- Greater force sensitivity
- Faster scan rates in air
- Superior imaging in liquids
- Imaging softer materials
- Reducing the effective quality factor in UHV experiments
- Applications in atomic force microscopy, SNOM, and shear force microscopy
The ActivResonance Controller is an invaluable accessory for scanning probe microscopes.
By electronically changing the effective properties of your force sensor, it allows you to fine tune its response to any interaction. It dramatically improves the performance of existing force microscopes, especially when working in liquid environment.
When operating a scanning force microscope in dynamic mode
it is the quality factor that controls the settle time of
the oscillating probe, and therefore how long the probe takes
to react to a change in interaction. With the ActivResonance
Controller it is possible to reduce the effective quality
factor, allowing faster scan rates without having an adverse
impact on force sensitivity.
When imaging in liquid using a dynamic mode, the cantilever's
oscillation is heavily damped. The lateral forces are reduced
considerably, but the normal forces are still high enough
to distort many samples, and make imaging impossible. The
phase angle measurement now a standard tool for enhancing
contrast in air, is almost completely lost in liquid. These
problems originate from the viscous damping of the AFM cantilever
by the liquid environment. The ActivResonance Controller's
patent pending technology electronically removes the effective
damping of the cantilever. This gives back to the user all
the advantages of tapping in air, without having to make any
compromises on environmental conditions.
The
ActivResonance Controller feeds back the processed amplitude
and phase response of the cantilever into the drive signal,
allowing the effective quality factor of the cantilever to
be enhanced. The force sensitivity of the oscillating cantilever
is proportional to its quality factor, so by increasing the
effective quality factor the force sensitivity can be similarly
improved. The effect of increasing the effective Q factor
in liquid is seen in the increased sharpness of the resonant
peak with the ActivResonance Controller enabled. The value
of the effective Q factor and peak sharpness can be selected
over a continuous range.
Reducing the Q factor in air (AFM)
Polymer
Crystallisation: Phase angle images showing the crystallisation
of polymer spherulite at room temperature, in which the effective
quality factor has been reduced from 270 to 90 (resonant frequency
272kHz). This enables faster imaging of the sample but maintains
good force sensitivity, particularly important when following
a process such as crystallisation from the molten phase; 50µm
scan area, tip velocity 403µm/s.
[Rollover
the image above to see a close up of a 1µm scan taken
at a scan rate of 22Hz showing lamellar resolution of the
growing spherulite. (arrowed above)]
Enhancing the Q Factor in liquid (AFM)
Phase-separated
Lipid Membrane: Phase angle images showing fused lipid
vesicles. Lower phase noise and higher phase contrast are
achieved on this soft specimen with ActivResonance Control
enabled.
[Rollover the image the image above to see the effect from enhancing the Q Factor.]
Polystyrene/dekalin
gel:
Topographic images showing the swollen surface of an isotactic
polystyrene/dekalin gel imaged under dekalin. Another example
of how the ActivResonance Controller may be used to image
samples that are too soft to image with
conventional liquid.
[Rollover the image above to see the effect from enhancing the Q Factor]
Reducing the Q Factor for Shear Force Microscopy
DNA:
Images of double-stranded DNA taken at the same scan speed.
(a) with a Q of 404, the true Q of the probe, (b) taken with a Q of 134.
The reduced Q allows the surface to be tracked much more successfully
- a similar image with the high Q took three times as long
to acquire. The inserts show the change in probe amplitude
with time after the drive force has been switched off at the
red arrow - i.e. the settle time of the probe. This is a common
problem in SNOM using shear force feedback systems where scanning
too fast, without good height control, can lead to topographic
artifacts ruining the optical information.
Other Applications
The ActivResonance Controller can be used to create new applications for conventional AFMs. In this example it is used in dynamic force spectroscopy using a conventional AFM, enabling the complex mechanics (elastic and viscous) of a molecule to be measured. With conventional AFM, in liquid, the microscope does not have sufficient sensitivity to shifts in the resonant frequency of the cantilever, or to changes in its amplitude which are needed to give the full complex mechanics of the molecule. These shifts can now be measured using the ActivResonance Controller. The figure shows a series of force vs. extension plots for a molecule of the polysaccharide, dextran. The bottom trace shows the simple deflection vs. extension data, the data that would be conventionally obtained, shows a hump before final extension and pull-off.
Uniquely, the ActivResonance Controller unravels the elastic and dissipative interactions.
It is believed that this peak is due to the chair to boat configuration transition in the glucose ring.
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