With the support of ray, diagrams explain how the images were obtained. Your explanation must include comments about the location and use of the objective and selected area apertures. A TEM consists of condenser lenses to focus the electron beam on the specimen, an objective lens to form diffraction in the BFP (Back focal plane) and the image in the image plane, and other lenses to magnify the image or the diffraction pattern on the screen. In order to obtain images in TEM, we either focus on the central spot (direct beam), or on the scattered electrons (diffracted beam). This is done by inserting an aperture (selected area aperture) into the BFP of the objective lens followed by selecting the appropriate beam. Bright field image (Figure 1) The given micrograph depicts a dark MgO crystal on a light and bright background. The topography on the face of the crystal is very clear. Such an image is called a bright field image and has a very high contrast. ...
The parts of the crystal in Bragg orientation appear dark, and the amorphous parts of the crystal are bright. The objective diaphragm is adjusted in such a way that an aperture appears in the back focal plane of the objective lens, allowing only the direct beam to enter and blocking the diffracted beam. The objective aperture, when inserted, controls the collection angle. The placement of the SAD (selected area aperture) is critical as it should be adjusted to obtain only the direct beam in this case. Darkfield image (Figure 2) The micrograph in figure two depicts a bright MgO crystal on a dark background. Such an image is obtained by selecting only the scattered electrons using a selected area aperture, enabling them to reach the image plane. The electrons that are not in the direct beam are selected to form a dark field image. The objective aperture is moved sideways to select the un-scattered electrons. This method is of high utility in case of observing certain specific crystallographic orientations of the specimen. The dark field image can also be obtained through another method, called centered dark field operation. In this case, the objective aperture is not shifted and the primary/direct beam is used instead. "The beam is tilted in order to allow only the scattered/diffracted electrons to go through the objective aperture (William and Carter 2009). Selected area diffraction pattern SAED (Figure 7) The given micrograph clearly depicts the symmetry of the lattice of MgO crystal through a selected area diffraction pattern. Selected area diffraction patterns are obtained by inserting the SAD aperture into the image plane of the objective lens and aperture on the optic axis in the middle of the viewing screen (William and Carter 2009).
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