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I am conducting some custom force spectroscopy tests and am interested in how much the tip penetrates the sample. I am measuring force relaxation to obtain viscoelastic properties. Presently, I am applying a specified Z ramp and I am holding this Z for a specified length of time while monitoring deflection error. I would like to know how deep the probe has penetrated the sample.
Can anyone recommend a calculation procedure for determining the deformation (from the surface) or as a possible clue, how is "separation" calculated from the available channels?
Also if anyone can recommend an experimental procedure to conduct a "force controlled" (creep) test where I hold load constant and monitor deformation with respect to time, that would also be great.
Many Thanks,
Christopher
Hi Christopher,
In normal ramping mode, giving a surface delay, measn indeed as Mike says a constant height (of the z-sensor) and you can measure the deflection variation by strip chart.
If you howver go into the PicoForce software, there you can create a specific script and really do force-clamping (keeping the deflection (=load) constant during a period of time and at the same time measure the variation in z-sensor).
Also a controlled variation in deflection is possible in this scripting mode.
regards,
Patrick Markus
Christopher,
Calculating separation requires knowledge of how much the probe deflects vs. how much the sample deforms under the tip when the z piezo is extending. First you must calibrate deflection sensitivity on a hard surface. This teaches the software how z piezo motion translates to photo diode voltage. Calibrating deflection sensitivity on a hard surface ensures that all of the z piezo motion translates into probe deflection. After this step is done the photo diode can be used to measure z distance in nm instead of volts.
After deflection sensitivity is calibrated you can ramp z and capture a force curve (deflection vs. z).
When you open the force curve offline you can choose to display "deflection vs. separation" instead of "deflection vs. z."
The horizontal axis on a deflection vs. z plot is just z piezo distance (nm). On a separation plot the horizontal axis becomes [(z piezo distance in nm) - (photo diode measured z distance in nm)]. In regions where there is no probe/sample interaction the plot looks the same (because 0 is subtracted for each value of z piezo distance). However in the separation plot looks different because not all of the z piezo motion causes probe deflection. Any z piezo motion that is not seen by the photo detector is presumably due to the sample deforming under the tip.
With regards to the force controlled experiment many people bring the probe down and hold it using a surface delay while monitoring deflection on a "strip chart." IIf I'm not mistaken this is sometimes called force clamping. To give you better advice on this please indicate the type of spm and version of software you are using.
I hope that helps. Please let me know if I can clarify more.
Regards, Mike Maybrun
Veeco Applications
Thanks for the response. I am using a newer Dimension Icon with NanoScope 8.10. I had tried the surface hold option, but I could not get data to be captured while the hold was taking place. Since then however, I think I understand the strip chart operations better and perhaps I can make it work now.
A second question: with Closed loop on, will the system hold the deflection as solidly as it does with the z piezo displacement?
Thanks again and I will post back with my results from using the surface hold and the strip chart.
With z closed loop turned on the system will feedback on the on the z sensor (not deflection). This will ensure that the z piezo position does not change. Assuming that nothing besides z piezo motion is causing probe deflection then the deflection would remain constant. However if the probe is curling due to thermal changes or the sample is drifting you can still observe deflection changes even while holding the z piezo position constant.
Hope that helps,
Mike
So that makes me think that a "creep" (force controlled) experiment is not possible. With closed loop on, the feedback is still keying in on a displacement (zsensor) not on a force (deformation). Does that sound correct?
Also I was unable to capture data with the strip chart using the surface hold option. I can get relaxation data (displacement controlled) data using the custom type features in the expanded view under ramp, in concert with the strip chart. If anyone can think of a way to run a Force controlled experiment, I would be interested, but for now, this will work.
Thanks again,
Is the PicoForce software usually installed? I do not see it on our computer. If not can I obtain a copy?
Here are some steps to follow to illustrate how scripting can be used to hold a constant deflection. The resolution of the images is poor so I've tried to highlight when appropriate.
Step 1. Enter Ramp mode.
Setp 2. Click on the icon circled in red to open the pico force environment. A new panel will appear to the right of the plot area. Click on the "Script" tab.
Write a script with 4 segments summarized as follows:
Segment 1 - Ramp Z +1 um (relative) towards surface and delay on surface for 1 second.
Segment 2 - Ramp Z -1 um (relative) away from surface and delay above surface for 1 second.
Segment 3 - Ramp Setpoint +1 V (relative) and then hold for 1 second. (+1 V sepoint chosen because it puts the z piezo in roughly the same position as the z ramp in segment 1 for illustration puposes).
Segment 4 - Ramp Setpoint another +1 V (relative) and then hold for 1 second.
Step 5. Configure the script segments using the following images (press the "Add" button to add a segment). Ideally the gain values used should be the same for all segments. 5 & 10 should work fine for this script. I neglected to set them the same in this experiment.
Step 5. Press the "Run" button. This will start the strip chart. When the segments complete press the "Pause" button to stop the strip chart.
Step 6. Press "Capture" to capture the strip chart data.
Step 7. Open the captured strip chart data.
The captured data should look similar to the following (I have added lines to show approximate beginning and end of each segment).
In segments 1 & 2 the script "Ramp Channel" was set to "Z Scan." Here the z position is held constant while deflection is allowed to change. This can be observed in the following image by noting the change in deflection and the constant z sensor position during the 1 second delays in segments 1 & 2. Deflection and z sensor during the delay of segment 1 is circled in red below.
In segments 3 & 4 the script "Ramp Channel" was set to "Setpoint." Now both the ramp and the z position during the delay are determined by adjusting z to maintain a constant deflection. This can be observed in the following image by noting the change in z sensor and the constant deflection during the 1 second delays in segments 3 & 4. I've zoomed in on the data during the surface delays for clarity.
I hope that this process is illustrative for the purposes of your clamping experiment.
Sincerely, Mike Maybrun
Thanks for the effort in repairing this demonstration. I hope that you saved it somewhere, for future reference! I have executed this experiment exactly, and then extended the idea to longer hold times which are more appropriate for my samples. I have three observations:
First about half the time, the software objects with a message indicating failure to obtain initial set point when the experiment segment is Set-point>relative.
Second, the system does not seem as proficient in holding the deflection constant as it does with the z piezo. For example, the applied deflection does not settle as quickly, and then "drifts" downward or upward during the hold.
Third, there seems to be some dependence on the ramp velocity and hold time that I have not totally figured out. If the velocity is set too fast, the system gives the failure to obtain initial set point error and if it is too slow, it seems that the segment gets cut off before the ramp is complete. I wonder when the hold is initiated, after the ramp ends or at the beginning of the ramp.
In any case the system is capable of running a displacement controlled experiment, which suits my needs well enough. Thanks again for your help with this question.