Direct Determination of Atomic Structure and Magnetic Coupling of Magnetite Twin Boundaries.

Various properties of ceramics can be significantly influenced by the presence of grain boundaries. The influence on the properties is closely related to the grain‐boundary atomic structures. As different grain boundaries have different atomic structure, different grain boundaries have different influence on the properties. It is difficult to examine the atomic structure and properties of individual grain boundaries in ceramics. In order to understand the atomic–structure–property relationships, well‐defined single grain boundaries should be characterized. In the present paper, we review our recent results on the investigations of atomic structures and electrical properties of ZnO single grain boundaries. The relationships between the atomic structures and the electrical properties were investigated using ZnO bicrystals, whose grain‐boundary orientation relationship and grain‐boundary planes can be arbitrarily controlled. The discussion focuses on the microscopic origin of nonlinear current–voltage ( I–V ) characteristics across ZnO grain boundaries. High‐resolution transmission electron microscopy (HRTEM) observations and lattice‐statics calculations revealed the atomic structures of the undoped ZnO [0001] Σ7 and Σ49 grain boundaries, enabling a comparison between coincidence site lattice (CSL) boundaries with small and large periodicity. These grain boundaries contained the common structural units (SUs) featuring atoms with coordination numbers that are unusual in ZnO. The Σ49 boundary was found to have characteristic arrangement of the SUs, where two kinds of the SUs are alternatively formed. It is considered that the characteristic arrangement was formed to effectively relax the local strain in the vicinity of the boundary. Such a relaxation of local strain is considered to be one of dominant factors to determine the SU arrangements along grain boundaries. I–V measurements of the undoped ZnO bicrystals showed linear I–V characteristics. Although the coordination and bond lengths of atoms in the grain boundaries differ from those in the bulk crystal, this does apparently not generate deep unoccupied states in the band gap. This indicates that atomic structures of undoped ZnO grain boundaries are not responsible for the nonlinear I–V characteristics of ZnO ceramics. On the other hand, the nonlinear I–V characteristic appeared when doping the boundaries with Pr. High‐angle annular dark‐field scanning transmission electron microscopy (HAADF‐STEM) image of Pr‐doped boundaries revealed that Pr segregates to specific atomic columns, substituting Zn at the boundary. However, the Pr itself was not the direct origin of the nonlinear I–V characteristics, as the Pr existed in the three‐plus state and would not produce acceptor states in the boundary. First‐principles calculations revealed that Pr doping instead promotes the formations of acceptor‐like native defects, such as Zn vacancies. We believe that such acceptor‐like native defects are microscopic origin of the nonlinear I–V characteristics. Investigations of various types of grain boundaries in the Pr and Co‐codoped ZnO bicrystals indicated that the amounts of Pr segregation and the nonlinear I–V characteristics significantly depend on the grain‐boundary orientation relationship. Larger amount of Pr segregation and, as a result, higher nonlinearity in I–V characteristics was obtained for incoherent boundaries. This indicates that Pr doping to incoherent boundaries is one of the guidelines to design the single grain boundaries with highly nonlinear I–V characteristics. Finally, a Pr and Co‐codoped bicrystal with an incoherent boundary was fabricated to demonstrate a highly nonlinear I–V characteristic. This result indicates that ZnO single‐grain‐boundary varistors can be designed by controlling grain‐boundary atomic structures and fabrication processes. Summarizing, our work firstly enabled us to gain a deeper understanding for the atomic structure of ZnO grain boundaries. Secondly, we obtained important insight into the origin of nonlinear I–V characteristics across the ZnO grain boundaries. And, finally, based on these results, we demonstrated the potential of this knowledge for designing and fabricating ZnO single‐grain‐boundary varistors.

