On-line image processing in high resolution electron microscopy

On-line processing and analysis of high resolution electron microscope (HREM) images has been implemented using a software controlled image processor based on a commercial digital video framestore. It directly digitizes the output from the low light TV camera on the HREM and applies any processing necessary. The conditions under which images from amorphous specimens are being obtained can be established on-line. The fast fourier transforms (FFTs) of 256 times 256 pixels produced in 15 s with the system off-line are comparable to those from diffraction analysis of the same data on a laser optical bench. The precision of analysis of defocus, aberrations and astigmatism on-line is discussed, together with possible approaches to routine analysis of crystalline specimens and for interactive control.     

Recent advances and applications of high resolution electron microscopy

This review covers several broad areas: firstly, recent developments in HREM instrumentation, and then novel techniques for imaging are discussed, including some of the problems of image interpretation. Applications of HREM techniques to a wide range of materials problems are described and include solid state chemistry, ceramics, semiconductors, metals and natural diamonds. The next generation of high resolution microscopes will operate in the 300–400 kV range, have low C_s objective lenses, and have sufficiently good vacuum to allow the combined use of CBED and EELS facilities with imaging in the sub-2 Å range. Microprocessor control of instrumental parameters such as astigmatism, alignment and defocus are seen as an important way forward in achieving the optimum performance of these instruments.     

Procedures for focusing,stigmating and alignment in high resolution electron microscopy

Several different approaches to the alignment, stigmating and focusing of a high resolution electron microscope are evaluated theoretically and experimentally. Ambiguities in the interpretation of diffractograms are pointed out which make it necessary to explore a range of incident beam directions before the correct alignment can be established. The variation of image contrast with the imaging conditions is examined in detail, and its global minimization is shown to be a reliable criteria for adjustment of all parameters. Recommendations are made as to the best procedures to adopt in various circumstances, and a computer-controlled procedure based on the image contrast is described which sets all parameters automatically in less than a minute.     

Analysis of high resolution transmission electron microscope images of crystalline-amorphous interfaces

For the analysis of images of homogeneous crystalline–amorphous interfaces we propose to average them along the interface obtaining the averaged interface image or the averaged intensity profile. Due to averaging, contrast components with the periodicity of the crystalline area of the image are extracted. Thus, the contrast features originating from the random overlap of the projected potentials of atoms in the amorphous layer are suppressed. It is shown that averaged images can be simulated by the multi-slice method using the novel approach to model the near interfacial amorphous structure by its mean atomic density distribution in front of the crystalline boundary. The crystalline structure is represented by its known atomic positions. We apply the proposed method to the investigation of the near interfacial short-range order in the c-Si/a-Ge crystalline–amorphous interface.     

The high resolution electron microscopy of stacking defects in Cu–Zn–Al shape memory alloy

The characteristic defects in the martensitic phase of the memory alloy Cu–Zn–Al have been investigated using the Cambridge University 600 kV High Resolution Electron Microscope. Conditions are found under which the images can be quantitatively simulated when this involves the transfer of structural information at resolutions beyond the first zero of the contrast transfer function. Defects of sequence-type were positively identified by the comparison of high-resolution micrographs with full dynamical image simulations.     

High resolution electron microscopy of partial dislocations in the Laves phase structure

A Mg-base Laves phase was investigated by high resolution electron microscopy (HREM). Linear defects found at terminations of stacking faults were classified into three groups. The first is a partial dislocation at a termination of a stacking fault, the second is a superposed partial dislocation which is defined as a defect produced by a superposition of terminations of two or more stacking faults lying on neighbouring layers, and the third is a combined linear defect which consists of a characteristic combination of terminations of stacking faults. In the last case the total stacking fault vector becomes equal to the translation vector in the basal plane, so that the defect needs no relaxation of the lattice. The Burgers vectors of the partial dislocations were estimated with the aid of modified Burgers circuits.     

High-resolution electron microscopy of human enamel crystals

The structure of enamel crystals obtained from four human premolars has been studied by high-resolution electron microscopy (HREM) in the [0001], [***2110], [***1540], [***0110] and [***1213] crystallographic directions at various microscope defocus and crystal thickness values. The resolution obtained has not previously been reported for human enamel crystals. In all cases, it was possible to match the experimental images to images calculated using the atomic positions of mineral hydroxyapatite. However, a deviation from hexagonal symmetry characterized by marked (***1010) planes of intensity different from the one of the (***3030) and {***1010}-type planes was observed. In this work, we present an improvement of Scherzer resolution of 0.25-0.20 nm over previous work on biological enamel crystals. This improvement of resolution has permitted the incorporation of crystallographic reflections of higher spatial frequencies into the imaging process of the microscope and has led to a more precise structure determination of the crystals studied.     

