Resolution in AFM

Atomic force microscope (AFM) is a powerful nanotechnology tool for molecular imaging and manipulations. One major factor limiting resolution in AFM to observe individual biomolecules such as DNA is the low sharpness of the AFM tip that scans the sample. Nanoscale 1,3,5,7-tetrasubstituted adamantane is found to serve as the molecular tip for AFM and may also find application in chemically well-defined objects for calibration of commercial AFM tips [113]. 

The concept of resolution in AFM is different from radiation-based microscopies because AFM imaging is a three-dimensional imaging technique. There is an important distinction between images resolved by wave optics and those resolved by scanning probe techniques. The former is limited by diffraction, whereas the latter is limited primarily by apical probe geometry and sample geometry. Usually the width of a DNA molecule is loosely used as a measure of resolution, because it has a known diameter of 2.0 nm in its B form. 

Many applications of AFM to pillared clays or zeolites have not specifically addressed the porosity characteristics, but rather the occurrence of adsorbed surface Al species in, e.g., pillared montmorillonite [41], or the crystal growth processes, adsorption on porous surfaces and the surface structure of natural zeolites [42]. Sugiyama et al. [43] succeeded to reveal the ordered pore structure of the (001) surface of mordenite after removal of impurities that clogged the pores. The authors indicated that resolution in AFM imaging of zeolites is significantly affected by the magnitude of the periodical corrugation on the crystal surface, so that if the surface contains deep pores only the pore structure, but not the atomic structure, can be resolved. 

If we are examining steps of atomic dimensions in figure 10.14, redraw the schematic to scale and thus explain the factors that determine vertical and lateral resolution in AFM. 

Fig. 14. Proposed atomic scale resolution in afm of PET. From Ref. 95, Copyright (1997). Reprinted by permission of John Wiley Sons, Inc.

One of the most important advances in electrochemistry in the last decade was tlie application of STM and AFM to structural problems at the electrified solid/liquid interface [108. 109]. Sonnenfield and Hansma [110] were the first to use STM to study a surface innnersed in a liquid, thus extending STM beyond the gas/solid interfaces without a significant loss in resolution. In situ local-probe investigations at solid/liquid interfaces can be perfomied under electrochemical conditions if both phases are electronic and ionic conducting and this... 

The STM uses this eflFect to obtain a measurement of the surface by raster scanning over the sample in a manner similar to AFM while measuring the tunneling current. The probe tip is typically a few tenths of a nanometer from the sample. Individual atoms and atomic-scale surface structure can be measured in a field size that is usually less than 1 pm x 1 pm, but field sizes of 10 pm x 10 pm can also be imaged. STM can provide better resolution than AFM. Conductive samples are required, but insulators can be analyzed if coated with a conductive layer. No other sample preparation is required. 

A very similar technique is atomic force microscope (AFM) [38] where the force between the tip and the surface is measured. The interaction is usually much less localized and the lateral resolution with polymers is mostly of the order of 0.5 nm or worse. In some cases of polymer crystals atomic resolution is reported [39], The big advantage for polymers is, however, that non-conducting surfaces can be investigated. Chemical recognition by the use of specific tips is possible and by dynamic techniques a distinction between forces of different types (van der Waals, electrostatic, magnetic etc.) can be made. The resolution of AFM does not, at this moment, reach the atomic resolution of STM and, in particular, defects and localized structures on the atomic scale are difficult to see by AFM. The technique, however, will be developed further and one can expect a large potential for polymer applications. 

Currently, both AFM and STM are in their infancies regarding applications to plant systems. Thus, the future most likely will involve adapting these methods to plant investigations. Furthermore, it is anticipated that cryo-AFM will be more widely employed as investigators continue to improve AFM resolution. In this regard, the future of probe-based microscopies is discussed by Paesler and Moyer (41). 

Figure 7.12 Images at atomic resolution of graphite obtained with scanning tunneling (left) and atomic force microscopy (middle). The graphite lattice contains two types of sites A-sites with a carbon atom neighbor in the second layer and B-sites without a neighbor in the next layer. STM detects the B-sites, whereas the A-sites show up better in AFM. (STM image courtesy of TopoMetrix AFM image courtesy of M.W.G.M. Verhoeven, Eindhoven).

With the advances in AFM, especially the optical beam deflection method in the repulsive-force regime, the AFM study of solid surfaces under an electrolyte becomes practical. Atomic resolution with AFM at a liquid-solid interface has been routinely achieved (Manne et al., 1990, 1991). A typical fluid cell for the AFM study of electrochemistry is shown in Fig. 15.10. The top of the cell is made of glass to allow light to go in and out. 

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