Dark-field electron spectroscopic imaging of aged microstructures in an aluminium lithium base alloy

1990 ◽  
Vol 48 (4) ◽  
pp. 444-445
Author(s):  
I. G. Solórzano ◽  
W. Probst
Keyword(s):  
Base Alloy ◽  
Spectroscopic Imaging ◽  
Chromatic Aberration ◽  
Dark Field ◽  
Objective Lens ◽  
Electron Spectroscopic Imaging ◽  
Bright Field ◽  
Field Mode ◽  
Dark Field Electron ◽  
Very High

The examination of microstructures make very high demands on the imaging quality and, therefore, on the instrumentation. In Al-Li base alloys it is of great interest to determine parameters such as size, distribution, morphology and coherency of precipitate phases as they dictate their mechanical behavior. In order to reveal morphological features with high quality the electron spectroscopic imaging (ESI) in dark field mode has shown to be quite a powerful technique.The ESI technique in the TEM is based on the possibility that accelerated electrons can be elastic and inelastically scattered by the sample atoms, as recently reviewed. The electron distribution in the transmitted and diffracted beams through a crystalline sample is such that both energy loss and elastic electrons will enter a typical objective aperture and thus contribute to both bright field and dark field images. The effect of the polyenergetic electrons is that the image is affected by chromatic aberration of the objective lens. In CTEM’s this effect is enhanced the lower the accelerating voltage and the thicker the sample.

Observation of Electron Energy Losses in Dark Field Images

1967 ◽  
Vol 25 ◽  
pp. 246-247
Author(s):  
J. S. Lally ◽  
R. M. Fisher ◽  
A. Szirmae ◽  
H. Hashimoto
Keyword(s):  
Spherical Aberration ◽  
Focal Length ◽  
Chromatic Aberration ◽  
Dark Field ◽  
Bragg Angle ◽  
Energy Losses ◽  
Objective Lens ◽  
Characteristic Energy ◽  
Crystalline Materials ◽  
Dark Field Electron

It is commonly assumed that the poor resolution of axially illuminated dark field electron micrographs of crystalline materials is due to the spherical aberration of the objective lens. Actually in many cases the lack of sharpness of the image results from the displacement by chromatic aberration of additional images formed by the electrons which have suffered large energy losses as a result of single or multiple plasmon scattering. In the case of very small diffracting particles, grains or other fine structures in the specimen it is possible to observe multiple images corresponding to these characteristic energy losses. The displacement (Δx) of the image is given by Δx = C f α ΔE/E where C is the chromatic aberration coefficient, f the focal length, α is twice the Bragg angle, E the accelerating potential, and ΔE the energy loss. For a typical plasmon loss of 20 ev the displacement at 100 kv is about 100 Å.

Experimental High Resolution Dark Field Electron Microscopy

1977 ◽  
Vol 35 ◽  
pp. 142-143
Author(s):  
Larry Pierce ◽  
Peter R. Buseck
Keyword(s):  
Electron Microscopy ◽  
High Resolution ◽  
Dark Field ◽  
Objective Lens ◽  
Aperture Size ◽  
Bright Field ◽  
Chromatic Aberrations ◽  
Field Electron Microscopy ◽  
Dark Field Electron ◽  
Diffraction Patterns

High resolution dark field (DF) images of the superstructures of the pyrrhotite (Fe1-xS) and bornite-digenite (Cu5FeS4-Cu9S5) series can be related to structure. Further, they provide more detail than bright field (BF) images. The same objective aperture size and stigmater settings were used for DF as for BF imaging; symmetrical arrangements of diffracted beams in the objective aperture were used. Images that can be related to structure were obtained at the defocus value giving the greatest image contrast, thereby enabling proper defocusing without requiring extensive through-focus series.For the minerals of interest, diffraction patterns consist of many superstructure reflections and a few subcell reflections. BF images contain primarily features of the superstructure, presumably because the subcell reflections fall far from the axis of the objective lens and thus are affected by spherical and chromatic aberrations and beam divergence. Likewise, DF images formed with a similar arrangement of beams as that in BF contain only features of superstructure, but with reverse contrast to BF.

