Technical information

42
11. FORCE CURVE DATA CONVERSION
(M. Rixman)
For data from the DI Multimode AFM
1. Export the raw data as an ASCII file
Do this in the Offline/Utility menu. Header information is known to cause
problems in some third party software applications and should therefore not be included in
the ASCII file. Be sure to save the raw data file. The header information will be required for the
data processing, and can be accessed from the raw data file.
2. Obtain necessary parameters from the header file
Required parameters include:
(from the \*Scanner list): \@Sens. Zscan (nm/V)
(from the \*Ciao scan list): \@Sens. Deflection (nm / V) (from
the (from the \*Ciao force list): Scan Rate (Hz)
Z scan start (V)
Z scan size (V)
(from the \*Ciao force image list): Samps/line
\@Z scale (V / LSB)
3. Open the ASCII file and begin data conversion from LSB to nm
The data will probably be in one long column of a length equal to twice the number
of samples per line. If Samps/line=512, the first 512 values are the extension values and the
second 512 values are the retraction values. The data points are in the order that you would
view them on the actual force curve from left to right. For ease in data analysis, it is best to
separate these into two adjacent columns. If you have converted the force curve into
something other than the default (deflection vs z-piezo position), such as force vs
separation, then there will be one column of a length equal to four times the number of
samples per line; the first two sets of data will be the default values (i.e. deflection vs z-piezo
position), and the second set will be the converted values (force vs separation, e.g.). In each
set, the extension data will precede the retraction data, and will be ordered as you would
view them on the force plot from left to right.
The data points are in LSB (8-bit two’s-compliment) format, and must first be
converted into A/B voltages and then into nanometers before further analysis may be done.
The process is outlined below:
{}{ } )/(@\)(int/ LSBVscaleZLSBpodatavoltageBA •=
{}{ } )/(.\)(/ VnmDeflectionSensVvoltageBAdeflection •=
Now that you have converted the data points on the ordinate of the force curve, you
must calculate the corresponding z-piezo position data points. Begin by converting Z scan
start and Z scan size from V to nm by multiplying them by the \@Sens. Zscan value
(nm/V). The first deflection point in both the retraction and extension data sets corresponds
to the piezo position at Z Scan Start. Each successive deflection data point is reported after