Ferroelectricity driven by Y d0-ness with rehybridization in YMnO3

We investigated electronic structure of hexagonal multiferroic YMnO3 using the polarization dependent x-ray absorption spectroscopy (XAS) at O K and Mn L(2,3) edges. The spectra exhibit strong polarization dependence at both edges, reflecting anisotropic Mn 3d orbital occupation. Moreover, the O K edge spectra show that Y 4d states are strongly hybridized with O 2p ones, resulting in large anomalies in Born effective charges on off-centering Y and O ions. These results manifest that the Y d(0)-ness with rehybridization is the driving force for the ferroelectricity, and suggest a new approach to understand the multiferroicity in the hexagonal manganites.     

First-principles calculations of K-shell x-ray absorption spectra for warm dense ammonia

The x-ray absorption spectroscopy is a powerful tool for the detection of thermodynamic conditions and atomic structures on warm dense matter. Here, we perform first-principles molecular dynamics and x-ray absorption spectrum calculations for warm dense ammonia, which is one of the major constituents of Uranus and Neptune. The nitrogen K-shell x-ray absorption spectrum(XAS) is determined along the Hugoniot curve, and it is found that the XAS is a good indicator of the prevailing thermodynamic conditions. The atomic structures at these conditions are ascertained. Results indicate that the ammonia could dissociate to NH_x(x = 0, 1, or 2) fragments and form nitrogen clusters, and the ratios of these products change with varying conditions. The contributions to the XAS from these products show quite different characteristics,inducing the significant change of XAS along the Hugoniot curve. Further model simulations imply that the distribution of the peak position of atomic XAS is the dominant factor affecting the total XAS.     

Production and characterization of Ti:sapphire thin films grown by reactive laser ablation with elemental precursors

Crystalline Ti:sapphire (Ti:Al(2)O(3)) thin films were grown at low temperatures upon Al(2)O(3) (0001) substrates by reactive crossed-beam laser ablation at 248 nm by use of a liquid Ti-Al alloy target and O(2) . The films were investigated ex situ by x-ray diffraction, x-ray photoelectron spectroscopy, and Rutherford backscattering spectrometry. Low-temperature luminescence was identical to that for Ti(3+) ions in bulk samples of Al(2)O(3) .     

Observation of selective surface element substitution in FeTe_(0.5)Se_(0.5) superconductor thin film exposed to ambient air by synchrotron radiation spectroscopy

A systematic investigation of oxidation on a superconductive Fe Te_(0.5)Se_(0.5)thin film,which was grown on Nb-doped SrTiO_3(001) by pulsed laser deposition,has been carried out.The sample was exposed to ambient air for one month for oxidation.Macroscopically,the exposed specimen lost its superconductivity due to oxidation.The specimen was subjected to in situ synchrotron radiation photoelectron spectroscopy(PES) and x-ray absorption spectroscopy(XAS) measurements following cycles of annealing and argon ion etching treatments to unravel what happened in the electronic structure and composition after exposure to air.By the spectroscopic measurements,we found that the as-grown FeTe_(0.5)Se_(0.5)superconductive thin film experienced an element selective substitution reaction.The oxidation preferentially proceeds through pumping out the Te and forming Fe–O bonds by O substitution of Te.In addition,our results certify that in situ vacuum annealing and low-energy argon ion etching methods combined with spectroscopy are suitable for depth element and valence analysis of layered structure superconductor materials.     

Ferrimagnetism in EuFe4Sb12 due to the interplay of f-electron moments and a nearly ferromagnetic host

We combine x-ray magnetic circular dichroism spectroscopy at Fe L2,3 edges, at Eu M4,5 edges, x-ray absorption spectroscopy (XAS) investigation of Eu valence, and local spin density calculations, to show that the filled skutterudite Eu0.95Fe4Sb12 is a ferrimagnet in which the Fe 3d moment and the Eu2+ 4f moment are magnetically ordered with dominant antiferromagnetic coupling. From Eu L3 edge XAS, we find that about 13% of the Eu have a formal valence of 3+. We ascribe the origin of ferrimagnetism at a relatively high transition temperature TC of 85 K in Eu0.95Fe4Sb12 to f-electron interaction with the nearly ferromagnetic [Fe4Sb12]2.2- host lattice.     

