Silicon nitride tips

Fig. 6. Lateral stiffness vs. load data for a silicon nitride tip vs. mica surface in ultra-high vacuum. Solid line is fit of the JKR model to the data. Reprinted with pennission from ref. [67].

Fig. 7. Voigt model analysis of (a) lateral contact stiffness and (b) the response time, t, for a silicon nitride tip vs. poly(vinylethylene) as a function of frequency and polymer aging times. Reprinted with permission from ref [71].

Amongst the earliest measurements involving chemical functionality of the probe were those of Nakagawa et al. [69]. They investigated octadecyltrichlorosilane (OTS) chemically modified tips against chemically adsorbed monolayers of different alkyl-trichlorosilanes in ethanol, as shown schematically in Figure 14. When both tip and surface were modified by OTS, a large adhesive force was observed that was not present for the case of an unmodified silicon nitride tip on an OTS-modified surface. Additionally there... 

Figure 6.13 Normalized force between a microfabricated silicon nitride tip of an atomic force microscope and a planar mica surface in 1-propanol at room temperature [174], The tip had a radius of curvature of R w 50 nm. The different symbols were recorded during approach (filled circles) and retraction (open circles) of the tip.

Example 11.4. McGuiggan et al. [492] measured the friction on mica surfaces coated with thin films of either perfluoropolyether (PFPE) or polydimethylsiloxane (PDMS) using three different methods The surface forces apparatus (radius of curvature of the contacting bodies R 1 cm) friction force microscopy with a sharp AFM tip (R 20 nm) and friction force microscopy with a colloidal probe (R 15 nm). In the surface force apparatus, friction coefficients of the two materials differed by a factor of 100 whereas for the AFM silicon nitride tip, the friction coefficient for both materials was the same. When the colloidal probe technique was used, the friction coefficients differed by a factor of 4. This can be explained by the fact that, in friction force experiments, the contact pressures are much higher. This leads to a complete penetration of the AFM tip through the lubrication layer, rendering the lubricants ineffective. In the case of the colloidal probe the contact pressure is reduced and the lubrication layer cannot be displaced completely. 

FIG. 7 Force vs. distance curves measured at different KC1 concentrations with a silicon nitride tip on mica. The pH was about 6 due to dissolved CO2. All curves were 16 times averaged Only the approach of the sample to the tip is shown. (From Ref. 41.)... 

Bottino et al. [1994] used silicon nitride tips and silicon nitride gold-coated cantilevers to obtain the images in the height mode (or constant force mode). The AFM technique has been compared quite well with SEM using alpha-alumina membrane samples. The found that AFM could determine the sizes of individual constituent gamma-alumin.. particles in the membranes and detect subtle differences in their dimension ratios. Base. 

Fig. 3.39 Force-displacement curves captured with silicon nitride tip on glass in buffer (10 mM Tris [pH 8], 100 mM NaCl, top) and ultrapure water (Millipore water (18.4 MW cm), bottom). The different range of the electrostatic repulsion, as well as a difference in pull-off force are obvious...

Fig. 4.4 F-d curves recorded with a silicon nitride tip in ambient on (a) CH3 and (b) COOH terminated areas of a patterned SAM model surface, (c) Pull-off force histogram calculated from 4096 individual f-d curves, (d) Corresponding quantitative pull-off force map [22]...

The limits of the celebrated Derjaguin approximation for predicting forces between submicron-sized particles have been argued for some time. Now the approximation can be validated using the force data obtained for the interaction between the AFM tips on microfabri-cated cantilevers and the flat surfaces. The radius of curvature of the AFM tips is about 10 nm and provides the ideal geometry with small interaction forces. Fig. 5 shows an example for the forces measured with the graphite (HOPG) flat surfaces and the silicon nitride tips with the radius of curvature of about 7 nm in solution with different pH. 

The silicon or silicon nitride tips were cleaned with ethanol for 5 min, followed by rinsing with Milli-Q water for 5 min. 

In the AFM, the two interacting surfaces are the planar sample surface and the surface of the AFM probe. For microscopy the most commonly used probes are the sharp microfabricated silicon nitride or silicon tips that provide a high lateral resolution. These probes also provide high resolution when measuring surface forces, but introduce the problem that the surface geometry is difficult to determine in the 10-nm regime [120-122]. This would, however, be necessary since typical radii of curvature are from 5 to 60 nm. In addition, the surface chemistry of silicon nitride tips, which are most frequently used for force measurements, is rather complex [51, 52, 123,124],... 

A more convenient setup consists of a conducting sample, which serves as a working electrode, and an insulating probe. Raiteri and coworkers and Doppen-schmidt and coworkers [55,178] measured the force between a gold, platinum, or graphite sample and a silicon nitride tip. 

The silicon nitride tip is used mostly for C-AFM. Measurements can be done in ambient air, controlled atmospheres, or in non-aggressive liquids. AFM also allows surface forces, and even molecular forces, to be directly quantified [23]. For example, the interaction forces between a silicon tip and microfiltration and ultrafiltration membranes in an electrolyte solution can be measured [24]. The geometry of the cantilever is not simple, and in some cases not even known, so comparison with theory is difficult. However, attaching a sphere to the cantilever instead of a tip enables the measurement of interaction between surfaces of known geometry [25]. This technique has been used to measure interactions between different materials in air... 

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