Quantitative high resolution transmission electron microscopy of nanostructured semiconductors

Peak‐finding procedures and the geometric phase method of quantitative high resolution electron microscopy (qHRTEM) were applied to determine the local strain and the chemical composition of nanostructured semiconductor materials. The growth of the structures investigated was induced by minimization of strain energy. The analysis of strain distribution is necessary for the understanding of the self‐organized formation of nanostructures. The possibilities and limitations of the methods are discussed in detail by analysing HRTEM images of (Si,Ge) islands and of a double layer of stacked quantum dots of (In,Ga)As and Ga(Sb,As).     

In situ observation of the initial growth process of carbon nanotubes by time-resolved high resolution transmission electron microscopy

We succeeded in plan-view dynamic observation of the initial formation process of carbon nanotubes from β-SiC( 1 1 1 ) surfaces by time-resolved high resolution transmission electron microscopy. At 1360 °C, the flakes of graphite layers of a fibre orientation were formed on the SiC( 1 1 1 ) surfaces. From the graphite layers, carbon nanotubes were formed perpendicular to the ( 1 1 1 ) plane of the SiC. A scanning tunnelling microscopy observation showed that the end of carbon nanotube was closed. These results indicate that the caps of the carbon nanotubes are formed by a lift of a part of the graphene along the [ 1 1 1 ] direction of the SiC through generation of pentagons and heptagons. Two types of carbon nanotube, single-wall and double-wall, were observed in plan-view images. Different image intensity between an outer ring and an inner ring in double-wall nanotubes suggests that the inner layers of multiwall nanotubes are formed after the outer ones.     

Practical considerations for high spatial and temporal resolution dynamic transmission electron microscopy

Although recent years have seen significant advances in the spatial resolution possible in the transmission electron microscope (TEM), the temporal resolution of most microscopes is limited to video rate at best. This lack of temporal resolution means that our understanding of dynamic processes in materials is extremely limited. High temporal resolution in the TEM can be achieved, however, by replacing the normal thermionic or field emission source with a photoemission source. In this case the temporal resolution is limited only by the ability to create a short pulse of photoexcited electrons in the source, and this can be as short as a few femtoseconds. The operation of the photo-emission source and the control of the subsequent pulse of electrons (containing as many as 5 x 10(7) electrons) create significant challenges for a standard microscope column that is designed to operate with a single electron in the column at any one time. In this paper, the generation and control of electron pulses in the TEM to obtain a temporal resolution <10(-6)s will be described and the effect of the pulse duration and current density on the spatial resolution of the instrument will be examined. The potential of these levels of temporal and spatial resolution for the study of dynamic materials processes will also be discussed.     

Advantages of visualization of organelle structure by high resolution scanning electron microscopy

Examination of subcellular structures in detail and in three dimensions (3D) by scanning electron microscopy (SEM) is now possible on a routine basis due to improvements in design of the modern scanning electron microscope and new methods of specimen preparation involving chemical removal of the cytosol and the cytoskeleton. Cells which have been fixed, frozen, cleaved, thawed, and subjected to cytosol removal exhibit constituents such as nuclear chromatin, cisternae of endoplasmic reticulum, mitochondria, and the Golgi complex in bold relief. This permits examination by SEM in 3D of these structures from several aspects at a resolution close to that of conventional transmission electron microscopy (TEM). As a result, minute changes in the 3D structure of subcellular components can now be easily and conveniently examined from many specimens and anatomical sites, in development, in a variety of physiological processes, and in disease. The SEM method offers many advantages over the various TEM techniques now used for similar purposes, since much larger areas of the specimen can be surveyed by SEM in a given time, sectioning is not required and minute 3D changes in nuclear and organelle structure can be identified and analyzed more easily. The advantages are such that a host of biological questions can now be answered by SEM which, so far, have resisted solution using only TEM techniques. In addition, a new field of pathological diagnosis using SEM may develop, using the advantages offered by the technique in exploring the cell's interior as well as cellular tissue organization.     

