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Treatment of disorder effects in X-ray absorption spectra beyond the conventional approach
摘要： The contribution of static and thermal disorder is one of the largest challenges for the accurate determination of the atomic structure from the extended X-ray absorption fine structure (EXAFS). Although there are a number of generally accepted approaches to solve this problem, which are widely used in the EXAFS data analysis, they often provide less accurate results when applied to outer coordination shells around the absorbing atom. In this case, the advanced techniques based on the molecular dynamics and reverse Monte Carlo simulations are known to be more appropriate: their strengths and weaknesses are reviewed here.
关键词： Reverse Monte Carlo,Extended X-ray absorption fine structure (EXAFS),Molecular dynamics,Static and thermal disorder,X-ray absorption spectroscopy
更新于2025-09-23 15:23:52
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The influence of nitrogen doping on the electronic structure of the valence and conduction band in TiO <sub/>2</sub>
摘要： X-ray emission spectroscopy (XES) and X-ray absorption spectroscopy (XAS) provide a unique opportunity to probe both the highest occupied and the lowest unoccupied states in matter with bulk sensitivity. In this work, a combination of valence-to-core XES and pre-edge XAS techniques are used to determine changes induced in the electronic structure of titanium dioxide doped with nitrogen atoms. Based on the experimental data it is shown that N-doping leads to incorporation of the p-states on the occupied electronic site. For the conduction band, a decrease in population of the lowest unoccupied d-localized orbitals with respect to the d-delocalized orbitals is observed. As confirmed by theoretical calculations, the N p-states in TiO2 structure are characterized by higher binding energy than the O p-states which gives a smaller value of the band-gap energy for the doped material.
关键词： TiO2 doping,X-ray absorption spectroscopy,electronic structure analysis,X-ray emission spectroscopy
更新于2025-09-23 15:23:52
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Operando observation of chemical transformations of iridium oxide during photoelectrochemical water oxidation
摘要： Iridium oxide is one of the few catalysts capable of catalyzing the oxygen evolution reaction (OER) in both acidic and basic conditions. Understanding the mechanism of IrOx under realistic photoelectrochemical conditions is important for the development of integrated water splitting systems. Herein, we have developed a highly efficient OER photoanode in pH 1 aqueous solutions based on a sputtered IrOx film and a p+n-Si light absorber, interfaced with sputtered Au layer. Operando high energy resolution fluorescence detection X-ray absorption spectroscopy (HERFD XAS) was employed to monitor the oxidation state changes of IrOx during both electrochemical and photoelectrochemical (PEC) water oxidation reactions in pH 1 aqueous solutions. We observed a gradual increase of the average oxidation state of Ir with increasing anodic potential in the pre-catalytic region, followed by a reduction of Ir under O2 evolution conditions. Consistent results were obtained on dark anodes and illuminated photoanodes. However, when the thickness of IrO2 was increased to 2 and 3 nm, the spectral changes became much less pronounced and the reduction of Ir oxidation state after the OER onset was not observed. This is due to the lower surface to bulk ratio, where lattice oxygen sites in the bulk are not accessible for the formation of hydroxide. More generally, the operando method developed here can be extended to other materials, thereby providing a powerful tool for mechanism discovery and an enabling capability for catalyst design.
