Contact-mode imaging

Fig. 3.5.11 AFM contact mode image of CdS particles made in CdAr bilayer film deposited on mica, exposed to H2S and subsequently immersed in CHC13 to remove excess surfactant.

In contact mode images in water only the border walls are dragged away, because they are softer and not very well anchored to the bulk PMMA. 

Fig. 4 The effect of the AFM tip on the nucleation of PEO crystallization in contact-mode AFM scanning. The micrograph exhibits a defection contact-mode image [79]...

In contact mode imaging AFM. attractive vdW interactions are the principal cause of mechanical spring instabilities at close approach of the tip and sur-... 

Fig. 12 AFM contact mode image showing a typical assembly of DODAB bilayer patches (a). The patches were self-assembled from a vesicle suspension and imaged in pure water. To the right, a series of approaching force curves are shown that were taken along a line across one of the patches in 5 mM LiN03(b). The different electrostatic interaction - repulsive on the mica substrate at the beginning and end of the line and attractive on the DODAB patch - is visible. (The figure is reproduced with kind permission from M. Radmacher [204].)...

Fundamental Principles of AFM. The principles of AFM as applied to geochemical systems have been described in detail in many previous publications (e.g., 9-13). Briefly, AFM works by rastering a sample underneath a sharp tip that is attached to or part of a cantilever. A number of forces, including Van der Waals dispersion forces, the so-called atomic force (related to the Pauli exclusion principle), electrostatic forces, and others, cause the tip to deflect as different surface features pass beneath it. By monitoring this deflection, a 3-dimensional map of the sample surface can be constructed. AFM is commonly operated in so-called contact mode, wherein the tip is in direct contact with the sample surface and repulsive forces are dominant. Non-contact mode imaging, which is conducted in an attractive regime, commonly in aqueous solution, also is possible and has resulted in more reliable and interpretable atomic-scale imaging (e.g., 14). 

Etched silicon probes provide the highest aspect ratio and the most consistent tip sharpness of the probes supplied at present. The silicon nitride probe is inexpensive, durable, and suitable for contact mode imaging. 

Figure 13.5 Surface morphology of Pebax 1657 membrane obtained by AFM in contact (a) and in friction (b) modes. The contact mode images give the surface topology...

Figure 2. AFM non-contact mode images of the surface of FUm-A (a) height image and (b) phase image.

Softer cantilevers are used for contact mode imaging of softer samples and tapping mode in liquid due to the lower resonance frequencies that reduce damping effects. In terms of force spectroscopy, the cantilever stiffness has to be... 

A natural extension of contact mode imaging is to operate in a mode sensitive to material properties, force modulation imaging [131]. The setup for this is similar to contact mode, with three differences ... 

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