X-ray Absorption Near-edge Structure Measurements Research Articles

Tailored Local Electronic Environment of Co-N4 Sites in Cobalt Phthalocyanines for Enhanced CO2 Reduction Reaction.

Atomically dispersed Co-N4-based catalysts have been recently emerging as one of the most promising candidates for facilitating CO2 reduction reaction (CO2RR). The local electronic environment of Co-N4 sites in these catalysts is considered to play a critical role in adjusting the catalytic performance, the effort of which however is not yet clearly verified. Herein, a series of cobalt phthalocyanines with different peripheral substituents including unsubstituted phthalocyanine Co(II) (CoPc), 2,9,16,23-tetramethoxyphthalocyaninato Co(II) (CoPc-4OCH3), and 2,9,16,23-tetranitrophthalocyaninato Co(II) (CoPc-4NO2) are supported onto the surface of the multi-walled carbon nanotubes (CNTs), affording CoPc@CNTs, CoPc-4OCH3@CNTs, and CoPc-4NO2@CNTs. X-ray photoelectron spectroscopy and X-ray absorption near-edge structure measurements disclose the influence of the peripheral substituents on the local electronic structure of Co atoms in these three catalysts. Electrochemical tests indicate the higher CO2RR performance of CoPc-4OCH3@CNTs compared to CoPc@CNTs and CoPc-4NO2@CNTs as exemplified by the higher Faraday efficiency of CO, larger part current densities, and better stability displayed by CoPc-4OCH3@CNTs at the applied voltage range from -0.6 to -1.0V versus RHE in both H-cell and flow cell. These results highlight the effect of the electron-donating -OCH3 substituent on the enhanced catalytic activity of CoPc-4OCH3@CNTs, which will help develop Co-N4-based catalysts with promising catalytic performance toward CO2RR.

Variable oxidizing capacity of slab-derived fluids: Insights from Fe and S speciation in glasses from the Troodos Ophiolite

Oxygen fugacity (ƒO2) varies systematically across tectonic environments. The typically high ƒO2 recorded in arc settings relative to oceanic ridges is attributed to recycling of oxidized materials derived from subducting slabs into regions of the mantle that undergo partial melting. To evaluate further the relationship between ƒO2 and mantle metasomatism, Fe and S X-ray Absorption Near-Edge Structure measurements were conducted on volcanic glass wafers sampled across the extrusive section of the Troodos Ophiolite, Cyprus, which is a type locality for studying the influence of subduction on mantle melting processes and the generation of oceanic crust. The glasses record ƒO2 values relative to the quartz-fayalite-magnetite (FMQ) buffer of FMQ+0.13±0.16(1σ) to FMQ+0.74±0.23(1σ) upon quenching at the seafloor. At a given MgO content, the ƒO2 values recorded by the Troodos glasses do not vary systematically based on type (i.e., boninitic versus tholeiitic) nor by sampling location, indicating that the lavas were derived from primary melts and mantle sources that were indistinguishable in their initial ƒO2 levels. The glass ƒO2 values do not vary with dissolved H2O contents, nor with trace element ratios (e.g., Ba/La, Ce/Pb) and Pb and Sr isotopic compositions, all used to monitor interaction of slab-derived materials with the Troodos mantle source. The decoupling of ƒO2 and H2O and fluid-mobile elements renders the Troodos an important endmember for interpreting global variations in the redox state of mantle melts formed in subduction-influenced settings. For example, the Troodos glasses are reduced relative to Izu-Bonin boninites and melt inclusions from Mariana and Cascades arc volcanoes, despite overlapping and elevated H2O contents, Ba/La, and S/Dy relative to mid-ocean ridge basalts. The lack of correlation between ƒO2 and volatile contents and with proxies for subduction influence in the Troodos glasses is similar to basalts formed in back-arc environments and in near-trench settings in Izu-Bonin-Mariana. Such comparisons suggest that the slab-derived fluids that infiltrated the Troodos melting region were derived from a shallow slab and had low oxidizing capacity relative to fluids or melts which interacted with the mantle in the Izu-Bonin-Mariana arc and in the Cascades, which represent arc environments with comparable glass Fe XANES measurements. A simple framework accounts for local and global variability in ƒO2 in fractionation-corrected glasses, where fluids of variable oxidizing capacity interact with depleted mantle to produce hydrous melts. The inheritance of volatile and fluid-mobile elements in regions of the mantle that undergo partial melting does not necessarily lead to higher ƒO2 if the oxidizing capacity of the infiltrating hydrous fluids or melts is low. In back-arcs and near-trench environments, partial melts formed by interaction with fluids of low oxidizing capacity are similar to arc melts in their volatile contents, trace element, and isotopic signatures, but modestly oxidized relative to mid-ocean ridge basalts.

