Top-bottom Effect Research Articles

Does the order of elastic and inelastic scattering affect an image or is there a top bottom effect from inelastic scattering?

Especially for light elements inelastic scattering is more probable than the elastic scattering that conveys the structural information. The question arises as to whether an image using inelastically scattered electrons is different depending on whether the elastic or inelastic scattering happens first, is there a top-bottom effect. We show that since inelastic scattering is concentrated in a narrow range of angles, much less than typical Bragg angles in light element materials, the inelastic and elastic processes are separable and, to a very good approximation, there is no top-bottom effect. For weakly scattering thin biological specimens that are phase objects the separation is exact and there can be no top-bottom effect.

Phase contrast imaging with inelastically scattered electrons from any layer of a thick specimen

A controversy exists as to whether the signal in a high resolution phase contrast electron micrograph of a particle in a thick specimen is the same irrespective of the particle’s position along the beam axis. Different conceptions of inelastic scattering and its effects on wave interference have led to radically different expectations about the degree of phase contrast vs. depth. Here we examine the information available from bright field phase contrast images of small crystalline particles on the top or bottom of a thick support. The support is an aluminium foil which has strong plasmon resonances that cause a large proportion of the electron beam to lose energy in transit. Phase contrast micrographs of the atomic lattice of two ensembles of platinum particles were measured in an energy loss window corresponding to the first plasmon resonance. The signal measured for particles on top was equal to that for particles on the bottom of the foil to within a 99% confidence interval, and the measurements exclude other models of depth dependent phase contrast in the literature to >5σ. These observations are consistent with quantum theory which considers dynamical effects as independent of event sequence and is distinct from the “top-bottom effect” observed in amplitude contrast. We thus confirm that phase contrast using inelastically scattered electrons can be obtained equally well from particles within any layer of a thick specimen.

Response of the soil food web to warming and litter removal in the Tibetan Plateau, China

Both climate warming and litter removal alter soil nematode communities and the structure, stability, and function of soil food webs. The soil nematode metabolic carbon footprint, based on nematode biomass and respiration, can be used to indicate ecosystem function and the response of nematodes to nutrient enrichment in the ecosystem. However, it is unclear whether or how climate warming and litter removal affect the metabolic carbon footprint of soil nematodes and soil food webs. In this study, we investigated four warming and litter removal treatments in the Qinghai-Tibet Plateau, China, to determine the effects of climate warming and litter removal on soil microbial food webs. The treatments included: (1) control (CK), (2) warming, (3) litter removal, (4) warming + litter removal. We found that warming and litter removal had significant negative effects on the diversity and richness of soil nematodes. In addition, our results showed warming indirectly and directly affected vegetation diversity, thereby altering soil nematode diversity. Vegetation played an important role in the maintenance of soil nematode community diversity. The enrichment and structural footprints reflected nematodes that could respond most rapidly to resource enrichment and those with higher trophic levels in soil food webs, respectively. Overall, warming and litter removal significantly negatively affect the enrichment metabolic footprint, indicating they could significantly reduce the external resource inputs into the food web. Both warming and litter removal reduced the structural metabolic footprint of the soil food web, although the changes were not significant, indicating that the metabolic activity of nematodes of high nutrient levels decreased and the ability to regulate the food web from top-bottom effects was weakened under warming and litter removal. The nematode functional metabolic footprints indicated that litter removal could aggravate the effect of warming on the soil food web and result in more severe degradation. The combination of warming and litter removal significantly inhibited the connectance of the bacterial channel, but had no significant effect on the fungal channel. These results suggest that the soil food web enhanced resistance to the environment by increasing the proportion of the fungal channel. It is critical to understand the responses of soil food webs to climate warming and litter removal to better predict how ecosystem functions will change in the future.

