Artifacts in Transmission Electron Microscopy - Sample Preparation Topic for Transmission Electron Microscopy - page 135

Techniques involved : direct replica, indirect replica, freeze fracture, and deco-
ration shadowing, “positive-staining” contrast.
Fusion : Change in solid-liquid physical state of a solid sample.
It can be caused by an increase in temperature during cryogenic preparation.
Techniques involved: cryo-fixation, substitution-infiltration-embedding in cry-
ogenic mode, and frozen hydrated film.
Phase transformation : Total or partial transformation of the crystallographic
and/or chemical structure of a sample, which can be caused by thermal
effects during physical deposition.
Techniques involved: direct replica, indirect replica, decoration-shadowing,
and “negative-staining” contrast.
Frost : Condensation of atmospheric water vapor on a very cold thin slice.
Techniques involved: cryo-ultramicrotomy and frozen hydrated film.
3 Artifacts Induced During TEM Observation
The investigation of microstructure using electron microscopy involves the interac-
tion of the electron beam with the sample. This interaction irradiates the sample,
creates charges in the slice, and can produce heat. The result is physical or chemical
changes or the destruction of the material. All of these effects are a function of the
acceleration voltage, the dose of electrons received, the size of the zone irradiated,
and of course, the nature of the sample. The different types of degradation include
artifacts caused by observation and secondary damage caused by thermal effects
during observation.
3.1 Artifacts Not Linked to Thermal Damages
Dehydration : The principal effect of the vacuum in the microscope is the instanta-
neous dehydration of materials in liquid solution or hydrated materials. We cannot
work directly on these samples; they must either be immobilized or the water must
be extracted.
On samples that are more lightly hydrated and chemically unstable (polymers,
biological materials, non-stoichiometric oxides, etc.), the vacuum combined with
irradiation can lead to the loss of light elements in a structure. This subsequently
leads to phase transformations, resulting in significant changes in shape, which may
ultimately result in the complete loss of structure.