Advantages of modern field emission Electron Microscopes for characterization of catalytic materials

1995 ◽  
Vol 53 ◽  
pp. 402-403
Author(s):  
L. F. Allard ◽  
S. A. Bradley ◽  
E. Völkl ◽  
S. J. Pennycook
Keyword(s):  
Field Emission ◽  
Collection Efficiency ◽  
Energy Dispersive Spectrometer ◽  
Dark Field ◽  
Supported Metal Catalysts ◽  
X Rays ◽  
Catalytic Materials ◽  
Scanning Transmission ◽  
Annular Dark Field ◽  
Electron Microscopes

The ability to characterize the structure of a variety of catalytic materials, especially supported metal catalysts, has been dramatically enhanced in recent years by the improved electron microscopy capabilities provided by the new generation of field emission gun transmission (FE-TEM) and dedicated scanning transmission electron microscopes (d-STEM). Although the reciprocity theorem says that a TEM and a dedicated STEM have equivalent, but reciprocal, electron optical geometries so that imaging conditions in one instrument can, in principal, be duplicated in the other, in reality the TEM and d-STEM offer column geometries that are optimized for different kinds of analytical output. For example, the d-STEM permits introduction of an energy dispersive spectrometer, which collects x-rays at a solid angle of 0.2 sr (but at a low take-off angle). A high take-off angle detector in a TEM may give a collection angle of only 0.013 sr. This x-ray collection efficiency coupled with use of annular dark-field techniques makes the imaging and analysis of nanometer-sized metal clusters on supports more effective.

Nano-Sized Catalytic Materials Investigated by a STEM-Based Mass Spectroscopic Technique

1997 ◽  
Vol 3 (S2) ◽  
pp. 443-444
Author(s):  
J. C. Yang ◽  
A. Singhal ◽  
S. Bradley ◽  
J. M. Gibson
Keyword(s):  
Bimetallic Catalyst ◽  
Structural Information ◽  
Scanning Transmission Electron Microscope ◽  
Dark Field ◽  
Spectroscopic Technique ◽  
Carbon Substrate ◽  
Support Material ◽  
Catalytic Materials ◽  
Scanning Transmission ◽  
Annular Dark Field

Knowledge of catalysts' sizes and shapes on their support material is crucial in understanding catalytic properties. With increasing interest in nanosized catalytic materials, it is vital to obtain structural information at the nanometer level in order to understand their catalytic behavior. We have recently demonstrated that very high angle (˜100mrad) annular dark-field (HAADF) images in a dedicated scanning transmission electron microscope (STEM) can be used to quantitatively measure the number of atoms of individual nano-sized clusters on a support material We are presently applying this technique to a bimetallic catalyst, PtRu5, where our data suggest that the shape of the PtRu5 particle is, surprisingly, oblate on the carbon substrate.PtRu5 is of interest for methanol oxidation for applications in batteries. PtRu5 compounds were produced by a molecular precursor method. Imaging was performed on a Field Emission Gun (FEG) Vacuum Generators HB501 STEM operated at 100kV.

Materials analysis by aberration-corrected STEM

2004 ◽  
Vol 839 ◽  
Author(s):  
Ondrej L. Krivanek ◽  
Neil J. Bacon ◽  
George C. Corbin ◽  
Niklas Dellby ◽  
Andrew McManama-Smith ◽  
...  
Keyword(s):  
Spherical Aberration ◽  
Dark Field ◽  
Optical Aberration ◽  
Transmission Electron ◽  
Scanning Transmission ◽  
Annular Dark Field ◽  
Powerful Research Tool ◽  
Electron Microscopes ◽  
Electron Energy Loss ◽  
Aberration Corrected

ABSTRACTElectron-optical aberration correction has recently progressed from a promising concept to a powerful research tool. 100–120 kV scanning transmission electron microscopes (STEMs) equipped with spherical aberration (Cs) correctors now achieve sub-Å resolution in high-angle annular dark field (HAADF) imaging, and a 300 kV Cs-corrected STEM has reached 0.6 Å HAADF resolution. Moreover, the current available in an atom-sized probe has grown by about 10x, allowing electron energy loss spectroscopy (EELS) to detect single atoms. We summarize the factors that have made this possible, and outline likely future progress.

