Total Fluorescence Yield Modes Research Articles

Origin of microscopically coupled ferromagnetic Cu-ions in a distorted system of Cu-doped ZnO and their synchrotron-based electronic structures

Spintronics-based studies have produced significant attention in the last decade while claiming the observation of room temperature ferromagnetism (RTFM). Nevertheless, there is a lack of consensus on a mechanism responsible for this phenomenon. In this study, we focus on Cu-doped ZnO (ZCO) to understand the microscopic origin of RTFM and the role of different oxidation states of Cu in RTFM. We have performed different spectroscopic techniques using synchrotron facilities. The values of spin-moment obtained from x-ray magnetic circular dichroism sum-rule truly exhibit a ferromagnetic interaction in the nanocrystalline powder of ZCO with ∼0.58 μB for 5% of Cu concentration in the total fluorescence yield mode. Such an enhanced magnetization is attributed to the presence of Cu2+, which is mainly localized in the bulk region. Cu in ZCO is mostly dominated by the presence of Cu2+. This is clearly reflected by the profiles of x-ray photoemission spectroscopy. Consequently, the weakly magnetized total electron yield mode is attributed to a state of magnetic frustration as the majority of Cu3+ is found on the surface. Some of these Cu3+ when come in the vicinity of Cu2+ ions result in a highly correlated state of double exchange mechanism, which is the microscopic origin of RTFM in ZCO. The coupling between Cu2+-Cu3+ is mediated via oxygen vacancies (VO), the presence of which is confirmed through the features of electron energy loss spectroscopy over different edges. The confirmation of VO is also supported by the deconvolution of E2high-phonon in the Raman spectra. Moreover, the defects in the local electronic structures of ZCO are demonstrated by the deconvoluted spectra of Cu L3 x-ray absorption spectroscopy. The images obtained from high-resolution transmission electron microscopy confirm the incorporation of Cu into the wurtzite crystal of ZnO. A clear enhancement in magnetization upon an increase in carriers of Cu in ZCO indicates carrier-induced ferromagnetism. Cu2+ and VO are the two attributes of RTFM in ZCO.

Origin of Enhancement of Orbital Magnetic Moment in SiO2-Coated Fe3O4 Nanocomposites Studied by X-ray Magnetic Circular Dichroism.

In this study, magnetic Fe3O4 nanoparticles (NPs) were dispersed uniformly by varying the thickness of the SiO2 coating, and their electronic and magnetic properties were investigated. X-ray diffraction confirmed the structural configuration of monophase inverse-spinel Fe3O4 NPs in nanometer size. Scanning electron microscopy revealed the formation of proper nonporous crystallite particles with a clear core-shell structure with silica on the surface of Fe3O4 NPs. The absorption mechanism studied through the zeta potential indicates that SiO2-coated Fe3O4 nanocomposites (SiO2@Fe3O4 NCs) possess electrostatic interactions to control their agglomeration in stabilizing suspensions by providing a protective shield of amorphous SiO2 on the oxide surface. High-resolution transmission electron microscopy images demonstrate a spherical morphology having an average grain diameter of ∼11-17 nm with increasing thickness of SiO2 coating with the addition of a quantitative presence and proportion of elements determined through elemental mapping and electron energy loss spectroscopy studies. Synchrotron-based element-specific soft X-ray absorption spectroscopy and X-ray magnetic circular dichroism (XMCD) techniques have been involved in the bulk-sensitive total fluorescence yield mode to understand the origin of magnetization in SiO2@Fe3O4 NCs. The magnetization hysteresis of Fe3O4 was determined by XMCD. At room temperature, the magnetic coercivity (Hc) is as high as 1 T, which is about 2 times more than the value of the thin film and about 5 times more pronounced than that of NPs. For noninteracting single-domain NPs with the Hc spread from 1 to 3 T, the Stoner-Wohlfarth model provided an intriguing explanation for the hysteresis curve. These curves determine the different components of Fe oxides present in the samples that derive the remnant magnetization involved in each oxidation state of Fe and clarify which Fe component is responsible for the resultant magnetism and magnetocrystalline anisotropy based on noninteracting single-domain particles.

