High-resolution Electron Microscopy Image Simulations Research Articles

A New Structure with Localized sp2 Bonding for Fivefold Twinning in Diamond.

Changes in atomic bonding configuration in carbon from sp3 to sp2 are known to exist in certain structural defects in diamond, such as twin boundaries, grain boundaries, and dislocations, which have a significant impact on many properties of diamond. In this work, the atomic structure of fivefold twinning in detonation synthesized ultra-dispersed diamonds is investigated using a combination of techniques, including spherical aberration-corrected high-resolution electron microscopy (HREM), HREM image simulations, and molecular mechanics (MM) calculations. The experimental HREM images reveal clearly that the fivefold twinning in diamond has two distinct structures. In addition to the concentric fivefold twins, where the core structure is the intersection of five {111} twinning boundaries, a new extended core structure with co-hybridization of bonding is identified and analyzed in fivefold twinning. The atomic structure forming these fivefold twinning boundaries and their respective core structures is proposed to involve both the tetrahedral sp3 and planar graphitic sp2 bonding configurations, in which a co-hybridized planar hexagon of carbon serves as a fundamental structural unit. The presence of this sp2 -bonded planar unit of hexagonal carbon rings in general grain boundaries is also discussed.

HRTEM Analysis of Crystallographic Defects in CdZnTe Single Crystal

In recent years, CdZnTe has attracted much attention due to its superior electrical and structural properties for room-temperature operable gamma and x-ray detectors. However, CdZnTe (CZT) material has often suffered from crystallographic defects encountered during the growth and post-growth processes. The identification and structural characterization of these defects is crucial to synthesize defect-free CdZnTe single crystals. In this study, Cd0.95 Zn0.05 Te single crystals were grown using a three-zone vertical Bridgman system. The single crystallinity of the material was ensured by using x-ray diffraction measurements. High-resolution electron microscopy (HRTEM) was used to characterize the nano-scale defects on the CdZnTe matrix. The linear defects oriented along the ⟨211⟩ direction were examined by transmission electron microscopy (TEM) and the corresponding HRTEM image simulations were performed by using a quantitative scanning TEM simulation package.

From phason planes to superstructures in an Al–Pd–Mn alloy

The atomic structures of four structurally complex phases of an Al–Pd–Mn alloy, namely the Ψ-, ξ1-, ξ2- and ξ3-phases, have been determined by means of high-resolution electron microscopy and by theoretical structure simulations based on the periodic stacking of two different types of phason planes. The structural characteristics of the Ψ-phase appears as a thin slab of ξ′-phase sandwiched between a pair of type-1 phason planes; in addition, three newly-found ξ i -phases (i = 1–3) were constructed by periodic stacking of the type-2 phason planes with an adjacent stacking (ξ1), centred by a flattened hexagon (ξ2), which are sandwiched around a thin slab of ξ-phase (ξ3). High-resolution electron microscopy image simulations of the Ψ-phase based on our structural model match very well with the experimental results and this agreement is also supported by electron diffraction and X-ray powder diffraction simulations based on the model. Our results show that structural models of periodic stacking of two types of phason planes can be used to construct the atomic structure of ξ′- and ξ-phase related superstructures in the Al–Pd–Mn alloy.

Crystal structure of the parent misfit-layered cobalt oxide [Sr 2O 2] qCoO 2

The crystal structure of our newly discovered Sr–Co–O phase is investigated in detail through high-resolution electron microscopy (HREM) techniques. Electron diffraction (ED) measurement together with energy dispersive X-ray spectroscopy (EDS) analysis show that an ampoule-synthesized sample contains an unknown Sr–Co–O ternary phase with monoclinic symmetry and the cation ratio of Sr/Co=1. From HREM images a layered structure with a regular stacking of a CdI 2-type CoO 2 sheet and a rock-salt-type Sr 2O 2 double-layered block is observed, which confirms that the phase is the parent of the more complex “misfit-layered (ML)” cobalt oxides of [ M mA 2O m +2] q CoO 2 with the formula of [Sr 2O 2] q CoO 2, i.e. m = 0 . It is revealed that the misfit parameter q is 0.5, i.e. the two sublattices of the CoO 2 sheet and the Sr 2O 2 block coexist to form a commensurate composite structure. We propose a structural model with monoclinic P2 1/ m symmetry, which is supported by simulations of ED patterns and HREM images based on dynamical diffraction theory.

Direct high-resolution electron microscopy of BN nanotubes with hexagonal zigzag network

Hexagonal zigzag network of boron nitride (BN) nanotubes was investigated by high-resolution electron microscopy (HREM) and image processing. From the HREM image, lattice planes of {0 0 2} and hexagonal rings of BN nanotube were confirmed by comparing image simulation and nanostructure models. The present work indicated that arrangement of hexagonal rings of BN nanotubes could be confirmed by combination of HREM image analysis and simulation.

