Inphase and Quadrature

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Stephen Minne replied on Tue, Jul 5 2011 11:19 AM

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Check out the FAQ on PFQNM and ScanAsyst. This is answered in question 33: PeakForce QNM FAQ_Edited2.pdf

If you are looking to get adhesion/material property information I would recommend you use the QNM channels rather than these.

33. What is the meaning for “Inphase”, “Quadrature” channels in ScanAsyst mode?
It has been known since 1998 that the TappingMode phase signal arises from a combination of different properties of the probe's vibrational dynamics as it interacts with the sample. Both dissipative and conservative interactions can play a role.
The dissipation (or work done during the tip-sample interaction) in Peak Force Tapping can be calculated by integrating the dot product of the force on the tip with the velocity of the tip over one period of the cantilever vibration. The result of this integration is pi*(amplitude of the vibration)*(the component of the force that is in phase with the tip position). One might suppose that this would lend a very simple interpretation of the "Inphase" channel, but two factors complicate things:
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(1) A phase shift that arises from delays in the electronics, piezo response, and viscous effects
(2) A background force that is not corrected in this calculation (it is corrected most everywhere else in Peak Force Tapping) that arises from viscous effects and optical interference
A large phase shift can cause the contrast of either the Inphase or Quadrature signals to invert. By adding the Inphase and Quadrature components in quadrature, one can calculate the dissipation, but the background will be added to it. The background can vary with position (X, Y, and Z), so the Inphase and Quadrature channels can only be qualitative.
The bottom line is that the Inphase and Quadrature channels can be used to map out the lateral distribution of materials that have different dissipations when they interact with the SPM probe, but it is not possible to say which material is more dissipative for certain. This is similar to TappingMode Phase imaging, where the phase signal can also invert depending upon the imaging conditions and the different material properties present in the sample. ScanAsyst and TappingMode are both good for showing that there is something different, but not so good at showing what the difference is. If a user needs to know what the difference is, Peak Force QNM can answer the question for them.
Tip: Lowering the deflection limit will significantly decrease the bit noise in the Inphase and Quadrature signals.
Note: It is not possible to adjust the drive phase to compensate for the phase shift because the drive phase is already used by the Peak Force Tapping feedback background subtraction algorithm.