Materials Problems and Approaches for TEM and TEM/STEM Analyses - Sample Preparation Topic for Transmission Electron Microscopy - page 77

from proteins provided by X-rays will be combined with that from TEM analyses
to refine the local structures of proteins in proteic complexes.
Protein or DNA crystals or virus films deposited in 2D are considered crystalline
materials. In this case, diffraction contrast can be used for imaging.
10.2.2 Structure of Bulk Frozen Samples
In the case of bulk frozen samples, a thin section obtained by cryo-ultramicrotomy
can be observed directly using cryomicroscopy in bright-field and low-dose mode
(low irradiation of electrons of maximal dose: 1 e - /Å 2 ) with a CCD camera at a
low light level. This method is the only one that allows access to the structure of a
biological material in its hydrated state.
11 Study of Properties
TEM can be used to investigate the local properties of a material in relation to the
ordered or amorphous structure in a bulk sample, fine particle, or a multilayer mate-
rial. Depending on the case, this requires a special specimen holder and/or specific
support grid in order to increase the temperature, to measure an electrical current (if
a stress must be applied), or if a particular atmosphere is needed.
11.1 Optical Properties
The local optical properties of a compound can be investigated using plasmons in
EELS to determine the dielectric constant of a compound. Thus, the local variations
of dielectric properties in an oxide can be highlighted near crystal defects such as
domain walls. This does not require a particular specimen holder.
11.2 Electrical Properties
The local electrical properties can be measured directly on grain boundaries in con-
ductive ceramics or bicrystal materials. This type of local measurement requires a
special specimen holder equipped with four current measurement points on both
sides of a grain boundary.
11.3 Electronic Properties
Local EELS analyses, EXELFS, and ELNES in particular are used to determine
the electronic structure of an atom in its environment. Thus, the proximity of an
interface or crystal defect disturbs the electronic structure (i.e., the chemical bonds)