Search

In a modified form of electron holography, as originally proposed by Gabor, a specimen illuminated by the focused, convergent beam of a scanning transmission electron microscope is followed by a thin crystal which acts as a periodic array of atomic focusers. Each of the broad diffraction spots of the crystal then contains a magnified image of the specimen with a resolution limit of 0.05 nm or less. The method is illustrated by images of crystal lattice planes and tungsten atoms in the diffraction patterns formed by crystals in the walls of carbon nanoshells.

Scanning transmission electron microscopy (STEM) instruments have some particular advantages as compared with the more common transmission electron microscopes for some applications to surface research. Imaging of surfaces and mapping of the elemental distributions on surfaces with spatial resolutions approaching 1 nm are possible in an ultrahigh-vacuum STEM instrument when the low-energy secondary electrons or the Auger-emitted electrons are collected with high efficiency. In the imaging of surface layers on thin-film substrates, viewed in transmission, the use of a thin annular detector in STEM may greatly enhance the contrast, as illustrated by the case of the imaging of very thin nanocrystalline carbon layers on much thicker amorphous SiO2 films. The scanning reflection mode in a STEM instrument can provide some useful forms of contrast in images of surface structure. Standing wave fields can be formed on the surfaces of crystals with electrons, as with X-rays, one advantage of the electron case being that the standing wave fields may be imaged. Two new forms of electron holography, involving a STEM instrument and suitable for the study of surface structure, are proposed.

We considered the analysis of bioassay data resulting from the treatment of fruit that has been naturally infested, when the treatment sample size is unknown. Natural infestation may cause considerable variability between fruit. We described a procedure to quantify this variability and account for it in the bioassay analysis. We also discussed the optimal numbers of control and treatment fruit.

In developing its quarantine policy for the importation of fruit-fly host material, New Zealand has used the concept of a maximum pest limit. An equation which links the maximum permissible numbers of fruit flies with lot size (N), mean number of mature fruit fly larvae per infested fruit (μ), proportion of infested fruit (p), and treatment efficacy (μ) has been suggested. We discuss the research which is needed to obtain accurate measurements of the parameters in the model. Decisions based on these parameters support the development of a quarantine policy that can be justified scientifically. Methods for collection of biological data to determine μ and p are detailed. It is suggested that efficacy trials to measure φ should be based on dose/mortality assessments. Experimental techniques used in efficacy trials should focus on assessment of variability and determination of confidence limits. Application of the maximum pest limit equation is discussed.

The strain fields in the vicinity of 90° domain boundaries in BaTiO3 ceramics and single crystals have been revealed by high resolution images and diffractograms. It is suggested that such strain fields may be relaxed in the thin ferroelectric films. The observation of 180° domain boundaries in bright field imaging is possible provided that the contrast due to the failure of Friedel's law is large enough. Reflection electron microscopy (REM) has been developed as a novel technique to study the domain boundaries in ferroelectrics. The contrasts of various ferroelectric domain boundaries are discussed. It is found that the contrast of 90° domain boundaries can arise from the differences of deviations from the Bragg condition, including those due to the lattice distortion around the domain boundaries. The contrast can be enhanced by surface resonance conditions.

We developed a probability model to quantify the quarantine risk from importing fruit fly hosts. Bayesian theory is used to show how sample size at point of entry is related to the number of test insects used to determine efficacy of the postharvest disinfestation treatment. The model assumes that other variables such as lot size, average number of insects in an infested fruit, and maximum pest limit (or tolerance) are fixed and known. Examples are given using data for the fruit fly species Bactrocera melanotus (Coquillet) and Bactrocera xanthodes (Broun).

Dry heat is proposed as a non-chemical postharvest disinfestation treatment for export persimmons (Diospyros kaki L.). A fruit surface temperature of 47°C at 55–60% RH for 15 min is recommended to effect 100% mortality of thrips, mealybugs, and leafrollers. On persimmon fruit these are all surface dwelling pests. Fruit tolerance trials showed that this fruit surface temperature/time exposure was not detrimental to persimmon fruit firmness, soluble solids concentration, and appearance; and did not reduce shelf life. A commercial scale confirmatory trial is planned.

