Electric Force (EFM) Imaging
Surface Potential Detection—Theory
262
MultiMode SPM Instruction Manual
Rev. B
Figure 14.5a
Simpli
fi
ed Block Diagram of Basic Extender Module in Surface Potential Mode
The key here is that the force on the cantilever depends on the product of the ac drive voltage and
the dc voltage difference between the tip and the sample. And, when the tip and sample are at the
same dc voltage (Vdc=0), the cantilever will feel no oscillating force. The Basic Extender Module
uses this fact to determine the effective surface potential on the sample, Vsample. The Extender
determines the local surface potential by adjusting the dc voltage on the tip, Vtip, until the
oscillation amplitude becomes zero. At this point the tip voltage will be the same as the unknown
surface potential. The voltage applied to the cantilever tip Vtip is recorded by the NanoScope III to
construct a voltage map of the surface.
RMS
Lock-in
Servo Controller
High Resolution
Oscillator
Oscillator Signal
Tapping
Piezo
Sample
Photodiode Signal
Signals to
NanoScope
Cantilever Deflection Signal
Laser
Photo-
Detector
Amplitude Signal
Reference
Signal
Beam
(Feedback loop
adjusts DC tip
voltage to zero lock-
in signal)
Basic Extender Module
Amplifier
detector
Sum
Potential Signal
DC Voltage
AC
GND
To
Tip