Specific Ion Effects at the Vapor-Formamide Interface: A Reverse Hofmeister Series in Ion Concentration Depth Profiles.

Employing neutral impact collision ion scattering spectroscopy (NICISS), we have directly measured the concentration depth profiles (CDPs) of various monovalent ions at the vapor-formamide interface. NICISS provides CDPs of individual ions by measuring the energy loss of neutral helium atoms backscattered from the solution interface. CDPs at the vapor-formamide interface of Cl-, Br-, I-, Na+, K+, and Cs+ are measured and compared to elucidate the interfacial specific ion trends. We report a reverse Hofmeister series in the presence of inorganic ions (anion and cation) at the vapor-formamide interface relative to the water-vapor interface, and the CDPs are found to be independent of the counterion for most ions studied. Thus, ions at the surface of formamide follow a "Hofmeister paradigm" where the counterion does not impact the ion series. These specific ion trends are complemented with surface tension and X-ray absorption near-edge structure (XANES) measurements on formamide electrolyte solutions.

Quantification and speciation of lead in air particulate matter collected from an urban area in Amman, Jordan

Lead (Pb) is a carcinogenic element, and it originates in the atmosphere from various natural and anthropogenic sources. This work aims to provide further insights about the presence of lead in Amman’s atmosphere. Air particulate matter (PM) samples were collected in two fractions (PM2.5 and PM10) simultaneously on Teflon filters in an urban area in Amman, Jordan. Elemental quantification and chemical speciation of Pb were performed using particle-induced X-ray emission (PIXE) and X-ray absorption near-edge structure (XANES) techniques, respectively. Elemental quantitative analysis using PIXE showed that Pb concentrations increased during workdays regardless of the aerosol particle size fraction. The average Pb concentration was 2.9 ± 1.7 ng/m3 and 3.9 ± 2.0 ng/m3 in PM2.5 and PM10, respectively. Based on XANES measurements, it was shown that both PM2.5 and PM10 in Amman contain divalent lead (Pb (II)), most likely as PbSO4, PbS and PbO compounds.

X‐ray absorption fine structure characterization of a multicomponent spinel catalyst

AbstractDemand for high‐performing, inexpensive catalysts has motivated the development of mixed 3d transition metal architectures. Here, x‐ray spectroscopies are uniquely positioned to inform intensive searches of compositional spaces by elucidating structure–function relationships. However, thorough analyses of complex systems are often non‐trivial. In this work, one such example was pursued, a chemical series of spinel‐based emissions catalysts of the general formula Mn0.1CuxCo2.9−xO4 for 0.1 ≤ x ≤ 0.8. The local electronic and atomic structures of these materials were characterized by x‐ray absorption near edge structure (XANES) and extended x‐ray absorption fine structure (EXAFS) analyses, respectively. EXAFS analyses required models to be constructed that captured the relevant structural changes across the chemical series. Building these models required insights from XANES, x‐ray diffraction (XRD), and inductively coupled plasma atomic emission spectroscopy analyses. XANES measurements at the Co K‐edge, when complemented with ab initio multiple scattering calculations, suggested that the fraction of Co atoms in the octahedral sites of the spinel increased as the concentration of substituted Cu increased. Similarly, XANES measurements at the Mn K‐edge suggested that the occupancy of the first coordination shell varied throughout the series. Finally, the XRD results revealed an impurity, a CuO phase, formed at higher copper concentrations. This is consistent with the results of the principal component analysis performed on the Cu K‐edge XANES spectra. These hypotheses were incorporated into the EXAFS fitting models and are consistent with the subsequent quantitative analyses.

