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Iterative reconstruction of magnetic induction using Lorentz transmission electron tomography
Intense ongoing research on complex nanomagnetic structures requires a fundamental understanding of the 3D magnetization and the stray fields around the nano-objects. 3D visualization of such fields offers the best way to achieve this. Lorentz transmission electron microscopy provides a suitable combination of high resolution and ability to quantitatively visualize the magnetization vectors using phase retrieval methods. In this paper, we present a formalism to represent the magnetic phase shift of electrons as a Radon transform of the magnetic induction of the sample. Using this formalism, we then present the application of common tomographic methods particularly the iterative methods, to reconstruct the 3D components of the vector held. We present an analysis of the effect of missing wedge and the limited angular sampling as well as reconstruction of complex 3D magnetization in a nanowire using simulations.
3D elemental mapping with nanometer scale depth resolution via electron optical sectioning
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Foreword for the special issue on the Tenth International Workshop on Low Energy Electron Microscopy and Photoemission Electron Microscopy
The Tenth International Workshop on Low Energy Electron Microscopy and Photoemission Electron Microscopy (LEEM/PEEM-10) was hosted in Monterey, California on September 11–15, 2016. This Workshop marked the 18th anniversary of the LEEM/PEEM series. The inaugural LEEM Workshop was organized by Ernst Bauer, the inventor of LEEM, and was held 1998 in Tempe, Arizona, USA. Since then this biennial series of meetings was held in locations rotating between America, Asia and Europe. The workshop series is designed to review the status of LEEM, PEEM, SPLEEM, XPEEM and related techniques, and to promote and disseminate applications of cathode lens microscopy to a broad audience of interested scientists. Highlighting the most recent scientific advances and instrumental developments, the LEEM/PEEM-10 meeting was attended by 114 researchers from 17 countries and featured sessions discussing new results on properties of surfaces, thin films, organic films, as well as surface chemistry, magnetism, time resolved methods and instrumental advances and novel applications of LEEM and PEEM to other subject areas.
Comparison of dislocation characterization by electron channeling contrast imaging and cross-correlation electron backscattered diffraction
In this paper, the relative capabilities and limitations of electron channeling contrast imaging (ECCI) and cross-correlation electron backscattered diffraction (CC-EBSD) have been assessed by studying the dislocation distributions resulting from nanoindentation in body centered cubic Ta. Qualitative comparison reveals very similar dislocation distributions between the CC-EBSD mapped GNDs and the ECC imaged dislocations. Approximate dislocation densities determined from ECC images compare well to those determined by CC-EBSD. Nevertheless, close examination reveals subtle differences in the details of the distributions mapped by these two approaches. Finally, the details of the dislocation Burgers vectors and line directions determined by ECCI have been compared to those determined using CC-EBSD and reveal good agreement.
Measurement Error in Atomic-Scale Scanning Transmission Electron Microscopy—Energy-Dispersive X-Ray Spectroscopy (STEM-EDS) Mapping of a Model Oxide Interface
Abstract With the development of affordable aberration correctors, analytical scanning transmission electron microscopy (STEM) studies of complex interfaces can now be conducted at high spatial resolution at laboratories worldwide. Energy-dispersive X-ray spectroscopy (EDS) in particular has grown in popularity, as it enables elemental mapping over a wide range of ionization energies. However, the interpretation of atomically resolved data is greatly complicated by beam–sample interactions that are often overlooked by novice users. Here we describe the practical factors—namely, sample thickness and the choice of ionization edge—that affect the quantification of a model perovskite oxide interface. Our measurements of the same sample, in regions of different thickness, indicate that interface profiles can vary by as much as 2–5 unit cells, depending on the spectral feature. This finding is supported by multislice simulations, which reveal that on-axis maps of even perfectly abrupt interfaces exhibit significant delocalization. Quantification of thicker samples is further complicated by channeling to heavier sites across the interface, as well as an increased signal background. We show that extreme care must be taken to prepare samples to minimize channeling effects and argue that it may not be possible to extract atomically resolved information from many chemical maps.
