Investigation of hole states near the Fermi level in Nb(1-)(x)Mg(x)B(2) by electron energy-loss spectroscopy and first-principles calculations

The fine structures of the electron energy-loss spectra (EELS) for the B-K edge have been examined in NbB(2) and superconducting Nb(0.75)Mg(0.25)B(2). The experimental results are analyzed based on the calculations of density functional theory (DFT) using the Wien2k code. The results of the EELS spectra and the angular decomposition of the density of states (DOS) reveal that both the B p(z) and B p(x)+p(y) states in NbB(2) have large weights at the Fermi energy due to intersheet covalent bonding with notable hybridization between the Nb 4d and B 2p states. This kind of hybridization also results in different core-hole behaviors for the B-K edge in two orthogonal crystallographic orientations. The best fit between experimental and theoretical data is achieved with consideration of the core-hole effect of the B 1s states, in particular for the q perpendicular c spectra. Analysis of the electronic structure of the Nb(1-)(x)Mg(x)B(2) superconductors suggests that confinement of the intersheet covalent bonding is likely to be favorable for the improvement of superconductivity in this kind of materials.     

Investigation of hexagonal and cubic GaN by high-resolution electron energy-loss spectroscopy and density functional theory

High-resolution electron energy-loss spectroscopy in a transmission electron microscope is a very powerful method for the study of electronic structure of materials. The fine structure of Ga L(2,3) and N ionization edges in c-GaN and h-GaN was studied using a TEM equipped with a monochromator and high-resolution energy spectrometer. The experimental results were compared with the results of calculation based on the density functional theory using the Wien2k code and show that the best fit is achieved when the core hole effect is taken into account. The effect of the core hole value and the supercell size on the energy-loss near-edge structure have been investigated. A different behaviour was found for c-GaN and h-GaN: better agreement is obtained for a 0.5 core hole for h-GaN and for a full core hole for c-GaN. The anisotropic behaviour of the experimental spectra and calculated spectra for h-GaN have been studied and the "magic" angle was determined.     

Orientation dependence of ionization edges in EELS

Anisotropy in the density of unoccupied states can be detected in the fine structure of ionization edges in angle-resolved EELS. It is shown that in a crystal an interference term occurs in the inelastic signal, and how it relates to electron channeling and site selection. The combination of orientation and site selection induces subtle variations in the ELNES. It is shown how this technique can be used to analyze local anisotropy related to the point group of the target atom. A second example shows how to extract non-dipole transitions at small scattering angles.     

Model-based quantification of EELS spectra: Including the fine structure

An extension to model-based electron energy loss spectroscopy (EELS) quantification is reported to improve the possibility of modelling fine structure changes in electron energy loss spectra. An equalisation function is used in the energy loss near edge structure (ELNES) region to model the differences between a single atom differential cross section and the cross section for an atom in a crystal. The equalisation function can be shown to approximate the relative density of unoccupied states for the given excitation edge. On a set of 200 experimental h-BN spectra, this technique leads to statistically acceptable models resulting into unbiased estimates of relative concentrations and making the estimated precisions come very close to the Cramér-Rao lower bound (CRLB). The method greatly expands the useability of model-based EELS quantification to spectra with pronounced fine structure. Another benefit of this model is that one also gets an estimate of the unoccupied density of states for a given excitation edge, without having to do background removal and deconvolution, making the outcome intrinsically more reliable and less noisy.     

Study of changes in L32 EELS ionization edges upon formation of Ni-based intermetallic compounds

EELS L₃₂ ionization edges in several Ni‐based intermetallic compounds have been studied and interpreted in terms of the distribution of electrons in the valence d‐bands. It is demonstrated that the integral EELS cross‐sections change only slightly upon the formation of intermetallic compounds and therefore the charge transfer between atoms is negligible. On the other hand, the changes in the fine energy‐loss near‐edge structure (ELNES) of the Ni L₃ edge can be readily detected indicating an important redistribution of d‐electrons at the Ni site with alloying. These features are well reproduced by ab‐initio calculations with a FLAPW method in its WIEN97 implementation. In contrast to the drastic effect of chemical environment, structural transformations in the investigated intermetallics result in smaller ELNES changes, which can be detected by only exceptional instruments with a higher energy resolution.     

