Simulation of transmission electron microscopy images during tensile fracture of metal foils

Based on computer simulations, we examined a new mechanism of plastic deformation that has been proposed to operate in tensile fracture of metal foils. We constructed a Au crystal containing high concentration of vacancies and/or one subjected to large elastic tensile strain using embedded atom method (EAM) potential, and then calculated transmission electron microscopy (TEM) images of the crystal based on multislice method. Randomly distributed vacancies did not cause a large distortion in the crystal lattice, and did not affect the TEM image intensity appreciably unless the vacancy concentration exceeded several percent. Under a large elastic tensile strain of about 10% along 100, a periodic displacement of atoms whose amplitude was 10% of the atomic distance was induced, reducing the intensity of equal thickness fringe by about half. At around 15% tensile strain along 110, the crystal transformed from fcc to bcc structure, releasing the distortion of crystal lattice.     

Review: transmission scanning electron microscopy

The availability of scanning attachments for transmission microscopes and the advent of very high resolution scanning microscopes now enables materials to be studied in both the back scattered and transmission scanning modes. It is the purpose of this review to present in outline the subject of transmission scanning microscopy, the advocated advantages in comparison with conventional transmission microscopy and some of the achieved and potential applications.     

Metallurgical applications of scanning transmission electron microscopy

It is shown that scanning transmission electron microscopy (STEM) has followed two main lines of development, the pure STEM based upon a field emission electron source in which the emphasis is given to high resolution, and a combined system in which STEM is an attachment to a conventional transmission microscope (TEM + STEM). When used in combination with an energy dispersive X-ray spectrometer, the combined TEM + STEM system is shown to be extremely versatile and possibly the more useful for the applied metallurgist. The high vacuum requirements of pure STEM, however, make this system suitable to be used in conjunction with an Auger spectrometer. Examples of the various microanalysis facilities of STEM are given in the article, including micro-diffraction, rocking-beam channelling patterns, qualitative and quantitative X-ray spectroscopy analysis, particle analysis and in situ experimentation. The controversial subject of whether thicker specimens can be studied in STEM compared with conventional TEM is also discussed.     

Analytical transmission electron microscopy at organic interfaces

Organic materials are ubiquitous in all aspects of our daily lives. Increasingly there is a need to understand interactions between different organic phases, or between organic and inorganic materials (hybrid interfaces), in order to gain fundamental knowledge about the origin of their structural and functional properties. In order to understand the complex structure–property–processing relationships in (and between) these materials, we need tools that combine high chemical sensitivity with high spatial resolution to allow detailed interfacial characterisation. Analytical transmission electron microscopy (TEM) is a powerful and versatile technique that can fulfil both criteria. However, the application of analytical TEM to organic systems presents some unique challenges, such as low contrast between phases, and electron beam sensitivity. In this review recent analytical TEM approaches to the nanoscale characterisation of two systems will be discussed: the hybrid collagen/mineral interface in bone, and the all-organic donor/acceptor interface in OPV devices.     

Scanning transmission electron microscopy and microdiffraction techniques

At the moment scanning transmission electron microscopy (stem) instruments are not competetive with conventionaltem instruments for high resolution bright field imaging. For studies of the structure and defects of crystalline materials, their special virtues lie in the application of dark field imaging modes combined with observations of microdiffraction patterns from regions of diameter comparable with the microscope resolution limit (currently about 5 Å). They also offer capabilities for microanalysis by use of energy dispersive x-ray spectroscopy (eds) or electron energy loss spectroscopy (els). In principle the spatial resolution of these microanalysis methods is comparable to that of the imaging modes but in practice it is limited by poor signal-to-noise ratios or by the nonlocalized nature of the inelastic scattering process. The capabilities for microdiffraction are illustrated by sequences of diffraction patterns obtained as the incident beam is moved within the unit cell of a crystal of large (20 Å) periodicity. Applications of more immediate practical significance include diffraction studies of small crystallites of gold 20 to 50 Å in diameter and of the near-amorphous, thin oxide layers formed on chromium and iron films at room temperature. Microdiffraction, combined with reflection electron microscopy andels analysis, provides a powerful new approach to the study of the surface structure of crystals, including bulk samples, and the investigation of surface reactions. In particular, if a beam of small diameter (10–20 Å) is made to run along the face of a small crystal, the diffraction pattern andels curves are very sensitive to the form of the potential distribution at the surface and the excitations of the surface states of the crystal.     

