Physical and Chemical Mechanisms of Preparation Techniques - Sample Preparation Topic for Transmission Electron Microscopy - page 119

In TEM, this technique is only used to deposit polycrystalline gold particles on
the surface of a carbon TEM support film. These gold particles help in the calibration
of diffraction diagrams.
7.2.3 Vacuum
The size of the grains deposited depends on the quality of the chamber vacuum and
the type of plasma. Classic pumping contains a roughing pump combined with an
oil diffusion pump that creates a vacuum on the order of 10 -4 Pa.
This vacuum can contain the vapors of residual oil coming from the pump
that contaminate the chamber and especially the surface of the specimen during
deposition. Contamination can be reduced by placing a liquid-nitrogen trap above
the diffusion pump. Pumping using a turbomolecular pump does not have this
drawback, since there is no heated oil in this system.
Pumping begins at an atmospheric pressure and continues until a pressure of
10 -4 Pa is reached with the same pump.
This is the ideal system, but it can be used only in small-volume chambers,
otherwise pumping would take too long.
Cryo-pumping is used for some freeze-fracture devices. It is the cleanest vacuum,
allowing acquisition of pressures down to 10 -4 Pa. The ionic vacuum can also be
used if a very high vacuum from 10 -5 to 10 -6 Pa is needed.
7.2.4 Substrate
The surface of the substrate or the specimen must be very clean. The increase in
substrate temperature during deposition increases the kinetic energy and therefore
the mobility of the particles. The result is increased particle size. To prevent this,
specimen holders cooled with either water or liquid nitrogen can be used.
The crystalline state of the substrate surface can be used wisely to make epitaxial
coatings.
For fine particles, the dispersion liquid can contain wetting agents, salts, or other
compounds that impede the proper distribution of metallic particles.
Eliminating them is recommended insofar as possible.
7.3 Techniques Involving a Physical Deposition: Continuous or
Holey Thin Film, Contrast Enhancement by Shadowing or
Decoration, Replicas, and Freeze Fracture
Continuous or perforated thin film : To obtain a thin and self-supporting carbon film,
an NaCl single crystal is used as a deposition substrate. The gem salt must then be
dissolved on the surface with water. A freshly cleaved sheet of mica can also be
used. The film is unstuck by dunking/floating it on the surface of some water. The