The structure of 1D CuI crystals inside SWNTs

Nanocomposites consisting of one‐dimensional CuI crystals inside single‐walled carbon nanotubes were obtained using the capillary technique. high‐resolution transmission electron microscopy investigations of the atomic structure of the encapsulated 1D CuI crystals revealed two types of 1D CuI crystals with growth direction <001> and relative to the bulk hexagonal CuI structure. Atomic structure models were proposed based on the high‐resolution transmission electron microscopy images. According to the proposed models and image simulations, the main contrast in the 1D crystal images arises from the iodine atoms whereas copper atoms, with lower atomic number giving lower contrast, are thought to be statistically distributed.     

The structure and continuous stoichiometry change of 1DTbBrx@SWCNTs

HRTEM and HAADF STEM of 1DTbBr_x@SWCNT meta‐nanotubes reveal three structural modifications of 1D nanocrystals within single wall carbon nanotube channels attributed to a different stoichiometry of the guest crystal. For SWCNTs with diameters D_m > 1.4 nm a most complete tetragonal unit cell is observed. When crystallization occurs inside SWCNT with D_m < 1.4 nm 1D TbBr_x crystal deforms a nanotube to elliptical shape in cross section. In this case the 1D crystal unit cell becomes monoclinic, with possible loss of a part of bromine atoms. Two modifications of a monoclinic unit cell appear. One of them is characterized by single or pair vacancies in the structure of the 1D crystal. Another structure is explained by peripheral and central bromine atoms loss. An appearance of such modifications can be stimulated by electron irradiation. The loss of bromine atoms is in agreement with chemical analysis data. Electronic properties of obtained meta‐nanotubes are investigated using optical absorption and Raman spectroscopy. It is shown that intercalation of terbium bromide into SWCNTs leads to acceptor doping of SWCNTs. According to local EDX analysis and elemental mapping this doping can arise from significant stoichiometry change in 1D nanocrystal indicating an average Tb:Br atomic ratio of 1:2.8 ± 0.1.     

HRTEM of 1DSnTe@SWNT nanocomposite located on thin layers of graphite

The method for imaging of highly sensitive nanostructures unstable under electron beam irradiation is introduced. To reduce charge and thermally generated beam damage, highly conductive multilayered graphene or thin graphite layers were used as supports for nanostructures. Well‐defined crystalline structure of graphite layers enables image reconstruction by Fourier filtering and allows maintaining high quality of images. The approach was tested for imaging of highly sensitive quasi one‐dimensional SnTe nanocrystals hosted inside single‐walled carbon nanotubes. Relying on the filtered images and the image simulation, the structure of one‐dimensional SnTe was established as a chain of fcc NaCl type unit cells, connected by the [001] edges with <110> direction coinciding with nanotube axis.     

A method for determining void arrangements in inverse opals

The periodic arrangement of voids in ceramic materials templated by colloidal crystal arrays (inverse opals) has been analysed by transmission electron microscopy. Individual particles consisting of an approximately spherical array of at least 100 voids were tilted through 90° along a single axis within the transmission electron microscope. The bright‐field images of these particles at high‐symmetry points, their diffractograms calculated by fast Fourier transforms, and the transmission electron microscope goniometer angles were compared with model face‐centred cubic, body‐centred cubic, hexagonal close‐packed, and simple cubic lattices in real and reciprocal space. The spatial periodicities were calculated for two‐dimensional projections. The systematic absences in these diffractograms differed from those found in diffraction patterns from three‐dimensional objects. The experimental data matched only the model face‐centred cubic lattice, so it was concluded that the packing of the voids (and, thus, the polymer spheres that composed the original colloidal crystals) was face‐centred cubic. In face‐centred cubic structures, the stacking‐fault displacementvector is . No stacking faults were observed when viewingthe inverse opal structure along the orthogonal <110>‐type directions, eliminating the possibility of a random hexagonally close‐packed structure for the particles observed. This technique complements synchrotron X‐ray scattering work on colloidal crystals by allowing both real‐space and reciprocal‐space analysis to be carried out on a smaller cross‐sectional area.     

Fretting on the cubic face of a single-crystal Ni-base superalloy at room temperature

Fretting experiments were conducted on the (1 0 0) crystal (cubic) face of a single crystal Ni-base superalloy in two directions, 〈1 0 0〉 and 〈1 1 0〉, to study the effect of crystallographic orientation on the fretting response in both partial slip and gross slip regimes. The study involved point contacts using a tungsten carbide (WC) ball with radius 10 mm tested at room temperature. Ball indentation was used to determine secondary crystallographic orientations. Under sufficiently high normal force, the friction on the cubic face in the 〈1 1 0〉 direction was larger than in the 〈1 0 0〉 direction. This difference can be explained by the extent of plastic deformation in the surface layer and associated microstructure changes, both of which depend on the coupling between the crystallographic orientation and the cyclic deformation field.     