GPU-based discrete element simulation on flow stability of flat-bottomed hopper

Abstract In this study, the flow stability of the flat-bottomed hopper was investigated via GPU-based discrete element method (DEM) simulation. With the material height inside the hopper reducing, the fluctuation of the flow rate indicates an unstable discharge. The flow regions of the unstable discharge were compared with that of the stable discharge, a key transformation zone, where the voidage showed the largest difference between unstable and stable discharge, was revealed. To identify the relevance of the key transformation zone and the hopper flow stability, the voidage variation of the key transformation zone with material height reducing was studied. A sharp increase in the voidage in the key transformation zone was considered to be the standard for judging the unstable hopper flow, and the ‘Top–Bottom effect’ of the hopper was defined, which indicated the hopper flow was unstable when the hopper only had the top area and the bottom area, because the voidage of particles in the top area and the bottom area were both variables.

The top–bottom effect of a tilted thick specimen and its influence on electron tomography

We present the top-bottom effect (TBE) of a 5 microm thick amorphous specimen in a 3 MV ultrahigh voltage electron microscope (ultra-HVEM) and its influence on the quality of electron tomography (ET). The 40-nm gold particles on the top surface of the specimen tilted at different angles have been observed to be of poorer image quality than those on the symmetrical bottom surface obtained by tilting and turning over the specimen. The point spread function of the gold-particle images was calculated using the increment Wiener filter and the image-quality variation was then evaluated. The TBE is shown to become more remarkable with the increase of the effective thickness of the tilted specimen or the decrease of the magnification of the ultra-HVEM. The ET simulation indicates that the TBE may generate approximately 4% distortion in the radial direction of a reconstructed sphere model.

Measuring the top–bottom effect of a tilted thick specimen in an ultrahigh-voltage electron microscope

From the 40 nm gold particles on the surface of a 5μm thick amorphous specimen tilted by a 360° tilt holder, we have observed the top–bottom effect (TBE) in a 3 MV ultrahigh-voltage electron microscope. Because of the TBE, object points located on the top surface of the specimen have poorer image quality than those on the bottom. Measurements of the diameter and contrast of gold-particle images show that the TBE becomes strong when increasing the effective thickness of the specimen by tilting the specimen. The TBE can produce additional image blurring of about 10, 27, and 74 nm in the gold-particle diameter for the effective thickness of 5.0, 10.0, and 14.6μm, respectively. The need for further investigations is thus suggested to understand and determine the TBE limitation on the resolution or fidelity of electron tomography for thick specimens.

Top-bottom effects in double diffraction

A top-bottom effect in double diffraction has been observed in selected-area diffraction (SAD) patterns recorded from epitactic overgrowths. In SAD patterns recorded from epitactic α-Fe 2O 3 islands grown on an (0001) Al 2O 3 substrate, the double-diffraction reflections were found always to surround the primary reflections associated with the bottom crystal. This observation is explained using simple geometric constructions and the concepts of kinematical diffraction in a deformed crystal. The effect may be useful as a simple method for determining on which side of a TEM specimen a particular epitactic feature lies.

Dynamical electron diffraction in substitutionally disordered column structures

For column structures, such as fcc-based alloys viewed along the cube direction, the concept of electron channelling through the atom columns is more and more used to interpret the corresponding HREM images. In the case of (partially) disordered columns, the projected potential approach which is used in the channelling description must be questioned since the arrangement of the atoms along the beam direction might affect the exit wave of the electrons. In this paper, we critically inspect this top-bottom effect using multi-slice calculations. A modified channelling theory is introduced which turns out to be very appropriate for the interpretation of these results. For substitutionally disordered column structures, it is also discussed how to link the chemical composition of the material to statistical data of the HREM image. This results in a convenient tool to discern images taken at different thicknesses and focus values.

