Calculations of Z-Contrast for organic and biological specimens

1983 ◽  
Vol 41 ◽  
pp. 284-285
Author(s):  
R.F. Egerton ◽  
M. Misra
Keyword(s):  
Atomic Number ◽  
Cross Sections ◽  
Phase Contrast ◽  
Detection System ◽  
Dark Field ◽  
Single Atom ◽  
Bright Field ◽  
Annular Dark Field ◽  
Z Contrast ◽  
Increasing Function

So-called "atomic-number contrast" is obtained in STEM by displaying a ratio signal formed by dividing the annular-dark-field signal Iad by the inelastic component Ii of the bright-field intensity (isolated by means of an electron spectrometer; see Fig. 1). Originally used for single-atom imaging, the technique has more recently been applied to polymer samples and biological tissue.We report here estimates of the ratio signal from organic specimens, based on the following assumptions:(1) That the specimen is amorphous and that phase contrast may be neglected for the electron-optical conditions and specimen features being considered; (2) That atomic cross sections may be used to estimate the amount of elastic and inelastic scattering. Modern calculations differ from simple Lenz theory in predicting that the cross section is not a smoothly-increasing function of atomic number (see Fig. 2), particularly for the 1ighter elements. (3) We assume a slightly idealized detection system in which all elastically scattered electrons contribute to Iad, while all electrons which have been inelastically (but not elastically) scattered contribute to Ii.

Imaging single platinum atoms on zeolites in the STEM

1990 ◽  
Vol 48 (4) ◽  
pp. 240-241
Author(s):  
Stephen B. Rice ◽  
Michael M. J. Treacy ◽  
Mark M. Disko
Keyword(s):  
Atomic Number ◽  
Scanning Transmission Electron Microscope ◽  
Dark Field ◽  
Supported Metal Catalysts ◽  
Bright Field ◽  
Fourier Filtering ◽  
Scanning Transmission ◽  
Annular Dark Field ◽  
Contrast Technique ◽  
Z Contrast

High angle annular dark-field (HAAD) imaging in the scanning transmission electron microscope has been shown in recent years to be a very effective tool in characterizing materials in which there are large differences in atomic number. Supported metal catalysts, in particular, have been explored extremely successfully using this Z-contrast technique. HAAD has very good sensitivity to high atomic number clusters on low atomic number supports, due to the approximately Z2 relationship. Furthermore, since the image contrast is due primarily to amplitude contrast, the resulting images are maps of mass thickness. Owing to the linear proportionality between intensity and the number of atoms probed, the intensity values integrated over metal clusters can be used as a measure of the cluster size.High resolution bright-field imaging is better suited for resolving structure in periodic specimens, and can be used to obtain structure images of zeolites. However, even with contrast enhancements such as Fourier filtering available from image processing, bright-field images are ineffective for detecting clusters containing fewer than about 20 Pt atoms in supports thicker than about 100Å. In comparison, we have demonstrated that the HAAD technique can be used successfully to detect single atoms of platinum on a 200Å thick zeolite support.

New detection system for HAADE and holography in STEM

1992 ◽  
Vol 50 (1) ◽  
pp. 102-103
Author(s):  
Marian Mankos ◽  
Shi Yao Wang ◽  
J.K. Weiss ◽  
J.M. Cowley
Keyword(s):  
High Efficiency ◽  
Detection System ◽  
Dark Field ◽  
Light Signal ◽  
Bright Field ◽  
Transmitted Beam ◽  
Wide Range ◽  
Annular Dark Field ◽  
Resolution Imaging ◽  
Diffraction Patterns

