Electron Energy Loss Spectroscopy Imaging Research Articles

Localization of inelastic electron scattering in the low-loss energy regime

The spatial resolution and contrast level in electron energy-loss spectroscopy (EELS) imaging depend on the delocalization of the inelastic electron scattering cross sections. Theoretical calculations within the dipole approximation provide the lower limit for the delocalization of low loss signals, and suggest that atomic resolution EELS imaging in the low loss energy regime (<50 eV) should be possible. Here, we directly measure the localization of the inelastic electron scattering at different energy loss in the low loss regime using a clean open edge of monolayer graphene. Our results demonstrate that the delocalization depends both on the energy loss and the specific electron excitation mode contributing to the energy loss. While the plasmons are delocalized over 1.2 nm, sub-nm enhancement is observed at the edge for the low-loss signal at 11 eV, indicating the possible formation of a one-dimensional plasmon (or inter-band transition) at the edge of monolayer graphene. Our results also suggest that if the initial states or final states are atomically localized, atomic resolution EELS imaging could be obtained even in the low loss region of the spectra.

Spatially resolved mapping of disorder type and distribution in random systems using artificial neural network recognition

The spatial variability of the polarization dynamics in thin film ferroelectric capacitors was probed by recognition analysis of spatially resolved spectroscopic data. Switching spectroscopy piezoresponse force microscopy (SSPFM) was used to measure local hysteresis loops and map them on a two dimensional (2D) random-bond, random-field Ising model. A neural-network based recognition approach was utilized to analyze the hysteresis loops and their spatial variability. Strong variability is observed in the polarization dynamics around macroscopic cracks because of the modified local-elastic and electric-boundary conditions, with the most pronounced effect on the length scale of \ensuremath{\sim}100 nm away from the crack. The recognition approach developed here is universal and can potentially be applied for arbitrary macroscopic and spatially resolved data, including temperature- and field-dependent hysteresis, I-V curve mapping, electron microscopy electron energy loss spectroscopy (EELS) imaging, and many others.

Application of Electron Energy-Loss Spectroscopy (EELS) and Energy-Filtered Transmission Electron Microscopy (EFTEM) to the Study of Mineral Transformation Associated with Microbial Fe-Reduction of Magnetite

AbstractElectron energy-loss spectroscopy (EELS), energy-filtered transmission electron microscopy (EFTEM), and high-resolution transmission electron microscopy (HRTEM) have been applied in mineralogy and materials research to determine the oxidation states of various metals at high spatial resolution. Such information is critical in understanding the kinetics and mechanisms of mineral–microbe interactions. To date, the aforementioned techniques have not been applied widely in the study of such interactions. In the present study, the three techniques above were employed to investigate mineral transformations associated with microbial Fe(III) reduction in magnetite. Shewanella putrefaciens strain CN32, a dissimilatory metal-reducing bacterium, was incubated with magnetite as the sole electron acceptor and lactate as the electron donor for 14 days under anoxic conditions in bicarbonate buffer. The extent of bioreduction was determined by wet chemistry and mineral solids were investigated by HRTEM, EFTEM, and EELS. Magnetite was partially reduced and biogenic siderite formed. The elemental maps of Fe, O, and C and red-green-blue (RGB) composite map for residual magnetite and newly formed siderite were contrasted by the EFTEM technique. The HRTEM revealed nm-sized magnetite crystals coating bacterial cells. The Fe oxidation state in residual magnetite and biogenic siderite was determined using the EELS technique (the integral ratio of L3 to L2). The integral ratio of L3 to L2 for magnetite (6.29) and siderite (2.71) corresponded to 71% of Fe(III) in magnetite, and 24% of Fe(III) in siderite, respectively. A chemical shift (~1.9 eV) in the Fe-L3 edge of magnetite and siderite indicated a difference in the oxidation state of Fe between these two minerals. Furthermore, the EELS images of magnetite (709 eV) and siderite (707 eV) were extracted from the electron energy-loss spectra collected, ranging from 675 to 755 eV, displaying different oxidation states of Fe in the magnetite and siderite phases. The results demonstrate that EELS is a powerful technique for studying the Fe oxidation-state change as a result of microbial interaction with Fe-containing minerals.

Scanning transmission electron microscopy imaging dynamics at low accelerating voltages

Motivated by the desire to minimize specimen damage in beam sensitive specimens, there has been a recent push toward using relatively low accelerating voltages (<100 kV) in scanning transmission electron microscopy. To complement experimental efforts on this front, this paper seeks to explore the variations with accelerating voltage of the imaging dynamics, both of the channelling of the fast electron and of the inelastic interactions. High-angle annular-dark field, electron energy loss spectroscopic imaging and annular bright field imaging are all considered.