High-temperature dislocation plasticity in the single-crystal superalloy LEK94

The evolution of the dislocation structure that forms during uniaxial creep deformation in the single‐crystal superalloy LEK94 of low density and with Re additions was analysed using transmission electron microscopy. The material has a γ/γ′‐microstructure consisting of γ′‐cubes (L12 phase, 80 vol.%) separated by thin γ‐channels (face‐centred cubic). <100> tensile creep tests were performed at 980 and 1020 °C at stresses of 200 and 240 MPa. The microstructure was investigated at three characteristic stages of creep (directly after loading, at 5% strain and after rupture) to show the evolution of the dislocation structure during high‐temperature creep. It was found that in the early stages of creep, a₀/2<011> dislocations form within the γ‐channels. Later on, dislocation networks form and γ′ cutting processes with a₀/<001> superdislocations are observed. The results are in line with observations made for other superalloy single crystals in the high‐temperature low‐stress creep regime.     

Frictional Characteristics of Quasicrystals at High Temperatures

AlCuFe quasicrystal coatings were deposited by unbalanced magnetron sputtering for study of their friction and wear properties at high temperatures. It has been shown that growth and annealing conditions can be controlled to produce icosahedral quasicrystal or the approximant cubic phase. The comparison of friction and wear properties between quasicrystal and an approximant with nearly the same stoichiometry permits assessment of the unique quasicrystalline structure for tribological applications. The goals of this study are to determine how crystal structure influences tribological properties and to study the general friction and wear behavior of quasicrystals. Tribological properties were evaluated using a pin-on-disk tribometer and crystal structure was characterized using an X-ray diffractometer. The tests specimens were 10 m thick films deposited on a 1 diameter steel disk and a 1/4 diameter steel ball. Data was collected over a range of temperatures from room to 600°C. Friction coefficients and wear rates of quasicrystals and approximants were nearly identical for room temperature tests. The wear process generated Al, Cu, Fe, and oxide debris on the side of the track, but overall wear was mild. The friction coefficient of the icosahedral phase was 25% lower than the cubic phase at 150 thru 450°C. Generally, only moderate differences in the friction and wear properties were observed between the icosahedral quasicrystal phase and the cubic phase.     

Structure projection retrieval by image processing of HREM images taken under non-optimum defocus conditions

A direct method for retrieval of the projected potential from a single HREM image of a thin sample is presented. Both out-of-focus and astigmatic images can be restored. The defocus and astigmatism values are first determined from the Fourier transform of the digitised HREM image. Then a filter is applied which reverts the phases of those Fourier components which have been reversed by the Contrast Transfer Function (CTF). The method has been incorporated into the CRISP image processing system. It can be applied on any sample, crystalline or amorphous. From thin crystalline areas the projected symmetry can be determined and a further improvement achieved by imposing the symmetry exactly. This compensates for the effects of crystal tilt. Five HREM images of a thin crystal of K(8-x)Nb(16-x)W(12+x)O80 (x = 1), taken with different defocus and astigmatism values, were processed. Only one, taken near Scherzer defocus, was directly interpretable before image processing. After processing, all images showed the projected potential of the structure. Using data to 2.77 angstroms resolution, all heavy (Nb/W) atom positions were found in every image, within on average 0.15 angstroms of the positions determined by single crystal X-ray diffraction. In the HREM images taken under non-optimum defocus conditions, also the potassium atoms in the tunnels of the structure were found.     

Reliability of atom column positions in a ternary system determined by quantitative high-resolution transmission electron microscopy

Employing an iterative structure refinement procedure, we have determined the atomistic structure of the Σ3 (111) grain boundary in strontium titanate (SrTiO₃) from high-resolution transmission electron microscopy (HRTEM) images. This grain boundary serves as a model system to study the effect of column occupancies on the reliability of the column positions. In this paper we introduce a method to derive confidence regions for the positions of individual atom columns at crystal defects. Based on a statistical approach we first determine the reliabilities of different types of atom columns in regions of unfaulted crystal. Next we extrapolate these reliabilities to obtain the reliabilities of individual atom columns at the grain boundary. The method accounts correctly for random errors and promises to be generally applicable provided that repetitive units of unfaulted crystal structure are contained in the HRTEM image. Under the conditions of the present study, the reliability of a column position correlates with the projected electrostatic potential of the column. Accordingly, the reliabilities of the column positions at the boundary vary with the column type: 0.008 nm for Sr–O columns, 0.014 nm for Ti columns, and 0.018 nm for O–O columns.     

