Electric Force (EFM) Imaging
Electric Field Gradient Detection—Theory
242
MultiMode SPM Instruction Manual
Rev. B
14.1.2 Surface Potential Imaging Overview
Surface potential imaging measures the effective surface voltage of the sample by adjusting the
voltage on the tip so that it feels a minimum electric force from the sample. (In this state, the
voltage on the tip and sample is the same.) Samples for surface potential measurements should have
an equivalent surface voltage of less than ±10V, and operation is easiest for voltage ranges of ±5V.
The noise level of this technique can be as low as a few mV. Samples may consist of conducting
and nonconducting regions, but the conducting regions should not be passivated. Samples with
regions of different materials will also show contrast due to contact potential differences. Semi-
quantitative voltage measurements can be made on samples if the system is carefully calibrated on
a sample at a known voltage.
This method requires one of the Electronics Modules or NanoScope
IV, as well as version 3.1 or later of the NanoScope software
.
14.2 Electric Field Gradient Detection—Theory
Electric
fi
eld gradient imaging 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 14.2a
Comparison of attractive and repulsive forces to action of a taut spring attached to the tip
Changes in cantilever resonant frequency can be detected in one of the following ways:
•
Phase detection—
with Basic Extender Module only
.
•
Frequency modulation (FM)—
with Quadrex Extender Module only
.
•
Amplitude detection—
not recommended due to artifacts
.
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
0
Frequency
∆
F
0
Amplitude
