DMT Modulus Calibration

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Mike Maybrun replied on Thu, Jul 15 2010 4:42 PM

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Suggested by Mike Maybrun

Sheila,

For the PDMS-1 the modulus value is around 2.5 MPa.  The DMT model requires, amoung other criteria, a sufficient amount of both indentation and probe deflection.  The TAP 525 is much to stiff to use with a 2.5 MPa sample.   It will deform the sample too much without deflecting the probe.  Use the ScanAsyst Air probe for measurements on PDMS-1. 

You have probably read that in most cases you should look to the deformation channel and adjust the setpoint until you have at least 2 nm of deformation.  While this tends to be true for samples with higher modulus (like the PS FILM) and stiffer cantilevers (like Tap 525) it is not a good technique for the ScanAsyst Air probe on PDMS-1 (or any sample < ~20 MPa).  The key is understanding that deformation is a value calculated in real time by NanoScope while indentation (the more important criteria) can only be calculated at this time by manual examination of deflection (force) vs. separation plots off line. 

It turns out that for samples with higher modulus and stiffer probes the deformation calculation is a pretty good approximation of indentation. 

For samples like PDMS-1 where there is a lot of adhesion the difference between calculated deformation data and indentation becomes significant (the deformation is substantially lower than indentation).

The following plots were obtained from raw deflection data captured with "High Speed Data Capture."  Individual force curves were exported and displayed as PeakF Deflection vs. Separation to highlight the way deformation is calculated vs. the indentation.

NanoScope calculates "deformation" in real time as around 85% of the separation from zero deflection to the peak force on the load curve:

This would represent 1 pixel value in the real-time deformation map shown below.  Note the average value of the deformation data in the data channel is around 3 nm.

The next plot shows what the real indentation is at the same pixel.  Indentation is measured as the separation from the minimum force to the peak force on the load curve:

In case it's not clear yet the next plot shows the difference at one pixel location between what would be displayed by NanoScope in the real-time deformation map and the actual indentation at that same location.

Another point worth noting is that the force monitor displayed in real time is PeakF Deflection vs. Z plot; not a PeakF vs. Separation.  When the horizontal axis is Z (not separation) the catilever compliance is not compensated for.  However the force monitor can still be used to get a better estimation of indentation depth for soft samples like PDMS-1 than you would get from the deformation channel and is much quicker than capturing raw data and analyzing off line.  If you use the force monitor you will probably over estimate the indentation by around 25-35%.  The next plot shows what the force monitor was displaying when the above force curves were captured. 

If you were using the "absolute" method to calibrate these would be important points because the indentation depth must be used for the "height from Apex 1" parameter in tip qual to estimate the tip radius.  If 3 nm was used (instead of 21 nm) as the height from the tip apex the tip radius would be under estimated and the modulus would be over estimated because modulus is proportional to cantilever spring constant over the square root of tip radius.

Since you are using the relative method I am assuming that you were simply following a generic procedure that reccommended 2 nm of "deformation" but didn't take the error in deformation into account with softer, adhesive samples.

The good news is that if you are engaged on PDMS-1 with a ScanAsyst Air probe you certainly will have more than 2 nm of indentation.  You don't need to be concerned with any adjustments to increase the setpoint unless there are tracking problems.  500pN setpoint usually works well with ScanAsyst Air probe on the PDMS samples from the sample kit.

You also had a few other concerns.

The problem with negative deformation probably means you do not have a good deflection sensitivity value.  Remember to calibrate deflection sensitivity on a clean, hard surface like sapphire.  Use ramp mode; not the "update sensitivity" button on the force monitor.  Do several calibrations and make sure the numbers agree closely and that they are not abnormally high for the type of probe you are using.  If they are move to a different area of the sample and try again.  Deflections sensitivity is the most important step in the calibration.

For problems with the DMT Modulus data looking digitized or off scale you just need to adjust the DMT Modulus Limit.  This parameter can be found by clicking on the expanded parameter list icon (4 red puzzle pieces) and then locating the PeakForce Limits tab.  Lower the DMT Modulus Limit to around 100 MPa or so for PDMS - 1.  This parameter is optimizing the range and resolution of the digital converter for modulus data.

Finally, if you use the "absolute" calibration method you will need the "tip check" sample (titanium oxide) included in the PFQNM sample kit.

Hope that helps,

-Mike Maybrun

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