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Building Nanowires
Posted Date: 12 Mar 2008 Resource Type: Articles/Knowledge Sharing Category: How things work
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Posted By: Olufemi Member Level: Diamond
Rating:  Points: 5
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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].
Growing Nanowires
Chemical vapor deposition (CVD) is an example of a bottom-up approach. In general, CVD refers to a group of processes where solids form out of a gaseous phase. Scientists deposit catalysts (such as gold nanoparticles) on a base, called a substrate. The catalysts act as an attraction site for nanowire formation. Scientists put the substrate in a chamber with a gas containing the appropriate element, such as silicon, and the atoms in the gas do all the work. First, atoms in the gas attach to atoms in the catalysts, then additional gas atoms attach to those atoms, and so on, creating a chain or wire. In other words, the nanowires assemble themselves.
A new way to build nanowires is to print them directly to the appropriate substrate. A team of researchers in Zurich pioneered this method. First, they carved a silicon wafer so that the raised portions on the wafer coincided with the way they wanted the nanowires arranged. They used the wafer like a stamp, pressing it against a synthetic rubber called PDMS. They then drew a liquid filled with gold nanoparticles, called a colloidal suspension, across the PDMS. The gold particles settled into the channels created by the silicon wafer stamp. Now the PDMS became a mold capable of transferring a "print" of gold nanowires onto another surface. PDMS molds can be used repeatedly and may play a role in the mass production of nanowire circuitry in the future [source: Nature Nanotechnology].
Several laboratories have created transistors using nanowires, but their creation requires a lot of time and manpower. Nanowire transistors perform as well or better than current transistors. If scientists can find a way to design a way to produce and connect nanowire transistors together efficiently, it will pave the way to smaller, faster microprocessors, which will allow the computer industry to keep up with Moore's Law. Computer chips will continue to get smaller and more powerful.
Research in nanowire production continues across the world. Many scientists believe it's just a matter of time before someone comes up with a viable way to mass produce nanowires and nanowire transistors. Hopefully, if and when we reach that point, we'll also have a way to arrange nanowires the way we want so that we can use them to their full potential.
Nature's Home-grown Nanowires
Until recently, scientists believed all nanowires were manmade, but a couple of years ago biologists discovered that bacteria that can grow their own nanowires. A bacterium called Geobacter sulfurreducens dumps electrons onto metal atoms (the electrons are a byproduct of the bacterium's fuel consumption). If there is a shortage of metal in the bacterium's environment, it will grow a nanowire appendage to conduct electrons to the nearest metal, allowing the bacterium to consume more fuel. Scientists hope to build organic fuel cells using bacteria like Geobacter sulfurreducens to produce electricity.
Olufemi
Nigeria
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Responses
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| Author: Karthika M 29 May 2008 | Member Level: Gold Points : 2 | I am always interesting to know about new findings in Science.
It's useful and interesting.Thank you.
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