It is of great academic significance to understand the influence that the atomic-scale structure of interfaces and boundaries within materials has on magnetic coupling characteristics and promote the innovation of advanced magnetic devices. Here, we carried out a systematic investigation of the atomic and electronic structures of twin boundaries (TBs) in Li0.5Fe2.5O4 (LFO) thin films and determined their concurrent magnetic couplings using atomic-resolution transmission electron microscopy and first-principle calculations at the atomic scale. The results show that ferromagnetic or antiferromagnetic coupling can exist across the different TBs in LFO thin films, and electrical structures within a few atomic layers directly rely on the atom arrangement across the TB. Uncovering one-to-one relationships between the magnetic coupling properties of individual TBs and atomic-scale structures can clarify a thorough comprehension of numerous fascinating magnetic properties of commonly utilized magnetic materials, which will undoubtedly encourage the progress of sophisticated magnetic materials and devices.

Three-Dimensional Atomic Structure of Grain Boundaries Resolved by Atomic-Resolution Electron Tomography

The atomic scale structure and chemistry of (111) twins in MgAl2O4 spinel crystals from the Pinpyit locality near Mogok (Myanmar, formerly Burma) were analysed using complementary methods of transmission electron microscopy (TEM). To obtain a three-dimensional information on the atomic structure, the twin boundaries were investigated in crystallographic projections $$ [\ifmmode\expandafter\bar\else\expandafter\=\fi{1}10] $$ and $$ [11\ifmmode\expandafter\bar\else\expandafter\=\fi{2}]. $$ Using conventional electron diffraction and high-resolution TEM (HRTEM) analysis we have shown that (111) twins in spinel can be crystallographically described by 180° rotation of the oxygen sublattice normal to the twin composition plane. This operation generates a local hcp stacking in otherwise ccp lattice and maintains a regular sequence of kagome and mixed layers. In addition to rotation, no other translations are present in (111) twins in these spinel crystals. Chemical analysis of the twin boundary was performed by energy-dispersive X-ray spectroscopy (EDS) using a variable beam diameter (VBD) technique, which is perfectly suited for analysing chemical composition of twin boundaries on a sub-nm scale. The VBD/EDS measurements indicated that (111) twin boundary in spinel is Mg-deficient. Quantitative analyses of HRTEM (phase contrast) and HAADF-STEM (Z-contrast) images of (111) twin boundary have confirmed that Mg2+ ions are replaced with Be2+ ions in boundary tetrahedral sites. The Be-rich twin boundary structure is closely related to BeAl2O4 (chrysoberyl) and BeMg3Al8O16 (taaffeite) group of intermediate polysomatic minerals. Based on these results, we conclude that the formation of (111) twins in spinel is a preparatory stage of polytype/polysome formation (taaffeite) and is a result of thermodynamically favourable formation of hcp stacking due to Be incorporation on the {111} planes of the spinel structure in the nucleation stage of crystal growth. The twin structure grows as long as the surrounding geochemical conditions allow its formation. The incorporation of Be induces a 2D-anisotropy and exaggerated growth of the crystal along the (111) twin boundary.

This talk will give an overview of methods for solving the atomic structure of nanostructured materials using focused electron beams. It will illustrate these methods with a range of applications, such as the determination of the atomic structure and stability of nanoparticle facets [1]; the local atomic structure of "chessboard' nanostructures in lithium-based titanate perovskites; and the measurement of local polarity, dopant concentration and atomic-scale morphology in semiconducting nanowire quantum wells. These methods take advantage of the fact that electron wavefields can be brought to a focal point smaller than an Ångström in diameter, enabling small volumes of matter to be probed and characterized. The wealth of information contained in the resulting diffraction patterns can be interrogated selectively to isolate and `image' specific structural information. Several methods using small focused electron beams will be described in this talk, including; (i) An approach for the determination of centrosymmetric structures from the direct observation of structure factor phases by inspection of features in convergent beam electron diffraction patterns [2]. The method can achieve high resolution from just a few phase observations and no intensity measurements or iterative refinements are required; (ii) Methods for the quantitative interpretation of the intensity in atomic resolution imaging and diffraction data for the measurement of local atomic and electronic structure; (iii) Pseudo-confocal scanning transmission electron microscopy methods for obtaining depth and chemical information which record the scattered intensity in a plane conjugate to the specimen (as opposed to the diffraction plane) [3].

