Atomic Resolution Electron Energy Loss Research Articles

Atomic Resolution Structural and Chemical Imaging Revealing the Sequential Migration of Ni, Co, and Mn upon the Battery Cycling of Layered Cathode.

Layered lithium transition metal oxides (LTMO) are promising candidate cathode materials for next-generation high-energy density lithium ion battery. The challenge for using this category of cathode is the capacity and voltage fading, which is believed to be associated with the layered structure disordering, a process that is initiated from the surface or solid-electrolyte interface and facilitated by transition metal (TM) reduction and oxygen vacancy formation. However, the atomic level dynamic mechanism of such a layered structure disordering is still not fully clear. In this work, utilizing atomic resolution electron energy loss spectroscopy (EELS), we map, for the first time at atomic scale, the spatial evolution of Ni, Co and Mn in a cycled LiNi1/3Mn1/3Co1/3O2 layered cathode. In combination with atomic level structural imaging, we discovered the direct correlation of TM ions migration behavior with lattice disordering, featuring the residing of TM ions in the tetrahedral site and a sequential migration of Ni, Co, and Mn upon the increased lattice disordering of the layered structure. This work highlights that Ni ions, though acting as the dominant redox species in many LTMO, are labile to migrate to cause lattice disordering upon battery cycling, while the Mn ions are more stable as compared with Ni and Co and can act as pillar to stabilize layered structure. Direct visualization of the behavior of TM ions during the battery cycling provides insight for designing of cathode with high structural stability and correspondingly a superior performance.

Tensor decompositions for the analysis of atomic resolution electron energy loss spectra

A selection of tensor decomposition techniques is presented for the detection of weak signals in electron energy loss spectroscopy (EELS) data. The focus of the analysis lies on the correct representation of the simulated spatial structure. An analysis scheme for EEL spectra combining two-dimensional and n-way decomposition methods is proposed. In particular, the performance of robust principal component analysis (ROBPCA), Tucker Decompositions using orthogonality constraints (Multilinear Singular Value Decomposition (MLSVD)) and Tucker decomposition without imposed constraints, canonical polyadic decomposition (CPD) and block term decompositions (BTD) on synthetic as well as experimental data is examined.

Resolving Active Sites and Porosity in PGM-Free Catalysts By Electron Microscopy

Electrodes based on platinum group metal-free (PGM-free) catalysts are under development as an alternative to costly Pt-based cathodes for automotive fuel cell systems.[1,2] Novel PGM-free catalysts with much improved activity and durability must be developed in order to become competitive with their PGM nanoparticle counterparts for transportation applications. Analytical characterization by transmission electron microscopy plays an important role in identifying electrochemically active sites at the atomic scale as well as to exploring electrode morphology and porosity at the microscale. In this work, a hybrid PGM-free catalyst developed at Los Alamos National Laboratory was studied by multiscale and multidimensional electron microscopy methods. This promising catalyst, branded CM-PANI-Fe-C, has been synthesized by heat-treating two nitrogen precursors, polyaniline (PANI) and cyanamide (CM). The catalyst is comprised of fibrous carbonaceous agglomerates intermixed with few-layer graphene sheets. Low voltage aberration-corrected scanning transmission electron microscopy (STEM) and atomic resolution electron energy loss spectroscopy (EELS) were utilized to identify single Fe atoms coordinated with nitrogen within the few-layer graphene sheets (Fig. 1a,b). These atomic features have been identified by modeling as a possible active site which exhibits high four-electron selectivity. The thick electrode structures required to accommodate high non-PGM loadings also play a critical role in fuel cell performance. Hierarchal pore structures (spanning the micro to macro) are necessary for maintaining high mass transport through the thick electrodes. Complementary imaging and compositional mapping by energy dispersive X-ray spectroscopy (EDS) has been utilized to study the bulk electrode structures, as shown in Fig. 1c. These analytical studies will be extended to understand the role of different process parameters on catalyst and electrode structures at multiple length scales. References G. Wu, K. L. More, C. M. Johnston, P. Zelenay, Science 332 (2011) 443.Z. Chen, D. Higgins, A. Yu, L. Zhang, J. Zhang, Energy Environ. Sci. 4 (2011) 3167. Acknowledgements Research sponsored by the Fuel Cell Technologies Office, Office of Energy Efficiency and Renewable Energy, U.S. Department of Energy (DOE), and through a user project supported by ORNL’s Center for Nanophase Materials Sciences (CNMS), which is a DOE Office of Science User Facility. Figure 1

Stabilisation of Fe2O3-rich Perovskite Nanophase in Epitaxial Rare-earth Doped BiFeO3 Films.

