Contact mode lateral force

A popularly employed imaging mode for biological samples is the intermittent contact mode (also oscillating or tapping mode). In this mode, lateral force components are eliminated by oscillating the AFM tip above the sample surface, so that the tip only directly touches the surface vertically and at the minimum of its oscillation amplitude. Oscillation is mediated via piezo contacts and is applied close to the resonance... 

To minimize effects of friction and other lateral forces in the topography measurements in contact-modes AFMs and to measure topography of the soft surface, AFMs can be operated in so-called tapping mode [53,54]. It is also referred to as intermittent-contact or the more general term Dynamic Force Mode" (DFM). A stiff cantilever is oscillated closer to the sample than in the noncontact mode. Part of the oscillation extends into the repulsive regime, so the tip intermittently touches or taps" the surface. Very stiff cantilevers are typically used, as tips can get stuck" in the water contamination layer. The advantage of tapping the surface is improved lateral resolution on soft samples. Lateral forces... 

Consequently, the lateral force between the tip and sample can be significantly reduced (Fig. 6.2B). Traditionally, contact mode typically could provide higher resolution, but recent advances in noncontact techniques have led to spatial resolution up to the atomic level in vacuums and liquids (Fukuma et al., 2005 Giessibl, 2003 Sugimoto et ah, 2007). Therefore, dynamic mode is preferred for soft and unstable samples. 

In the contact mode the tip scans the sample in close contact with the surface. The force on the tip is repulsive with a mean value of 10 N. This force is set by pushing the cantilever against the sample surface with a piezoelectric positioning element. In contact mode AFM the deflection of the cantilever is sensed and compared in a DC feedback amplifier to some desired value of deflection. If the measured deflection is different from the desired value, the feedback amplifier applies a voltage to the piezo to raise or lower the sample relative to the cantilever in order to restore the desired value of deflection. The voltage that the feedback amplifier applies to the piezo is a measure of the height of features on the sample surface. It is displayed as a function of the lateral position of the sample. 

In the contact mode, the probe senses forces F acting both perpendicular and parallel to the surface plane, i.e. normal forces and lateral forces, respectively (Fig. 5). Figure 6 shows a typical force-distance curve obtained when the tip is brought into close proximity with the sample surface. The normal force is proportional to the deflection of the tip(Z(), while the distance is given by the position of the cantilever (Zc) relative to the sample surface. Zc=0 corresponds to the straight cantilever (Z(=0) in contact. 

The lateral force microscope (LFM) is a modification of the standard contact mode SFM [87-90]. In addition to the normal forces, the friction forces exerted on the probe are measured via torsion of the cantilever (Fig. 5). This mode is sometimes called friction force microscopy . LFM can be used in combination with topographic imaging as it shows changes in material as well as enhanced contrast on sharp edges (Fig. 9). In addition to morphology, it provides information on the friction and wear properties (Sect. 3.4). 

For example, chemical contrast images were obtained by lateral force microscopy (LFM) from a topologically flat surface of a self assembled monolayer consisting of chemically different domains. In order to make the chemical adhesion the dominant contribution to the friction signal, the tip was modified by a monolayer with appropriate terminal groups [149-155]. However, since LFM operates in contact mode, the surface deformation is inevitable. 

Complementary to the indentation experiments, the lateral force microscope has been introduced to measure lateral forces exerted on the tip [388]. Somewhat latter, different modifications of contact-mode SFM have been developed to investigate indentation and wear in thin films [ 115]. On the lateral force measurement, one has to distinguish between operation at very small lateral displacements of the cantilever when the shear forces dominate in the net force, and a... 

Both AM and FM modes were initially meant to be non-contact modes, i.e., the cantilever was far away from the surface and the net force between the front atom of the tip and the sample was clearly attractive. The AM mode was later used very successfully in ambient conditions at a... 

Potential for image distortion due to lateral shear forces (in contact mode)... 

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