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.     

High resolution and conventional electron microscopy studies of repeated wedge microtwins in monoclinic rare earth sesquioxides

High resolution and conventional studies have been made on the inclined twin boundaries occurring near the tip of wedge microtwins in the case of repeated wedge microtwins occurring at twin intersections. These inclined boundaries appear to be continuous and all the atomic planes are also continuous so that we suggest the use of the term ‘inclined coherent boundaries’ instead of ‘incoherent’ which one generally employs, in analogy to the case of precipitates in a matrix. This is different from what occurs in the case of isolated wedge microtwins for the same material and for the same kind of twins where the inclined twin boundary is really not coherent, being made of non-inclined coherent regions separated by steps containing a dislocation. The coherent inclined boundaries of repeated wedge microtwins appear to be rotated about two perpendicular axes of the habit plane (tilt and twist) giving rise to a distribution of microdislocations whose role has been studied. The values of the tilt and twist angles are related to a minimization of the total energy by a mechanism that is likely to appear only in the case of thin foils. A comparative study of the different roles related to the different structures of inclined twin boundaries for isolated and repeated wedge microtwins has been made, and also a comparative study of the mechanisms of stress relaxation in each case. The studies have been made on (313) twins of a monoclinic samarium sesquioxide.     

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).     

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.     

Optimized conditions for imaging the effects of bonding charge density in electron microscopy

We report on the observability of valence bonding effects in aberration-corrected high resolution electron microscopy (HREM) images along the [0 1 0] projection of the mineral Forsterite (Mg₂SiO₄). We have also performed exit wave restorations using simulated noisy images and have determined that both the intensities of individual images and the modulus of the restored complex exit wave are most sensitive to bonding effects at a level of 25% for moderately thick samples of 20-25 nm. This relatively large thickness is due to dynamical amplification of bonding contrast arising from partial de-channeling of 1s states. Simulations also suggest that bonding contrast is similarly high for an un-corrected conventional electron microscope, implying an experimental limitation of signal to noise ratio rather than spatial resolution.     

The application of image pickup and storage-display techniques to the high resolution electron microscope

The advantages of using image-intensifiers and other image pickup systems as an aid to the observation of the image in the electron microscope have been known for many years, and a review of a number of the systems employed so far is presented here. The additional advantages to be gained by the use of a sensitive TV pickup tube in conjunction with a storage tube are discussed, with particular reference to a proposed system for use with a high voltage, high resolution, electron microscope. It is shown that the prime limitation to the signal-to-noise ratio obtainable in the image pickup-display chain is due to the noise in the electron microscope signal itself, and that with the proposed pickup and storage tube arrangement a field of 800 by 600 picture points, with adequate signal-to-noise ratio, should be obtainable for a current density of 10^?9 A/m² in the image plane of the microscope if an integration period of about 3 s is used.     

The application of analytical electron microscopy to the study of diseased biological tissue

A routine method is described for X-ray microanalysis of thin specimens of biological tissue containing mineral particles in cancerous growths. Such a method allows information to be obtained that relates pathological history to histology, electron microscopy and X-ray microanalysis. Classification of minerals is possible in a way that is not provided by bulk analysis. The technique provides baselines of elemental data of minerals from various sources that may be used to classify particles found present in certain tumour growths.     

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.     

A simple computer program to obtain digital pseudo-colored high resolution electron microscopy images

Computer programs which allow one to simulate high resolution electron microscopy images generally deliver the output on a line printer with the overprinting technique. Although this method suffers from poor resolution and limited contrast dynamics, it is still in widespread use. A more effective way is to represent the images in digital form on a color video display, and several solutions have been proposed recently, which often, however, make necessary use of rather expensive pictorial video terminals. In this work we describe a simple program which allows one to represent the output of a multislice program (projected potentials, lattice images, and diffraction patterns) on the display of an inexpensive color video terminal. Useful operations on the image, such as color selection over a wide range, image magnification, and filtering (to enhance specific details of interest) have been implemented. The displayed image can be either photographed from the screen or recorded on a good quality hard copier.     

Visualization of bloch waves of high energy electrons in high resolution electron microscopy

By applying the Bloch wave theory of electron diffraction to the problem of lattice images in high resolution electron microscopy, it is shown that those characteristic type of images as found for Ge <110> can be interpreted as images of individual Bloch waves formed by the incident electron inside the lattice.     

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.