Recent developments in dynamic transmission electron microscopy

One of the current major driving forces behind instrument development in transmission electron microscopy (TEM) is the ability to observe materials processes as they occur in situ within the microscope. For many processes, such as nucleation and growth, phase transformations and mechanical response under extreme conditions, the beam current in even the most advanced field emission TEM is insufficient to acquire images with the temporal resolution (∼1 μs to 1 ns) needed to observe the fundamental interactions taking place. The dynamic transmission electron microscope (DTEM) avoids this problem by using a short pulse laser to create an electron pulse of the required duration through photoemission which contains enough electrons to form a complete high resolution image. The current state-of-the-art in high time resolution electron microscopy in this paper describes the development of the electron optics and detection schemes necessary to fully utilize these electron pulses for materials science. In addition, developments for future instrumentation and the experiments that are expected to be realized shortly will also be discussed.     

Oxide-oxide interactions studied by transmission electron microscopy

The use of electron diffraction to study the interface region of thin, composite oxide films provides a sensitive means of investigating the mechanism of the solid-state reactions between these oxide layers.An investigation of the reaction between CuO and Al₂O₃ and NiO and Al₂O₃ by this method indicates the formation of an aluminate layer by a mechanism involving cation counterdiffusion.     

High-resolution transmission electron microscopy of AlAs-GaAs semiconductor superlattices

A new high-resolution transmission electron microscopy (HREM) technique is proposed for observing interfacial structures in semiconductor superlattices in the <1 1 0> cross-section, and is used to characterize AlAs-GaAs interfacial step structures. Analysis of diffracted beam amplitudes shows that the {1 1 1| beam amplitudes of GaAs are minimized at an extinction distance of 14.4 nm, whereas those of AlAs show significantly higher values. This remarkable difference in the {1 1 1| beam amplitudes leads to a marked contrast between HREM images of GaAs and AlAs around this specimen thickness, allowing edge-on observation of the interfacial atomic steps running along the <1 1 0> direction. Artifacts produced by ion milling prevent atomic-scale observations of the interfaces so an artifact-free TEM specimen preparation technique is also presented. The HREM method provides information on the step intervals and straightness of step edges at AlAs-GaAs interfaces fabricated on just-cut and vicinal GaAs (0 0 1) substrates. The effect of the growth interruption method during molecular beam epitaxy on interface smoothing is also demonstrated.     

Measurement of foil thickness in transmission electron microscopy

Methods for the measurement of the thickness of thin-foil specimens used in transmission electron microscopy are either difficult to carry out or have been subject to criticism. In particular, the contamination spot method is said to overestimate the thickness because the region of rapidly changing contrast marking the apparent edge of the spot is not on the foil surface but is on a broad contamination deposit whose thickness is changing much more slowly. A new method for measuring foil thickness is proposed, based on contamination deposits on the foil surfaces. The problems of the contamination spot method, in which the deposit is of circular form, are avoided by using one of the condenser lenses to focus the electron beam in a thin line on the foil during deposition. Adequate contrast can be obtained with a line whose width is one-third to one-fifth of the foil thickness and having a height equal to or less than its width. The error, being a fraction of the line width, is then very small. After rotation of the foil, the lines separate into two and the corresponding edges of the lines provide distinct features whose separation can be measured to determine thickness. The axis of rotation, perpendicular to which the separation of the lines has to be measured to calculate foil thickness, is determined by depositing two contamination lines at right angles. The method allows a number of measurements of thickness covering a relatively large area of foil to be made per contamination experiment. Near the edge of the foil, the upper and lower lines of contamination can join around the foil edge to form a U shape which can be used to measure thickness profile of the foil right up to the edge.     

Transverse transmission electron microscopy of sputtered NbN films

A technique of preparing transverse sections of sputtered NbN films for transmission electron microscopy is presented. Microstructural details of grain morphologies, orientations, and phase compositions can be readily analysed using transverse specimens over the entire film thickness including the deposited film-substrate interface.     

