Selection of Preparation Techniques Based on Material Problems and TEM Analyses - Sample Preparation Topic for Transmission Electron Microscopy - page 192

The origin of this problem may be linked to the nature of the etchant or to the pres-
ence of heavy atoms in the material that are difficult to polish. The way to resolve
this problem is to complete the thinning of the thin slice using ionic thinning fol-
lowing chemical thinning, or by using a short chemical thinning after the ionic or
mechanical thinning.
11.2 Increasing Contrast
For materials composed of light atoms, e.g., polymers or biological materials, con-
trast can be increased by adding contrasting agents using the “positive-staining”
contrast technique. This will reveal the different phases.
For fine particles with a low atomic number, contrast can be increased using
“negative staining” or through shadowing. These techniques highlight the reliefs
and surface-morphology details.
11.3 Reducing Charge Effects
During TEM observation, if the charge dissipation is insufficient, as is the case for
insulating materials, the sample becomes unstable under the beam and observation
is not possible. The way to solve this problem is to coat the sample with a very thin
carbon film in order to eliminate these effects.
11.4 Limitation of Strain Hardening
For brittle or ductile materials, thinning techniques can result in strain hardening of
the sample. The way to limit strain hardening is to carry out a final ion milling or
FIB thinning, or chemical or electrochemical etching.
11.5 Removal of Surface Amorphization
Ion milling, and to a lesser extent FIB, creates amorphization of the surface layers,
which can reach a few nanometers and interfere with HRTEM observations. These
amorphous surface layers can be reduced by brief chemical etching; this is possible
principally for single-phase materials.
11.6 Removal of Surface Contamination
Following the electrolytic and electrochemical thinning preparations, a layer of sur-
face contamination may form. It can be eliminated after a final brief ion milling
process at low energy (100-500 eV, low-angle ions, sample cooled down with liquid
nitrogen).