Imaging mode setup
The first component to consider is apertures. There are a number of movable (adjustable) apertures in the TEM column.
There are two other apertures to be concerned with when imaging. The objective aperture is the first aperture situated below the sample plane. It is used to increase contrast and is introduced into the column for bright field and dark field imaging.
Sitting below the sample, the objective aperture is designed to improve the contrast of the image since it excludes scattered electrons that serve to reduce contrast.
There are a range of sizes to choose from: the smaller the aperture, the greater the contrast and darker the image (because more electrons are excluded from the image).
Note that the greater the mass in a sample or the thicker the sample, the more the beam scatters as it passes through the sample.
For bright and dark field imaging, it is necessary to adjust the objective lens (focus knob) to produce a crisp image of what is in the plane of the sample (specimen). This means that the electron beam is adjusted so that the beam cross-over occurs at the level of the sample.
When undertaking diffraction imaging, the focus point drops below the sample to the back focal plane. So for diffraction imaging the cross-over of the beam needs to occur in the back focal plane. When this happens, the objective aperture, if left in place, is seen to be in focus. It becomes obvious as a hole in a black platform with a crisply focused edge.
Often, in diffraction mode, it is necessary to isolate a local region so that only this region produces a diffraction pattern. This is achieved by introducing the selected area diffraction (SAD) aperture into the column. When the selected area diffraction aperture is used to limit the area which is used for obtaining a diffraction pattern, this is called selected area diffraction (SAD).
The beam paths for the different imaging modes are shown in the diagram.
Dark Field imaging is achieved by blocking the unscattered beam and only allowing scattered electrons to form the image.
There are a few ways this can be achieved. Methods 1 and 2 involve putting the objective area into to column.
Method 1: (DF1 in the image) is where an objective aperture is introduced and moved so that it excludes the unscattered beam but allows some signal to pass through from a specific area of interest in the diffraction pattern, for example a specific intensity spot.
Method 2: (DF2 in the image) is where the beam is tilted so that the unscattered beam path is blocked by the objective aperture. In this way a particular intensity spot in a diffraction pattern can be centred as the new “main beam”. The dark field image will be produced only from those electrons being diffracted along this axis. The image is then focussed in the sample plane (on the sample).
Convergent Beam Electron Diffraction (CBED)
When the electron beam is converged on the sample to a point (method = convergent beam), instead of using a parallel stream of electrons through the sample, the diffraction pattern forms discs instead of points ("Focused Convergent" in the image below). These discs can contain detail that provides information about the crystal symmetry of the specimen.
For this type of imaging the focus (objective lens) is adjusted so that the sample is in focus (not the objective aperture).