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Dear All,
Does anyone know why the DMT modulus image have negative value (no flatten)?
Thanks,
Iwei
Most likely you have a too high deflection sensitivity, you can try reducing it and modulus image would back to normal.
Ang Li
As Ang Li says, the problem is that your deflection sensitivity is too high. This causes the force vs. separation plot to bend back so that it appears that the sample deformation decreases with increasing force. When the DMT model is applied to this data it results in a negative modulus.
If you want to work with a qualitative measurement, you can just decrease the number (look in the 'Calibrate>Detector...' menu for the 'Deflection Sensitivity'). If you want accurate numbers you need to calibrate the deflection sensitivity in ramp mode on a sample that is clean and very hard (saphire, silicon, etc depending on how stiff your probe is).
--Bede
Thanks Ang Li and Bede Pittenger.