Parallel Electron Energy Loss Spectroscopy Research Articles

Analysis of the electronic properties of CoCrPt thin films using parallel electron energy loss spectroscopy (PEELS)

The effects of the Pt content in CoCrPt thin films on the appearance of the near-edge structure of the Cr and Co L2,3 absorption edges were studied using parallel electron energy loss spectroscopy (PEELS). These reflect local changes in the 3d band of the material. It was found that addition of Pt leads to local fluctuations in the 3d band occupancy in CoCrPt films which causes a decrease of the number of 3d electrons per Co atom. An increase of the L3 to L2 white lines intensities ratio on the L2,3 Co edge with Pt content probably indicates an increase of the magnitude of exchange interactions in the CoCrPt thin films.

The microstructure of inclusions in nanocrystalline carbon films deposited at low temperature

Nanocrystalline growth has been observed in carbon films deposited from a CH 4-Ar r.f. plasma. The films were deposited on Si and glass substrates held at low temperature. X-ray analysis of the films grown on n-type Si (100) indicates the presence of diamond crystals up to 50 nm in diameter, with some evidence for oriented growth in the [111] direction parallel to Si [100]. The X-ray data also indicate the presence of SiC with the diamond. Transmission electron microscopy of the films deposited on glass shows smaller crystals up to 10 nm in diameter which can group themselves into large dendrite-like features. Parallel electron energy loss spectroscopy of these smaller crystals and the surrounding amorphous matrix confirms that the crystals are predominantly carbon. However, the electron diffraction data are not consistent with those expected for diamond or graphite. Instead, the five diffraction rings observed from these crystals can be fitted either to a tetragonal structure with a = 5.00 A ̊ and c = 3.26 A ̊ or to a hexagonal structure with a = 4.46 A ̊ and c = 3.26 A ̊ . Further work is required to be able either to distinguish between these two possibilities or perhaps to explain the ring sequence in terms of a completely different crystal structure.

Electron-beam-induced reduction of Mn 4+ in manganese oxides as revealed by parallel EELS

Chemical reduction of Mn 4+ in the mineral asbolan was examined using parallel electron energy-loss spectroscopy (PEELS). Changes on the oxidation state of the manganese were monitored by recording the electron energy-loss near-edge structure (ELNES) for the Mn M 2,3- and L 2,3-edges. The edges in the undamaged material were consistent with Mn 4+. Electron beam irradiation of the mineral caused the ionisation edges to change in shape and energy consistent with the decrease in the proportion of the Mn 4+ and the increase in the proportions of Mn 3+ and Mn 2+. The final edge is very similar in energy and shape to that of Mn 2+ in rhodochrosite (MnCO 3) and manganosite (MnO) and the differences in shape can be accounted for by the presence of a small amount of residual Mn 3+.

EELS of colloids in Mg+ implanted MgAl2O4 spinel

Because magnesium aluminate spinel (MgAl2O4) shows a strong resistance to void swelling during neutron irradiation at elevated temperatures, it is a candidate material for specialized applications in proposed fusion reactors. During implantation at 25°C with 2 MeV Mg+ ions to ∼2.8 × 1021 Mg+/m2, dislocation loops are formed at midrange depths (∼0.5 - 1.0 μm) on {110} and {111}. The microstructurc in the implanted ion region (∼1.5 - 2.0 μm) is shown in cross-section in Fig. 1. Within this implanted ion region, small features (4 - 10 nm diam.) were observed in dark field (DF) images using a spinel 222 reflection (Fig. 2). No evidence was found in electron diffraction patterns to suggest these features are (hexagonal) metallic Mg. However, in an earlier study, similar features in Al+ implanted spinel were identified by parallel electron energy loss spectrometry (PEELS) as metallic Al colloids. Phase identification of metallic Al within this spinel by electron diffraction is complicated because the lattice parameter of spinel (0.8083 nm) is almost exactly twice that of aluminum (0.4049 nm) and the phases are oriented cube-on-cube.

