UNIMO Technology UDR-204 Manual de instrucciones Pagina 214

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Force Imaging
Force Calibration (Contact Mode AFM)
194 MultiMode SPM Instruction Manual Rev. B
11.4.3 Advanced Techniques
Sensitivity Determination
The Deection Sensitivity allows conversion from the raw photodiode signal (in Volts) to
deection of the cantilever (in nm), and is normally set from the Force Calibration mode. The
sensitivity must be calibrated before accurate deection data can be obtained. Sensitivity is equal to
the inverse of the slope of the force curve while the cantilever is in contact with a hard sample
surface. Complete the following steps to calculate the sensitivity:
1. Obtain a good force curve on the display monitor using a hard sample so that the sample does
not deform.
2. Position the cursor on one end of the contact portion of the curve.
3. Click on the left mouse button to fix the line segment.
4. Drag the mouse to position the “rubber band line” parallel to the contact portion of the force
curve (see Figure 11.4c).
Figure 11.4c Set the Sensitivity Parameter
5. The second click on the mouse causes the system to calculate the slope of the line segment
and enter -1/slope as the Sensitivity in the panel.
6. A click of the right mouse button will remove the line segment from the screen.
If Sensitivity is calibrated on a material much stiffer than the cantilever, it measures the inverse of
the value of the AFM’s optical lever sensitivity; i.e., how many volts of deection signal are
produced by a given deection of the cantilever tip. The sensitivity will change for different
cantilever lengths and styles (shorter cantilevers give lower Sensitivities). Sensitivity will also
change with the position of the laser on the cantilever and the quality of the laser beam reection
from the cantilever.
Note: It is important to calibrate the Sensitivity parameter on a hard substrate as
described here BEFORE using the force curve’s vertical scale for quantitative
measurements.
Click and drag line parallel to plot
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