Recent advances in electron tomography: TEM and HAADF-STEM tomography for materials science and semiconductor applications.

Electron tomography is a well-established technique for three-dimensional structure determination of (almost) amorphous specimens in life sciences applications. With the recent advances in nanotechnology and the semiconductor industry, there is also an increasing need for high-resolution three-dimensional (3D) structural information in physical sciences. In this article, we evaluate the capabilities and limitations of transmission electron microscopy (TEM) and high-angle-annular-dark-field scanning transmission electron microscopy (HAADF-STEM) tomography for the 3D structural characterization of partially crystalline to highly crystalline materials. Our analysis of catalysts, a hydrogen storage material, and different semiconductor devices shows that features with a diameter as small as 1-2 nm can be resolved in three dimensions by electron tomography. For partially crystalline materials with small single crystalline domains, bright-field TEM tomography provides reliable 3D structural information. HAADF-STEM tomography is more versatile and can also be used for high-resolution 3D imaging of highly crystalline materials such as semiconductor devices.     

透射电镜三维重构确定金纳米粒子的立体结构

贵金属纳米颗粒具有局域表面等离激元这一特性使其具有丰富的光学性质,而这一特性受制于纳米颗粒所形成的立体几何形状,而透射电镜和扫描电镜的二维图像不能真切地观测和确定纳米颗粒所形成的立体几何结构。透射电镜三维重构技术可作为一种确定纳米颗粒立体结构的直观有效的方法。本文利用透射电镜的三维重构技术,选择合适的参数进行二维图像的采集、图像匹配对中及重构、立体模型的构建,从而通过构建的模型对两种金纳米颗粒样品的不同几何形状所产生的边界形态进行了确认和分析。     

Mapping the chemistry in nanostructured materials by energy-filtering transmission electron microscopy (EFTEM)

Energy-filtered transmission electron microscopy (EFTEM) can be used to acquire elemental distribution maps at high lateral resolution within short acquisition times, which makes it quite efficient for a detailed characterization of nanostructures, as illustrated with examples concerning a nanostructured substituted La-based cermet compound and a nanoscale multilayer. In the first example, we show how phases in a rapidly cooled substituted LaNi5 can be visualized by recording jump ratio images. Secondly, EFTEM was capable of imaging individual nanoscale layers in a magnetic multilayer consisting of 2 nm terbium and 3 nm iron.     

Nanotomography in the chemical, biological and materials sciences

Nanotomography is a technique of growing importance in the investigation of the shape, size, distribution and elemental composition of a wide variety of materials that are of central interest to investigators in the physical and biological sciences. Nanospatial factors often hold the key to a deeper understanding of the properties of matter at the nanoscale level. With recent advances in tomography, it is possible to achieve experimental resolution in the nanometre range, and to determine with elemental specificity the three-dimensional distribution of materials. This critical review deals principally with electron tomography, but it also outlines the power and future potential of transmission X-ray tomography, and alludes to other related techniques.     

Minerals and aligned collagen fibrils in tilapia fish scales: structural analysis using dark-field and energy-filtered transmission electron microscopy and electron tomography

The mineralized structure of aligned collagen fibrils in a tilapia fish scale was investigated using transmission electron microscopy (TEM) techniques after a thin sample was prepared using aqueous techniques. Electron diffraction and electron energy loss spectroscopy data indicated that a mineralized internal layer consisting of aligned collagen fibrils contains hydroxyapatite crystals. Bright-field imaging, dark-field imaging, and energy-filtered TEM showed that the hydroxyapatite was mainly distributed in the hole zones of the aligned collagen fibrils structure, while needle-like materials composed of calcium compounds including hydroxyapatite existed in the mineralized internal layer. Dark-field imaging and three-dimensional observation using electron tomography revealed that hydroxyapatite and needle-like materials were mainly found in the matrix between the collagen fibrils. It was observed that hydroxyapatite and needle-like materials were preferentially distributed on the surface of the hole zones in the aligned collagen fibrils structure and in the matrix between the collagen fibrils in the mineralized internal layer of the scale.     

