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As in CTEM, the image in EFTEM is produced by electron scattering in the specimen. However, EFTEM also utilises additional interactions which are not taken into account in CTEM.
In CTEM, the electrons used for imaging are selected only via the aperture diaphragm according to their scattering angles. Therefore, only electrons with a large scattering angle contribute to contrast generation.
The energy of the electrons and the difference in their energies remain unaccounted for, despite the fact that the bandwidth of the energy differences caused by the chromatic aberration of the objective lens has consider-able influence on contrast and resolution.
In EFTEM, the contrast is optimised by a special technique, e.g. by filtering out the contrast-reducing electrons from the spectrum of transmitted electrons, or by only allowing electrons containing specific information to be used for imaging. This method has no negative effects on resolution. Even unstained, thin specimens, frozen hydrated specimens or conventionally thick specimens can be imaged with superb contrast in the physically exact focus, which is the Gaussian focus.
By imaging energy selected electrons, new information can be obtained with contrasts such as the structure and element sensitive contrast. In addition, elemental distribution images with utmost lateral resolution are possible.
Image contrast by electron selection
EFTEM selects electrons not only according to their scattering angle, but also according to energy and energy bandwidth. The conditions for contrast generation are there-fore much better than in CTEM, particularly with specimens consisting of light elements.
The filtering out according to energy and energy bandwidth does not have an adverse effect on the resolution, unlike smaller lens aperture diaphragms.
Element information of scattered electrons
If specimen atoms are ionised during the inelastic scattering process, energy losses which are characteristic of the element will result. These appear as absorption edges in the energy loss spectrum.
Fine structures in the energy loss spectra convey information on adjacent atoms and permit chemical bonds to be analysed. This makes energy loss analysis ideal for the chemical characterisation of minute specimen structures.
The X-ray quantum hn generated during the inelastic scattering process is used for X-ray microanalysis (EDX) both in EFTEM and CTEM. | |
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