Observations on the structure of amorphous arsenic by high resolution electron microscopy

Results are presented of a preliminary investigation of the structure of sputtered films of amorphous arsenic, using both high resolution bright field and tilted illumination dark field imaging modes. It is demonstrated that the material is not microcrystalline but appears to show a different form of short range order than is seen in films of amorphous carbon or germanium. The low angle scattering for the thin films was found to be very variable and it is suggested that this is associated with variability of a ‘cavern-like’ structure and of the normal triple coordination.     

石墨在高温形成的TEM原位动态观察

New carbon materials such as nanotubes and fullerenes are currently in the spotlight because of their remarkable properties and their potential to affectbroad areas of science and teclmology.Therefore,it is important to clarify the roles of metal catalysts on the synthesis of new carbons for controlling their structures and properties.However,the formation mechanisms of new carbons are still unclear despite the large amount of progress that has been reported concerning synthesis and applications.This is partly because only a few direct observations of graphite formation have been reported under ordinary synthesis conditions[1-2].     

Kodirex for high resolution electron microscopy.

Measurement of the important performance parameters shows that Kodak Kodirex film is more suitable than conventional ones for dark-field transmission electron microscopy of molecules at very high mignification. Results are cited for 120 kV; but the relation ship is valid up to 3 MV.     

Structural characterization of Ba2YCu3O7 by high resolution transmission electron microscopy

We show that high-resolution transmission electron microscopy can be used directly to reveal the structure of the metal framework and its defects in Ba₂YCu₃O₇. The use of dynamical effects under carefully selected and controlled conditions allows the detection of long-range order in the disposition of the oxygen vacancies, but inevitable misalignments make the quantification of the order parameter difficult. Specimen preparation methods are compared, and a wide-angle convergent-beam pattern is demonstrated.     

The importance of beam alignment and crystal tilt in high resolution electron microscopy

The relative influences of crystal tilt and beam alignment on high-resolution electron-microscopic imaging have been investigated. With the use of contrast transfer theory in generalised dimensionless form, the major effect of slight beam misalignment has been shown to be the introduction of an antisymmetric phase shift in the diffracted beams so that the presence of any such misalignment cannot be detected by the standard diagnostic tool of high-resolution electron microscopy, namely the optical diffractogram. Specific image simulations, at 100 and 500 keV, for materials of both small and large unit cells (SnO₂ and Ti₂Nb₁₀O₂₉ respectively) show, however, that even slight beam tilt can have a marked effect on the images of crystalline materials, causing considerable spurious detail and a loss of expected symmetry. The various options for ensuring accurate beam and crystal alignment are briefly reviewed, and some aspects of the alignment problems are demonstrated using some recent experimental images recorded at 500 kV.     

Low-noise boron supports for high resolution electron microscopy

Very thin (～1.5 nm) amorphous boron films are useful as specimen supports for the study of biochemical molecules by dark-field transmission electron microscopy. The noise factor is significantly smaller than for carbon films, probably because of the tendency to bond in icosahedral B₁₂ subunits. Signal-to-noise ratio of about 2 to 3 have been obtained with bromine and strontium atoms in test molecules.     

Specimen coating for high resolution scanning electron microscopy

Thin conducting films, produced by evaporation or soft vacuum sputtering generally show cracks and grain formation, when examined under high resolution scanning electron microscopy (SEM). These artefacts can obscure surface features of coated specimens or cause confusion in the interpretation of micrographs. No such structures have been observed in films produced by ion beam deposition. Ion beam deposition equipment is described in which a cold cathode saddle field ion source, operating at low pressure (15mPa), produces a 2 mm diameter beam of energetic ions (5 keV) and neutrals. With the beam directed onto a target at 30° to glancing incidence, the sputtered material coats the specimens, which are held in a planetary system for good coverage. Conditions favouring fine grain growth are a high nucleation density and low energy transfer to the substrate by thermal conduction or radiation or by particle or photon radiation. These conditions are satisfied by ion beam deposition but evidently not by evaporation or soft vacuum sputtering. With the specimen stationary, sharp shadowing is obtained because the target acts almost as a point source, because of the small diameter of the beam and because there is little scatter at the operating pressure.     