The problems of satisfactorily describing fruit fly host status for quarantine purposes are explored. A definition is provided based on the ability of a commodity to sustain fruit flies from the egg laying stage through to the next generation of reproducing adults. Experimental procedures required to determine host status for multivoltine fruit flies are described. These methods include both laboratory and field cage trials, field collection of fruit, and methods demonstrating fruit fly presence in a study area.

A methyl bromide quarantine fumigation schedule of 32 g /m3 for 4 h at 21-26°C and under normal atmospheric pressure is reported for watermelons grown in Tonga and shipped to New Zealand. This fumigation schedule is effective against Bactrocera xanthodes (Broun), which was recorded from watermelons for the first time in 1985. A work plan permitting resumption of watermelon trade from Tonga to New Zealand specifies production, culling, inspection, treatment, holding, and transport conditions which must be fulfilled to ensure that defined maximum pest limits are not exceeded.

The effectiveness of 2 traps for Queensland fruit fly, Bactrocera tryoni was compared for 1 year amongst relatively low populations at Applethorpe-Stanthorpe in southeast Queensland. The Lynfield trap which is an inexpensive, non-sticky, pot-type trap was significantly more effective than the Jackson trap and was at least as reliable in the detection of fruit fly populations. Use of Lynfield traps in quarantine surveillance programs should allow for a greater trap density and a more sensitive detection system to be established without vastly increased operational costs.

New Zealand is the only major fruit-producing nation that has remained free of economically important fruit fly species. Maintenance of this status is essential not only to guarantee horticultural exports but also to minimize production costs associated with field control and disinfestation treatments against fruit flies after harvest. To guarantee continued freedom from fruit flies, the concept of a maximum pest limit is developed as an alternative to a very restrictive import policy. The maximum pest limit is defined as the maximum number of immature fruit flies that may be present in consignments imported during a specified time to a specified location. To preclude the establishment of fruit flies, we propose a maximum pest limit of three live larvae per day. Applying a treatment of known efficacy ensures that this limit is not exceeded only if the infestation level is below a predetermined value. We describe a sampling model for accurate assessment of the infestation level and examine the practical implications of setting the maximum pest limit.

By using a nanoprobe electron beam 4 A in diameter, the [001] nanodiffraction patterns of AlGaAs‐GaAs multiple‐quantum‐well structures have been formed and the epitaxial relationship between AlGaAs and GaAs is confirmed. The intensities of the (200) diffraction disk, monitored by a spot detector, are displayed in two ways: (1) the (200) dark‐field scanning transmission electron microscopy (STEM), which shows the layers of AlGaAs and GaAs in contrast, and (2) the (200) line‐scan profile, which reveals the (200) intensity distribution of a specimen region of uniform thickness. The thickness and the absolute Al concentration of AlGaAs layers are, respectively, determined from the contrast of, and the (200) thickness contour position in, the (200) dark‐field STEM images. The microanalysis on the (200) line‐scan profile is used to find the local Al concentrations in AlGaAs layers and to study the interface boundary between the layers of AlGaAs and GaAs. Diffusion of the Al atoms from the AlGaAs layer into the ...

A new system of fruit fly surveillance trapping has been implemented in New Zealand. The Jackson lure trap has been replaced with the Lynfield lure trap and Nakagawa bait trap.

Conditions for observation of GS bands for the 137 space groups exhibiting these extinctions

The thickness dependence of ferroelectric domains in thin free‐standing BaTiO3 crystalline films has been studied by transmission electron microscopy. It is found that the widths of ferroelectric domains decrease as the film thickness decreases. This phenomenon may indicate that the ferroelectric properties of thin films are weakened due to surface relaxation effects, including lattice relaxation and a change of spontaneous polarization and charge compensation. The weakening of ferroelectric domains is suggested as a transition state from ferroelectric to paraelectric phase of the BaTiO3 thin film. The thickness of the surface relaxation layer of totally nonferroelectric film is on the order of 10 nm.

