scholarly journals The 1s-State Analysis Applied to High-Angle, Annular Dark-Field Image Interpretation—When Can We Use It?

2004 ◽  
Vol 10 (1) ◽  
pp. 4-8 ◽  
Author(s):  
Geoffrey R. Anstis
Keyword(s):  
Image Interpretation ◽  
Dark Field Image ◽  
Dark Field ◽  
High Angle ◽  
Atomic Column ◽  
Thermal Diffuse Scattering ◽  
Small Probe ◽  
Annular Dark Field ◽  
Electron Intensity ◽  
Thermal Diffuse

A small probe centered on an atomic column excites the bound and unbound states of the two-dimensional projected potential of the column. It has been argued that, even when several states are excited, only the 1sstate is sufficiently localized to contribute a signal to the high-angle detector. This article shows that non-1sstates do make a significant contribution for certain incident probe profiles. The contribution of the 1sstate to the thermal diffuse scattering is calculated directly. Sub-Ångstrom probes formed by Cs-corrected lenses excite predominantly the 1sstate and contributions from other states are not very large. For probes of lower resolution when non-1sstates are important, the integrated electron intensity at the column provides a better estimate of image intensity.

Preliminary Study of Domain Boundaries in Barium Titanate with High Angle Annular Dark Field(HAADF) Imaging

1991 ◽  
Vol 49 ◽  
pp. 940-941
Author(s):  
Feng Tsai ◽  
Shi-Yao Wang ◽  
John M. Cowley
Keyword(s):  
Dark Field ◽  
Ferroelectric Domain ◽  
Crystal Defects ◽  
High Angle ◽  
Thermal Diffuse Scattering ◽  
Angle Scattering ◽  
Domain Boundaries ◽  
Annular Dark Field ◽  
Preliminary Study ◽  
Thermal Diffuse

BaTiO3 have been studied for years by electron microscopists. Those studies have revealed the existence of 90° and 180° ferroelectric domain boundaries. The high angle annular dark field(HAADF) STEM technique is known to have strong sensitivity to atomic number Z. The signal is generated by the channelling of electrons along rows or planes and high angle scattering(mostly thermal diffuse scattering) from the atoms. Contrast in HAADF images can be given by crystal defects as a result of changes in channeling or changes in the high angle scattering process. This work reports a preliminary study of ferroelectric domain boundaries in BaTiO3 with HAADF imaging and a qualitative discussion of the observed domain wall contrast.

Stem Dark-Field Image Using A 200-kV AEM

1996 ◽  
Vol 54 ◽  
pp. 444-445
Author(s):  
T. Tomita ◽  
T. Honda ◽  
M. Kersker
Keyword(s):  
High Resolution ◽  
Field Emission ◽  
Thermal Field ◽  
Imaging System ◽  
Dark Field Image ◽  
Dark Field ◽  
Specimen Thickness ◽  
High Angle ◽  
Long Term Stability ◽  
Annular Dark Field

Interpretation of the high resolution transmission image typically requires simulation since the contrast changes in a complicated way due to changes in focus and specimen thickness. The contrast in images formed by collecting high angle forward scattered electrons in STEM does not change with changes in thickness or defocus.Until recently, high angle annular dark field (HADF) images were obtained only from instruments using cold field emission guns. Recently we have attempted to obtain HADF images using Schottky (ZrO/W(100)) thermal field emission and using a 200kV instrument designed as a comprehensive TEM/STEM. Advantages of the ZrO/W emitter are easy operation, very good short and long term stability, high brightness, and narrow energy spread. This microscope, The JEM2010F with thermal field emission, allows subnanometer analysis with EDS(spot, line, and mapping), EELS, holograms, etc, and has a standard TEM imaging system for high resolution imaging and for various diffraction modes, viz., CBED, selected area, Tanaka, etc.

Imaging dislocations with an annular dark-field detector

1992 ◽  
Vol 50 (2) ◽  
pp. 1224-1225 ◽  
Author(s):  
J. Liu ◽  
J. M. Cowley
Keyword(s):  
Supported Catalysts ◽  
Dark Field ◽  
Contrast Effects ◽  
Thermal Diffuse Scattering ◽  
Transmission Electron ◽  
Scanning Transmission ◽  
Annular Dark Field ◽  
Active Research ◽  
Thermal Diffuse

High-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) has been recently the subject of active research because of its successful applications to the characterization of supported catalysts, interface problems in MBE grown semiconductors, superconductors and X-ray multilayers. The characteristics of HAADF images are different from those of the TEM images. For perfect single crystals the HAADF signal is mainly generated from thermal diffuse scattering. HAADF technique has also been used to study dopant contrast effects in semiconductors and dislocations have also been observed with an ADF detector. In this paper we report a study of dislocation contrasts and their dependence on the inner collection angle of the ADF detector.The STEM instrument used for the observations was the HB5 from VG Microscopes, Ld., modified by the addition of an ultra-high resolution pole piece (Cs = 0.8 mm) and a two-dimensional detector system. Post specimen lenses and various beam stops were used to change the inner (α) and outer (β) collection angles of the ADF detector.

Local structure of Pb(Zr0.53Ti0.47)O3

2014 ◽  
Vol 47 (5) ◽  
pp. 1688-1698 ◽  
Author(s):  
K. Z. Baba-Kishi ◽  
A. M. Glazer
Keyword(s):  
Phase Boundary ◽  
Local Structure ◽  
Dark Field ◽  
Chemical Inhomogeneity ◽  
Monoclinic Space Group ◽  
High Angle ◽  
Cell Level ◽  
Bright Field ◽  
Atomic Column ◽  
Annular Dark Field

High-angle annular dark-field (HAADF) and annular bright-field (ABF) images recorded from the Pb(ZrxTi1−x)O3morphotropic phase boundary (PZTmpb) showB-site displacements along the 〈110〉 directions and prominent distortions in the oxygen cages surrounding both theBsites and the Pb environments. The measured range ofB-site displacements is about 0.25–0.4 Å. Oxygen cage distortions appear to be variable in shape and dimensions at the unit-cell level. Comparison of the observed displacements with the structural projections based on the established monoclinic space groupCm(Cs3) shows a good overall agreement. A qualitative match betweenCm(Cs3) and the reported observations is inconclusive because of inaccuracy in the measurements, originating from imprecise identification of atomic column centres inherent in the HAADF and ABF images. In most of the observed cases,B-site displacements in HAADF images, and oxygen cage distortions in ABF images, appear pronounced compared with the structural projections inCm(Cs3). Columnar chemical inhomogeneity has been commonly observed in bothB-site and Pb columns in PZTmpb. Weak 〈110〉 diffuse streaking along the [001], [110] and [111] zone axes has been imaged, suggestive of correlation with the systematic ion disorder along 〈110〉.

EM of rapidly solidified permanent magnets

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.

Simulation of high-resolution annular dark field STEM images

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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