Characterization of BN Rich Layer on Ammonia Treated Nextel™312 Fibers

ABSTRACTA BN rich layer grown on Nextel™312 fibers by appropriate ammonia treatments was evaluated using various complimentary techniques including X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Scanning Electron Microscopy (SEM), and Transmission Electron Microscopy (TEM)/ Parallel Electron Energy Loss Spectroscopy (PEELS in TEM). Three different ammonia treatments were studied. Ammonia treatment resulted in crystallization of the Nextel™312 fiber. The BN rich surface layer formed due to ammonia treatment was clearly detected in XPS and PEELS both before and after oxidation. The layer thickness was estimated to be between 5–10 nm. The layer was stable after oxidation treatment at 600°C for 100 hours. High resolution TEM observations of the fiber surface revealed a variable BN rich layer thickness. Patches of turbostratic BN were observed under certain conditions, however mostly the layer appeared to be amorphous.

High Resolution Tem Study of Diamond Formation on Silicon and Molybdenum Field Emitter Surfaces

AbstractDiamond is an attractive material for coating microfabricated metal and semiconductor field emitters, since it enhances the stability and emission characteristics of the emitter. In the present study, polycrystalline diamond thin films were grown on silicon and molybdenum field emitters by microwave plasma chemical vapor deposition, using the bias-enhanced nucleation technique. High resolution transmission electron microscopy (TEM) was used to analyze the morphology of the diamond film and the structure of the diamond/emitter interface. Electron diffraction patterns and high resolution images indicate the presence of a polycrystalline diamond film, as well as a polycrystalline SiC layer between the diamond film and the Si emitter. A carbide interlayer was also found to exist between the diamond and the Mo emitter surface. Parallel electron energy loss spectroscopy confirms the TEM identification of a polycrystalline diamond film.

Diffusion Bonding Of Ti-48A1-2Mn-2Nb (at. %)

AbstractThe diffusion bonding behaviour of Ti-48at.%Al-2at.%Mn-2at.%Nb has been studied as a function of temperature (in the range 1200–1350°C), time (15–45 minutes) and starting microstructure (lamellar, duplex and near γ) at constant bonding pressure of 10 MPa. It was found, that under the above conditions, small twin related γ grains, approximately 10–20 μm in size, nucleated at the original interface and grew into the matrix forming a double necklace grain structure. Particles of α2 were observed around the interface, the formation of α2 particles was believed to be related to oxygen partitioning and stabilisation effects from dissolved oxide films during the bonding process. Evidence for this mechanism was obtained from parallel electron energy loss spectroscopy (PEELS), which identified oxygen partitioning in the α2 particles. For the fully lamellar structure bonded at 1250°C for 45 minutes the failure strength of the bond was found to be 250 MPa, approximately 50 MPa lower than the failure strength of the base material.

Applications of high-spatial-resolution microanalysis in materials science

The development of advanced materials frequently requires control over the presence and propertiesof interfaces. In order to establish the relationship between the microstructure and the macroscopic behaviour of the material both, the structure and the composition of the interfaces have to be revealed at a resolution at or close to the atomic level. In this paper new techniques in analytical electron microscopy with high spatial resolution will be discussed. The analysis is based on electron energy-loss spectroscopy (EELS). Spatial resolution can either be obtained by applying an energy filteringTEM (EFTEM) or a small probe in a field emission dedicated STEM. Both techniques have been applied toa number of model systems and the achievable detection and resolution limits will be compared. The experimental studies were performed at (i) a Zeiss EM 912 Omega instrument equipped with a 1024 × 1024 slow scan CCD camera and (ii) a VG HB 501 equipped with parallel EELS.

