in the on/off ratio of the current in the MIM capacitor can be attributed to the decrease in the number of nitrogen vacancies in the AlScN.

NF3 and F2 gas fluorination of GaN surface and Pt/GaN interface analyzed by hard X-ray photoelectron spectroscopy

The effects of NF3 or F2 gas annealing on epitaxially grown GaN and its interface with sputter-deposited Pt were investigated using hard X-ray photoelectron spectroscopy. Annealing GaN and Pt/GaN samples in an NF3 atmosphere led to the emergence of prominent F 1s peaks and chemically shifted Ga 2p peaks, indicating the efficient formation of Ga–Fx species not only in the bare GaN surface but also in the Pt/GaN interface, even when the NF3 treatment was performed after the Pt was deposited. By contrast, F2 annealing also led to the fluorination of the GaN surface and nonfluorination of the Pt/GaN interface. Although the plasma sputtering process removed F from the surface, band shifts were observed when the treatment conditions were varied. The findings in this study suggest that NF3 treatment is an effective post-processing method for fluorinating GaN-based systems before or after metal deposition.

Quantification of aluminium and silicon‐containing materials using Ag Lα X‐rays

In recent years, instrument manufacturers have been providing laboratory‐based HAXPES (hard X‐ray photoelectron spectroscopy) instruments that have monochromated silver Lα, chromium Kα and gallium Kα X‐ray sources. To be more than a qualitative tool, two things are required: calibration of the signal intensity as a function of kinetic energy (spectrometer response function) and relative sensitivity factors. This is now possible because of routines being available to generate the spectrometer response functions for dual Al Kα/Ag Lα X‐ray sources and the availability of Cant average matrix relative sensitivity factors (AMRSFs). However, to improve accuracy when using the Cant AMRSF care needs to be taken in the definition of the peak areas being used. This is because they need to include the total intrinsic signal that often extends beyond the main peak. This is a particular challenge when quantifying using the Al 1s peak generated with Ag Lα X‐rays as some of the intrinsic signal overlaps with the Al KLL Auger peak. When materials include both aluminium and silicon atoms, the Si 2s and 2p peaks sit upon extended signals coming from the Al 2s and 2s peaks, respectively. Thus, quantification should be limited to using the main peak area, which necessitates different relative sensitivity factors (RSFs). Using polydimethylsiloxane (PDMS) and sapphire (Al2O3) as model systems, experimental RSFs have been determined for use with the main peaks for O 1s, Al 1s, Al2s Al 2p, Si 1s, Si 2s and Si 2p subshells normalized to the RSF of the carbon 1s main peak.

Atomic position and the chemical state of an active Sn dopant for Sn-doped β-Ga2O3(001)

We investigated the atomic position and the chemical state of an active Sn dopant for Sn-doped β-Ga2O3(001) using x-ray absorption near the edge structure (XANES) and hard x-ray photoelectron spectroscopy. We found that the Sn dopant had only one chemical state, which was a Sn4+ oxidation state. The bond length around the Sn dopant atom became longer due to the relaxation effect after the Sn dopant insertion. Comparison of the experimental and simulated XANES spectra showed that the octahedral Ga substitutional site in the β-Ga2O3(001) is an active Sn dopant site.

Effect of Al2O3 thickness and oxidant precursors on the interface composition and contamination in Al2O3/GaN structures

Effect of Al₂O₃ thickness and oxidant precursors on the interface composition and contamination in Al₂O₃/GaN structures

Unveiling the Potential of Ambient Air Annealing for Highly Efficient Inorganic CsPbI3 Perovskite Solar Cells.

Here, we report a detailed surface analysis of dry- and ambient air-annealed CsPbI3 films and their subsequent modified interfaces in perovskite solar cells. We revealed that annealing in ambient air does not adversely affect the optoelectronic properties of the semiconducting film; instead, ambient air-annealed samples undergo a surface modification, causing an enhancement of band bending, as determined by hard X-ray photoelectron spectroscopy measurements. We observe interface charge carrier dynamics changes, improving the charge carrier extraction in CsPbI3 perovskite solar cells. Optical spectroscopic measurements show that trap state density is decreased due to ambient air annealing. As a result, air-annealed CsPbI3-based n-i-p structure devices achieved a 19.8% power conversion efficiency with a 1.23 V open circuit voltage.

Formation Processes of a Solid Electrolyte Interphase at a Silicon/Sulfide Electrolyte Interface in a Model All-Solid-State Li-Ion Battery.

