Getting Wired with Nanowires
Nanowires are simply very tiny wires. They are composed of metals such as silver, gold, or iron, or semiconductors such as silicon, zinc oxide, and germanium. Nanoparticles are used to create these little nanowires, which can have a diameter as small as 3 nanometers.
The production of nanowires is similar to nanotubes; it requires using a catalyst particle in a heated reaction chamber. To grow nanowires composed of gallium-nitride, researchers at Harvard University flow nitrogen gas and vaporized gallium through the reaction chamber containing an iron target. Iron nanoparticles are vaporized from the target by a laser to act as a catalyst. Both gallium and nitrogen molecules dissolve in the iron nanoparticle. When you get so much gallium and nitrogen in the particle that it starts to sweat off of the surface, molecules precipitate onto the surface of the particle where they combine to grow the nanowire.
When you grow any nanowire, the materials you use must be soluble in the catalyst nanoparticle. For example, to grow silicon nanowires, a gold catalyst nanoparticle is used because silicon dissolves in gold.
To grow arrays of nanowires — great for making electronic devices or sensors — you can use catalyst nanoparticles positioned on a solid substrate, rather than nanoparticles in a vapor. For example, researchers at the National Institute of Standards and Technology have used gold nanoparticles on a sapphire surface as catalysts to grow arrays of nanowires composed of zinc oxide. By changing the size of the gold nanoparticles, they are able to control whether the nanowires grow tilted vertically at a 60-degree angle up from the surface, or horizontally along the surface.
Nanowires at work
Several research groups have demonstrated the use of nanowires to create memory devices and transistors. Researchers at Hewlett-Packard and the University of California at Los Angeles have demonstrated that a memory cell can be formed at the intersection of two nanowires. Using a somewhat more complicated array of nanowires, they have also come up with a transistor-like device called a crossbar latch.
Folks at the University of Southern California and the NASA Ames Research Center have demonstrated a memory device that uses indium oxide nanowires. They are predicting that this device will be able to store 40 gigabits per square centimeter, which is a lot of data by anybody's standards.
Building transistors and memory devices used in computer chips from materials about the width of a nanometer, such as nanowires, is called molecular electronics.
Meanwhile, over at Harvard University they've demonstrated a nanowire-based sensor that can detect diseases in blood samples. The working part of the sensor is a nanowire that has been functionalized by attaching certain nucleic acid molecules to it. The nucleic acid molecules bond to a cystic fibrosis gene if it is present in a blood sample. When this happens, the conductance of the nanowires changes. The change in the nanowire conductance causes a current to flow.
This type of sensor has the potential to provide immediate analysis of blood samples for a variety of diseases, possibly right in your doctor's office with just a pinprick in your finger. That's much more convenient than giving vials full of blood and waiting for a test to come back from a lab. Add to that, this sensor is highly sensitive and might detect diseases we've never even been able to detect before, or detect viruses at an earlier stage.
But there's a major challenge for researchers developing this technique, either with nanowires or nanotubes: They have to find a way to make the sensors selective and prevent false signals. In the Harvard demonstration, they did this by using a specific nucleic acid that would only bond to the cystic fibrosis gene.
Finally, researchers at the National Institute of Standards and Technology, as well as the folks at theMax Planck Institute, are investigating the use of nanowires to increase the density of a magnetic recording medium (such as the disk drives used in computers). Both groups have been able to deposit arrays of magnetic nanowires — and their work shows that it's feasible to use this type of structure to store information at a much higher density than current disk drives can. However, other researchers are investigating the idea of using certain arrangements of nanoparticles to do the same thing as nanowires. It's a toss-up as to which idea will win out.