Materials Problems and Approaches for TEM and TEM/STEM Analyses - Sample Preparation Topic for Transmission Electron Microscopy - page 59

2.1 Macroscopic Characterization

Structural analyses begin at the macroscopic scale. This corresponds to what can
be observed with the naked eye alone or by using a magnifying lens. This type of
analysis helps to determine whether the material is bulk, porous, or in liquid phase;
homogeneous or heterogeneous; either mixed or with large grains.

2.2 Microscopic Characterization

Structural analyses at these scales can be carried out using optical and confocal
microscopy, as well as by scanning or transmission electron microscopy. They may
be combined with X-ray imaging and SIMS imaging.
Photon characterization (down to 400 nm) is used to identify morphology, e.g.,
the size of grains composing the material, their orientation, or their texture. Classic
optical microscopy techniques are used to observe the different phases. Crystalline
materials have a strong phase contrast related to their high scattering power (high
atomic number or Z ), and they can be directly observed. In the case of a poorly
diffusive material such as organic or biological materials, preliminary staining
often precedes observation in order to increase the optical density of phases to be
observed.
At these scales, observations under polarized light can clearly show the pres-
ence of grains (from 1

m) and their various orientations and can also highlight
a preferential orientation. The polarizing microscope is equipped with a polarizer
in the lighting system, which selects a polarization orientation, and a second rotat-
ing polarizer (called an analyzer), which selects the light rays once again based on
another polarization direction. When the polarizer and the analyzer are oriented at
a90 ◦ angle, which corresponds to phase-contrast imaging conditions (also called
“crossed Nichols” conditions), the crystal planes selectively diffuse the polarized
light depending on their orientation. Under these conditions, an amorphous mate-
rial that has uniform contrast (i.e., the same polarization color in all directions) can
be distinguished from a crystalline or polycrystalline material, which changes color
depending on its orientation to the light (Fig. 4.1) .
Confocal microscopy uses a highly convergent laser beam to produce images
with very little depth of field. Usually the laser beam scans the sample surface,
helping to improve resolution. By focusing the objective at different depth levels
in the sample, a series of images can be produced, from which a three-dimensional
representation of the specimen can be made. This can be used to make virtual cross
sections of the sample. Coupled with fluorescent labeling, this type of observation
allows for the in situ viewing of dynamic phenomena in live tissues.
For materials in solid-state physics, the overall crystalline structure of the sample
must be determined using X-ray diffraction or neutron diffraction techniques. These
analyses average the information of the analyzed sample over a volume of 1

µ

m 3 .
They provide information on all crystalline phases present in the material, but do

µ