Energy band structure and X-ray spectra of phenakite Be<Subscript>2</Subscript>SiO<Subscript>4</Subscript>

The electronic structure of crystalline phenakite Be₂SiO₄ is investigated using x-ray emission spectroscopy (XES) (Be K _α XES, Si L _{2, 3} XES, O K _α XES) and x-ray absorption spectroscopy (XAS) (Be 1s XAS, Si 2p XAS, O 1s XAS). The energy band structure is calculated by the ab initio full-potential linearized augmented-plane-wave (FLAPW) method. The total and partial densities of states and the dispersion curves for the Be₂SiO₄ compound are presented. It is shown that the top of the valence band and the bottom of the conduction band of the Be₂SiO₄ compound are predominantly formed by the oxygen 2p states. According to the results obtained, the electron transition with the lowest energy supposedly can occur at the center of the Brillouin zone. The effective masses of electrons (0.5m _e ) and holes (3.0m _e ) for the Be₂SiO₄) compound are estimated.     

Characterization of an organic field-effect thin-film transistor in operation using fluorescence-yield x-ray absorption spectroscopy

In?situ element-specific observation of electronic states of organic films beneath metal electrodes is achieved by x-ray absorption spectroscopy (XAS) in the bulk-sensitive fluorescence-yield (FY) mode. The molecular orientation in Au-covered oligo-thiophene films is confirmed by the C K-edge FY-XAS spectra and the applied bias dependence of the spectra is successfully detected for the first time. The present method can give deeper insights into the electronic-state investigation of various real-device systems under operational conditions.     

Single impurity Anderson model versus density functional theory for describing Ce L3 x-ray absorption spectra of CeFe2: resolution of a recent controversy

We resolved a recent controversy on the structure of the Ce L(3) x-ray absorption spectra (XAS) of CeFe(2); i.e., which of the single impurity Anderson model (SIAM) and the first-principles band calculations based on the density-functional theory (DFT) describes more appropriately the Ce 4f states and their contribution to the Ce L(3) XAS? For this purpose, we examined the core-hole effect in Ce L(3) XAS as an application of our new method taking advantage of resonant x-ray emission spectroscopy. Our result clearly shows that the Ce L(3) XAS structure is caused by the mixed valence 4f character revealed by the core-hole potential effect as indicated by SIAM, but denies the possibility that the L(3) XAS structure is caused by the 5d band structure with a very small core-hole effect as predicted by band calculations based on DFT.     

Spatial chemical inhomogeneity and local electronic structure of Mn-doped Ge ferromagnetic semiconductors

We have investigated the chemical distributions and the local electronic structure of potential diluted magnetic semiconductor Ge0.94Mn0.06 single crystals using scanning photoelectron microscopy (SPEM), x-ray absorption spectroscopy (XAS), and photoemission spectroscopy (PES). The SPEM image shows the stripe-shaped microstructures, which arise from the chemical phase separation between the Mn-rich and Mn-depleted phases. The Mn 2p XAS shows that the Mn ions in the Mn-rich region are in the divalent high-spin Mn2+ states but that they do not form metallic Mn clusters. The Mn 3d PES spectrum exhibits a peak centered at approximately 4 eV below E(F) and the negligible spectral weight near E(F). This study suggests that the observed ferromagnetism in Ge1-xMnx arises from the phase-separated Mn-rich phase.     

Proton transfer in a two-dimensional hydrogen-bonding network: water and hydroxyl on a pt(111) surface

The time scale of proton transfer between H(2)O and OH adsorbed on a Pt(111) surface was determined by a combination of laser-induced thermal desorption (LITD) and microscale x-ray photoelectron spectroscopy (micro-XPS). The patterned distribution OH+H(2)O/H(2)O/OH + H(2)O was initially prepared on the Pt(111) surface by the LITD method and the time evolution of the spatial distribution of H(2)O and OH was observed by the micro-XPS technique. From quantitative analyses based on a diffusion equation, we found two proton-transfer pathways with different time scales of 5.2+/-0.9 ns and 48+/-12 ns at 140 K, which were attributed to direct proton transfer to the neighbor site and H(3)O(+)-mediated transfer to the next-nearest site, respectively.     

Unraveling the symmetry of the hole states near the Fermi level in the MgB2 superconductor

We use x-ray absorption spectroscopy (XAS) and electron energy loss spectroscopy (EELS) to study the fine structure at the K edge of boron in MgB(2). We observe in XAS a peak of width 0.7 eV at the edge threshold, signaling a narrow energy region with empty boron p states near the Fermi level. The changes in the near edge structure observed in EELS with direction of the momentum transfer imply that these states have p(x)p(y) symmetry. Our observations are consistent with electronic structure calculations indicating a narrow energy window of empty p(x)p(y) states that falls to zero at 0.8 eV above the Fermi level. The disappearance of the p(x)p(y) feature in EELS at grain boundaries suggests that this signature may become powerful in probing superconductivity at nanoscale.     

