33.9: Atomic Force Microscopy
Atomic force microscopy (AFM) is a type of scanning probe microscopy that can analyze topographic details of various specimens like ceramics, glass, polymers, and biological samples. AFM offers over 1000 times more resolution than the optical imaging system. Images generated from AFM are three-dimensional surface profiles, offering an advantage over the flat, two-dimensional images from other imaging techniques.
The AFM Probe
The probe is regarded as the heart of any AFM setup and comprises the cantilever and tip assembly. Probes are the most commonly replaced part on this type of microscope because the constant interaction with the samples wears down the tip. Therefore, the choice of material for the probe depends on the properties of the sample. Silicon probes, used to analyze hard samples, are stiffer and sharper than silicon nitride probes, which are better suited to scan softer samples. These sharp tips are produced using electrochemical etching or carbon nanotubes for higher accuracy analysis.
Imaging Modes of AFM
In AFM, surface topography is studied using the interaction between the probe tip and the sample surface. There are two main imaging modes — a static mode, also referred to as the contact mode, and a dynamic mode.
In the static or contact mode, the tip of the probe is in continuous contact with the sample surface. As the tip drags over the surface, repulsive forces between the sample and the tip result in the cantilever bending, which is recorded. The entire specimen surface is scanned back and forth in both x- and y-axes, called raster scanning, while the vertical movement of the cantilever records the z-axis, thus generating a 3D image.
In the dynamic mode, the probe oscillates just above the sample surface, coming close to, but not touching the surface. Attractive and repulsive forces determine the variation in distance between the tip and the sample, affecting the amplitude of cantilever oscillation. This feedback is recorded to construct the surface topography of the sample.
