AFM Cantilevers

Fig. 4. Typical AFM rupture experiment (top) Receptor molecules are fixed via linker molecules to a surface (left) in the same way, ligand molecules are connected to the AFM cantilever (right). When pulling the cantilever towards the right, the pulling force applied to the ligand can be measured. At the point of rupture of t he ligand-receptor complex the measured force abruptly drops to zero so that the rupture force can be measured.

Fig. 3. (a) Illustration of various AFM cantilever configurations for indentation experiments and (b) simple mechanical model for AFM-based indentation (by sample displacement). 

The shape of AFM cantilevers (much thinner than the width) results in torsional deflection when forces push the tip laterally as in friction measurements (when the tip is sliding) or lateral stiffness measurements (when the tip is stuck). 

Sagvolden et al. [86] also combined the use of colloids with AFM force sensors to study adhesion. In their case, instead of attaching the colloid to the end of the AFM probe and applying a normal force, they approached the free colloids from side on, with the AFM cantilevered at an angle of approximately 30° to the surface normal. Thus, they applied a predominantly lateral force to the colloid particles. The colloids were coated with protein molecules, and their adhesion was studied against three nonbiological surfaces, consisting... 

Figure 2.4 A scanning electron microscopy image of an AFM cantilever tip covered with a thin silver film.

A representative example of the upd process is copper on gold and an extremely illuminating study of this system using repulsive AFM was reported by Manne et al. (1991). The authors employed a commercially available AFM, the essentials of which are shown in Figure 2.33. The reference electrode was a copper wire in contact with the electrolyte at the outlet of the cell. The counter electrode was the stainless steel spring clip holding the AFM cantilever in place. The working electrode was a 100 nm thick evaporated Au film (which is known to expose mainly the Au(111) surface) mounted on an (x, v) translator. 

Fig. 4 Spring model of the AFM cantilever. Reprinted with permission from http //stm2. nrl.navy.mU/how-afm/how-afm.html...

Fig. 14 Two topview AFM images of A-DNA in AFM tapping mode imder ambient conditions a before and b after mechanically cutting a dsDNA strand in the center of the image (circles) with the AFM cantilever tip [106]. Reprinted with permission...

Studies on fundamental interactions between surfaces extend across physics, chemistry, materials science, and a variety of other disciplines. With a force sensitivity on the order of a few pico-Newtons, AFMs are excellent tools for probing these fundamental force interactions. Force measurements in water revealed the benefits of AFM imaging in this environment due to the lower tip-sample forces. Some of the most interesting force measurements have also been performed with samples under liquids where the environment can be quickly changed to adjust the concentration of various chemical components. In liquids, electrostatic forces between dissolved ions and other charged groups play an important role in determining the forces sensed by an AFM cantilever. 

Fig.9 Typical F/R vs. D curve between the PMMA brush (L = 87nm, Mn = 121700, Mw/Mn = 1.39) and the sUica probe (attached on an AFM cantilever). The arrowheads indicate critical distances is the equilibrium thickness at which a repulsive force is detectable, and Do is the offset distance beyond which the brush was no more compressible...

In the early years of AFM operation, the cantilevers were cut from a metal foil, and the tips were made from crushed diamond particles, picked up by a piece of eyebrow hair, and painstakingly glued manually on the cantilevers. This situation has changed completely since the methods for mass production of cantilevers with integrated tips were developed. A review of various methods for making cantilevers using standard microfabrication techniques was published by Albrecht et al. (1990), and an improved method is described by Akamine et al. (1990). Those AFM cantilevers with integrated tips are now available commercially. 

Figure 8. A molecular system of extension x is connected at its leftmost end to a bead trapped in an optical well (or to the tip of an AFM cantilever) and at its rightmost end to an immobilized surface (or a bead fixed to the tip of a micropipette). The position of the bead relative to the center of the trap, xt, gives a readout of the acting force / = KXi,. The control parameter in this setup is Z = Xb + X, whereas both xt and x are fluctuating quantities.

Preserving only the low-frequency terms in the AFM cantilever response (since the high-frequency terms will be filtered out by the mechanical response, and possibly also the electronic circuitry), the additional force due to the vibration of the tip and the surface is... 

Fig. 10 (a) Molecular imprinting of cytochrome c immobilized on mica, and AFM images of the surfaces, (b) Molecular force spectroscopy with a cytochrome c-derivatized AFM cantilever of the cytochrome c-imprinted polymer (MIP) and the non-imprinted control polymer (NIP). Adapted from [104] with permission from Elsevier... 

Figure 17.10 SEM picture of Pt coated AFM cantilever tip after measurement...

Figure 19.14 Experimental setup for study of the polarization distribution in FeCaps with size down to sub micron range. AFM cantilever is used only for piezoresponse mapping, whereas AC and DC voltage is applied through the independent probe.

Fig. 11. Series of steps used to fabricate a small light source onto the end of a commercial AFM cantilever (A). (B) A small hole is cut into the cantilever and (C) an 8 pm high-index glass sphere is glued in place. (D) The glass sphere is then shaped into a pyramid (E), coated with aluminum and an aperture formed to produce small light source on the end of the cantilever (F). Reproduced with permission from Ref. [29]. Copyright 2001 American Institute of Physics.

A similar design that coupled fiber optic NSOM probes with AFM cantilevers was also reported [28]. In this approach, shown in Fig. 14, a standard fiber optic NSOM probe was carefully inserted through the 7 pm hole that was cut into the end of an AFM probe. Once glued into place, the excess fiber on the backside of the AFM cantilever was removed using FIB milling. This approach... 

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