High-resolution electron microscopy of grain boundaries

Our ability to observe atomic-scale features of grain boundaries has tremendously improved during the past decade. In this paper we give, aided by a number of examples, a select overview, on progress in the field of grain boundary research directly related to the advent of modern high-resolution electron microscopy (HREM) instruments (point-to-point resolution better than 0.2 nm). Examples of grain boundary issues addressed by atomic structure observations of grain boundaries in oxides and metals will be given with emphasis on systematic investigations of the role of macroscopic and microscopic grain boundary parameters. Since comparisons between observed interface structures and atomistic computer modeling results are quite important, considerable efforts towards quantification have been undertaken recently by a number of authors. Most valuable insights have been obtained by the systematic examination of a range of grain boundary structures, using a combination of experimental observations and computer modeling results. In this manner HREM observations have been invaluable not only as a test of theoretical models, but also by exposing common atomic-scale features of high-angle grain boundaries. This has brought us closer to the goal of generating a general understanding of the interface structure and its connection to properties. Such studies have given valuable insights regarding the correlations between macroscopic grain boundary geometry, interfacial energy, and atomic relaxation modes.Work supported by the U.S. Department of Energy, Basic Energy Sciences, under contract W-31-109-Eng-38.     

High-resolution electron microscopy of heterostructures and interfaces

High-resolution transmission electron microscopy (HREM) allows to study a wide range of device-relevant topics in heteroepitaxial layer structures. Quantitative HREM may be used to obtain chemical information on a near-atomic scale from interfacial transition zones. The physical background is described and demonstrated on several examples in the Al _x Gal_1–x As/GaAs system. The HREM contrast of antiphase boundaries in InP grown on Si was studied by image simulations and has been compared to experimental images. Silicon carbide precipitates were identified by HREM at the homoepitaxial Si/Si interface. They stem from carbon contamination prior to Si layer growth.     

High-resolution transmission electron microscopy on silicon carbide whiskers

SiC whiskers were grown from the reaction of silicon monoxide (SiO) with activated carbon containing iron impurities. Growth proceeds through a VLS growth mechanism with SiO and CO as reacting gases. HRTEM combined with EDS shows that the SiC whisker is topped by a Fe₃Si catalyst droplet. The SiC whisker is found to be one-dimensionally disordered along the [111] growth direction of an fcc crystal structure. Although the catalyst droplet is usually larger than the top face of the whisker, we observed a number of situations where the diameter of the droplet was smaller. The study of the SiC-Fe₃Si interface showed that the growth is nucleated from the edges.     

High-resolution analytical electron microscopy of catalytically etched silicon nanowires

We report on the characterization of hexagonally ordered, vertically aligned silicon nanowires (SiNW) by means of analytical transmission electron microscopy. Combining colloidal lithography, plasma etching, and catalytic wet etching arrays of SiNW of a sub-50 nm diameter with an aspect ratio of up to 10 could be fabricated. Scanning transmission electron microscopy has been applied in order to investigate the morphology, the internal structure, and the composition of the catalytically etched SiNW. The analysis yielded a single-crystalline porous structure composed of crystalline silicon, amorphous silicon, and SiO _x with x≤2.     

High-resolution electron microscopy in structural studies of inorganic materials

A summary is given of the use of high-resolution electron microscopic imaging methods in structure determination, stressing their advantage over conventional diffraction methods, and the instrumental and specimen-dependent limitations on their applicability. Some examples of the use of this technique are given, both in systems where the unit-cell structure is known and only long-range order is lacking, and in cases of completely unknown phases.     

High-resolution transmission and scanning electron microscopy of boride-nitride nanostructured films

The results of electron-microscopic studies of grain boundaries and the structure of fractures of titanium boride-and nitride-based films obtained by nonreactive magnetron sputtering are considered. The chemical and phase composition of the films is analyzed with the help of Auger electron spectroscopy and microscopic electron diffraction analysis. The structure of boundaries and the presence of amorphous inclusions, dislocations, and other structural distortions are discussed and the nature of the deformation under indentation is considered.     

