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Calibrating rectangular silicon probe (0.95N/m) with 4.5um polystyrene spherical tip

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sedeleo posted on Tue, Jul 9 2013 12:52 PM
Hello,
Thanks for reading, hopefully some of you have encountered this sort of thing before. Our current probes are 0.95N/m rectangular cantilevers with 4.5um spherical tips - and we haven't been able to calibrate them successfully. Because these cantilevers are silicon probes, they are not coated - so our working voltage is only about 2.5-3.5 (as opposed to a usual 6-7V).  Though they are for use in fluid, I have tried thermal tuning in both fluid and air. For the silicon 0.95 N/m with 4.5um polystyrene sphere attached, the resonance frequency range is 65-150 kHz with the nominal value being 105 kHz.
When I calibrate the probe in PBS, I have measured a deflection sensitivity of about 16-17nm/V consistently but have trouble measuring a consistent spring constant. I have been conducting the thermal calibration at 5-2000kHz. Additionally, we have adjusted the value for the cantilever (because it is rectangular - in the software program) and tried to lower the voltage at which we determine the deflection setpoint (0.6V is as low as we could go). As you will see in the image included, I am concerned because the baseline of our thermal tuning is not level. Current measured k values range from 2.5-10N/m.
Inline image 1
-
When I calibrate the probe in air there is also a tilt (though more slight).  This is an example of thermal tuning in air:
Inline image 2

I repeated the calibration in air yesterday. I got a deflection sensitivity measurement of about 22.44nm/V and a spring constant of 9.5907N/m - 10x the expected value, but the curve looks very similar to that pictured above. We have also been using a force trigger of 2V and centering the detector as much as possible. It seems I would have to get a deflection sensitivity of approximately 70nm/V to get a spring constant of ~0.95N/m. Thus far I have had deflection sensitivities range from approximately 16nm/V to 23nm/V. These values are consistent with those I obtained with a 0.35 N/m cantilever with a 4.5um polystyrene sphere tip that we were able to calibrate and use successfully.

Do you know of any way to export the data collected during thermal tuning for alternate analysis (i.e. Matlab)? We currently use NanoScope Analysis software, but this may enable me to better tease out parts of the problem. Additionally our system may be quite noisy, however we have been able to calibrate our first probes effectively via thermal tuning (0.35N/m triangular cantilever with 4.5um polystyrene sphere).
Thanks in advance!
Sarah
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Polis Tyrol replied on Wed, Jul 10 2013 9:44 AM

I cannot view your images. Did you try to upload them using "My Files" (bottom right of your profile homepage) and to include it using "Insert Media"?

Dietmar

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sedeleo replied on Wed, Jul 10 2013 9:58 AM

This is the first image, calibration in fluid

And the second, calibration in air

Thank you!

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Joop de Vries replied on Thu, Jul 11 2013 9:29 AM

Before you do the fit you should zoom in a lot more on the peak so that your x-axis is only ~10 to 30 eV. Than your baseline is "straight"and the fit will be good because with the axis like this you will get very wrong fittings and as a result wrong spring constants. You zoom in with "control" mouse click, but I think you will know that  

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Polis Tyrol replied on Wed, Jul 17 2013 9:19 AM

Please let me know if you managed to solve your problem.

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sedeleo replied on Fri, Aug 9 2013 12:50 PM

We were able to zoom in and fit the curve more closely but spring constant still measures a high spring constant. It seems we will need a reference probe in order to correctly calibrate this probe. 

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Ang Li replied on Thu, Aug 22 2013 8:07 PM

I guess the spring constant was specified to the original cantilever and since you have extra mass attached, the resonance frequency will shift and back calculation of spring constant from thermal tune will be offseted. 

Ang Li

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