Aggregates of grain boundaries: structure and properties

The formation and anisotropic/isotropic diffusion behaviors of vacancy in typical twin boundaries of α-Ti: An ab initio study

Developing novel catalysts with high efficiency and selectivity is critical for enabling future clean energy conversion technologies. Interfaces in catalyst systems have long been considered the most critical factor in controlling catalytic reaction mechanisms. Interfaces include not only the catalyst surface but also interfaces within catalyst particles and those formed by constructing heterogeneous catalysts. The atomic and electronic structures of catalytic surfaces govern the kinetics of binding and release of reactant molecules from surface atoms. Interfaces within catalysts are introduced to enhance the intrinsic activity and stability of the catalyst by tuning the surface atomic and chemical structures. Examples include interfaces between the core and shell, twin or domain boundaries, or phase boundaries within single catalyst particles. In supported catalyst nanoparticles (NPs), the interface between the metallic NP and support serves as a critical tuning factor for enhancing catalytic activity. Surface electronic structure can be indirectly tuned and catalytically active sites can be increased through the use of supporting oxides. Tuning interfaces in catalyst systems has been identified as an important strategy in the design of novel catalysts. However, the governing principle of how interfaces contribute to catalyst behavior, especially in terms of interactions with intermediates and their stability during electrochemical operation, are largely unknown. This is mainly due to the evolving nature of such interfaces. Small changes in the structural and chemical configuration of these interfaces may result in altering the catalytic performance. These interfacial arrangements evolve continuously during synthesis, processing, use, and even static operation. A technique that can probe the local atomic and electronic interfacial structures with high precision while monitoring the dynamic interfacial behavior in situ is essential for elucidating the role of interfaces and providing deeper insight for fine-tuning and optimizing catalyst properties. Scanning transmission electron microscopy (STEM) has long been a primary characterization technique used for studying nanomaterials because of its exceptional imaging resolution and simultaneous chemical analysis. Over the past decade, advances in STEM, that is, the commercialization of both aberration correctors and monochromators, have significantly improved the spatial and energy resolution. Imaging atomic structures with subangstrom resolution and identifying chemical species with single-atom sensitivity are now routine for STEM. These advancements have greatly benefitted catalytic research. For example, the roles of lattice strain and surface elemental distribution and their effect on catalytic stability and reactivity have been well documented in bimetallic catalysts. In addition, three-dimensional atomic structures revealed by STEM tomography have been integrated in theoretical modeling for predictive catalyst NP design. Recent developments in stable electronic and mechanical devices have opened opportunities to monitor the evolution of catalysts in operando under synthesis and reaction conditions; high-speed direct electron detectors have achieved sub-millisecond time resolutions and allow for rapid structural and chemical changes to be captured. Investigations of catalysts using these latest microscopy techniques have provided new insights into atomic-level catalytic mechanisms. Further integration of new microscopy methods is expected to provide multidimensional descriptions of interfaces under relevant synthesis and reaction conditions. In this Account, we discuss recent insights on understanding catalyst activity, selectivity, and stability using advanced STEM techniques, with an emphasis on how critical interfaces dictate the performance of precious metal-based heterogeneous catalysts. The role of extended interfacial structures, including those between core and shell, between separate phases and twinned grains, between the catalyst surface and gas, and between metal and support are discussed. We also provide an outlook on how emerging electron microscopy techniques, such as vibrational spectroscopy and electron ptychography, will impact future catalysis research.

