Atomic Force Microscopy: a beginners guide to nano-scale surface imaging

I wanted to give a brief description about Atomic Force Microscopy as I wanted to give people a bit of easily consumable scientific info.

What is it exactly?

Atomic Force Microscopy or AFM for short is a surface imaging technique, in which You basically drag a very pointy needle across the surface of the sample and move the sample in such a way, as to keep the needle from moving up or down. If we record the movements of the sample, we can generate 3D image of the surface. Of course, this is a very simple explanation, and I'll try to elaborate.

The Physics behind AFM.

AFM uses attractive Wan-der-Waals (a type of attractive force between molecules) and repelling electromagnetic forces between the needle and the surface. As a result, the needle is attracted to the surface, when the distance is large enough, but repelled from the surface, when it is close enough.

How it works?

The needle has a typical point radius of 2-10 nm and is attached to a cantilever. The other end of the cantilever is attached firmly to the microscope frame. When we drag the needle across the surface of the sample, the cantilever bends in order to account all the bumps and valleys on the surface of the sample (the sample roughness). To detect the bending of the cantilever, we shine a laser on to the needle end of the cantilever, which bounces of and then hits a set of photodiodes. The cantilever bends, the reflected laser beam hits the photodiodes in a different place. BOOM, we can detect the bending. A final peace of the puzzle is moving the sample, because in AFM the cantilever/laser side of the microscope is stationary. To achieve this, the sample is placed on a movable sample stage, capable of very fine movements. By moving the stage, we can keep the spot of the reflected laser beam in the same place. All this allows us to measure the surface in all 3 axes: x, y and z.

What can we do with it?

With AFM, we can image the surface of the sample, of course. However, this is only the simplest of options. By modifying the needle, we can measure adhesive, electric and magnetic properties to name a few. It is also a non-destructive method, which is all a plus.

Drawbacks of AFM.

As with anything, there are some drawbacks. The biggest one being we don't really get a "true" image of the surface, but a 3D model and that the surface needs to be relatively smooth in order to get a good result. The "true" image problem stems from the fact, that particles on the surface can be picked up in AFM as hills, not particles. This is because we don't really "see" the surface, but rather, we "feel" it.

If anyone is interested, there is a lot of literature available on the internet for further reading. If people will be interested, I’ll try to make more of these short introductions.