Hierarchical assembled Ag2CuMnO4 nanoflakes as a novel electrode material for energy storage applications

In this work, a novel electrode material of hierarchical assembled silver copper manganese oxide (Ag2CuMnO4) nanoflakes was successfully prepared by a hydrothermal process at temperatures of 120 and 140 ºC. Influences of hydrothermal temperatures on the structure, morphology, porosities, and electrochemical performance of the prepared samples were thoroughly investigated. At 120 ºC, the flower-like microspheres composed of smaller Ag2CuMnO4 nanoflakes delivered the specific capacitance as high as 617.06 F g−1 at a current density of 1 A g−1 with the corresponding energy density of 145.17 Wh Kg−1. By using in-situ X-ray absorption near-edge structure (XANES) measurements, the charge storage mechanisms of Ag2CuMnO4 in 3 M KOH aqueous electrolyte were revealed for the first time through the multiple redox reactions of Ag/Ag1+, Cu/Cu1+, Cu1+/Cu2+, and Mn3+/Mn4+. For practical application, the symmetric supercapacitor device was fabricated by using Ag2CuMnO4 as both positive and negative electrodes. As a result, the fabricated supercapacitor device could deliver maximum energy density and power density of 2.40 Wh Kg−1 and 60.06 W Kg−1 at 0.1 A g−1, respectively. Therefore, this work provides some new insights into the charge storage mechanisms of Ag2CuMnO4 and its potential use as novel electrode material in supercapacitors.

Single-crystal thin film growth of the Mott insulator EuVO3 under biaxial substrate strain

We report the single-crystal thin film growth of EuVO3 of a Mott insulator with V3+ (d2,S=1). We used the pulsed laser deposition (PLD) and molecular beam epitaxy (MBE) methods and obtained the well-oriented epitaxial thin films with a pseudo-tetragonal crystal structure of EuVO3 which is distinct from the orthorhombic structure in bulk. The high quality of EuVO3 films is verified by the valence state of europium and vanadium ions with X-ray absorption near-edge structure (XANES) measurements showing clear signs from Eu3+ and V3+. The electrical resistivity exhibits the insulating temperature dependence; however, the magnitude of the resistivity is reduced from that of bulk EuVO3, possibly due to biaxial strain from substrates.

Niobium speciation in minerals revealed byL2,3-edges XANES spectroscopy

AbstractThe systematic mineralogy of niobium (Nb) is complex, with more than one hundred species dominated by multicomponent oxides of similar chemistry. The determination of Nb speciation in solids (i.e., the distribution between the phases and the crystal-chemical environment of Nb) is thus a challenge in geological contexts. Here, we present the first Nb L2,3-edges X-ray absorption near-edge structure (XANES) measurements on various Nb minerals and synthetic oxides with geological relevance. The interpretation of Nb L2,3-edges XANES spectra in the light of crystal-field theory shows the sensitivity of spectra to local site symmetry and electronic environment around Nb atoms. Crystal-field multiplet simulations give estimates of the 10Dq crystal-field parameter values for Nb5+, which range from 2.8 to 3.9 eV depending on Nb coordination and Nb—O distances. Rather than a 10Dq vs. R–5 relationship (where R represents the average Nb-O bond distance) expected in a point-charge model, we find a R–3 dependence with the crystal-field splitting for reference materials with octahedrally coordinated Nb. Complementary ligand-field multiplet simulations provide evidence of charge transfer between Nb and O. The contribution of the ionic and covalent characters to the Nb-O bonds is equivalent, unlike more ionic 3d metal–O bonds. This systematic characterization of the L2,3-edges XANES spectral properties of Nb provides information on the mechanisms by which Nb5+ substitutes for Fe3+, Ti4+, or Ce4+ in oxides common in geological contexts. Whereas the substitution of Nb5+ for Ce4+ does not modify the local structure of the cation site in cerianite, the substitution of Nb5+ for Ti4+ in rutile and anatase results in an increase of the cation-ligand distance and a decrease in the symmetry of the cation site. Conversely, the substitution of Nb5+ for Fe3+ in hematite and goethite results in a smaller cation site distortion. Our study demonstrates the usefulness of L2,3-edges XANES spectroscopy to determine Nb speciation in minerals to understand the processes of enrichment of this critical metal.

