Feb 3, 2012
Probing delicate samples: stiff AFM mechanics at soft forces
Atomic force microscopy (AFM) measurements of nanoscale mechanical properties can require relatively large forces to probe elastically stiff materials such as metals or ceramics. Such forces are potentially damaging, particularly to thin or delicate samples. Recently, researchers have demonstrated a solution to this dilemma. The new method enables AFM measurements of elastic modulus with forces less than 20 nN, roughly 10 times the force needed to break a single atomic bond.
Researchers Jason Killgore and Donna Hurley at the National Institute of Standards and Technology in Boulder, US, achieved these results using an AFM technique called contact resonance spectroscopy. The method involves analysing the resonance frequency of a vibrating AFM cantilever when the probe tip is in physical contact with the sample of interest.
From measurements of the contact-resonance frequencies, information is obtained about the interaction forces between the tip and the sample – for example, contact stiffness. Models for the tip-sample contact mechanics can then be used to relate the contact stiffness to mechanical properties such as elastic modulus.
A reduction in applied force
By exciting higher-order resonance modes, Killgore and Hurley showed that the applied force could be reduced by almost two orders of magnitude. At the same time, the researchers noted that measurement sensitivity was significantly enhanced. The scientists used a combination of experimental results and finite element analysis to demonstrate their findings. The accuracy of the technique was shown by quantitatively differentiating two glass specimens with moduli in the 50 to 75 GPa range using an applied force of only 12 nN.
Next, the team will apply higher-mode contact resonance spectroscopy to thin films. Benefits include the elimination of complicated mathematical corrections required by high-force measurements due to substrate effects.
A full description of the technique can be found in the journal Nanotechnology.
About the author
Jason P Killgore and Donna C Hurley are scientists in the Materials Reliability Division of the National Institute of Standards and Technology in Boulder, Colorado, US. Their research focuses on the development and application of new atomic force microscopy techniques for more accurate measurement of nanoscale mechanical properties.