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Electric Techniques

Electric Force Microscopy

Rev. D Dimension 3100 Manual 275

16.2 Electric Force Microscopy

16.2.1 Electric Force Microscopy Theory

Electric Force Microscopy is analogous to standard MFM, except that gradients being sensed are

due to electrostatic forces. In this method, the cantilever is vibrated by a small piezoelectric element

near its resonant frequency. The cantilever’s resonant frequency changes in response to any

additional force gradient. Attractive forces make the cantilever effectively “softer,” reducing the

cantilever resonant frequency. Conversely, repulsive forces make the cantilever effectively “stiffer,”

increasing the resonant frequency. A comparison of these force additives is shown in Figure 16.2a.

Figure 16.2a Comparison of Attractive and Repulsive Forces

Changes in cantilever resonant frequency are detected in one of the following ways:

• Phase detection (with Extender Electronics Module only)

• Frequency modulation (with Extender Electronics Module only)

• Amplitude detection (not recommended due to artifacts)

All of the above methods rely on the change in resonant frequency of the cantilever due to vertical

force gradients from the sample. Figure 16.2b shows a diagram of how the Extender electronics

module provides signal enhancement and feedback allowing gradient detection. The best

candidates for electric ﬁeld gradient imaging are samples that have large contrasts in the electric

force gradient due to material differences or regions at substantially different potentials. For other

samples having rough surface topography or small voltage variations, this technique may be

undesirable because topographic features appear in the LiftMode image.

Attractive gradient equivalent to additional spring in tension attached

to tip, reducing the cantilever resonance frequency.

Repulsive gradient equivalent to additional spring in compression attached

to tip, increasing the cantilever resonance frequency.

Frequency

Amplitude

∆F

Frequency

∆F

Amplitude

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