A novel detection system has been designed and realized experimentally on the HB5 STEM instrument. Shadow images, diffraction patterns as well as high-angle annular dark field and bright field images are observed simultaneously with high efficiency using CCD and TV cameras. The microscope can be operated in a wide range of instrument modes which includes the implementation of new techniques for high resolution imaging.As shown in Fig. 1, the detection system has three triple choice stages. Diffracted beams can be collected by three P47 fast phosphor annular detectors inclined at 45 degree to the axis and having different inner and outer acceptance angles, which can be adjusted by the postspecimen lenses. The detector is observed through a window by a photomultiplier. The annular detectors have been used also for a new bright field STEM technique which utilizes the inner rim of the detectors to collect only the outermost annular part of the central beam and promises an improvement in resolution by a factor of about 1.6. Initial results show some promise (Fig. 2). The transmitted beam is then converted into a light signal in YAG and P47 detectors; optionally the central part of the beam can be detected in the EELS spectrometer. The generated light signal is reflected through a system of mirrors, exits the vacuum chamber and is collected with high efficiency by high aperture optical lenses.

Prospects For Imaging of Single Dopant Atoms in Silicon by ADF Stem

1998 ◽  
Vol 4 (S2) ◽  
pp. 646-647
Author(s):  
Richard R. Vanfleet ◽  
John Silcox
Keyword(s):  
Atomic Number ◽  
Misfit Strain ◽  
Scanning Transmission Electron Microscope ◽  
Dark Field ◽  
Technology Roadmap ◽  
Transmission Electron ◽  
Scanning Transmission ◽  
Annular Dark Field ◽  
Dopant Atoms ◽  
Z Contrast

The demands of the National Technology Roadmap for Semiconductors will necessitate measurement of dopant concentrations with greater spatial resolution than now possible. Current experimental and simulation experience indicate that Annular Dark Field (ADF) imaging in a Scanning Transmission Electron Microscope (STEM) should be able to determine dopant distributions with near atomic resolution. The ADF signal is derived from electrons diffusely scattered to high angles, resulting in contrast due to atomic number (Z-contrast) and defects in the crystal lattice. Thus, heavy atoms can be imaged by their Z-contrast and small atoms by the misfit strain induced in the silicon lattice. Atomic number scattering is proportional to Zn where n is between 1.5 and 1.9 depending upon the inner detector angle of the ADF detector.

Possibility of Single Atom Resolution in Single-Sideband Imaging

1977 ◽  
Vol 35 ◽  
pp. 16-17
Author(s):  
K. H. Downin
Keyword(s):  
Electron Microscopy ◽  
Image Processing ◽  
Phase Contrast ◽  
Dark Field ◽  
Single Atom ◽  
Bright Field ◽  
Heavy Atoms ◽  
Single Atoms ◽  
Single Sideband ◽  
Light Components

The ability to distinguish between heavy and light components of a specimen, and especially to distinguish single heavy atoms, would be a great benefit in many instances. Even with the demonstrated ability of the STEM, and of the CTEM especially in dark field, to image single atoms, it is of interest to know the possibility of using bright field in a conventional microscope with subsequent image processing to obtain images of single heavy atoms separated from the image of a lighter supporting film.Single-sideband image reconstruction, involving the combination of two images obtained using opposite halves of the diffraction pattern, offers in principle the ability to separate images of heavy and light specimen components, or specimen components giving rise to amplitude and phase contrast, respectively. This has been demonstrated in electron microscopy^, but under conditions or low resolution, where inelastic scattering, as well as scattering outside the effective objective aperture, contribute heavily to the component with amplitude contrast.

Imaging of Silicon (111) at 1.92Å Resolution using a 100keV STEM

1990 ◽  
Vol 48 (1) ◽  
pp. 34-35
Author(s):  
Peirong Xu
Keyword(s):  
High Resolution ◽  
Phase Contrast ◽  
Threshold Energy ◽  
Dark Field ◽  
Observation Time ◽  
Operating Voltage ◽  
Pole Piece ◽  
Field Phase ◽  
Bright Field ◽  
Annular Dark Field