Ring defects-rich and pyridinic N-doped graphene aerogel as floating adsorbent for efficient removal of tetracycline: Evidence from NEXAFS measurements and theoretical calculations

The rational design of carbon-based adsorbents with a high uptake efficiency for polar organic molecules is a key challenge in water purification research. Herein, we report a graphene aerogel that is doped with pyridinic-N and has abundant ring defects (denoted by DNGA). The aerogel sample exhibits a high adsorption capacity of 607.1 mg/g toward tetracycline (TC), a fast adsorption process (20 min), and good reusability (with a declining efficiency < 10.0% after five cycles), while being easy to recycle. C/N K-edge X-ray absorption near-edge structure (XANES) measurements demonstrate that the efficient adsorption capacity of the DNGA sample is related to the presence of ring defects and the pyridinic-N species. Density functional theory (DFT) calculations demonstrate that ring defects of type 5–8–5 and the pyridinic-N species at the edge location are primarily responsible for TC removal. In this study, we resolve a controversial issue regarding the origin of the adsorption performance origin of N-doped carbon-based adsorbents. The findings of this study can guide the development of novel and improved N-doped carbon-based adsorbents for the removal of target contaminants.

Simultaneous Sequestration of Co2+ and Mn2+ by Fungal Manganese Oxide through Asbolane Formation

Biogenic manganese oxides (BMOs) have attractive environmental applications owing to their metal sequestration and oxidizing abilities. Although Co readily accumulates into Mn oxide phases in natural environments, the Co2+ sequestration process that accompanies the enzymatic Mn(II) oxidation of exogenous Mn2+ remains unknown. Therefore, we prepared newly formed BMOs in a liquid culture of Acremonium strictum KR21-2 and conducted repeated sequestration experiments in a Mn2+/Co2+ binary solution at pH 7.0. The sequestration of Co2+ by newly formed BMOs (~1 mM Mn) readily progressed in parallel with the oxidation of exogenous Mn2+, with higher efficiencies than that in single Co2+ solutions when the initial Co2+ concentrations (0.16–0.8 mM) were comparable to or lower than the exogenous Mn2+ concentration (~0.8 mM). This demonstrates a synergetic effect on Co sequestration. Powder X-ray diffraction showed a typical pattern for asbolane only when newly formed BMOs were treated in Mn2+/Co2+ binary systems, implying that the enzymatic Mn(II) oxidation by newly formed BMOs favored asbolane formation. Cobalt K-edge X-ray absorption near-edge structure measurements showed that both Co(II) and Co(III) participated in the formation of the asbolane phase in the binary solutions, whereas most of the primary Co2+ was sequestered as Co(III) in the single Co2+ solutions, which partly explains the synergetic effects on Co sequestration efficiency in the binary solutions. The results presented here provide new insights into the mechanism of Co interaction with Mn oxide phases through asbolane formation by enzymatic Mn(II) oxidation under circumneutral pH conditions.