Local crystal structure analysis with several picometer precision using scanning transmission electron microscopy

We report a local crystal structure analysis with a high precision of several picometers on the basis of scanning transmission electron microscopy (STEM). Advanced annular dark-field (ADF) imaging has been demonstrated using software-based experimental and data-processing techniques, such as the improvement of signal-to-noise ratio, the reduction of image distortion, the quantification of experimental parameters (e.g., thickness and defocus) and the resolution enhancement by maximum-entropy deconvolution. The accuracy in the atom position measurement depends on the validity of the incoherent imaging approximation, in which an ADF image is described as the convolution between the incident probe profile and scattering objects. Although the qualitative interpretation of ADF image contrast is possible for a wide range of specimen thicknesses, the direct observation of a crystal structure with deep-sub-angstrom accuracy requires a thin specimen (e.g., 10 nm), as well as observation of the structure image by conventional high-resolution transmission electron microscopy.     

Image deconvolution in spherical aberration-corrected high-resolution transmission electron microscopy

The method of image deconvolution developed previously for FEG high-resolution transmission electron microscope (HRTEM) without a spherical aberration (C(s)) corrector was for the first time applied to FEG HRTEM with a C(s)-corrector. The principle and the procedure of image deconvolution are briefly described. Four qualified [1 1 0] images of Si were selected from a through-focus series to perform image deconvolution. The projected potential is successfully derived from all the images, and the obtained "dumbbell" structure maps of Si [1 1 0] are in good agreement with the calculated potential map. The criterion of selecting qualified images for performing image deconvolution is indicated. The possibility of applying image deconvolution to defect study and to ab initio crystal structure determination is discussed.     

Structure of nanometre-sized Xe particles embedded in Al crystals

The structure and lattice parameters of Xe particles about 1 nm to about 6 nm in size embedded in Al were investigated with off‐Bragg condition high‐resolution transmission electron microscopy. An Xe particle about 1 nm in size had different structural properties from those 2–6 nm in sizes. Some 1‐nm Xe particles had an face‐centred cubic (f.c.c.) structure with the same orientation as the Al matrix, whereas others of the same size had a non‐f.c.c. structure. The lattice parameters of a 1‐nm f.c.c. Xe particle were about 20% smaller than the average value obtained from electron diffraction, i.e. the particle was compressed by about 80%. The lattice parameters of Xe crystals about 2 nm to about 6 nm in size were almost the same as those obtained from diffraction results. One of the reasons for the extra compression seen with a 1‐nm Xe particle is the increase in pressure inside an Xe particle with decreasing particle size.     

In situ observation of the initial growth process of carbon nanotubes by time-resolved high resolution transmission electron microscopy

We succeeded in plan-view dynamic observation of the initial formation process of carbon nanotubes from β-SiC( 1 1 1 ) surfaces by time-resolved high resolution transmission electron microscopy. At 1360 °C, the flakes of graphite layers of a fibre orientation were formed on the SiC( 1 1 1 ) surfaces. From the graphite layers, carbon nanotubes were formed perpendicular to the ( 1 1 1 ) plane of the SiC. A scanning tunnelling microscopy observation showed that the end of carbon nanotube was closed. These results indicate that the caps of the carbon nanotubes are formed by a lift of a part of the graphene along the [ 1 1 1 ] direction of the SiC through generation of pentagons and heptagons. Two types of carbon nanotube, single-wall and double-wall, were observed in plan-view images. Different image intensity between an outer ring and an inner ring in double-wall nanotubes suggests that the inner layers of multiwall nanotubes are formed after the outer ones.     

The effect of inelastic scattering on crystal structure refinement from electron diffraction patterns recorded under almost parallel illumination

Recently a number of crystal structures were determined using electron diffraction data with an almost parallel electron beam. In many cases no energy filtering was applied. On the other hand, the contrast in convergent beam electron diffraction patterns is greatly improved by energy filtering of the electrons. To investigate whether energy filtering will improve the accuracy of the structure analysis from diffraction data recorded under an almost parallel beam condition, we recorded diffraction patterns of the [100] zone of YBa(2)Cu(3)O(7) using unfiltered electrons, zero-loss electrons and plasmon-loss electrons, respectively. Subsequently, the structure is refined based on these different electron diffraction datasets, using the program MSLS (Acta Crystallogr. A 54 (1998) 91). The results show that the obtained atomic positions are not significantly different for the chosen filter conditions. Even with amorphous carbon deposited on the specimen, which will cause a significant increase (>40 times) of energy-loss electrons, the structure refinement led to the same atomic positions.     

