About Atomic Force Microscopy (AFM)

“Rome wasn’t built in one day.” – AFM took me much longer than a day to learn. (Actually I believe there is not a single microscopy allows a fast pass.) Thanks to my professor, Dr. Schniepp, and his graduate students, Sean and Laura, I am finally able to scan samples independently, as well as process images. But I am still at the stage of building up my experience.

One task before each scanning would be asking myself: what mode I should use to scan the particular sample. Different equipments provide multiple choices of modes, among which the fundamental ones would be contact mode and tapping mode.

As you can imagine, ‘contact’ means the tip touches the surface of sample. But let me remind you, in the microscopic world, when molecules get way too close to each other, they push each other away; when they are a little bit further, but not very far, they tend to be attracted to each other. When AFM is in contact mode, the tip is almost always in the former situation, which means both the tip and the surface experience repulsive forces. And the compression may lead to damages to both sides – the last thing we want. Even though we know the feedback system tries its best to keep the tip a particular distance from the surface, we cannot forget that when pressing too hard, both sides get ‘hurt’, especially when the sample is soft. (Imagine you press your finger on a balloon, the balloon will deform; and when you press too hard – boom.)

On the other hand, when we have very small fractures under the tip (I am talking about less-than-1 nm-small), the tip can drag the fractures along when they are in contact. The surface, as well as the image, can become a mess.

Thus, even if contact mode is straightforward and simple, we still need tapping mode to come in. The tip constantly oscillates when it scans over the sample. The cantilever could be maintained within the attractive regime, or, the amplitude could be larger that go down to the repulsive regime as well. Either way, tapping mode exposes softer sample to lower risk of being damaged.