[Spm] Tip height in NC-AFM

[allard katan]

Unfortunately, "tip altitude" cannot be measured with AFM. Frequency shift
in NC AFM is related to a force versus distance profile. How the two are
related depends on cantilever parameters (resonance frequency, quality
factor, spring constant) and user-tunable parameters (amplitude, phase shift
in the PLL) . 

Just a shortlist of many papers about how to calculate force profiles from
frequency shifts: Albrecht et al, J. Appl Phys 1991, small amplitude limit;
Giessibl, PRB 1997, or more recently Rev. Mod. Phys 2003, large amplitude
limit and exact solutions for certain types of potentials; Sader & Jarvis,
Nanotechnology 2005, arbitrary amplitudes. 

 

You need to do a frequency shift versus distance measurement to be able to
calculate the force profile. Once you know the force profile, you can decide
where your sample is. Common choices are either the onset of repulsive force
or the position of a vertical asymptote in the force profile. If you already
know the force profile in advance, either from theoretical arguments or
previous experimental work, calculating the frequency shift is pretty
straightforward, as you had already guessed.

 

If all that is too much work, here's a more direct (but less exact)
technique: Just approach your tip to the sample until your frequency shift
goes through the roof and call that distance zero. Then go back x nm to
where your favorite frequency shift setpoint is. Your "tip altitude" is then
x. Not the method of choice if you care about damage to your tip and/or
sample.

 

Best regards,

 

Allard Katan

Interface Physics Group

Kamerlingh Onnes Laboratory

Leiden Institute of Physics

Niels Bohrweg 2

Leiden, The Netherlands

 

 

  _____  

From: Matt Brukman [mailto:mbrukman at seas.upenn.edu] 
Sent: Monday, October 01, 2007 20:04
To: spm at spmlist.di.com
Subject: [Spm] Tip height in NC-AFM

 

SPMers-

 

Is anyone aware of any work that relates frequency shift in NC AFM to tip
altitude? I guess you could do it from first principles if you know the vdW
force profile etc, but I'm hoping there are more direct techniques
available....

 

Thanks in advance,

Matt Brukman

 

Postdoc Mat Sci -- Bonnell Group

University of Pennsylvania

3231 Walnut St -- 113 LRSM

Philadelphia, PA 19104

215-746-2373

 





 

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