Preparation of frozen-hydrated specimens for high resolution electron microscopy

A method is presented for preserving the high resolution structure of biological membranes in a frozen-hydrated environment for electron microscopy. The technique consists of sandwiching a specimen between two carbon films and then waiting while some of the solvent evaporates. When the solvent layer is judged to be of an appropriate thickness, the specimen is then frozen in liquid nitrogen. The specimen can then be inserted into the precooled stage of an electron microscope. Electron diffraction studies of the purple membrane of Halobacterium halobium recorded at -120 degrees C have shown that the structure can be preserved to a resolution of 3.5 A. The main advantage of this method over previous techniques is that the hydrating conditions can be accurately controlled.     

High resolution electron microscopy of carbon structure

An investigation is made of the inherent performance of a high-resolution transmission electron microscope applied to the study of developing graphitic-sheet structures in heat-treated, coal-tar pitch carbons. Image detail is shown to be highly dependant on instrumental defocus. It is not obvious which form will be assumed by artefacts in these images; consequently, anomalous features are illustrated by reference to a specific electron-optical case and a corresponding light-optical analogy. Despite the difficulties associated with locating an optimum level of focus, optical diffractometry confirms that adequate conditions of microscopy are attainable on a routine basis. In a quantitative analysis of morphology in coal-tar pitch carbons, the technique reveals the cause of incipient, thermally-induced, molecular distortions which evidently result in a regression in the improvement of order otherwise developing with progressive heat-treatment.     

Specimen preparation technique for high resolution transmission electron microscopy studies on model supported metal catalysts

A new technique is described which can be used for preparing transmission electron microscopy (TEM) specimens suitable for high resolution studies on supported metal catalysts. By conventional silicon processing techniques 200 × 200 μm² Si₃N₄ membranes on Si wafers are produced. These membranes are extremely flat and have a uniform thickness of 13 nm. They can be used as a support in various kinds of thin film deposition. A TiO₂ film, optimally structured with respect to the requirements for high resolution TEM work in TiO₂–metal cluster systems, is deposited on the Si₃N₄ layer. It consists of one monolayer of 10–25 nm TiO₂ crystallites. TiO₂ lattice images show that a line resolution down to 0.19 nm is possible. Examples of TiO₂–Pd and TiO₂–Rh are given using respectively photodeposition and impregnation reduction to produce l.5–4 nm metal clusters.     

The use of histograms for transmission electron microscopy: Defocus and astigmatism correction at high resolution

A real-time method for optimizing the defocus of a conventional transmission electron microscope in the phase contrast imaging mode has been investigated using image histogram data. This method can also be used to minimize the objective lens astigmatism. It will be shown both theoretically and empirically, using a digital television frame store, that a histogram will give the largest peak when an image has a broad and flat contrast transfer function. This method has distinct advantages of speed and minimal computational requirements over obtaining the power spectrum of an image.     

Problems of interpretation in high resolution electron microscopy

Current assumptions in wave aberration theory and specimen scattering theory are reviewed. More quantitative image simulations would be valuable as well as use of a wider range of imaging techniques, particularly STEM. The severe difficulties of high resolution three-dimensional reconstruction are described and illustrated.     

Investigations of the structure of amorphous and partially crystalline metallic alloys by high resolution electron microscopy

A number of amorphous and partially crystalline palladium-silicon alloys have been examined by high resolution transmission electron microscopy at 500 kV. With the directly interpretable resolution extending beyond the first peak in the structure factor at 0?23 nm, details of the local microstructure at the atomic level are visible. Images of small metallic particles show a well-defined pattern of fringes over local regions. In some instances, especially in partially-ordered alloys, neighbouring or overlapping fringe patterns appear to be orientationally-related in a similar manner to fringe systems seen in symmetrically multiply-twinned particles. The significance of this type of structural examination for amorphous metals is discussed.