Grappling with the problem

In a recent Nanotechnology article, Keith Brown and Robert Westervelt from Harvard University, US, have proposed a nanoscale pick-and-place tool using a triaxial AFM probe. The triaxial probe consists of a conducting AFM tip with two conducting shells separated by insulating layers. Radio-frequency voltages are applied to the tip and inner shell while the outer shell is grounded. An opening at the end of the tip allows the electric field to escape. The tip creates an electric field zero away from its surface that can trap nanoscale objects in water using negative dielectrophoresis. The trapping works because water has a dielectric constant much larger than most objects and so water pushes nanoparticles into electric field minima.

Matching the trap size to the object

The Triaxial AFM Contact-free Tweezer (TACT) operates like a hand reaching into a bag full of baseballs and grabbing a single ball. The trap is size-matched to the object, like a hand and a baseball. The attractive trap is surrounded by a repulsive region, which ensures that one particle is grabbed at a time. In addition, the non-contact design prevents van der Waals forces from accidentally sticking the particle to the tip.

The TACT can be used to pick up single semiconductor quantum dots, carbon nanotubes, semiconductor nanowires and even biological particles such as viruses. Particles as small as 4 nm in diameter can be held with this technique.