Atomic Scale Structure and Chemistry of Interfaces by Z-Contrast Imaging and Electron Energy Loss Spectroscopy in the Stem

AbstractThe macroscopic properties of many materials are controlled by the structure and chemistry at grain boundaries. A basic understanding of the structure-property relationship requires a technique which probes both composition and chemical bonding on an atomic scale. High-resolution Z-contrast imaging in the scanning transmission electron microscope (STEM) forms an incoherent image in which changes in atomic structure and composition across an interface can be interpreted directly without the need for preconceived atomic structure models (1). Since the Z-contrast image is formed by electrons scattered through high angles, parallel detection electron energy loss spectroscopy (PEELS) can be used simultaneously to provide complementary chemical information on an atomic scale (2). The fine structure in the PEEL spectra can be used to investigate the local electronic structure and the nature of the bonding across the interface (3). In this paper we use the complimentary techniques of high resolution Zcontrast imaging and PEELS to investigate the atomic structure and chemistry of a 25° symmetric tilt boundary in a bicrystal of the electroceramic SrTiO3.

Atomic-resolution characterization of an SrTiO3 grain boundary in the STEM

High-resolution Z-contrast imaging in the scanning transmission electron microscope (STEM) forms an incoherent image in which changes in atomic structure and composition across an interface can be interpreted intuitively without the need for preconcieved atomic structure models. Since the Z-contrast image is formed by electrons scattered through high angles, parallel detection electron energy loss spectroscopy (PEELS) can be used simultaneously to provide complementary chemical information on an atomic scale. The fine structure in the PEEL spectra can be used to investigate the local electronic structure and the nature of the bonding across the interface. In this paper we use the complimentary techniques of high resolution Z-contrast imaging and PEELS to investigate the atomic structure and chemistry of a 25 degree symmetric tilt boundary in a bicrystal of the electroceramic SrTiO3.Figure 1(a) shows a Z-contrast image of a symmetric region of the tilt boundary. The brightest spots in the image correspond to the increased scattering power of the Sr atomic columns (Z=38) with theless bright spots corresponding to the Ti atomic columns (Z=22). The lighter O atomic columns are notvisible in a Z-contrast image.

Extending the limits of molecular microanalysis

Modern polymer blends are frequently composed of domains and interfacial phases having submicron dimensions. Previously, die characterization of specific unknown submicron polymer phases has been limited to selective staining methods that help to classify the phase but rarely lead to chemical identification. The present procedure uses parallel detection electron energy loss spectroscopy (PEELS) to perform submicron molecular microanalysis on beam sensitive materials. Polymer domains are first differentiated by their elemental composition and then by their characteristic carbon core loss edge structure. These spectra are compared to spectra recorded from polymers of known composition.A polymer film composed of alternating 0.5 μm layers of polycarbonate (PC) and polymethylmethacrylate (PMMA) was used as a test specimen (Fig. 1). Ultra-thin sections (<50 nm) were prepared by microtomy, collected on unsupported 600 mesh copper grids and examined at −160°C usinga VG HB601UX dedicated STEM fitted with a Gatan 666 UHV PEELS. The combination of beam blanking, simplecontrol of electron dose, UHV, low energy spread FEG, stage stability and the ability to produce a high contrast image at a very low electron dose makes this instrument ideally suited for this experiment.

Parallel electron energy loss spectroscopy study of al-substituted calcium silicate hydrate (CSH) phases present in hardened cement pastes

The local environment of aluminium in Al-substituted calcium silicate hydrate phases in cement pastes has been determined by measurement and modelling Al K-ELNES. The results are in agreement with bulk 27Al and 29Si MAS NMR data and predictions derived from compositional trends.

Microchemical Analysis of the SCS‐6 Silicon Carbide Fiber

A detailed quantitative study of the microstructural variation of chemical composition of chemically vapor‐deposited commercial SiC SCS‐6 fiber is presented. Chemical etching and various electron‐optical techniques including scanning electron microscopy, transmission electron microscopy, transmission electron microscopy, scanning Auget microscopy, Auger electron spectroscopy, and parallel electron energy loss spectroscopy are used to analyze the chemical composition of the as‐received fiber. In addition, some results on stereology of the high‐temperature annealed fiber are presented. The results show that the carbon‐to silicon atom ratio in the SiC layers decreases in a stepwise fashion from ∼ 3:2 to ∼ 1:1 in going from the inner‐most layer to outermost layer.

