In their experiments, the two Wisconsin researchers use a solution of the nickel isotope to fill sets of microscopic channels in a silicon plate.

ECONOMIST: Micromachines: The engines of Lilliput | The

The transistor itself is composed of a single phosphorous-31 isotope, which has been precisely placed on a base of silicon using a Scanning Tunneling Microscope in an ultra-high vacuum chamber.

FORBES: Scientists Create A One-Atom Transistor