Technical information

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a determined distance from the previous z-position has been traveled. The Z distance
between data points may be calculated as follows:
lineSamps
nmsizescanZ
spodatabetweencedisZ
/
)(
inttan =
The raw ASCII data has now been converted and is ready for further analysis.
4. Convert the deflection vs z-piezo position data into force vs separation
Begin by plotting the raw data; there is some key information to be gotten from the
plot. First locate the jump-to-contact point of the extension curve. Offset all of the data
points such that the initial point of contact is at the origin of the graph.
The slope of the retraction curve to the left of the point of pull-off is the sensitivity of the
piezo. Divide all deflection data points by this value; the resulting values are the actual
deflection data:
slopecurveretraction
nmpodataraw
nm
)(int
)( = δ
Next convert the z-piezo position values into separation from the surface by subtracting
each of the retraction and extension δ values from their corresponding z-piezo data points:
δ −=
positionZpiezonmSeparation )(
**NOTE: If you wish to plot the deflection vs z-position curve before proceeding to conversion to
force vs separation, e.g., the above calculations should be modified such that the deflection is found
by dividing the raw data points by the negative of the retraction curve slope. When calculating
separation, then, you should add rather than subtract the deflection value from the z-piezo position.
Doing so will give you the desired curves in both plots.
Finally, convert the deflection data into force data by multiplying each of the points
by the spring constant k (in N/m) of the cantilever tip used to make the measurement:
knNForce
×= δ
)(
And voila! Plot the force vs separation and you may obtain the forces involved in the
jump-to-contact and the pull-off. Over a series of force curves of the same sample a
statistical analysis if these forces may be performed to obtain well-supported experimental
information on your sample. In addition, the slope of a line connecting the last point of
adhesion to the first point after pull-off should be equal to the spring constant of your
cantilever tip.