Energy-filtered transmission electron microscopy of biological samples on highly transparent carbon nanomembranes

Ultrathin carbon nanomembranes (CNM) comprising crosslinked biphenyl precursors have been tested as support films for energy-filtered transmission electron microscopy (EFTEM) of biological specimens. Due to their high transparency CNM are ideal substrates for electron energy loss spectroscopy (EELS) and electron spectroscopic imaging (ESI) of stained and unstained biological samples. Virtually background-free elemental maps of tobacco mosaic virus (TMV) and ferritin have been obtained from samples supported by ∼1nm thin CNM. Furthermore, we have tested conductive carbon nanomembranes (cCNM) comprising nanocrystalline graphene, obtained by thermal treatment of CNM, as supports for cryoEM of ice-embedded biological samples. We imaged ice-embedded TMV on cCNM and compared the results with images of ice-embedded TMV on conventional carbon film (CC), thus analyzing the gain in contrast for TMV on cCNM in a quantitative manner. In addition we have developed a method for the preparation of vitrified specimens, suspended over the holes of a conventional holey carbon film, while backed by ultrathin cCNM.

Chemical Imaging at Atomic Resolution as a Technique To Refine the Local Structure of Nanocrystals

The challenging problem of mapping the chemical composition of cation columns in individual nanocrystals at atomic resolution is addressed by using a method based on aberration-corrected electron microscopy, core-loss electron energyloss spectroscopy, and simulations. The potential of this novel approach to provide unique structural information, which is the key to rationalizing macroscopic behavior, is illustrated with the analysis of ceria–zirconia mixed oxides, which are nanomaterials with substantial technological impact. Metal nanoparticles supported on this family of oxides are currently materials of interest as catalysts in a variety of chemical transformations in the area of environmental protection, such as low-temperature water-gas shift, selective oxidation of CO in the presence of large amounts of hydrogen, or three-way catalysis. Strong variations in the chemistry of ceria–zirconia mixed oxide catalysts have been observed after they have undergone redox cycles involving reduction treatments at high temperatures ( 1173 K) then oxidation at mild temperatures ( 823 K). In particular their reducibility is significantly enhanced after such aging treatments. Scanning transmission electron microscopy (STEM) techniques have provided crucial information to account for these changes in the redox behavior. High-resolution electron microscopy (HREM) combined with high-angle annular dark-field (HAADF) imaging and tomography have revealed the occurrence of a disorder–order transformation in the cationic sublattice of these oxides, which tend to rearrange into a distribution characteristic of the so called pyrochlore phase. This phase is an archetype structure for A2B2O7 (A= + 3 cation, B=+ 4 cation) compounds and can be considered a fluorite superstructure. The structural transformation takes place during the reduction step of the cycle, in which the fully reduced mixed oxide with Ce/Zr molar ratio 1:1 adopts the Ce2Zr2O7 stoichiometry. Nevertheless, HAADF studies have clearly shown that, in the case of ceria–zirconia mixed oxides, this cation-ordered arrangement is preserved even after full reoxidation, that is, in the oxide with Ce2Zr2O8 stoichiometry, whenever the oxidation temperature does not exceed 823 K. Electron-microscopy studies have also revealed another remarkable feature of the ceria–zirconia aged oxides with the pyrochlore-type cation sublattice: the occurrence of compositional heterogeneities at the nanometer scale. Taking these observations into account and also considering that the disorder–order transition may not be completed in the time scale and under the temperature conditions used in the redox-cycling treatments, the important question arises whether these heterogeneities are in fact occurring on a finer scale, that is, at the atomic level. Such heterogeneities, compatible with the HREM and HAADF observations, will strongly influence the details of the counterpart oxygen sublattice and, consequently, the chemical and catalytic response of these oxides. To date, the atomic-column by atomic-column compositional analysis of the oxidized pyrochlore required to justify such a possibility has not been accomplished. Herein, using the capabilities of an aberration-corrected Nion UltraSTEM microscope (operated at 100 kV) we not only provide the first direct chemical evidence of the cationic order present in the Ce2Zr2O8 oxidized pyrochlore but we also show how atomicresolution electron energy-loss spectroscopy (EELS) mapping, based on core–shell ionization, can be combined with EELS image simulation to detect quite subtle local deviations in the cation sublattice from the completely ordered structure. This information provides a much more accurate structural description of the active catalyst nanocrystals, which must be considered to model both their oxygen-exchange capabilities and, eventually, their catalytic performance. [*] Dr. S. Trasobares, Dr. M. L pez-Haro, Dr. J. A. Perez-Omil, Dr. J. J. Calvino Departamento de Ciencia de los Materiales e Ingenier a Metalfflrgica y Qu mica Inorg nica Facultad de Ciencias, Universidad de C diz Campus Rio San Pedro, 11510-Puerto Real, C diz (Spain) Fax: (+34)956-016286 E-mail: susana.trasobares@uca.es Homepage: http://www.uca.es/tem-uca

