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Hallo everybody,
I am using a multimode afm with a nanoscope IIIa controller, AS-130VLR scanner and nanoscope 5.31r1 software.
When recording force-distance-curves at different ramp-sizes for determination of deflection-sensitivity i observe a dependence of deflection-sensitivity on ramp-size.
This dependence is not an effect of scanner calibration. I tested this by calibrating the scanner at different heights (52 nm & 200 nm) and recording force-distance-curves with the proper z-sensitivity. The deflection-sensitivity in this case differs about a factor of 3!
To determine the "real" deflection-sensitivity i scanned a calibration-grating in "constant-height-mode" (I-Gain = 0.0001; P-Gain = 0) and calculated the deflection-sensitivity from the deflection-image.
An example of this dependence is shown in the graph. Especially confusing are the two occurring jumps at 700 nm and 1400 nm.
Please could you tell me how to determine the deflection-sensitivity in the right way?
Thanks a lot in advance :)
Best regards
Jonas
Jonas,
This idea about a bad Z board makes a lot of sense. NanoScope controllers use three digital-analog converters (DACs) per axis. One controls offset, another controls the scale, and another performs the actual scan. The concept is that the full bit resolution of the DACs is available no matter if you are doing a very large scan or a very small scan. If the scaling DAC were not functioning then it would affect the calibration for that axis. The less obvious point is that this scaling DAC is used in ramp mode, set to something a bit larger than the selected ramp size. So as you change the ramp size the scaling DAC changes too. This theory also explains why a second scanner didn't fix the issue.
This is actually pretty easy to test. If you scan the calibration grating with different "Z limit" values you would expect to measure the same pit depth at each Z limit value. If the values are off considerably then it indicates a problem with the Z scaling DAC. Be sure to set gains to track the pits, not zero gains like you did before.
If it's Tech Support suggesting this cause then they've probably seen it before.
Regards,
-Ben
A somewhat related issue I am having. I see a variation in sensitivity from curve to curve without changing anything! This can be 10-15% variance over the course of 10 minutes - random numbers, not drift. I am using a standard DNP probe (standard tip) and trying to calibrate the sensitivity on cleaned glass in pure water (Nanoscope IV controller and MM-AFM and liquid cell). I'm sticking to +/- 2V deflection. Ultimately I'll be trying to measure between a droplet on my cantilever and one on the surface within various aqueous solutions to try to observe differences in the forces in different solutions. But with such variation in the sensitivity (let alone the force constant) it's just random what the force curves look like. Does anyone think I have any hope at all of maintaining sensitivity throughout the experiments looking at the same droplets after repeatedly changing the aqueous environment?!
Many thanks in advance,
James K