thermal tune for cantilever stiffness disagree /questions on 'practical advice on the determination of cantilever spring constants'

Fei Liu posted on Fri, Jun 22 2012 8:03 AM

rated by 0 users

Hi,

I am trying to  rectangular calibrate cantilevers by thermal tune in air. The fit model is Lorentan. The correcting factor I used in the thermal tune dialog box is 1.106 as suggested Ben Ohler .The stiffness I get is ~5.6N/m. 

Then I  record the thermal noise by HSDC at 500kHz and do the analysis myself. But what I get is 8.2N/m. The error is over 40%.

Here is how I analyze the recorded thermal noise in air.

0. import HSDC file to matlab by following the suggestion from the link :http://nanoscaleworld.bruker-axs.com/nanoscaleworld/forums/p/372/561.aspx

and convert  is form  volt to nm, by multiplying the data with the deflection sensitivity.

1. Fourier transform to get power spectrum of the thermal noise. The data length for Fourier Transform is 64K.

2. Fit to Lorentian Equation.

3. Calculate the area under the fitted curve subtracting the noise baseline.

4. Use the equation 16 from 'practical advice on the determination of cantilever spring constants' 

http://nanoscaleworld.bruker-axs.com/nanoscaleworld/media/p/143.aspx

The cantilever resonance frequency is 150kHz.

Why is the stiffness different from that given by the software?

My AFM is Multimode with Nanoscope V controller.

Does the software do something different?

Best,

Fei