Mar 9, 2010
TR-mode AFM shares STM success
In scanning tunnelling microscopy (STM), the tip-sample interaction – the tunnelling current – is a monotonic function of the tip-sample distance and increases sharply with decreasing distance. Even with a relatively large tip, it is possible to get atomic resolution as long as a single atom is somewhat more protruded than the other tip atoms. In contrast, typical atomic force microscopy (AFM) imaging relies on a tip-sample interaction that is composed of long- and short-range forces and is not monotonic. The feedback operation is more complicated. In particular, the long-range forces degrade the spatial resolution because many atoms near the tip apex contribute to the measured force. That is why typical AFM tips are much smaller than STM tips, but the spatial resolution is not as good.
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.
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.
About the author
The work was performed in the Surface and Nanoscience Laboratory of the Institute of Physics, Academia Sinica (IPAS), Taiwan, ROC. Chih-Wen Yang is a postdoctoral research fellow at IPAS. Dr Ing-Shouh Hwang is a research fellow at IPAS and an adjunct professor at NTHU.