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Questions about the "Fast retrace" option and the Peak Force tapping

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Philippe Herman posted on Fri, Dec 21 2012 5:35 AM



I'm wondering  what does the "Fast retrace" option (located in the image scan tab) enables or disables during a force volume image in contact mode besides the fact that it divides the time required to obtain the FV by 2.

Secondly, I tried to use the peak force tapping mode also. I used a peak force frequency of 1Khz, and a scan rate of 1hz. Wich means that every second a line (one trace and one retrace) is achieved by the tip. Meaning also that on one line, 1000 force curves are recorded. But the tricky part is that only a maximum of 256 samples/line are recorded. So the question is 3/4 of the original info is lost, why? What does the program use to discriminate the force curve it will take and "reject" the others? It would certainly have a link with the "32MB limit" for a file. Why do we have that limit and is it possible to overcome that limitation?


Thank you!



Philippe Herman

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Bruker Employee

Hello Philippe,

When Fast Retrace is enabled in FV mode, the system does not perform force curves during retrace only during the trace. This is why your time to collect the FV image decreases by half.

With respect to PeakForce Tapping, it is my understanding that the system does not 'record' each force curve because of the amount of time and computer memory/storage that would be required to do so. If you would like to see 'consecutive' force curves, you can do a high speed data capture (HSDC). This will allow you to record all the force curves obtained on a certain location of your sample. The HSDC cannot record all force curves for an entire image - only as many the buffer can hold (which would be equilavent to a couple of scan lines). The software can also show you the location in a captured image where the curves were taken. 




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Hello Andrea,


Ok, for the explanation for the fast retrace, but when I do a FV and disable the fast retrace, I don't have 2 force curves for 1 pixel, so what happens with the second force curve?

Actually, for the peak force tapping, when 1000 curves are recorded and when I ask 256 samples/line, it means that the system does 256 curves on the trace and 256 on the retrace, so 512 curves.Thus, 488 curves are "missing" and the system discriminate them in some way, but how? I understand that there is a limitation of memory/storage, but considering the fact that the size of a recorded file cannot be superior to 32MB, I think that even if the files grows even 10 times bigger, it shouldn't be a problem for a brand new computer to handle it.

HSDC allows me to have the 1000 curves for one or two lines, so this option is not suitable since I'm looking for the information on a large area (living cell) so for me one or two scan lines is not interesting.


Thank you very much for the explanation, it helps to understand how the system works.





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Hey Philippe,

I answered (correctly I hope) a previous question which might help your understanding of how peakforce tapping determines sample topography.

I'll happily stand corrected, but my understanding is that peakforce tapping isn't performing "controlled" force curves at each point to determine the height used for topography.

The scanner is oscillated (in your case at 1kHz) and the probe taps the surface accordingly, in the same way as it would in tapping mode but in this case well below resonance. The deflection of the probe is then monitored and the force curve recorded for each tap is detected and analysed by the electronics to determine the peak force that the probe experiences. The system uses this as the feedback mechanism and adjusts the scanner extension to maintain the peakforce setpoint.

The topography you obtain is simply the extension of the Z-scanner whilst the system is maintaining constant peakforce as the system scans, so if there are four force curves performed at a pixel then you are simply averaging the height data at that point.

As far as retaining force curves goes; I believe that the latest Nanoscope v8.15R3 software introduced the "Peakforce capture" feature which actually allows you to record the force curve for each pixel (in this case I don;t know if it records all of them or just picks one).

See the description below taken from the following link:

Enhancements to PeakForce QNM

  • The new PeakForce Capture function now allows a force curve to be captured at each pixel in the PeakForce QNM images, providing researchers full access to the PeakForce QNM data to explore alternate analysis models and validate their results against other techniques


Hopefully this helps!

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