Nanobacteria and Psammoma Bodies: Ultrastructural Observations in a Case of Pathological Placental Calcification

Nanobacteria are controversial infectious agents with nanometric size, the capacity to nucleate hydroxyapatite and grow in culture, and present in human diseases associated with calcification and psammoma bodies. The authors report a case of pathological placental calcifications associated with nanobacteria. Electron microscopy and electron energy loss spectroscopy imaging were used to recognize 160-nm-sized calcium-free bodies mainly presenting as extracellular fibrillary tangles and 500-nm-sized calcified bodies; they encrusted the syncito-trophoblast basal membrane and aggregated into miniaturized psammoma bodies. Nanobacteria may be composed of a prionoid protein with self-assembling and self-propagating abilities whose growth is associated with the formation of psammoma bodies.

Suppression of Octahedral Tilts and Associated Changes in Electronic Properties at Epitaxial Oxide Heterostructure Interfaces

Epitaxial oxide interfaces with broken translational symmetry have emerged as a central paradigm behind the novel behaviors of oxide superlattices. Here, we use scanning transmission electron microscopy to demonstrate a direct, quantitative unit-cell-by-unit-cell mapping of lattice parameters and oxygen octahedral rotations across the BiFeO3-La0.7 Sr0.3 MnO3 interface to elucidate how the change of crystal symmetry is accommodated. Combined with low-loss electron energy loss spectroscopy imaging, we demonstrate a mesoscopic antiferrodistortive phase transition near the interface in BiFeO3 and elucidate associated changes in electronic properties in a thin layer directly adjacent to the interface.

Smart Acquisition EELS

Electron energy loss (EEL) spectroscopy and high angle annular dark field (HAADF) imaging in aberration-corrected electron microscopes are powerful techniques to determine the chemical composition and structure of materials at atomic resolution. We have implemented Smart Acquisition, a flexible system of scanning transmission electron microsocpy (STEM) beam position control and EELS collection, on two aberration-corrected dedicated cold field emission gun (FEG) STEMs located at SuperSTEM, Daresbury Laboratory. This allows the collection of EEL spectra from spatially defined areas with a much lower electron dose possible than existing techniques such as spectrum imaging.

Enrichment of boron at grain boundaries of platinum-based alloys determined by electron energy loss spectroscopy in a transmission electron microscope

AbstractPolycrystalline platinum-based alloys show very good creep properties at high temperatures. Small amounts of boron (less than 1 at.%) considerably increase the creep strength. Transmission electron microscopy measurements were conducted to localize the element boron in the samples. With image electron energy loss spectroscopy spectra were extracted revealing an enrichment of B at grain boundaries. The boron distribution was plotted with the three window method.

Synthesis of Quaternary Chalcogenide Nanocrystals: Stannite Cu2ZnxSnySe1+x+2y

A successful synthesis of Cu(2)Zn(x)Sn(y)Se(1+x+2y) (CZTSe) nanoparticles is presented. Nearly monodisperse and highly faceted CZTSe nanoparticles were prepared through reaction of metal amino complexes with saturated solutions of selenium in trioctylphosphine at high temperature. High-resolution transmission electron microscopy and X-ray diffraction analysis showed the prepared nanocrystals to have the tetragonal Cu(2)ZnSnSe(4) crystallographic structure. Nanoscale-resolved electron energy loss spectroscopy images showed the presence of the four elements in each individual nanocrystal. Further analyses of the particles' chemical composition showed the nanocrystals to be consistently Zn- and Sn-poor.

