Hollow Cone Dark Field Research Articles

Hollow-cone Dark Field (HCDF) Imaging for Nano-grained Mg: Experimental and Simulated Contrast

Hollow-cone Dark Field (HCDF) Imaging for Nano-grained Mg: Experimental and Simulated Contrast

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.

Hollow Cone Electron Imaging for Single Particle 3D Reconstruction of Proteins.

The main bottlenecks for high-resolution biological imaging in electron microscopy are radiation sensitivity and low contrast. The phase contrast at low spatial frequencies can be enhanced by using a large defocus but this strongly reduces the resolution. Recently, phase plates have been developed to enhance the contrast at small defocus but electrical charging remains a problem. Single particle cryo-electron microscopy is mostly used to minimize the radiation damage and to enhance the resolution of the 3D reconstructions but it requires averaging images of a massive number of individual particles. Here we present a new route to achieve the same goals by hollow cone dark field imaging using thermal diffuse scattered electrons giving about a 4 times contrast increase as compared to bright field imaging. We demonstrate the 3D reconstruction of a stained GroEL particle can yield about 13.5 Å resolution but using a strongly reduced number of images.

B11-O-12Development of Hollow Cone Dark Field environmental Electron Microscopy and their Biological Application

B11-O-12Development of Hollow Cone Dark Field environmental Electron Microscopy and their Biological Application

Characterization of crystallite morphology for doped strontium fluoride nanophosphors by TEM and XRD

Crystallite morphology for Eu-doped and undoped SrF2 nanophosphors have been determined by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The values for average crystallite size obtained by the application of the Scherrer equation and the full width at half maximum (FWHM) values for XRD peaks are compared to the results obtained using the hollow cone dark field (HCDF) TEM imaging technique. In the case of the TEM analysis, a bimodal crystallite size distribution was revealed with one of the distributions having a measured range of crystallite sizes which was in good agreement with the XRD data. HCDF in combination with FIB specimen preparation was found to be a promising technique for the determination of crystallite size distributions in nanophosphors which might facilitate a better understanding of their scintillation properties.

Strain-induced grain growth of cryomilled nanocrystalline Al in trimodal composites during forging

Abstract Grain growth of nanocrystalline aluminum ( nc Al) in trimodal Al metal-matrix-composites (MMCs) during hot forging was investigated. The nc Al phase formed through cryomilling of inert gas-atomized powders in liquid nitrogen has an average grain size down to 21 nm, exhibits excellent thermal stability. However, substantial grain growth of nc Al up to 63 nm was observed when the Al MMCs were thermo-mechanically processed even at relatively low temperatures. Grain growth of the cryomilled nc Al phase in trimodal Al MMCs after hot forging was documented with respect to temperature ranging from 175 °C to 287 °C, true strain ranging from 0.4 to 1.35 and strain rate ranging from 0.1 to 0.5 s −1 . Hollow cone dark field imaging technique was employed to provide statistically confident measurements of nc Al grain size that ranged from 21 to 63 nm. An increase in forging temperature and an increase in true strain were correlated with an increase in grain size of nc Al. Results were correlated to devise a phenomenological grain growth model for forging that takes strain, strain rate and temperature into consideration. Activation energy for the grain growth during thermo-mechanical hot-forging was determined to be 35 kJ/mol, approximately a quarter of activation energy for bulk diffusion of Al and a half of activation energy for static recrystallization.

Grain size determination in nano-scale polycrystalline aggregates by precession illumination-hollow cone dark field imaging in the transmission electron microscope

Abstract Precession illumination hollow cone dark field (PI-HCDF) transmission electron microscopy (TEM) provides high contrast multi-beam dark field images, which are suitable for effective and robust grain size measurements in nano-scale polycrystalline aggregates. Precession illumination with slightly converged electron beam probes and precession angles up to 3° has been produced using a computer-controlled system using a JEOL JEM 2000FX TEM instrument. Theoretical and practical aspects of the experimental technique are discussed using example precession illumination hollow cone diffraction patterns from single crystalline NiAl and the importance of selecting the appropriate precession angle for PI-HCDF image formation and interpretation is described. Results obtained for precession illumination are compared with those of conventional parallel beam illumination experiments. Nanocrystalline Al has been used to evaluate the influence of the precession angle on PI-HCDF image contrast with a focus on grain size analysis. PI-HCDF imaging has been applied for grain size measurements in regions of a nanocrystalline Al thin film adjacent to the edge of a pulsed laser melted and rapidly solidified region and determined the dimensions of a heat-affected zone.

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.