Atomic resolution electron microscope images formed by SiL 3-ionization electrons

Contrast of energy-filtered electron microscope images with atomic resolution of Si atoms producing SiL 3-ionization electrons in a Si crystal is discussed. The localization width of the inelastic scattering potential is deduced from the inelastic scattering factor for SiL 3-ionization, which is based on a program for a modified hydrogenic L-shell cross section. The image contrast at the bottom surfaces of the Si crystals containing a single inelastic scattering layer at the top, middle and bottom layers showed diffuse, sharp and very sharp image contrast, i.e. top-bottom effect is predominant. Similar images of an atom which scattered electrons inelastically with a very small energy loss in an aluminum crystal were calculated and it is noted that the atomic structure of aluminum is observed.

Registry and UHV transmission electron diffraction of surfaces

A model using a distorted wave analysis coupled with considerations of registry and channeling is developed for understanding high-energy electron diffraction from surfaces in a plan-view geometry. We show that this model explains strong effects due to inelastic scattering, particularly for the 1×1 surface as a function of beam orientation with respect to the underlying bulk substrate and with respect to the effect of registry on intensities. Numerical simulations using the multislice algorithm are correlated with the model, and show additional complications due to a top-bottom effect and coupling.

Rigor, and plan-view simulation of surfaces

This paper discusses the rigor of simulating plan-view images of surfaces using numerical multislice calculations. In particular, the validity of using a conventional multislice approach with individual atomic slices is tested versus a three-dimensional multislice with the top and bottom surfaces specifically included; the result is that the two are identical when inelastic scattering is neglected and almost identical when it is included. These results also demonstrate that inelastic scattering cannot be neglected for surface plan-view simulations. For instance, the bulk-forbidden, surface-allowed spots are shown to correspond to true bulk Bloch waves which are damped in thicker crystals consistent with experimental observations. The existence of a strong top-bottom effect is also pointed out, which means that plan-view imaging is more sensitive to the bottom surface except in exceedingly thin crystals.

The top-bottom iV-beam phase contrast effect from finite crystals II. Theory

Abstract The top-bottom contrast effect which has been observed in dark-field images of aperiodic surface detail and which was discussed in Part I is shown to arise as a result of N-beam refraction reversal in one or more of the important Bloch wave components, formed by high-energy transmission electron diffraction. The effect is shown to arise in two-beam as well as N-beam diffraction. In particular, the zero-contrast condition, whereby the contrast from one or other surface is suppressed, and found experimentally in Part I, is found to arise as a consequence of a finite-crystal Bloch wave degeneracy.

A top–bottom contrast effect in dark-field images arising fromN-beam dynamic refraction

The refraction effect associated with an off-Bragg diffracted beam in high-energy transmission electron diffraction is analysed. It is found that the wave vectors of Bloch-wave components of a beam associated with a finite excitation error are significantly shortened or lengthened as a consequence of dynamic scattering, with the direction of change being determined by the sign of the excitation error. By consideration of the dark-field image formed from the off-Bragg reflection and its associated refractive fine structure it is shown that a top-bottom effect exists, which can be demonstrated by comparing images of the same field for positive and negative excitation errors. This is discussed in two dimensions for a crystal of triangular cross section and one horizontal face, in the two orientations of apex up and apex down, with numerical and graphical evaluations from linear three-beam diffraction.

Utilization of the effect in the high-resolution electron microscopy of surfaces

Abstract A top-bottom contrast effect which arises in dark-field images of aperiodic surface detail may be utilized to achieve separate imaging of either surface. The upper or lower crystal surface is brought into contrast according to whether the excitation error of the diffracted beam being used is positive or negative. This has been found to apply almost independently of the N-beam diffraction condition and to be effective in imaging at the highest instrumental resolution. In this investigation, an MgO platelet with a distinctive surface structure has been used as a test sample. The preliminary results presented show how the technique may allow the dynamics of a surface structure to be studied under conditions where high contrast and high resolution are compatible entities, close to Gaussian focus.