Magnetic properties of Cr-doped VO2 thin films in tetragonal phase of rutile and their synchrotron-based electronic structures

The present study is focused on the investigation at 400 K of the tetragonal-rutile phase of Cr-doped VO2 (CVO) thin films grown by pulsed laser deposition. Synchrotron-based x-ray measurements of both the surface-sensitive total electron yield (TEY) and bulk-sensitive total fluorescence yield (TFY) modes were used to investigate the pristine and Cr-doped VO2 (5%, 10%, 20%, and 30% of atomic weight). The structural analysis and purity of the crystalline phase of the as-deposited films are manifested via grazing incidence x-ray diffraction patterns, which confirm the tetragonal-rutile phase. The purity of the phase is also confirmed by the presence of Eg-mode phonons in the Raman spectra and its deconvolution reflects on the oxygen-mediated electronic/vibrational transitional effect. A clear hysteretic behavior obtained through vibrating sample magnetometry strongly suggests the ferromagnetic interaction in the thin films of CVO. The local-electronic property of the samples is examined using x-ray absorption spectroscopy (XAS) in TEY and TFY modes where the difference in the configured helicity photons resulted in the fine spectra of x-ray magnetic circular dichroism (XMCD). XAS and XMCD measurements performed at V L2,3 and Cr L2,3 edges explicitly demonstrate the ferromagnetism in the thin films of CVO. The strong hybridization between V 3d and Cr 3d states with O 2p states is evident from the spectra of the O K-edge, resulting in the onset of the cation-pair formation V5+–Cr3+, which is ferromagnetic by means of double-exchange interaction. The theoretical calculation of density functional theory made upon Vienna ab initio simulation package suggests that CVO is in a mixed state of a ferromagnetic-insulator and a half-metallic ferromagnet.

Origin and Quantitative Description of the NESSIAS Effect at Si Nanostructures

AbstractThe electronic structure of SiO2‐ versus Si3N4‐coated low nanoscale intrinsic silicon (Si) shifts away from versus toward the vacuum level Evac, originating from the Nanoscale Electronic Structure Shift Induced by Anions at Surfaces (NESSIAS). Using the quantum chemical properties of the elements involved to explain NESSIAS, an analytic parameter Λ is derived to predict the highest occupied energy level of Si nanocrystals (NCs) as verified by various hybrid‐density functional calculations and NC sizes. First experimental data of Si nanowells (NWells) embedded in SiO2 versus Si3N4 were measured by X‐ray absorption spectroscopy in total fluorescence yield mode (XAS‐TFY), complemented by ultraviolet photoelectron spectroscopy (UPS), characterizing their conduction band and valence band edge energies EC and EV, respectively. Scanning the valence band sub‐structure over NWell thickness yields an accurate estimate of EV shifted purely by spatial confinement, and thus the actual EV shift due to NESSIAS. Offsets of ΔEC = 0.56 eV and ΔEV = 0.89 eV were obtained for 1.9 nm thick NWells in SiO2 versus Si3N4, demonstrating an intrinsic Si type II homojunction. This p/n junction generated by NESSIAS eliminates any deteriorating impact of impurity dopants, offering undoped ultrasmall Si electronic devices with much reduced physical gate lengths and CMOS‐compatible materials.

Turning Low-Nanoscale Intrinsic Silicon Highly Electron-Conductive by SiO2 Coating.

Impurity doping in silicon (Si) ultra-large-scale integration is one of the key challenges which prevent further device miniaturization. Using ultraviolet photoelectron spectroscopy and X-ray absorption spectroscopy in the total fluorescence yield mode, we show that the lowest unoccupied and highest occupied electronic states of ≤3 nm thick SiO2-coated Si nanowells shift by up to 0.2 eV below the conduction band and ca. 0.7 eV below the valence band edge of bulk silicon, respectively. This nanoscale electronic structure shift induced by anions at surfaces (NESSIAS) provides the means for low-nanoscale intrinsic Si (i-Si) to be flooded by electrons from an external (bigger, metallic) reservoir, thereby getting highly electron- (n-) conductive. While our findings deviate from the behavior commonly believed to govern the properties of silicon nanowells, they are further confirmed by the fundamental energy gap as per nanowell thickness when compared against published experimental data. Supporting our findings further with hybrid density functional theory calculations, we show that other group IV semiconductors (diamond, Ge) do respond to the NESSIAS effect in accord with Si. We predict adequate nanowire cross-sections (X-sections) from experimental nanowell data with a recently established crystallographic analysis, paving the way to undoped ultrasmall silicon electronic devices with significantly reduced gate lengths, using complementary metal-oxide-semiconductor-compatible materials.