Photon-Assisted Electron Energy Loss Spectroscopy and Ultrafast Imaging

A variety of ways is described in which photons can be used not only for ultrafast electron microscopy but also to enormously widen the energy range of spatially-resolved electron spectroscopy. Periodic chains of femtosecond laser pulses are a particularly important and accurately timed source for single-shot imaging and diffraction as well as for several forms of pump-probe microscopy at even higher spatial resolution and sub-picosecond timing. Many exciting new fields are opened up for study by these developments. Ultrafast, single shot diffraction with intense pulses of X-rays supplemented by phase retrieval techniques may eventually offer a challenging alternative and purely photon-based route to dynamic imaging at high spatial resolution.

Evidence for an organic origin of pedogenic calcitic nanofibres

Calcium carbonate nanofibres are found in numerous terrestrial environments, often associated with needle fibre calcite. This study attempts to mimic the natural system and generate comparable crystalline structures. A comparison of natural and synthesized nanofibre structures, using HRTEM as well as electron energy loss spectroscopy (EELS) and electron spectroscopic imaging (ESI), has demonstrated that this type of nanocrystal can result from precipitation on organic templates, most likely cellulose nanofibres. This study emphasizes the fundamental role of organic templates in the precipitation of calcium carbonate in vadose environments, even at the nanoscale.

Reactive Multilayers Examined by HRTEM and Plasmon EELS Chemical Mapping

We examine chemical mapping of reaction phases in a Cu-Al multilayer system using low-loss electron energy loss spectroscopy spectrum imaging and image spectroscopy techniques. The sensitivity of the plasmon peak position and shape to various crystal structures and phases is exploited using postprocessing of spectra into second derivative plasmon maps and line scans. Analytical transmission electron microscopy is complemented by studies of the orientation relationship of the multilayer system using high-resolution electron microscopy of interfaces and selected area diffraction. The techniques have been applied to the Cu-Al multilayer sample and sharply bound epitaxial phases are found, before and after heat treatment.

TEM studies of nanostructure in melt-spun Mg–Ni–La alloy manifesting enhanced hydrogen desorbing kinetics

The hydrogen storage properties of a magnesium-rich Mg–Ni–La alloy prepared by melt-spinning are significantly improved by nanostructure formation during crystallization and activation. It can absorb and desorb ∼5 wt% hydrogen at temperatures as low as 200 °C in moderate time periods. Transmission electron microscopic (TEM) studies on this alloy indicate that the nanostructure, consisting of LaH 3 and Mg 2NiH 4 nano-particles dispersed homogeneously in MgH 2 matrices after hydrogenation, is rather stable at temperatures below 300 °C but undergoes coarsening and segregation of these particles and matrices above ∼400 °C. These structural changes have been confirmed by electron energy-loss spectroscopic (EELS) imaging as well as high-resolution TEM techniques. A new EELS peak associated with a plasmon excitation in the MgH 2 phase (H-plasmon) is found for the first time in this study. By imaging the H-plasmon peak, the hydrogen distribution in the alloy has been clearly visualized. We have succeeded in observing the hydrogen desorption process at ∼400 °C in-situ in the microscope using this EELS imaging technique.

Pectin‐like carbohydrates in the green alga Micrasterias characterized by cytochemical analysis and energy filtering TEM

Pectins are the major matrix polysaccharides of plant cell walls and are important for controlling growth, wall porosity and regulation of the ionic environment in plant cells. Pectic epitopes recognized by the monoclonal antibodies JIM5, JIM7 and 2F4 could be localized in the primary wall during development of the green alga Micrasterias. As the degree of pectin esterification determines the calcium-binding capacity and thus the physical properties of the cell wall, chemical and enzymatic in situ de-esterification was performed. This resulted in displacement of epitopes recognized by JIM5, JIM7 and 2F4, respectively, in changes in the intensity of the antibody labelling as visualized in CLSM. In addition, calcium-binding capacities of cell walls and components of the secretory apparatus were determined in transmission electron microscopy by electron energy loss spectroscopy and electron spectroscopic imaging. These analyses revealed that pectic polysaccharides are transported to the cell wall in a de-esterified form. At the primary wall, pectins get methyl-esterified at the inner side, thus allowing flexibility of the wall. At the outer side of the wall they become again de-esterified and bind high amounts of calcium which leads to cell wall stiffening. Mucilage vesicles possess the highest calcium-binding capacity of all structures observed in Micrasterias, indicating that the pectic polysaccharides of mucilage are secreted in a de-esterified, compact form. When mucilage is excreted through the cell wall, it loses its ability to bind calcium. The esterification of pectins involved is obviously required for swelling of mucilage by water uptake, which generates the motive force for orientation of this unicellular organism in respect to light. Incubation of Micrasterias in pectin methylesterase (PME), which de-esterifies pectic polymers in higher plants, resulted in growth inhibition, cell shape malformation and primary wall thickening. A PME-like enzyme could be found in Micrasterias by PME activity assays.

