Physical and Chemical Mechanisms of Preparation Techniques - Sample Preparation Topic for Transmission Electron Microscopy - page 120

film is recovered on TEM support grids (see Volume “Techniques”, Chapter 2 ,
Sections 13 and 14).
To deposit carbon on support grids coated with a thin polymer film (continuous
or holey), the classic Joule-effect evaporation techniques are used.
Contrast enhancement using shadowing or decoration is essentially used for
fine particle materials. To obtain a shadowing effect on the sample, an evapora-
tion angle of a few degrees with regard to the plane of the thin slice must be
used. The angle will be lower if the relief of the structure to be highlighted is low.
This requires a special setup in the evaporation chamber. Generally, two evapo-
ration heads are used. The first is used to carry out low-angle evaporation of the
metal in order to produce a shadowing effect and the second is used to reinforce
the film obtained by a layer of amorphous carbon evaporated perpendicular to the
surface. The use of masks enables two successive evaporations without disrupt-
ing the vacuum in the chamber. The specimen holder must be tiltable from the
outside.
The decoration technique uses either evaporation by the Joule effect of a polymer
or evaporation of gold. This latter technique highlights the intrinsic crystal organi-
zation of the object. The deposited atoms will select preferential sites and provide
indications of the surface state of the material.
7.3.1 Replica Techniques
Direct replica : The direct-replica technique requires the use of the heavy metal
shadowing technique followed by a carbon coating in order to form a continuous
film, constituting the print. In this case, the object is dissolved and the microscopic
investigation is conducted on this print.
Indirect replica : In this case, the initial object is replaced by a polymer mold,
which is then shadowed. A carbon film is deposited on it as in the direct-replica
technique. The polymer mold is dissolved and TEM observation is carried out on
the print of the mold.
Extractive replica : The extractive replica technique uses the evaporation of a
carbon film deposited onto the surface of the object in order to enable the extraction
of particles found there, enabling their observation in the microscope.
Freeze fracture : This is also a replica technique, but instead it is applied to the
fractured face of an object. It applies to bulk, soft, and hydrated materials or fine par-
ticle materials in liquid solution. It is conducted on a frozen sample. After fracturing,
and while the object is still cold, the shadowing technique is applied, followed by
the evaporation of a carbon film. Investigation under the microscope is carried out
on the print obtained after the sample is dissolved.
7.3.2 Contrast Enhancement by Physical Coating: “Negative-Staining”
Contrast
In the case of the negative-staining contrast enhancement technique, the stain will
not react chemically with the material, but create a dark background on which the