Photon-in/photon-out endstation for studies of energy materials at beamline 02B02 of Shanghai Synchrotron Radiation Facility*Project supported by the National Natural Science Foundation of China (Grant No. 11227902) as part of NSFC ME2 beamline project, Science and Technology Commission of Shanghai Municipality, China (Grant No. 14520722100), and the National Natural Science Foundation of China (Grant Nos. 11905283 and U1632269).

A new photon-in/photon-out endstation at beamline 02B02 of the Shanghai Synchrotron Radiation Facility for studying the electronic structure of energy materials has been constructed and fully opened to users. The endstation has the capability to perform soft x-ray absorption spectroscopy in total electron yield and total fluorescence yield modes simultaneously. The photon energy ranges from 40 eV to 2000 eV covering the K-edge of most low Z-elements and the L-edge of 3d transition-metals. The new self-designed channeltron detector allows us to achieve good fluorescence signals at the low photon flux. In addition, we synchronously collect the signals of a standard reference sample and a gold mesh on the upstream to calibrate the photon energy and monitor the beam fluctuation, respectively. In order to cross the pressure gap, in situ gas and liquid cells for soft x-ray absorption spectroscopy are developed to study the samples under realistic working conditions.

The surface chemistry of metallurgical aluminas

We have studied the evolution of surface basicity as a function of calcination temperature in a range of transition alumina‐rich analogues of metallurgical grade aluminas produced by calcination of Bayer gibbsite. Specific basicity, the number of basic sites per unit surface area, was determined by thermometric back‐titration and found to increase with calcination temperature up to ~700°C to 800°C, decreasing dramatically thereafter. The population of tetrahedrally coordinated Al3+, which exhibits a qualitatively similar evolution, was determined by Al K‐edge near‐edge X‐ray absorption fine structure (NEXAFS) spectroscopy but found not to correlate with basicity changes. Interestingly, a shift to higher and then lower binding energy of Al 2p photoelectrons by X‐ray photoelectron spectroscopy and changes in the intensity of O K‐edge NEXAFS spectra do appear to correlate with surface basicity. Exploiting the differing surface sensitivities of NEXAFS spectra collected in partial electron and total fluorescence yield modes, we find O K‐edge spectra intensities, Al 2p X‐ray photoelectron spectrometer binding energies, and surface basicity all reflect the reorganisation of internal surfaces rather than changes in AlO4:AlO6 occupation. Copyright © 2017 John Wiley & Sons, Ltd.

Tuning inner-layer oxygen functional groups of reduced graphene oxide by potentiostatic oxidation for high performance electrochemical energy storage devices

The electrochemical lithiation/delithiation of oxygen-containing functional groups (OCFGs) of nanocarbon materials, particularly graphene, have attracted intensive interest in recent years. Here, we propose a controllable potentiostatic oxidation approach to tune the OCFGs of as-prepared reduced graphene oxide (rGO) in a carbonate-based electrolyte to improve the specific capacity and rate capability. By X-Ray absorption spectroscopy in total fluorescence yield mode and X-Ray diffraction, we confirm that potentiostatic oxidations generate new OCFGs in the inner-layer of rGO. The content of OCFGs increases as oxidation potential being elevated. Such increasing of OCFGs in quantity significantly enhances the capacity. For instance, the specific capacity of 170.4mAhg−1 for pristine rGO electrode is increased to 290.5mAhg−1 after the oxidation at 5.0V. We demonstrate that oxidations at moderate potentials can reduce the electrochemical and ohmic polarizations of rGO electrodes without deteriorating diffusion dynamic, thereby improving rate capability. After the optimal oxidation at 4.7V, rGO electrode exhibits an excellent rate capability, delivering 58.4mAhg−1 at 20Ag−1.

Electronic structure of magnetic Fe/MgO/Fe/Co multilayer structure by NEXAFS spectroscopy

The present work investigates the local electronic structure of magnetic Fe/MgO/Fe/Co multilayer structure by using angle dependent near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. The multilayer stack was grown on Si(100) substrates using electron-beam evaporation. X-ray diffraction study reveals the polycrystalline nature of different layers in the stack. Transmission electron microscopy shows different layers separated with each other except for upper Fe and Co layers. Magnetic nature of deposited structure was confirmed using vibration sample magnetometer. While the Fe L-edge NEXAFS measurements with total electron yield mode exhibit the spectral features that are indicative for formation of FeOx, the total fluorescence yield mode measurements exhibit onset of metallic nature of the Fe layer. These measurements at the Co L-edge reflect the presence of metallic nature of cobalt. Mg K-edge measurements performed at different angles exhibit local electronic structure of MgO layers very much similar to that of bulk MgO. Further, O K-edge fine structure supports the results obtained from various edges for the elements in each layer.