High-resolution electron microscopy of boron nitride nanotube with yttrium nanowire

Boron nitride (BN) nanotubes were investigated by high-resolution electron microscopy (HREM) and image processing. From the HREM image, a BN nanotube encapsulating yttrium nanowire was confirmed by comparing calculated diffraction and a nanostructure model. The present work indicated that yttrium elements could be confined in BN nanotube with large energy gap.     

High-resolution electron microscopy studies of antiphase boundaries in Cu3Au

We report the results of a crystallographic analysis of antiphase boundaries (APB) in Cu₃Au obtained by using high-resolution electron microscopy. Our observations corroborate the conclusions of previous work, based on conventional electron microscopy, which indicate that antiphase boundaries are mainly located in the cube planes. We found that the deviation of the average APB orientation from the cube planes is related to the formation of kinks on an atomic scale. Moreover, the identification of the boundary plane shows that the APBs are mostly conservative. These results are discussed and interpreted in the framework of a pairwise potential approach with predominant nearest neighbor interactions.     

The dimer-trimer model for heterogeneous catalysis

We study a dimer-trimer lattice model for heterogeneous catalysis for the reaction 1/2A₂+1/3B₃AB. The A₂ and B₃ particles require two and three active sites for their adsorption onto the lattice, respectively. The model is unusual in that it possesses an infinite number of absorbing states whereby the lattice is poisoned and reactions must stop. Previously studied models have only two absorbing states. In one dimension, the lattice poisons with mostly dimers and a few trimers even at vanishingly small dimer-adsorption probabilities and there is a discontinuity when this probability is zero. On the triangular lattice, the poisoned phases consist of only one component and vacancies, and the phase diagram is similar to that of the monomer-dimer model of Ziff, Gulari, and Barshad. However, the second-order transition belongs to a different universality class than Reggeon field theory, contrary to previous models. Finally, we present results for the Kagomé lattice, for which the poisoned phases consist of two components due to its smaller connectivity.     

High-resolution transmission electron microscopy of phase formation and growth in metal-Si-Ge systems

High-resolution transmission electron microscopy (HRTEM) in conjunction with nano-beam (NB) analysis as well as energy dispersive spectrometry analysis have been fruitfully utilized to study the interfacial reactions in the metal-Si-Ge systems. In this paper we report the results of TEM study of the phase formation and growth in Ti-Si-Ge, Cu-Si-Ge and Ni-Si-Ge systems.     

High-resolution electron microscopy of individual metallofullerene molecules on the dipole orientations in peapods

Electron microscopy with atomic sensitivity enables us to obtain a direct image of the intra-molecular structure of metallofullerenes encapsulated inside single-walled carbon nanotubes. By a comparison of high-resolution images with a simulation to extract the relative atom positions for encaged metal atoms in each molecule, the distribution of the molecular orientations and interactions between adjacent molecules in metallofullerene peapods have been statistically analyzed. The results are suggestive of strong interactions between fullerene–fullerene and fullerene–tube in peapods at room temperature. Received: 10 October 2002 / Accepted: 25 October 2002 / Published online: 10 March 2003 RID="*" ID="*"Corresponding author. Fax: +81-298/61-6310, E-mail: suenaga-kazu@aist.go.jp     

High-resolution electron microscopy of Si cones formed on Ar+-bombarded Si wafers

Conical Si projections generated on Si wafers bombarded with obliquely incident Ar⁺ ions were studied by high-resolution transmission electron microscopy. The cones were composed of an [111]-oriented bulk phase covered with a disordered thin layer, but the bulk phase was not perfectly ordered, containing an amorphous domain underneath the outermost area. Such a multi-phase structure is inexplicable in terms of ion erosion, suggesting interplay of the redeposition of sputtered Si atoms on the bombarded area with the ion-erosion process so as to promote cone evolution. The cones were also characterized by the development of web-like platelets at their acute angles, an indication of a crystal growth process involved in the surface phenomenon observed.     

