Soft X-ray Spectroscopy Techniques Research Articles

Design of RIXS beamline at Shenzhen innovation light facility

An optical design of the RIXS beamline for the Shenzhen Innovation Light Facility (SILF) was reported. The beamline will cover an energy range from 200 to 1500 eV, with three endstations: a Q-RIXS for momentum-dependent high-resolution resonant inelastic x-ray scattering, an e-RIXS for high-throughput resonant inelastic x-ray scattering and an APXPS. All endstations share the same undulator, front end optics and the main branch of beamline until the deflecting mirrors after the monochromator. The Q-RIXS and e-RIXS will be located in two separate branches, while the APXPS endstation is mounted on a free port located right between the exit slit and the refocusing optics for the e-RIXS endstation. This design allows for the mounting of different techniques and the preparation of sample environments without disrupting the operation of the other branch, thus maximizing the utilization of beamtime. The combined energy resolution is expected to be better than 50 meV for Q-RIXS and better than 100 meV for e-RIXS. Once available, it will provide in-demand, versatile and high-quality soft x-ray spectroscopy techniques to users in fields of quantum, energy and surface science.

Electronic Structure of Orthorhombic Bi4V2O11 and Role of Gap States in Photoelectrochemical Water Splitting

A group of bismuth vanadium oxides shows promise as an excellent material for photoanodes in water splitting. However, the efficiency of Bi4V2O11 in water splitting is not satisfactory compared to BiVO4. In this study, we investigated the electronic structure of orthorhombic Bi4V2O11 using density functional theory (DFT) and synchrotron soft X-ray spectroscopy techniques to clarify the causes of inefficiency. Our DFT calculations of the two-fold superlattice, as confirmed by near-edge X-ray absorption fine structure measurements, revealed its indirect-band-gap nature. An angle-resolved photoelectron spectroscopy study identified the states within the band gap, and the resonant photoemission technique and DFT simulation clarified that the origin of the gap states was anisotropic localization of electrons around V–O tetrahedra. Water absorption enhances the gap states, which dramatically impedes the kinetics of water oxidation reactions. Our results reveal the role of gap states in water oxidation reactions and offer new insights into manipulating gap states in 3d transition metal oxides.

Annealing Temperature Dependence of Various Properties of ZnO Nanoparticles Investigated with Soft XAS

In this paper, structural, optical and electronic properties of ZnO nanoparticles were investigated with the variation in the annealing temperature. X-ray diffraction, High-Resolution Transmission Electron Spectroscopy, Absorption spectroscopy, Raman Spectroscopy and Soft X-ray spectroscopy techniques were invoked to investigate and gauge the effect of annealing temperature on these properties. Intact Hexagonal Wurtzite phase of the nanoparticles with increasing crystallinity and spherical shape with particle size between 20[Formula: see text]nm and 23[Formula: see text]nm [using both X-ray Diffractometer (XRD) and High-resolution Transmission of electron microscopy (HRTEM)], blue shift of absorption peak from 372[Formula: see text]nm to 366[Formula: see text]nm and Optical Band Gap from 3.24[Formula: see text]eV to 3.17[Formula: see text]eV (UV-Vis-NIR), narrowing of [Formula: see text] mode in Raman Spectra; all indicated enhanced crystallinity with increased annealing temperature. Effects produced by the sole variable such as annealing temperature on the indicative parameters in a definite order catapult the annealing conditions as a significant candidate for customized properties. To have a deeper insight at the electronic levels, Synchrotron-based X-ray absorption spectroscopic studies of ZnO nanoparticles were undertaken. The analysis of Soft XAS spectra showed the change in number of O 2[Formula: see text] and Zn 4[Formula: see text] unoccupied states with increasing particle size and crystallinity of ZnO nanoparticles for varying annealing temperature.

Impact of Nitrogen, Boron and Phosphorus Impurities on the Electronic Structure of Diamond Probed by X-ray Spectroscopies

Doping diamond with boron, nitrogen or phosphorus enables a fine tuning of its electronic properties, which is particularly relevant for applications involving electron emission. However, the chemical nature of the doping sites and its correlation with electron emission properties remain to be clarified. In this work, we applied soft X-ray spectroscopy techniques to probe occupied and unoccupied electronic states in undoped, boron-, phosphorus- and nitrogen-containing single crystal diamonds. X-ray absorption, X-ray emission and X-ray photoemission spectroscopies, performed at the carbon K-edge, provide a full picture of new electronic states created by impurities in diamond. The different probing depths of fluorescence- and electron-based detection techniques enable a comparison between surface and bulk contributions.