Structure of Aqueous Scandium(III) Nitrate Solution by Large-Angle X-ray Scattering Combined with Empirical Potential Refinement Modeling, X-ray Absorption Fine Structure, and Discrete Variational Xα Calculations

Abstract Large-Angle X-ray scattering (LAXS) and Sc K X-ray absorption near-edge structure (XANES) measurements are made at room temperature on a 1 M (= mol dm−3) Sc(NO3)3 aqueous solution. The X-ray interference function is subjected to an empirical potential structure refinement (EPSR) modeling to extract the site-site pair correlation functions, the coordination number distribution, and the spatial density functions (three-dimensional structure). The LAXS analysis combined with EPSR reveals that Sc3+ is surrounded by six or seven water molecules and one oxygen atom of NO3− with an Sc3+-Ow (H2O) and Sc3+-ON (NO3−) distance of 2.14 Å. NO3− has a weak solvation shell with a broad distribution of coordination numbers with the highest population of nine at an N-Ow distance of 3.66 Å. Solvent water forms the tetrahedral network structure. The XANES spectrum is compared with simulated spectra on various coordination geometries of an ScO7 moiety with a discrete variational Xα (DV-Xα) molecular orbital (MO) method. A distorted monocapped trigonal prism structure of Sc3+ best reproduced the experimental pre-peak due to the Sc 1s → 3d transition in the XANES spectrum.

In situ beam reduction of Pu(IV) and Bk(IV) as a route to trivalent transuranic coordination complexes with hydroxypyridinone chelators.

The solution-state interactions of plutonium and berkelium with the octadentate chelator 3,4,3-LI(1,2-HOPO) (343-HOPO) were investigated and characterized by X-ray absorption spectroscopy, which revealed in situ reductive decomposition of the tetravalent species of both actinide metals to yield Pu(III) and Bk(III) coordination complexes. X-ray absorption near-edge structure (XANES) measurements were the first indication of in situ synchrotron redox chemistry as the Pu threshold and white-line position energies for Pu-343-HOPO were in good agreement with known diagnostic Pu(III) species, whereas Bk-343-HOPO results were found to mirror the XANES behavior of Bk(III)-DTPA. Extended X-ray absorption fine structure results revealed An-OHOPO bond distances of 2.498 (5) and 2.415 (2) Å for Pu and Bk, respectively, which match well with bond distances obtained for trivalent actinides and 343-HOPO via density functional theory calculations. Pu(III)- and Bk(III)-343-HOPO data also provide initial insight into actinide periodicity as they can be compared with previous results with Am(III)-, Cm(III)-, Cf(III)-, and Es(III)-343-HOPO, which indicate there is likely an increase in 5f covalency and heterogeneity across the actinide series.

High-Pressure Synthesis of Light Lanthanide Dodecaborides (PrB12 and CeB12): Effects of Valence Fluctuation on Volume and Formation Pressure.

Light lanthanide dodecaborides, RB12 (R = Pr and Ce), were synthesized from a stoichiometric mixture of hexaborides and boron using a laser-heated diamond anvil cell under high-pressure and high-temperature conditions. Contrary to the expectation that lighter lanthanide elements require higher pressure to crystallize RB12, in situ X-ray diffraction experiments reveal that cerium dodecaboride crystallizes at 26 GPa, which is significantly lower than that required to form the heavier praseodymium dodecaboride (35 GPa). In addition to the lower formation pressure, an anomalous volume reduction is also observed in CeB12, which can be explained by a valence fluctuation between Ce3+ and Ce4+ indicated by X-ray absorption near-edge structure measurements. A polyhedral coordination change from a truncated cube in RB6 to a truncated octahedron in RB12 and associated shortening of the R-B bond length result in an increase in bulk modulus and hardness.

