Sep 2, 2011
Force modulation improves nanoscale conductivity
It is known that the force modulation of atomic force microscope (AFM) probes reduces friction and wear. Now, scientists have shown that conductive probes display an additional benefit; the dithering motion improves the ability of the tip to make electrical contact with a surface.
Researchers from the University of Twente, in collaboration with scientists at IBM Research - Zurich, have studied the effect of force modulation on conductive probes. The team used probes made of platinum silicide in a custom-built AFM.
Force modulation is achieved using a dither piezo as found in a regular tapping mode AFM. The tip is operated in contact-mode and a very small motion is added by the piezo. This causes the tip to slightly bounce on the surface without actually losing contact with the sample.
The motion of the tip produces not only less friction, but also less tip wear, and interestingly the contact resistance between the tip and the sample goes down. The current is up to six times larger for the same average load force.
This surprising result is of great benefit for conductive probes. The probes can now be used at a lower load force without losing electrical contact with the sample. A lower load force means that an even larger reduction in wear can be achieved.
Reporting its study in the journal Nanotechnology, the team elaborates on why the conductance increases. The group argues that the tip effectively pierces the layer of contaminants that is commonly present on a sample surface under ambient conditions. The surface contaminants prevent a conductive tip from making immediate electrical contact when it touches a sample.
Conductive AFM usually requires that the tip is pushed into the sample more strongly to make a good electrical contact, leading to extra tip wear. The new technique shows that the conductance is now much less dependent on the load force and keeps performing after more than 1 m of tip travel.
Additional information can be found in the journal Nanotechnology.
About the author
Wabe W Koelmans is a postdoctoral researcher at the MESA+ Institute for Nanotechnology, University of Twente, the Netherlands. He works on the readout of cantilevers and arrays. The study described here was performed at the IBM Research's Zurich laboratory. Recently, he started a new activity on the interaction of ultrasharp silicon needles and soft tissue.