Identifying hexagonal boron nitride monolayers by transmission electron microscopy

Multislice simulations in the transmission electron microscope (TEM) were used to examine changes in annular-dark-field scanning TEM (ADF-STEM) images, conventional bright-field TEM (BF-CTEM) images, and selected-area electron diffraction (SAED) patterns as atomically thin hexagonal boron nitride (h-BN) samples are tilted up to 500 mrad off of the [0001] zone axis. For monolayer h-BN the contrast of ADF-STEM images and SAED patterns does not change with tilt in this range, while the contrast of BF-CTEM images does change; h-BN multilayer contrast varies strongly with tilt for ADF-STEM imaging, BF-CTEM imaging, and SAED. These results indicate that tilt series analysis in ADF-STEM image mode or SAED mode should permit identification of h-BN monolayers from raw TEM data as well as from quantitative post-processing.     

High-resolution transmission electron microscopy: the ultimate nanoanalytical technique

To be able to determine the elemental composition and morphology of individual nanoparticles consisting of no more than a dozen or so atoms that weigh a few zeptograms (10(-21) g) is but one of the attainments of modern electron microscopy. With slightly larger specimens (embracing a few unit cells of the structure) their symmetry, crystallographic phase, unit-cell dimension, chemical composition and often the valence state (from parallel electron spectroscopic measurements) of the constituent atoms may also be determined using a scanning beam of electrons of ca. 0.5 nm diameter. Nowadays electron crystallography, which treats the digital data of electron diffraction (ED) and high-resolution transmission electron microscope (HRTEM) images of minute (ca. 10(-18)g) specimens in a quantitatively rigorous manner, solves hitherto unknown structures just as X-ray diffraction does with bulk single crystals. In addition, electron tomography (see cover photograph and its animation) enables a three-dimensional picture of the internal structure of minute objects, such as nanocatalysts in a single pore, as well as structural faults such as micro-fissures, to be constructed with a resolution of 1 nm from an angular series of two-dimensional (projected) images. Very recently (since this article was first written) a new meaning has been given to electron crystallography as a result of the spatio-temporal resolution of surface phenomena achieved on a femtosecond timescale.     

Quantitative chemical phase imaging by means of energy filtering transmission electron microscopy

Electron spectroscopic imaging (ESI) in the transmission electron microscope (TEM) is a powerful method to produce 2-dimensional elemental distribution maps. These maps show in a clear way the chemical situation of a small specimen region. In this work we used a Gatan Imaging Filter (GIF) attached to a 200 kV TEM to investigate a Ba-Nd-titanate ceramic. The three phases occuring in this material could be visualized using inner-shell ionization edges (Ba M₄₅, Nd M₄₅ and Ti L₂₃). We applied different image correlation techniques to the ESI elemental maps for direct visualization of the chemical phases. First we simply overlaid the elemental maps assigning each element one colour to form an RGB image. Secondly we used the technique of scatter diagrams to classify the different phases. Finally we quantified the elemental maps by dividing them and multiplying them by the appropriate inner-shell ionization cross-sections which gave atomic ratio images. By using these methods we could clearly identify and quantify the various phases in the Ba-Nd-titanate specimen.Dedicated to Professor Dr. rer. nat. Dr. h.c. Hubertus Nickel on the occasion of his 65th birthday     

Electron tomography of Nafion ionomer coated on Pt/carbon black in high utilization electrode for PEFCs

To confirm the superiority of newly developed electrocatalyst layer (ECL) for polymer electrolyte fuel cells, three-dimensional dispersion states of Nafion ionomer in Pt/carbon black agglomerates were analyzed by electron tomography based on multiple TEM images taken at different tilt angles. Uniform distribution of the ionomer has been first observed, proving the high catalyst utilization in the new ECL distinctive from that of the conventional one.     

A coincidence transmission electron microscope with digital waveform measuring system for analytical microscopy

We have been developing a new analytical transmission electron microscope (TEM), called a coincidence TEM, which in principle enables elemental mapping images to be observed at a high signal‐to‐noise (S/N) ratio under very low dose radiation conditions. In this paper, we report the development of a coincidence TEM with a digital waveform measuring system for obtaining a coincidence elemental mapping image. In this system, analog signals detected by a Si(Li) detector and a multianode, position‐sensitive photomultiplier (PSPM) are continuously converted into 12‐bit digital waveform data at a rate of 100 MHz, and transferred to a PC. From the transferred digital waveform data, information on X‐ray photon energy, electron incident position, and detection times of both X rays and electrons are calculated by digital waveform measurement, which lead to the observation of a successful coincidence elemental mapping image. Copyright © 2005 John Wiley & Sons, Ltd.     