A Perspective on Studying Electronic Structure of Batteries through Soft X-ray Spectroscopy

Author(s): Zhuo, ZQ; Pan, F; Yang, WL | Abstract: Understanding electronic structure is crucial to enhance the battery performance. Soft X-ray spectroscopy (SXS) is one of the most effective methods to provide direct probe of electronic states. Here, spectroscopic measurements of transition metal 3d and oxygen 2p states are simply reviewed. Then, we mainly focus on the perspective of the development direction of modern SXS techniques. Although the true power of recently developed high efficiency mapping of resonant inelastic X-ray scattering (mRIXS) has been apparent for materials and chemistry studies, great challenges remain for mRIXS spectroscopic interpretation, and the understanding of the battery materials on novel redox activities remains elusive.

Atomic-scale understanding of the electronic structure-crystal facets synergy of nanopyramidal CoPi/BiVO4 hybrid photocatalyst for efficient solar water oxidation

The observation of a spontaneous electron enrichment at the apexes of bismuth vanadate (BiVO4) nanopyramidal arrays is achieved by performing synchrotron-based angular dependent X-ray absorption spectroscopy. In accordance with density function theory calculations, the formation of (112) facets enriched apexes is proposed. Such intrinsically electron-enriched (112) facets at the apexes produce a potential facilitating the diffusion of photoexcited holes, which not only mitigates the recombination with the photoexcited electrons, but also provide an active reaction site for the photoassisted selective growth of cobalt phosphate (CoPi), a well known and highly active co-catalyst for water oxidation at the apex of BiVO4 nanopyramids. Benefiting from the element-resolved properties of synchrotron-based X-ray spectroscopy, 3d electrons on the vanadium site are directly resolved by resonant inelastic X-ray scattering measurements. Due to this unique morphology, the charge at the apex is expected to induce a strong interaction with CoPi, as a result of the metal-to-ligand charge transfer spectroscopy feature observed from the cobalt site. Substantial evidences are collected by means of comprehensive soft X-ray spectroscopy techniques with high spectral resolution revealing the atomic-scale origin and the nature of the synergy as well as the strong correlation between its unique structural properties and the electronic structure of this novel highly-ordered hybrid photocatalyst and its significantly improved photoelectrochemical performance.

Elemental-sensitive Detection of the Chemistry in Batteries through Soft X-ray Absorption Spectroscopy and Resonant Inelastic X-ray Scattering.

Energy storage has become more and more a limiting factor of today's sustainable energy applications, including electric vehicles and green electric grid based on volatile solar and wind sources. The pressing demand of developing high-performance electrochemical energy storage solutions, i.e., batteries, relies on both fundamental understanding and practical developments from both the academy and industry. The formidable challenge of developing successful battery technology stems from the different requirements for different energy-storage applications. Energy density, power, stability, safety, and cost parameters all have to be balanced in batteries to meet the requirements of different applications. Therefore, multiple battery technologies based on different materials and mechanisms need to be developed and optimized. Incisive tools that could directly probe the chemical reactions in various battery materials are becoming critical to advance the field beyond its conventional trial-and-error approach. Here, we present detailed protocols for soft X-ray absorption spectroscopy (sXAS), soft X-ray emission spectroscopy (sXES), and resonant inelastic X-ray scattering (RIXS) experiments, which are inherently elemental-sensitive probes of the transition-metal 3d and anion 2p states in battery compounds. We provide the details on the experimental techniques and demonstrations revealing the key chemical states in battery materials through these soft X-ray spectroscopy techniques.

Surface Modifications of Na and K Metal Incorporated Cu(In,Ga)Se2 Absorbers Investigated by Synchrotron‐Based Spectroscopies

Na and K metals have been evaporated on a Cu(In,Ga)Se2 (CIGSe) thin film solar cell absorber at 400 °C in order to investigate the effect of alkali metal incorporation on the very near-surface (up to 5 nm) region of the CIGSe absorber using soft X-ray spectroscopy techniques, focusing on the main compositional and electronic modifications of the absorber surface. Quantitative X-ray photoelectron spectroscopy (XPS) showed Cu deficiency and Se enrichment on the CIGSe surface after alkali treatment which may play a role in assisting Na diffusion away from the surface, leaving behind a significantly higher K content than Na along the entire range of the CIGSe surface region probed, although nominally equal amounts of Na and K metal have been evaporated onto the CIGSe surface. A [K]/([K]+[Cu]) concentration ratio of 0.99 ± 0.01 at an information depth of ≈1.7 nm from the surface may indicate the formation of a wide band gap compound like KInSe2 (Eg ∼ 2.67 eV) on the CIGSe surface as a result of alkali metal deposition. Ultra-violet photoemission spectroscopy (UPS) and near edge X-ray absorption fine structure spectroscopy (NEXAFS) measurements further confirm a 1.09 eV surface band gap increase along with a type-inversion at the surface of the alkali metal-incorporated CIGSe as compared to the untreated CIGSe with a surface band gap of 1.3 ± 0.2 eV. These changes in the surface composition and electronic structure of the modified CIGSe surface as a result of the alkaline treatment could be attributed to the increase in alkali-treated CIGSe-based thin film solar cell efficiencies seen in recent years.

