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The DMT model is based on the spherical tip rather than the circular contact. Therefore as the indentation goes deeper the increased contact area is taking into consideration in the model. There is no further correction needed. On the other hand, if the tip continue indenting beyond the spherical part, entering the linear side wall of the tip, the DMT
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One way to judge if the probe is appropriate is to look at the deformation channel (assuming you have calibrated the optical sensitivity). If you have to use lots of deflection setpoint (up to volts) to generate fraction of 1 nm deformation, the lever is too soft. Vise versa, if you produce large deformation ( high 10s of nm) even at low set point
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PFT has close similarity to force volume, where the trigger force controls the maximum interaction force. There are two major differences. The first one is the level of force control. While force volume operates at a trigger force well in excess of nN in order to gain a stable trigger, PFT can control down to pN level. The second difference is the speed
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The limitation comes from the spring constants of the cylindrical probes which are usually much stiffer, limiting the precision of force control and the ability of the system to preserve the sharp apex. A sharp SNL (0.2-0.4 N/m) can have a half angle as high as 80 degree in the apex portion. Judging form the angle of you sample, it may worth to try
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Another possibility is to image DNA and look at the width expansion. If you do have DNA width near 2 nm, your tip must be very close to 1 nm in radius. In case you don't want to bother working with DNA you could look at HOPG step and see how sharp you can get to.