The resolution that may be achieved in an image formed by reconstruction from an in-line electron hologram, without aberration correction, is governed by the source size. Computer simulations show that ultrahigh resolution, of better than 0.05 nm for 100 keV electrons, may be possible if the electron source is a crossover formed at the exit face of a thin atomic-focuser crystal when electrons from a beam focused in a scanning transmission electron microscope (STEM) instrument are channeled along a line of atoms through the crystal. Simulations also show that, because of the channeling effect, the resolution of the reconstructed image is not degraded by translations or oscillations of the STEM beam by 0.1 nm or more. We suggest a scheme for removing the unwanted background and conjugate image from the reconstructed image by combining the in-line holography with an off-axis mode.

The resolution in scanning transmission electron microscopy may be enhanced by taking advantage of the information contained in the nanodiffraction patterns recorded for each position of the scanning incident beam. We have demonstrated the first production of ultrahigh resolution, better than 0.1 nm, by this method, in the imaging of an essentially one-dimensional object, the wall of a multiwalled carbon nanotube, using an instrument for which the resolution for normal imaging is about 0.3 nm.

Electron nanodiffraction, particularly as performed in a dedicated STEM instrument with coherent illumination, provides, in principle, a means for obtaining information on structural detail in the range between that of STEM image resolution, about 0.2 nm, and the limits of information possible from elastic scattering from atoms, about 0. 03 nm. In practice, most nanodiffraction work has been concerned with finding the crystal structure, crystal defects, and sometimes crystal shapes for nanoparticles in the size range of 1-2 nm or for regions of thin crystalline films of about these dimensions. Available equipment allows for the recording of nanodiffraction patterns at the rate of 30 per second, or faster, providing means for detailed study of extended areas or of dynamical processes.

Shadow images, magnified by projection from a small electron source, were amongst the first images to be obtained by electron microscopes.24 In the same year (1939) pointprojection shadow images of carbon grids were also reported at a few hundred volts using a field-emission electron tip.320 The authors comment on the absence of aberrations in their lensless system, the removal of which has proved a major obstacle for conventional electron holography at higher voltages. Fig. 14.1 (a) shows the optical arrangement for point-projection microscopy (PPM). A tungsten field emitter S is placed at a distance z1 (a few hundred nanometers) from a thin sample T. and an image recorded on a channel plate D about z2 = 10 cm away. The magnification is approximately M = z2/z2 ≈ 106 and the resolution in the image is about equal to the source size. Fig. 14.1(b) emphasises that the resulting image is equivalent to a TEM image with defocus z1 (under certain conditions). The conducting sample, grounded, acts as the anode. Since field emitters of atomic dimensions can be obtained,107 atomic resolution should be possible. However, at the very low voltages (about 100 V) needed to sustain field emission with z1 = 100 nm, electron penetration in carbon is only about 0.8 nm, and most samples are completely opaque. Then the main features seen in the images are Fresnel fringes around edges, and occasionally interference fringes between waves passing through different pinholes. This arrangement constitutes exactly Gabor’s original proposal for electron holography, and also corresponds to the in-line STEM Fresnel holography case described in Chapter 2, Section 3 (with sub-nanometer aberration coefficients appropriate to the PPM virtual field-emission source). For STEM, the probe-forming lens shown in Fig. 2.6(a) is used. A low voltage PPM instrument was built at NIST in 1968302 and several more have been built more recently23,108,318,418 using STM-based construction methods.

The sign of the exponential in the Fourier transform is something that we have been concerned with for many years. Of course, there are two conventions that have been used with almost equal frequency but we have been trying to get people to stick to one of them to avoid confusion. In our books and papers (Refs. 65,207, etc.) we have used the convention of the positive sign in the exponential for the forward transform which represents the Fraunhofer diffraction pattern for a real-space object. This is consistent with assuming that a plane wave, going in positive direction in real space is written exp \([i\,(\omega t - \overrightarrow k \overrightarrow r )]\) rather than a minus sign before the i, so that the phase advances with time. This is also consistent with making the Scherzer defocus negative, and also implies that the phase shift in a solid with a positive potential (accelerating the electrons) is negative so that the transmission function is exp [−iσAE(x,y)]. If the other convention is to be used for the Fourier transform exponent sign, then all authors should be advised of all these other implications, which are not immediately obvious. Otherwise, we might find ourselves producing a treatment of positron holography! There is a summary of our conventions and definitions in the appendixes of Refs. 65 and 207.

