A short introduction to Atomic Force Microscopy

The first AFM microscopy was invented by IBM scientists Gerd Binnig, Calvin Quate, and Christoph Gerber in 1985 (Binnig et al., 1986). The AFM built upon its precursor, the Scanning Tunnelling Microscope (STM), which was developed by IBMs Gerd Binning and Heinrich Rohrer in the early 1980s and earned them the 1986 Nobel Prize for Physics (Binnig & Roher, 1983).

The breakthrough came when they replaced the conducting tip used in STM with a sharp probe attached to a flexible cantilever. This probe could interact with the sample surface through various forces, such as van der Waals forces, without the need for a conductive sample. As the probe scanned across the surface, its motion was detected by monitoring the deflection of the cantilever. By precisely controlling the position of the probe and measuring its interactions with the sample, AFM could generate detailed images of surfaces at the atomic scale (Binnig & Quate, 1986)

The first AFM images were obtained in 1986, showcasing the technique’s ability to resolve individual atoms on a surface. Since then, AFM has undergone continuous refinement, with advancements in probe technology, control mechanisms, and data analysis techniques, further improving its resolution and versatility. Today, AFM is a widely used tool in various scientific fields, providing invaluable insights into the structure, properties, and behaviour of materials at the nanoscale.

How it works

The principle of AFM is closely related to STM, Where a very sharp tip scans off the sample surface. Whereas, STM creates topographic images based on tunnelling effects between the surface and the probe tip, in AFM the tunnelling tip is replaced by a force sensitive cantilever (Giessibl, 2003). The AFM works on the principle of attractive and repulsive forces between atoms, often described with the Lennard-Jones potential. As the AFM scans across the surface, any variations in topography will result in bending of the cantilever as a consequence of these attractive or repulsive forces. An incident laser-beam reflected off the flat top of the cantilever will record any deflection of the cantilever on a position sensitive photodiode detector, thus creating a topographic

image of the sample surface. AFM analysis can be operated in three different modes. In Contact mode, the distance between the AFM tip and the sample is very short, which repels the cantilever. This, results in a very high resolution. However, the AFM probe might crash into the crystal surface, damaging the sample and increasing wear of the AFM tip. In Non-contact mode, the AFM tip hovers higher above the sample surface, attracting the cantilever to the surface. The resolution will be lower, but the sample and AFM tip will remain undamaged. And in Tapping mode the cantilever oscillates occasionally touching the sample surface. This will improve resolution over non-contact mode, while also preserving the sample and AFM tip.

AFM allows obtaining properties of surfaces and materials at the nanometre scale. With this technique, it is possible to determine the topography, including the height and roughness of the surface, as well as mechanical properties such as elasticity, adhesion, hardness, and viscoelasticity. In addition, AFM is used for morphological studies and chemical-physical properties of molecular nanostructures, as well as for the analysis of dynamic processes, dissolution, and growth kinetics. AFM is also used in the analysis of protein aggregates, biofilms, and colloids, as well as in nanolithography, electronic conductivity, and mechanical properties of electronic circuits. This technique is widely used in biology, physics, materials science, and nanotechnology.