  Applying ac voltages, we trapped gold nanoparticles between microfabricated
electrodes under well-defined conditions. We demonstrate that the nanoparticles
can be controllably fused together to form homogeneous gold nanowires with
pre-defined diameters and conductance values. Whereas electromigration is known
to form a gap when a dc voltage is applied, this ac technique achieves the
opposite, thereby completing the toolkit for the fabrication of nanoscale
junctions.
