EELS analysis of cation valence states and oxygen vacancies in magnetic oxides

Transition metal oxides are a class of materials that are vitally important for developing new materials with functionality and smartness. The unique properties of these materials are related to the presence of elements with mixed valences of transition elements. Electron energy-loss spectroscopy (EELS) in the transmission electron microscope is a powerful technique for measuring the valences of some transition metal elements of practical importance. This paper reports our current progress in applying EELS for quantitative determination of Mn and Co valences in magnetic oxides, including valence state transition, quantification of oxygen vacancies, refinement of crystal structures, and identification of the structure of nanoparticles.     

Comparison of the electronic structure of a thermoelectric skutterudite before and after adding rattlers: an electron energy loss study

Skutterudites, with rattler atoms introduced in voids in the crystal unit cell, are promising thermoelectric materials. We modify the binary skutterudite with atomic content Co(8)P(24) in the cubic crystal unit cell by adding La as rattlers in all available voids and replacing Co by Fe to maintain charge balance, resulting in La(2)Fe(8)P(24). The intention is to leave the electronic structure unaltered while decreasing the thermal conductivity due to the presence of the rattlers. We compare the electronic structure of these two compounds by studying the L-edges of P and of the transition elements Co and Fe using electron energy loss spectroscopy (EELS). Our studies of the transition metal white lines show that the 3d electron count is similar for Co and Fe in these compounds. As elemental Fe has one electron less than Co, this supports the notion that each La atom donates three electrons. The L-edges of P in these two skutterudites are quite similar, signalling only minor differences in electronic structure. This is in reasonable agreement with density functional theory (DFT) calculations, and with our multiple scattering FEFF calculations of the near edge structure. However, our experimental plasmon energies and dielectric functions deviate considerably from predictions based on DFT calculations.     

Fe valence determination and Li elemental distribution in lithiated FeO₀.₇F₁.₃/C nanocomposite battery materials by electron energy loss spectroscopy (EELS)

Electron energy loss spectroscopy (EELS) is a powerful technique for studying Li-ion battery materials because the valence state of the transition metal in the electrode and charge transfer during lithiation and delithiation processes can be analyzed by measuring the relative intensity of the transition metal L₃ and L₂ lines. In addition, the Li distribution in the electrode material can be mapped with nanometer scale resolution. Results obtained for FeO_0.7F_1.3/C nanocomposite positive electrodes are presented. The Fe average valence state as a function of lithiation (discharge) has been measured by EELS and results are compared with average Fe valence obtained from electrochemical data. For the FeO_0.7F_1.3/C electrode discharged to 1.5 V, phase decomposition is observed and valence mapping with sub-nanometer resolution was obtained by STEM/EELS analysis. For the lowest discharge voltage of 0.8 V, a surface electrolyte inter-phase (SEI) layer is observed and STEM/EELS results are compared with the Li-K edges obtained for various Li standard compounds (LiF, Li₂CO₃ and Li₂O).     

Resonant electron exchange scattering in late transition metal monoxides

Intra-atomic d-d transitions in NiO(100) and CoO(100) have been investigated with angle-resolved electron energy loss spectroscopy (EELS) at primary energies close to the metal 3s excitation threshold. Electron exchange scattering was found to be resonantly enhanced at the 3s threshold due to the temporary formation of a negative ion core state and its subsequent decay via Auger-like transitions. In both oxides the threshold is lowered several eV due to a strong electron- core hole interaction. Angle-dependent studies reveal a different dependency of exchange processes on the scattering angle as compared with nonresonant measurements and reveal a different angle dependence for each specific d-d transition. It is suggested that in these oxides large-angle single-step inelastic scattering contributes significantly to the EELS measurements in reflection mode.     

