Electric Force (EFM) Imaging
Electric Field Gradient Detection—Theory
Rev. B
MultiMode SPM Instruction Manual
243
All of the above methods rely on the change in resonant frequency of the cantilever due to vertical
force gradients from the sample.
shows a diagram of how the Basic Extender Module
provides the signal enhancement and feedback allowing gradient detection. The best candidates for
electric
fi
eld gradient imaging are samples that have large contrasts in the electric force gradient
due to material differences or regions at substantially different potentials (voltage differences of 1.0
or more). For other samples having rough surface topography or small voltage variations, this
technique may be undesirable because topographic features will appear in the
LiftMode
image.
Figure 14.2b
Diagram of Extender Electronics Modules in phase and frequency measurement mode
Topographic features appear in the
LiftMode
image because local force gradients are heavily
in
fl
uenced by surface structure. That is, sharp features on sample surfaces concentrate the local
force gradient. This happens on all roughness scales, so the local force gradient will also vary in
much the same way as does the surface topography. Thus, electric
fi
eld
LiftMode
images measured
by amplitude, phase or frequency detection often show contrast that is very similar to the surface
topography. Samples with rough surface topography or with smaller potential variations are more
successfully imaged by the surface potential measurement method described below.
Note:
In most cases, it is necessary to apply a voltage across the tip or sample to
achieve a high-quality image. Various methods for applying voltages to the tip
and sample are included in the sections that follow. Samples with permanent
electric
fi
elds may not require the application of voltage.
RMS
Phase Detector
Servo Controller
High Resolution
Oscillator
Oscillator Signal
Frequency Signal
Tapping
Piezo
Sample
Photodiode signal
Phase Signal
Signals to
NanoScope
Cantilever Deflection Signal
Frequency
Control lines
Laser
Photodetector
Conditioning
Detector
Amplitude Signal
Reference
Signal
Beam
(Feedback loop
adjusts oscillation
frequency until
phase lag is zero)
Basic Extender Module