Atomic structure imaging using bright field phase contrast at less than 2Å resolution has become routinely possible in medium and high voltage microscopes (>200 keV). Radiation damage at these elevated voltages can be serious and this limits the length of useful observation time. For example, the knock-on threshold energy for silicon is 120-190keV. Recently, a VG HB501A STEM equipped with a newly developed ultra-high resolution pole piece (Cs=0.7mm) has demonstrated the capability of achieving sub-2Å resolution in imaging the (111) silicon latticer using both bright field (BF) and annular dark field (ADF) modes at an operating voltage of l00keV (Fig.1).A thin silicon specimen was prepared through successive steps of chemical etching, anodic etching and reactive ion etching. Large flat thin areas about 100Å thick were produced in the specimen. Since there is no tilting mechanism for the stage used with this ultra-high resolution pole piece, the specimen was not oriented exactly along the (111) zone axis as indicated by CBED but was less than 1-2° off.

Which is Better for Protein Imaging: Phase Contrast TEM or Annular Dark Field STEM?

2001 ◽  
Vol 7 (S2) ◽  
pp. 382-383
Author(s):  
P. Rez
Keyword(s):  
Phase Contrast ◽  
Protein Structure Determination ◽  
Dark Field ◽  
Biological Molecules ◽  
Contrast Imaging ◽  
Bright Field ◽  
Low Contrast ◽  
X Ray ◽  
Contrast Microscopy ◽  
Annular Dark Field

In a landmark paper Henderson compared X-ray, neutrons and electrons for protein structure determination. He showed that electron microscopy should be superior to X-ray or neutron diffraction in terms of dose for a given resolution. in addition he presented a theoretical analysis to determine the smallest size molecule whose structure could be determined by phase contrast microscopy. Although he qualitatively considered amplitude contrast mechanisms and concluded they were inferior to phase contrast, no explicit numerical analysis was performed. It has been implicitly assumed that bright field phase contrast imaging is the optimal technique for imaging small biological molecules. Protein specimens are usually embedded in some medium such as ice or glucose. Since they must give a very low contrast it seems reasonable to expect that bright field techniques for these weakly scattering objects would be inferior, given that a weak signal is sitting on large background.

Removal of inelastically scattered electrons substantially increases phase contrast on frozen-hydrated molecules

1987 ◽  
Vol 45 ◽  
pp. 652-653
Author(s):  
John P. Langmore ◽  
Brian D. Athey
Keyword(s):  
Electron Diffraction ◽  
Phase Contrast ◽  
Low Dose ◽  
Dark Field ◽  
Biological Structure ◽  
Bright Field ◽  
Low Contrast ◽  
Negative Stain ◽  
The Difference ◽  
Better Than

Although electron diffraction indicates better than 0.3nm preservation of biological structure in vitreous ice, the imaging of molecules in ice is limited by low contrast. Thus, low-dose images of frozen-hydrated molecules have significantly more noise than images of air-dried or negatively-stained molecules. We have addressed the question of the origins of this loss of contrast. One unavoidable effect is the reduction in scattering contrast between a molecule and the background. In effect, the difference in scattering power between a molecule and its background is 2-5 times less in a layer of ice than in vacuum or negative stain. A second, previously unrecognized, effect is the large, incoherent background of inelastic scattering from the ice. This background reduces both scattering and phase contrast by an additional factor of about 3, as shown in this paper. We have used energy filtration on the Zeiss EM902 in order to eliminate this second effect, and also increase scattering contrast in bright-field and dark-field.