Carbon–metal interfaces analyzed by aberration-corrected TEM: How copper and nickel nanoparticles interact with MWCNTs

Experimental confirmation for the stronger interaction of Ni with multi-walled carbon nanotubes (MWCNTs) compared to Cu with MWCNTs is presented. The interfaces between Cu (Ni) nanoparticles side-on oriented onto MWCNTs are analyzed with high spatial resolution electron energy-loss spectroscopy (EELS) of the carbon K-edge. The EEL spectra reveal a rehybridization from sp² to sp³ hybridized carbon of the outermost MWCNT layer at the Ni interface, but no such rehybridization can be observed at the Cu interface. The EELS results are supported by transmission electron microscopy (TEM) images, which show a better wetting behavior of Ni and a smaller gap at the Ni–MWCNT interface, as compared to the corresponding Cu interfaces. The different behavior of Cu and Ni can be explained in terms of differing valence d-orbital occupancy. For the successful experimental demonstration of this effect the use of a soft chemical metal deposition technique is crucial.     

A STEM/EELS method for mapping iron valence ratios in oxide minerals

The valence state of iron in minerals has useful applications in the geosciences for estimating redox conditions during mineral formation or re-equilibration. STEM/EELS techniques offer the advantage over other methods of being able to measure Fe valence with very high spatial resolution across mineral grains and grain boundaries. We have modified an EELS method for point analyses of iron valence ratios (Fe(3+)/SigmaFe) making it possible to generate line scans and maps of Fe valence. We demonstrate this method with measurements at an interface between iron-bearing oxides in a finely intergrown sample of magnetite and ilmenite. The STEM/EELS method is based on a calibrated relationship between Fe(3+)/SigmaFe and the relative intensities of the Fe L(3) and L(2) white lines in core energy-loss spectra for oxide and silicate minerals. Our method overcomes problems of energy drift in spectrum images by aligning energy-loss edges at a fixed energy position prior to background removal. An automated routine for batch processing of core loss spectra, including additional background removal and calculation of Fe L(3)/L(2) intensity ratios, allows for rapid Fe(3+)/SigmaFe determinations of multiple point analyses or spectrum images and the preparation of Fe valence maps, with an analytical error of +/-0.05 to +/-0.09 in the Fe(3+)/SigmaFe measurements.     

Intrinsic ferromagnetism in insulating cobalt doped anatase TiO2

Using complementary experiments we show that the room temperature ferromagnetism observed in anatase Co:TiO(2) films is not carrier mediated, but coexists with the dielectric state. TEM and x-ray absorption spectroscopy reveal a solid solution of Co in anatase, where Co is not metallic but in the +2 state substituting for Ti. Measurements at 300 K yield a M(S) of 1.1 mu(B)/Co atom, while all films are highly insulating. The evidence of intrinsic ferromagnetism in the dielectric ground state of Co:TiO(2) leads to new considerations for the origin of ferromagnetism in transition metal doped oxides.     

Electron energy loss spectroscopy analysis of the interaction of Cr and V with MWCNTs

The presented scanning transmission electron microscopy (STEM) and electron energy-loss spectroscopy (EELS) results show the strong reaction of Cr and V with the graphitic walls of MWCNTs. For Vanadium, an interfacial VC layer could be observed at the interface between VN and MWCNTs, when the samples were heated in situ to 750 °C. Knowledge about this interfacial VC layer is important for the formation of VN-MWCNT hybrid materials, used in supercapacitor electrodes, often synthesized at high temperatures. Chromium reacts at 500 °C with the MWCNTs to form Cr₃C₂ and in some cases, dissolved the MWCNT completely. Together with the previously published results about the interaction of MWCNTs with Cu (no interaction) and Ni (a slight rehybridisation trend for the outermost MWCNT-wall observed with EELS) (Ilari et al., 2015) the influence of the valence d-orbital occupancy of 3d transition metals on the interaction strength with CNTs is shown experimentally. For a transition metal to form chemical bonds towards CNT-walls, unoccupied states in its valence d-orbitals are needed. While Ni (2 unoccupied states) interacts only slightly, Cr (5 unoccupied states) and V (7 unoccupied states) react much stronger and can dissolve the MWCNTs, at least partially.     