Top-bottom effect in energy-selecting transmission electron microscopy

Energy-selecting TEM can avoid the chromatic error of large specimen thicknesses when selecting an energy window at the most probable energy loss. The resolution is limited by the spatial beam broadening for structures at the top (electron entrance) of the specimen layer. A test experiment with polystyrene spheres of 1.1 μm in diameter shows a blurring of 8 nm when imaging with Δ E = 250 eV and E = 80 keV.

Fundamental concepts of stem imaging

This review paper introduces some basic concepts of STEM imaging. The important Principle of Reciprocity is considered in detail. The quantitative breakdown of reciprocity when inelastic scattering is significant is demonstrated and the top-bottom effect briefly discussed. The signal-to-noise ratio in STEM images is considered, and the implications this has for the imaging of defects in crystals is discussed.

Limiting factors in specimen thickness in conventional and scanning transmission electron microscopy

Abstract Specimens of silicon containing grown-in stacking faults have been examined in conventional transmission electron microscopy (CTEM) and scanning transmission electron microscopy (STEM) at voltages between 80 and 200 kV. It is found that the use of a tungsten filament STEM severely limits the penetration because of the small current available in the (~50 A) probe. In addition, the large beam divergence which is used generally causes the orientation of best transmission to dominate the contrast. When using a field emission (FEG) STEM this limitation is found to be largely overcome although the useful intensity is still lower in FEGSTEM than in CTEM. The factor limiting usable specimen thickness is found to be associated with a top-bottom effect in both CTEM and FEGSTEM; contrast information is lost from the top† of the specimen in CTEM and from the bottom in STEM. The maximum usable foil thickness corresponding to the loss of fringe contrast associated with this top-bottom effect is approximately t...

Measurement of the top bottom effect in scanning transmission electron microscopy of thick amorphous specimens

SummaryIn Scanning Transmission Electron Microscopy (STEM) details at the top of thick specimens can be imaged with a better resolution than those at the bottom, because the scanning electron probe is broadened by multiple scattering. For quantitative measurement we used a test specimen consisting of a substrate film with an island‐structured indium film and coated with polystyrene spheres of different diameters up to 1.1 μm. The fine detail contained in the indium film was observed and the beam broadening was that caused by the polystyrene spheres. This specimen was imaged with the spheres both above and below the layer, in a JEOL 100B electron microscope with a scanning device. Electron energies from 20 to 100 keV were used. Images with the sphere below show a higher resolution than those with the sphere above and also those of conventional electron microscopical images at 100 kV and 200 kV. The broadening of the edges on the photographic film was recorded with a microdensitometer. The results can be compared with theoretical calculations by a Monte Carlo method. There is a good agreement with the experiments if one uses an effective aperture obtained by transmission experiments.

Test on Resolution of Electron Microscope Applied to Thick Specimens

The popularity of the ultra high voltage electron microscope has resulted in a tendency to use thick specimens. R. Uyeda, Y. Kamiya, H. Hashimoto, and V. E. Cosslett reported the results of experiments and calculations determining to what thickness the electron microscope can observe specimens.Generally, in the case of thick specimens, the resolution in the microscope image depends on chromatic ‘aberration and when the energy distribution of electron beams, after being transmitted through the specimen, is measured, the resolution due to chromatic aberration can be obtained byIn practice, however, resolution depends on the position of the objects (e.g., irregularities such as lattice defects) in the specimen. This may be attributable to phase contrast due to interference phenomena and also attributable to the top-bottom effect pointed out by H. Hashimoto.

Packing of Spheres

SCOTT1 has described experiments with equi-sized steel balls of 0.3175 cm. diameter in loose and in dense random stable packings, having volume packing densities of 0.60(1) and 0.63(7), corrected for wall and top–bottom effects. The specific gravity of steel is high, and this may have had an influence on the arrangement of the spheres, as the impact of the falling or rolling balls will have been rather high. Some experiments have been done with nylon spheres of comparable diameter, 0.287 ± 0.002 cm., and a specific gravity of 1.15(6) ± 0.02. These were a loan from Prof. H. Kramers of Delft.