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Conducting morphology scan at different tip-sample distance

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keith1019 posted on Tue, Apr 10 2012 5:09 AM

Dear all,

I am now using tapping-mode to conduct morphology scan and tip amplitude against time measurement by using the MultiMode IVa, the controller NanoScope IVa and lock-in amplifier (SR830) from Stanford Research Systems.

 

My setup is to illuminate chopped laser onto the sample surface to create force about 0.X or to several piconewton which may affect the tip vibration motion as well as the tip amplitude.

 

What I have tried is to record the tip amplitude against time at different tip-sample distance so that we can illustrate the force magnitude indirectly and also the morphological dependent force.

 

Thus I have to control tip-sample distance precisely as well as record the tip amplitude against time.

 

What I have tried is to fed signal from the In0 port on the top panel of the SAMIII to my lock-in amplifier and oscilloscope and to do:

  1. Conduct a simple morphology scan to get the information of the sample surface with feedback loop 
  2. Conduct another morphology scan and to record result from the oscilloscope without feedback loop

Finally I compare the result from 1) and 2) to get an indirect evidence of the created force in piconewton scale and the direct morphological dependent force.

 

The above is what I expect to do and get but

Questions

1)The step motor provided in the control software can't precisely to control the tip-sample distance and can't tell any information about the magnitude of tip sample distance. Are there any tools or functions can help me to control the tip sample distance and also give me the precise value of the distance?

2) If I keep the feedback loop on, can I determine the feedback frequency or reaction time? Since I have introduced lock-in amplifier to my setup, I don't want to capture the feedback modified signal.

Thank you very much for spending time on my post.

Keith

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replied on Tue, Apr 10 2012 5:59 PM

Hi Keith,

I am not really sure what you are attempting to do but:

1. The tip-sample distance is set by selecting a "setpoint" in Tapping Mode. I.e. the tip will be a a distance to the sample where the reduction in amplitude from the free amplitude equals the setpoint. At the end of the oscillation cycle the tip will typically be in repulsive contact with the sample. The "average" distance of the tip of course will depend on the oscillation amplitude that you selected.

2. What you can do to offset the tip in "z" is using a functionality referred to as LiftMode. In LiftMode your MultiMode would first acquire a line (trace) of the sample topography. The system would then turn the feedback off and retrace it plus a user selected z-offsett. That offsett can be zero or even negative.

So even though I am not completly sure what you intend to do, I have the feeling that using LiftMode will get you there.

Best,

Stefan

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Thank you for Stefan's help

What I would like to do is as follow

My sample is a metallic grating as shown in the above pic.

What I expect to be measured is that the tip amplitude above the region A is different from the region B when the force induced by the chopped laser at the interface of my sample.

Thus, I attempt to do:

I first conduct a morphology scan to get information about my sample surface.

Secondly, I repeat scanning again and again at different height shown as the dot lines in different colors with respect to the surface level labelled as region A to capture the tip amplitude against time as long as the scanning.

Thirdly, my expected result should be 

Hence, I seek for tools or settings to control the "height level"~ tip-sample distance.

or I should say to control the mean distance between tip and the surface of region A.

As described in the user manual, lift mode seems not exactly suit my use.

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Answered (Not Verified) replied on Wed, Apr 11 2012 1:58 PM

Hi Keith,

I still think that LiftMode does exactly what you intend to do, i.e. repeat scanning your sample at user controlled heights above the sample. During the Liftline you can monitor the amplitude or phase shift of your cantilever which you expect to be influenced by your laser.

Stefan

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Janne replied on Tue, Apr 24 2012 8:15 AM

I think he wants to do constant height scanning with feedback turned off. LiftMode doesn't keep the height constant.

I would also be interested whether this is possible.

 

Best,

Janne

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Ang Li replied on Tue, Apr 24 2012 8:26 AM

Hi, Keith and Janne,

I guess Nanoman can partially do what you are trying to do. In Nanoman, you can lift z to certain height and draw a line over xy plain in either contact mode or tapping mode, you can also define the speed of lift and line drawing, though this is only for one line scan but not a whole image scan.

Ang Li

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replied on Tue, Apr 24 2012 10:37 AM

Janne,

LiftMode does exactly that: Keeping the height above the sample constant by tracing back the previously acquired topography plus/minus an optional height offset. If you would use LiftMode you would actually trace the sample at varying heights depending on the topographic features on your samples. If your sample is atomically flat and has no tilt this is of course not an issue but the moment your sample exhibits tilt and/or topographic features not using LiftMode can introduce a significant error in your measurements.

There might be a possibility to use Force Volume for the experiments. When analysing the data one just has to pay attention to the actual topography measured at each pixel location in order to compensate for it.

Stefan

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