关键词： oxygen evolution reaction (OER),electrochemical and photoelectrochemical (PEC),high energy resolution fluorescence detection X-ray absorption spectroscopy (HERFD XAS),iridium oxide,Operando method
更新于2025-09-23 15:23:52
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Fingerprints of sp1 Hybridized C in the Near-Edge X-ray Absorption Spectra of Surface-Grown Materials
摘要： Carbon structures comprising sp1 chains (e.g., polyynes or cumulenes) can be synthesized by exploiting on-surface chemistry and molecular self-assembly of organic precursors, opening to the use of the full experimental and theoretical surface-science toolbox for their characterization. In particular, polarized near-edge X-ray absorption fine structure (NEXAFS) can be used to determine molecular adsorption angles and is here also suggested as a probe to discriminate sp1/sp2 character in the structures. We present an ab initio study of the polarized NEXAFS spectrum of model and real sp1/sp2 materials. Calculations are performed within density functional theory with plane waves and pseudopotentials, and spectra are computed by core-excited C potentials. We evaluate the dichroism in the spectrum for ideal carbynes and highlight the main differences relative to typical sp2 systems. We then consider a mixed polymer alternating sp1 C4 units with sp2 biphenyl groups, recently synthesized on Au(111), as well as other linear structures and two-dimensional networks, pointing out a spectral line shape specifically due to the presence of linear C chains. Our study suggests that the measurements of polarized NEXAFS spectra could be used to distinctly fingerprint the presence of sp1 hybridization in surface-grown C structures.
关键词： density functional theory,self-assembly,carbynes,on-surface chemistry,near edge X-ray absorption spectroscopy,C 1s absorption
更新于2025-09-23 15:22:29
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Charge-state analysis of small barium-oxide clusters by X-ray absorption spectroscopy
摘要： Small barium-oxide clusters BamOn+ are studied by mass spectrometry and X-ray absorption spectroscopy to discuss stability of the clusters and oxidation state of constituent atoms. It is found that clusters with bulk composition, n = m, are stable, which can accommodate one or two excess oxygen atoms additionally as manifested by n = m + 1 and m + 2 species in the mass spectrum. XAS spectra of Ba2O2+ and Ba2O3+ reveal that the oxidation state of barium atoms stays at +2 (the bulk BaO value) even after binding excess oxygen, whereas spectral features originating from oxygen exhibit composition dependence. The present finding suggests that stoichiometric small barium-oxide clusters bind less-negatively-charged oxygen atoms without change in the charge state of barium.
关键词： X-ray absorption spectroscopy,charge state,barium-oxide clusters,oxidation number
更新于2025-09-23 15:22:29
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Soft X-ray Absorption Spectroscopy Study of Spin Crossover Fe-Compounds: Persistent High Spin Configurations under Soft X-ray Irradiation
摘要： Metal-organic complex exhibiting spin crossover (SCO) behavior has drawn attention for its functionality as a nanoscale spin switch. The spin states in the metal ions can be tuned by external stimuli such as temperature or light. This article demonstrates a soft X-ray–induced excited spin state trapping (SOXEISST) effect in Hofmann-like SCO coordination polymers of FeII(4-methylpyrimidine)2[Au(CN)2]2 and FeII(pyridine)2[Ni(CN)4]. A soft X-ray absorption spectroscopy (XAS) study on these polymers showed that the high spin configuration (HS; S = 2) was prevalent in Fe2+ ions during the measurement even at temperatures much lower than the critical temperatures (>170 K), manifesting HS trapping due to the X-ray irradiation. This is in strong contrast to the normal SCO behavior observed in FeII(1,10-phenanthroline)2(NCS)2, implying that the structure of the ligand chains in the polymers with relatively loose Fe-N coordination might allow a structural adaptation to stabilize the metastable HS state under the soft X-ray irradiation.
关键词： soft X-ray induced excited spin state trapping,X-ray absorption spectroscopy,high spin,spin crossover
更新于2025-09-23 15:21:21
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Time-resolved Element-selective Probing of Charge Carriers in Solar Materials
摘要： We review our recent results on the implementation of picosecond (ps) X-ray absorption spectroscopy to probe the electronic and geometric structure of centres formed by photoexcitation of solar materials such as polymorphs and inorganic Cs-based perovskites. The results show electron localization at Ti defects in TiO2 anatase and rutile and small hole polaron formation in the valence band of CsPbBr3, all within 80 ps. This method is promising for the study of the ultrafast time scales of such processes, especially with the advent of the Swiss X-ray Free Electron Laser (SwissFEL).
