Tiny Functional Nanowires

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.

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.

According to the researchers, the method - on-wire lithography (OWL) - allows them 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 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, the OWL method has been able to produce gaps as small as 2.5 nanometers wide.

The 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.

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