Electron holography was conceived as a means for overcoming the limitation of the resolution of electron microscopes due to the unavoidable presence of aberrations of electromagnetic lenses.129,130 The essential means for doing this was the formation of an interference pattern between the electron wave scattered by the specimen and a reference wave of known form. The practical realization of this aim had to await the technological developments of smaller, brighter electron sources to give greatly improved coherence of the illumination and more effective, quantitative recording media. With the advent of these aids, rapid progress is now being made toward realization of the potential of electron holography for providing important advances in the power and range of applications of electron microscopy.

Most mineralogical applications of transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) have involved the imaging of extended defects, such as dislocations, stacking faults, Wadsley defects, and antiphase boundaries. Modern electron microscopes, however, are capable of imaging individual, isolated atoms (e.g., Isaacson et al. 1979), raising the question of whether individual or clustered point defects can be resolved within mineral crystals.

In recent years the surfaces of Cu3Au, a typical ordering alloy, have attracted interest from many aulhors[l-6], because the surface is a good example for testing general theories of the 2-D order-disorder phase transition, and also because it is a good material for studying the behavior of bimetallic alloy surfaces and their reactions with surface impurities. Previous work on Cu3Au surface mostly used macroscopic techniques and provided only information averaged over a largearea. In this work the microscopic imaging and diffraction methods are used to study the (111) surface on a more localized scale. Reflection Electron Microscopy (REM) was used to directly image the surface, and the selected-area RHEED was used to determine the local surface crystal structure and orientation.The samples used for observation are single crystal spheres prepared by melting a small piece of Cu3Au suspended on a thin Mo wire. The actual compositions of the samples are about Cu0.7Au0.3. The (111) facets on these spheres were then cleaned by the conventional ion-milling-heating method in a UHV environment.

The long-period structure (LPS) has long been known to exist in bulk ordering alloys such as CuAu and Cu3Au. in the structure the ordered lattice shifts back and forth by a vector every M unit cells and forms a super cell 2M unit cells long. The planes where the lattice shifts are usually called antiphase boundaries (APBs) and the LPS can be regarded as periodically arranged antiphase domains. This superstructure can be detected by the splitting (∝1/M) of some superlattice spots in the diffraction panern. One may ask whether the LPS also occurs on the surface; if so, what is its effect onthe diffraction pattern. The answer to them is important to explain some unknown features observed in the RHEED experiment. They also have significance for the surface transition theories.Ideal termination of a LPS Cu3Au crystal gives a surface first-layer atomic arrangement shown in fig.1a. In this arrangement the basic vectors a and b of the surface hexagonal ordered structure are two bulk {110} vectors. Every M unit cells in a direction the lattice shifts by(1/2)b. This forms the 2-D super unit cell with a size of 2Ma×b. The relatively shifted areas are actually 2-D antiphase domains. The boundaries between them are the periodically arranged linear APBs. All these features indicate that the ideal Cu3Au(111) surface is indeed a 2-D LPS. Fig.1b and 1c show the 2-D reciprocal lattice calculated from the above surface arrangement.

Transmission electron microscopy (TEM) has been used to study the microstructures, especially the ferroelectric domains and boundaries, of ferroelectric crystals and free-standing thin films because they are closely related to the ferroelectric properties of thin films. But, for a long time observations have been made on the assumption that the properties of ferroelectric domains and domain boundaries are the same as those of the bulk crystal, even though sometimes some evidence may have been shown that ferroelectric domains in thin films could be different from those in the bulk material. In fact, weak ferroelectricity of thin films has been noticed. It was found that the ferroelectric characteristics of thin BaTiO3 ferroelectric films deposited on substrates are different from those of bulk materials. Although in their analysis interfaces and grain boundaries are involved, the mechanism of weak ferroelectricity of thin films and the microstructures which are related to the ferroelectricity of the film remains unclear.

