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What are the criteria for the cantilever selection for absolutely unknown sample, especially inhomogeneous one, when elasticity of components may vary by order of magnitude?
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 (10s of mV), the lever is too hard. A good lever generally gives you around 50 to a few hundreds mV control range for the set point and generate a few nm to low 10s of nm deformation.
I did not use force unit in this answer because the system is ultimately controlled by analog voltages. They reflect the generic performance of the system.
We recommend to use the Tap150A probe first as it covers the middle range of modulus. If the modulus measured are beyond or below the Tap150A capability range, then choose the stiffer or soft probe as described in the manual.
For the inhomogeneous sample, try to use the probe that can cover the range of component. But if it is not possible, then different probes are needed to measure different component of the material accurately. We also recommend user to zoom into a smaller area composed of just one component of material, perform AUTO CONFIG operation to identify and subtract the parasitic background and get accurate material property for this component of material.