High-resolution electron microscopy and its applications

A review of research on high-resolution electron microscopy (HREM) carried out at the Institute of Physics, the Chinese Academy of Sciences, is presented. Apart from the direct observation of crystal and quasicrystal defects for some alloys, oxides, minerals, etc., and the structure determination for some minute crystals, an approximate image-contrast theory named pseudo-weak-phase object approximation (PWPOA), which shows the image contrast change with crystal thickness, is described. Within the framework of PWPOA, the image contrast of lithium ions in the crystal of R-Li₂Ti₃O₇ has been observed. The usefulness of diffraction analysis techniques such as the direct method and Patterson method in HREM is discussed. Image deconvolution and resolution enhancement for weak-phase objects by use of the direct method are illustrated. In addition, preliminary results of image restoration for thick crystals are given.     

High resolution imaging and interpretation of regular and irregular structures in n-niobium pentoxide crystal

High resolution structure images of crystal wedges of n-Nb₂O₅ were obtained with the Kyoto 500 kV electron microscope. The two-dimensional arrangement of contrasty dots in the image taken at a very thin area of the fragment was found to coincide with the positions of Nb ions in the a-c projection. The variations in the image feature due to the thickness increase and defocusing showed a fairly close correlation with those of a multislice image simulation. As an application of HREM observation to the study of irregular structure, the sporadic appearance of cross-type images was detected and was interpreted in terms of the existence of interstitial ions. The reliability of intuitive interpretation of images was also discussed on the basis of multislice simulations.     

Computer simulation of HREM images of network models of SiO2 glass

The possibilities and limitations of HREM for evaluating structural details in amorphous silica were investigated by computer simulation. The calculated defocus series of HREM images for an unstrained and a linearly strained network model of vitreous SiO₂ have shown that microvoids, density fluctuations as well as strained regions can be interpretably imaged for thin specimens (t < 5 nm).     

The microstructure of experimental SiC fibre-reinforced yttrium magnesium aluminosilicate (SiCf-YMAS) materials

Two experimental SiC fibre-reinforced yttrium magnesium aluminosilicate (SiCf-YMAS)-type ceramic-matrix composite (CMC) materials fabricated (i) by the glass process and (ii) by chemical precursor infiltration have been studied by light microscopy, transmission electron microscopy (TEM), high-resolution electron microscopy (HREM) and energy-dispersive X-ray spectroscopy (EDS). The distribution of the fibres inside the composite as well as the average diameter of fibres have been determined by image analysis. The microstructure of the YMAS matrices has been characterized by TEM observations. YMAS matrices are formed of two main phases, cordierite and β-yttrium silicate (Y₂Si₂O₇). Two minor phases (mullite and spinel) have been found to crystallize inside the cordierite and the yttrium silicate crystals. Fibre-matrix interfaces have been observed in HREM. A thin turbostratic carbon layer (20–30 nm) has been imaged in both composites at the fibre-matrix interface. It crystallizes along the matrix interface and grows inside the fibre, forming a diffuse interphase. The carbon layer is believed to be the consequence of reaction between oxygen in the matrix and SiC nanocrystals of the Nicalon fibres.     

The effect of mechanical vibration and drift on the reconstruction of exit waves from a through focus series of HREM images

Experimental HREM images can show a limited resolution as a result of mechanical vibration and drift. In this paper the effect of such mechanical vibrations on the accuracy of the through focus exit wave reconstruction method is investigated for different thicknesses of a test structure of La₃Ni₂B₂N₃. A through-focus series of HREM images for this structure is simulated for different kinds of mechanical vibration corresponding to an information limit g of about 7 nm⁻¹: (1) no mechanical vibration, (2) isotropic mechanical vibration, and (3) several anisotropic mechanical vibrations. From these through-focus series the reconstructed exit wave is calculated (Ultramicroscopy 64 (1996) 109). The above isotropic and anisotropic mechanical vibrations have a large effect on the reconstructed exit waves when compared with the reconstructed exit wave without mechanical vibration, i.e. the range of amplitudes and phases in a reconstructed exit wave decreases and the background intensity increases. The initial thickness and orientation can be obtained using a least-squares refinement procedure (Acta Crystallogr. A 54 (1998) 91) when there is no mechanical vibration present. In the case of isotropic or anisotropic vibration, the refined thickness and orientation are likely to give wrong results depending on the size of the vibrations and on the number of significant reflections (which is related to the size of the unit cell, the thickness and the misorientation).     