The chemical nature of electron waves in metals — A basis for the study of heterogeneous catalysis

The tight-binding method is used to describe the electronic waves of metals in the atomic orbital language familiar to chemists. It is shown that (in both fcc and bcc crystals) k vectors with components (0, 0, 1) or (1, 1, 1) carry hybrids between the s waves and Z² waves whose axis Z is parallel to the direction of the wave vector. In particular the higher-energy hybrid waves are strongly pointed along the wave vector direction. Furthermore, using the finite cube model, with excellent analytical approximations to the orbitals, the existence of privileged wave vector directions perpendicular to the surface approached by an incoming substrate is demonstrated. A preliminary discussion of the favorable conditions for catalysis emphasizes the extremely favorable role of such pointed hybrids if their sub-band lies (empty) just above the Fermi level of the metal.     

The study of water in heterogeneous media using environmental scanning electron microscopy

The Environmental scannning electron microscope has opened the door to examining a whole new range of water- and other liquid-containing specimens at unprecedented resolution and depth of field. In particular, the technique provides the opportunity to examine the nature and distribution of water in heterogeneous systems in both static and dynamic experiments. Obtaining useful information from these experiments, though, requires careful experimental design and a thorough understanding both of the chamber environment and of the many interactions between the imaging electrons, the specimen, and the chamber gas. Here, we present an introduction to the technique, followed by a selection of examples of experiments wherein the distribution and behaviour of all three thermodynamic phases of water may be of interest. It is our hope that this paper should inspire others to consider the merits of developing this technique.     

Microporous heteropoly oxometalate heterogeneous catalysis

A new class of microporous catalysts has been found. Certain of the salts of heteropoly oxometalates with monovalent cations show characteristics expected for microporous solids: relatively large quantities of absorbed gas at low relative pressures, high C_BET values and limited linearity ranges of the BET equation. Pore size analysis shows that the distributions are broad, ranging from approximately 6–13 Å. The micropores are believed to result from the translation and rotation of the Keggin anions.     

High-resolution microscopy of plasmon field distributions by scanning tunneling luminescence and photoemission electron microscopies

The exploitation of plasmon resonances to promote the interaction between conjugated molecules and optical fields motivates intensive research. The objectives are to understand the mechanisms of plasmon-mediated interactions, and to realize molecularly- or atomically-precise metal nanostructures, combining field enhancements and optical antenna effects. In this review paper, we present examples of plasmonic-field mappings based on scanning tunneling microscope (STM)-induced light emission or multiphoton photoemission (PEEM), two techniques among those which offer todayʼs best spatial resolutions for plasmon microscopy. An unfamiliar property of the junction of an STM is its ability to behave as a highly localized source of light. It can be exploited to probe optoelectronic properties, in particular plasmonic fields, with ultimate subnanometer spatial resolution, an advantage balanced by a sometimes delicate deconvolution of local-probe influence. Alternatively, local-probe disadvantages can be overcome by imaging the photoemitted electrons, using well-established electron optics. This allows obtaining two-dimensional intensity maps reflecting the unperturbed distribution of the optical near field. This approach provides full field spectroscopic images with a routine spatial resolution of the order of 20 nm (down to 5 nm with recent aberration corrected instruments).     

High-resolution transmission electron microscopy of heteroepitaxial barium strontium titanate films grown on MgO substrates

Nanoscale (5–100 nm) ferroelectric heteroepitaxial Ba_0.8Sr_0.2TiO₃ films on MgO(001) substrates were studied by methods of high-resolution transmission electron microscopy using Tecnai G² and FEI Titan 80–300 microscopes. Misfit dislocations resulting from stress relaxation in films and insignificant local misorientation of film regions (block structure) were detected. The heterostructure was numerically simulated, which allowed the detection of the formation of oxygen (substrate)-titanium (film) bonds or the oxygen (substrate)-barium (film) bond at the film-substrate interface.