X ray absorption spectroscopy investigation of phase transition in Ge,GaAs and GaP

Abstract

The phase transitions of GaAs, GaP and Ge under pressure have been investigated by x-ray absorption spectroscopy (XAS). At the onset of the transition the Debye-Waller factor increases and the x-ray absorption near edge structure (XANES) is progressively modified. A mixing of the low and high pressure phase can be determined by XAS as well as amorphization of the sample on pressure release.     

Switching the electronic properties of Co-octaethylporphyrin molecules on oxygen-covered Ni films by NO adsorption

Using x-ray absorption spectroscopy, we demonstrate that the electronic properties of Co-octaethylporphyrin (CoOEP) molecules on oxygen-covered ultrathin Ni films can be reversibly manipulated by a chemical stimulus. This is achieved by adsorption of nitrogen monoxide (NO), leading to the formation of a NO-CoOEP nitrosyl complex, and subsequent thermal desorption of the NO from the Co ions. The integration of the absorption spectra of the Co L(2,3) edges reveals a partial oxidation of the Co ions after dosing with NO compared to the pristine sample, for which a valency of 2?+ and a low-spin state of the Co ions can be deduced from the Co L(2,3) XAS line shape. By means of x-ray magnetic circular dichroism the magnetic moments of the Co ions were found to be coupled parallel to the magnetization of the Ni films across the intermediate layer of atomic oxygen, before and after NO uptake.     

Insight into growth of Au–Pt bimetallic nanoparticles: an in situ XAS study

Au–Pt bimetallic nanoparticles have been synthesized through a one‐pot synthesis route from their respective chloride precursors using block copolymer as a stabilizer. Growth of the nanoparticles has been studied by simultaneous in situ measurement of X‐ray absorption spectroscopy (XAS) and UV–Vis spectroscopy at the energy‐dispersive EXAFS beamline (BL‐08) at Indus‐2 SRS at RRCAT, Indore, India. In situ XAS spectra, comprising both X‐ray near‐edge structure (XANES) and extended X‐ray absorption fine‐structure (EXAFS) parts, have been measured simultaneously at the Au and Pt L₃‐edges. While the XANES spectra of the precursors provide real‐time information on the reduction process, the EXAFS spectra reveal the structure of the clusters formed in the intermediate stages of growth. This insight into the formation process throws light on how the difference in the reduction potential of the two precursors could be used to obtain the core–shell‐type configuration of a bimetallic alloy in a one‐pot synthesis method. The core–shell‐type structure of the nanoparticles has also been confirmed by ex situ energy‐dispersive spectroscopy line‐scan and X‐ray photoelectron spectroscopy measurements with in situ ion etching on fully formed nanoparticles.     

The temperature dependence of Cu(2)O formation on a Cu(110) surface with an energetic O(2) molecular beam

We report a study of the surface temperature (T(s)) dependence of Cu(2)O formation on a Cu(110) surface induced by a hyperthermal O(2) molecular beam (HOMB), using x-ray photoemission spectroscopy in conjunction with synchrotron radiation. From the T(s) dependence of the O uptake curves, the direct dissociative adsorption process mainly contributes to the formation of the p(2?×?1)-O structure with an O coverage (Θ) of 0.5?ML for 2.2?eV HOMB incidence. On the other hand, the rate of oxidation at Θ?>?0.5?ML, particularly in Cu(2)O formation, strongly depends on the T(s). Thicker Cu(2)O islands were found inhomogeneously at 400 and 500?K, suggesting the dominant role of the migration of Cu atoms in the Cu(2)O formations on the Cu(110) surface.     