Transmission Electron Microscopy Studies of Electron-Selective Titanium Oxide Contacts in Silicon Solar Cells
Abstract In this study, the cross-section of electron-selective titanium oxide (TiO 2 ) contacts for n -type crystalline silicon solar cells were investigated by transmission electron microscopy. It was revealed that the excellent cell efficiency of 21.6% obtained on n -type cells, featuring SiO 2 /TiO 2 /Al rear contacts and after forming gas annealing (FGA) at 350°C, is due to strong surface passivation of SiO 2 /TiO 2 stack as well as low contact resistivity at the Si/SiO 2 /TiO 2 heterojunction. This can be attributed to the transformation of amorphous TiO 2 to a conducting TiO 2- x phase. Conversely, the low efficiency (9.8%) obtained on cells featuring an a-Si:H/TiO 2 /Al rear contact is due to severe degradation of passivation of the a-Si:H upon FGA.
Net Electronic Charge as an Effective Electronic Descriptor for Oxygen Release and Transport Properties of SrFeO 3 -based Oxygen Sorbents
Perovskite oxides, as oxygen sorbents, exhibit excellent potential in thermochemical redox applications such as chemical looping air separation (CLAS), resulting from their excellent redox properties and high tunability. The structural and compositional flexibility of perovskites, while making these mixed oxides highly versatile, also poses challenges for their design and optimization. Moreover, the elevated operating temperature and dynamic oxygen partial pressure swings during redox processes add additional complications for oxygen sorbent design. From a fundamental standpoint, finding simple yet effective descriptors for the effect of cation substitutions on oxygen sorbents’ redox and oxygen transport properties are of significant importance for rational optimization of perovskite-based oxygen sorbents. In the present study, a series of A/B-site-doped SrFeO 3-δ perovskites are investigated using density functional theory (DFT) + U simulations. From these simulations, an effective electronic structural descriptor, the net electronic charge (Δe) of oxygen anions, is extracted based on its excellent correlations with vacancy formation energy, experimentally derived oxygen capacity, and oxygen vacancy migration barrier. Overall, these findings provide an effective tool to correlate the oxygen sorbents’ redox performances with a simple yet physically meaningful descriptor for rational design and optimization of perovskites in the context of CLAS.
Potential Skyrmion Host Fe(IO3)3: Connecting Stereoactive Lone-Pair Electron Effects to the Dzyaloshinskii-Moriya Interaction
Magnetic skyrmions, which are topologically distinct magnetic spin textures, are gaining increased attention for their unique physical properties and potential applications in spintronic devices. Here we present a design strategy for skyrmion host candidates based on combinations of magnetic spin, asymmetric building units having stereoactive lone-pair electrons, and polar lattice symmetry. To demonstrate the viability of the proposed rational design principles, we successfully synthesized a Fe(IO 3 ) 3 polycrystalline sample and single crystals by using a new simplified low-temperature pathway, which is experimentally feasible for extending materials growth of transition metal iodates. Single crystal X-ray and powder synchrotron X-ray diffraction measurements demonstrated that Fe(IO 3 ) 3 crystallizes in the polar chiral hexagonal lattice with space group P63. The combined structural features of the macroscopic electric polarization along the c-axis stemming from the coalignment of the stereoactive lone-pairs of the IO 3 – trigonal pyramid and the magnetic Fe 3+ cation residing on the 3-fold rotation axis were selected to promote asymmetric exchange coupling. We find evidence of a predicted skyrmion phase at 14 K ≤ T ≤ 16 K and 2.5 T ≤ μ 0 H ≤ 3.2 T driven by a Dzyaloshinskii–Moriya (DM) interaction, a conclusion supported by the appreciable DM exchange and the zero-field spiral antiferromagnetic ground state of Fe(IO 3 ) 3 deduced from neutron diffraction experiments. The associated magnetic modulation wavelength of the putative skyrmions is expected to be short ~18 nm, comparable to the period of the DM-driven incommensurate order. This work links stereoactive lone-pair electron effects to enhanced DM interaction, demonstrating a new approach for chemical guidelines in the search for skyrmionic states of matter.
Dominance of interface chemistry over the bulk properties in determining the electronic structure of epitaxial metal/perovskite oxide heterojunctions
We show that despite very similar crystallographic properties and work function values in the bulk, epitaxial Fe and Cr metallizations on Nb:SrTiO3(001) generate completely different heterojunction electronic properties. Cr is Ohmic whereas Fe forms a Schottky barrier with a barrier height of 0.50 eV. This contrast arises because of differences in interface chemistry. In contrast to Cr [Chambers, S. A. et al., Adv. Mater. 2013, 25, 4001.], Fe exhibits a +2 oxidation state and occupies Ti sites in the perovskite lattice, resulting in negligible charge transfer to Ti, upward band bending, and Schottky barrier formation. The differences between Cr and Fe are understood by performing first-principles calculations of the energetics of defect formation which corroborate the observed interface chemistry and structure.