Charge/discharge mechanism of a new Co-doped Li2O cathode material for a rechargeable sealed lithium-peroxide battery analyzed by X-ray absorption spectroscopy

Soft X-ray absorption spectroscopic studies are carried out to clarify the charge/discharge reaction mechanism of Co-doped Li2O (CDL, Co/Li = 0.1 molar ratio) as a cathode material for a new rechargeable lithium-peroxide battery. Upon charging CDL in an aprotic electrolyte, a drastic change can be seen in the O K-edge spectra, with a new, strong peak assignable to σ*(O–O) of peroxide at photon energy of 531.0 eV. This peak is reduced during subsequent discharging, causing the spectrum to essentially return to that of pristine CDL recorded in total fluorescence yield mode. The Co L2,3-edge spectra do not show a remarkable change during charging, with the exception of the disappearance of a Co2+ shoulder peak. The spectrum of charged CDL is in reasonable agreement with the calculated spectrum, assuming that the fraction of Co3+–L (where L indicates a hole state in the oxygen 2p band) is dominant in the electronic configuration of the ground state. This suggests that, to a certain extent, a redox reaction involving a ligand hole state (Co3+–L) participates in generation of the capacity.

Observation of magnetically hard grain boundaries in double-perovskite Sr2FeMoO6

Unusual low-temperature magneto-resistance (MR) of ferromagnetic Sr2FeMoO6 polycrystals has been attributed to magnetically hard grain boundaries which act as spin valves. We detected the different magnetic hysteresis curves for the grains and the grain boundaries of polycrystalline Sr2FeMoO6 by utilizing the different probing depths of the different detection modes of x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD), namely, the total electron yield (TEY) mode (probing depth ) and the total fluorescence yield (TFY) mode (probing depth ). At 20 K, the magnetic coercivity detected in the TEY mode was several times larger than that in the TFY mode , indicating harder ferromagnetism of the grain boundaries than that of the grains. At room temperature, the grain boundary magnetism became soft and and were nearly the same. From the line-shape analysis of the XAS and XMCD spectra, we found that in the grain boundary region the ferromagnetic component is dominated by Fe2+ or well-screened signals, while the non-magnetic component is dominated by Fe3+ or poorly screened signals.

Ferromagnetism of cobalt-doped anatase TiO2 studied by bulk- and surface-sensitive soft x-ray magnetic circular dichroism

We have studied magnetism in anatase Ti1−xCoxO2−δ (x = 0.05) thin films with various electron carrier densities, by soft x-ray magnetic circular dichroism (XMCD) measurements at the Co L2,3 absorption edges. For electrically conducting samples, the magnetic moment estimated by XMCD was <0.3 μB/Co using the surface-sensitive total electron yield mode, while it was 0.3–2.4 μB/Co using the bulk-sensitive total fluorescence yield mode. The latter value is in the same range as the saturation magnetization 0.6–2.1 μB/Co deduced by SQUID measurement. The magnetization and the XMCD intensity increased with carrier density, consistent with the carrier-induced origin of the ferromagnetism.

X-ray spectra and specific features of the structure of lithium-sodium cobaltite Li x Na y CoO2

The electronic structure and specific features of the structure of nonstoichiometric cobaltite Li x Na y CoO2 (x = 0.42, y = 0.36) have been studied comprehensively. The calculated multiplet for the lowspin state of the Co3+ ion agrees with the experimental spectra. It has been established using X-ray absorption spectra measured in the total photoelectric effect yield and total fluorescence yield modes that the Li x Na y CoO2 cobaltite is stoichiometric with respect to the alkali metal near its surface and is defective inside. It has been demonstrated that the charge compensation in the case of an alkali metal deficit in LixNayCoO2 is due to holes in O 2p states.

Orbital and bonding anisotropy in GaFeO 3

The polarization-dependent O K-edge X-ray absorption spectra on GaFeO 3 were simultaneously measured in the total electron yield and total fluorescence yield modes. The spectra measured in both modes display nearly identical polarization dependence, confirming that the polarization dependence is not due to extrinsic artifacts such as the oxygen non-stoichiometry or degradation at the surface but due to the intrinsic nature of this system. We also obtained identical magnetic hysteresis loops in both modes by using the X-ray magnetic circular dichroism at Fe L-edges.