Simulation of electron energy loss near-edge structure at the Al and N K edges and Al L(23) edge in cubic aluminium nitride

Calculations of electron energy near edge structures (ELNES) are compared with experimental data obtained in a high-resolution transmission electron microscope. This study concerns small precipitates of aluminium nitride in low carbon steel. The ELNES technique allows to clearly establish that these precipitates crystallize in a cubic rather than in a hexagonal crystallographic cell. The influence on simulated spectra of different parameters are investigated: the size of the atomic shell and its relation with the electron inelastic mean free path. We also examine the influence of the core hole and the sensitivity to cell parameters. We particularly examine the Al L(23) near edge structure and features relating to the different transition channels (A(1g), E(g) and T(2g)). Results of a multiple scattering and band structure calculations using ICXANES and WIEN97 codes, respectively, are compared in the region from 0 to 30 eV above the edge onsets. Both calculations are in a rather good agreement.     

Electron energy loss near edge structure on the nitrogen K-edge in vanadium nitrides

This paper presents electron energy-loss near-edge structure (ELNES) data for the N K edges of vanadium nitrides. By rapid thermal processing of vanadium layers in pure nitrogen at high temperatures the two known vanadium nitrides, VN and V₂N, have been prepared. The phases have been checked by electron diffraction and quantitative electron energy-loss spectroscopy (EELS) analysis. Because their crystallographical structures are different, they also exhibit different ELNES features, which can be used as fingerprints for rapidly distinguishing between VN and V₂N. The experimental findings are supported by modelling the N K edge using a band structure approach (full linearized augmented plane wave method).     

Preparation of location-specific thin foils from Fe–3% Si bi- and tri-crystals for examination in a FEG-STEM

Bi-crystals and tri-crystals of a nominal Fe–3% Si (wt%) of well-defined orientations have been grown using a floating-zone technique with optical heating. The manufacture of these unique crystals and the preparation technique involved in harvesting thin foils from specific locations for transmission electron microscopy are described in detail. In particular, the grain boundary triple junction has been extracted from the tri-crystal and examined in high-resolution aberration-corrected FEG-STEM instruments. To achieve the necessary resolution, the foils have to be uniformly thin, in the range 50–100 nm over large areas of the specimen. For ferromagnetic materials, there are further challenges arising from the magnetic field interaction, with the electron beam placing significant demands on the aberration correction system. One way to minimise this interaction is to reduce the total mass of magnetic material. To achieve this, an in situ focused ion beam lift-out technique has been combined with an additional precision ion-polishing stage to reproducibly provide thin-foil specimens suitable for high-resolution EELS and EDX analysis. Examination of the foils reveals that the final precision ion-polishing stage removes residual damage arising from the use of focused ion beam milling procedures.     

Effect of laser pulsing on the composition measurement of an Al–Mg–Si–Cu alloy using three-dimensional atom probe

The effect of laser pulse energy on the composition measurement of an Al–Mg–Si–Cu alloy (AA6111) specimen has been investigated over a base temperature range of 20–80 K and a voltage range of 2.5–5 kV. Laser pulse energy must be sufficiently higher to achieve pulse-controlled field evaporation, which is at least 0.9 nJ with a beam spot size of about 5 μm, providing an equivalent voltage pulse fraction, ∼14% at 80 K for the alloy specimen. In contrast to the cluster composition, the measured specimen composition is sensitive to base temperature and laser energy changes. The exchange charge state under the influence of laser pulsing makes the detection of Si better at low base temperature, but detection of Cr and Mn is better at a higher temperature and using higher laser energy. No such effect occurs for detection of Mg and Cu under laser pulsing, although Mg concentration is sensitive to the analysis temperature under voltage pulsing. Mass resolution at full-width half-maximum is sensitive to local taper angle near the apex, but has little effect on composition measurement.     

