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Force-distance curves can be used to measure the elstic property of samples. In Dimension ICON, force-distance curves are generated in Ramp mode after engaging the tip to the smaple surface. In experiments, hundreds of curves need to be obtained autonatically to get relatively accurate measurements. The tip velocity, peneration depth or maximum force before tip retraction are important parameters especially when the sample is nonhomogeneous.
How to control the penetration depth into the sample OR the maximum force just before the tip retraction in ramp mode? It seems that the Ramp size and Trig threshold both can affect the penetration depth but to what extent? Do the tip retract if either one of them is meet when ramping?
Thanks!
The non-contact value in the baseline region of force curves is arbitrary. It depends on how you have adjusted your vertical deflection signal. Actually, since it is normally "deflection error" it will be the raw deflection signal minus the deflection setpoint. All forces in the curve must be measured relative to the baseline force, which yes, should normally be zero.
The new NanoScope Analysis has a couple features that are useful for both of your questions. One is a "baseline correction" feature. It can offset a force curve so that a selected portion of the baseline is redefined as zero. It can also remove tilt, if present.
The other feature is "change parameters", which as the name suggests allows you to change saved parameter values in ramp files. These include spring constant, deflection sensitivity, tip radius, tip half angle and Poisson ratio.
Download instructions are in the post pinned at the top of the list in these forums.
-Ben
You can use a "relative trigger" on the deflection signal to control the maximum tip-sample force in a ramp mode force-distance curve. For instance, if you set a relative trigger of 1nN, you will find that the force curve extends until the force reached 1nN and then retracts from that point. I know it seems overconstrained by the ramp size parameter, but for single force curves we first go into feedback (using the imaging setpoint) in order to find the surface and then use that to estimate where we need to start the force curve (the Z offset) in order to reach the desired trigger within the desired ramp size. This isn't perfect. Sometimes we will need to ramp a bit farther than one ramp size before the trigger is reached. In this case, Z will retract exactly one ramp size from the trigger point and the extra distance on the extend side of the curve is truncated so that it matches the ramp size. You might also reach the trigger before ramping the complete ramp size. In this case, Z will retract one ramp size from the trigger and the extend curve will simply be shorter than the retract curve. If instead of a single ramp you use continuous ramping, then this process tends to be self-correcting because the immediately preceding ramp has very closely set the Z offset to the required value. But if you look closely, there are still cases where the trigger is reached a little early or a little late because the trigger is ultimately in control. You will notice that the Z offset parameter (Z Scan Start) is grayed out (disabled) when the trigger is enabled.
There is currently not a trigger mode that supports using indentation depth as the trigger, only force. I agree that this could be useful though.
Hi Ben,
Your answer is very helpful!! thanks a lot!
I have two additional questions about FD curves:
1. In many curves, the steady force when tip is far away from the surface is not zero. Why this is happening? I checked the sensitivity and spring constant and kept the set point a constant value when the tip was engaged to the surface each time.
2. Is the force value linear to the spring constant value? If in the software the spring constant K is set as 0.5 N/m while the actual value is 0.8, can we correct theexperimental force values by timeing (0.8/0.5) in data process?
Ning
I just checked a FD curve in metric and force units. The force and deflection is liear, force equals spring constant times deflection. But I still dont understand why the stead force can be positive or negative value insteand of zero.
Thank you veru much, Ben！