Electron spectroscopic imaging of thick crystalline specimens

1989 ◽  
Vol 47 ◽  
pp. 412-413
Author(s):  
L. Reimer ◽  
R. Rennekamp ◽  
A. Bakenfelder
Keyword(s):  
Angular Distribution ◽  
Lattice Defect ◽  
Spectroscopic Imaging ◽  
Chromatic Aberration ◽  
List Type ◽  
Electron Spectroscopic Imaging ◽  
Field Mode ◽  
Energy Filtering ◽  
Plasmon Loss ◽  
Electron Energy Loss

Electron spectroscopic imaging (ESI) by an energy-filtering electron microscope (EFEM, Zeiss EM902) shows the following advantages when compared with the unfiltered bright-field mode:1.The zero-loss image does not contain the contribution of inelastically scattered electrons. Though plasmon scattering shows a conversation of Bragg contrast - edge and bent contours and lattice defect images -, the angular distribution of inelastically scattered electrons results in a broader spectrum of excitation errors and a blurring of Bragg contrast.2.The zero-loss image avoids the chromatic aberration of inelastically scattered electrons for medium specimen thicknesses and can be applied so long as the intensity of the zero-loss peak in the electron energy-loss spectrum (EELS) is high enough for an exposure in a reasonable time (<100 s).3.Thick specimens with negligible zero-loss intensity can be imaged with an energy window at the highest multiple plasmon loss of the Poisson distribution or at the most probable energy of a Landau distribution. The angular distribution of electrons with these energy losses is so broad that the Bragg contrast is blurred, and the contrast is only caused by anomalous absorption effects similar to multi-beam images in the STEM mode when using a large probe aperture.

Inelastic Scattering in Electron Microscopy

1973 ◽  
Vol 31 ◽  
pp. 254-255
Author(s):  
M. Isaacson ◽  
J. Langmore ◽  
J. Wall ◽  
A. V. Crewe
Keyword(s):  
Electron Microscopy ◽  
High Resolution ◽  
Chromatic Aberration ◽  
Dark Field ◽  
Biological Molecules ◽  
Objective Lens ◽  
Transmission Electron ◽  
Field Electron Microscopy ◽  
Dark Field Electron ◽  
Conventional Transmission Electron Microscope

The effort to image biological molecules by high resolution (2-4 Å) dark field electron microscopy has stimulated interest in those factors Which influence image contrast. It is known that elastically scattered electrons can be used to obtain high resolution information about a specimen. On the other hand, most inelastically scattered electrons cannot contribute any high resolution information about the specimen since they are the result of a nonlocalized interaction of the incident electrons with the electrons in the specimen. Moreover, in the conventional transmission electron microscope (CTEM) without a chromatic aberration corrector or an energy filter between the specimen and recording plane, inelastically scattered electrons blur the image, due to the chromatic aberration of the objective lens. This has particular importance in biological electron microscopy, since the ratio of total inelastic to elastic scattering for carbon is 1.6.

DNA organization in nucleosomes

1982 ◽  
Vol 60 (3) ◽  
pp. 364-370 ◽  
Author(s):  
D. P. Bazett-Jones ◽  
F. P. Ottensmeyer
Keyword(s):  
Spectroscopic Imaging ◽  
Dark Field ◽  
Electron Spectroscopic Imaging ◽  
Nucleosome Core ◽  
Dna Organization ◽  
Field Electron Microscopy ◽  
Dark Field Electron ◽  
A New Technique ◽  
Beaded Appearance ◽  
Active Genes

A new technique known as electron spectroscopic imaging has allowed the direct visualization of DNA within the nucleosomes of chromatin. The results presented here confirm the model which suggests that approximately two supercoil turns of DNA are wound about the nucleosome core. The structure of nucleosomes from putative transcriptionally active genes, fractionated by preferential sensitivity to DNAase II and solubility in 2 mM MgCl2, has been examined using both dark field electron microscopy and electron spectroscopic imaging. Oligomeric strands of nucleosomes in this fraction have a less distinct beaded appearance than those of bulk chromatin. The phosphorus distribution in this chromatin suggests that the DNA has a less recognizable organization, lacking a two-turn supercoil per subunit. The unique appearance of this fraction in 30 mM NaCl is reversibly changed to the classical beaded appearance when dialyzed into 0.4 M NaCl.

