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Methods for Driving the AFM Cantilever for Intermittent Contact AFM

MAC Mode
Magnetically Actuated AFM (MAC Mode) is an implementation of Intermittent Contact Mode AFM. In MAC Mode, the AFM cantilever is partially coated with a magnetic thin film. An alternating current is driven through a coil that is positioned such that the cantilever’s free (and the attached tip) may be driven into (near-) vertical oscillations via the inductively-induced magnetic field interacting with the magnetic coating.

In MAC Mode, the drive force is applied directly to the cantilever, close to the cantilever’s free end. This feature of MAC Mode makes it substantially more suitable than Acoustic Mode for Intermittent Contact Mode AFM imaging and Spectroscopy in liquid. The difference between the resonance response of an AFM cantilever in a liquid as driven with MAC Mode and with Acoustic Mode reveals this quite clearly.

AC Mode
Alternating Current (AC) Mode AFM is another implementation of Intermittent Contact Mode AFM. In AC Mode AFM, the AFM cantilever is driven at its fixed end with a piezoelectric actuator that is positioned under the substrate to which the cantilever is attached. An alternating current (AC) voltage is applied to this actuator, whose resulting motion is then amplified at the cantilever’s free end, where the tip is.

Acoustic Mode
This mode is yet another implementation of Intermittent Contact Mode AFM, used to drive the cantilever in a liquid. The energy required to move the cantilever comes primarily from the liquid that surrounds it. The entire liquid cell on which the cantilever is mounted is driven by a piezoelectric actuator. The motion of the liquid cell couples to the liquid, and indirectly to the cantilever. The cantilever’s frequency response is a convolution of its own resonance response with the response of the liquid and the liquid cell. The mass of the cantilever is negligible compared with that of the liquid in motion. The resonance response of the cantilever is overshadowed by other peaks in the frequency response curve in Acoustic Mode. Phase Images in liquid using Acoustic Mode drive are hardly ever as useful as their counterparts in air, or as in liquid with MAC Mode. This lack of clarity in the apparent resonance response also makes the interpretation of the recorded images subject to more uncertainty than images taken with MAC Mode.

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