| 1. |
- Kersell, Heath, et al.
(författare)
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Some Future Perspectives in Ambient Pressure X-ray Spectroscopies : Atmospheric Pressure, Spatially Resolved and Multi-modal Experiments
- 2021
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Ingår i: Ambient Pressure Spectroscopy in Complex Chemical Environments. - Washington, DC : American Chemical Society. - 1947-5918 .- 0097-6156. - 9780841298125 - 9780841298118 ; 1396, s. 333-358
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Bokkapitel (refereegranskat)abstract
- This chapter introduces a selection of novel and emerging concepts in ambient pressure X-ray spectroscopies. Technical considerations and first photoemission experiments performed in and above 1 bar pressure are described. Then, X-ray spectroscopy experiments that provide high lateral (microscopy) and depth (X-ray standing-waves) resolution in ambient environments are discussed. The final part of the chapter introduces recent developments in multimodal instrumentation, such as the combination of ambient pressure X-ray Photoelectron Spectroscopy with Infrared spectroscopy and X-ray scattering.
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| 2. |
- Baeumer, Christoph, et al.
(författare)
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Tuning electrochemically driven surface transformation in atomically flat LaNiO3 thin films for enhanced water electrolysis
- 2021
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Ingår i: Nature Materials. - : Springer Science and Business Media LLC. - 1476-1122 .- 1476-4660. ; 20:5, s. 674-682
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Tidskriftsartikel (refereegranskat)abstract
- Structure–activity relationships built on descriptors of bulk and bulk-terminated surfaces are the basis for the rational design of electrocatalysts. However, electrochemically driven surface transformations complicate the identification of such descriptors. Here we demonstrate how the as-prepared surface composition of (001)-terminated LaNiO3 epitaxial thin films dictates the surface transformation and the electrocatalytic activity for the oxygen evolution reaction. Specifically, the Ni termination (in the as-prepared state) is considerably more active than the La termination, with overpotential differences of up to 150 mV. A combined electrochemical, spectroscopic and density-functional theory investigation suggests that this activity trend originates from a thermodynamically stable, disordered NiO2 surface layer that forms during the operation of Ni-terminated surfaces, which is kinetically inaccessible when starting with a La termination. Our work thus demonstrates the tunability of surface transformation pathways by modifying a single atomic layer at the surface and that active surface phases only develop for select as-synthesized surface terminations.
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| 3. |
- Conti, Giuseppina, et al.
(författare)
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X-ray standing-wave spectroscopy : A powerful method for probing buried interfaces
- 2023
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Ingår i: Encyclopedia of Solid-Liquid Interfaces. - 9780323856690 ; 1-3, s. 1-335
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Bokkapitel (refereegranskat)abstract
- In this chapter we present a non-destructive method for characterizing buried interfaces with high depth resolution combining X-rays standing wave and photoemission electron spectroscopy. This method incorporates the power of the X-rays photoelectron spectroscopy, which is a surface sensitive technique in the nanometer and sub-nanometer regime, with the depth selectivity of X-ray standing wave. This method has been successfully applied to study solid/solid interfaces and, more recently, solid/gas and solid/liquid interfaces, as we demonstrate on several examples. In addition, we also describe a few examples of liquid/solid interface characterization by standing wave X-ray fluorescence relevant for catalysis, energy science and biomedical applications.
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| 4. |
- Duchon, Tomas, et al.