Potassium titanate nanowires: Structure, growth, and optical properties

A simple one step hydrothermal reaction among ${\mathrm{TiO}}_{2}$ nanoparticles and KOH solution was found to result in potassium titanate nanowires. The diameters of these nanowires are about 10 nm and the lengths range from 500 nm to $2\ensuremath{\mu}\mathrm{m}.$ The nanowires were analyzed by a range of methods including powder x-ray diffraction (XRD), high resolution electron microscopy (HREM), selected area electron diffraction, electron energy loss spectroscopy, XRD and HREM image simulations. The structure of the nanowires is determined to be of the type of ${\mathrm{K}}_{2}{\mathrm{Ti}}_{6}{\mathrm{O}}_{13}.$ Based on HREM observations of the growth process of the nanowires, we propose that the growth of the namowire was initiated by the formation of the ${\mathrm{K}}_{2}{\mathrm{Ti}}_{6}{\mathrm{O}}_{13}$ nuclei inside the anatase matrix following the crystallographic relation ${(200)}_{\mathrm{nanowire}}{//(101)}_{\mathrm{anatase}}.$ These nuclei subsequently grew to form one-dimensional nanowires via preferential growth along the [010] direction. Absorption experiments show that the potassium titanate nanowires are wide-band semiconductors with a band width ${E}_{g}\ensuremath{\sim}3.45\mathrm{eV}.$

Transversely modulated crystal structure of charge-orbital ordered manganitesNd1−xSr1+xMnO4(x=2/3,3/4)

The crystallographic superstructures of the charge-orbital ordered phases were investigated for layered manganites, Nd 1 - x Sr 1 + x MnO 4 (x = 2/3, 3/4), by high-resolution electron microscopy (HREM) at low temperature. Transverse and sinusoidal structural distortion giving rise to the superstructure were successfully observed in HREM images at 80 K. We also performed the simulations of electron diffraction patterns and HREM images, which support the transverse modulations. The observation implies that a new kind of Wigner-crystal model-accompanied with a charge-density wave (CDW) of e g electrons-best explains the charge-orbital ordered state in the manganites.

Influence of structural variations on high-resolution electron microscopy images of poly[1,6-di(N-carbazolyl)2,4-hexadiyne] nanocrystals

Abstract High-resolution electron microscopy (HREM) was used to obtain information about structural variations in poly[1,6-di(N-carbazolyl)-2,4-hexadiyne) (polyDCHD) nanocrystals. Several particles in different crystallographic zones were imaged with high spatial resolution of up to 0.19nm, resulting in image information on a subunit-cell level (a = 1.74nm, b = 1.29nm and c = 0.49nm; β = 108.3°, P21/a; Z = 2). The average nanocrystal had a nominally rectangular shape and a size of about 40 nm × 80 nm with the major growth direction along the polymer chains (001). Most of the HREM images correspond to the [100] or [120] zone. In a few cases, it was also possible to image crystals oriented in the [001] zone with the polymer chain axis normal to the carbon substrate. Image interpretation was done by comparison with multislice HREM image simulations based on an approximation for the crystal structure and imaging conditions. These simulations performed for polyDCHD indicate that the HREM image is sensitive to ...

Behaviour of a nanometric SnO2 powder under swift heavy-ion irradiation: From sputtering to splitting

Abstract Tin oxide nanometric powders have been irradiated with several swift heavy ions (Ar, Cd, Ta, Pb and U) and observed by transmission electron microscopy (TEM) and high-resolution electron microscopy (HREM). Except for Ar ions, cylindrical tracks are visible at low fluences. The mean track radius is all the more important since the electronic stopping power S e is high. Furthermore, the high S e irradiating ions (Pb, Ta and U) create cylindrical holes while, in the case of Cdion irradiations, the track cores remain crystallized with a difference of contrast from the surrounding crystal. In this case, simulations of HREM images allow us to assert that an amount of matter is lost in Cd track cores. Assuming that the observed latent tracks result from a thermal process, the thermal spike model gives a good description of the observed effects in the high-S e regime. Cylindrical holes correspond to a local area where the maximum temperature reached along the ion path exceeds the boiling point. Concernin...

