The Different Observation Modes in Electron Microscopy (SEM, TEM, STEM) - Sample Preparation Topic for Transmission Electron Microscopy - page 37

2.4 Signals Used for Structure

2.4.1 Transmitted Electrons: Thin Samples with Thickness <100 nm

When the sample is thin, the energetic electrons will cross over the sample after
undergoing elastic scattering without energy loss and inelastic scattering with
energy loss. A small portion of them are absorbed. Elastically transmitted electrons
are used by the TEM (Fig. 3.3) to produce structural imaging, crystallography, and
spectroscopy (see Section 5) . Inelastically transmitted electrons are used to perform
various chemical analyses.

Incident Electron
Beam

Transmission Electron Microscopy

Thin Slice Sample

Image and Diffraction Modes
Structure, Crystallography, Defects
Site Locations

Inelastically Scattered
Electrons

Unscattered
Electrons

Elastically Scattered
Electrons

Chemical Analysis Modes
EELS Chemical Composition
EELS Chemical Bond Transmitted Electrons

Fig. 3.3 Signals resulting from the electron-matter interaction with a thin sample in transmission
electron microscope

After elastic scattering ( E 0 energy), the incident beam may either be diffracted
according to Bragg's law (2 d sin ) by the atomic planes of the crystalline

material or scattered by an amorphous material.
The interference between scattered and diffracted electrons coming from the
object interacting with the incident beam through an electromagnetic lens results
in the formation of images containing structural information on the specimen down
to the atomic or molecular scale. These images can be obtained using a TEM, a
TEM/STEM, and a STEM.

θ = n λ

3 Microscopes and Observation Modes

3.1 Illumination Sources

An electron gun is composed of a hot cathode at zero voltage, placed in a tube with
a voltage varying around -50 eV in order to regulate the beam intensity (Wehnelt
cylinder).