AFM Spectroscopies

SFM can be used not only as tool for topography acquisition but also can be used to produce spatially resolved maps of the surface or material properties of a sample; these include charge density, adhesion and stiffness, as well as the force required to break specific igand–receptor bonds. SFM can also be used as a tool for force spectroscopy – measuring forces as a function of distance. For oscillating cantilever tip-sample force can affect some other characteristics of cantilever oscillation – amplitude, frequency, phase, dissipation etc. Correspondingly dependence of these characteristics upon tip-sample distance can also be regarded as spectroscopic data.
Force is measured in an SFM by collecting a force curve, which is a plot of cantilever deflection, d_c, as a function of sample position along the z-axis (i.e. towards or away from the probe tip; the z-piezo position). It assumes a simple relationship (i.e. Hooke’s Law) between the force, F, and the cantilever deflection:

F = - k d_c

where k is the spring constant of the cantilever. Some other forces included in tip-sample interaction under dc approach or retracting motion are presented on the figure left [1]. Used definitions see below.
The interpretation of AFM force curves relies almost entirely on established force laws, particularly those determined using the SFA [2]. These force laws describe force as a function of the probe–sample separation distance (D) rather than as a function of the z-piezo position. Thus, to be useful, the force curves must be transformed into descriptions of force as a function of distance, F(D). However, current SFMs do not have an independent measure of D. Instead, the transformation to D is achieved by subtracting the cantilever deflection from the z-piezo movement.

For a very hard surface, zero separation is defined as the region in the force curve in which the cantilever deflection is coupled 1:1 with the sample movement; this appears in the force curve as a straight line of unit slope. A corrected curve is called a force–distance curve. Notice that determining D by this approach requires that the tip make contact with the sample.