摘要 :
The low-temperature superstructure of YFe_2O_(4_δ)was carefully investigated by transmission electron microscopy. We found that the unique superstructure at about 100 K is characterized by 1/14 2/7 1/14-type superlattice reflecti...
展开
The low-temperature superstructure of YFe_2O_(4_δ)was carefully investigated by transmission electron microscopy. We found that the unique superstructure at about 100 K is characterized by 1/14 2/7 1/14-type superlattice reflection spots, suggesting the presence of charge-reordering process. The low-temperature high-resolution images clearly show superlattice modulations in Y-O layers as well as Fe-0 layers. The modification of intrabilayer charge interactions due to lattice distortions in Y-O layers is discussed to play a crucial role in the stabilization of long-periodic superstructures at low temperatures.
收起
摘要 :
Computer image simulations provide a crucial aid to high-resolution transmission electron microscopy (HRTEM) in gaining fundamental understanding of the structure of materials. Interpretation of HRTEM images is, however, complicat...
展开
Computer image simulations provide a crucial aid to high-resolution transmission electron microscopy (HRTEM) in gaining fundamental understanding of the structure of materials. Interpretation of HRTEM images is, however, complicated due to continuous structure deformation caused by the imaging electron beam. A computational methodology has been implemented that takes into account the effects of the electron beam on deformation of sample structure during observation and imaging in HRTEM. The evolution of the sample structure is described as a sequence of externally initiated discrete damage events with a frequency determined by the cross section, which depends on the energy of the electron beam. A series of images showing structure evolution with time is obtained by coupling molecular dynamics simulations with the image simulation. These simulation parts are linked by two experimental parameters: the energy of the electron beam and the electron dose rate. As the energy of the electron beam also determines resolution and contrast of the obtained HRTEM image, a careful selection of its value is required to achieve a fine balance between reduction of the sample damage caused by the electron beam and the quality of the acquired image. The proposed computational approach is used to simulate the recently observed process of structural transformation of a small graphene flake into a fullerene cage. The simulated series of images showing the evolution of a graphene flake under the 80 keV electron beam closely reproduces experimental HRTEM images with regard to the structure evolution route, evolution rate, and signal-to-noise ratio. We show that under the increased electron beam energy of 200 keV a similar observation will be obscured by high damage rate or low signal-to-noise ratio.
收起
摘要 :
The folding of paper, hide, and woven fabric has been used for millennia to achieve enhanced articulation, curvature, and visual appeal for intrinsically flat, two-dimensional materials. For graphene, an ideal two-dimensional mate...
展开
The folding of paper, hide, and woven fabric has been used for millennia to achieve enhanced articulation, curvature, and visual appeal for intrinsically flat, two-dimensional materials. For graphene, an ideal two-dimensional material, folding may transform it to complex shapes with new and distinct properties. Here, we present experimental results that folded structures in graphene, termed grafold, exist, and their formations can be controlled by introducing anisotropic surface curvature during graphene synthesis or transfer processes. Using pseudopotential-density-functional-theory calculations, we also show that double folding modifies the electronic band structure of graphene. Furthermore, we demonstrate the intercalation of C_(60) into the grafolds. Intercalation or functionalization of the chemically reactive folds further expands grafold's mechanical, chemical, optical, and electronic diversity.
收起
摘要 :
This study addresses the influence of static atomic displacements (SAD) and chemical bonding in (InGa)(NAs) alloys on structure factors for electron scattering and stresses their importance for compositional analysis. First, SAD a...
展开
This study addresses the influence of static atomic displacements (SAD) and chemical bonding in (InGa)(NAs) alloys on structure factors for electron scattering and stresses their importance for compositional analysis. First, SAD are derived using valence force field (VFF) methods and their reliability is demonstrated by calculating residual atomic forces using density-functional theory (DFT). A systematic study of structure factors for low indium and nitrogen contents is given by means of full DFT calculations on the one hand and atomistic models on the other. We show that the consideration of SAD via VFF together with the inclusion of bonding via modified atomic scattering amplitudes is in best agreement with a full DFT calculation, providing the possibility to include bonding effects also if large cells containing 10~6 atoms are considered. Second, a technique is presented, which allows the extraction of atomically resolved indium and nitrogen composition maps by evaluating strain and contrast in transmission electron microscopic two-beam images simultaneously. As the fringe contrast is compared with Bloch wave simulations, which in turn require accurate structure factors, we exemplify the influence of SAD and bonding on composition profiles regarding an In_(0.08)Ga_(0.92)N_(0.03)As_(0.97) quantum well structure. In this respect, imaging conditions may be chosen for which SAD are of minor influence for conventional transmission electron microscopy, whereas relative errors larger than 25% can arise for the compositions if bonding is neglected.
收起
摘要 :
Calcium cobaltite thin films with a nominal ratio Ca/Co=l were grown on (101)-NdGaO_3 substrate by the pulsed laser deposition technique. The structure of the as-deposited metastable phase is solved using a precession electron dif...
展开
Calcium cobaltite thin films with a nominal ratio Ca/Co=l were grown on (101)-NdGaO_3 substrate by the pulsed laser deposition technique. The structure of the as-deposited metastable phase is solved using a precession electron diffraction three-dimensional data set recorded from a cross-sectional sample. We found that iin ordered oxygen-deficient Ca_2Co_2O_5 perovskite of the brownmillerite type with lattice parameters a≈a_p 2~(1/2) ≈ 5.46 A, b≈4a_p≈ 14.88 A, and c ≈ a_p 2~(1/2) ≈ 5.46 A (space group: Ibm2) has been stabilized using the substrate-induced strain. The structure and microstructure of this metastable cobaltite are further discussed and compared to related bulk materials based on our transmission electron microscopy investigations. The present results open the route to the resolution of metastable phases prepared in the form of a thin film using the precession electron diffraction and should be useful for the solid state chemistry community.
