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How Nanowires Work
Posted Date: 12 Mar 2008 Resource Type: Articles/Knowledge Sharing Category: How things work
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Posted By: Olufemi Member Level: Gold
Rating:  Points: 5
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In 1965, engineer Gordon Moore predicted that the number of transistors on an integrated circuit -- a precursor to the microprocessor -- would double approximately every two years. Today, we call this prediction Moore's Law, though it's not really a scientific law at all. Moore's Law is more of a self-fulfilling prophecy about the computer industry. Microprocessor manufacturers strive to meet the prediction, because if they don't, their competitors will [source: Intel].
To fit more transistors on a chip, engineers have to design smaller transistors. The first chip had about 2,200 transistors on it. Today, hundreds of millions of transistors can fit on a single microprocessor chip. Even so, companies are determined to create increasingly tiny transistors, cramming more into smaller chips. There are already computer chips that have nanoscale transistors (the nanoscale is between 1 and 100 nanometers -- a nanometer is one billionth of a meter). Future transistors will have to be even smaller.
Enter the nanowire, a structure that has an amazing length-to-width ratio. Nanowires can be incredibly thin -- it's possible to create a nanowire with the diameter of just one nanometer, though engineers and scientists tend to work with nanowires that are between 30 and 60 nanometers wide. Scientists hope that we will soon be able to use nanowires to create the smallest transistors yet, though there are some pretty tough obstacles in the way.
In this article, we'll look at the properties of nanowires. We'll learn how engineers build nanowires and the progress they've made toward creating electronic chips using nanowire transistors. In the last section, we'll look at some of the potential applications for nanowires, including some medical uses.
Nanowire Properties
Depending on what it's made from, a nanowire can have the properties of an insulator, a semiconductor or a metal. Insulators won't carry an electric charge, while metals carry electric charges very well. Semiconductors fall between the two, carrying a charge under the right conditions. By arranging semiconductor wires in the proper configuration, engineers can create transistors, which either acts as a switch or an amplifier.
Some interesting -- and counterintuitive -- properties nanowires possess are due to the small scale. When you work with objects that are at the nanoscale or smaller, you begin to enter the realm of quantum mechanics. Quantum mechanics can be confusing even to experts in the field, and very often it defies classical physics (also known as Newtonian physics).
For example, normally an electron can't pass through an insulator. If the insulator is thin enough, though, the electron can pass from one side of the insulator to the other. It's called electron tunneling, but the name doesn't really give you an idea of how weird this process can be. The electron passes from one side of the insulator to the other without actually penetrating the insulator itself or occupying the space inside the insulator. You might say it teleports from one side to the other. You can prevent electron tunneling by using thicker layers of insulator since electrons can only travel across very small distances.
Another interesting property is that some nanowires are ballistic conductors. In normal conductors, electrons collide with the atoms in the conductor material. This slows down the electrons as they travel and creates heat as a byproduct. In ballistic conductors, the electrons can travel through the conductor without collisions. Nanowires could conduct electricity efficiently without the byproduct of intense heat.
At the nanoscale, elements can display very different properties than what we've come to expect. For example, in bulk, gold has a melting point of more than 1,000 degrees Celsius. By reducing bulk gold to the size of nanoparticles, you decrease its melting point, because when you reduce any particle to the nanoscale, there's a significant increase in the surface-to-volume ratio. Also, at the nanoscale, gold behaves like a semiconductor, but in bulk form it's a conductor.
Other elements behave strangely at the nanoscale as well. In bulk, aluminum isn't magnetic, but very small clusters of aluminum atoms are magnetic. The elemental properties we're familiar with in our everyday experience -- and the ways we expect them to behave -- may not apply when we reduce those elements down to the size of a nanometer.
Scientists re still learning about the different properties of various elements at the nanoscale. Some elements, like silicon, don't change much at the nanoscale level. This makes them ideal for transistors and other applications. Others are still mysterious, and may display properties that we can't predict right now.
Olufemi
Nigeria
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