Optimization of imaging conditions for composition determination by annular dark field STEM

Many functional materials are difficult to analyse by scanning transmission electron microscopy (STEM) on account of their beam sensitivity and low contrast between different phases. The problem becomes even more severe when thick specimens need to be investigated, a situation that is common for materials that are ordered from the nanometre to micrometre length scales or when performing dynamic experiments in a TEM liquid cell. Here we report a method to optimize annular dark-field (ADF) STEM imaging conditions and detector geometries for a thick and beam-sensitive low-contrast specimen using the example of a carbon nanotube/polymer nanocomposite. We carried out Monte Carlo simulations as well as quantitative ADF-STEM imaging experiments to predict and verify optimum contrast conditions. The presented method is general, can be easily adapted to other beam-sensitive and/or low-contrast materials, as shown for a polymer vesicle within a TEM liquid cell, and can act as an expert guide on whether an experiment is feasible and to determine the best imaging conditions.

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Effect of strain on ADF-STEM high-resolution images

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100087641 ◽

1991 ◽

Vol 49 ◽

pp. 666-667

Author(s):

M. Kelly ◽

D.M. Bird

Keyword(s):

High Resolution ◽

Strong Influence ◽

Intensity Variation ◽

Dark Field ◽

Atomic Resolution ◽

Strain Fields ◽

High Resolution Image ◽

Annular Dark Field ◽

High Resolution Images ◽

Interesting Alternative

It is well known that strain fields can have a strong influence on the details of HREM images. This, for example, can cause problems in the analysis of edge-on interfaces between lattice mismatched materials. An interesting alternative to conventional HREM imaging has recently been advanced by Pennycook and co-workers where the intensity variation in the annular dark field (ADF) detector is monitored as a STEM probe is scanned across the specimen. It is believed that the observed atomic-resolution contrast is correlated with the intensity of the STEM probe at the atomic sites and the way in which this varies as the probe moves from cell to cell. As well as providing a directly interpretable high-resolution image, there are reasons for believing that ADF-STEM images may be less suseptible to strain than conventional HREM. This is because HREM images arise from the interference of several diffracted beams, each of which is governed by all the excited Bloch waves in the crystal.

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Analysis of Dark-Field Images of Disordered Materials

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100096059 ◽

1972 ◽

Vol 30 ◽

pp. 560-561

Author(s):

J. M. Cowley

Keyword(s):

Amorphous Materials ◽

Careful Analysis ◽

Dark Field ◽

Minimum Diameter ◽

Statistical Fluctuations ◽

Bright Spots ◽

Sufficient Detail ◽

Random Arrangement ◽

Diffraction Patterns ◽

Imaging Conditions

Recently a number of authors have reported detail in dark-field images obtained from diffuse-scattering regions of electron diffraction patterns. Bright spots in images from short-range order diffuse peaks of disordered binary alloys have been interpreted as evidence for the existence of microdomains of ordered lattice or of segragated clusters of one component. Spotty contrast in dark field images of near-amorphous materials has been interpreted as evidence for the existense of microcrystals. Without a careful analysis of the imaging conditions such conclusions may be invalid. Usually the conditions of the experiment have not been specified in sufficient detail to allow evaluation of the conclusions.Elementary considerations show that even for a completely random arrangement of atoms the statistical fluctuations of density will give a spotty contrast with spots of minimum diameter determined by the dark field aperture size and other factors influencing the minimum resolvable distance under darkfield imaging conditions, including fluctuations and drift over long exposure times (resolution usually 10Å or more).

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Design of a new objective lens for an HB-501A STEM

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100086386 ◽

1991 ◽

Vol 49 ◽

pp. 416-417

Author(s):

Earl J. Kirkland ◽

Robert J. Keyse

Keyword(s):

High Resolution ◽

Spherical Aberration ◽

Scanning Transmission Electron Microscope ◽

Dark Field ◽

Objective Lens ◽

Specimen Holder ◽

Transmission Electron ◽

Scanning Transmission ◽

Annular Dark Field ◽

High Resolution Microscopy

An ultra-high resolution pole piece with a coefficient of spherical aberration Cs=0.7mm. was previously designed for a Vacuum Generators HB-501A Scanning Transmission Electron Microscope (STEM). This lens was used to produce bright field (BF) and annular dark field (ADF) images of (111) silicon with a lattice spacing of 1.92 Å. In this microscope the specimen must be loaded into the lens through the top bore (or exit bore, electrons traveling from the bottom to the top). Thus the top bore must be rather large to accommodate the specimen holder. Unfortunately, a large bore is not ideal for producing low aberrations. The old lens was thus highly asymmetrical, with an upper bore of 8.0mm. Even with this large upper bore it has not been possible to produce a tilting stage, which hampers high resolution microscopy.

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EM of rapidly solidified permanent magnets

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100121399 ◽

1992 ◽

Vol 50 (1) ◽

pp. 198-199

Author(s):

Raja K. Mishra

Keyword(s):

Melt Spinning ◽

Permanent Magnets ◽

Dark Field Image ◽

Dark Field ◽

Maximum Energy ◽

Quench Rate ◽

Maximum Energy Product ◽

Energy Product ◽

Annular Dark Field ◽

Rapidly Solidified

The discovery of a new class of permanent magnets based on Nd2Fe14B phase in the last decade has led to intense research and development efforts aimed at commercial exploitation of the new alloy. The material can be prepared either by rapid solidification or by powder metallurgy techniques and the resulting microstructures are very different. This paper details the microstructure of Nd-Fe-B magnets produced by melt-spinning.In melt spinning, quench rate can be varied easily by changing the rate of rotation of the quench wheel. There is an optimum quench rate when the material shows maximum magnetic hardening. For faster or slower quench rates, both coercivity and maximum energy product of the material fall off. These results can be directly related to the changes in the microstructure of the melt-spun ribbon as a function of quench rate. Figure 1 shows the microstructure of (a) an overquenched and (b) an optimally quenched ribbon. In Fig. 1(a), the material is nearly amorphous, with small nuclei of Nd2Fe14B grains visible and in Fig. 1(b) the microstructure consists of equiaxed Nd2Fe14B grains surrounded by a thin noncrystalline Nd-rich phase. Fig. 1(c) shows an annular dark field image of the intergranular phase. Nd enrichment in this phase is shown in the EDX spectra in Fig. 2.

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Simulation of high-resolution annular dark field STEM images

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s042482010013657x ◽

1995 ◽

Vol 53 ◽

pp. 40-41

Author(s):

E. J. Kirkland

Keyword(s):

Electron Beam ◽

Atomic Layer ◽

Fresnel Diffraction ◽

Dark Field ◽

Objective Lens ◽

Image Simulation ◽

Small Probe ◽

Annular Dark Field ◽

Stem Image ◽

Uniform Plane

In a STEM an electron beam is focused into a small probe on the specimen. This probe is raster scanned across the specimen to form an image from the electrons transmitted through the specimen. The objective lens is positioned before the specimen instead of after the specimen as in a CTEM. Because the probe is focused and scanned before the specimen, accurate annular dark field (ADF) STEM image simulation is more difficult than CTEM simulation. Instead of an incident uniform plane wave, ADF-STEM simulation starts with a probe wavefunction focused at a specified position on the specimen. The wavefunction is then propagated through the specimen one atomic layer (or slice) at a time with Fresnel diffraction between slices using the multislice method. After passing through the specimen the wavefunction is diffracted onto the detector. The ADF signal for one position of the probe is formed by integrating all electrons scattered outside of an inner angle large compared with the objective aperture.

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

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s042482010017520x ◽

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,

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

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100174448 ◽

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.

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