The intersections of ferroelectric domain boundaries with crystal surfaces have been studied by optical microscopy. The method is widely used but usually of low resolution. Transmission electron microscopy (TEM) can provide high-resolution images but may not be appropriate for studying crystal surfaces. Scanning electron microscopy (SEM) has also been used to study the intersections of ferroelectric domain boundaries with the surfaces of ferroelectric crystals. However, the resolution is still low and is destructive if an etched crystal surface is used. Other alternatives have also been attempted to study ferroelectric domain boundaries on surfaces, such as scanning tunneling microscopy (STM), atomic force microscopy (AFM). But, no reports have been known so far.On the other hand, reflection electron microscopy (REM), as a branch of dark-field imaging technique dedicated for surface studies in TEM, has been developed to study crystal surfaces, surface reconstruction and surface defects with a resolution of about 10Å. It has been considered as a powerful technique to study surface defects and may be used to study the ferroelectric domain boundaries emerging on surfaces.

The behavior of impurities on the surfaces is an important topic in surface science. Cu3Au is a typical ordering bimetalic alloy whose bulk and clean surface have been extensively studied.This make it a good material for studying the effect of impurities such as sulfur on the properties of the alloy surfaces. Some work has been done by the present authors on the structure and compositionchange of the Cu3Au (110) surface with the sulfur adsorption . It was found that when S isadsorbed on the surface the ratio of the surface Au concentration over Cu concentration decreases drastically. The reason of this change was explained as movement of Au atoms into the bulk and Cu atoms from the bulk to the surface due to the difference in the affinity of Au and Cu for S.The previous observations were based on macroscopic RHEED and AES techniques, the information was averaged over a large area. In order to obtain more localized information such as surface morphology andthe element distribution, secondary electron microscopy (SEM) and scanning Auger microscopy (SAM) were used to study the clean and S-adsorbed Cu3Au (110) surfaces. The experiment was carried out in a modified UHV VG HB501.

In reflection electron microscopy (REM), the surface reflection electrons undergo both elastic and inelastic scattering within a crystal. The dominant inelastic processes are phonon scattering, valence electron excitation, bulk and surface plasmon excitation and combinations of these processes. Multiple inelastic scattering processes are also probable as the mean traveling distance of surface reflection electrons is about 10 to 100 nm. In reflection high energy electron diffraction pattern (RHEED), 50% to 90% of the electrons contributing to surface reflection spots used for imaging have suffered energy loss of more than 10 eV, thus the main limitation on REM image resolution is due to the chromatic aberration effects given by the energy spread from inelastic scattering. An energy filter fitted inside a TEM microscope can remove most of the inelastic scattering contribution and so improve the contrast and resolution. Oxygen-annealed rutile (001), (100) and (110) surfaces were previously studied by REM and RHEED techniques without energy filtering.

Reflection electron microscopy (REM) has been used to study surface defects such as surface steps, dislocations emerging on crystal surfaces, and surface reconstructions. However, only a few REM studies have been reported about the planar defects emerging on surfaces. The interaction of planar defects with surfaces may be of considerable practical importance but so far there seems to be only one relatively simple theoretical treatment of the REM contrast and very little experimental evidence to support its predications. Recently, intersections of both 90° and 180° ferroelectric domain boundaries with BaTiO3 crystal surfaces have been investigated by Tsai and Cowley with REM.The REM observations of several planar defects, such as stacking faults and domain boundaries have been continued by the present authors. All REM observations are performed on a JEM-2000FX transmission electron microscope. The sample preparations may be seen somewhere else. In REM, the incident electron beam strikes the surface of a crystal with a small glancing angle.

