Materials Problems and Approaches for TEM and TEM/STEM Analyses - Sample Preparation Topic for Transmission Electron Microscopy - page 68

Fig. 4.8 TEM image of a
thin section of a neuron,
showing its organelles,
nucleus, mitochondria,
endoplasmic reticulum,
ribosomes, and intercellular
junctions (synapses)
( J. Boumendil CMEABG,
Université Claude
Bernard-Lyon 1
Villeurbanne )
The tomography technique consists of making a series of eucentric TEM images
by tilting the sample from +70 ◦ to -70 ◦ around an axis. This technique is used
in conjunction with very powerful software to make 3D reconstructions of cellular
organelles.
Textured materials (bone, tooth, collagen, etc.) are considered to be poorly orga-
nized or crystalline materials. These types of materials are observed in diffraction
contrast.
For isolated macromolecules , analysis is performed in either bright-field or dark-
field imaging mode, depending on the case. It is sometimes coupled with molecular
modeling to build a 3D reconstruction of these systems. These analyses are generally
made in cryomicroscopy (see Section 10.1) .
6.2 Atomic Structure
These analyses are generally applicable to crystalline materials as well as to certain
poorly organized materials. They also apply to crystal defects such as disloca-
tions, stacking faults, grain boundaries, interfaces, etc. The HRTEM, Z -contrast
(HAADF), and STEM modes enable this type of investigation. These analyses
require rigorous sample orientation in order to achieve diffraction of the atomic
planes. In the case of poorly organized or amorphous materials with a local crys-
talline order, no sample orientation is required since all orientations are present at
the same time in this type of sample. They often only yield lattice fringes (Fig. 4.9) .
The atomic structure of phases or grain boundaries and interfaces may be deter-
mined by HRTEM. The optimum resolution of HRTEM varies depending on the
microscope, accelerating voltage, and spherical aberration of the objective lens. Two
types of high-resolution images can be produced, resulting in either an intensity
image (Fig. 4.10a ) that is obtained directly or a phase image that is calculated from