Spatially resolved electron energy-loss studies of metal–ceramic interfaces in transition metal/alumina cermets

Composites consisting of an alumina matrix and 20 vol.% transition metal (Ni or Fe) particles, prepared by hot pressing powder blends, have been studied using spatially resolved transmission electron energy-loss spectroscopy (EELS), and, to a lesser extent, by high-resolution electron microscopy (HREM). Particular attention was paid to the elucidation of the chemical bonding mechanisms at the metal-ceramic interface; EELS spectra from interfacial regions being obtained via a spatial difference technique. From both qualitative and quantitative interpretation of EELS near-edge structures, as well as observed HREM images, the data appear to be consistent with the presence of an Al-terminated alumina at the interface and the formation of direct transition metal – aluminium bonds in Al(O₃M) (M = Ni or Fe) tetrahedral units, possibly as a result of the dissolution and interfacial reprecipitation of Al during processing. These results correlate well with similar model studies on diffusion-bonded Nb/Al₂O₃ interfaces and may, in the light of recent theoretical electronic structure calculations, have implications for the resultant interfacial bond strength in such materials.     

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.     

Emergence of small defect contrast within HREM images of nonstoichiometric rutile

Calculations of HREM images have been used to define the conditions for which interpretable information may be obtained concerning the nature of small defects (traditionally called “point defects”) and their aggregation to form extended defects in nonstoichiometric rutile. At specific crystal thickness and lens defocus conditions as few as three small defects (aligned along the projection axis) become visible in lattice images. The emergence of stronger contrast, as extended defects develop, should allow the interstitial or vacancy nature of the defects to be distinguished. Experimental HREM images are also presented which show a variety of “small defect contrasts” for [001], [010] and [111] projections of nonstoichiometric rutiles (TiO_2-x; 0.006 ≦ x ≦ 0.02). None of these allow interstitial versus vacancy defects to be distinguished directly, although some show evidence for aggregates of less than 10 small defects.     

Precise measurements of nanostructure parameters

The precision of measurements performed by atomic-force microscopy (AFM) and high-resolution electron microscopy (HREM) for solving problems of metrology and diagnostics of solid nanostructures is discussed. The HREM-measured height of a monatomic step on a Si(111) surface covered by a thin natural oxide film is demonstrated to be 0.314 ± 0.001 nm. The same accuracy is ensured by AFM measurements through controlling the Si surface relief with heating in ultra-high vacuum on specially created test objects with the distance between the steps being approximately 2 μm. It is shown that the geometric phase method can be used to quantify the strains in the crystal lattice of strained heterostructures on the basis of HREM images with accuracy to 10⁻⁴%, and in situ irradiation by electrons in HREM measurements can be used to visualize ordered clusterization of vacancies and self-interstitial atoms in {113} planes in Si samples.     

High-resolution rlectron microscopic investigation of supported platinum particles reduced at high temperatures

High-resolution electron microscopy has been used to characterize the platinum particles supported on TiO₂ or ZnO. After reduction at elevated temperatures, the metallic particles display a regular, faceted shape, and several superstructures, Pt₃ Ti(C), Pt₃ Ti(H), PtTi, and PtZn, have been found. These results, which may involve strong metal-support interaction, have been confirmed by optical diffraction and image simulation.     

An electron microscopy study of the phase Al3Fe

The phase Al₃Fe (monoclinic C2/m, a = 1·549 nm, b = 0·808 nm, c = 1·248 nm, β = 107·8°) has been studied by transmission electron microscopy (TEM) and high resolution electron microscopy (HREM). Crystals were obtained from a direct chill-cast ingot of an Al-0·25 wt% Fe-0·13 wt% Si alloy. Extracted crystals were prepared by dissolving the aluminium phase in butanol and filtering off the particles. The extracted Al₃Fe crystals were mainly (100) platelets. The monclinic lattice was confirmed by tilt experiments and the mirror plane was confirmed by convergent beam electron diffraction. Experimental HREM images from the [100] and [110] projection agreed with images calculated by the multislice method. The interpretation of images in terms of a projected crystal structure is discussed. Common defects in Al₃Fe crystals were: twins on (100) and faults on (001). The (001) faults could be described by a displacement 1/2·[100] on a fault plane at z = 0·5 in the unit cell. A model for (001) faults, based on multiple twinning, is proposed.     