Calcia stabilized zirconia(CSZ) and ceria(CeO2) doped with rare-earth oxides have been used as electrolytes in solid oxide fuel cells(SOFC). Such ceramics are usually good ionic conductors for oxygen ions at elevated temperatures. A few studies have attempted to correlate the microstructures with ionic conductivity. It was suggested that the grain boundaries play a role in depressing the ionic conductivity. In their study, it was found that continuous “thick” boundary layers of secondary phases existed on the grain boundaries and blocked ion transport.In the present studies, high resolution transmission electron microscopy has been used to investigate the microstructures of grain boundaries in zirconia doped with calcia (ZrO2 + 13%CaO) and ceria ceramics doped with calcia (CeO2 + 10% CaO). The ceramics was made in our Solid State Ionics Laboratory. Transport number measurements on zirconia samples showed that the transport number tO2- =1 from PO2 = 10-17 atm to PO2 =1 atm.

Shadow images are obtained on the detector plane when the specimen in a STEM instrument is illuminated by a stationary beam formed with a very large or no objective aperture. It has been recognized for some time that the shadow image formed after a thin crystal specimen in the diffraction plane of a STEM instrument with a field emission gun can be treated as an in-line hologram. Since the transmitted and diffracted beams are added to give interference effects, all the information on the relative phase of the scattered beams is recorded in the shadow image. In fact, this is basically equivalent to the original scheme envisioned by Gabor, where he proposed that a reconstruction could be made from a hologram so that the imperfection due to spherical aberration of the objective lens could be corrected and improved resolution could be achieved.

Single crystal TiO2 (rutile) (110) surface has been characterized by several experimental techniques. In this paper, we report the investigations of “optically polished” as well as high temperature oxygen annealed rutile (110) surfaces by using reflection electron microscopy (REM) and reflection high energy electron diffraction (RHEED) techniques.The crystal was purchased, “optically polished” as-received, from Commercial Crystal Laboratories, Inc.. The details in specimen cutting and surface cleaning procedures have been reported previously. The samples were annealed in pure oxygen at 1425°C for 36 h. The experimental observations were carried out in a Philips 400T microscope operated at 120 kV. The REM images were obtained by selecting the specular reflection spots satisfying surface resonance conditions.Figure 1 is a REM image of as-received rutile (110) surface. The corresponding RHEED pattern is shown in the inset. The azimuthal angle of the incident beam was at 3.9° away from [001] zone axis and the image was formed by choosing (440) specular reflection spot under surface resonance condition.

Lead zirconium titanates(PZT) are ferroelectric materials of considerable current interest because their potential for applications in memory devices and electrooptics has drawn the attention of industry. The performance of the devices made of PZT largely depends on the behaviors of ferroelectric domains and domain boundaries. TEM has been proved to be a very powerful tool in the study of ferroelectric domains and domain boundaries in ferroelectric materials. It is suggested that around the domain boundaries in BaTiO3 there is a distribution of lattice distortion and strain field. However, few references have been found on the study of ferroelectric domains and domain boundaries in PZT materials by TEM.

Reflection high energy electron diffraction (RHEED) has been used extensively to observe changes in surface reconstructions by analyzing the geometry of the RHEED pattern and to monitor growth of layers in MBE systems by measuring the changes of the intensity of the specular spot with time. RHEED is also capable of yielding the structure of the surface by using dynamical diffraction theory to analyze experimental reflection rocking curves. These rocking curves trace the change in the intensity of the RHEED spots as a function of the angle of incident illumination. They are equivalent to the intensity vs. voltage curves (I-V) obtained in low energy electron diffraction (LEED) which have been used to determine most of the known surface structures. The LEED I-V curves are energy filtered and the theoretical calculations consider only the elastically-scattered electrons. The RHEED theory is also developed only for elastic scattering, but the experimental measurements so far have not been energy filtered.

Reflection high energy electron diffraction (RHEED) has been widely used to study surface reconstructions, surface morphology and crystal growth. In a transmission electron microscope RHEED can be combined with reflection electron microscopy (REM) to study the structures of surfaces with high spatial resolution. In this paper we report some preliminary results of RHEED investigation of the reconstruction of the oxygen annealed sapphire surfaces.The optically polished sapphire samples were obtained from Saphikon Inc. The specimens were cut, cleaned and chemically etched in boiling phosphoric acid before annealing in pure oxygen. Details of the sample preparation procedure was reported elsewhere. Oxygen annealing prevented change of surface stoichiometry and surface reactions with elements other than oxygen. The RHEED experiments were conducted in a Philips 400T transmission electron microscope operated at 120 kV with a vacuum pressure about 10-7 Torr and equipped with cold and hot stages.