Volcano structure in atomic resolution core-loss images

A feature commonly present in simulations of atomic resolution electron energy loss spectroscopy images in the scanning transmission electron microscope is the volcano or donut structure. In the past this has been understood in terms of a geometrical perspective using a dipole approximation. It is shown that the dipole approximation for core-loss spectroscopy begins to break down as the probe forming aperture semi-angle increases, necessitating the inclusion of higher order terms for a quantitative understanding of volcano formation. Using such simulations we further investigate the mechanisms behind the formation of such structures in the single atom case and extend this to the case of crystals. The cubic SrTiO 3 crystal is used as a test case to show the effects of nonlocality, probe channelling and absorption in producing the volcano structure in crystal images.

Interpreting atomic-resolution spectroscopic images

Core-loss electron energy loss spectroscopy is a powerful experimental tool with the potential to provide atomic-resolution information about electronic structure at defects and interfaces in materials and nanostructures. Interpretation, however, is nonintuitive. Comparison of experimental and simulated compositional maps in $\mathrm{La}\mathrm{Mn}{\mathrm{O}}_{3}$ shows good agreement, apart from an overall scaling of image contrast, and shows that the shape and width of spectroscopic images do not show a simple variation with binding energy, as commonly assumed, or with the size of the orbital excited. For the low lying La ${N}_{4,5}$ edge with threshold at around $99\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, delocalization does not preclude atomic resolution, but reduces the image contrast. The image width remains comparable to that of the much higher lying O $K$ edge with threshold at around $532\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. Both edges show a volcanolike feature, a dip at the column position not previously seen experimentally. In the case of the O $K$ edge, this represents an experimental verification of nonlocal inelastic scattering effects in electron energy loss spectroscopy imaging. In the case of the ${N}_{4,5}$ edge, the volcanolike feature is due to dynamical channeling and absorption of the probe through the specimen thickness. Simulation is therefore critical to the interpretation of atomic-resolution elemental maps.

Imaging Si nanoparticles embedded in SiO 2 layers by (S)TEM-EELS

Fabrication of systems in which Si nanoparticles are embedded in a thin silica layer is today mature for non-volatile memory and opto-electronics applications. The control of the different parameters (position, size and density) of the nanoparticles population is a key point to optimize the properties of such systems. A review of dedicated transmission electron microscopy (TEM) methods, which can be used to measure these parameters, is presented with an emphasis on those relying on electron energy-loss spectroscopy (EELS). Defocused bright-field imaging can be used in order to determine topographic information of a whole assembly of nanoparticles, but it is not efficient for looking at individual nanoparticles. High-resolution electron imaging or dark-field imaging can be of help in the case of crystalline particles but they always provide underestimated values of the nanocrystals population. EELS imaging in the low-energy-loss domain around the Si plasmon peak, which gives rise to strong signals, is the only way to visualize all Si nanoparticles within a silica film and to perform reliable size and density measurements. Two complementary types of experiments are investigated and discussed more extensively: direct imaging with a transmission electron microscope equipped with an imaging filter (EFTEM) and indirect imaging from spectrum-imaging data acquired with a scanning transmission electron microscope equipped with a spectrometer (STEM-PEELS). The direct image (EFTEM) and indirect set of spectra (STEM-PEELS) are processed in order to deliver images where the contribution of the silica matrix is minimized. The contrast of the resulting images can be enhanced with adapted numerical filters for further morphometric analysis. The two methods give equivalent results, with an easier access for EFTEM and the possibility of a more detailed study of the EELS signatures in the case of STEM-PEELS. Irradiation damage in such systems is also discussed.

Reconstruction of 3D morphology of polyhedral nanoparticles

The three-dimensional (3D) faceting morphology of ceria nanoparticles is analysed usingtransmission electron microscopy (TEM)-based computed tomography on the nanometrescale. A novel tomography mode of electron energy loss spectroscopic imaging usinga single energy window for inelastically scattered electrons is introduced andfound to be reliable and fast for freestanding nanoparticles. To compare the newtomographic method with other methods, we provide the first comprehensiveapplication of three complementary TEM-based imaging techniques, including brightfield TEM and annular dark field specific TEM (STEM). Traditional bright-fieldTEM tomography is found to be applicable, in spite of obvious artefacts, forcrystalline particles of constant composition. However, the safest interpretationis achieved by a combined recording of bright field and spectroscopic images.