Electric Force (EFM) Imaging
Surface Potential Imaging—Procedure
Rev. B
MultiMode SPM Instruction Manual
267/(270 Blank)
7. Select the
Interleave Controls
command. This brings up a new set of scan parameters that
are used for the interleaved scan where surface potential is measured. Different values from
those on the main scan may be entered for any of the interleaved scan parameter. To fix any
of the parameters so they are the same on the main and interleave scans, click on the green
bullets to the left of particular parameter. The green bullet changes to “off” (gray) and the
parameter value changes to the main Feedback Controls value. Set the interleave
Drive
frequency
to the main feedback value. Enter an interleaved
Setpoint
of
0V
. Set
Interleave
scan
to
Lift
.
8. Enter an Interleave Controls
Drive amplitude
. This is the ac voltage that is applied to the
AFM tip. Higher
Drive amplitude
produces a larger electrostatic force on the cantilever and
this makes for more sensitive potential measurements. Conversely, the maximum total
voltage (ac + dc) that may be applied to the tip is ±10V. So a large
Drive amplitude
reduces
the range of the DC voltage that can be applied to the cantilever. If the sample surface
potentials to be measured are very large, it is necessary to choose a small
Drive amplitude
,
while small surface potentials can be imaged more successfully with large
Drive
amplitudes
. To start choose a
Drive amplitude
of
2V
.
9. Set the Channel 2 image
Data type
to
Potential
. Set the scan
Line direction
for the main
and interleave scans to
Retrace
. Remember to choose the
Retrace
direction because the lift
step occurs on the trace scan and data collection occurs on the retrace.
10. Choose a
Lift start height
of
0nm
and a
Lift scan height
of
50nm
. The
Lift scan height
can be readjusted later. Set the drive phase to -90°. This compensates for mechanical lay in
the cantilever as it responds to the oscillating electric field.
11. Switch
Interleave mode
to
Enable
to start LiftMode. Now, when the microscope completes
a topographic scan line (trace and retrace) the system turns off the TappingMode piezo and
switches the oscillator signal to the cantilever. The cantilever is driven electrostatically
according to the interleave
Drive amplitude
that has been selected. Also, when
Potential
is
selected as the
Data type
for the Channel 2 image, a feedback circuit is enabled in the
Extender box which adjusts the dc voltage on the tip to maintain the cantilever oscillation
amplitude at zero. To do this, the feedback circuit uses the lock-in signal of the cantilever
oscillation and tries to keep this value at zero volts. As detailed in the
when the cantilever oscillation amplitude has returned to zero, the dc voltage on the tip and
sample are the same. The NanoScope records the dc voltage applied to the tip and this signal
is displayed in the
Potential
data type.
12. Adjust the Input gains. The feedback loop that is used by the Basic Extender for surface
potential measurements is the same as the one used in Frequency Modulation (FM) for
magnetic and electric force gradient detection, as described previously. The feedback loop
should be tuned in a similar manner to the FM setup. Select
Other Controls
and adjust the
FM gains. Setting both
Input
igain
and
Input
pgain
to
15.0
is a good starting point. As with
the topography gains, the scan can be optimized by increasing the gains to maximize
feedback response, but not so high that oscillation sets in. The gains often need to be much
lower for potential measurements than for standard FM measurements. More information on
tuning the feedback loop is given in
below.
Note:
In older versions of software, Input igain = FM-igain and Input pgain =
FM-pgain)
