Selection of Preparation Techniques Based on Material Problems and TEM Analyses - Sample Preparation Topic for Transmission Electron Microscopy - page 177

6.3 Fine Particles
In materials science and in biology, fine particles may be composed of fibers,
platelets, powders, spheres, tubes, viruses, bacteria, or macromolecules. They can
be observed either after being deposited on a TEM support film using the fine par-
ticle dispersion technique or embedded in a resin in order to make a block that
can be handled and then sectioned using the ultramicrotomy technique. Some bulk
materials can be made into fine particles after crushing (for materials in solid-state
physics).
7 Selection Criteria Based on Material Properties
7.1 Based on the Physical State of the Material
Compact material : Does not present any particular difficulty, regardless of the
technique used: electrochemical, chemical, mechanical, or ionic.
Porous materials : Depending on the size of the porosity, it will either be consid-
ered as a dense bulk material if the pore size is nanometric or as a porous material.
In the latter case, it is important to fill the pores using infiltration before thinning
the material. When the material is made bulk, it can be thinned using tripod pol-
ishing and ultramicrotomy mechanical techniques or the FIB technique. The best
results will be obtained using a soft tripod polishing technique for hard materials,
ultramicrotomy or cryo-ultramicrotomy for soft materials, or by the FIB technique,
enabling us to maintain the internal porosities. For porous biological materials, it is
necessary to replace the air contained in the pores with water or a solvent, as is the
general case for hydrated materials.
Materials with a liquid solution (mainly biological materials) : This type of mate-
rial concerns all bulk or dispersed biological materials. The preparation of bulk
materials will require the elimination of water through dehydration, substitution
using a solvent, and then using an embedding resin before using the ultramicro-
tomy technique. One can also transform the water into solid phase in order to use
the cryo-ultramicrotomy or the freeze-fracture technique.
When the material is composed of fine particles in aqueous phase, it can contain
viruses, bacteria, or macromolecules (DNA, proteins, etc.). It can be observed either
directly after deposition onto a TEM support film or after the carbon support film
is treated. This treatment is positive ionization for depositing DNA and a negative
ionization for proteins. These fine particles can also be prepared using the frozen-
hydrated film technique and can be observed using cryo-TEM after cryo-transfer.
7.2 Based on the Chemical Phases in the Material
Knowing the chemical nature of the material, in particular the type of chemical
bonds and its chemical stability (from its phase diagram), helps to predict the sec-