Effects of Defocus on EELS
- Practical Electron Microscopy and Database -
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This book (Practical Electron Microscopy and Database) is a reference for TEM and SEM students, operators, engineers, technicians, managers, and researchers.
 

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It seems that the TEM imaging mode appears to allow the selection of a small specimen area (using selected area aperture) for microanalysis by EELS while a larger area of the sample is being illuminated. This might seem a useful way to get a spectrum from a small particle or interface when it is impossible to get a small enough probe. A user with a LaB6 instrument would be tempted to believe they could do analysis similar to an owner of an FEG (field emission gun) TEM. However, they would be incorrect. The image seen on the TEM screen is from the region of the EELS spectrum that has the lost electrons, on a thin sample, the zero loss. However, the electrons that have lost energy are focused by the TEM objective at a different locations due to its chromatic aberration (Cc). This means the image at the energy loss being observed in the spectrum will come from a different area of the specimen. Thus this method will always give a false result.

Quantitative analysis is almost impossible in the TEM imaging mode because the electrons with different energies are spread over areas of different size in the image. If the illumination is focused to a small area, or the specimen is not homogeneous, the intensity ratio between low-energy edges and high energy edges can be changed by as much as a factor of ~ 10 in the imaging mode simply by changing the focus of the objective lens. As a consequence, attempting to quantify the specimen composition without taking account of this effect is likely to be highly spurious. It is therefore almost always better to collect EELS spectra for quantitative analysis in the diffraction mode.

An exception occurs when the illumination is highly defocused (over tens of microns), the specimen is homogeneous over tens of microns and the size of the illuminated area 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 enter the aperture in error also due to chromatic aberration. Operating under these conditions is sometimes useful on contamination-prone specimens, or when one needs to spread the illumination over a large are to minimize radiation damage, while maintaining some spatial resolution or a high collection angle.

 

 

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