From amorphous to crystalline silicon nanoclusters: structural effects on exciton properties

Synchrotron x-ray absorption spectroscopy (XAS) and electron spin resonance (ESR) experiments were performed to determine, in combination with Raman spectroscopy and x-ray diffraction (XRD) data from previous reports, the structure and paramagnetic defect status of Si-nanoclusters (ncls) at various intermediate formation stages in Si-rich Si oxide films having different Si concentrations (y = 0.36-0.42 in Si(y)O(1-y)), fabricated by electron cyclotron resonance plasma-enhanced chemical vapor deposition and isochronally (2 h) annealed at various temperatures (T(a) = 900-1100 °C) under either Ar or (Ar + 5%H(2)) atmospheres. The corresponding emission properties were studied by stationary and time dependent photoluminescence (PL) spectroscopy in correlation with the structural and defect properties. To explain the experimental data, we propose crystallization by nucleation within already existing amorphous Si-ncls as the mechanism for the formation of the Si nanocrystals in the oxide matrix. The cluster-size dependent partial crystallization of Si-ncls at intermediate T(a) can be qualitatively understood in terms of a 'crystalline core-amorphous shell' Si-ncl model. The amorphous shell, which is invisible in most diffraction and electron microscopy experiments, is found to have an important impact on light emission. As the crystalline core grows at the expense of a thinning amorphous shell with increasing T(a), the PL undergoes a transition from a regime dominated by disorder-induced effects to a situation where quantum confinement of excitons prevails.     

碱性介质中正丁醇在铂电极上吸附和氧化的电化学原位FTIR反射光谱和EQCM研究

运用电化学循环伏安(CV)、原位FTIR反射光谱和电化学石英晶体微天平(EQCM)等方法研究了碱性介质中正丁醇在Pt电极表面吸附和氧化行为。结果表明：正丁醇电氧化过程与溶液酸碱性有着密切的关系。酸性介质中正丁醇在Pt电极上的CV曲线有2个正向氧化峰,而碱性介质中只有1个正向氧化峰, 第2个氧化峰的消失可能是由于碱性介质中Pt电极表面钝化引起的。原位FTIR反射光谱检测到,在实验条件下,碱性介质中正丁醇电氧化过程的最终产物只有丁酸根。EQCM研究还从电极表面质量定量变化的角度提供了正丁醇反应机理的新数据。     

甲酸在Pt(100)/Sb电极上氧化的时间分辨原位FTIR反射光谱研究

以电化学原位时间分辨ＦＴＩＲ反射光谱和循环伏安方法研究甲酸在不同Ｓｂ覆盖度修饰的Ｐｔ（１００）单晶电极上的氧化。发现Ｓｂａｄ的修饰抑制了甲酸的解离吸附,使反应经活性中间体直接氧化至ＣＯ２。电化学和红外光谱数据表明,θＳｂ＝０．２４的Ｐｔ（１００）／Ｓｂ电极具有最高的电催化活性。     

表面合金电催化剂上甲酸氧化的原位FTIR反射光谱研究

运用原位红外反射光谱(FTIRS)和电化学循环伏安法(CV)研究了甲酸在三种不同电极上的电催化特性。结果表明甲酸在碳载铂电极(Pt/GC)上的电催化氧化机理与本体铂电极(Pt)相类似,即可以通过活性中间体或毒性中间体氧化至CO_2。Pt/GC对甲酸的氧化比Pt具有更高的电催化活性。Pt/GC表面以Sb吸附原子修饰的电极(Sb-Pt/GC)上,甲酸氧化的起始电位(E;)提前至-0.10V,氧化电流峰电位(Ep)提前至0.34V,氧化峰电流(jp)值增加了7.28倍,半峰宽(FWHM)为0.30V。同样,Surface al-loy/GC电极上,E_I为-0.12V,E_p为0.32V和j_p为7.25mA·cm~(-2),相对Pt/GC分别负移了0.22,0.02V和增大了8.15倍,半峰宽(FWHM)为0.5V。表明Sb-Pt/GC和Surface alloy/GC电极不仅能够有效地抑制毒性中间体CO的生成,而且还可以显著地提高其对活性中间体的氧化的电催化活性。     

Increased surface spin stability in γ-Fe2O3 nanoparticles with a Cu shell

γ-Fe(2)O(3) nanoparticles were coated with a Cu shell in situ during synthesis. An interfacial monolayer of CuO in the Cu-coated γ-Fe(2)O(3) nanoparticles was discovered that stabilized the disordered surface spins of γ-Fe(2)O(3) nanoparticles. Element-specific x-ray absorption spectroscopy at the L-edges for Cu and Fe indicated the magnetic moment of the Cu in the shell interacted with the γ-Fe(2)O(3) nanoparticle's surface magnetic moments. This exchange interaction between the Fe and Cu at the interface permitted an overall Cu moment in CuO (an antiferromagnet typically) that altered the γ-Fe(2)O(3) nanomagnetism. Increasing the Cu shell thickness also increased the total Fe magnetism of the nanoparticles.