In spectroscopy, the projector lens crossover image is magnified and projected onto the camera. When properly adjusted, the observed CCD image consists of sharp vertical lines representing electrons of a particular energy. Line displacement in the horizontal (or dispersive) direction indicates a difference in energy between electrons.
The projector crossover is present under a typical transmission electron microscope (TEM) or scanning (S)TEM operating conditions (in some dedicated STEMs, a virtual crossover is formed). The movement of the crossover up and down in the column slightly depends on the exact operating mode. Still, you can easily compensate for this movement by making small adjustments to the pre‐prism quadrupole Focus X. Thus, you can tune the spectrometer for any operating condition of the microscope.
The spectra you acquire will correspond to the feature in the center of the microscope viewing screen; this is the area selected by the spectrometer entrance aperture when you lift the screen up. Depending on the operational mode, this area can be a precipitate image in TEM (e.g., a Bragg beam from a diffraction pattern). Three basic operation modes are possible: TEM Imaging, TEM Diffraction, and STEM, which will be discussed below.
Note: Due to the mechanical variability of the TEM, viewing screen, and spectrometer mount, the center of the TEM screen (or viewing camera) is not at the exact position of the spectrometer entrance aperture. This is normal and will not affect the operation of the system. For STEM electron energy loss spectroscopy (EELS) acquisition, the alignment of the ADF STEM detector and the entrance aperture is important. Detectors from Gatan should be concentric with the entrance aperture and are adjusted at installation. For detectors from 3rd party or TEM suppliers, please contact the appropriate organization for assistance.
With the TEM in the Imaging mode, an image is formed on the viewing screen, while the projector crossover contains a small diffraction pattern. Because the spectrometer projects an energy-dispersed diffraction pattern onto the detector, this mode is also known as diffraction‐coupled. The camera length \(L\) of the pattern in the projector crossover is given by
\(L = \frac{h}{M}\)
and the diameter of the projector crossover, \(d_{p}\), is
\(d_{p} = \frac{2 ß h}{M}\)
where
In practical terms, with \(h\) = 50 cm and at \(M\) = 10,000x, \(L\) = 50 μm. A diffraction pattern encompassing angles to 50 mrad is, therefore, only 5 μm in diameter at the projector back focal plane, and it becomes even smaller at larger microscope magnifications. This means you can attain good energy resolution in the TEM imaging mode while it accepts practically all the scattered electrons from a particular specimen area. If you require a smaller range of scattering angles, you can select this when you use the objective aperture.
Operating in the TEM imaging mode is convenient because you can easily modify the spectrum intensity via the condenser aperture's illumination setting and/or size. The selected specimen area is directly visible on the viewing screen, and you can change it by translating the specimen, shifting the image electronically, changing the microscope magnification, and/or changing the spectrometer entrance aperture. The collection efficiency can be very high in this mode. These characteristics make the TEM imaging mode ideal for the initial examination of any specimen.
Note: Only operate in TEM imaging mode for spectroscopy during the initial examination; otherwise, the analysis results may be misleading.
The TEM imaging mode, at first sight, appears to allow the selection of a small specimen area for microanalysis by the spectrometer. In contrast, a larger area of the sample is illuminated. This might seem useful to get a spectrum from a small particle or interface when getting a small enough probe is impossible. A user with a LaB6 instrument may believe they can do analysis similar to an owner of an FEG instrument. However, they would be incorrect. The image on the TEM screen is from the spectrum region with the most electrons on a thin sample, the zero-loss. Unfortunately, electrons that have lost energy are focused by the TEM objective at a different location due to their chromatic aberration (\(C_{c} \)). This means the image of the observed energy loss in the spectrum will come from a different area of the specimen. Thus, this method will always yield a false result, as shown below. Electrons that have been scattered by large angles and also suffer an energy loss will be displaced laterally in the image by a distance \(d\) given by
\(d = q\cdot \Delta f\)
where
The defocus error depends on the energy difference between electrons for which the objective lens was focused and the energy loss of electrons of interest. It is equal to
\(\Delta f = C_{c} \cdot \frac{\Delta E}{E_{o}}\)
where
Focus is normally done while you look at the image formed by the zero-loss electrons. For a 100 kV primary energy electron that lost 1000 eV while being scattered over 5 mrad, the displacement (referred back to the sample) amounts to 100 nm (\(C_{c}\) = 2 mm). Thus, to obtain information from areas smaller than 100 nm, you should use a small probe, as in the STEM mode.