HREM structure characterization of interfaces in semiconducting multi-layers using molecular-dynamics-supported image interpretation

Multi-layer structures in binary systems, for example InAs/AlSb and AlSb/GaSb, as well as ternary InGaAs/AlGaAs quantum wells grown by molecular beam epitaxy (MBE) are investigated by high-resolution electron miscroscopy (HREM). Interpretation of the micrographs requires methods of image analysis and the computer simulation of the HREM contrast. The relaxed atomic structure of the interfaces is modelled by molecular dynamics (MD) and energy minimization. The importance of strain relaxations to the image interpretation will be demonstrated and compared with the image spread and shift caused by the microscope aberrations. The possibility of revealing the compositional variations and the elastic deformations at the interfaces by HREM imaging under special defocus and sample thickness conditions will be discussed.     

TEM and HREM study of Al3Zr precipitates in an Al-Mg-Si-Zr alloy

The structure of Al₃Zr precipitates in Al‐1.0Mg‐0.6Si‐0.5Zr (in wt.%) alloy was investigated using conventional transmission electron microscopy (TEM) and high‐resolution TEM (HREM). After annealing of the alloy in the temperature range 450–540 °C, spherical precipitates of metastable L1₂‐Al₃Zr phase appeared nearly homogeneously within the matrix, and elongated particles were found at grain boundaries. L1₂‐structured Al₃Zr were about 20–30 nm in diameter and coherent with the matrix. Inside some of them, planar faults parallel to {100} planes were revealed by use of HREM. Most probably, these faults are an indication of the transition stage of transformation to the stable D0₂₃‐type Al₃Zr phase. The elongated precipitates (about 100 nm) were identified as D0₂₂‐type Al₃Zr. Energy‐dispersive X‐ray analysis showed that they contain, apart from Al, mainly Zr with small amounts of Si. The substitution of Al by Si increased the stability of the D0₂₂‐Al₃Zr as compared with D0₂₃‐Al₃Zr.     

Estimation of the state of rod structural elements based on recording of deformation responses to serial impact excitations

Based on the example of welded and one-piece-cut steel (Steel 10) frame triangular elements (4 × 8 × 245 mm) using the methods of thermal unloading up to the temperature T _max = 625°C, serial impact testing of parent (stressed) and unloaded elements, and comparative analysis of Fourier images of deformation responses with respect to unloaded elements, it was found that a nonlinear region of the Fourier image degenerates into a linear one and the maximum of the linear regions of the Fourier image shifts up to 2 Hz. The dynamic deformation responses of the elements were recorded using superbroadband (from 10⁻² to 10⁸ Hz) ferroelectric microsensors based on thin Pb(Zr_0.53Ti_0.47)O₃ films with a thickness of ∼2.0 μm with a 0.8 mm² contact pad. The results can provide a the signal for distinguishing stressed states of structures and their elements.     

Structure projection retrieval by image processing of HREM images taken under non-optimum defocus conditions

A direct method for retrieval of the projected potential from a single HREM image of a thin sample is presented. Both out-of-focus and astigmatic images can be restored. The defocus and astigmatism values are first determined from the Fourier transform of the digitised HREM image. Then a filter is applied which reverts the phases of those Fourier components which have been reversed by the Contrast Transfer Function (CTF). The method has been incorporated into the CRISP image processing system. It can be applied on any sample, crystalline or amorphous. From thin crystalline areas the projected symmetry can be determined and a further improvement achieved by imposing the symmetry exactly. This compensates for the effects of crystal tilt. Five HREM images of a thin crystal of K(8-x)Nb(16-x)W(12+x)O80 (x = 1), taken with different defocus and astigmatism values, were processed. Only one, taken near Scherzer defocus, was directly interpretable before image processing. After processing, all images showed the projected potential of the structure. Using data to 2.77 angstroms resolution, all heavy (Nb/W) atom positions were found in every image, within on average 0.15 angstroms of the positions determined by single crystal X-ray diffraction. In the HREM images taken under non-optimum defocus conditions, also the potassium atoms in the tunnels of the structure were found.     