Interfacial electronic structure of Cl6SubPc non-fullerene acceptors in organic photovoltaics using soft X-ray spectroscopies.

In organic photovoltaics (OPVs), determining the energy-level alignment of a donor and an acceptor is particularly important since the interfacial energy gap between the highest occupied molecular orbital (HOMO) level of a donor and the lowest unoccupied molecular orbital (LUMO) level of an acceptor (E-E) gives the theoretical maximum value of the open-circuit voltage (VOC). To increase the E-E, non-fullerene acceptors, which have a lower electron affinity (EA) than C60, are receiving increasing attention. In this study, we investigated the energy-level alignment at the interface of a boron chloride subphthalocyanine (SubPc) donor and a halogenated SubPc (Cl6SubPc) acceptor using soft X-ray spectroscopy techniques. The estimated E-E of Cl6SubPc/SubPc was 1.95 eV, which was significantly higher than that of 1.51 eV found at the interface of C60/SubPc. This increased E-E was the origin of the enhanced VOC in OPVs. Additionally, we studied the molecular orientation of Cl6SubPc using angle-dependent X-ray absorption spectroscopy. The highly disordered Cl6SubPc molecules result in low carrier mobility, which contributes to the lower short-circuit current density of the Cl6SubPc acceptor OPVs than the C60 acceptor OPVs.

Feasibility of a ring FEL at low emittance storage rings

A scheme for generating coherent radiation at latest generation low emittance storage rings such as PETRA III at DESY (Balewski et al., 2004 [1]) is proposed. The scheme is based on focusing and subsequent defocusing of the electron beam in the longitudinal phase space at the undulator location. The expected performance characteristics are estimated for radiation in the wavelength range of 500–1500eV. It is shown that the average brightness is increased by several orders of magnitude compared to spontaneous undulator radiation, which can open new perspectives for photon-hungry soft X-ray spectroscopy techniques.

Recent progress on synchrotron-based in-situ soft X-ray spectroscopy for energy materials.

Soft X-ray spectroscopy (SXS) techniques such as photoelectron spectroscopy, soft X-ray absorption spectroscopy and X-ray emission spectroscopy are efficient and direct tools to probe electronic structures of materials. Traditionally, these surface sensitive soft X-ray techniques that detect electrons or photons require high vacuum to operate. Many recent in situ instrument developments of these techniques have overcome this vacuum barrier. One can now study many materials and model devices under near ambient, semi-realistic, and operando conditions. Further developments of integrating the realistic sample environments with efficient and high resolution detection methods, particularly at the high brightness synchrotron light sources, are making SXS an important tool for the energy research community. In this progress report, we briefly describe the basic concept of several SXS techniques and discuss recent development of SXS instruments. We then present several recent studies, mostly in situ SXS experiments, on energy materials and devices. Using these studies, we would like to highlight that the integration of SXS and in situ environments can provide in-depth insight of material's functionality and help researchers in new energy material developments. The remaining challenges and critical research directions are discussed at the end.

Development of near atomically perfect diffraction gratings for EUV and soft x-rays with very high efficiency and resolving power

Multilayer-coated Blazed Gratings (MBG) can offer high diffraction efficiency in a very high diffraction order and are therefore of great interest for high-resolution EUV and soft x-ray spectroscopy techniques such as Resonance Inelastic X-ray Scattering. However, realization of the MBG concept requires nano-scale precision in fabrication of a saw-tooth substrate with atomically smooth facets, and reproduction of the blazed groove profile in the course of conformal growth of a multilayer coating. We report on recent progress achieved in the development, fabrication, and characterization of ultra-dense MBGs for EUV and soft x-rays. As a result of thorough optimization of all steps of the fabrication process, an absolute diffraction efficiency as high as 44% and 12.7% was achieved for a 5250 l/mm grating in the EUV and soft x-ray regions respectively. This work now shows a direct route to achieving high diffraction efficiency in high order at wavelengths throughout the soft x-ray energy range with revolutionary applications in synchrotron science.