Researchers have demonstrated that BiFeO3 exhibits ferroelectric hysteresis but none have shown a strong ferromagnetic response in either bulk or thin film without significant structural or compositional modification. When remanent magnetisations are observed in BiFeO3 based thin films, iron oxide second phases are often detected. Using aberration-corrected scanning transmission electron microscopy, atomic resolution electron energy loss spectrum-mapping and quantitative energy dispersive X-ray spectroscopy analysis, we reveal the existence of a new Fe2O3-rich perovskite nanophase, with an approximate formula (Fe0.6Bi0.25Nd0.15)3+ Fe3+O3, formed within epitaxial Ti and Nd doped BiFeO3 perovskite films grown by pulsed laser deposition. The incorporation of Nd and Bi ions on the A-site and coherent growth with the matrix stabilise the Fe2O3-rich perovskite phase and preliminary density functional theory calculations suggest that it should have a ferrimagnetic response. Perovskite-structured Fe2O3 has been reported previously but never conclusively proven when fabricated at high-pressure high-temperature. This work suggests the incorporation of large A-site species may help stabilise perovskite-structured Fe2O3. This finding is therefore significant not only to the thin film but also to the high-pressure community.

Magnetism-Driven Ferroelectricity in Double Perovskite Y₂NiMnO₆.

We report the discovery of multiferroic behavior in double perovskite Y2NiMnO6. X-ray diffraction shows that the material has a centrosymmetric crystal structure of space group P2(1)/n with Ni(2+)/Mn(4+) ordering. This result is further confirmed by aberration-corrected scanning transmission electron microscopy combined with atomic resolution electron energy loss spectroscopy. The appearance of ferroelectric polarization coincides with the magnetic phase transition (∼67 K), which indicates that the ferroelectricity is driven by magnetism, and this is further confirmed by its strong magnetoelectric (ME) effect. We proposed the origin of the ferroelectricity is associated with the combination of Ni(2+)/Mn(4+) charge ordering and the ↑↑↓↓ spin ordering. When compared with other known magnetic multiferroics, Y2NiMnO6 displays several attractive multiferroic properties, including high polarization (∼145 μC/m(2)), a high multiferroic transition temperature (∼67 K), and strong ME coupling (∼21%).

Data Processing for Atomic Resolution Electron Energy Loss Spectroscopy

The high beam current and subangstrom resolution of aberration-corrected scanning transmission electron microscopes has enabled electron energy loss spectroscopy (EELS) mapping with atomic resolution. These spectral maps are often dose limited and spatially oversampled, leading to low counts/channel and are thus highly sensitive to errors in background estimation. However, by taking advantage of redundancy in the dataset map, one can improve background estimation and increase chemical sensitivity. We consider two such approaches--linear combination of power laws and local background averaging--that reduce background error and improve signal extraction. Principal component analysis (PCA) can also be used to analyze spectrum images, but the poor peak-to-background ratio in EELS can lead to serious artifacts if raw EELS data are PCA filtered. We identify common artifacts and discuss alternative approaches. These algorithms are implemented within the Cornell Spectrum Imager, an open source software package for spectroscopic analysis.

Seeing the atoms more clearly: STEM imaging from the Crewe era to today

This review covers the development of scanning transmission electron microscopy from the innovations of Albert Crewe to the two-dimensional spectrum imaging in the era of aberration correction. It traces the key events along the path, the first atomic resolution Z-contrast imaging of individual atoms, the realization of incoherent imaging in crystals and the role of dynamical diffraction, simultaneous, atomic resolution electron energy loss spectroscopy, and finally the tremendous impact of the successful correction of lens aberrations, not just in terms of resolution but also in single atom sensitivity.

Quantitative statistical analysis, optimization and noise reduction of atomic resolved electron energy loss spectrum images

In this work we investigate methods of statistical processing and background fitting of atomic resolution electron energy loss spectrum image (SI) data. Application of principal component analysis to SI data has been analyzed in terms of the spectral signal-to-noise ratio (SNR) and was found to improve both the spectral SNR and its standard deviation over the SI, though only the latter was found to improve significantly and consistently across all data sets analyzed. The influence of the number of principal components used in the reconstructed data set on the SNR and resultant elemental maps has been analyzed and the experimental results are compared to theoretical calculations.

Locating La atoms in epitaxial Bi3.25La0.75Ti3O12 films through atomic resolution electron energy loss spectroscopy mapping

Atomic resolution high-angle annular dark-field imaging of La-doped bismuth titanate (BLT), Bi3.25La0.75Ti3O12, has been carried out with an aberration-corrected transmission electron microscope. The HAADF image revealed the presence of defects in the [Bi2O2]2+ layers and extra atomic rows between the [Bi2O2]2+ layers and the [Bi2Ti3O10]2− perovskite slabs. Electron energy loss spectroscopy elemental mapping at atomic resolution revealed the exact location of La dopants in the bismuth titanate parent unit cell. These results are discussed in terms of large remanent polarization and enhanced fatigue resistance in BLT.

