Nov 9, 2011
Vibrometer-based model enables accurate measurements in liquid with existing AFM hardware
The use of a piezoelectric element (acoustic excitation) to vibrate the base of a microcantilever is a popular way to perform dynamic atomic force microscopy (AFM). In air or vacuum, the base motion is so small (relative to the tip motion) that it can be neglected. However, in liquid environments the base motion can be large and cannot be neglected. To further complicate things, in most AFM setups the base motion cannot be observed directly. One way around this is to rely on theoretical formulas and models to estimate its magnitude.
Unfortunately, such formulas can be inaccurate. For example, a significant component of the piezo excitation does not mechanically excite the cantilever, but rather transmits acoustic waves through the surrounding liquid, which in turn indirectly excites the cantilever. What's more, resonances of the piezo, chip and holder can obscure the true cantilever dynamics even in well designed liquid cells.
Now, by integrating an entire AFM with a scanning laser Doppler vibrometer, researchers at Purdue University, US, have developed a method to overcome these limitations. The vibrometer allowed the team to visualize the motion of the entire cantilever while the AFM is scanning in liquids (see the animation above).
Simple calibration procedure
Based on this data, the group has been able to develop an accurate mathematical model of the cantilever's motion in liquid when the base is excited by the piezoelectric crystal. The mathematical model shows that quantitative results are in fact possible with the acoustic excitation method.
A simple calibration procedure, which takes just five minutes or less, allows users to determine the necessary parameters. The result will enable researchers to obtain significantly more accurate quantitative results using their existing hardware.
Further information can be found in the journal Nanotechnology.
About the author
Daniel Kiracofe is a doctoral student in Mechanical Engineering at Purdue University and a researcher at the Birck Nanotechnology Center, advised by Arvind Raman. The Raman lab group focuses on developing fundamental understanding of the dynamics of Atomic Force Microscopy and then using that knowledge to develop new techniques.