文档介绍：Chapter 5 TEM imaging
Before 1960s, people can not make specimen thin enough to be electron transparent. The only type of specimen used for TEM observation is replica specimen.
Replica specimen can only be used for observation of surface morphology. It can not be used for observation of interior of materials.
In 1960s, Hirsch et al did TEM observed directly on the thin foil specimen. They developed diffraction contrast technique and theory which is based on the Bragg diffraction effect.
Using diffraction contrast technique you can not only observe the diffraction contrast image of material, but also obtain many other material information (. crystal structure, phase relationship, defect status etc.).
Contrast
Contrast (C) is defined in terms of the difference in intensity between two adjacent areas
In practice, your eyes can’t detect intensity changes <5%, and even <10% is difficult. So unless the contrast from your specimen exceeds 5-10% you won’t see anything on the screen or on the photograph.
If your image is digitally recorded, you can enhance low contrast electronically to levels at which your eyes can perceive it.
TEM image
TEM image is the result of interaction of incident electron beam with specimen.
When the electron beam go out from the low surface of the specimen, intensity of the transmitted electron beam changes due to the interaction of electron beam with the specimen. So the intensity of electron beam hit the screen is different. This electron image with non-uniform intensity is called contrast image.
Type of TEM contrast
Amplitude contrast
mass-thickness contrast
diffraction contrast (most often used)
Phase contrast
high resolution image
Z contrast
mass-thickness contrast
arises due to the difference in mass-thickness of materials.
mainly used for noncrystalline materials such as polymers and biological specimen.
dominates when specimen thickness is greater than 10nm.
mass-thickness contrast
mass-thickness contrast image of an extra