Selection of Preparation Techniques Based on Material Problems and TEM Analyses - Sample Preparation Topic for Transmission Electron Microscopy - page 181

properties. They are often prepared using mainly chemical or electrochemical
methods in order to prevent any mechanical-type artifacts.
8 Selection Criteria Related to the Type of TEM Analysis
The type of analysis to be performed will condition the selection of one preparation
over another. Selection criteria are ultimately related to the following question that
must be answered: Is it an investigation of topography, structure, structural defects,
crystallography, chemical composition, or chemical bonds or is it an investigation
of physical, chemical, and functional properties of the material?
Topography Investigation : When a bulk material cannot be thinned, replicas of
its surface can be made using direct- or indirect-replica techniques in order to inves-
tigate its topography using CTEM. For hydrated organic materials, the internal
topography of the material can be investigated using freeze fracture.
Structural Analysis : Organization and Structure, Interface, and Volume: To inves-
tigate microstructure, a sample preparation technique must be used that allows all
the components to remain in place until the thin slice is made. Mechanical thin-
ning techniques such as tripod polishing, ultramicrotomy, ion bombardment, and
FIB make this possible. The investigation of a single crystal atomic structure can
be done using the simplest technique, e.g., crushing, which will yield a statistic of
its crystal projections. In biology, only the ultramicrotomy technique can be used to
obtain a thin slice.
Crystallographic Analysis : Crystallographic analyses can be made following
electrochemical, mechanical, and ionic preparations. However, the best diffraction
patterns are obtained on samples prepared with mechanical preparations such as tri-
pod polishing, ultramicrotomy, or FIB thinning, due to the constant thickness of the
slice. Under these conditions, qualitative analysis of the stresses at the interfaces
can be performed and combined with the crystallographic structure and possible
quantitative chemical analyses.
The material can have an orientation dictated by its processing. Depending on
the need to investigate longitudinal plane orientation in cross section with regard to
the preferential orientation or a particular crystallographic direction, bulk samples
have to be pre-oriented; fine particles must be pre-embedded before thinning. This
problem is not present for fine particles, where the orientation is random. As a con-
sequence, due to the statistical distribution of all of the particular orientations on the
support film, crystallographic analysis is possible.
Crystal Defect Analysis : The investigation of crystal defects dictates a particular
orientation of the thin slice so that the defects can be oriented. In the case of thin
films or heterostructures, the full structural analysis of the interfaces and growth
defects requires the preparation of a longitudinal-plane section in order to identify
the interfaces in the film plane and a cross section to identify the interfaces between