A study on CFD data compression using hybrid supercompact wavelets

A hybrid method with supercompact multiwavelets is suggested as an efficient and practical method to compress CFD dataset. Supercompact multiwavelets provide various advantages such as compact support and orthogonality in CFD data compression. The compactness is a crucial condition for approximated representation of CFD data to avoid unnecessary interaction between remotely spaced data across various singularities such as shock and vortices. But the supercompact multiwavelet method has to fit the CFD grid size to a product of integer and power of two,m _X ². To resolve this problem, the hybrid method with combination of 3, 2 and 1 dimensional version of wavelets is studied. With the hybrid method, any arbitrary size can be handled without any shrinkage or expansion of the original problem. The presented method allows high data compression ratio for fluid simulation data. Several numerical tests substantiate large data compression ratios for flow field simulation successfully.     

A mirror electron microscope for surface analysis

The principle of mirror microscopy has been adapted to provide a relatively low resolution surface microscope (<1000 ×), a large transfer width low energy electron diffractometer and a photoelectron analyser in k_|| space. A focused electron beam of ? 10 kV is decelerated through a Johansson lens, reflected in front of the sample and reaccelerated back through the lens to produce an electron image over a field of view of a few microns. The image can be interpreted as a micrograph of work function variations on the surface if other effects (geometry, magnetic field) are uniform. In the LEED mode, diffracted beams virtually retain their positions on the screen over the whole impact energy range used (0.160 V). Secondary electrons are preferentially focused around the lens-gun electro-optic axis, thus effectively filtering them out from the diffraction pattern. The design has an inherently large coherence length, of up to 10⁴ Å. Photoelectrons can similarly be imaged in k_|| space on the detector plane. The addition of energy filtering at the screen allows the two-dimensional Fermi surface to be imaged.     

Molecular dynamics simulation of polyethylene under cyclic loading: Effect of loading condition and chain length

Two nano-blocks of polyethylene (PE) are made and subjected to cyclic deformation with various loading conditions, i.e., strain vs. stress control, zero lateral strain vs. zero lateral stress, and different load amplitude by using the coarse grained molecular dynamics simulation. The one block is filled with 1000 random coil chains of [-CH₂-]³⁰⁰ (long chain), while the other 10 000 chains of [-CH₂-]³⁰ (short chain). The random coil chains are freely grown and relaxed in a simulation box of , then compressed to and relaxed to obtain a stress-free equilibrium. Under the zero lateral stress condition, σ^′=0, the long-chain block shows a leaf-like hysteresis curve both in the stress- and strain-controlled cyclic loading. The area of the hysteresis loop increases as the maximum load is changed to , and , respectively. The “Mullin's effect” is also observed, i.e., the stress-strain curve depicts lower path in the 2nd or later loading, although the target is never a rubber with filler. Under the zero lateral strain condition, ε^′=0, the long-chain block shows little hysteresis with the stress amplitude of 100 MPa, while it shows rapid or unstable elongation around at ε=0.35 in the simulation of ε_max=0.5 and . The short-chain block also shows unstable elongation under the ε^′=0 condition even with the stress amplitude of 100 MPa, noting that it has an upper yield point of at ε=0.35 and lower one around at ε=1.6. On the other hand, the short-chain block is stretched without remarkable stress increase up to the strain around 1.0, under the lateral condition of σ^′=0. Then the block shows “strain hardening” and comes up to the external stress of 100 MPa. It is worth noting that the block shows a leaf-like hysteresis in the 2nd or later cycle; the stress goes back to zero around ε=1.0 in the unloading process and rises up immediately when the load is reversed, as same as the long-chain block.     

The contribution of phonon scattering to high-resolution images measured by off-axis electron holography

The contribution of electrons that have been phonon scattered to the lattice fringe amplitude and the background intensity of a high-resolution electron microscope (HREM) image is addressed experimentally through the analysis of a defocus series of energy-filtered off-axis electron holograms. It is shown that at a typical specimen thickness used for HREM imaging approximately 15% of the electrons that contribute to an energy-filtered image have been phonon scattered. At this specimen thickness, the phonon-scattered electrons contribute a lattice image of opposite contrast to the elastic lattice image. The overall lattice fringe contrast is then reduced to 70% of the value that it would have in the absence of phonon scattering. At higher specimen thickness, the behaviour is defocus-dependent, with the phonon image having lattice fringe contrast of either the same or the opposite sense to the elastic image as the defocus is varied.     

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.