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Force displacement curve
> F(z) z
This is a force-displacement curve, showing the change in the force acting on the tip as it approaches and withdraws from the surface.

The horizontal axis is the extension of the piezo in the z-direction.

The y-axis is the force experienced by the cantilever. Since force is calculated from Hooke's law: F = -kx, the y-axis can also be considered as cantilever deflection.
Force displacement curve
> F(z) z F(z) z
When the tip is far away from the sample, the cantilever hardly feels any force from the sample surface, so the deflection is almost zero.
water layer adsorbed on surface