Thickness measurement of glucose-embedded crotoxin complex crystals by EELS

In three-dimensional structure determination of macromolccules by electron crystallography it is necessary to combine diffraction patterns and images recorded at various tilt angles from different crystals. In order to merge these data sets successfully all the crystals should have the same thickness. It has been proposed that parallel electron energy loss spectroscopy (EELS) might provide a useful means of assessing the thickness of a beam-sensitive organic crystal prior to recording the high-resolution structural data. This can be achieved by measuring the fraction of the total transmitted electrons that do not lose energy, i.e., the zero-loss intensity. If Izis the integrated zero-loss intensity and Itotis the total integrated intensity in the energy loss spectrum, then the specimen thickness, t, is given in terms of the total inelastic mean free path, λi, by, t/λi= ln(Itot/Iz). Results recently obtained fromn-paraffin crystals have shown that it is feasible to determine the number of unit cell layers under low electron dose conditions.

Energy-filtered microdiffraction in a dedicated scanning transmission electron microscope

Microdiffraction patterns have been recorded using existing parallel electron energy-loss spectroscopy instrumentation in a field emission scanning transmission electron microscope (STEM) operating at 100 kV. The patterns are recorded by a low-noise CCD camera coupled to a scintillator which is placed after the exit slit of a spectrometer. Three sets of quadrupoles between the scintillator and spectrometer are used to control the camera length. The patterns are recorded at extremely high resolution (0.06 mrad) and very low noise with an energy window of down to 1 eV. As recording of the whole pattern is undertaken in parallel, acquisition times are very short (a few seconds) compared to lower-resolution, higher-noise, serial methods. Using this technique zero-energy-loss microdiffraction patterns have been obtained from individual layers in Si/Si 1- x Ge x multilayers using the sub-nanometer-size probe of STEM. The high resolution of the patterns permits strain quantification in these layers to a few parts in ten thousand using higher-order Laue zone (HOLZ) deficit lines. Using a defocussed probe, strain maps of the superlattice structure can be obtained. Patterns taken from near the edge of single crystals show coherence effects in both the zero-order disc and HOLZ lines. Energy-filtered microdiffraction patterns have been acquired from single crystals for losses of up to 200 eV.

Quantitative Scanning Transmission Electron Microscopy of Ultrathin Cryosections: Subcellular Organelles in Rapidly Frozen Liver and Cerebellar Cortex

Freeze-dried, ultrathin cryosections of directly frozen mouse liver and brain have been prepared and characterized by low-dose dark-field scanning transmission electron microscopy (STEM). These improved cryosections gave images comparable to those from conventional plastic sections. They were thin enough (⪡1.0 elastic mean free path) to use established dark-field techniques, modified for thickness-dependent nonlinearities, to measure the dry mass fraction of individual organelles, and hence to deduce their water content. Digital STEM imaging in combination with electron and X-ray spectroscopy has important biological applications, as illustrated by studies on calcium regulation in Purkinje neurons. Calcium concentrations per unit dry weight of dendritic compartments were determined by the peak/continuum method of energy-dispersive X-ray spectroscopy (EDXS), which necessarily overstates elemental concentrations because of beam-induced mass loss. The dry mass content of organelles at low dose and the percentage of dry mass retained after analysis at high dose were as follows: mitochondria (46.0 g dry mass/100 g hydrated mass, 67% mass retained); endoplasmic reticulum (27.9 g/100 g, 57%); and cytoplasm (16.3 g/100 g, 41%). These values were used to correct elemental concentrations for mass loss. Results indicated that the major calcium storage organelle in Purkinje cell dendrites is the endoplasmic reticulum, of which there are two types distinguished by their levels of calcium. Parallel electron energy loss spectroscopy of dendritic organelles corroborated EDXS measurements, with an improved sensitivity that indicates the feasibility of quantitative calcium mapping.