In addition, quantitative analysis is made nearly impossible in the TEM imaging mode for the same reasons. Electrons of different energies are spread over areas of different sizes in the image. The intensity ratio between low and high-energy edges can change dramatically if the illumination area is too small or the specimen is not homogeneous. It is, therefore, almost always better to collect spectra for quantitative analysis in the diffraction mode unless the specimen is homogeneous over tens of µm and the illumination area size on the viewing screen is much larger than the spectrometer entrance aperture. In this case, the electrons that miss the spectrometer entrance aperture due to chromatic aberration will be replaced by a similar number of electrons that also enter the aperture in error due to chromatic aberration. Operation under these conditions is sometimes useful on contamination‐prone specimens or when one needs to spread the illumination over a large area to minimize radiation damage.
With the TEM in Diffraction mode, a diffraction pattern is formed on the viewing screen, while the projector crossover contains a small image of the illuminated specimen area. Because the spectrometer projects this energy-dispersed image onto the detector, this mode is also known as image‐coupled. If a small area of the sample is illuminated, the size at the projector crossover, \(d_{p}\), is given by
The magnification of the image at the projector crossover is given by
\(d_{p} = Md_{s}= \frac{h}{L}d_{s}\)
where
and you, therefore, determine the diameter in eV at the beam trap of the projector crossover by
\(D_{p} = d_{p}K_{s} = \frac{h}{L}d_{s}K\)
where
For example, a 3 μm diameter illuminated specimen area will give rise to a 3 μm projector crossover size for \(L\) = 70 cm and \(h\) = 70 cm. This permits better than 1 eV energy resolution at 200 kV. Therefore, the TEM Diffraction mode is useful to acquire spectra from large areas. However, you must carefully monitor the size of the illuminated area when you use a small camera length to ensure that the energy resolution does not worsen considerably.
The entrance aperture limits the spectrometer's angular acceptance range in the TEM diffraction mode. It can be varied when you change the camera length or select a different aperture size. The diffraction pattern on the viewing screen makes monitoring the specimen phase, thickness, and diffraction conditions possible. When you do not use a selected area diffraction aperture, the spectrum is collected from the whole illuminated specimen area, and the spatial resolution is therefore determined purely by the probe‐forming performance of the microscope. In this case, unlike in the imaging mode, chromatic effects do not appreciably change the collection efficiency of the spectrometer in the Diffraction mode. This makes quantitative chemical analysis of small sample areas much more reliable.
Diffraction mode is ideal for acquiring the high-quality spectra necessary to look for low-concentration elements or when you want to perform extended fine structure (EXELFS) analysis. As in channeling experiments, it is also useful to closely monitor the diffraction condition and/or collection angles.
In a correctly set-up STEM mode, a diffraction pattern is displayed on the viewing screen, and the projector crossover contains an image of the probe. Electron‐optically, this mode is, therefore, similar to the TEM Diffraction mode.
In the STEM mode, it is easy to image a large sample area even though the illumination is focused on a small probe (via the collection of a STEM image) and to position the probe accurately on the feature of interest (when you use the scan coils). This makes the STEM mode ideal for acquiring EELS data from precise areas of the sample (e.g., precipitates, interfaces, cell membranes).
When the STEM probe scans over large areas of the specimen, the probe motion will transfer to the spectrum, thus giving a motion of the energy loss peaks. This effect is not apparent at high magnifications but becomes increasingly important as the field of view increases. Descan coils are recommended to remove the motion of the probe in the energy loss spectrum.