Chemical disorder in polycrystalline Ni2MnGa thin films

The chemical disorder in magnetic shape memory alloys it is an important parameter that impacts on its magneto-mechanical and electronic properties. In this work, Ni2MnGa thin films were deposited on flexible glass substrates by using DC magnetron sputtering technique. The short-range order/disorder was investigated by means of Extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES) measurements using at Mn K-edge. Polycrystalline samples of Ni2MnGa with fixed stoichiometry exhibit L21 structure with low residual stress and similar grain sizes. A phenomenological model taking into account three chemical order parameters associated with main chemical anti-site disorders and reliable lattice tetragonal distortions extracted from X-ray diffraction analyses was used to perform systematic computational simulations. The results reveal the persistence of local tetragonal distortions of the crystal structure with L21 structure under when increasing annealing temperatures that are accompanied by a systematic decrease of the anti-site disorder between Ni and Mn atoms and substitutions between Ni and Ga atoms, with the orderly occupation of the atomic sites of Mn and Ga remaining unstable. This work also shows presents a consistent model to perform quantitative analyses of the chemical anti-site disorders through EXAFS measurements in the on Ni2MnGa polycrystalline thin films that preserve both the stoichiometry and the L21 crystalline structure.

Single-Cell Chemistry of Photoactivatable Platinum Anticancer Complexes.

The Pt(IV) prodrug trans, trans, trans-[Pt(pyridine)2(N3)2(OH)2] (Pt1) and its coumarin derivative trans, trans, trans-[Pt(pyridine)2(N3)2(OH)(coumarin-3-carboxylate)] (Pt2) are promising agents for photoactivated chemotherapy. These complexes are inert in the dark but release Pt(II) species and radicals upon visible light irradiation, resulting in photocytotoxicity toward cancer cells. Here, we have used synchrotron techniques to investigate the in-cell behavior of these prodrugs and visualize, for the first time, changes in cellular morphology and Pt localization upon treatment with and without light irradiation. We show that photoactivation of Pt2 induces remarkable cellular damage with extreme alterations to multiple cellular components, including formation of vacuoles, while also significantly increasing the cellular accumulation of Pt species compared to dark conditions. X-ray absorption near-edge structure (XANES) measurements in cells treated with Pt2 indicate only partial reduction of the prodrug upon irradiation, highlighting that phototoxicity in cancer cells may involve not only Pt(II) photoproducts but also photoexcited Pt(IV) species.

In situ N K-edge XANES study of iron, cobalt and nickel nitride thin films.

A prototype in situ X-ray absorption near-edge structure (XANES) system was developed to explore its sensitivity for ultra-thin films of iron-nitride (Fe-N), cobalt-nitride (Co-N) and nickel-nitride (Ni-N). They were grown using DC-magnetron sputtering in the presence of an N2 plasma atmosphere at the experimental station of the soft XAS beamline BL01 (Indus-2, RRCAT, India). XANES measurements were performed at the N K-edge in all three cases. It was found that the N K-edge spectral shape and intensity are greatly affected by increasing thickness and appear to be highly sensitive, especially in low-thickness regions. From a certain thickness of ∼1000 Å, however, samples exhibit a bulk-like behavior. On the basis of the obtained results, different growth stages were identified. Furthermore, the presence of a molecular N2 component in the ultra-thin regime (<100 Å) was also obtained in all three cases studied in this work. In essence, this prototype insitu system reveals that N K-edge XANES is a powerful technique for studying ultra-thin films, and the development of a dedicated in situ system can be effective in probing several phenomena that remain hitherto unexplored in such types of transition metal nitride thin films.

Regulating electronic structure and adsorptivity in molybdenum selenide for boosting electrocatalytic water splitting