Electron Enrichment in 3d Transition Metal Oxide Hetero-Nanostructures

Direct experimental observation of spontaneous electron enrichment of metal d orbitals in a new transition metal oxide heterostructure with nanoscale dimensionality is reported. Aqueous chemical synthesis and vapor phase deposition are combined to fabricate oriented arrays of high-interfacial-area hetero-nanostructures comprised of titanium oxide and iron oxide nanomaterials. Synchrotron-based soft X-ray spectroscopy techniques with high spectral resolution are utilized to directly probe the titanium and oxygen orbital character of the interfacial region's occupied and unoccupied densities of states. These data demonstrate the interface to possess electrons in Ti 3d bands and an emergent degree of orbital hybridization that is absent in parent oxide reference crystals. The carrier dynamics of the hetero-nanostructures are studied by ultrafast transient absorption spectroscopy, which reveals the presence of a dense manifold of states, the relaxations from which exhibit multiple exponential decays whose magnitudes depend on their energetic positions within the electronic structure.

Magnetism at spinel thin film interfaces probed through soft X-ray spectroscopy techniques

Magnetic order and coupling at the interfaces of highly spin polarized Fe 3O 4 heterostructures have been determined by surface sensitive and element specific soft X-ray spectroscopy and spectro-microscopy techniques. At ambient temperature, the interface between paramagnetic CoCr 2O 4 or MnCr 2O 4 and ferrimagnetic Fe 3O 4 isostructural bilayers exhibits long range magnetic order of Co, Mn and Cr cations which cannot be explained in terms of the formation of interfacial MnFe 2O 4 or CoFe 2O 4. Instead, the ferrimagnetism is induced by the adjacent Fe 3O 4 layer and is the result of the stabilization of a spinel phase not achievable in bulk form. Magnetism at the interface region is observable up to 500 K, far beyond the chromite bulk Curie temperature of 50–95 K.

Model study of soft x-ray spectroscopy techniques for observing magnetic circular dichroism in buried SmCo magnetic films

X-ray emission and absorption spectroscopy (XES and XAS, respectively) are important and powerful techniques for determining the electronic properties of materials. Both are used to study magnetic circular dichroism (MCD) which is especially useful for analyzing the magnetic properties of materials. We present XAS and XES measurements and a MCD model study of two thin film layered samples containing SmCo layers in order to report on the applicability of soft x-ray spectroscopic techniques to determine the composition, layer thickness, and electronic structure of such materials. Using a transmission by fluorescence attenuation (TFA) technique we determined the composition and thickness of the SmCo layer to be consistent with the intended composition and thickness. We also confirmed the thickness of the other layers by comparing the XES from the thin film with that of a bulk sample. We showed by a model study that TFA could be used to obtain MCD, and thus the anisotropy of the sample, for film thicknesses between about 400 and 800 Å.

Valence band structure and electron transport properties in rhombic triacontahedron and Mackay icosahedral types of Al-Mg-Pd and other quasi-crystals

The valence band structure of both rhombic triacontahedron (RT) and Mackay icosahedron (MI) types of quasi-crystal (QC) is studied by means of X-ray photoemission spectroscopy and the Al K and Pd L beta 2,15 soft X-ray spectroscopy techniques. Information about the density of states at the Fermi level EF is supplemented by the data on the electronic specific-heat coefficient. The Hall coefficient is also studied. The Al-Mg-Pd, Al-Mg-Zn and Al-Mg-Cu QCs are selected as representative of the RT-type QCs and Al-Mg-Pd, Al-Ru-Cu and Al-Re-Pd as representative of the MI-type QCs. The valence band structure of the RT-type QCs is characterized by a narrow d band in the middle of the valence band and a quasi-free-electron-like smooth density of states in the vicinity of EF. On the other hand, the d states of the transition-metal elements are more widely spread in the MI-type QCs. The depletion minimum in the Al 3p electron distribution occurs at the binding energies 3-5 owing to the interaction with the d states of the late-transition-metal elements. The presence of the pseudo-gap across EF has been confirmed in both RT- and MI-type Al-Mg-Pd QCs. We propose also that the effective carrier concentration derived from the Hall coefficient may be used as a universal parameter for both RT- and MI-type QCs in place of the conventionally used electron concentration per atom. We found that the resistivity and electronic specific-heat data exhibit universal relationships when plotted as a function of the effective carrier concentration. From this we conclude that either electrons or holes dominate at EF in most RT- and MI-type QCs studied so far. Exceptions to this will be those whose Hall coefficient is extremely small and changes its sign with temperature.