How stuff works: Atomic Force Microscope

Picture source: Wikimedia

Since the beginning of science itself, mankind has been driven to understand the nature of matter. During that difficult quest the building blocks of all matter in the universe was discovered - the atom. The atom was first postulated in 400BC by Democritus and later refined by Dalton in the early 1800s. As time went on, more complicated experiments actually showed that atoms were not the most fundamental building block after all. In fact atoms are made of a nucleus with sub-atomic particles called protons and neutrons, and this nucleus was surrounded by a cloud of electrons.

Mankind is now creating more and more complex technology, and as our technology gets better, it is generally getting smaller. We are now at a stage where understanding nanoscale properties of materials ("Nano" means about 1 billionth of a metre, or about 10 atoms across) is important for the future development of mankind itself. So how can we start to understand materials on this nanoscale? Well intuitively as humans we generally like to observe things, after all, seeing is believing. However, you can go to the shop today and buy the best light microscope that money can buy and I can promise you, you will not be seeing atoms anytime soon. In order to characterise materials on this nanoscale we need a new type of microscope.

Actually there are now a number of microscopes that allow use to see on the nanoscale. The one I will be talking about today is the "Atomic force microscope". The atomic microscope does not allow us to see using light or in fact any other type of electomagnetic radiation. Instead it essentially "feels" the surface. Imagine closing your eyes and using your hands to run over the back of an egg carton. You know there bumps on the surface of the egg carton because you run your hand across the surface and can feel the your hand going up and down. The atomic force microscope is doing something similar but on a much smaller scale. It consists of an atomically sharp tip which is mounted on a cantilever. The cantilever-tip system can be scanned across the surface of a material. As this occurs the tip feels features in the surface and essentially moves up and down with these features. The distance the tip moves up and down is detected using a laser which is reflected off the surface of the cantilever. As the cantilever moves up and down in response to the surface, the position of the reflected laser spot also moves and is converted into a voltage depending upon its position. This allows the mapping of features on a surface in the nanoscale, and in fact quite recently the atomic force microscope has been used to map individual atoms.