Understanding the electrochemical reactions at the interface between a Si anode and a solid sulfide electrolyte is essential in improving the cycle stabilities of Si anodes in all-solid-state batteries (ASSBs). Highly dense Si films with very low roughnesses of <1 nm were fabricated at room temperature via cathodic arc plasma deposition, which led to the formation of a Si/sulfide electrolyte model interface. Li (de)alloying through the model interface hardly occurred during the first cycle, whereas it proceeded stably in subsequent cycles. Hard X-ray photoelectron spectroscopy and neutron reflectometry directly revealed that the reduction or oxidation of the interfacial component or Li3PS4 electrolyte occurred during the first cycle. Consequently, an interfacial layer with a thickness of 13 nm and primarily composed of Li2S, SiS2, and P2S5 glasses was formed during the first cycle. The interfacial layer acted as a Li-conductive, electron-insulating solid electrolyte interphase (SEI) that provided reversible (de)lithiation. Our model interface directly demonstrates the electrochemical reaction processes at the Si/Li3PS4 interface and provides insights into the structures and electrochemical properties of SEIs to activate the (de)lithiation of Si anodes using a sulfide electrolyte.

HAXPES reference spectra of CeO2 with Cr Kα excitation

Hard x-ray photoelectron spectroscopy (HAXPES) using monochromatic Cr Kα radiation (5414.8 eV) has been used to acquire XPS and Auger data on an argon gas cluster-sputtered CeO2 sample. Survey data, high-resolution scans of all observed photoelectron peaks, and high-resolution scans of Auger lines are presented herein.

Effect of hydrogen on the chemical state, stoichiometry and density of amorphous Al2O3 films grown by thermal atomic layer deposition

Amorphous oxide thin films grown by thermal atomic layer deposition (ALD) typically contain high impurity concentrations of hydrogen, which affects both chemistry and structure and thereby the functional properties, such as the barrier properties in, for example, microelectronic and photovoltaic devices. This study discloses the effect of H incorporation in amorphous Al2O3 ALD oxide films on the local chemical binding states of Al, O and H, as well as the oxide density and stoichiometry, by a combined analytical approach using elastic recoil detection analysis, Rutherford backscattering spectroscopy and full chemical state analysis by dual‐source X‐ray photoelectron spectroscopy (XPS)/hard X‐ray photoelectron spectroscopy (HAXPES). The experimental findings are compared with crystalline anhydrous α‐Al2O3 and hydroxide α‐Al (OH)3 reference phases and further supported by molecular dynamic simulations. It is shown that H preferably forms covalent –OH hydroxyl bonds with O in the nearest‐neighbour coordination spheres of interstitial Al cations, which affects both the ligand electronic polarizability and the bond length of the randomly interconnected [AlOn] polyhedral building blocks in the amorphous ALD oxide films.

Assessing hard X-ray photoelectron spectroscopy as a new evaluation method for studying protein adsorption on anisotropic hydroxyapatite ceramics model

Assessing hard X-ray photoelectron spectroscopy as a new evaluation method for studying protein adsorption on anisotropic hydroxyapatite ceramics model

Effect of annealing on the molecular ordering and crystal structure of iron phthalocyanine thin films on Si (111) surface

The Effect of annealing on molecular ordering in Iron Phthalocyanine (FePc) thin films deposited on Si (111) substrate surface was investigated using lab source (Cu Kα) based x-ray diffraction, synchrotron based 2-dimensional grazing incidence x-ray diffraction, polarization dependent x-ray absorption spectroscopy and hard x-ray photoelectron spectroscopy. Room temperature deposition of FePc on Si substrate resulted in amorphous thin films with randomly oriented FePc molecules. However, annealing of these amorphous films at 200°C led to crystallization with highly ordered FePc molecules having monoclinic crystal lattice structure. The ordered FePc molecules followed edge-on stacking on Si substrate surface with an azimuthally averaged tilt angle of 75±5° between molecular plane and substrate surface. Also, the b-c plane of the unit cell was lying almost parallel to the substrate surface. The edge-on stacking ordering of FePc molecules in annealed thin films led to higher charge conduction as compared to as-deposited films.

