Principles of atomic force microscopy AFM

We will discuss the operating principle of atomic force microscopy (AFM) and other scanning force microscopies in more detail in Chapter 7. At this point, simply think of this technique as analogous to an antiquated record player, in which the needle gently touches the surface of the record to produce music. Similarly, the AFM tip either gently taps, or hovers immediately above, the surface of a planar substrate. 

Figure 9.5 Principle of atomic force microscopy (AFM). The movement of the tip over the surface can be followed by the use of a laser beam and a photodiode...

The very new techniques of scanning tunnelling microscopy (STM) and atomic force microscopy (AFM) have yet to establish themselves in the field of corrosion science. These techniques are capable of revealing surface structure to atomic resolution, and are totally undamaging to the surface. They can be used in principle in any environment in situ, even under polarization within an electrolyte. Their application to date has been chiefly to clean metal surfaces and surfaces carrying single monolayers of adsorbed material, rendering examination of the adsorption of inhibitors possible. They will indubitably find use in passive film analysis. 

A new alternative to solve this problem is atomic force microscopy (AFM) which is an emerging surface characterization tool in a wide variety of materials science fields. The method is relatively easy and offers a subnanometer or atomic resolution with little sample preparation required. The basic principle involved is to utilize a cantilever with a spring constant weaker than the equivalent spring between atoms. This way the sharp tip of the cantilever, which is microfabricated from silicon, silicon oxide or silicon nitride using photolithography, mechanically scans over a sample surface to image its topography. Typical lateral dimensions of the cantilever are on the order of 100 pm and the thickness on the order of 1 pm. Cantilever deflections on the order of 0.01 nm can be measured in modem atomic force microscopes. 

On the other hand, optical microscopy, confocal microscopy, ellipsometry, scanning electron microscopy (SEM), scanning tunneling microscopy (STM), atomic force microscopy (AFM) and total internal reflection fluorescence (TIRF) are the main microscopic methods for imaging the surface structure. There are many good books and reviews on spectroscopic and chemical surface analysis methods and microscopy of surfaces description of the principles and application details of these advanced instrumental methods is beyond the scope of this book. 

Thus, the STM reveals itself as a powerful structural technique to be added to already standard ones routinely in use in the different research fields. The same can be stated for the Atomic Force Microscopy (AFM) as it has proved the same capabilities to study either conducting or insulating disordered surfaces [48]. Finally, it should be noted that STM also presents capabilities to analyze the electronic structure of the surface and to modify the surface. These applications have not been considered in this text but the main principles and examples can be found in the literature [2, 16]. 

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