Force Spectroscopy

To investigate the sample properties by the AFM, the force spectroscopy mode is widely applied [7, 8]. In this mode, information about the sample properties is derived from deflection changes as a function of a cantilever separation from the surface, recorded at a single point rather than by scanning the lateral position of the tip (from a so-called force curve). In the single force curve, the base of the cantilever is moved in the vertical direction towards the surface and then retracted again. During the motion, the deflection of the cantilever is recorded as a function of relative sample position (Fig. 3.10).

The force curve consists of two parts: the one recorded during approaching the tip to the sample surface (an approach curve, red line in Fig. 3.10) and the other one, collected during opposite motion (a retract curve, black line in Fig. 3.10). The character (shape) of the force curve depends on physical and chemical properties of two interacting surfaces, namely the investigated sample, the probing tip, and the surrounding environment.

Regardless of the sample type, there are several features that are present in all curves. When the cantilever is away from the surface, its deflection should be zero since there is no detectable interaction force. Actually, due to thermal vibrations, the cantilever oscillates around its free position, reflecting the noise present in a particular AFM system (this is represented by a horizontal base line (A)). During the approach, if both the tip and the surface are charged with the same sign, at close distances—prior to the contact—the cantilever can be repulsed from the surface (region R). It is represented by the slight raise in the baseline, as visible in

Fig. 3.10. The presence of attractive forces between the tip and a surface is reflected by a jump-in (point B), i.e., the moment when the cantilever is suddenly attracted. At this moment, the gradient of the attractive force is larger than the cantilever spring constant. When the tip is in contact with the sample surface, the electron clouds of atoms of both the tip and the sample are overlapping and repulsing. The further approach results in the cantilever bending (up to the certain maximum value, point C), which character depends on the material properties of the investigated sample (linear or not). During the retraction, the interacting repulsive force decreases and, the tip does not separate from the surface exactly at the same point where it started to touch the surface. Forces that are responsible for such behavior arise from adhesive properties of investigated surfaces. In further retract when the elastic force of the cantilever exceeds the gradient of the adhesive force, the tip is rapidly separated from the surface. The point D corresponds to the maximum value of the force (a so-called pull-off force). Further separation yields in cantilever fluctuations around its free position (base line).

Figure 3.10 A schematic view of the force curve recorded by the AFM working in the force spectroscopy mode. Arrows indicate direction of the movement of the AFM cantilever. Adapted with permission from [9].