Observation of the loss of the hydroxyapatite sixfold symmetry in a human fetal tooth enamel crystal

A deviation from the hydroxyapatite hexagonal symmetry of a human tooth enamel crystal observed by high-resolution electron microscopy is reported. This symmetry deviation is characterized by: (1) ‘preferential’ planes that can be indexed as (100) with an intensity that differs from the (300) and the other {100} hexagonal equivalent planes; and (2) streaking of higher order reflections in the optical diffractogram of the image of the crystal. Computer simulations show that similar ‘preferential’ planes can also be observed at specific crystal tilt angles (and/or beam tilt and/or objective aperture misalignment) and at crystal thickness/microscope defocus values in images of hydroxyapatite crystals observed along the [0001] or [] zone axes. The streaking of higher order reflections in the optical diffractogram is related to a deformation of the crystal itself and does indeed show a symmetry deviation of the crystal under observation.     

A protocol for 3D image reconstruction from a single image of an oblique section.

Oblique section 3D reconstruction can produce a 3D image of a sectioned crystal from a single electron micrograph. We describe here in detail a reconstruction protocol applicable to an electron micrograph of an oblique section through a 3D crystal. The protocol is described in six steps: (1) selection criteria for images, (2) preprocessing steps to correct for image defects, (3) determination of unit cell coordinates, (4) interpolation of strip images with correction for image distortions and crystal disorder, (5) production of a crystallographic serial section reconstruction, (6) correction for skewed sampling to produce an oblique section reconstruction. In addition, we explore Wiener filter deconvolution of the section thickness. We describe a method for determining the section thickness by comparing data from projections of the oblique section reconstruction with corresponding data from a thick longitudinal section. Several schemes for Wiener filter deconvolution are described that differ in the way information on the signal-to-noise ratio is used in the filter.     

Image matching between experimental and simulated high‐resolution electron micrographs of sapphire on the orientation

The effects of imaging parameters have been studied on their roles of the severe mismatches between experimental and simulated high‐resolution transmission electron micrographs of sapphire along the direction. Image simulation and convergent‐beam electron diffraction techniques have been performed on misalignments of the electron beam and the crystal specimen. Based on this study, we have introduced an approach to achieve reliable simulation for experimental images of sapphire on the projection by the use of iterative digital image matching.     

Transmission electron microscopy at 20 kV for imaging and spectroscopy

The electron optical performance of a transmission electron microscope (TEM) is characterized for direct spatial imaging and spectroscopy using electrons with energies as low as 20 keV. The highly stable instrument is equipped with an electrostatic monochromator and a C_S-corrector. At 20 kV it shows high image contrast even for single-layer graphene with a lattice transfer of 213 pm (tilted illumination). For 4 nm thick Si, the 200 reflections (271.5 pm) were directly transferred (axial illumination). We show at 20 kV that radiation-sensitive fullerenes (C₆₀) within a carbon nanotube container withstand an about two orders of magnitude higher electron dose than at 80 kV. In spectroscopy mode, the monochromated low-energy electron beam enables the acquisition of EELS spectra up to very high energy losses with exceptionally low background noise. Using Si and Ge, we show that 20 kV TEM allows the determination of dielectric properties and narrow band gaps, which were not accessible by TEM so far. These very first results demonstrate that low kV TEM is an exciting new tool for determination of structural and electronic properties of different types of nano-materials.     

A simple 2D composite image analysis technique for the crystal growth study of L‐ascorbic acid

This work was destined for 2D crystal growth studies of L‐ascorbic acid using the composite image analysis technique. Growth experiments on the L‐ascorbic acid crystals were carried out by standard (optical) microscopy, laser diffraction analysis, and composite image analysis. For image analysis, the growth of L‐ascorbic acid crystals was captured as digital 2D RGB images, which were then processed to composite images. After processing, the crystal boundaries emerged as white lines against the black (cancelled) background. The crystal boundaries were well differentiated by peaks in the intensity graphs generated for the composite images. The lengths of crystal boundaries measured from the intensity graphs of composite images were in good agreement (correlation coefficient “r” = 0.99) with the lengths measured by standard microscopy. On the contrary, the lengths measured by laser diffraction were poorly correlated with both techniques. Therefore, the composite image analysis can replace the standard microscopy technique for the crystal growth studies of L‐ascorbic acid.     

A practical approach to the periodic continuation method for the simulation of high resolution TEM images of isolated crystal defects

The smallest spacing of the artificial structure necessary for a proper simulation of TEM images of isolated defects using the method of periodic continuation is studied for periodically repeated antiphase boundaries (APB) in a Au₄Mn crystal. The “net wavefield” of an APB at the exit face of the crystal is visualized by subtracting from the APB image the perfect crystal image (i.e. electron density) on both sides of the APB. It follows that, even for strongly scattering material, such as gold, the width of this “image” at the exit face does not exceed 1/100 of the foil thickness. In the electron microscope, however, the image widens further as a result of defocus, also with a proportionality factor of about 1/100. Hence it is suggested to use the smallest spacing for the dynamical diffraction calculations, which require most of the computing time, and to increase the repeat distance for the calculation of the electron microscope transfer.