Non-contact AFM

Figure 8.5 a) Schematic of the 7x 7 reconstruction of the Si(l 11) surface, b) Image of a Si 7x 7 surface obtained with a combination of STM and a special non-contact AFM mode [331] (see Section 8.6.3). Only the adatoms are visible under the chosen imaging conditions. Single defects of the surface structure are resolved (circles). The picture was kindly provided by E. Meyer. 

Fig 7. Non-contact AFM image of a Ti02(l 10)(lxl) surface. Reprinted with permission from ref [54],... 

A model involving discrete bond breaking was proposed for the formation of the (1x3) surface [107,108]. Surfaces were prepared that exhibited both, the (1x1) termination and the ridges typical for the (1x3) surface. STM and non-contact AFM images showed an intermediate phase, which had a (1x3) symmetry, but did not possess the characteristics of the microfaceted structure. Raza et al. [107] suggested that the bonds labeled a in Fig. 15 are broken, which allows the Ti atoms to relax towards the other rows of bridging oxygen atoms. 

Zeolite beta has been widely studied as a Bronsted acid catalyst and has been shown to be highly active for reactions such as alkylation and acylation [8,9]. The effect of the crystallite size on the catalytic activity of beta has been investigated [10] but since betas often have very high external surface areas, it is possible that acid sites associated with framework aluminium close to the outer surface will contribute to the overall catalytic activity of the zeolite. This may adversely affect the shape-selectivity of the reaction. In this study a series of beta zeolites with differing Si/Al ratio and ESA were investigated by means of non-contact AFM and N2 absorption measurements and the catalytic activity was tested by an acylation reaction capable... 

Figure 2 Non-contact AFM of (a) calcined BET102B and (b) PBloxA2, top view presentation, scan area 1x1...

Fig. 5.10 Non-contact AFM image of a silica glass fracture prepared 1 x 10 11 mbar and imaged at 1 x 10-8 mbar [43],...

Non-contact AFM is performed in liquid, under vacuum and in air [90]. In the non-contact regime, the water film present on a sample in air will typically not be penetrated. If for a particular sample this is a problem, and contact mode also happens to be a problem (for example a soft sample could be damaged by a dragging tip) then a tapping mode operation of the cantilever in the intermittent regime may be the best choice. 

Figure 5.7 Non-contact AFM images of DCNDBQT without template layer evaporated on Ti02 (a) 5 pm image of the prepared sample, (b) zoomed image showing the terrace structures, (c) high resolution image with a unit cell of a = 2.0 nm, b = 0.1 nm, c = 1.5 nm.

Fig. 1 - Large scale non-contact-AFM topography of CW (part a 1.6 x 1.6 pm ) and CP (part b 4.0 X 4.0 pm ) supports. Part c reports the Raman spectra of CP (solid line), CW (dotted line) and graphite model compound (gray line).

Figure Bl.19.26. Highly resolved, non-contact AFM image of the Ti02(l 10)-(1 x i) surface (8.5 x 8.5 nm ) with a single step. The two-dimensional order of the bright spots (0.65 x 0.3 nm ) reproduces the alignment of the bridging oxygen atoms. (Taken from [121], figure 3.)...

Even without atomic resolution, AFM has proved its worth as a technique for the local surface structural determination of a number of bio-inorganic materials, such as natural calcium carbonate in clam and sea-urchin shells [123]. minerals such as mica [124] and molybdenite [125] as well as the surfaces of inorganic crystals, such as silver bromide [126] and sodium decatungstocerate [127]. This kind of information can prove invaluable in the understanding of phenomena such as biomineralization, the photographic process or catalysis, where the surface crystallography, especially the presence of defects and superstructures, can play an important role, but is difficult to determine by other methods. AFM has the considerable advantage that it can be used to examine powdered samples, either pressed into a pellet, if the contact mode is employed, or loosely dispersed on a surface, if intermittent or non-contact AFM is available. 

Fig. 5.7. Three-dimesional non-contact AFM image of an ES625 ultrafiltration membrane. Reprinted from [18]. Copyright 1996, with kind permission from Elsevier...

Szymonska, J. and Krok, F. 2003. Potato starch granule nanostructure studied by high resolution non-contact AFM. Int. J. Biol. Macromol. 33 1-7. 

Tanaka and co-workers observed two stages in the deposition of polymer by non-contact AFM in vacuum [192], Initially, rapid diffusion of molecules allows association and organization before laying down on the surface. Bulk solvent dries, but solvent trapped between the polymer and the surface takes much longer. This allows adsorbed aggregates to reorient for favorable correspondence with the substrate lattice. 

Non-contact AFM is one of several vibrating cantilever techniques in which an AFM cantilever is vibrated near the surface of a sample. Tip-sample spacing is in the order of tens to hundreds of angstroms. 

Non-contact AFM is desirable where httle or no contact between the tip and sample is required. The total force between the tip and sample is relatively low at about 10 N which makes it advantageous for studying soft or elastic samples. As no contact is made, there are no tip contamination issues to consider. 

Intermittent Contact AFM is similar to the non-contact AFM mode except that the vibrating cantilever is brought closer to the sample so that at the maximum amplitude it makes slight contact with the surface (i. e., taps the surface). The intermittent contact region is shown on Fig. 2. As with non-contact measurement, the change in cantilever oscillation amplitude is in response to variation in tip-to-sample spacing. 

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