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Dynamic
Force Mode
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| Dynamic
Force Mode is very similar to Non-contact mode AFM in many ways such as
the applied force and the measurement principle. Dynamic Force Mode is a
hybrid of the two most fundamental measurement methods, represented by
contact mode and non-contact mode. In LFM, the cantilever vibrates in
free-space in the vicinity of the resonant frequency like in non-contact
mode. At the same time, since the vibrating cantilever gets very close to
the sample surface, it taps the surface repeatedly, and the tip contacts
the sample surface as in contact mode. If you measure the amplitude
of vibration of the cantilever used in Dynamic Force Mode while changing
the frequency, as shown in Figure 1, there appears a special frequency
where the amplitude resonates and amplifies greatly. This is called the
intrinsic frequency (f0).
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| Figure 1. Resonant frequency |
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| Dynamic
Force Mode uses the non-contact mode feedback circuit with keeping the vibrating
frequency (f1) a little bit lower than the resonant frequency while oscillating
in free-space. Then, as the tip is lowered, the real spring constant reduces due
to the attractive van der Waals force which becomes larger as the tip comes
closer to the sample surface, as shown in Figure 2 (a). Therefore the resonant
frequency changes to effective frequency (feff) in non-contact regime and the
amplitude at the frequency f1 increases. Since the amplitude increases, the
non-contact mode feedback circuit decreases the distance between the tip and the
sample surface, indicated in the graph of vibration amplitude vs tip-sample
distance and z-feedback as shown in Figure 2 (b). Therefore, the vibrating
cantilever, which is oscillating above the sample, approaches the sample almost
in contact or in collision with the surface. This method, keeping intermittent
contact between the sample surface and the vibrating cantilever is called
Dynamic Force Mode AFM. |
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| Figure 2. (a) Resonant
frequency shift (b) Amplitude vs. Z-feedback |
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| Similar
to the initial approach of making contact with the sample, while scanning,
larger amplitude reduces the distance between the tip and sample, and
smaller amplitude increases the distance depending on the surface
roughness to determine the surface topology. For certain samples, Dynamic
Force Mode yields better measurements than contact mode or non-contact
mode AFM. Dynamic Force Mode AFM has an advantage over contact mode in the
sense that it will damage the sample less since there is no drag,
frictional, or lateral force to pull the sample sideways. It is also more
effective than non-contact mode when measuring a sample with a relatively
rough surface and a large height difference over a large area. The
importance and the application of Dynamic Force Mode AFM becomes more
significant than before as it overcomes the limit of both contact mode and
non-contact mode, while maintaining the merits of both modes.
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| Basic
contact & DFM |
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| Phase imaging |
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