Nanomaterial electronic structure investigation by valence electron energy loss spectroscopy - an example of doped ZnO nanowires

The effects of doping (by ion implantation) on the electronic structure of ZnO nanowires, particularly on the defect states generation in the band gap of ZnO, are investigated using valence electron energy loss spectroscopy (VEELS) performed in a transmission electron microscope (TEM). The improved spectrum energy resolution via the introduction of a gun monochromator, together with the reduced intensity in the zero loss peak tail as realized by spectrum acquisition at non-zero momentum transfer, enable us to extract such electronic structure information from the very low loss region of the EEL spectra. We have compared the doping effects of several dopant elements, i.e., Er, Yb, and Co, and found that generation of the band tail states ( approximately 2-3.3eV) is a common consequence of the ion implantation process. On the other hand, specific mid-gap state(s) in the lower energy range are created only in the rare earth element doped ZnO nanowires, suggesting the dopant-sensitive nature of such state.     

Characteristic chemical shifts of quasicrystalline Zn–Mg–Zr alloys studied by EELS and SXES

Chemical shifts of the constituent atoms of primitive icosahedral quasicrystal (P-QC), face-centred icosahedral quasicrystal (F-QC) and 1/1-approximant (1/1-AP) of F-QC Zn–Mg–Zr alloys were investigated for the first time using high energy-resolution electron energy-loss spectroscopy (EELS) and soft-X-ray emission spectroscopy (SXES). Among Zn M-shell and Mg L-shell excitation EELS spectra of P-QC, F-QC and 1/1-AP alloys, only the quasicrystalline alloys showed a chemical shift towards the larger binding energy side. In Zn-L and Zr-L emission SXES spectra, the P-QC and F-QC alloys showed a chemical shift towards larger binding energy side. The magnitudes of the shifts in the Zn-L emission spectra of the quasicrystalline alloys were almost the same as for ZnO. These results strongly suggest a decrease in valence charge in quasicrystalline states. Therefore, it should be concluded that bonding in quasicrystalline states involves a characteristic increase in covalency compared with bonding in corresponding approximant and standard metal crystals.     

典型磁性材料价电子结构研究面临的机遇与挑战

目前在磁性材料磁有序现象研究中广泛使用的交换作用、超交换作用和双交换作用模型形成于1950年代及其以前,这些模型都涉及材料中的价电子状态,但那时还没有充分的价电子状态实验依据.1970年代以来,有关价电子结构实验结果的报道越来越多,这些实验结果表明传统的磁有序模型需要改进.首先,大量电子谱实验表明,在氧化物中除存在负二价氧离子之外,还存在负一价氧离子,并且负一价氧离子的含量可达30%或更多.这说明以所有氧离子都是负二价离子为基本假设的超交换和双交换作用模型需要改进.其次,一些实验证明,铁、钴、镍自由原子的一部分4s电子在形成铁磁性金属的过程中变成了3d电子,这为探讨金属磁性与电输运性质的关系提供了依据.此外,即使在现代的密度泛函计算中,仍不能给出磁性交换作用能的函数表达式,只能采取各种不同模型进行模拟计算,从而使磁性材料的模拟计算遇到严重困难.寻求一个磁有序能的函数表达式可能是解决这个困难的途径.这些研究表明磁性材料价电子结构研究面临着重大的机遇与挑战.本文首先介绍一些典型的实验例证,然后介绍了基于这些实验结果的一套典型磁性材料的磁有序新模型,随后介绍了基于新模型的磁性材料价电子结构与旧模型的主要区别,最后指出了未来研究工作面临的挑战.     

Carrier mobility of iron oxide nanoparticles supported on ferroelectrics studied by Mössbauer spectroscopy

⁵⁷Fe Mössbauer spectroscopy was performed on two types of Fe oxide nanoparticles supported on a typical ferroelectric, BaTiO3. It was found that the valence state of FeO nanoparticles changed to a mixed 2+/3+ state at high temperature where BaTiO₃ shows paraelectric behaviour. We attribute this phenomenon to the fluctuation of electric dipoles which realizes carrier injection into the Fe oxides. This is the first report which discusses a dynamical valence state of transition metal oxides supported on ferroelectrics.     