Quantitative structural assessment of heterogeneous catalysts by electron tomography

We present transmission electron microscope (TEM) tomography investigations of ruthenium-based fuel cell catalyst materials as employed in direct methanol fuel cells (DMFC). The digital three-dimensional representation of the samples not only enables detailed studies on number, size, and shape but also on the local orientation of the ruthenium particles to their support and their freely accessible surface area. The shape analysis shows the ruthenium particles deviate significantly from spherical symmetry which increases their surface to volume ratio. The morphological studies help to understand the structure formation mechanisms during the fabrication as well as the high effectiveness of these catalysts in the oxygen reduction reaction at the cathode side of fuel cells.     

A negative stain for electron microscopic tomography

While negative staining can provide detailed, two-dimensional images of biological structures, the potential of combining tomography with negative staining to provide three-dimensional views has yet to be fully realized. Basic requirements of a negative stain for tomography are that the density and atomic number of the stain are optimal, and that the stain does not degrade or rearrange with the intensive electron dose (~10? e/nm2) needed to collect a full set of tomographic images. A commercially available, tungsten-based stain appears to satisfy these prerequisites. Comparison of the surface structure of negatively stained influenza A virus with previous structural results served to evaluate this negative stain. The combination of many projections of the same structure yielded detailed images of single proteins on the viral surface. Corresponding surface renderings are a good fit to images of the viral surface derived from cryomicroscopy as well as to the shapes of crystallized surface proteins. Negative stain tomography with the appropriate stain yields detailed images of individual molecules in their normal setting on the surface of the influenza A virus.     

电子断层三维重构技术在材料微观结构分析中的应用

电子断层三维重构技术是在透射电镜基础上发展起来的,用以解析材料三维结构的一种技术。本文以美国FEI公司Tecnai G~2 F20透射电镜三维重构系统——Xplore 3D系统为例,探讨了样品制备与取向的选择、样品漂移问题的成因与校正、降低缺失锲存在造成的模型失真及空间分辨率的提高等问题,从以上四个方面详细介绍电子断层三维重构技术的要点及在材料微观结构方面的应用经验。     

Development of aberration-corrected electron microscopy

The successful correction of spherical aberration is an exciting and revolutionary development for the whole field of electron microscopy. Image interpretability can be extended out to sub-Angstrom levels, thereby creating many novel opportunities for materials characterization. Correction of lens aberrations involves either direct (online) hardware attachments in fixed-beam or scanning TEM or indirect (off-line) software processing using either off-axis electron holography or focal-series reconstruction. This review traces some of the important steps along the path to realizing aberration correction, including early attempts with hardware correctors, the development of online microscope control, and methods for accurate measurement of aberrations. Recent developments and some initial applications of aberration-corrected electron microscopy using these different approaches are surveyed. Finally, future prospects and problems are briefly discussed.     

Transmission electron microscopy observations of composite polymer colloid nucleated by functionalized silica

In this work, functionalized nanometric silica particles were engaged in emulsion polymerization of ethyl acrylate. The morphological characterization of this composite material was performed by transmission electron microscopy (TEM) and small angle X-ray scattering (SAXS). We were particularly interested in the state of encapsulation of the silica particles and their distribution in the latex film. Initialy, we successively studied both components of the composite: polymer beads and silica particles differ by their size and also by their contrast. In addition, it was possible to perfrom dark field TEM to study this system of two amorphous phases because their respective amorphous halos are not too close. Hence, we investigated the colloid material in aqueous media and after film formation. Although no ideal encapsulation is observed in the colloid in aqueous media, the distribution of silica in the latex film is good. SAXS results are in good agreement with TEM observations.     

Three-dimensional pore structure of chromatographic adsorbents from electron tomography

The pore structure of chromatographic adsorbents directly influences macromolecular partitioning and transport in chromatography. Quantitative structural characterization of chromatographic media has generally been performed in terms of the mean pore size or, at best, the pore size distribution (PSD), but more detailed information on, e.g., connectivity has been lacking. We have applied electron tomography, a 3D TEM technique that views a sample from multiple perspectives and allows reconstruction of the volumetric structure, to capture the internal details of microporous chromatographic media with nanometer-scale resolution. Visualization of reconstructions of three adsorbents, Toyopearl SP-650 C, SP-550 C, and CM Sepharose FF, provides thorough and direct information on the geometry and the interconnectivity of the pore network. The structures are qualitatively consistent with in situ AFM images, and quantitative data for the porosities and PSDs from the analysis of tomographic data agree reasonably well with inverse size-exclusion chromatography results. For a more straightforward representation of the networking and size features of the disordered pore space, a 3D thinning algorithm was used to derive pore skeletons and consequently quantitative data on distributions of local path lengths, widths, tortuosities, and connectivities. Such enriched structural information can be instrumental in more discriminate structural evaluation and construction of engineered pore models for the study of solute intraparticle transport.     

