Nano Bits, Engineering Management, April 2004

NANOTECHNOLOGY

nanobits

This section was written by Editor Jeffrey Winters.

NANOCASTER

What can you do with a guitar that's the size of a single blood cell? Jam.

That's the word from Cornell University in Ithaca, N.Y., where the 40-micrometer-long guitar was fabricated in November 2003.

This 40 nm guitar really jams.

Lidija Sekaric, now a researcher at IBM's Watson Research Center in Yorktown Heights, N.Y., built the nanoguitar with Cornell graduate student Keith Aubin and undergraduate researcher Jingqing Huang. It was a reprise of the first nanoguitar, constructed at Cornell in 1997. That guitar was built as a demonstration of fabrication techniques.

The new nanoguitar is not as small as the original, which was only 10 µm. But unlike the original, the microscopic strings of this new guitar can be played. When light from a laser hits the strings, they begin to oscillate, much the way that strings on a standard instrument vibrate when plucked. The vibrating strings of the nanoguitar create an interference pattern in the light as it is reflected back to the source and detected. Since the frequencies of vibration are some 17 octaves higher than those of a real guitar, the frequencies must be converted into audible notes.

Of course, no one expects to make a living playing such a tiny instrument. But the technology has many other possible applications. Since the frequency at which an object oscillates depends upon its mass and length, nanoscale objects can be made to vibrate at extraordinary frequencies. In theory, a nanoscale rod could vibrate at the frequency used as the carrier wave for a cell phone. Such a rod might one day replace the quartz crystal currently used in such phones, but take up much less space and use only a fraction of the power.

A vibrating device might also be used to modulate light, work now done by lasers in fiber-optic communications networks. Such networks carry voice, data, and Jimi Hendrix solos.

SILICON METAL

Chemistry used to be easy. Iron and cobalt were metals; oxygen and carbon were not. But when researchers start measuring material at the atomic level, all bets are off. The latest finding blurring the distinctions is that at the extremes in terms of size, silicon may behave like a metal.

Xiao Cheng Zeng, a chemist at the University of Nebraska-Lincoln, led a team that created a computer model of silicon nanotubes, hypothetical molecules that are less than 1 nanometer in length. The team discovered that the arrangement of the silicon atoms in the tube walls determines the tube diameter. Placing the atoms in a square configuration—as opposed to hexagonal or pentagonal—yields the thinnest known nanotube, a mere half-nanometer in diameter.

But the real surprise came when Zeng analyzed the quantum mechanical properties of tubes that thin, so skinny that they behave like lines rather than like three-dimensional objects. Zeng found that instead of acting like a semiconductor—the property that makes silicon chips the basis of modern electronics—these one-dimensional silicon nanotubes would likely conduct electricity like a normal metal.

The results were published in February in the Proceedings of the National Academy of Sciences.

It may take some time before Zeng's computational analysis can be tested in the lab. Silicon is highly reactive at the temperatures needed to melt it. That means that making silicon nano- tubes will require techniques different from the ones used to make the now-familiar carbon nanotubes.

It's also not clear whether silicon nanotubes would have any advantages over their carbon-based cousins. Carbon nanotubes can also take on conducting or semiconducting properties, depending on their configuration. And they are much easier to make.

FLAT LENS

Researchers at Northeastern University in Boston have developed a flat lens crystal capable of producing a real image. The team, led by physicist Srinivas Sridhar, based their work on recent discoveries of material that can bend light negatively. Normal transparent material, like glass or water, have positive indices of refraction: As a beam of light enters the material from a vacuum, the beam is bent toward the perpendicular. But in some photonic crystals, which are designed with specific light-controlling properties, the index of refraction can be made negative.

Sridhar and his team fabricated a photonic crystal out of aluminum rods. Though optically opaque, the rods transmit microwaves, and the researchers were able to use a flat slab made of the aluminum rods to focus a microwave beam.