Spring Constant - JPK instruments nanowizard afm Handbook

properties. There are many different options, depending on the particular
application, but a couple are introduced here.
Hydrophobic cantilevers
Hydrophobic tips can be used to image hydrophilic surfaces (e.g. proteins) under
high-humidity conditions with small forces and thus high resolution. The adhesion
between hydrophobic and hydrophilic surfaces is lower than between similar
surfaces, so the sample disruption should be less. Silanization is one method to
change the chemical groups on the surface of the cantilever, and make it more
hydrophobic. An example of a silanization protocol is given in this handbook, see
Section 9.2.
EBD tips
Electron beam deposited (EBD) tips are AFM tips that have been modified to grow
a narrow tip-on-the-tip. Normally shaped silicon intermittent-contact cantilevers are
modified in scanning electron microscopes. The electron beam focused on top of
the tip deposits a small column of carbon, leading to a tip with narrow radius and
high aspect ratio. The carbon is from the hydrocarbon contamination in the
vacuum chamber of the SEM.
The high aspect ratio of the tip is one advantage of this method, and allows
Knapp, HF, Wiegräbe, W, Heim, M,
imaging of structures such as grooves or troughs, that are not imaged well by tips
Eschrich, R, Guckenberger, R.
with conventional shapes. The amorphous carbon surface may also be less
Biophys. J. 69 (1995) 708-715.
adhesive than the bare silicon tips for imaging samples such as proteins. Each tip
"Atomic force measurements and
must be individually modified, however, and this is not practical for most AFM users
manipulation of Langmuir-Blodgett
on grounds of either access to equipment, or cost to purchase such tips.
films with modified tips"

4.5 Spring constant

Background information
The analysis of force curves uses force against tip-sample separation. This means
the vertical deflection signal from the detector has to be converted from Volts into
Newtons, and the z-position signal needs to be corrected for the deflection of the
AFM tip along the same axis.
The spring constant calibration (N/m) is generally the most difficult part of the
calibration. There are various different methods to calibrate the spring constant of
an AFM cantilever, and unfortunately all of them have significant problems. If
experiments are compared where different methods are used, sometimes
differences of 10-20% can be seen. The methods are reasonably consistent if they
are used carefully, so it is often good to pick one particular method and calibrate all
cantilevers as consistently as possible. This means the data from different
cantilevers can be combined well to give good statistics. Then it is just important to
realize that there will be some systematic difference between the results from
different methods.
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JPK Instruments NanoWizard Handbook Version 2.2a 21