(författare)
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Establishing structure-sensitivity of ceria reducibility : real-time observations of surface hydrogen interactions
- 2020
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Ingår i: Journal of Materials Chemistry A. - : ROYAL SOC CHEMISTRY. - 2050-7488 .- 2050-7496. ; 8:11, s. 5501-5507
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Tidskriftsartikel (refereegranskat)abstract
- The first Layer of atoms on an oxide cataLyst provides the first sites for adsorption of reactants and the Last sites before products or oxygen are desorbed. We employ a unique combination of morphological, structural, and chemical analyses of a model ceria cataLyst with different surface terminations under an H2 environment to unequivocally establish the effect of the Last Layer of atoms on surface reduction. (111) and (100) terminated epitaxiaL isLands of ceria are simultaneously studied in situ allowing for a direct investigation of the structure reducibility relationship under identical conditions. Kinetic rate constants of Ce4+ to Ce3+ transformation and equilibrium concentrations are extracted for both surface terminations. Unlike the kinetic rate constants, which are practically the same for both types of isLands, more pronounced oxygen release, and overall higher reducibility were observed for (100) isLands compared to (111) ones. The findings are in agreement with coordination -Limited oxygen vacancy formation energies calculated by density functional theory. The results point out the important aspect of surface terminations in redox processes, with particular impact on the catalytic reactions of a variety of catalysts.
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| 5. |
- Eren, Baran, et al.
(författare)
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Contamination Issues in Ambient Pressure Experiments
- 2021
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Ingår i: Ambient Pressure Spectroscopy in Complex Chemical Environments. - Washington, DC : American Chemical Society. - 1947-5918 .- 0097-6156. - 9780841298125 - 9780841298118 ; 1396, s. 267-295
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Bokkapitel (refereegranskat)abstract
- Contamination is the most common and arguably the most significant problem scientists are facing in experimental surface science research that is practiced in the presence of gases. It is fair to say that contamination problems are often worse with ambient pressures compared to conventional experiments in vacuum. It is one of the main reasons for poor reproducibility in this field and in relevant basic and applied research fields like heterogeneous catalysis and electrochemistry. Whilst some type of contaminants are more innocent and only hinder quantitative analysis, some are harmful as they change the outcome of the experiments. In this chapter, the potential sources of contamination are summarized and some solutions are suggested. Examples of commonly observed contaminants such as hydrocarbons, oxygenated hydrocarbons, and adsorbed species of traces gases are presented. The scope of this chapter is restricted to ambient pressure x-ray photoelectron spectroscopy and infrared reflection absorption spectroscopy studies on single crystal surfaces, but similar problems exist on other sample surfaces or with other techniques such as x-ray absorption spectroscopy and sum frequency generation spectroscopy.
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| 6. |
- Kalha, Curran, et al.
(författare)
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Hard x-ray photoelectron spectroscopy : a snapshot of the state-of-the-art in 2020
- 2021
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Ingår i: Journal of Physics. - : Institute of Physics Publishing (IOPP). - 0953-8984 .- 1361-648X. ; 33:23
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Forskningsöversikt (refereegranskat)abstract
- Hard x-ray photoelectron spectroscopy (HAXPES) is establishing itself as an essential technique for the characterisation of materials. The number of specialised photoelectron spectroscopy techniques making use of hard x-rays is steadily increasing and ever more complex experimental designs enable truly transformative insights into the chemical, electronic, magnetic, and structural nature of materials. This paper begins with a short historic perspective of HAXPES and spans from developments in the early days of photoelectron spectroscopy to provide an understanding of the origin and initial development of the technique to state-of-the-art instrumentation and experimental capabilities. The main motivation for and focus of this paper is to provide a picture of the technique in 2020, including a detailed overview of available experimental systems worldwide and insights into a range of specific measurement modi and approaches. We also aim to provide a glimpse into the future of the technique including possible developments and opportunities.
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| 7. |
- Karslloǧlu, Osman, et al.