An Efficient Annular Dark-Field Detector Capable of Single-Electron Counting Adapted to a High-Resolution Field-Emission SEM

1988 ◽  
Vol 46 ◽  
pp. 190-191
Author(s):  
R. Reichelt ◽  
U. Aebi ◽  
A. Engel
Keyword(s):  
High Resolution ◽  
Electron Energy ◽  
Field Emission ◽  
Major Elements ◽  
Dark Field ◽  
Primary Electron ◽  
Transmission Electron ◽  
Annular Dark Field ◽  
Electron Microscopes ◽  
Scanning Electron Microscopes

Various high resolution scanning electron microscopes (HRSEM) are now commercially available providing probe sizes in the range of 0.5 to 1.5 nm at 30 keV due to their field emission gun 1.2. Equipped with efficient detector systems (which collect different signals and applied to specifically prepared samples) HRSEM challenge the conventional transmission electron microscope (TEM) with high resolution surface images of biological specimens collecting secondary (SE) or backscattered (BSE) electrons. However, the yield of (SE) carrying high resolution information is rather small, i.e. the SE-I yield at 20 keV primary electron energy amounts to < 1% for the major elements (H; C; N; O; P) constituting biological matter. The yield of BSE is greater than the corresponding total SE yield (electron energy >15 keV), but BSE emerge due to high angle elastic scattering from a surface area with a diameter of typically 30% of the deepest electron penetration R (e.g. R≈10 μm for elements mentioned above at 30 keV).

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.

scholarly journals Quantification of critical particle distance for mitigating catalyst sintering

2021 ◽  
Vol 12 (1) ◽  
Author(s):  
Peng Yin ◽  
Sulei Hu ◽  
Kun Qian ◽  
Zeyue Wei ◽  
Le-Le Zhang ◽  
...  
Keyword(s):  
Carbon Black ◽  
Dark Field ◽  
Critical Distance ◽  
Supported Metal Catalysts ◽  
Carbon Supports ◽  
Thermally Induced ◽  
Surface Areas ◽  
Scanning Transmission ◽  
Annular Dark Field ◽  
Supported Metal

AbstractSupported metal nanoparticles are of universal importance in many industrial catalytic processes. Unfortunately, deactivation of supported metal catalysts via thermally induced sintering is a major concern especially for high-temperature reactions. Here, we demonstrate that the particle distance as an inherent parameter plays a pivotal role in catalyst sintering. We employ carbon black supported platinum for the model study, in which the particle distance is well controlled by changing platinum loading and carbon black supports with varied surface areas. Accordingly, we quantify a critical particle distance of platinum nanoparticles on carbon supports, over which the sintering can be mitigated greatly up to 900 °C. Based on in-situ aberration-corrected high-angle annular dark-field scanning transmission electron and theoretical studies, we find that enlarging particle distance to over the critical distance suppress the particle coalescence, and the critical particle distance itself depends sensitively on the strength of metal-support interactions.

Description of a New High Resolution Scanning Transmission EM

1972 ◽  
Vol 30 ◽  
pp. 418-419
Author(s):  
Michael Beer ◽  
J. W. Wiggins ◽  
David Woodruff ◽  
Jon Zubin
Keyword(s):  
High Resolution ◽  
Field Emission ◽  
Energy Dispersive Spectrometer ◽  
Scanning Transmission Electron Microscope ◽  
Objective Lens ◽  
Condenser Lens ◽  
Transmission Electron ◽  
Pattern Size ◽  
Scanning Transmission ◽  
Under Construction

A high resolution scanning transmission electron microscope of the type developed by A. V. Crewe is under construction in this laboratory. The basic design is completed and construction is under way with completion expected by the end of this year.The optical column of the microscope will consist of a field emission electron source, an accelerating lens, condenser lens, objective lens, diffraction lens, an energy dispersive spectrometer, and three electron detectors. For any accelerating voltage the condenser lens function to provide a parallel beam at the entrance of the objective lens. The diffraction lens is weak and its current will be controlled by the objective lens current to give an electron diffraction pattern size which is independent of small changes in the objective lens current made to achieve focus at the specimen. The objective lens demagnifies the image of the field emission source so that its Gaussian size is small compared to the aberration limit.

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

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.

High spatial resolution x-ray microanalysis of interfaces

1995 ◽  
Vol 53 ◽  
pp. 284-285
Author(s):  
J. R. Michael
Keyword(s):  
Spatial Resolution ◽  
Electron Probe ◽  
Solid Angle ◽  
Collection Efficiency ◽  
Spatial Scales ◽  
Energy Dispersive Spectrometer ◽  
X Rays ◽  
X Ray ◽  
Probe Size ◽  
Brightness Field

X-ray microanalysis in the analytical electron microscope (AEM) refers to a technique by which chemical composition can be determined on spatial scales of less than 10 nm. There are many factors that influence the quality of x-ray microanalysis. The minimum probe size with sufficient current for microanalysis that can be generated determines the ultimate spatial resolution of each individual microanalysis. However, it is also necessary to collect efficiently the x-rays generated. Modern high brightness field emission gun equipped AEMs can now generate probes that are less than 1 nm in diameter with high probe currents. Improving the x-ray collection solid angle of the solid state energy dispersive spectrometer (EDS) results in more efficient collection of x-ray generated by the interaction of the electron probe with the specimen, thus reducing the minimum detectability limit. The combination of decreased interaction volume due to smaller electron probe size and the increased collection efficiency due to larger solid angle of x-ray collection should enhance our ability to study interfacial segregation.

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,

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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