Graphene, a two-dimensional honeycomb lattice of carbon atoms of great interest in (opto)electronics and plasmonics, can be obtained by means of diverse fabrication techniques, among which chemical vapour deposition (CVD) is one of the most promising for technological applications. The electronic and mechanical properties of CVD-grown graphene depend in large part on the characteristics of the grain boundaries. However, the physical properties of these grain boundaries remain challenging to characterize directly and conveniently. Here we show that it is possible to visualize and investigate the grain boundaries in CVD-grown graphene using an infrared nano-imaging technique. We harness surface plasmons that are reflected and scattered by the graphene grain boundaries, thus causing plasmon interference. By recording and analysing the interference patterns, we can map grain boundaries for a large-area CVD graphene film and probe the electronic properties of individual grain boundaries. Quantitative analysis reveals that grain boundaries form electronic barriers that obstruct both electrical transport and plasmon propagation. The effective width of these barriers (∼10-20nm) depends on the electronic screening and is on the order of the Fermi wavelength of graphene. These results uncover a microscopic mechanism that is responsible for the low electron mobility observed in CVD-grown graphene, and suggest the possibility of using electronic barriers to realize tunable plasmon reflectors and phase retarders in future graphene-based plasmonic circuits.

Structure and energy of the (101) twin boundary in hot-deformed Nd-Fe-B magnets

Micro four-point probe investigation of individual ZnO grain boundaries in a varistor ceramic

General relations between the local atomic structure and properties of grain boundaries (GBs) in superconducting copper oxides are described. The results are based on the complex experimental and theoretical study, which includes: (1) the transmission electron microscopy characterization of GBs in the bulk materials, (2) GB atomic structure simulation, (3) first-principles calculations and modelling of the electron structure modification, crystal lattice relaxation and intergrain bond formation near the GBs simulated. The number of broken Cu-O bonds, the degree of electron state perturbation at the GB and the role of crystal lattice relaxation are found to be strongly dependent on the GB topology.

Local atomic and electronic structure of tungsten ions in AWO 4 crystals of scheelite and wolframite types

Disentangling the limitations of dipole activity at the atomic scale in the dielectric materials is key to boosting the intrinsic polarization ability and electromagnetic (EM) energy dissipation. Herein, an atomic structure regulation in molybdenum disulfide (MoS 2 ) nanosheets (NSs) connected by multi‐wall carbon nanotubes (MoS 2 ‐MWCNTs) is studied to explore the correlation between the local atomic structure and polarization properties at the atomic scale. Induced by the phase transition engineering, 1T‐phase dominated MoS 2 NSs with centrosymmetric D3d point groups are evolved from the 2H‐phase structure. Atomic‐scale and visualized electric field distribution in the MoS 2 NSs are verified by the advanced integrated differential phase contrast technology and related scanning transmission electron microscopy (iDPC‐STEM) imaging. Periodic atomic structure in the 1T‐phase MoS 2 is constructed to enhance symmetrical electric field distribution and those phase interlaced regions promoted a significant increase in dielectric relaxation. Benefiting the boosted dielectric ability and polarization behaviors, 1T‐phase dominated MoS 2 @MWCNTs (TMC) exhibit superior EM wave energy dissipation with the minimum reflection loss (RL min ) value of −74.3 dB at only 1.7 mm thickness. Changes in atomic structure and electric field study in the MoS 2 NSs help to further elucidate their polarization mechanism and to explore the for studying the mixed‐phase 2D functional nanosheets.

The atomic structure of amorphous alloys plays a crucial role in determining both their glass-forming ability and magnetic properties. In this study, we investigate the influence of adding the Y element on the glass-forming ability and magnetic properties of Fe86−x Y x B7C7 (x = 0, 5, 10 at.%) amorphous alloys via both experiments and ab initio molecular dynamics simulations. Furthermore, we explore the correlation between local atomic structures and properties. Our results demonstrate that an increased Y content in the alloys leads to a higher proportion of icosahedral clusters, which can potentially enhance both glass-forming ability and thermal stability. These findings have been experimentally validated. The analysis of the electron energy density and magnetic moment of the alloy reveals that the addition of Y leads to hybridization between Y-4d and Fe-3d orbitals, resulting in a reduction in ferromagnetic coupling between Fe atoms. This subsequently reduces the magnetic moment of Fe atoms as well as the total magnetic moment of the system, which is consistent with experimental results. The results could help understand the relationship between atomic structure and magnetic property, and providing valuable insights for enhancing the performance of metallic glasses in industrial applications.