Hollow-cone Dark-field Transmission Electron Microscopy Research Articles

The effect of annealing on optical transmittance and structure of ZLANI fluorozirconate glass thin films

We report the effect of thermal annealing on structure, composition, optical transmittance and thickness of a novel fluorozirconate glass (ZLANI) containing Zr, La, Al, Na and In fluorides. In this work, pulsed laser deposition was used to grow thin films of ZLANI, and thermal annealing at different temperatures was performed on the films. Annealing did not change the composition, but a clear structural evolution of the ZLANI glass was observed by transmission electron microscopy (TEM), showing that we can control microstructure independent of composition. An increase in transmittance after the film was subject to a 100 °C thermal anneal was ascribed to the removal of defects and structural relaxation in the amorphous state. Following an anneal of 200 °C, the transmittance decreases due to heterogeneous formation of crystalline nuclei and changes in the local bonding. After the final annealing at 300 °C, a wider-scale crystallization occurred, with some major crystal phases formed as Zr2F8(H2O)6 and ZrO2, which alters the shape of the transmittance curve. The crystalline content of the crystal phases that form in the annealed films was quantified using hollow cone dark field TEM imaging. The 100 °C or 200 °C annealing decreases the film thickness by inducing structural relaxation and densification of the amorphous films, while the thickness increase of the 300 °C annealed film resulted from the formed large crystals. These results provide insights for the design of multilayer nanocomposites with a ZLANI glass matrix, which have potential applications as up-/down-conversion luminescent materials and X-ray storage phosphors.

Medium range order of bulk metallic glasses determined by variable resolution fluctuation electron microscopy

Variable resolution fluctuation electron microscopy (FEM) experiments are implemented with hollow-cone dark-field transmission electron microscopy. Medium range order lengths of zirconium and iron based bulk metallic glasses and amorphous silicon nitride are determined from the FEM results. It shows that maximum normalized intensity variances of FEM images occur when their nominal resolution approaches the correlation length Λ of the amorphous materials. Additionally, differences in the length and magnitude of medium range order are compared between metallic and covalent bond amorphous materials.

Hollow-cone dark-field transmission electron microscopy for dislocation density characterization of trimodal Al composites

This paper describes a methodology based on hollow-cone dark-field (HCDF) transmission electron microscopy (TEM) to study dislocation structures in both nano- and micro-crystalline grains. Although the conventional approach based on a two-beam condition has been commonly used to acquire weak-beam dark-field (WBDF) TEM images for dislocation structure characterization, it is very challenging to employ this technique to study nanocrystalline materials, especially when the grains are less than 100 nm in diameter. Compared to the conventional two-beam approach, the method described in this paper is more conducive for obtaining high-quality WBDF-TEM images. Furthermore, the method is suitable for studying samples with both nanocrystalline and coarse-grains. A trimodal Al metal-matrix-composite (MMC) consisting of B 4C particles, a nanocrystalline Al (NC-Al) phase, and a coarse-grained Al (CG-Al) phase has been reported to exhibit an extremely high strength and tailorable ductility. The dislocations in both NC-Al and CG-Al phases of the trimodal Al MMCs at different fabrication stages were examined using the HCDF method described. The influence of the dislocation density in both NC-Al and CG-Al phases on the strength and ductility of the composite is also discussed.

3-D Observation of Cu Particles Precipitated in Si by High-Angle Hollow-Cone Dark-Field Transmission Electron Microscopy

The performance of electronic devices, such as dynamic random access memories, is degraded by contamination due to impurity atoms as well as crystalline imperfections created during processing. The evaluation of those degradation causes is generally done using an analytical transmission electron microscope. The information obtained, however, is limited to two-dimensional images of the specimen as seen from a single direction. Advanced semiconductor devices with finer-pattern structures are expected to exhibit larger fluctuations in device performance due to the spatial distribution of the faults. A new method has thus been examined to determine the atomic species and to reconstruct three-dimensional (3-D) images of the specimen structure by using high-angle hollow-cone dark-field transmission electron microscopy (HADF-TEM).A incident angle controller was added to a conventional TEM to control the electron-beam deflection coils. This enables the incident electron beam to be inclined and rotated, providing hollow-cone illumination of the specimen, as shown in Fig. 1.

