A giant step toward tiny functional nanowires

Carving a telephone pole is easy if you have the right tools, say a power saw and some large chisels. And with some much tinier tools you could even carve a design into a paper clip if you wanted to. But shrink your sights down to the nanoscale, to a nanowire that is 1,000 times smaller than the diameter of a paper clip, and you find there are no physical tools to do the job properly.

So a team of Northwestern University scientists turned to chemistry and developed a new method that can routinely and cheaply produce nanowires with gaps as small as five nanometers wide — a feat that is unattainable using conventional lithographic techniques. The results will be published in the July 1 issue of the journal Science.

Carved gaps are essential to a nanowire’s function, and controlling those gaps would allow scientists and engineers to design with precision devices ranging from tiny integrated circuits to gene chips and protein arrays for diagnostics and drug discovery.

“With miniaturization happening across so many fields, our existing tools — our chisels of a sort — can’t control the shapes and spacing of these small structures,” said Chad A. Mirkin, director of Northwestern’s Institute for Nanotechnology, who led the research team. “Our method allows us to selectively introduce gaps into the wires. These gaps can be filled with molecules, making them components for building small electronic and photonic devices or chemical and biological sensors.”

The development of sophisticated nanoelectronics, said Mirkin, depends on the ability to fabricate and functionalize electrode gaps less than 20 nanometers wide for precise electrical measurements on nanomaterials and even individual molecules. While conventional techniques can’t make gaps much smaller than 20 nanometers wide, Mirkin’s method, called on-wire lithography, or OWL, has been able to produce gaps as small as 2.5 nanometers wide.

Mirkin and his team made the notched structures by first depositing into a porous template segmented nanowires made of two materials, one that is resistant to wet-chemical etching (gold) and one that is susceptible (nickel). The template is then dissolved, releasing the nanowires. Next, the wires are dispersed on a flat substrate, and a thin layer of glass is deposited onto their exposed faces. They are then suspended in solution, and a selective wet-chemical etching removes the nickel, leaving gold nanowires with well-defined gaps where the nickel used to be. (The glass is used as a bridging material, to hold the nanowire together.)

Using the OWL method, the researchers prepared nanowires with designed gaps of 5, 25, 40, 50, 70, 100, 140 and 210 nanometers wide. (A nanometer is one billionth of a meter or roughly the length of three atoms side by side. A DNA molecule is 2.5 nanometers wide.) In recent days, they have refined the technique to be able to make gaps as small as 2.5 nanometers wide.

“With dip-pen nanolithography, we can then drop into these gaps many different molecules, depending on what function we want the structure to have,” said Mirkin, also George B. Rathmann Professor of Chemistry. “This really opens up the possibility of using molecules as components for a variety of nanoscale devices.”

In addition to Mirkin, other authors on the Science paper are Lidong Qin (lead author), Sungho Park and Ling Huang of Northwestern University.