Mapping surface: stiffness imaging

Force modulation imaging is the dynamic contact mode that identifies and maps differences in surface stiffness or elasticity. These techniques use a variety of surface properties to differentiate among materials where topographical differences are small or not measurable. 

In the contact mode, there are static modes (de-modes), and dynamic modes (ac-modes). In the former, a cantilever-type spring bends in response to the force which acts on the probing tip until a static equilibrium is established [1]. In the dynamic mode, the lever oscillates close to its resonance frequency. A distance-dependence force shifts the resonance curve. Another technique is to modulate the position of the sample at a frequency below the cantilever resonance but above the feedback-response frequency and send the response signal to a lock-in amplifier to measure the signal s amplitude and phase [4]. The lock-in output is connected to the auxiliary data acquisition channels to form an image - this approach is popularly known as force modulation (FM-mode). FM-mode imaging or force cmve is an AFM technique that identifies and maps differences in surface stiffness or elasticity. 

The tip is thus engaged similar to contact mode AFM. Scan rates of 1 Hz and low imaging forces are advisable to minimize possible impact of lateral forces. During each scan line the tip periodically indents the sample surface. From the feedback loop correction for the z-component for each pixel a topography (height) image is obtained the simultaneously recorded rms cantilever amplitude provides a stiffness map [117]. 

---