Lateral thinking

Luckily, the long-range forces are usually acting only in the normal direction, which invites the use of different imaging modes. Researchers in Taiwan excite torsion resonance (TR) in AFM cantilevers during imaging. Here, the tip vibrates in the lateral direction and the oscillation is affected by the lateral force gradient only. The normal forces have no effect on the torsional oscillation. The oscillation behaviour starts to change only when the tip begins to make contact with the sample, which allows clear detection of the contact point and enables a soft contact between the tip apex and the sample.

The team from Academia Sinica's Surface and Nanoscience Laboratory uses the frequency shift of the torsion resonance as the basis for feedback control. The frequency shift exhibits a sharp jump from zero upon contact and is a monotonic function of the tip-sample distance, similar to the behaviour of the tunnelling current in STM.

Underwater operation

The TR-mode AFM is especially useful in liquid and the group has demonstrated atomic resolution on a mica surface in water with a relatively blunt tip and negligible lateral force.

For the same cantilever, the resonance frequency and the quality factor of the torsion resonance are about two orders of magnitude higher than those of the fundamental flexural resonance. As a result, the TR mode can achieve a higher force sensitivity and allow a higher scanning speed than typical flexural modes.

Using this new soft-contact TR mode, the scientists plan to image soft biological molecules in liquid and investigate properties related to the solid-water interfaces.

Full results are available in the journal Nanotechnology.