Rationally designing highly efficient, cost-effective electrocatalysts towards overall water splitting to substitute noble-metal-based catalysts currently is still an outstanding challenge for renewable energy conversion. Herein, we demonstrate multiscale regulation of MoSe2 by CoSe tailoring for achieving significantly increased activity as a bifunctional electrocatalyst for highly efficient overall water splitting. X-ray absorption near-edge structure (XANES) and ultraviolet photoelectron spectroscopy (UPS) measurements unravel electron transfer from CoSe to MoSe2 as well as the increase of atomic disorder degree and the decrease of coordination number for MoSe2, thus facilitating efficient and fast interfacial charge transport and meanwhile providing more electrocatalytic active sites. Moreover, the introduced CoSe species not only improves the water adsorption/dissociation capability of MoSe2 by providing additional water adsorption sites, which is crucial for subsequent fast H2 generation, but also promotes MoSe2 to realize bifunctional behavior. Besides, density functional theory (DFT) calculations further reveal that CoSe modification for MoSe2 could evidently tailor d-band center of MoSe2 and accordingly significantly reduce hydrogen-adsorption Gibbs free energy (ΔGH*) of MoSe2 from 0.78 to 0.16 eV, along with an enhanced electronic conductivity owing to the increased electronic states near the Fermi level based on density of states (DOSs). This work gives a deeper insight for the relationship between multiscale regulation strategy and intrinsic activity of materials.

Relationship between Width and Height of π* Peak in C K-XANES of Graphitic Carbons.

The relationship between the width and height of the π* peak on X-ray absorption near edge structure (XANES) in the C K region of graphitic carbons is investigated from XANES measurements and theoretical analysis. This relationship is herein named the "characteristic diagram of π* peak profile". Mechanically ground (MG) graphite experimentally exhibits a linear correlation on the diagram. To understand the linear correlation, C K-XANES of the graphitic hexagonal carbon layers are calculated using first-principles calculations by focusing on the edge carbon atoms. The linear correlation is well explained by the ratio of the edge carbon atoms of the graphitic hexagonal carbon layers. The characteristic diagram of π* peak profile is applied to industrial carbon black (CB) for identification. CBs exhibit identical distributions on the diagram, which depend on their uses. It is therefore confirmed that the proposed characteristic diagram of π* peak profile can be a useful tool to identify graphitic carbons from the edge carbon atoms.

X-ray absorption study of defects in reactively sputtered GaN films displaying large variation of conductivity

Undoped GaN films were epitaxially grown on c-sapphire by rf magnetron reactive sputtering of GaAs at different N2 percentages in Ar–N2 sputtering atmosphere. These films display a large increase in conductivity from ∼10−6 Ω−1cm−1 to ∼102 Ω−1cm−1 with decrease of N2 in sputtering atmosphere from 100% to 10%. X-ray photoelectron spectroscopy reveals a monotonous decrease of N/Ga ratio, concurrently with increase of uncoordinated/interstitial Ga in the films, with decrease of N2 percentage in sputtering atmosphere. Extended x-ray absorption fine structure measurements and x-ray absorption near edge structure (XANES) simulation at Ga K-edge demonstrate the absence of N vacancies in sputtered GaN films and point towards the presence of Ga vacancies, which increase with decrease of N2 percentage in sputtering atmosphere. XANES measurements at O K-edge reveal that O interstitials are present in most of the films, except for the film grown at low N2 percentages (∼10%), in which case, O substitution on N-site may also take place. A monotonous increase of n-type conductivity is observed with increase of Ga interstitials, which suggests that Ga interstitials are the main source of n-type conductivity in sputtered GaN films. The low conductivity of the films grown at high N2 percentages (≳75%) is attributed to the limited presence of Ga interstitials and a strong compensation, dominated by N and O interstitials. The high conductivity GaN films grown at low N2 percentages may be partly contributed by O substituted on N site, along with Ga vacancies and possibly VGa-ON complexes, as the compensating defects.

Origin of room-temperature ferromagnetism in Co-doped CeO2

Co-doped CeO2 fabricated using the conventional solid-state reaction method demonstrates ferromagnetism at room temperature (RT). To investigate the origin of ferromagnetism at RT, powder X-ray diffraction analysis is first performed, in which pattern changes due to Co doping are not indicated. Subsequently, local environment analysis is carried out using X-ray absorption near-edge structure (XANES) measurements at the Co K-edge, demonstrating the metallic valence state of Co. Hence, it is confirmed that Co exists as nanosized small particles in CeO2, which demonstrates ferromagnetism at RT.