Building Nanowires from the Top Down
Nanoscience specialists talk about two different approaches to building things in the nanoscale: the top-down approach and the bottom-up approach. A top-down approach essentially means that you take a bulk amount of the material you plan on using for nanowires and carve away until you are down to the right size. A bottom-up approach is an assembly process where smaller particles join to make a larger structure.
Although we can build nanowires using either approach, no one has found a way to make mass production feasible. Right now, scientists and engineers would have to spend a lot of time to make a fraction of the number of nanowires they would need for a microprocessor chip. An even greater challenge is finding a way to arrange the nanowires properly once they are built. The small scales make it very difficult to build transistors automatically -- right now, engineers usually manipulate wires into place with tools while observing everything through a powerful microscope.
An example of a top-down approach is the way scientists make fiber-optic nanowires. Fiber-optic wires carry information in the form of light. To make a fiber-optic nanowire, engineers first start with a regular fiber-optic cable. There are a few different approaches to reduce a fiber-optic cable to the nanoscale. Scientists could heat up a rod made out of sapphire, wrap the cable around the rod, and pull the cable, stretching it thin to create a nanowire. Another method uses a tiny furnace made from a small cylinder of sapphire. Scientists draw the fiber-optic cable through the furnace and stretch it into a thin nanowire. A third procedure called flame brushing uses a flame under the fiber-optic cable while scientists stretch it [source: Gilberto Brambilla and Fei Xu].
In the next section, we'll look at the ways scientists can grow nanowires from the bottom up.