HANCOCK, N.H. An approach to creating single-crystal nanowires from just about any semiconducting material is being pioneered by Charles Lieber and his group at Harvard University in tandem with the nanotech startup he co-founded, Nanosys Inc.
Together with methods for placing wires in arrays and multilayers, the technique promises to create complex systems at the nanoscale.
While the process details are a closely held secret at Nanosys, some idea of how to control the growth of nanowires using a catalyst can be gleaned from the Harvard work, which is in the public domain.
The key idea behind the process is to use a nanocluster that is the same diameter as the wire to be grown.
A catalyst and the compounds that form the nanowire must coexist in a liquid state, which will then allow the continuous growth of a nanowire from the reaction site of the nanocluster. For example, iron will dissolve both gallium and nitrogen, so an iron nanocluster with a 10-nanometer diameter can grow gallium-nitride nanowires of the same diameter.
Catalytic nanoclusters of a specific size can be made by blasting particles off a substrate with a laser. The size of the nanoclusters can be closely controlled by adjusting the pressure of a neutral gas in the chamber.
Nanosys has turned this basic idea into a manufacturable process for producing nanowires from silicon and all of the known compound semiconductors. In addition, ferroelectric and piezoelectric materials have formed nanowires, the company said.
The process also allows one type of material to follow another type so that a single nanowire can include sequential material structures with well-defined interfaces. Combined with special quantum effects that occur at these small sizes, the process has created a wide variety of electronic and optical functions from a single technique.
The process is directed by a special CAD design system built at Nanosys that lets system designers specify what physical effects they need. The manufactured nanowires then are incorporated into arrays using a variety of techniques.
For example, polymer matrices can hold the wires, and the polymer solution can then be microprinted onto a substrate. The company is developing assembly processes based on the type of system desired.
The important flexible aspect of this systems approach to nanostructures is that the functions built into the nanowires are independent of the methods used to build systems from them.