Behaviour of a nanometric SnO 2 powder under swift heavy-ion irradiation: from sputtering to splitting

Abstract Tin oxide nanometric powders have been irradiated with several swift heavy ions (Ar, Cd, Ta, Pb and U) and observed by transmission electron microscopy (TEM) and high-resolution electron microscopy (HREM). Except for Ar ions, cylindrical tracks are visible at low fluences. The mean track radius is all the more important since the electronic stopping power S e is high. Furthermore, the high S e irradiating ions (Pb, Ta and U) create cylindrical holes while, in the case of Cdion irradiations, the track cores remain crystallized with a difference of contrast from the surrounding crystal. In this case, simulations of HREM images allow us to assert that an amount of matter is lost in Cd track cores. Assuming that the observed latent tracks result from a thermal process, the thermal spike model gives a good description of the observed effects in the high-S e regime. Cylindrical holes correspond to a local area where the maximum temperature reached along the ion path exceeds the boiling point. Concerning Cd-ion tracks, the temperature reached is not high enough to lead to vaporization. A sputtering effect on surface may explain the fact that some matter remains crystallized in the track cores. Moreover, a grain size effect during irradiation has been evidenced by TEM observations and confirmed by thermal spike calculations.

Molecular dynamics simulation of the atomic structure of a NiO tilt grain boundary at high temperature

The detailed atomic structure of the 5(310)[001] tilt grain boundary is still the subject of controversy since the structures deduced from high-resolution electron microscopy (HREM) experiments differ from those derived from static calculations. The presence of defects combined with the effects of temperature have been proposed as possible reasons for this discrepancy. To elucidate this question we examined several possible configurations of the grain boundary structural unit in which we incorporated defects. Some of these configurations were selected after energy minimization at T = 0 K and then studied at high temperature. We chose the lowest energy configurations, in which there are still some defects remaining after relaxation, to perform high temperature simulations using constant temperature molecular dynamics. The detailed analysis of the results shows that the defects have a tendency to agglomerate; in some cases this agglomeration can even locally modify the periodicity of the grain boundary. Using the average location of the atoms resulting from the high-temperature simulations, we performed HREM image simulations. Several images present features which are also observed in the experimental HREM images. Indeed the degree of agreement between our results and the HREM experimental images is improved with respect to the structures resulting from the calculations performed on configurations without defects.

Strain relaxation and thermal stability of the 3C-SiC(001)/Si(001) interface: A molecular dynamics study

Molecular dynamics (MD) and high-resolution electron microscopy (HREM) imaging are used to investigate the mechanism of strain relaxation of a model 3C-SiC(001)/Si(001) interface. It is found that the essential atomic mechanism governing this process is the formation of undulations in planes parallel and perpendicular to the interface. The net effect is the generation of misfit-accommodating dislocations, of the <110> type, which allow for structure relaxation at 2, 700 and 1000 K. MD configurations are then used for HREM image simulations. Comparisons with actual HREM images of the interface support the model interface and relaxation mechanisms proposed herein.

Structure of V–Al2,O3interfaces grown by molecular beam epitaxy

Abstract V films were deposited on the (102) R-plane of sapphire by molecular beam epitaxy. Cross-sectional and plan-view specimens were prepared and the V–Al2O3 interface was investigated from different directions by conventional and high-resolution electron microscopy (HREM). It was found that the film grows in an epitaxial manner with respect to the substrate with a tilt angle of about 2°. Using selected-area diffraction patterns and HREM, the orientation relationship between V and Al2O3 was determined and the geometrical misfit dislocations and interfacial stacking faults were analysed. The misfit dislocations are a geometrical consequence of the large mismatch between interatomic spacings at the two sides of the V–Al2O3 interface and exist from the start of deposition. Because of the large lattice mismatch, the V–Al2O3 interface is expected to be incoherent; however, because of bonding between the atoms on the two sides of the interface, the atoms in the vicinity of the interface are displaced from their bulk positions, giving rise to very localized strains at the interface. These strains result in dislocation-like contrast in micrographs obtained by conventional electron microscopy. The V film is tilted because it grows so as to maintain a unique three-dimensional orientation relationship between V and Al2O3. Basically the V atoms follow the same spatial orientation as the Al cations in the underlying sapphire substrate. As a result of the tilt, periodic stacking faults at the interface are observed in plan-view electron microscopy specimens. A comparison of HREM image simulations with experimental images of the interface indicates that the V atoms may be bonded to O atoms on the R-plane of Al2O3; these O atoms, however, are not from the original O-terminated R-plane but may have formed as an extra monolayer on this plane just prior to deposition.

Epitaxial growth defects and interfacial structures of Cu deposited on TiO2

Epitaxial growth defects and the interfacial structure between vapor deposited Cu and TiO2(110) have been studied by combined high-resolution electron microscopy (HREM) and image simulations. The Cu film was found to grow epitaxially with an orientation given by Cu(111)//TiO2(110) and Cu〈110〉//TiO2 [001]. With this relationship, there exist two equivalent domain orientations which are rotated with respect to each other by 180°. Localized misfit dislocations have not been detected, but {111} stacking faults and microtwins were observed which may occur as a result of 3-D island coalescence. HREM observations and image simulations have been used to study the interface atomic structure. The dominant interfacial structure has a stoichiometric (110) TiO2 surface with bridging rows of O atoms and occasionally, an interfacial structure having a reduced (110) TiO2 surface terminated by both Ti and O atoms has been observed locally.

