AFM Today
Atomic force microscopy (AFM) was developed in 1986 by collaborating research teams from IBM and Stanford University. Within a few years, it was applied as a new, high-resolution surface and structural analysis technique at a wide range of institutes around the world, ultimately complementing scanning electron microscopy (SEM) and transmission electron microscopy (TEM).
Some Attractive Features of AFM
- For most common applications of AFM, the instrument is
relatively inexpensive (compared to SEM or TEM) and a vast infrastructure of existing tools are available for rent;
- 3D images of surface topography are obtainable at nanometer resolution, regardless of surface conductivity;
- Measurements can be done in air, liquid or controlled atmosphere and special sample preparation is
not necessary;
- AFM physically interacts with the sample, opening up a world of capabilities beyond simply imaging;
- AFM is a non-destructive technique that accommodates the use of complementary analysis methods.
How Does AFM Work?
 A schematic showing the key components of a typical AFM assembly is shown in Figure 1. A sharp tip at the end of a flexible cantilever scans the sample surface while maintaining a small constant force between the tip and the sample. A piezoelectric tube scanner scans either the sample or tip with respect to the other. The tip-sample interaction is monitored by measuring the deflection of the cantilever using a laser beam reflected off the back of the cantilever onto a split photo diode (detector). As the tip moves over the sample’s topography, a constant force between the tip and sample is maintained by keeping the deflection of the cantilever constant through a feedback loop mechanism that adjusts the height of the piezoelectric tube scanner. The computer records these horizontal and vertical movements of the scanner, thus creating a contrasted 3D topographic image of the sample.
There are two AFM modes commonly used to image samples, the contact mode and the tapping mode. In contact mode, the tip continuously touches the sample surface. A constant force between the tip and sample is maintained by keeping the deflection of the cantilever constant through a feedback loop mechanism. In tapping mode, the probe tip makes only intermittent contact with the sample, oscillating at its resonant frequency (i.e., lightly tapping the surface) as it scans the sample’s surface. The piezoelectric scanner adjusts the height of the probe from the surface as it scans the sample to maintain constant damped amplitude. The main advantage of using the tapping mode rather than the contact mode is that the former eliminates the lateral shear forces and thus, allows imaging of soft samples or those not tightly immobilized on the surface.
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