What can you see using a Transmission Electron Microscope?

A transmission electron microscope (TEM) is something that allows you to see really tiny things. You can even use it to see atoms (the stuff everything is made of). Instead of using light to view things, TEMs use electrons to view things.
In a TEM, a beam of electrons is sent through a very thin sample. Some of those electrons pass directly through and produce an image based on how the electrons are absorbed by the sample. You can actually see atoms under the right conditions (they kind of look like ping pong balls).

Twin boundaries in a CZTSe grain

Some of the electrons are diffracted. The electrons hit an ordered array of atoms (think of how you have seen fruit or balls packed together) and, maintaining the same amount of energy, bounce off in such a way that scattered electrons are sent to specific positions related to the planes of atoms in the array. Here is an image of this boundary under this mode

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 You can even look at the image associated with a specific diffraction plane.

Later, I will talk about how a TEM works.
(Citation: Samples from Scarpulla Research Group at University of Utah)