Surlyn®/silicate nanocomposite materials via a polymer in situ sol–gel process: Morphology

Surlyn®/silicate hybrid materials were produced via diffusion‐controlled polymer in situ sol–gel reactions for tetraethylorthosilicate. The heterogeneous morphologies of these materials were inspected with transmission electron (TEM), atomic force (AFM), and environmental scanning electron microscopic methods. The silicate uptake was highly dependent on the water affinity of the particular Surlyn® form (acid or ionic) rather than on the affinity of the solvent. The morphology consisted of silicate particles with diameters that were on the order of tens of nanometers. Hence, these materials can be classified as nanocomposites. The particle size distributions in both the TEM and AFM images for all composites appeared to be narrow, with un‐neutralized Surlyn® exhibiting a broader distribution. Larger particles were found near the film surfaces, and the silicon elemental distribution across the film thickness indicated higher concentrations near the surfaces, which is most likely due to the fact that the sol–gel reaction is diffusion controlled in these polymeric media. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 1563–1571, 2003     

Three-dimensional characterization of pigment dispersion in dried paint films using focused ion beam-scanning electron microscopy

The combination of integrated focused ion beam-scanning electron microscope (FIB-SEM) serial sectioning and imaging techniques with image analysis provided quantitative characterization of three-dimensional (3D) pigment dispersion in dried paint films. The focused ion beam in a FIB-SEM dual beam system enables great control in slicing paints, and the sectioning process can be synchronized with SEM imaging providing high quality serial cross-section images for 3D reconstruction. Application of Euclidean distance map and ultimate eroded points image analysis methods can provide quantitative characterization of 3D particle distribution. It is concluded that 3D measurement of binder distribution in paints is effective to characterize the order of pigment dispersion in dried paint films.     

Highly reproducible secondary electron imaging under electron irradiation using high-pass energy filtering in low-voltage scanning electron microscopy

The reproducibility of contrast in secondary electron (SE) imaging during continuous electron irradiation, which caused surface contamination, was investigated using SE high-pass energy filtering in low-voltage scanning electron microscopy (SEM). According to high-pass energy-filtered imaging, dopant contrast in an indium phosphide remained remarkably stable during continuous electron irradiation although the contrast in unfiltered SE images decreased rapidly as a contamination layer was formed. Charge neutralization and the SE energy distributions indicate that the contamination layer induces a positive charge. This results in a decrease of low-energy SE emissions and reduced dopant contrast in unfiltered SE images. The retention of contrast was also observed in high-pass energy-filtered images of a gold surface. These results suggest that this imaging method can be widely used when SE intensities decrease under continuous electron irradiation in unfiltered SE images. Thus, high-pass energy-filtered SE imaging will be of a great assistance for SEM users in the reproducibility of contrast such as a quantitative dopant mapping in semiconductors.     

The Cross‐Sectional Structure of Vanadium Oxide Nanotubes Studied by Transmission Electron Microscopy and Electron Spectroscopic Imaging

Vanadium oxide nanotubes (VO_x‐NTs) are easily accessible in pure form from vanadium(V) alkoxides and amines by a sol‐gel reaction and a subsequent hydrothermal treatment. The wall structure of VO_x‐NTs containing hexadecylamine or dodecylamine as the structure‐directing template has been characterised by transmission electron microscopy (TEM). A standard method for preparing TEM specimens was modified in order to investigate the cross‐sectional structure of the tubes. The elemental distribution in the layered structure inside the tube walls has been visualised by electron spectroscopic imaging: vanadium oxide builds up the layers that appear with dark contrast in the TEM images while carbon, i. e., the organic template, is present in between. The bent VO_x layers inside the tube walls are preferentially scrolls rather than concentric cylinders. Moreover, some tubes are formed by a combination of both types. The layer structure inside the tube walls is frequently disordered, and several types of defects appear.