Although an atomic force microscope (AFM) can be used to produce extraordinarily detailed surface plots, the underlying principles are extremely simple.
Basic Parts Of An AFM
An Atomic Force Microscope is simply an extremely small, sharp probe attached to a spring loaded cantilever that is moved over the surface of a sample. Light from a laser shining on the cantilever is detected by a photodiode and the signal interpreted by specialist electronics. The tip is normally made from a ceramic or semiconducting material but tips are now being made from carbon nanotubes.
Contact Mode and Tapping Mode
In order to ‘read’ the sample, the tip can be dragged smoothly over the surface or repeatedly tapped on the surface. Dragging the tip on the surface is knows as “Contact Mode” and tapping the surface is “Tapping Mode”. Another mode is also available, Non-Contact Mode. In non-contact mode the tip doesn’t actually touch the material surface but rather measures attractive forces between to sample and tip.
Figure 1. Basic parts of an Atomic Force Microscope
AFM tips are normally shaped like a pyramid and manufactured using microlithographic techniques. The sharper the tip, the better the resolution of the final image. The problem with these tips is that they can become snagged in narrow valleys in the sample and be broken, damaged or simply remain stuck. Tips are now being made from cylindrical carbon nanotubes that are much stronger, yet flexible and the shape of the tip helps them avoid getting snagged.
AFM tips also succumb to wear and over time will become blunt and require replacement as image resolution is lost.
Although AFMs are primarily designed to provide a 3D image of the surface of a sample, the tip can be used to move, bend or alter tiny objects or particles. This is known as nanomanipulation.
Advantages of Atomic Force Microscopes
When compared with electron microscopes, AFMs have two main advantages; the don’t require a vacuum and they give 3D images. Because AFMs don’t require a vacuum to operate samples can be imaged in air, or a required atmosphere or even under water. An electron microscope simply gives a 2D image or photo of the surface of an image. An AFM plot is a 3D topographical representation of the surface of the sample and can give additional information about the physical characteristics of the material like co-efficients of friction.