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Topic: Transmission electron microscopy Mechanism  (Read 1517 times)

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Offline Bias Binte Kamal

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Transmission electron microscopy Mechanism
« on: January 21, 2025, 01:44:08 PM »
I'm having a bit of a problem understanding the principle here. When the incident electron beam passes through the sample, does it knock out the electrons of the specimen? Is it the specimen electron that reaches the screen?

Offline Corribus

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Re: Transmission electron microscopy Mechanism
« Reply #1 on: January 21, 2025, 05:04:03 PM »
In a nutshell, when a beam of electrons are focused on a sample (containing atoms), there is a chance that electrons in the beam *Ignore me, I am impatient* into electrons in the atoms. Therefore some electrons in the beam make it through without hitting anything, and others are deflected by collisions with the atoms.

Generally, in bright field mode you're looking at electrons that are transmitted through the sample without interacting with anything. Bright spots correspond to areas where lots of electrons hit the detector, i.e., areas where there is nothing to impede transmission of electrons from the gun, and dark spots correspond to the opposite, i.e., areas where atoms do impede transmission of electrons. Heavier elements have more electrons, so they're more likely to impede electrons. In Z-contrast mode (Z being a measure of the nuclear weight), the contrast correlates to the weight of the atom at the spot being measured.  In dark field mode you are looking at electrons that are scattered to the periphery rather than those that pass right through, so the bright and dark spots are reversed, with bright spots showing places where electrons are more likely to bounce off of something and dark spots showing where electrons passed direct through.

Note that are other contrast modes beside Z-contrast. E.g., phase contrast, which looks at slight difference in phases of scattered electrons. So, you can get lots of different information depending on what contrast mode you select.
What men are poets who can speak of Jupiter if he were like a man, but if he is an immense spinning sphere of methane and ammonia must be silent?  - Richard P. Feynman

Offline Bias Binte Kamal

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Re: Transmission electron microscopy Mechanism
« Reply #2 on: January 22, 2025, 02:25:32 AM »
Thank u

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