Nanoprobe Network

Huabin - I'm glad you are using the wedge calibration method for lateral force calibration. For everyone: the latest software for that is available here on the Nanoprobe Network in the Software Library. I have just updated the files there.

If you are using a Veeco/Bruker AFM, then the file "friction_v_load_new.m" is set up to import and process the raw data. The user is prompted for an 'Offset' value that is applied to the Normal force signal. This is because often the photodiode is not set so that the normal force signal is actually zero when there is no force on the cantilever. When the tip is out of contact from the sample, the signal should be zero. If it is not, then enter a value close to what you see in the data (assuming your data includes a portion where the tip is out of contact) to offset the normal signal to make it equal to zero when out of contact.

This could be a little confusing since the term "offset" is also used to describe the sum of the lateral force trace and lateral force retrace signals.

I'm looking for a volunteer to update the Matlab file with some clearer language - any takers? You can post it in the Software Library.

If you're only doing lateral calibration, then you probably only want to take the data at positive loads anyway (avoids possible artifacts from the nonlinearity at low/tensile loads) so there may not be any out-of-contact data to subtract out, and you can ignore it. (But it is useful for plotting actual friction vs load measurements.)

As I recall, the wedge method relies only on the slopes of the four (T+R up, T+R down, T-R up, T-R down; all vs normal force) lines, and those are independent of the fixed normal force offset. So the Matlab script would definitely ignore it.

Matt Brukman

EDIT:

The four lines are (T+R up, T+R down, T-R up, T-R down; all/2 and vs normal force)

The 'offset' here really means any normal external load being added to the cantilever. So maybe we can rename the 'offset' to 'external load setpoint', and use the variable 'setpoint' in the code.

-Jingjing

JingJing,

There is no external load applied to the cantilever, rather the feedback loop maintains a load equivalent to ([deflection_setpoint] - [out_of_contact_deflection]) x [optical_sensitivity] x [spring_constant].

The "offset" referenced in the code is a correction to the out of contact deflection value and is necessary because cantilever drift causes the OOCD to vary over time. It absolutely, positively should not be renamed "setpoint" in the code.

The "offset" you're talking about is another thing entirely, if I'm understanding you correctly. You're talking about something like a function generator output that gets added to either the setpoint or the real-time deflection signal, depending on your setup. This can effectively increase or decrease the tip-sample contact forces, but it is not a load itself. I don't believe this value ever enters into the code at all.