1 July 2005: Minding the nanogaps aboard a molecular circuit
Fully-functioning molecular circuits could be a step closer thanks to a new nanofabrication technique developed by chemists at Northwestern University, Evanston, US. The chemists, led by Chad Mirkin, have created nanowires that contain tiny gaps, only a few nanometres wide, which can house single molecules.
Electrical contact with single molecules is an essential requirement of molecular circuits, which means researchers must manufacture electrode gaps less than 20nm wide. But it is difficult to produce structures under 100nm in size with conventional lithographic techniques.
Mirkin's team used template-based electroplating to create nanowires that were 360nm in diameter and 5?m in length. Electroplating is an efficient method for controlling the quantity of material that is deposited in a template and it allowed the creation of wires consisting of long segments of one metal divided by short segments of another. One of these metals (gold) is resistant to nitric acid, while the other (nickel) dissolves in it.
The researchers released the nanowires from the template and applied nitric acid, causing the nickel segments to dissolve away. This left gold nanowires with nanometre-scale gaps in them. To hold the nanowires together, the chemists coated one side with silica prior to applying the acid. Mirkin's team used the technique to create nanowires with gaps that were 5, 25, 40, 50, 70, 100, 140 and 210nm wide.
They also used dip-pen nanolithography to place compounds into the gaps, thereby altering the properties of the nanowire. The presence of a gap normally prevents a nanowire from conducting electricity, but the chemists showed they could fix this by depositing a mixture of the conducting polymers polyethylene oxide and polypyrrole into the gap.
Mirkin's team are continuing with this research and recently refined the technique to create gaps only 2.5nm wide, which is the width of a DNA molecule. 'This really opens up the possibility of using molecules as components for a variety of nanoscale devices,' said Mirkin. Jon Evans
L Qin et al, Science, 2005, 309, 113