Li 3 Mo 4 P 5 O 24 : A Two-Electron Cathode for Lithium-Ion Batteries with Three-Dimensional Diffusion Pathways
Abstract not provided
Indolo-naphthyridine-6,13-dione Thiophene Building Block for Conjugated Polymer Electronics: Molecular Origin of Ultrahigh n-Type Mobility
Abstract not provided
A Chemical Understanding of the Band Convergence in Thermoelectric CoSb 3 Skutterudites: Influence of Electron Population, Local Thermal Expansion, and Bonding Interactions
N-Type skutterudites, such as Yb x Co 4 Sb 12 , have recently been shown to exhibit high valley degeneracy with possible band convergence, explaining the excellent thermoelectric efficiency of these materials. Using a combined theoretical and experimental approach involving temperature-dependent synchrotron diffraction, molecular orbital diagrams, and computational studies, the chemical nature of critical features in the band structure is highlighted. We identify how n-type doping on the filler site induces structural changes that are observed in both the diffraction data and computational results. Additionally, we show how chemical n-type doping slightly alters the electronic band structure, moving the high-valley degeneracy secondary conduction band closer to the primary conduction band and thus inducing band convergence.
Nuclear–Electronic Orbital QM/MM Approach: Geometry Optimizations and Molecular Dynamics
Hybrid quantum mechanical/molecular mechanical (QM/MM) methods allow simulations of chemical reactions in atomistic solvent and heterogeneous environments such as proteins. Herein, the nuclear–electronic orbital (NEO) QM/MM approach is introduced to enable the quantization of specified nuclei, typically protons, in the QM region using a method such as NEO-density functional theory (NEO-DFT). This approach includes proton delocalization, polarization, anharmonicity, and zero-point energy in geometry optimizations and dynamics. Expressions for the energies and analytical gradients associated with the NEO-QM/MM method, as well as the previously developed polarizable continuum model (NEO-PCM), are provided. Geometry optimizations of small organic molecules hydrogen bonded to water in either dielectric continuum solvent or explicit atomistic solvent illustrate that aqueous solvation can strengthen hydrogen-bonding interactions for the systems studied, as indicated by shorter intermolecular distances at the hydrogen-bond interface. We then performed a real-time direct dynamics simulation of a phenol molecule in explicit water using the NEO-QM/MM method. Furthermore, these developments and initial examples provide the foundation for future studies of nuclear–electronic quantum dynamics in complex chemical and biological environments.
Multicomponent Cholesky Decomposition: Application to Nuclear–Electronic Orbital Theory
The Cholesky decomposition technique is commonly used to reduce the memory requirement for storing two-particle repulsion integrals in quantum chemistry calculations that use atomic orbital bases. However, when quantum methods use multicomponent bases, such as nuclear–electronic orbitals, additional challenges are introduced due to asymmetric two-particle integrals. This work proposes several multicomponent Cholesky decomposition methods for calculations using nuclear–electronic orbital density functional theory. To analyze the errors in different Cholesky decomposition components, benchmark calculations using water clusters are carried out. The largest benchmark calculation is a water cluster (H 2 O) 27 where all 54 protons are treated quantum mechanically. Furthermore, this study provides energetic and complexity analyses to demonstrate the accuracy and performance of the proposed multicomponent Cholesky decomposition method.
Effect of Anisotropic Confinement on Electronic Structure and Dynamics of Band Edge Excitons in Inorganic Perovskite Nanowires
Inorganic lead halide perovskite nanostructures show promise as the active layers in photovoltaics, light emitting diodes, and other optoelectronic devices. They are robust in the presence of oxygen and water, and the electronic structure and dynamics of these nanostructures can be tuned through quantum confinement. Here we create aligned bundles of CsPbBr3 nanowires with widths resulting in quantum confinement of the electronic wave functions and subject them to ultrafast microscopy. We directly image rapid one-dimensional exciton diffusion along the nanowires, and we measure an exciton trap density of roughly one per nanowire. Using transient absorption microscopy, we observe a polarization-dependent splitting of the band edge exciton line, and from the polarized fluorescence of nanowires in solution, we determine that the exciton transition dipole moments are anisotropic in strength. Furthermore, our observations are consistent with a model in which splitting is driven by shape anisotropy in conjunction with long-range exchange.