Oxidation states of Mn and Fe in various compound oxide systems

Energy-loss near-edge structure (ELNES) data of Mn-L(2,3) and Fe-L(2,3) ionization edges have been measured by means of quantitative electron energy-loss spectroscopy (EELS) for two series of Mn and Fe oxides with known formal cation oxidation states. In both series the absolute energy positions of Mn-L(2,3) and Fe-L(2,3) white-lines, as well as the white-line intensity ratio (L3/L2) vary with cation oxidation states. Additionally, spin-orbit spitting, i.e. the energy difference deltaE(L2-L3) between Mn-L(2,3) white-lines decreases with increasing Mn oxidation state. With these data from known standards calibration curves on white-line intensity ratio Mn(L3/L2) vs. Mn oxidation state, and Fe(L3/L2) vs. Fe oxidation state were established. EELS measurements on Mn and Fe doped ZnO thin films showed that the valence states of the dopants can unambiguously be determined by calibrating the Mn-L(2,3) and Fe-L(2,3) ELNES data against the measured standards. It is revealed that Mn in ZnO adopt a divalent state, thus Mn2+ ions substitute for Zn2+, whereas Fe dopants retain a trivalent oxidation state in the ZnO host lattice. Measurements on (Ba, Fe, Mn)-oxides revealed that both Fe and Mn cations are in a trivalent state. Thus, it is assumed that Mn3+ can partially be substituted for Fe3+ in barium hexaferrites.     

Electron spectroscopy of chemically synthesized ZnS clusters

Thin films of ionic compounds of ZnS clusters were measured by electron energy loss spectroscopy (EELS) and ultraviolet photoelectron spectroscopy (UPS). A size effect was observed in the valence plasmon energy measured by EELS from which the coherence length of the plasmon excitation can be estimated. The difference between the lowest excitations observed in UPS and EELS can be explained by the final state charging effect of a single cluster ion in UPS, which strongly depends upon the nominal charges of the clusters.     

Investigating carbonization and graphitization using electron energy loss spectroscopy (EELS) in the transmission electron microscope (TEM)

Electron energy loss spectroscopy (EELS) in the transmission electron microscope (TEM) is explored as a useful characterization technique in the study of carbonization and graphitization of organic precursors. A model series of carbon materials was prepared from highly graphitizable petroleum pitch heat treated in the range 200–2730°C. Initial characterization was performed using the established techniques of X-ray diffraction (XRD), He pycnometry, TEM, electron diffraction and high-resolution lattice imaging (HREM). EELS in the TEM was then examined. Two routes are presented to quantify the change in the proportion of sp ² type hybridization accompanying the heat treatment as the material transforms to the graphitic state. Both routes suggest an initial relative sp ² content of ～70%, rapidly increasing to ～90% during mesophase development and carbonization, and then slowly increasing to 100% during graphitization. The peak position of the bulk valence plasmon (π?+?σ) is shown to be an excellent measure of the degree of graphitic character, and its fundamental dependence upon sample density (ρ) is confirmed. The appearance and definition of features within the core loss region representing the density of unoccupied σ* states are demonstrated to be an excellent measure of the extent of order. Finally, a method is established by which to extract the C–C bond length from core loss EELS spectra with an accuracy of ±0.1?pm. This method suggests an average bond length of 1.44?Å in samples with low heat treatment temperatures, decreasing to the theoretical length of 1.42?Å as both the heteroatom content and proportion of non-sp ²-type hybridized carbon atoms decrease.     

