Materials Problems and Approaches for TEM and TEM/STEM Analyses - Sample Preparation Topic for Transmission Electron Microscopy - page 70

improved by using higher voltages (300, 400, 800, and 1.2 MeV), allowing for
the use of thicker samples. The use of a FEG-equipped microscope enables a limit
resolution of 0.07 nm.
Due to a very small probe size of about 0.14 nm (a size on the order of inter-
atomic distances), the HAADF technique (or Z -contrast imaging) makes it possible
to view the chemical contrast at the atomic scale in TEM/STEM. This contrast is
proportional to the square of the atomic number ( Z 2 ). In Fig. 4.10c , the atomic
columns correspond to the columns of Sr and TiO; the oxygen columns are not vis-
ible. In this mode, the statistical error of the measurement of the crystal parameter
is approximately 2%. At the atomic scale, two types of atoms may be differentiated,
but similarly small crystal lattice variations cannot be measured.
7 Crystallographic Analysis
Crystallographic analysis consists of determining the orientation of a material and
its crystalline structure, i.e., the crystal lattice, symmetry group, and point group,
which contains all the symmetry elements of the crystal (position and nature of
atoms). Electron diffraction is used to identify a structure known ahead of time
through X-ray diffraction (SAED diffraction, nanodiffraction, and microdiffraction)
or to determine the crystal lattice and the symmetry class (microdiffraction) or the
symmetry class and the point group (CBED). LACBED enables the characteri-
zation of crystal defects and quantitative measurement of stresses in a material.
Figure 4.11 shows the different types of electron diffraction patterns for single
crystal, polycrystal and amorphous structures.
Whereas techniques involving the diffraction of X-rays or neutrons provide infor-
mation on atomic structure down to the scale of the micron, electron diffraction
is used to achieve smaller spatial resolution of the structural analysis at nanomet-
ric scales. Convergent-beam electron diffraction also enables the investigation of
the structure, the quantification of local parameter variations (near an interface, for
example), the characterization of crystal defects (1D, 2D, and 3D), and the localiza-
tion of defects using large-angle convergent beam mode. Determination of the local
structure using CBED is equivalent to that obtained using X-ray diffraction, with
the addition of an image of the associated structure. Thus, the diffraction techniques
simultaneously associated with the TEM image mode allows the identification of the
atomic structure or a defect of the material at the nanometric scale. In some cases
using filtered imaging modes, it is possible not only to locate the structural defects
at the atomic scale but also to measure the local crystal potential. This will be the
challenge in the upcoming years for the new FEG microscopes equipped with C s
aberration correctors, a beam monochromator, and an energy filter (located either
inside or outside the TEM column).
Structural analysis using electron diffraction is used to locally identify the struc-
ture or structural changes. With electrons, only heavy atoms with a periodic structure
are visible. The unambiguous determination of the structure of the sub-lattice con-