High contrast hollow-cone dark field transmission electron microscopy for nanocrystalline grain size quantification

In this paper, we describe hollow-cone dark field (HCDF) transmission electron microscopy (TEM) imaging, with a slightly convergent beam, as an improved technique that is suitable to form high contrast micrographs for nanocrystalline grain size quantification. We also examine the various factors that influence the HCDF TEM image quality, including the conditions of microscopy (alignment, focus and objective aperture size), the properties of the materials imaged (e.g., atomic number, strain, defects), and the characteristics of the TEM sample itself (e.g., thickness, ion milling artifacts). Sample preparation was found to be critical and an initial thinning by wet etching of the substrate (for thin film samples) or tripod polishing (for bulk samples), followed by low-angle ion milling was found to be the preferred approach for preparing high-quality electron transparent samples for HCDF imaging.

Thickness dependence of structure and magnetic properties of annealed [Fe/Pt]n multilayer films

The L10 FePt phase material has potential applications for magnetic recording and permanent magnets due to its high magnetocrystalline anisotropy energy density. The heat treatment of [Fe/Pt]n multilayer films is one approach to form the L10 FePt phase at a lower processing temperature, which is highly desirable for its applications. This paper reports the influence of film total thickness (8–100 nm) on the structure and magnetic properties of annealed [Fe/Pt]n multilayer films. A novel technique based on hollow cone dark field transmission electron microscopy is used to determine the L10 phase fraction and grain size in the annealed films. It was found that the L10 phase fraction and grain size, ordering, and magnetic properties are strongly dependent on the total film thicknesses. An L10 FePt phase fraction close to 100% is achieved for a thick [Fe/Pt]n multilayer film (100 nm) annealed at 400 °C for 1 h, while a value of only 36% is achieved for a thin film (8 nm) annealed at the same condition. In this work the L10 FePt phase nucleation density is also presented. These results suggest that the L10 phase formation is nucleation limited, and that the subsequent growth of L10 phase grains also strongly influences the structure and magnetic properties of the annealed films.

Quantification of L1 phase volume fraction in annealed [Fe/Pt]n multilayer films

The heat treatment of [Fe/Pt]n multilayer films is a promising approach to form the L10 FePt phase at a reduced temperature, which is of interest for applications as high-density magnetic recording media and high-energy permanent magnets. The volume fraction of the L10 FePt phase in the annealed films strongly influences their magnetic properties. This paper introduces a novel method based on hollow cone dark field transmission electron microscopy imaging to quantify the L10 FePt phase volume fraction. This method is used to characterize two sets of [Fe/Pt]n multilayer films with varying deposition temperature and periodicity. It was found that both the deposition temperature and bilayer periodicity are significant to the structure and magnetic properties of annealed films and to the extent to which the L10 phase is formed. A correlation between L10 phase volume fraction, grain size, and magnetic properties was also observed.

Investigation of surface crystallites on C54 titanium silicide thin films using transmission electron microscopy

This paper discusses the occurrence of crystallites on the surface of low resistivity C54 titanium silicide thin films, which will influence the estimation of contact resistance in multi-layer ohmic contacts. Cross-sectional transmission electron microscopy analysis indicates the existence of a uniform thin film with crystallites on the surface at regular intervals increasing surface roughness. Jump ratio maps and hollow cone dark field imaging have shown that the crystallites have the same composition and orientation as the underlying micron-sized grains.

Critical assessment of the speckle statistics in fluctuation electron microscopy and comparison to electron diffraction

The technique of fluctuation electron microscopy (FEM) is applied to thin films of amorphous germanium and of polycrystalline gold in a transmission electron microscope. Even though the method was introduced as a tool for quantitative analysis of structural fluctuations in amorphous materials, the basic principles are applicable to any disordered specimen independent of the dimension of disorder. Hence, we extended the technique of FEM to a well-known specimen, gold, whereby it was possible to reinterpret the results of the measurements on amorphous germanium. The hollow-cone dark field images, the statistical analysis of which is the basis of FEM, are examined with respect to the effects of frequency filtering, and are compared to electron diffraction. We find that the angular dependence of the normalised variance, as measured by FEM, yields information similar to the average intensity of hollow-cone dark field image series. Both plots are basically identical to a scan through a selected-area diffraction pattern convoluted by the corresponding angular resolution function. Hence, it appears questionable whether standard FEM analysis provides more information than the classical pair distribution function, which is experimentally limited to short-range order. Frequency selective analysis of the normalised variance, however, gives supplementary information on preferred inter-atomic distances related to the medium-range order of the specimen.

Fluctuation Microscopy in the STEM

Fluctuation microscopy is an electron microscopy technique sensitive to medium-range order (MRO) in disordered materials. It has been applied to study amorphous germanium and silicon, leading to the conclusion that these materials exhibit more MRO than the conventional continuous random network model for their structure.As originally proposed by Treacy and Gibson, fluctuation microscopy utilizes mesoscopicresolution (1.5 nm) hollow-cone dark field (HCDF) imaging in a TEM. The normalized variance of such images,is a measure of the magnitude of fluctuations in the diffracted intensity from mesoscopic volumes of the sample and is sensitive to MRO via the three- and four-body atom distribution functions. Studying V as a function of the diffraction vector magnitude k gives information about the degree of MRO and the internal structure of ordered regions. V as a function of the inverse resolution Q gives information about the characteristic MRO length scale.