Analysis of chemical-vapour-deposited diamond grain boundaries using transmission electron microscopy and parallel electron energy loss spectroscopy in a scanning transmission electron microscope

Grain boundaries of unthinned chemical-vapour-deposited (CVD) diamond films have been examined using high-resolution transmission electron microscopy (TEM) and parallel electron energy loss spectroscopy (EELS) in a scanning transmission electron microscope. EELS analysis shows the presence of amorphous carbon at the grain boundaries and in a surface layer on the CVD diamond film. The amorphous carbon is shown to contain almost complete sp 2 bonding and to be non-hydrogenated. The amorphous carbon at the grain boundary and in a surface layer explains many of the properties and measurements of CVD diamond films.

Structure, Composition, and Uniformity of Plasma Deposited Nitride Thin Films

Transmission electron microscopy (TEM) and high resolution (15 Å) parallel electron energy loss spectroscopy (PEELS) are used to investigate the chemical and structural properties of a 70 Å amorphous nitride film deposited at 300°C by remote plasma‐enhanced chemical vapor deposition. The nitride films were found to have a concentration gradient across the width. The films were silicon rich near the substrate/film interface and approached a silicon‐to‐nitrogen concentration ratio of stoichiometrically correct near the film surface. This nonuniform structure was imaged using TEM. The concentration gradient appeared as a contrast gradient. PEELS was used in conjunction with TEM in an attempt to relate the chemical composition of the film to its image contrast in order to obtain a qualitative measure of the density. The films were determined to be less dense near the interface than near the surface. Ellipsometry measurements indicate that the films were, on the average, less dense than their thermal counterparts.

The effectiveness of electron holography, microscopy, and energy-loss spectroscopy in characterizing thin silicon oxide-nitride-oxide structures

High-resolution transmission electron microscopy, electron holography, and high-spatial- resolution (19 Å) computer-controlled parallel electron-energy-loss spectrometry (PEELS) were used to probe the structure of and chemical profile across two thin silicon oxide-nitride-oxide layered structures of nominal widths of 10 Å-50 Å-10 Å and 30 Å-50 Å-30 Å. It was found that the individual layers of the stacked structures could be clearly imaged using electron holography, but not with electron microscopy due to the behavior of the microscope transfer function and the shape of the potential profile of the structure. Holography revealed that the layers of the 10 Å-50 Å-10 Å system were in fact 14 Å-28 Å-18 ű5 Å, and the oxide layer in contact with the substrate (first oxide layer) was discontinuous. PEELS showed that the second oxide layer contained nitrogen, and the nitride layer had a silicon-to-nitrogen concentration ratio of 1.0±0.1. The 30 Å-50 Å-30 Å system was in fact 30 Å-20 Å-40 Å-15 ű5 Å. The second oxide layer was SiO1.6±0.2, and nitrogen was found near the film surface which made the structure appear to be four layers. These results show the power of holography in characterizing thin, light-element, amorphous layers and the importance of computer-controlled parallel energy-loss line scans for obtaining analytical information at the highest spatial resolution with minimum dose.

Iterative reduction of gain variations in parallel electron energy loss spectrometry

We discuss a method of reducing intrinsic gain variations of the detector array in parallel electron energy loss spectrometry (PEELS). Two or more spectra are acquired independently, but under the same experimental conditions, except that the spectra are shifted over the array with respect to each other by a certain amount. For stochastic gain variations the problem is solved in closed form. It is shown that the reduction of the variance of gain variations increases with the squareroot of number of iterations. Simulations for a purely stochastic gain spectrum prove the predictions. Experimental gain spectra turn out to be non-random. The consequence of non-stochasticity (correlations between channels in the array) can be understood within the theory. The proposed method is useful for any experiment involving a linear or two-dimensional detector array where it is not possible to obtain a “flat field” spectrum.