Thesis-PDF - IAP/TU Wien

a vibration isolation table, the TableStable TS 150, to actively damp vibration
frequencies of 0.7 − 1000 Hz by up to 40 decibel.
The force signal generated at the photodetector diode was sampled at 5 MHz
bandwidth and all further signal conditioning and processing (i.e. filtering) done
digitally under software control. For controlling the AFM, the software "Igor Pro"
was used (see below).
Measurement method
Olympus AC 240 TS-E cantilevers with a resonance frequency between 61 and 77
kHz in air and a spring constant of 1.2 to 2.5 N/m were used for imaging. The
data acquisition for most of the presented images was performed at the slowest
possible rate of 0.1 Hz as it rendered the most details. Various attempts have
been made for scanning under liquid, but the results were falling short of those of
ambient AFM imaging. A number of problems have been identified, such as the
difficulty to firmly attach cells to the surface under liquid, the cell surface motion
even when attached to the substrate, and several problems related to the instability
of cantilever resonance frequency and deflection drift. For the experiments under
liquid another Olympus cantilever (around 13 kHz resonance frequency) with a
low force constant was used.
For the mode of operation mostly used in the experiments, the intermittent
contact mode, the driving frequency of the cantilever is an important parameter.
For the Olympus AC 240 TS-E a typical setting is resumed here:
Driving Frequency: 58,595 kHz (the frequency at which the cantilever is oscillated.
Setting it about 5% lower than the cantilever’s resonance frequency achieves
good sensitivity for surface measurements),
Piezo Voltage: 1,044 V (a relative measuring unit, related to the voltage needed
to oscillate the cantilever),
Amplitude: 1 V (this corresponds to the voltage measured at the photodiode
receiving the laser beam reflected from the cantilever. 1 Volt on the photodiode
corresponds to the set target amplitude of the cantilever that should be achieved),
Q: 56,8 (the Q-factor, a measure for the quality of the cantilever. It based onto
the shape of the resonance curve, and amongst other things takes into account the
proportion of peak height and width at half height. Values range from about 10
to 200 where higher is better.)
AFM software
The software used to operate the AFM at the Institut für Allgemeine Physik in
Vienna is IgorPro by Wavemetrics. All standard AFM techniques are available
through the IgorPro user interface that is open for customized routines and userdefined
functions. Direct access of all important controller in- and outputs also
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