Atomic Species Analysis and Three-Dimensional Observation by High-Angle Hollow-Cone Dark-Field Transmission Electron Microscopy

Methods of determining atomic species and reconstructing three-dimensional images of the specimen structure were examined by high-angle hollow-cone dark-field transmission electron microscopy (HADF-TEM). The contrast of the HADF image systematically changed depending on the atomic number and the size of particles deposited on carbon films. The relationship between the contrast and the atomic number was well simulated using the theory of multiple electron scattering. The specimens, which were Cu particles precipitated in Si crystal, were observed with an inclination from -50 to 50° by steps of 2°. The images were processed using computed tomography. It became clear that the spatial distribution of the Cu particles differed depending on whether the precipitated position was a dislocation or a (111) stacking fault.

Dislocation contrast in high-angle hollow-cone dark-field TEM

Dislocations and grain boundaries have been imaged by high-angle hollow-cone dark-field transmission electron microscopy (HADF-TEM). The characteristics of image contrast in HADF-TEM are similar to those in high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM, or Z-contrast STEM), but different contrast mechanisms could be involved in each case because of differences in the respective scattering geometries. In HADF-TEM, the image contrast depends not only sensitively on the local mass-thickness of the specimen (i.e., Z-contrast), but also much more sensitively on the diffracting conditions that are initially set up for the corresponding on-axis bright-field TEM imaging, as well as on the position of the dislocation in the foil. The dislocations appearing in the HADF-TEM images have a one-to-one correspondence with those present in the corresponding BF-TEM image. The technique thus shares many properties with conventional diffraction contrast imaging. The dislocation contrast in HADF-TEM is generated by a two-step mechanism — the creation of diffuse scattering due to lattice distortion around the dislocation cores and the subsequent channelling propagation of the diffusely scattered electrons parallel to the optic axis, as governed by dynamical diffraction effects. The creation of local diffuse scattering is determined by the first step, and the final image contrast is essentially determined by the second step. It is the last step that makes HADF-TEM imaging analogous in many respects to conventional diffraction contrast imaging. The bright contrast of grain boundaries in HADF-TEM and HAADF-STEM images is at least in part due to high-angle diffuse scattering created by the local atomic rearrangement or relaxation. This is Huang scattering contrast, which usually does not carry atomic-number-sensitive information. It has been demonstrated that HADF- TEM is an easy method for determining the positions of dislocations in the foil thickness direction.

Dislocation contrast in high-angle hollow-cone dark-field TEM

Dislocations were imaged using the high-angle hollow-cone dark-field transmission electron microscopy (HADF-TEM) technique. Experiments were performed using a Philips CM30 TEM at 300 kV Dislocations and grain boundaries show bright contrast in HADF-TEM images and there are no contrast reversals with thickness or defocus. The dislocation contrast shows no dramatic variation when the average semi-conical angle θ was increased from 40 to 110 mrad, but does show strong dependence on the diffracting conditions that are set up for the corresponding on-axis bright-field (BF) TEM image (Fig. 1). Under the "one-beam" (random orientation without strong diffraction) reflection condition (Figs, 1a and 1a'), the visibility of the dislocations is poor in either the BF-TEM or the HADF-TEM image. Under the two-beam diffracting condition (Figs, 1b and 1b') both BF-TEM and HADF-TEM images show optimized contrast. The features appearing in the HADF-TEM images have a good one-to-one correspondence with the features shown in the corresponding BF-TEM images; the dislocation contrast disappears in the HADF-TEM images if the condition g·b = 0 is satisfied in the BF-TEM images, where b is the Burgers vector (Fig 2).