Imaging of platinum-shadowed macromolecular complexes in dark field

1984 ◽  
Vol 42 ◽  
pp. 146-147
Author(s):  
D.W. Andrews ◽  
F.P. Ottensmeyer
Keyword(s):  
Thin Film ◽  
Three Dimensional ◽  
Average Diameter ◽  
Dark Field ◽  
Bright Field ◽  
Field Mode ◽  
Macromolecular Complexes ◽  
Average Mass ◽  
Topological Features ◽  
Projection Images

Shadowing with heavy metals has been used for many years to enhance the topological features of biological macromolecular complexes. The three dimensional features present in directionaly shadowed specimens often simplifies interpretation of projection images provided by other techniques. One difficulty with the method is the relatively large amount of metal used to achieve sufficient contrast in bright field images. Thick shadow films are undesirable because they decrease resolution due to an increased tendency for microcrystalline aggregates to form, because decoration artefacts become more severe and increased cap thickness makes estimation of dimensions more uncertain.The large increase in contrast provided by the dark field mode of imaging allows the use of shadow replicas with a much lower average mass thickness. To form the images in Fig. 1, latex spheres of 0.087 μ average diameter were unidirectionally shadowed with platinum carbon (Pt-C) and a thin film of carbon was indirectly evaporated on the specimen as a support.

Prospects for Bright Field and Dark Field Electron Tomography on a Discrete Grid

2004 ◽  
Vol 10 (S03) ◽  
pp. 44-45 ◽  
Author(s):  
J. R. Jinschek ◽  
K. J. Batenburg ◽  
H. A. Calderon ◽  
D. Van Dyck ◽  
F.-R. Chen ◽  
...  
Keyword(s):  
Electron Tomography ◽  
Dark Field ◽  
Bright Field ◽  
Field Electron ◽  
Materials Research ◽  
Dark Field Electron ◽  
Free Environment ◽  
Discrete Grid

Extended abstract of a paper presented at the Pre-Meeting Congress: Materials Research in an Aberration-Free Environment, at Microscopy and Microanalysis 2004 in Savannah, Georgia, USA, July 31 and August 1, 2004.

Objective Lens Properties of Very High Excitation

1968 ◽  
Vol 26 ◽  
pp. 320-321 ◽  
Author(s):  
S. Suzuki ◽  
K. Akashi ◽  
H. Tochigi
Keyword(s):  
Thermal Emission ◽  
Spherical Aberration ◽  
Chromatic Aberration ◽  
Objective Lens ◽  
Technical Advances ◽  
Single Field ◽  
Very High ◽  
Illuminating System ◽  
Excellent Stability

Recently the image quality of the electron microscope has been highly improved, because of technical advances made for the exclusion of mechanical and electrical instabilities in the instrument itself. To further improve the resolution, it is important to minimize the chromatic aberration, as well as the spherical aberration. This is true, even though the accelerating voltage has excellent stability; because of the unavoidable velocity fluctuations of electrons, resulting from thermal emission from the cathode, and the energy-loss in the specimen.Therefore, the specimen should be immersed deeply into the lens field, to make these aberrations small. The result will be very high lens excitation. In 1962, Professor E. Ruska and Dr. W. Riecke had investigated the single field condenser-objective lens at the center of which the specimen is placed. Prior to that, in 1960, Dr. S. Suzuki, one of the authors, had pointed out that this new objective lens, in which the specimen is placed at the image side from the center of the lens field,(of the condenser-objective lens: but no special illuminating system is necessary.

Structures and defects identified by dark-field HREM

1992 ◽  
Vol 50 (1) ◽  
pp. 124-125
Author(s):  
J.P. Zhang
Keyword(s):  
High Resolution ◽  
Structural Information ◽  
Image Interpretation ◽  
Dark Field Image ◽  
Dark Field ◽  
Bright Field ◽  
Field Mode ◽  
Transmitted Beam ◽  
Structure Information ◽  
The Difference

The tilted illumination dark field high resolution imaging technique was applied to structures and defects of semiconductors and superconductors. We used a Hitachi-H9000 top entry microscope with a high resolution pole-piece of Cs=0.9 mm, operated at 300 Kv. Proper apertures, tilting angle and imaging conditions were chosen to minimize the phase shift due to aberrations. Since the transmitted beam was moved outside the aperture, the noise ratio was greatly reduced, which resulted in a significant enhancement of image contrast and apparent resolution. Images are not difficult to interpret if they have a clear correspondence to structure - information from image simulations in bright field mode can be used to assist in dark field image interpretation.An example in a semiconductor, GaAs/Ga0.49In0.51P2 superlattice imaged along [110] direction is shown in Figure 1. In this dark field image the GaAs and GaInP layers can be easily distinguished by their different contrast, and the difference in quality between both sides of interfaces is clear. An enlarged image in Figure 1 shows the defective area on the rough side of interface. Since this image shows the same pattern as the [110] projection of an fee structure, the major structural information about {111}, {200}, {220} planes can be obtained from this zone. Note that in bright field mode, [110] is not a good zone for imaging such multilayers.

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