Credit: © 2007 ACS

The quick and dirty method of measuring the resistance of a material is to deposit two metal contacts on it, pass a current from one to the other and measure the voltage between them. Done in this way, however, the resistance of the contacts contributes to the overall measured resistance. Making a 'four-probe' measurement — where the voltage is measured across different contacts than the ones through which the current passes — solves this problem, but depositing numerous contacts on a nano-object is no easy task.

Now, Timo Schwamb and colleagues1 at the Swiss Federal Institute of Technology and the Institute of Materials Science and Technology, both in Zurich, Switzerland are exploring ways to do this with carbon nanotubes, so that their electronic properties can be accurately determined. A technique known as dielectrophoresis was used to position suspended carbon nanotubes onto 'test chips'. The method involves applying a high-frequency alternating electric field between metal electrodes, which electrically polarizes the nanotube and forces it to lie across them.

However, instead of applying the field between two electrodes on a flat chip, the group tweaked the technique to position the nanotube across four metal contacts on a specially designed three-dimensional chip. Previously, this kind of nanotube positioning involved a number of painstaking steps and often required the use of an atomic force microscope.