(författare)
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Prospects for the expansion of standing wave ambient pressure photoemission spectroscopy to reactions at elevated temperatures
- 2022
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Ingår i: Journal of Vacuum Science and Technology A: Vacuum, Surfaces and Films. - : American Vacuum Society. - 0734-2101 .- 1520-8559. ; 40:1
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Tidskriftsartikel (refereegranskat)abstract
- Standing wave ambient pressure photoemission spectroscopy (SWAPPS) is a promising method to investigate chemical and potential gradients across solid-vapor and solid-liquid interfaces under close-to-realistic environmental conditions, far away from high vacuum. Until now, these investigations have been performed only near room temperature, but for a wide range of interfacial processes, chief among them being heterogeneous catalysis, measurements at elevated temperatures are required. One concern in these investigations is the temperature stability of the multilayer mirrors, which generate the standing wave field. At elevated temperatures, degradation of the multilayer mirror due to, for example, interdiffusion between the adjacent layers, decreases the modulation of the standing wave field, thus rendering SWAPPS experiments much harder to perform. Here, we show that multilayer mirrors consisting of alternate B4C and W layers are stable at temperatures exceeding 600 °C and are, thus, promising candidates for future studies of surface and subsurface species in heterogeneous catalytic reactions using SWAPPS.
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| 8. |
- Lischner, Johannes, et al.
(författare)
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Accurate determination of the valence band edge in hard x-ray photoemission spectra using GW theory
- 2016
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 119:16
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Tidskriftsartikel (refereegranskat)abstract
- We introduce a new method for determining accurate values of the valence-band maximum in x-ray photoemission spectra. Specifically, we align the sharpest peak in the valence-band region of the experimental spectrum with the corresponding feature of a theoretical valence-band density of states curve from ab initio GW theory calculations. This method is particularly useful for soft and hard x-ray photoemission studies of materials with a mixture of valence-band characters, where strong matrix element effects can render standard methods for extracting the valence-band maximum unreliable. We apply our method to hydrogen-terminated boron-doped diamond, which is a promising substrate material for novel solar cell devices. By carrying out photoemission experiments with variable light polarizations, we verify the accuracy of our analysis and the general validity of the method. Published by AIP Publishing.
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| 9. |
- Marinova, Maya, et al.
(författare)
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Depth Profiling Charge Accumulation from a Ferroelectric into a Doped Mott Insulator
- 2015
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Ingår i: Nano letters (Print). - : American Chemical Society (ACS). - 1530-6984 .- 1530-6992. ; 15:4, s. 2533-2541
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Tidskriftsartikel (refereegranskat)abstract
- The electric field control of functional properties is a crucial goal in oxide-based electronics. Nonvolatile switching between different resistivity or magnetic states in an oxide channel can be achieved through charge accumulation or depletion from an adjacent ferroelectric. However, the way in which charge distributes near the interface between the ferroelectric and the oxide remains poorly known, which limits our understanding of such switching effects. Here, we use a first-of-a-kind combination of scanning transmission electron microscopy with electron energy loss spectroscopy, near-total-reflection hard X-ray photoemission spectroscopy, and ab initio theory to address this issue. We achieve a direct, quantitative, atomic-scale characterization of the polarization-induced charge density changes at the interface between the ferroelectric BiFeO3 and the doped Mott insulator Ca1-xCexMnO3, thus providing insight on how interface-engineering can enhance these switching effects.
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| 10. |
- Nemsak, Slavomir, et al.
(författare)
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Element- and momentum-resolved electronic structure of the dilute magnetic semiconductor manganese doped gallium arsenide
- 2018
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Ingår i: Nature Communications. - : Springer Science and Business Media LLC. - 2041-1723. ; 9
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Tidskriftsartikel (refereegranskat)abstract
- The dilute magnetic semiconductors have promise in spin-based electronics applications due to their potential for ferromagnetic order at room temperature, and various unique switching and spin-dependent conductivity properties. However, the precise mechanism by which the transition-metal doping produces ferromagnetism has been controversial. Here we have studied a dilute magnetic semiconductor (5% manganese-doped gallium arsenide) with Bragg-reflection standing-wave hard X-ray angle-resolved photoemission spectroscopy, and resolved its electronic structure into element-and momentum-resolved components. The measured valence band intensities have been projected into element-resolved components using analogous energy scans of Ga 3d, Mn 2p, and As 3d core levels, with results in excellent agreement with element-projected Bloch spectral functions and clarification of the electronic structure of this prototypical material. This technique should be broadly applicable to other multi-element materials.
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