AFM Principles

Riquicha A.A.G. et al. 2001. Manipulation of Nanoscale Components with the AFM Principles and Applications, lEEEInt. Conf Nanotechnol, Maui, HI, October 28-30,2001. 

Detection of cantilever displacement is another important issue in force microscope design. The first AFM instrument used an STM to monitor the movement of the cantilever—an extremely sensitive method. STM detection suffers from the disadvantage, however, that tip or cantilever contamination can affect the instrument s sensitivity, and that the topography of the cantilever may be incorporated into the data. The most coimnon methods in use today are optical, and are based either on the deflection of a laser beam [80], which has been bounced off the rear of the cantilever onto a position-sensitive detector (figme B 1.19.18), or on an interferometric principle [81]. 

Fig. 5.2. Principle of AFM. The sample symbolized by the circles is scanned by means of a piezoelectric translator. The piezo crystal and the oscillator is only needed for tapping mode operation.

From the general principles described in the previous section the following basic components of an AFM can be identified ... 

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. 

More recently, the method of scanning near-field optical microscopy (SNOM) has been applied to LB films of phospholipids and has revealed submicron-domain structures [55-59]. The method involves scanning a fiber-optic tip over a surface in much the same way an AFM tip is scanned over a surface. In principle, other optical experiments could be combined with the SNOM, snch as resonance energy transfer, time-resolved flnorescence, and surface plasmon resonance. It is likely that spectroscopic investigation of snbmicron domains in LB films nsing these principles will be pnrsned extensively. 

Theories and simulation of the operation of AFM in liquid have been attempted [102-104], In principle, molecular dynamics or NEMD may be a suitable method to mimic the operation of a scanning tip. The time scale, however, precludes simulating a long-enough scan to see a complete atom. Most studies, therefore, were made with equilibrium conditions and a fixed position of the AFM tip. Explicit consideration of electrolytes and electrostatic effects has not been modeled. 

Explain the principles of the scanning probe microscopies STM and AFM, and discuss the type of information these techniques provide. What are the major differences between the two ... 

FIGURE 6.1 Diagram of AFM work principle. Reprinted with permission from Liu and... 

AFM in principle gives images of all surfaces, including those of a supported catalyst, provided the powder particles can be immobilized, for example in glue. It remains difficult, however, to tell the difference between a catalyst particle and features of the support. Model systems of particles on a flat support offer better opportunities, as Fig. 7.15 and the following example shows. 

DNA arrays have been also generated by Dip-Pen Nanolithography (DPN) [80]. DPN involves the transfer of NAs directly from a coated Atomic Force Microscope (AFM) tip to the substrate of interest by virtue of direct molecular diffusion. Using this technique, thiol-modified ONDs have been patterned onto gold substrates and acrylamide-modified ONDs onto glass sHdes that were previously modified with mercaptopropyltrimethoxysilane. Feature sizes ranging from many micrometers to less than 100 nanometers could be obtained. The deposition of two different OND sequences onto the same substrate has also been reported [80], but the appHcation of this principle to the fabrication of high-density arrays remains to be addressed. 

In principle, there are two methods for the AFM probing of amorphous layers, namely (i) an active probing, which probes the surface chemistry, and (ii) a passive probing, which probes the surface physics. The active AFM probing, which is essentially an extension of the AFM-based single molecule studies, relies on the measurement of LF by an AFM tip that is fimctionahsed with molecular recognition-active molecules (e.g., a potentially complementary DNA). 

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