HREM study of dispersoids in cryomilled oxide dispersion strengthened materials

Abstract Thin platelets (dispersoids) only a few atomic layers thick formed in cryomilled oxide dispersion strengthened aluminium, have been investigated by high-resolution electron microscopy (HREM). In order to interpret the experimental images, computer simulations of HREM images based on different structure models have been carried out. The image simulations have been used to determine the composition and the structure of the dispersoids. Two types of dispersoids have been found: one type consists of a single layer of nitrogen with the nitrogen atoms in tetrahedral positions in aluminium, the other type consists of one single layer of oxygen with the oxygen atoms in octahedral positions in aluminium.

High-resolution electron microscopy image simulations on the R-Al5CuLi3icosahedral approximant phase

Abstract The study of crystalline approximant structures is an important way to understand the local order in quasicrystalline phases. We compare here high-resolution electron microscopy (HREM) image simulations of the cubic approximant phase R-Al5CuLi3 with experimental HREM images of the icosahedral Al5CuLi3 phase. Local two-dimensional (2D) arrangements such as Penrose tiles of different scales and projections of icosahedral clusters (triacontahedra) are found to be common to both phases, confirming the close similarity of their short and medium-range order. The shape and decoration of three-dimensional (3D) tiles is suggested from their 2D projection. A 2D and 3D inflation rule, comparable with that expected in the icosahedral phase is demonstrated in the approximant phase.

A high-resolution electron microscopy study of mgo/al2o3interfaces and mgal2o4spinel formation

Abstract MgO films were deposited on the Al2O3 prism plane and the (0001)Al2O3 basal plane of sapphire by molecular beam epitaxy (MBE). Cross-sectional specimens with thicknesses of less than 15nm were prepared so that the MgO/Al2O3 interface could be investigated by high resolution electron microscopy (HREM). Two different orientation relationships were obtained for MgO films deposited on the differently oriented sapphire substrates. Misfit dislocations at the interface were analysed and a structural model for the atomic arrangement of the interface was proposed. Good matches between simulated and observed images of the MgO/Al2O3 interfaces were obtained. The HREM observations, image simulations and optical diffractograms confirm that MgAl2O4 (spinel) forms at the interface by a solid state reaction under electron beam irradiation.

Physical and mathematical foundations of multiwave electron diffraction

The dynamical problem of multiwave electron diffraction in perfect and imperfect crystals is treated on the basis of the Bloch wave formalism. In the case of a perfect crystal the classification of the electron Bloch states in terms of the transverse energy is introduced. A perturbation theory for the analytical construction of electron Bloch functions is suggested, using the symmetry of the dynamical matrix. The contribution of bound, valence and free Bloch states to the reflections and to high-resolution electron-microscopy (HREM) images are analysed. Using the column approximation, the equations describing the propagation and the excitation of the electron Bloch waves due to defects in a crystal lattice are derived from the Schrödinger equation. A solution of these equations is obtained in the first Born approximation. The theoretical foundation of the Cowley-Moodie multislice method for computer simulations of HREM images is presented. As an illustration of the Cowley-Moodie method, HREM images of Au in the (110) orientation and Y3Al5O12 (111) are calculated for different defocusing parameters Δf. In the case of Au the fine structure of Au lattice images, when the Au atom strings are surrounded by 'halos', is clearly revealed for certain values of Δf The Y-Al garnet images are shown to have either a sixfold or a threefold axis of symmetry, depending whether or not the dimensionless thickness in terms of the lattice spacing along (111) is an integer number.

A method for the simulation of high-resolution electron microscope images from non-periodic objects

Abstract It is shown that the calculation of wavefunctions using periodic extension of the sampling boundary points leads to an artificial error in the high-resolution electron microscopy image simulation for a non-periodic object. In a thorough study of this problem, a new method has been proposed for the dynamical image simulation of a non-periodic object.

Shot-noise simulations of HREM images of polymer crystals

High Resolution Electron Microscopy (HREM) is a very powerful technique for the study of crystal defects at the molecular level. Unfortunately polymer crystals are beam sensitive and are destroyed almost instantly under the typical HREM imaging conditions used for inorganic materials. Recent developments of low dose imaging at low magnification have nevertheless permitted the attainment of lattice images of very radiation sensitive polymers such as poly-4-methylpentene-1 and enabled molecular level studies of crystal defects in somewhat more resistant ones such as polyparaxylylene (PPX) [2].With low dose conditions the images obtained are very noisy. Noise arises from the support film, photographic emulsion granularity and in particular, the statistical distribution of electrons at the typical doses of only few electrons per unit resolution area. Figure 1 shows the shapes of electron distribution, according to the Poisson formula :