(Invited) Operando Approaches for Observing Reactions at Buried Electrochemical Interfaces

The performance of materials for electrochemical energy storage and conversion depends critically on reactions taking place at the interfaces between these materials and their operating environment: from the reversible incorporation of ions at electrode−electrolyte interfaces in rechargeable batteries, to the conversion of the waste carbon dioxide into liquid fuels. As the reaction conditions change so these interfaces change, with often dramatic effects on their structure and chemical state. Understanding the evolution of these interfaces under realistic conditions is critical to the selection and design of improved electrochemical materials. However, it is extremely challenging to extract this information during operation, due to interference from the bulk phases either side of these buried interfaces, which disturb most interface-sensitive probes.1,2 I will give an overview of our recent efforts to develop operando approaches for resolving the structural and chemical evolution of electrochemical interfaces.3-8 This includes enclosed reaction cells sealed with X-ray/electron-transparent membranes such as thin (<100 nm) silicon nitride or graphene membranes that remain impermeable liquids. We thereby use soft X-ray absorption spectroscopy (sXAS) to observe solid-electrolyte interphase (SEI) formation on Li-ion battery anodes,4 and changes in the chemical state of transition metal electrocatalysts under electrochemical control. I will further discuss our recent progress in depositing thin lithium layers onto solid electrolytes,5,6 and using hard X-ray Photoelectron Spectroscopy (XPS) to study electrode-electrolyte interfaces in all-solid-state batteries.7,8 The development of these interface-sensitive operando approaches is expected to provide valuable tools for revealing a wide range of interfacial reactions. Collaboration across European and international partners has been critical to this work, and I will highlight how European funding has helped make this possible and the benefits associated with this. References Wu et al. Phys. Chem. Chem. Phys. 2015, 17, 30229.Weatherup et al. Top. Catal. 2018, 61, 2085.Velasco-Velez et al. Angew. Chemie 2015, 54, 14554.Swallow et al. Nature Commun. 2022, 13, 6070Weatherup et al. J. Phys. Chem. B 2018, 122, 737.Khatun et al. Electrochimica Acta 2022, 431, 141145Narayanan et al. Nature Commun. 2022, 13, 7237Gibson et al. Faraday Discuss., 2022 , 236, 267

(Invited) In Situ Photoelectron Spectroscopy Reveals the Chemical Nature of Semiconductor Surface States

The accessible photovoltage of semiconducting photoabsorbers is typically 0.5-1 V below the theoretically achievable values predicted by the Shockley-Queisser limit [1]. Although the reason for this is still not well understood, surface and interface states within the photoabsorbers energy band gap may play a crucial role as they generally induce Fermi level pinning [2]. Within the PEC community, two key elements have been identified for the maximization of the photovoltage in photoelectrodes for water splitting: (i) the passivation of surface defects which is needed to avoid Fermi level pinning, and (ii) the increase of the minority carrier concentration at the interface to improve contact selectivity and optimize carrier extraction [3,4].To understand the role of surface defects on Fermi level pinning, detailed information on the chemical nature and electronic properties of surface states is needed. Such information is usually obtained with UHV surface science techniques such as XPS, but the photoelectrode surface under UHV conditions has little, if any, relevance to the electrified surface when immersed in the electrolyte. Moreover, recent studies have shown that semiconductor surfaces are not static, but undergo extensive structural and chemical transformations during PEC device operation [5,6]. In this talk, we will show our recent development of state-of-the-art techniques to study the structure and dynamics of semiconductor/water interfaces under practically relevant conditions [7]. The chemical nature of the electronic states for selected semiconductors prepared at the Institute for Solar Fuels has been explored using synchrotron-based resonant, ambient pressure soft and hard X-ray photoelectron spectroscopy (AP-XPS and AP-HAXPES) [6,8-11], and with in situ/operando Raman and photoluminescence spectroscopy. In particular, we will show that it is possible to study photon-induced chemical changes at solid/liquid interfaces using AP-XPS and AP-HAXPES [6,8].We will conclude this contribution by discussing about future perspectives and technical implementations for multimodal in situ/operando investigations of photoelectrocatalytic processes.[1] M.T. Mayer. Curr. Opin. Electrochem. 2017, 2, 104.[2] A. J. Bard et al. J. Am. Chem. Soc. 1980, 102, 3671.[3] A.G. Scheuermann et al., Nat. Mater. 2016, 15, 99.[4] M. Schleuning et al., Sustainable Energy Fuels, 2022, 6, 3701.[5] F.M. Toma et al., Nat. Commun. 2016, 7, 12012.[6] M. Favaro et al., J. Phys. Chem. B 2018, 122, 801.[7] M. Favaro et al., Surf. Sci. 2021, 713, 121903.[8] M. Favaro et al., J. Phys. D: Appl. Phys. 2021, 54, 164001.[9] M. Favaro et al., J. Phys. Chem. C 2019, 123, 8347.[10] W. Wang et al., J. Am. Chem. Soc. 2022, 144, 17173.[11] P. Schnell et al., Sol. RRL 2023, 2201104.