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.