Atomic resolution Z-contrast imaging of bulk crystal surfaces in Scanning Reflection Electron Microscopy

1990 ◽  
Vol 48 (4) ◽  
pp. 414-415
Author(s):  
Z. L. Wang ◽  
J. Bentley
Keyword(s):  
Electron Microscopy ◽  
Elastic Scattering ◽  
Dark Field ◽  
Contrast Imaging ◽  
Crystal Lattices ◽  
Small Probe ◽  
Scanning Transmission ◽  
Annular Dark Field ◽  
Image Position ◽  
Z Contrast

The success of obtaining atomic-number-sensitive (Z-contrast) images in scanning transmission electron microscopy (STEM) has shown the feasibility of imaging composition changes at the atomic level. This type of image is formed by collecting the electrons scattered through large angles when a small probe scans across the specimen. The image contrast is determined by two scattering processes. One is the high angle elastic scattering from the nuclear sites,where ϕNe is the electron probe function centered at bp = (Xp, yp) after penetrating through the crystal; F denotes a Fourier transform operation; D is the detection function of the annular-dark-field (ADF) detector in reciprocal space u. The other process is thermal diffuse scattering (TDS), which is more important than the elastic contribution for specimens thicker than about 10 nm, and thus dominates the Z-contrast image. The TDS is an average “elastic” scattering of the electrons from crystal lattices of different thermal vibrational configurations,

Systematic zone-Axis orientation of NM-scale particles for microdiffraction

1990 ◽  
Vol 48 (4) ◽  
pp. 262-263
Author(s):  
E D Boyes ◽  
L Hanna
Keyword(s):  
Real Space ◽  
Dark Field ◽  
Zone Axis ◽  
High Symmetry ◽  
Lattice Imaging ◽  
Cell Level ◽  
Bright Field ◽  
Axis Orientation ◽  
Annular Dark Field ◽  
Target Designation

A VG HB501 FEG STEM has been modified to provide track whilst tilt [TWIT] facilities for controllably tilting selected and initially randomly aligned nanometer-sized particles into the high symmetry zone-axis orientations required for microdiffraction, lattice imaging and chemical microanalysis at the unit cell level. New electronics display in alternate TV fields and effectively in parallel on split [+VTR] or adjacent externally synchronized screens, the micro-diffraction pattern from a selected area down to <1nm2 in size, together with the bright field and high angle annular dark field [HADF] STEM images of a much wider [˜1μm] area centered on the same spot. The new system makes it possible to tilt each selected and initially randomly aligned small particle into a zone axis orientation for microdiffraction, or away from it to minimize orientation effects in chemical microanalysis. Tracking of the inevitable specimen movement with tilt is controlled by the operator, with realtime [60Hz] update of the target designation in real space and the diffraction data in reciprocal space. The spot mode micro-DP and images of the surrounding area are displayed continuously. The regular motorized goniometer stage for the HB501STEM is a top entry design but the new control facilities are almost equivalent to having a stage which is eucentric with nanometric precision about both tilt axes.

Towards 1-Ångstrom-resolution STEM

1993 ◽  
Vol 51 ◽  
pp. 996-997
Author(s):  
H.S. von Harrach ◽  
D.E. Jesson ◽  
S.J. Pennycook
Keyword(s):  
High Resolution ◽  
Field Emission ◽  
Phase Contrast ◽  
Spherical Aberration ◽  
A Priori ◽  
Dark Field ◽  
Image Resolution ◽  
Objective Lens ◽  
Annular Dark Field ◽  
First Results

Phase contrast TEM has been the leading technique for high resolution imaging of materials for many years, whilst STEM has been the principal method for high-resolution microanalysis. However, it was demonstrated many years ago that low angle dark-field STEM imaging is a priori capable of almost 50% higher point resolution than coherent bright-field imaging (i.e. phase contrast TEM or STEM). This advantage was not exploited until Pennycook developed the high-angle annular dark-field (ADF) technique which can provide an incoherent image showing both high image resolution and atomic number contrast.This paper describes the design and first results of a 300kV field-emission STEM (VG Microscopes HB603U) which has improved ADF STEM image resolution towards the 1 angstrom target. The instrument uses a cold field-emission gun, generating a 300 kV beam of up to 1 μA from an 11-stage accelerator. The beam is focussed on to the specimen by two condensers and a condenser-objective lens with a spherical aberration coefficient of 1.0 mm.

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