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... Screw dislocations and grain boundaries have also been visualized at similar resolution (Fryer, 1980;Kobayashi, Fujiyoshi, Iwatsu & Uyeda, 1981;Van Tendeloo, Amelinckx, Muto, Verheijen, Loosdrecht & Meijer, 1993). Aided by computations, there have been numerous descriptions of disorder for inorganic materials (Busek & Veblen, 1988;Eyring, 1988;Smith & Barry, 1988). ...
The idea of solving unknown crystal structures from experimental electron-diffraction intensities and high-resolution electron micrographs has remained a controversial topic in the 60 year history of electron crystallography. In this review it will be shown that the application of modern direct phasing techniques, familiar to X-ray crystallographers, has decisively proven that such ab initio determinations are, in fact, possible. This statement does not, by any means, refute the existence of the several significant scattering perturbations identified by diffraction physicists. Rather, it does affirm that experimental parameters can be controlled to ensure that a 'quasi-kinematical' data set can be collected from many types of specimens. Numerous applications have been made to various types of specimens, ranging from small organics to proteins, and also some inorganic materials. While electron crystallography may not be the optimal means for determining accurate bonding parameters, it is often the method of choice when only microcrystalline specimens are available.
... Screw dislocations and grain boundaries have also been visualized at similar resolution (Fryer, 1980;Kobayashi, Fujiyoshi, Iwatsu & Uyeda, 1981;Van Tendeloo, Amelinckx, Muto, Verheijen, Loosdrecht & Meijer, 1993). Aided by computations, there have been numerous descriptions of disorder for inorganic materials (Busek & Veblen, 1988;Eyring, 1988;Smith & Barry, 1988). ...
The idea of solving unknown crystal structures from experimental electron-diffraction intensities and high-resolution electron micrographs has remained a controversial topic in the 60 year history of electron crystallography. In this review it will be shown that the application of modern direct phasing techniques, familiar to X-ray crystallographers, has decisively proven that such ab initio determinations are, in fact, possible. This statement does not, by any means, refute the existence of the several significant scattering perturbations identified by diffraction physicists. Rather, it does affirm that experimental parameters can be controlled to ensure that a 'quasi-kinematical' data set can be collected from many types of specimens. Numerous applications have been made to various types of specimens, ranging from small organics to proteins, and also some inorganic materials. While electron crystallography may not be the optimal means for determining accurate bonding parameters, it is often the method of choice when only microcrystalline specimens are available.
... The structure models for several cases were derived from HRTEM images which were then further optimized using DLS modeling to provide useful starting models for Rietveld refinement. Within the past twenty years, transmission electron microscopy has advanced to the point where it can provide images of the atomic arrangement in a crystal, often easily yielding insights into general structural schemes and, in some cases, solving apparently intractable problems (e.g., Veblen, 1985; Buseck and Veblen, 1988). It is important to note, however, that HRTEM images must be used with caution. ...
The Rietveld method was originally developed (Rietveld, 1967, 1969) to refine crystal structures using neutron powder diffraction data. Since then, the method has been increasingly used with X-ray powder diffraction data, and today it is safe to say that this is the most common application of the method. The method has been applied to numerous natural and synthetic materials, most of which do not usually form crystals large enough for study with single-crystal techniques. It is the ability to study the structures of materials for which sufficiently large single crystals do not exist that makes the method so powerful and popular. It would thus appear that the method is ideal for studying clays and clay minerals. In many cases this is true, but the assumptions implicit in the method and the disordered nature of many clay minerals can limit titsapplicability. This chapter will describe the Rietveld method, emphasizing the assumptions important for the study of disordered materials, and it will outline the potential applications of the method to these minerals. These applications include, in addition to the refinement of crystal structures, quantitative analysis of multicomponent mixtures, analysis of peak broadening, partial structure solution, and refinement of unit-cell parameters.
... The elastic scattering of high-energy electrons within a material, as detected by electron diffraction , electron holography or high resolution transmission electron microscopy (HRTEM), probes the distribution of the internal electrostatic potential [1, 2] of a sample. This potential is given by the Coulomb potential of the nuclei, screened by the electrons. ...