Analysis of computational EELS modelling results for MgO-based systems

The ab-initio density functional theory (DFT) code CASTEP was used to model oxygen K edges in various magnesium oxide systems. Firstly, for the bulk material the process of geometry optimisation was carried out. Predicted oxygen K edges were found for a single cell with experimental lattice parameters, and parameters obtained after geometry optimisation, both with single electron core-holes in place. After geometry optimisation, a different predicted result was obtained, although it was qualitatively similar to the result for experimental lattice parameters in some respects. For example, approximately the same sets of peaks are observed, though in different energy positions, and with different relative peak intensities within those sets. Ultimately for the single cell results the experimental lattice parameters generated the predicted result that was in the closest agreement with experiment. It was further observed that a large supercell result (based on the experimental lattice parameters, utilising a core-hole) led to a slightly improved comparison with experiment as compared to the corresponding single cell result, although the latter result, and indeed a ground state calculation also give reasonable agreement with experiment. To rationalise these observations it was necessary to investigate the density of states (DOS) for the MgO cell and its constituent atoms, and it was observed that the conduction bands were of predominantly magnesium character. Furthermore, the core-hole’s introduction had relatively little overall effect on the p DOS prediction for oxygen, though there is a significant localised change close to the Fermi level. This work also considers interface and surface results. The principal aim of the study was to explore the interface of Fe (0 0 1)/MgO (0 0 1), crucial in certain classes of magnetic tunnel junctions (MTJs), which have significant technological applications. An initial step was to consider a MgO (0 0 1) surface. It was verified that a surface could be constructed such that within that surface a theoretical result could be found that matched the bulk result. It was then valid to use this surface as part of an interface with iron. Theoretical results obtained at that interface compare well with experimental results from an epitaxially grown MTJ, and various conclusions are drawn with regard to the nature of the interface.     

Electron energy-loss near edge structure (ELNES) of InGaN quantum wells

Lasers and light‐emitting diodes (LEDs) that emit in the blue to green region are often based on In_xGa_1–xN quantum well structures. Ionization edges in the electron energy‐loss spectrum contain fine structures (called the energy‐loss near edge structure (ELNES)) and provide information about the electronic structure. In this paper we compare the experimental and calculated ELNES for the N‐K ionization edge of In_xGa_1–xN quantum wells. When the effects of the core‐hole are included in the calculations, agreement between experimental and calculated spectra is very good. Strain has been shown to accentuate the effects of In on the ELNES and moves the ionization edge onset down in energy, relative to the other features. These results suggest that ELNES may provide an alternative method to lattice imaging to determine the presence of strain in this system.     

Experimental study of ELNES at grain boundaries in alumina: intergranular radiation damage effects on Al-L23 and O-K edges

The need for understanding the structural and chemical properties of interfaces and grain boundaries in materials is being paralleled by new developments in transmission electron microscopy methods. The extraction of data on grain boundaries has to be carefully evaluated, freed from artefacts and allowing fairly direct interpretation. Besides improvements in data processing, a primary requirement is an improved knowledge of the ELNES fine structures of the relevant absorption edge. For the case of alumina, we first present a compilation of Al-L(23) edges with an energy resolution of 0.5-0.6eV. This allows identification of the ELNES features which are more likely to vary as a function of the aluminium atomic environment, i.e. the excitonic peaks between 77 and 79eV and the near-edge features between 83 and 86eV, which both clearly depend on the Al site environment. Our second investigation concerns the likely occurrence of electron radiation damage at interfaces which may adversely interfere with the identification of new bonding types. The Al-L(23) and O-K ELNES changes associated with several cases of damage are detailed.     

Study of structures at the boundary and defects in organic thin films of perchlorocoronene by high-resolution and analytical transmission electron microscopy

Both the periodic and non-periodic structures of perchlorocoronene (C₂₄Cl₁₂) crystals were characterized by high-resolution transmission electron microscopy (HRTEM), electron diffraction (ED), electron energy-loss spectroscopy (EELS), and energy-filtered transmission electron microscopy (EFTEM). The HRTEM images at the boundary of the C₂₄Cl₁₂ crystals exhibit the flexibility of defect structures, where molecules align to compensate for the discontinuity between two different domains. Emphasized by the filtered images, it was found that the non-periodic regions are created everywhere with a small electron beam irradiation (∼10⁶ electrons nm⁻²) and then spread over the entire regions to completely destroy the periodic structures after a higher electron dose (∼2×10⁶ electrons nm⁻²). The effect of the electron beam irradiation was monitored by ED, EELS, and EFTEM, where periodic structures and content elements are well preserved up to 10⁶ electrons nm⁻², but chlorine atoms decreased with a much higher electron dose. This is explained by the breakage of the C–Cl bond to detach chlorine atoms, confirmed by energy-loss near the edge structures (ELNES) of carbon π? peaks and chlorine loss at the edge of the specimen, as well as by theoretical simulation. The detachment of chlorine is localized at the peripheral edge around a hole confirmed by core-loss EFTEM imaging.     

