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Hi,
We have a Multimode 8 and Nanoscope V controller with PFQNM. I have used PFQNM on polystyrene and blend samples in the PFQNM kit as well as on other polymers to obtain the correct elastic modulus value using DMT for mapping and force curves. One day the values were correct and the next day they were wrong and have been since. The force vs separation curves look different now (screenshot attached) compared to before, which in most cases was the ideal force curve. Any ideas as to what may be causing this? I have attempted many absolute calibrations since this started using different tips and operated using varying levels of PeakForce setpoint but all of the force curves look like the screenshot. I tested the piezo with a calibration grid and X, Y, and Z are all sub-2% error.
I am assuming the issue with the force curve is also causing the issue in modulus determination as well.
Thanks,
David
It looks as if your deflection sensitivity is too large, causing the calculation of the separation (horizontal axis) to be incorrect.
Did you calibrate the deflection sensitivity on a hard sample prior to acquiring this curve?
--Bede
I have used sapphire in the past, tried the fused silica, and recently moved to both to determine the deflection sensitivity. The cantilever spring constant is 5 N/m nominal.
One issue that has cropped up recently, and why I started looking at the fsilica sample, is the sapphire sample has what I am assuming water build-up on the surface, which is not being relieved with the use of a desiccator for storage, due to how the sample appears now as compared to a year ago and the sample images provided by Bruker. I have searched for application notes and internet references regarding this but nothing was found. However, both the sapphire sample and the fsilica sample produce the same deflection sensitivity.
Let me know if you figure out a good way to clean the sapphire. Maybe plasma cleaning?
oops, now I see you have a Multimode. That answers 1 &2...
I do not move the laser position once calibrated since the deflection sensitivity is a function of laser position on the cantilever.
I calibrated (minus radius) the system and obtained a force curve on silica using a sharp tip. Same kind of bad results though there was no negative aspect to the curve. I will do what you've suggested regarding the polystyrene then sapphire force curve acquisition.
If the sapphire sample contamination is just adsorbed atmospheric water then heating near 100 C should clean it. I have yet to see any reports about this sort of issue but I do not know how many people use the sapphire standard.
What do you mean by "Same kind of bad results though there was no negative aspect to the curve."? I thought your issue was the backwards slope of the contact part of the curve.
What was the ramp rate of the ramp you provided? The ramp rate during calibration?
I don't think water alone is the issue -- my feeling is that the tip would go through the water and we wouldn't really see it except in adhesion. I think there are organics adsorbing on the surfaces and that is the bigger issue.
Hi Bede,
Is there any documentation for plasma cleaning procedures of the sapphire sample in terms of time and or conditions? Also, will the cleaning itself damage the adhesive used to bond the sapphire to the metal puck?
Thanks
Bede,
At one point you had mentioned did the deflection sensitivity change with use and it appears that it does.
The deflection sensitivity changes through either increasing instrument use time or 2 degrees F temperature change. Whenever we calibrate the instrument and scan any given sample the deflection sensitivity decreases for a few hours then stabilizes. Once the deflection sensitivity stabilizes I have been able to get near-correct modulus maps for polystyrene (3.35 vs 2.7 GPa). I am attempting to determine if this is instrument use time or room/instrument temperature change by running the instrument after the temperature has stabilized. The temperature issue has only cropped up as a function of the university A/C cycle and we're eliminating it as a possibility since the room temperature and deflection sensitivity stabilized at the same time.
My deflection sensitivity calibration settings are set the same as in the powerpoint by Michael Maybrun: The ramp size is 500 nm, the ramp rate is 1.03 Hz, and the trigger threshold is 0.5 V.