We report on the detection and charge distribution analysis for nitrogen substitutional dopants in single layer graphene membranes by aberration-corrected high-resolution transmission electron microscopy (HRTEM). Further, we show that the ionicity of single-layer hexagonal boron nitride can be confirmed from direct images. For the first time, we demonstrate by a combination of HRTEM experiments and first-principles electronic structure calculations that adjustments to the atomic potentials due to chemical bonding can be discerned in HRTEM images. Our experiments open a way to discern electronic configurations in point defects or other non-periodic arrangements or nanoscale objects that can not be analyzed in an electron or x-ray diffraction experiment. Comment: Article and supplementary information, v2 one more figure
... Increasing vibrational amplitudes, in fact, leave the average lattice constants unchanged and therefore simply result in a decrease of the diffraction intensity. 53 Our results are in agreement with the theory of Fasolino, which predicts that phonon-induced corrugations in graphene should occur on length scales below 20 nm. Such corrugations, which may be related to the different bond lengths arising from the multiplicity of the carbon bond, display a well-defined dependence on temperature. ...
Low-energy electron microscopy and microprobe diffraction are used to image and characterize corrugation in SiO(2)-supported and suspended exfoliated graphene at nanometer length scales. Diffraction line-shape analysis reveals quantitative differences in surface roughness on length scales below 20 nm which depend on film thickness and interaction with the substrate. Corrugation decreases with increasing film thickness, reflecting the increased stiffness of multilayer films. Specifically, single-layer graphene shows a markedly larger short-range roughness than multilayer graphene. Due to the absence of interactions with the substrate, suspended graphene displays a smoother morphology and texture than supported graphene. A specific feature of suspended single-layer films is the dependence of corrugation on both adsorbate load and temperature, which is manifested by variations in the diffraction line shape. The effects of both intrinsic and extrinsic corrugation factors are discussed.
... For example, the investigations conducted (Akhmadeev et al., 1992; Akhmadeev et al., 1993) showed that the Young's modulus in CuAkhmadeev et al., 1992; Akhmadeev et al., 1993; Lebedev et al., 1996a,b). An assumption that a non-equilibrium state of grain boundaries attributed to high density of grain boundary dislocations leading to the formation of a low modulus phase near grain boundaries could be the main reason for a decrease of the Young's modulus in ultrafine-grained materials produced by severe plastic deformation (Valiev et al., 1994b) was also put forward. On the other hand, it is known that severe plastic strains accompanied by the formation of a crystallographic texture (Alexandrov et al., 1996a) being the main factor determining the level and anisotropy of elastic properties in coarsegrained materials (Davies, 1976). ...
It was shown that in ultrafine-grained nanostructured Cu processed by severe plastic deformation and subjected to cold rolling and annealing, the level and character ofYoung's modulus anisotropy is significantly different from values corresponding to cold rolled and annealed coarse-grained Cu. The crystallographic texture formation processes are investigated in these states in parallel. The comparative study of the elastic behaviour and crystallographic texture lets us draw conclusions concerning the leading role of not only developing crystallographic texture but a specific defect structure of grain boundaries as well in the formation of unusual elastic properties ofultrafine-grained materials processed by severe plastic deformation.
The recent discovery of graphene has sparked much interest, thus far focused on the peculiar electronic structure of this material, in which charge carriers mimic massless relativistic particles. However, the physical structure of graphene--a single layer of carbon atoms densely packed in a honeycomb crystal lattice--is also puzzling. On the one hand, graphene appears to be a strictly two-dimensional material, exhibiting such a high crystal quality that electrons can travel submicrometre distances without scattering. On the other hand, perfect two-dimensional crystals cannot exist in the free state, according to both theory and experiment. This incompatibility can be avoided by arguing that all the graphene structures studied so far were an integral part of larger three-dimensional structures, either supported by a bulk substrate or embedded in a three-dimensional matrix. Here we report on individual graphene sheets freely suspended on a microfabricated scaffold in vacuum or air. These membranes are only one atom thick, yet they still display long-range crystalline order. However, our studies by transmission electron microscopy also reveal that these suspended graphene sheets are not perfectly flat: they exhibit intrinsic microscopic roughening such that the surface normal varies by several degrees and out-of-plane deformations reach 1 nm. The atomically thin single-crystal membranes offer ample scope for fundamental research and new technologies, whereas the observed corrugations in the third dimension may provide subtle reasons for the stability of two-dimensional crystals.
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