Artifacts in Transmission Electron Microscopy - Sample Preparation Topic for Transmission Electron Microscopy - page 136

Charging : If the charge dissipation is insufficient, the electrons and ions produced
under the electron beam cause the appearance of a localized current proportionate
to the intensity of the irradiation. This effect results in the instability of the sample
under the beam.
The effects are even greater if the beam is focused and the material is insulating,
i.e., if it does not allow the flow of electronic charges on the material via the support
grid and the specimen holder.
One way to reduce this effect is to deposit a carbon (or metal) film on the surface
of the sample.
Destruction : Sample destruction can be caused by the combined effects of
irradiation and temperature rise during observation.
Irradiation results in the degradation of the sample, the appearance of stresses
causing sample contraction, and the formation of cracks that eventually destroy the
sample.
The electron-matter interaction produces heat by plasmon deexcitation. The tem-
perature rise is proportionate to the current density of the beam and is inversely
proportionate to the thermal conductivity of the sample.
These effects can result in the amorphization of the material, crystallization,
phase demixing, and sample destruction. These effects can be reduced or eliminated
by the use of a liquid-nitrogen-cooled specimen holder.
During electronic irradiation, atomic displacement reduces when the inci-
dent kinetic energy increases, i.e., when the TEM acceleration voltage increases.
Therefore, at high energy, the effective cross section of electron interaction with the
matter is greater. The resulting elastic diffusion only leads to weak atomic displace-
ment. For most elements, the probability of atomic displacement is 100 times less
than the probability of all elastic interactions. Voltages ranging between 100 and
200 keV represent the best practical compromise for all materials in order to limit
irradiation damage.
The contrast of samples composed of light elements is even lower if the acceler-
ation voltage is high. Voltages ranging between 75 and 120 keV represent the best
practical compromise for organic materials.
Contamination : Contamination occurs under the action of the electron beam and
is caused by the combustion of hydrocarbons present in the TEM column or on
the surface of the sample. Hydrocarbons essentially come from oil vapors from the
distribution pumps. The hydrocarbon deposits formed are composed of carbon and
are more significant when working under a focused probe and an intense probe.
Contamination is greatest in a STEM. It results in the formation of dark zones that
cover the irradiated portion. Contamination is reduced by an anti-contamination cold
trap placed near the sample, in the air gap of the objective lens. This contamination
is irreversible and introduces a carbon signal into the chemical analysis. It increases
when the TEM vacuum decreases.
In cryomicroscopy, the specimen is very cold and can be contaminated very
quickly. In this case, the anti-contamination trap must be placed very close to the
specimen.