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Why DMT modulus change a lot with Peak force setpoint in peak force QNM mode?

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psuxi posted on Mon, Sep 3 2012 2:00 PM

The DMT modulus change a lot when I scan the same position on my specimen repeatedly with different setpoint values. The mean modulus from the image obtained with 2 nN can be five times larger than the one with 1 nN. Based on my scanning results, it seems that the modulus usually increases with the setpoint values.  Apparently the modulus of the scanning material doesn't change. How do I know which value is truthful?

Thanks!

 

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There are several reasons that this might occur:

  1. The elastic model is inappropriate due failure of geometric assumptions.  The DMT model assumes that the tip shape is spherical with a radius R>>deformation depth.  If this is not the case, you may want to try a Sneddon cone model, or some other model.
  2. During the experiment, your tip shape may be changing.  You can check for this by changing the order of the different setpoints to see if the trend still holds.
  3. A combination of using a cantilever that has too low of a spring constant and an inaccurate deflection sensitivity could cause the separation axis of the force curves to be calculated incorrectly and cause error in the fit.
  4. The elastic model is inappropriate due to other deformation mechanisms such as plastic creep.  You can check for this by changing the order of the different setpoints and/or changing the modulation rate.
  5. The sample has a varying modulus as a function of depth.

Note that other indentation and contact resonance type techniques are also suceptible to these sorts of issues. 

--Bede

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Bede,

 

We acquired Dimension Icon AFM in July with PF-QNM capabilities. We were provided with standards for PF-QNM such as PDMS soft 1 (2.5MPA), PDMS soft 2 (3.5 MPA) and Polystyrene (2.7GPA). I took the polystyrene and did measurements on it using the Hysitron nanoindenter, and I found the modulus to be 106GPA.

1.       Why such disparity?  I have read somewhere in the Bruker’s application notes that the standards age with time. However I don’t expect them to have aged that fast.

2.       I have also done both absolute and relative methods for PF-QNM. It has been stated in the application note that absolute method works for soft materials. It has also been indicated that relative method is good for both soft and hard materials. However, the application note does not state the range for absolute method. Please give me the range which absolute method can be applied with acceptable measurements.

Thank you,

 

Wilson

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Hi Wilson,

There is no way that the PS could have aged that much.  Either there is a problem with the measurement, or your sample is not PS, or the PS is so thin that the nanoindenter is feeling the substrate.  Can you scratch the sample with the AFM probe using nanoman or contact mode?

The absolute method can work over a similar range to the relative method if you can calibrate the spring constant and end radius accurately with the probe that you want to use.  This becomes a problem with stiffer probes.  Accuracy of the spring constant measurement really starts to drop for K>20N/m or so.

--Bede

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Wilson replied on Fri, Oct 12 2012 10:48 AM

Bede,

Thank you Bede for the answers. What the the thickness of the standard PS film provided upon acquisition of Icon AFM. What subtrate is the PS film deposited on?

Thank you,

Wilson 

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Wilson replied on Fri, Oct 12 2012 10:53 AM

Bede,

Thank you Bede for your response. What is the thickness of the standard PSFilm normally provided upon acquisition of Icon AFM with PF-QNM?

What substrate is the PSFilm deposited on?

Thank you,

Wilson

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Suggested by Bede Pittenger

The PS film is deposited on Silicon.  The thickness is not controlled very accurately since it is spin-cast.  I think it is typically 100-500nm thick.  You can test this using a razor blade to make a scratch and then imaging the area with the AFM.  The thickness will vary a bit over the sample area, so you might want to try a few spots.

--Bede

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