Determination of chromium valence over the range Cr(0)–Cr(VI) by electron energy loss spectroscopy

Chromium is a redox active 3d transition metal with a wide range of valences (−2 to +6) that control the geochemistry and toxicity of the element. Therefore, techniques that measure Cr valence are important bio/geochemical tools. Until now, all established methods to determine Cr valence were bulk techniques with many specific to a single, or at best, only a few oxidation state(s). We report an electron energy loss spectroscopy (EELS) technique along with an extensive suite of affined reference spectra that together, unlike other methods, can determine Cr valence (or at least constrain the possible valences) at high-spatial resolution (tens-of-nanometer scale) across a wide valence range, Cr(0)–Cr(VI). Fine structure of Cr-L_2,3 edges was parametrized by measurement of the chemical shift of the L₃ edge and the ratio of integrated intensity under the L₃ and L₂ edges. These two parameterizations were correlated to Cr valence and also the dⁿ orbital configuration which has a large influence on L-edge fine structure. We demonstrate that it is not possible to unambiguously determine Cr valence from only one fine-structure parameterization which is the method employed to determine metal valence by nearly all previous EELS studies. Rather, multiple fine-structure parameterizations must be used together if the full range of possible Cr valences is considered. However even with two parameterizations, there are limitations. For example, distinguishing Cr(IV) from Cr(III) is problematic and it may be difficult to distinguish low-spin Cr(II) from Cr(III). Nevertheless, when Cr is known to be divalent, low- and high-spin dⁿ orbital configurations can be readily distinguished.     

The relative importance of multiple inelastic scattering in the quantification of EELS

A comparison is given of energy loss results obtained for the L and K edges of aluminium as a function of specimen thickness, crystallographic orientation and collection angle. It is demonstrated that as the thickness is increased post-loss elastic scattering is generally important in reducing the fraction of electrons collected. The implications for the quantification of EELS data are discussed while a comparison of the Fe/C ratio in cementite demonstrates the improved consistency which can be obtained when comparing K and L losses at lower energy separation than are the losses for aluminium.     

Characterisation of the early stages of solute clustering in 1Ni–1.3Mn welds containing Cu

Microstructural characterisation of neutron irradiated low alloy steels is important for developing mechanistic understanding of irradiation embrittlement. This work is focused on the early stages of irradiation-induced clustering in a low Cu (0.03 wt%), high Ni (∼1 wt%) weld. The weld was irradiated at a very high dose rate and then examined by atom probe (energy-compensated position-sensitive atom probe (ECOPoSAP) and local electrode atom probe (LEAP)) with supporting microstructural information obtained by small angle neutron scattering (SANS) and positron annihilation (PALA).     

Development of a high energy resolution electron energy-loss spectroscopy microscope

We have developed a high energy resolution electron energy-loss spectroscopy (EELS) microscope, which can take spectra from specified small specimen areas and specified small reciprocal space areas to investigate detailed electronic structures. The EELS microscope is equipped with retarding Wien filters as the monochromator and the analyser. The filters are designed to achieve a stigmatic focus. The energy resolutions are 12 meV and 25 meV for cases without and with a specimen, respectively. Spatial and momentum resolutions are 30–110 nm in diameter and 1.1 nm⁻¹ in angular diameter, respectively. EELS spectra are presented to show the performance of this instrument.     

Laterally resolved EELS for ELNES mapping of the Fe L 2,3 - and O K-edge

Nowadays fingerprinting techniques are well established for phase analysis. One of the common problems is the accurate calibration of the energy scale to compare the electron energy loss (ELNES) and to determine the energy shift precisely. One solution to this problem is laterally resolved electron energy loss spectroscopy (EELS), which involves orienting the specimen area or structure of interest, parallel to the energy dispersive direction and dispersing the intensity across the interface as a function of energy. This ELNES information can now be used to quantify and map changes in the electronic environment. The most critical instrumental performance for ELNES investigations is the available energy resolution, which for our instrument was estimated using the 0.5eV splitting of the Mn L(3)-edge of the mineral bixbyite. An ideal test sample for the ELNES investigations is a titanohematite, a solid solution between ilmenite (FeTiO(3)), with Fe in a divalent oxidation state, and hematite (Fe(2)O(3)) with Fe in a trivalent oxidation state. Using energy filtered imaging with a slit width of 4eV it is possible to map the Fe(2+)/Fe(3+) ratio as well as the near-edge structure of the O(K) signal and correlate these ELNES maps with a spatial resolution of a few nanometres. Quantitative compositional mapping on a nanometre scale was obtained by electron spectroscopic imaging. Quantitative point analyses also yield the chemical composition and the valence states. The precise knowledge of the energy shift and near edge structure enables us to select the characteristic ELNES structure and calculate jump ratio images. This yields quantitative valence state maps by using the Fe L(2,3)-edge, as well as phase maps by using the O K-edge.