Spin-dependent valence-band density of states of 3d ferromagnets

Spin-resolved X-ray photoelectron spectroscopy (SRXPS) and high resolution X-ray photoelectron spectroscopy (HRXPS) studies of the valence bands of ferromagnetic Fe, Co, Co₆₆Fe₄Ni₁B₁₄Si₁₅ and Ni are reported. The SRXPS and HRXPS spectra are compared with theoretical densities of states (DOS) that are corrected for photoelectric cross section variations within the valence band. Agreement between theory and experiment is very good for ferromagnetic Fe and Co₆₆Fe₄Ni₁B₁₄Si₁₅. For Co metal, experiment agrees poorly with theory incorporating a 1.5 eV exchange splitting. Agreement is improved if a reduced Co exchange splitting of 1.2 eV is adopted theoretically. The reduced exchange splitting is attributed to valence electron correlation in Co metal. Ferromagnetic Ni shows poor agreement between experiment and theory. The SRXPS Ni spectra demonstrate that most of the disagreement concerns the ↑-spin channel.     

Theory of core-level spectroscopy in f and d electron systems

A review is presented of many body effects in core-level spectroscopy (CLS) of f and d electron systems from a theoretical point of view. Historical developments and the most recent topics in this field are described. The impurity Anderson model (IAM) has been successfully applied to the analysis of X-ray photoemission spectra (XPS) and X-ray absorption spectra (XAS) in f and d electron systems, where the f and d electron states are treated as being on a single atomic site and they are hybridized with valence or conduction electron states. The effect of a core-hole potential in the final state of CLS plays an important role. Typical examples of calculated results for XPS in rare-earth compounds and transition metal compounds are given. Recent developments in the study of resonant X-ray emission spectra (RXES) are also introduced. A theoretical approach beyond the IAM is discussed mainly for the analysis of RXES of transition metal compounds.     

Co:BaTiO3/Si(100)纳米复合薄膜制备、微结构及其紫外光电子能谱研究

采用脉冲激光气相沉积(PLD)方法，在Si(100)晶面上制备了Co:BaTiO₃纳米复合薄膜.采用X射线衍射(XRD)结合透射电镜(TEM)方法研究了两种厚度Co:BaTiO₃纳米复合薄膜的晶体结构，当薄膜厚度约为30 nm时，薄膜为单一择优取向；当薄膜厚度约为100nm时，薄膜呈多晶结构.原子力显微镜(AFM)分析表明，当膜厚为30nm时，薄膜呈现明显的方形晶粒.采用紫外光电子能谱(UPS)研究了Co的价态和Co:BaTiO₃纳米复合薄     

Co:BaTiO3/Si(100)纳米复合薄膜制备、微结构及其紫外光电子能谱研究

采用脉冲激光气相沉积(PLD)方法,在Si(100)晶面上制备了Co:BaTiO₃纳米复合薄膜.采用X射线衍射(XRD)结合透射电镜(TEM)方法研究了两种厚度Co:BaTiO₃纳米复合薄膜的晶体结构,当薄膜厚度约为30 nm时,薄膜为单一择优取向;当薄膜厚度约为100nm时,薄膜呈多晶结构.原子力显微镜(AFM)分析表明,当膜厚为30nm时,薄膜呈现明显的方形晶粒.采用紫外光电子能谱(UPS)研究了Co的价态和Co:BaTiO₃纳米复合薄 关键词： 3')" href="#">BaTiO₃ 纳米复合薄膜 紫外光电子能谱     

Intensity inversion between main and satellite lines in atomic photoionization

The 1s photoionization of atomic Li was studied by photoelectron spectroscopy in the photon energy region between 85 and 140 eV for the ground state and the three lowest excited configurations Li(*) 1s(2)nl, nl=2p, 3s, 3p. The importance of electron correlations was investigated by comparing the multielectron transitions, so-called shake-up and conjugate shake-up satellites, and the direct process, so-called main lines. The relative intensity of the satellites increases with the level of initial excitation of the Li atom. The shake-up process dominates for states with an n=3 valence electron and the satellites become stronger than the main lines. This spectacular effect can be explained by the spatial overlap of the initial and final state wave functions. Surprisingly, the spatial overlap affects shake-up and conjugate shake-up lines in the same way.