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.

Studying the metal-support interaction in Pd/γ-Al2O3 catalysts by atomic-resolution electron energy-loss spectroscopy

A 2 wt% Pd/γ-Al2O3 catalyst, reduced in hydrogen at different temperatures, has been studied by combined atomic resolution Z-contrast imaging and electron energy loss spectroscopy (EELS) techniques. Z-contrast images have been used primarily to evaluate the particle -size distribution and morphology as a function of the catalyst reduction temperature. Energy-loss spectra obtained from specific locations in and around the metal particles show a shift in the Pd M2,3 edge, which is an indication of the Pd nanoparticles getting negatively charged. This acquired charge on the Pd particles is the result of electron transfer from the alumina support and appears to be dependent on the size of the Pd nanoparticles. No aluminum signal is observed on the surface of the metallic nanoparticles, and the observed oxygen signal on the Pd nanoparticles is probably due to oxygen that gets adsorbed on the reduced catalyst.

Analysis of The Electronic Structure of Threading Dislocations in GaN Using Multiple Scattering Simulations

The promise of advanced technological applications in optical and electronic devices has led to a significant recent research effort in the structure-property relationships of defects in GaN. in particular, the major scientific issues arise from the high density of threading dislocations induced during thin film growth by film-substrate lattice mismatch. There is still debate as to the exact effect of these dislocations on the overall properties; they may or may not be electrically active and are thought to decrease the lifetime of devices. As such, a fundamental understanding of the electronic properties of these defects will facilitate the development of new and improved devices.The analysis of the electronic structure of dislocations in GaN is performed here by a combination of atomic resolution electron energy loss spectroscopy (EELS) in the scanning transmission electron microscope (STEM) and multiple scattering (MS) simulations. Experimentally, a Z-contrast image of the dislocation core is obtained first1 and used to position the probe for EELS.

A Combined Experimental and Theoretical Approach to Atomic Structure and Segregation at Ceramic Interfaces

In the last few years, the scanning transmission electron microscope has become capable of forming electron probes of atomic dimensions. This makes possible the technique of Z-contrast imaging, a method of forming direct images at atomic resolution with high compositional sensitivity. Atomic column positions can be determined to high accuracy from the image, and columns containing high-Z impurities will be visible. Atomic resolution electron energy loss spectroscopy is possible by locating the probe over particular atomic columns or planes seen in the image. This provides complementary information on low-Z species and chemical bonding. Such data represents an ideal starting point for first-principles theoretical calculations of energetics and dynamics, avoiding time-consuming searches of trial structures. Examples are shown of ordering in relaxor ferroelectrics, interfacial termination in oxide–oxide and metal–oxide interfaces, and an impurity-induced structural transformation of a ceramic grain boundary.

Atomic Resolution Electron Energy Loss Spectroscopy of Interfaces

The ability of high resolution STEM instruments to provide EELS data at the ultimate atomic resolution offers significant new insights into interfacial phenomena. Not only can composition profiles now be collected plane-by-plane across an interface, but in addition, the EELS near edge fine structure can be analysed to determine the valence states of atoms at the interface, and hence the nature of the bonding across the interface plane. by applying the spatial difference technique, these effects can be studied quantitatively with high sensitivity.Atomic resolution EELS played a critical role in explaining a completely unexpected interfacial structure found recently at CdTe/Si interfaces grown by a particular molecular beam epitaxy growth procedure. The high resolution Z-contrast image in Fig. 1 shows the CdTe film to be terminated by Te, but in addition, for several monolayers into the Si substrate, occasional columns show brighter than the surrounding Si, although remaining in the correct position.

High Angle Dark Field STEM for Advanced Materials

Incoherent imaging in the scanning transmission electron microscope (STEM) provides enhanced image resolution, strong Z-contrast, and the capability for direct inversion to the projected structure. It relies on the use of a high angle annular detector to average phase correlations between diffracted beams, while the coherent incident probe leads to columnar channelling in thick specimens. The Z-contrast image also provides a convenient, high intensity, reference image for atomic resolution electron energy loss spectroscopy, since incoherent imaging conditions may be established simultaneously for both signals. Recent results from a 300 kV STEM are reviewed, including the determination of dislocation core structures in compound semiconductors, distinguishing between oxygen and metal termination at a metal/ceramic interface, and resolving the atomic structure of supported catalyst clusters.