Selected area aperture

For a long time, Selected-Area Electron Diffraction (SAED) performed with a parallel incident beam and a selected-area aperture was the only experimental method available. During the three last decades, new diffraction techniques based on a convergent electron incident beam (CBED Convergent-Beam Electron Diffraction, LACBED Large-Angle... 

Nevertheless, this technique has a main disadvantage the minimum size of the diffracted area, which is selected by means of the selected-area aperture, is about 500 nm. It becomes difficult to prevent some thickness variations and/or some orientation variations in the diffracted area. The SAED patterns are, in fact, average patterns and the diffracted intensities can be strongly affected. For that reason, it is recommended to use Microdiffraction or CBED because the diffracted area is directly defined by the incident beam and can reach a few nanometers with recent microscopes. 

Fig. 1. Photo and illustration of the HRTEM allowing acquisition of images of catalysts under working conditions (4). The microscope is equipped with an FEG, a quadrupole mass spectrometer (QMS), a Gatan image filter (GIF), and a Tietz F144 CCD for data acquisition. The differential pumping system consists of IGPs, turbo molecular pump units (TMP, MDP), and an oil diffusion pump (ODP). The differential pumping stages are set up by apertures inside the TEM column (denoted by black bars) at the objective lens (OL), the first (Cl) condenser aperture, the second (C2) condenser aperture, and the selected area aperture (SA).

For a local analysis of the crystal structure, i.e., for the study of individual grains or selected phases, a selected area aperture can be inserted. Only the sample area selected by the aperture contributes to the diffraction pattern (Fig. 1). A plane-wave illumination, i.e., the appearance of sharp diffraction spots, can thus be maintained (Fig. 4). This technique is called SAD.f The smallest area that can be selected is about 0.5 pm given by the smallest selected area aperture. 

For all compositions, the polyacetylene domains are crystalline as is revealed by X-ray diffraction. This is further confirmed by electron diffraction from thin microtomed sections of blends with polyacetylene compositions of 40% or higher. Micro-toming of samples with less than 40% polyacetylene without sample cooling is difficult due to the rubbery nature of the composite. The polycrystalline nature of the polyacetylene domains is established from the observation that even selected area aperture of a few hundred angstroms produces Debye ring patterns. 

The present method, as well as the LACBED method, does not use the condenser aperture, but uses the selected area aperture to determine the angular disk size, whereas the ordinary method and the many-beam LACBED method use the condenser aperture. Therefore, this method allows the observation of the specimen with less contamination even in specimens which are not sufficiently clean. 

Selected-area electron diffraction (SAD) is a basic TEM technique to obtain diffraction information from a part of the specimen. A selected-area aperture is inserted below the sample holder and in the image plane of the objective lens. Only the area selected by the aperture on the screen contributes to the SAD pattern. In case of polycrystalline specimens, if more than one crystal contributes to the SAD pattern, it can be difficult or impossible to analyze. As such, it is useful to select a single crystalline region for analysis at a time. It may also be useful to select two crystals at a time, in order to examine the crystallographic orientation between them. 

There are two major problems using SAD to obtain electron diffraction patterns from a nanomaterial area selection and signal-to-noise ratio (S/N). The selected area from the specimen is defined by the size of the selected-area aperture in the first image plane of the objective lens. This leads to an effective probe size of a few 100 nm, which makes SAD unsuitable for obtaining diffraction information from individual nanostrucmres. Even if it is possible to select an individual nanostructure to form a SAD pattern, the S/N of diffraction information from the nanostructure is very small. Moreover, if the microscope is not perfectly aligned, the selected-area... 

Experimentally, obtain a conventional CBED first by focusing a probe on the TEM specimen. Then move the TEM specimen up or down, a diffraction pattem-like image can be observed in the image mode. Use a selected-area aperture to choose either a BE or a DF spot as shown in the inset of Fig. 5.10c. The height change is determined by the size of the selected-area aperture, such that only one diffraction spot is visible inside the aperture. Finally, switch to the diffraction mode and use the largest condenser aperture or even take the condenser aperture completely out. An example of a LACBED pattern of Si (111) is shown in Fig. 5.10. 

In selected area electron diffraction (SAED) in the TEM, an aperture (the intermediate or selected area aperture) is used to select a region of the specimen for diffraction (for an example, see Fig. 5.146). A near-parallel beam of electrons illuminates the specimen. Generally, the region contributing to the pattern is several micrometers in diameter. This is a large area compared to that in STEM microdiffraction but very much smaller than that needed for normal x-ray diffraction. 

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