关键词： Solar materials,Ultrafast,Perovskites,X-ray absorption spectroscopy,TiO2
更新于2025-09-23 15:21:01
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Influence of Halide Ions on the Structure and Properties of Copper Indium Sulphide Quantum Dots
摘要： In the synthesis of CuInS2 quantum dots (QDs), the halide ions present in the copper salts influence the QD growth and optical properties. X-ray absorption spectroscopy allowed rationalizing the halide incorporation in the lattice and the dependency of electronic properties of the material on the ion’s polarizability and interaction with hydrophobic moieties.
关键词： optical properties,halide ions,CuInS2 quantum dots,X-ray absorption spectroscopy
更新于2025-09-23 15:19:57
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Decomposing Electronic and Lattice Contributions in Optical Pump–X-ray Probe Transient Inner-Shell Absorption Spectroscopy of CuO
摘要： Electronic and lattice contributions to picosecond time-resolved X-ray absorption spectra (trXAS) of CuO at the oxygen K-edge are analyzed by comparing trXAS spectra, recorded using excitation wavelengths of 355 nm and 532 nm, to steady-state, temperature-dependent XAS measurements. The trXAS spectra at pump-probe time-delays ≥150 ps are dominated by lattice heating effects. Insight into the temporal evolution of lattice temperature profiles on timescales up to 100’s of nanoseconds after laser excitation are reported, on an absolute temperature scale, with a temporal sensitivity and a spatial selectivity on the order of 10’s of picoseconds and 10’s of nanometers, respectively, effectively establishing an 'ultrafast thermometer'. In particular, for the 532 nm experiment at ~5 mJ/cm2 fluence, both the initial sample temperature and its dynamic evolution are well captured by a one-dimensional thermal energy deposition and diffusion model. The thermal conductivity k = (1.3±0.4) W m–1 K–1 derived from this model is in good agreement with the literature value for CuO powder kpowder = 1.013 W m–1 K–1. For 355 nm excitation, a quantitative analysis of the experiments is hampered by the large temperature gradients within the probed sample volume owing to the small UV penetration depth. The impact of the findings on mitigating or utilizing photoinduced lattice temperature changes in future X-ray free electron laser (XFEL) experiments is discussed.
关键词： X-ray absorption spectroscopy,Ultrafast X-ray spectroscopy,CuO,lattice heating,thermal conductivity
更新于2025-09-19 17:15:36
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Internal Atomic-Scale Structure Determination and Band Alignment of IIa??VI Quantum Dot Heterostructures
摘要： This work shows that ZnTe/CdSe core/shell quantum dots synthesized by a standard literature procedure in actuality have an alloyed CdxZn1?xTe core. We employ X-ray absorption spectroscopy (XAS) at all four K-shell ionization edges (Zn, Te, Cd, and Se) and perform global ?tting analysis to extract the ?rst-shell bond distances. We combine our XAS results with transmission electron microscopy (TEM) sizing and elemental analyses, which allows us to propose models of the internal particle structure. Our multimodal characterization approach con?rms (1) the presence of Cd?Te bonds, (2) cation alloying in the particle core (and the absence of anion alloying), and (3) a patchy pure-phase CdSe shell. We synthesize particles of di?erent shell thicknesses and performed synthetic control studies that allowed us to discard a ZnTe/CdTe/CdSe core/shell/shell structure and con?rm the alloyed core/shell structure. Our structural analysis is extended with electronic band structure calculations and UV/vis absorption spectroscopy, demonstrating that the alloyed CdxZn1?xTe/CdSe core/shell quantum dots exhibit a direct band gap, di?erent from the predicted type II band alignment of the intended ZnTe/CdSe core/shell quantum dots. This study highlights the challenges with synthesizing II?VI quantum dot heterostructures and the power of XAS for understanding the internal structure of heterogeneous nanoparticles.
关键词： X-ray absorption spectroscopy,alloyed core,core/shell,band alignment,quantum dots
更新于2025-09-19 17:13:59