The Different Observation Modes in Electron Microscopy (SEM, TEM, STEM) - Sample Preparation Topic for Transmission Electron Microscopy - page 46

few nanometers in size using the transmitted elastic and inelastic electron imaging
modes (HAADF and EELS).
The purpose of the STEM as an accessory to the TEM is to be able to bring
together all of the options for analyzing a sample with good resolution in one
device. The STEM has the same functions as the TEM but with more incident beam
deflection coils. This makes it possible to scan the specimen with the probe.
In a TEM/STEM microscope we can, in parallel, detect X-rays emitted by the
sample and use the inelastic transmitted electrons for chemical analysis and imaging
(Fig. 3.9) using EDS and PEELS detectors.
5 Different TEM Observation Modes
5.1 Origin of Contrast
The electron beam is characterized by a wave-particle duality of the electrons. The
passage of the beam through the sample results in electron transmission deficiency
and a phase change of the exit wave. Depending on the type of sample, one of two
phenomena is predominant. In amorphous samples, the absorption phenomenon is
dominant, whereas in crystalline samples, phase change is essential.
Absorption Contrast (Fig. 3.10) : Elastic diffusion electron transmission increases
the opening angle of the incident electron beam, which is initially parallel. By plac-
ing a so-called contrast aperture at the exit surface of the specimen the extremely
high-angle scattered electrons are eliminated, keeping only the electrons that are
transmitted practically without scattering. It is then possible to discriminate two
points on the sample that have different scattering powers (either the nature of
the constituent atoms or the number of these atoms and consequently the local
thickness).
Fig. 3.10 Introduction of
contrast using an objective
aperture
The basis for viewing structures in any “amorphous” specimen is absorption con-
trast. An example of this includes samples of polymers or biological materials that
are essentially composed of light carbon, hydrogen, oxygen, and nitrogen atoms. In