收起
摘要 :
Carbon nanotubes (CNTs) filled completely with polycrystalline Cu nanowires were synthesized by laser vaporization of Cu and graphite under high-pressure Ar gas atmosphere. Depending on the Ar gas pressure (0.1-0.9 MPa) and the Cu...
展开
Carbon nanotubes (CNTs) filled completely with polycrystalline Cu nanowires were synthesized by laser vaporization of Cu and graphite under high-pressure Ar gas atmosphere. Depending on the Ar gas pressure (0.1-0.9 MPa) and the Cu content (1-40 at.%) in graphite targets for laser vaporization, various products with different morphologies were observed by scanning and transmission electron microscopy. The ratios of the Cu-filled CNTs and carbon nanocapsules particularly increased as Ar gas pressure was increased. The maximum ～60% fraction of Cu-filled CNTs with outer diameter of 10-50 nm and length of 0.3-3 μm was achieved at 0.9 MPa from graphite containing 20 at.% Cu. Most of the encapsulated Cu-nanowires were surrounded by single, double, or triple graphitic layers. Although the yield of the Cu-filled CNTs was also dependent on the Cu content in the graphite targets, no unfilled CNTs were produced even for low Cu content. The growth of Cu-filled CNTs is explained by the formation of molten Cu-C composite particles with an unusually C-rich composition in a space confined by high-pressure Ar gas, followed by precipitating Cu and C from the particles and subjecting them to phase separation.
收起
摘要 :
Images of flux vortices in superconductors acquired by transmission electron microscopy should allow a quantitative determination of their magnetic structure but, so far, only visual comparisons have been made between experimental...
展开
Images of flux vortices in superconductors acquired by transmission electron microscopy should allow a quantitative determination of their magnetic structure but, so far, only visual comparisons have been made between experimental images and simulations. Here, we make a quantitative comparison between Fresnel images and simulations based on the modified London equation to investigate the magnetic structure of flux vortices in MgB_2. This technique gives an absolute, low-field (～ 30 Oe) measurement of the penetration depth from images of single vortices. We found that these simulations gave a good fit to the experimental images and that if all the other parameters in the fit were known, the penetration depth for individual vortices could be measured with an accuracy of ±5 nm. Averaging over 17 vortices gave a penetration depth of Λ_(ab) = 113 ± 2 nm at 10.8 K assuming that the entire thickness of the sample was superconducting. The main uncertainty in this measurement was the proportion of the specimen which was superconducting. Allowing for a nonsuperconducting layer of up to 50-nm thickness on the specimen surfaces gave a penetration depth in the range Λ_(ab) = 100-115 nm, close to values of 90 ± 2 nm obtained by small-angle neutron scattering and 118-138 nm obtained by radio-frequency measurements. We also discuss the use of the transport of intensity equation which should, in principle, give a model-independent measure of the magnetic structure of flux vortices.
收起
摘要 :
Co_2FeSi/GaAs(110) and Co_2FeSi/GaAs(111)B hybrid structures were grown by molecular-beam epitaxy and characterized by transmission electron microscopy (TEM) and x-ray diffraction. The films contained inhomogeneous distributions o...
展开
Co_2FeSi/GaAs(110) and Co_2FeSi/GaAs(111)B hybrid structures were grown by molecular-beam epitaxy and characterized by transmission electron microscopy (TEM) and x-ray diffraction. The films contained inhomogeneous distributions of ordered L2_1 and B2 phases. The average stoichiometry was controlled by lattice parameter measurements; however, diffusion processes led to inhomogeneities of the atomic concentrations and the degradation of the interface, influencing long-range order. An average long-range order of 30-60% was measured by grazing-incidence x-ray diffraction, i.e., the as-grown Co_2FeSi films were highly but not fully ordered. Lateral inhomogeneities of the spatial distribution of long-range order in Co_2FeSi were found using dark-field TEM images taken with superlattice reflections.
收起
摘要 :
Boron nitride (BN) sheets were exfoliated, and proof of the presence of single and double layers was obtained via electron diffraction and plasmon electron energy loss spectroscopy. A plasmon structure unique to mono-and bi-layer ...
展开
Boron nitride (BN) sheets were exfoliated, and proof of the presence of single and double layers was obtained via electron diffraction and plasmon electron energy loss spectroscopy. A plasmon structure unique to mono-and bi-layer BN was established, and was accompanied by WIEN2K DFT calculations. The latter reproduced plasmon energies and general plasmon structure well; however, the detailed shape of the 7r-plasmon of the calculated pure BN spectra shows discrepancies with the experimental data. The theoretical models were then modified to include impurity atoms. Both oxygen and carbon impurities were considered, as well as different structures, including singular oxygen atoms and oxygen next to carbon atoms. These various configurations were obtained by analyzing atomic-resolution high-angle annular dark field (HAADF) images. Using these modified models, a π-plasmon structure close to the experimentally observed structure could be simulated.
收起
摘要 :
In YH_(2+x) hydrides stoichiometric deviations are associated with various interesting phenomena such as magnetic transitions and metal-insulator transitions; ordering of hydrogen on the octahedral sites (O sites) was suggested to...
展开
In YH_(2+x) hydrides stoichiometric deviations are associated with various interesting phenomena such as magnetic transitions and metal-insulator transitions; ordering of hydrogen on the octahedral sites (O sites) was suggested to be responsible. Long-range order was discovered in hydrogenated Y films by electron diffraction and high-resolution transmission electron microscopy. Ordering on interstitial O sites in the fee-based Y_(fcc)H_(2+x) solid solution (0收起