Rutile (single crystal TiO2) is widely used in electrochemistry, photochemical energy conversion and photocatalytic reactions of gases as a catalytic material. It is important to characterize the surface properties of rutile in order to understand its catalytic behavior. The rutile (001) surface is extremely unstable, forming facets on annealing as revealed by the LEED results. In this paper we report some preliminary results on the investigation of oxygen annealed rutile (001) surface, obtained by reflection high energy electron diffraction (RHEED) and reflection electron microscopy (REM) techniques.The crystal was cut into strips, finely polished, cleaned and chemically etched in NaOH and H2SO4 before annealing. The samples were annealed in pure oxygen at 1473 K for 36 h. The purposes of annealing the samples in pure oxygen are to preserve surface stoichiometry and to prevent surface reactions with elements other than oxygen during the annealing process. The RHEED and REM observations were performed in a Philips 400T microscope operated at 120 kV.

Scanning transmission electron microscopy (STEM) with a high angle annular detector has become a useful technique in material science. The atomic number sensitive contrast (Z-contrast) of the Annular Dark-Field (ADF) image is good for looking at the distribution of heavy elements in a relatively light substrate. In many cases the impurity distributions are substantially affected by the defects in the materials and their interaction with the impurities. Therefore it is also desirable to observe defects with ADF images. This is possible and has some advantages over normal STEM. We have studied the ADF imaging of dislocations, its contrast mechanism and visibility in the ADF image.

The (100) surface of magnesium oxide is of considerable interest because of its frequent use as substrate for epitaxial growth of metal films, high Tc superconductors and model catalyst systems. A large number of surface characterization techniques have been used to determine the atomic structure of the cleaved (100) surface. Clean surfaces have been produced either by cleaving MgO crystals in-situ under ultra-high vacuum (UHV) conditions, or more frequently, by cleaving in air and subsequent annealing in UHV. A wide variety of annealing temperatures and times have been used by different researchers, the upper limit on the temperature being set at ≈900°C to avoid segregation of Ca to the surface. Calcium is the main impurity in even the purest MgO crystals and a few studies have dealt with the structure of the Ca-rich (100) surface of MgO. All of the existing studies have used diffraction and spectroscopy techniques without imaging of the surface. It is the purpose of this work to study the topography of the UHV-annealed (100) surface by reflection electron microscopy (REM).

High resolution secondary electron (SE) images can be obtained in a scanning transmission electron microscope (STEM) by collecting the type I secondary electrons generated directly by the incident electron beam. The high resolution SE signal is, in general, localized to about 1 nm in the generation process. The escape depth of the secondary electrons, however, varies from about 1 nm for most metals to about 10 nm for some insulating materials. Surface steps can be imaged by collecting high resolution secondary electrons Among other possible contrast mechanisms for SE imaging of surface steps, topographic contrast plays an important role. The topographic contrast of surface steps depends on: (1) the incident electron probe size; (2) step height to SE escape depth ratio; (3) localization of the generated SE signals and (4) the incident beam angle relative to the specimen surface.In our UHV STEM (MIDAS) instrument, both the entrance and exit surfaces of the sample can be imaged by collecting secondary electrons. For thin samples, the resolution is < lnm for SE images of entrance and exit surfaces but for thicker samples the resolution of the SE image obtained from the exit surface will be degraded by the beam broadening effects. For studying steps on thick or bulk crystal surfaces, secondary electrons emitted from the entrance surfaces are, in general, collected to form images with high contrast and high resolution. In this report, all the SE images are obtained, in MIDAS, by collecting secondary electrons emitted from the entrance surfaces of the samples. Figure 1 is a SE image of a MgO smoke crystal revealing the surface growth steps with high contrast. The parallel straight steps, along directions, constitute the edges of unfinished atomic planes on the MgO {100} crystal surfaces. This image indicates that the {100} surfaces of MgO smoke crystals are flat surfaces. Figure 2 shows a SE image of a MoO3 crystal with surface growth steps spiraling from an end-on screw dislocation (the black dots in the SE image represent electron beam induced reduction products). The dark lines (indicated by D) represent up-steps and the bright lines (indicated by B) represent down-steps. It is interesting to note that the two-dimensional crystal growth around the dislocation is anisotropic, forming terraces with long edges parallel to [010] direction.