Interreaction of cu/au thin films observed by high angle hollow cone darkfield electron microscopy

Monodisperse hollow Ag and Ag/Au submicrometer spheres have been synthesized by a facile colloidal templating method. In this method, the silver nanoparticle seeds are synthesized on the silica colloids surface through the electroless plating approach to avoid the complex surface functionalization, and the subsequent seeding growth and dissolving silica core are carried out simultaneously. The as-prepared hollow spheres remain mostly the fine monodispersity and spheric shape of the silica core, and can self-assemble directly into ordered structure, which is realized for the first time. The method is simple and versatile, and can be used to prepare other monodisperse hollow composite metallic or bimetallic spheres.

HOLLOW CONE DARK FIELD MICROSCOPY OF GP ZONES IN AlAg

In spite of its low technological interest AlAg is well accepted as a binary model for unmixing systems due to its small lattice misfit and its high mass contrast favorable to X-ray scattering and electron microscopy. But, although often investigated, the composition and internal structure of the metastable GP zones is still controversial. For the interpretation of X-ray scattering data always an a-priori microstructure model is necessary, which might be the reason for the inconsistent results stated above. Therefore, the authors are looking for a more direct microscopic approach to get at least semi-quantitative information on the zone`s composition. Here, the ``high angle hollow cone dark field`` imaging using a conventional transmission electron microscope is proposed for composition analysis. Results on the internal structure of the GP zones in AlAg are presented.

Measurement of interdiffusion by high angle hollow cone illumination

High angle hollow cone darkfield or Z-contrast imaging is used to supress dynamical contributions like bend or extinction contours to electron imaging. Using a multiple scattering model it is possible to interpret image intensities quantitatively and to analyze specimen composition on a nanometer scale without specialized detectors like energy-energy-loss spectroscopy or energy dispersive spectroscopy. This method is practically tested on the interdiffusion of Cu-Au couples. Measured thin film interdiffusion coefficents, are comparable to literature data of the bulk diffusion coefficent in this system.

In Z-contrast, not all that glistens is high-Z

It is by now a given that high resolution Z-contrast is a useful imaging technique for detecting high atomic number (Z) domains in nanophase materials. However, image intensity also depends on specimen orientation, electron-optical geometry, thickness and local strain fields, as well as local structural ordering. In this paper we will present examples from our materials studies where contrasts from strain and short-range order in hollow-cone dark-field images provide important structural insights. Strain fields were found to be responsible for the unusual "blister' contrasts observed in hollow-cone dark field images of delaminated sheets of calcium sodium niobate layered materials. Wherever two sheets overlap, the average intensity increases and dark disks of diameter ~20 nm, with bright central spots, appear (Fig. 1). For large hollow-cone angles, q1 > 20 nm-1, the blisters vanish, although faint fine-scale features persist. Elasticity analysis and image simulations (Fig. 2) reveal that the blisters are pockets of gas or liquid that became trapped between sheets during the drying stage of the specimen preparation process, when individual sheets come into contact at random mutual angles and reseal (Fig. 3).

Hollow cone illumination—I. Dark-field electron microscopy of biological specimens

Fixed beam, hollow cone dark-field electron microscopy was set out to investigate the applicability to various biological specimens. The image quality of plain carbon films showed a significant improvement compared to those illuminated with tilted beam. Nucleic acid specimens, unstained or stained, were recorded with far better contrast than their stained counterparts. Apart from the fact that the dark-field micrographs were free from defocus-dependent granulation, microbiological specimens did not show features other than those already known from bright-field imaging. Specimens positively stained with heavy atom compounds, can provide complementary information to the negative staining technique employed in bright-field. Aldehyde-fixed material was successfully imaged in hollow cone dark-field. If sections were cut thin enough (≤ 40nm), post-fixation and staining was not required. In these specimens, dark-field micrographs revealed membrane-like structures not observed in bright-field.

Bright field and hollow cone dark field electron microscopy of palladium catalysts

SUMMARYThe usefulness of hollow cone dark field imaging for the study of Pd catalyst particles on γ‐alumina substrates is investigated. By varying the depth of the specimen in the objective lens field, different Bragg reflections can be conveniently selected. Typically, about 70% of the particles appear in either the 111 or 200 Bragg reflections. Rather more particles can be visible in bright field because of other Bragg reflections as well as absorption effects. Size distributions in bright field and dark field are compared.