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TappingMode AFM

This chapter details procedures for operating the Dimension FastScan SPM in TappingMode in air. For information regarding TappingMode in fluids, see Fluid Imaging.

TappingMode™ AFM, a Bruker-patented technique maps topography by lightly tapping the surface with an AFM probe that is oscillated at its resonant frequency.

In TappingMode, a piezo excites the cantilever substrate vertically, causing the cantilever to oscillate vertically. As the cantilever oscillates vertically, the reflected laser beam deflects in a regular pattern over a photodiode array, generating a sinusoidal, electronic signal or “detector signal”.

Before the AFM probe is engaged on the sample, the cantilever oscillates in free air close to its resonant frequency. As the probe approaches and encounters the sample surface, the amplitude of this oscillation decreases. By monitoring changes in amplitude and continuously feeding back on a user-specified amplitude setpoint, the tip moves up and down to maintain constant oscillation amplitude. By this means, a high-resolution three-dimensional image of the sample surface topography is produced.


Figure 1: Cantilever motion in TappingMode

TappingMode has become an important AFM technique, as it overcomes some of the limitations of contact AFM. By eliminating lateral forces that can distort or damage samples, TappingMode allows routine imaging of delicate and soft samples, as well as samples that are weakly attached to a substrate, that are often difficult to image in contact AFM. TappingMode can be performed in both air and fluid environments.

When the drive frequency is at or close to the resonant frequency of the cantilever, a small amount of shaking by the tapping piezo (a few angstroms of amplitude) can result in much larger cantilever oscillation (10-100 nm) in free air. However, if the drive frequency is away from cantilever resonant frequency, the cantilever amplitude drops very quickly because the cantilever’s natural response is out of phase with the tapping piezo shaking and the energy from the tapping piezo is less efficiently transferred to cantilever.

If the cantilever is driven at constant drive amplitude at its resonant frequency in free air, the cantilever will oscillate with constant amplitude. When the cantilever is brought close to the sample surface, the force gradient between tip and sample causes the effective resonant frequency of cantilever to shift, and the oscillation amplitude drops. Attractive forces will shift the cantilever resonant frequency lower, and repulsive forces will shift the cantilever resonant frequency higher. See Figure 2.

Figure 2: Cantilever oscillation amplitude vs. drive frequency

If the drive amplitude is kept constant, the energy going to the cantilever is constant. In a steady-state, the energy supplied to the cantilever in each tapping cycle by the tapping piezo equals the energy dissipated into the environment via hydrodynamic viscous interactions (e.g. air damping) and the energy dissipated into the sample by inelastic interactions at the tip-sample interface.

dissipated into the sample by inelastic interactions at the tip-sample interface.

The closer the average position of the cantilever is to the sample, the stronger the interaction forces between the tip and sample. As a result, more energy is dissipated into the sample during each tapping cycle and hence the cantilever oscillation amplitude decreases even with same drive amplitude.

In TappingMode, the drive frequency is usually set to about 5% below the cantilever natural resonant frequency. When the AFM probe moves close to the sample surface, it first feels the attractive force, and the cantilever effective resonant frequency shifts to a lower frequency, and the cantilever oscillation amplitude slightly increases. As the probe gets closer to sample surface, the tip starts to touch the sample surface at the end of the downswing in each tapping cycle. At this point the repulsive force becomes dominant. As the result, the effective resonant frequency of the cantilever further shifts to a higher frequency and more energy is transferred into the sample in each tapping cycle. The cantilever oscillation amplitude drops when the average tip-sample distance decreases. See Figure 3.

Figure 3: Cantilever oscillation amplitude vs. tip-sample distance

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