A transmission electron microscopy study of hexagonal ice

The morphologies of fast-frozen, thin-film samples of pure and dilute solutions of salts and surfactants in hexagonal ice are investigated with transmission electron microscopy. The cold-stage microscopy technique is described briefly and limitations imposed by the equipment and the sample itself are discussed. Ice grains, grain boundaries, dislocations, and stacking faults are imaged before radiolysis from the electron beam can alter their structures. The technique shows that screw dislocations in the ice basal plane are common, in accord with observations from X-ray topography and etch-replication microscopy. It also makes visible nonbasal dislocations in hexagonal ice, including dislocations in first prismatic planes, nonprismatic dislocation loops, and stacking faults on first pyramidal planes; heretofore, these defects have not been confirmed experimentally. Implications of the work for cold-stage microscopy of microstructured fluids are mentioned.     

纳米负载催化剂的透射电子显微镜表征

利用透射电子显微镜(TEM)的明场、暗场和扫描透射3种表征技术对纳米负载催化剂Cu-Ag/SiO2的微观形貌、结构进行研究表征,并将所得结果进行对比,讨论这3种技术在表征负载催化剂微观结构方面的优缺点。结果表明:STEM不仅能表征纳米颗粒的粒径分布,还能得到样品的元素分布信息,结果最为直观全面。     

Microstructural examination of layered coatings by scanning electron microscopy, transmission electron microscopy, and atomic force microscopy

The structure of r.f. sputtered multilayer Ti-BN coatings was investigated by low-voltage scanning electron microscopy, energy-dispersive X-ray spectrometry, transmission electron microscopy, and atomic force microscopy. Appropriate specimen preparation methods are described for each technique; these included fracture of the substrate, masking the growing film to produce a taper section, and ion-beam milling of embedded cross sections. Correlation of scanning electron micrographs with atomic force images was facilitated by the presence of similar composition contrast in both cases. Quantitative X-ray microanalysis of the layers was performed using the φ(z) approach. The crystal structures of nanocrystalline grains nucleated as a result of interdiffusion reactions during thermal annealing were identified by selected-area electron diffraction and convergent-beam microdiffraction as -titanium and f.c.c. titanium nitride.     

Clean electromigrated nanogaps imaged by transmission electron microscopy

Electromigrated nanogaps have shown great promise for use in molecular scale electronics. We have fabricated nanogaps on free-standing transparent SiN(x) membranes which permit the use of transmission electron microscopy (TEM) to image the gaps. The electrodes are formed by extending a recently developed controlled electromigration procedure and yield a nanogap with approximately 5 nm separation clear of any apparent debris. The gaps are stable, on the order of hours as measured by TEM, but over time (months) relax to about 20 nm separation determined by the surface energy of the Au electrodes. A major benefit of electromigrated nanogaps on SiN(x) membranes is that the junction pinches in away from residual metal left from the Au deposition which could act as a parasitic conductance path. This work has implications to the design of clean metallic electrodes for use in nanoscale devices where the precise geometry of the electrode is important.     

Portland cement clinker viewed by transmission electron microscopy

Thin sections of Portland cement clinker have been prepared by ion-beam thinning and examined in the electron microscope. The three most abundant phases, alite, belite and tricalcium aluminate have been identified. Features of interest include unexplained reflections in the diffraction patterns from alite and internally twinned or faulted martensite plates in belite. Hydrate gel coatings are obtained on the silicate phases but not on the aluminate phase, by immersing the thinned clinker in water. Dislocations in the aluminate phase do not appear to affect its reaction with water. On alite, easily observable hydrate coatings are obtained after immersion times as short as 5 min.     

Standardless atom counting in scanning transmission electron microscopy

We demonstrate that high-angle annular dark-field imaging in scanning transmission electron microscopy allows for quantification of the number and location of all atoms in a three-dimensional, crystalline, arbitrarily shaped specimen without the need for a calibration standard. We show that the method also provides for an approach to directly measure the finite effective source size of a scanning transmission electron microscope.     

The thinning of SpangoldTM for transmission electron microscopy studies

Abstracts are not published in this journal