The structures and properties of the prismatic faces and of as-grown single-crystal sapphire are still not well understood. Known information have been limited to calculated surface energies and TEM characterization . The TEM characterization has been limited to identification of the orientation of the structures. For better understanding of the behavior of these surfaces, the use of the RHEED and REM techniques, which have proved their value for purely surface analysis, is necessary. In this paper we report the characterization of these surfaces by the RHEED and REM techniques.The sapphire ribbons were purchased, as-grown, from Saphikon, Inc.. The method of preparation of the REM samples has been reported. The samples were oxygen-annealed for 3 6h at 1500°C. The experiments were performed in a Philips 400T TEM with a vacuum pressure of 10-6 Torr.Fig. 1(a) is a REM image showing the structural features that characterize the surface in the beam direction. The surface can be seen to exhibit periodic bright and dark domains, with parallel small steps (S) in average direction oriented 7° from the (after de-foreshortening). The appearence of two domains may be due to a difference in termination of the lattice at the surface. The THREED pattern (insert) shows the specular reflection used for imaging. Fig. 1(b) shows the structural features on the surface in the beam direction. Two domains (grey, G and white, W) can also be seen here, and may be attributed to the same effect. A well defined step can be seen at the boundary of the two domains. The faceted steps in the white domain can be seen to adopt average direction oriented 5° from the . Strain field of dislocation (D) can be seen to run across the steps, and is displaced at the steps. The RHEED pattern (insert) shows the specular spot used for imaging. Fig. 2 shows the structural features on the surface in the beam direction, with the spot (insert) used for imaging. Emerging dislocation (D) can be seen terminating at kink of the large step.

Franckeite (˜FeSn3Pb5Sb2Si14) has a complex modulated structure containing alternating layers of different composition. The complex composition and structure, lack of a well-defined unit cell, incommensurate character, and poor quality of crystals has made it difficult to study by standard X-ray and electron-diffraction methods. High-angle annular dark-field (HAADF) STEM imaging provides a way of mapping the compositional distributions directly and with atomic resolution. For thin regions of the sample, the intensity of the electrons scattered at high angles depends strongly on the atomic number of the scatterer. This unique property makes HAADF imaging powerful for studying compositional modulations on the atomic scale. By comparing the structure as viewed by high-resolution imaging to the compositional information provided by the HAADF technique, we have been able to obtain an improved understanding of crystal structures with displacive and compositional modulations, such as franckeite. We use the HAADF technique to test the assumption that in franckeite two kinds of layers (SnS2 and PbS) are stacked along the a axis with a 1.73 nm repeat, and that Sb and Fe produce structural modulations in the c direction.

Scanning transmission electron microscopy (STEM) offers a powerful extension to the conventional transmission electron microscopy (TEM) by combining high resolution microanalysis with a variety of imaging and diffraction modes. Bright-field (BF) and dark-field STEM imaging (with or without energy filtering) can be used to obtain structural images or to locate small particles and other inhomogeneous sample areas for microanalysis. The microdiffraction technique can provide crystallographic information on nanometer-scale small particles. Secondary electron (SE) images with a resolution approaching the probe size (0.5 nm) can be obtained in STEM instruments, providing topographic information on the samples studied. High-angle annular dark field (HAADF) imaging is also available in dedicated STEM instruments. The intensity of high-angle scattered electrons depends strongly on the atomic number (Z). This unique property makes the HAADF imaging a powerful microscopic technique in characterizing supported catalyst systems which often comprize high atomic number particles supported on low atomic number substrates.