C&EN: TODAY'S HEADLINES - NANOSIZED LITE BRITE

MITCH JACOBY

A nanoscale light source based on a single carbon nanotube has been demonstrated by researchers at IBM [Science, 300, 783 (2003)]. The electrically driven emitter, which exploits unique semiconducting properties of carbon nanotubes, may enable development of miniature photonic and optoelectronic devices and other applications that depend on fine control of light.


	Avouris PHOTO BY MITCH JACOBY

Light emission from various types of nanowires and nanotubes has been reported previously by a number of research groups. For example, scientists have examined fluorescence emission from solution-phase samples of carbon nanotubes. And nanowires composed of conventional semiconductor materials, such as gallium arsenide, have also served as the active element in light-emitting devices.

But while some progress has been made toward developing nanoscale light sources, the current study--carried out by scientists at IBM's T. J. Watson Research Center, Yorktown Heights, N.Y., including Phaedon Avouris, manager of nanoscale science and technology; James A. Misewich; and Richard Martel--differs from the earlier work in several important ways.

For example, unlike the nanotube fluorescence emission, which was induced by laser excitation of solubilized bulk samples, the IBM group induces light emission from individual carbon nanotubes (single molecules) fixed on solid supports--electrically. Electrical control is essential for applications.

And unlike the nanowire devices, which were constructed by forming p-n junctions between separate positive-charge-carrying (p-type) nanowires and negative-charge-carrying (n-type) nanowires or by growing multicomponent wires with built-in p-n junctions, the single-nanotube device doesn't require special doping and fabrication procedures.


	MEET YOU HALFWAY By injecting positive and negative charge carriers (holes and electrons) simultaneously at opposite ends of a carbon nanotube channel in a specially configured field-effect transistor, IBM researchers cause the particles to interact near the center of the tube. The recombination process produces polarized infrared light. IBM T. J. WATSON RESEARCH CENTER

To make the tiny light source, the IBM team constructed a circuit element known as a field-effect transistor (FET), in which a single carbon nanotube serves as the semiconducting channel. In general, FETs consist of a channel and three electrodes referred to as source, drain, and gate. By applying a voltage to the gate electrode, a current is made to flow from the source to the drain--through the channel. Avouris points out that the unique feature of the new IBM device is that it is designed as an ambipolar transistor--an FET capable of transmitting positive or negative charge carriers, depending on the bias voltage applied to the gate.

"We adjusted the operating conditions so that both electrons and holes are injected into the nanotube simultaneously from the source and drain electrodes," Avouris explains. As the electrons and holes come together, they neutralize each other in an energetic annihilation process. The energy appears as polarized infrared light with a wavelength of roughly 1.5 µm. Avouris notes that the frequency of the emitted light matches that used in optical telecommunication technology.

The IBM team members propose a number of ways in which device performance can be customized and improved. For example, they note that it should be possible to control the wavelength of emitted light by using nanotubes of various diameters. The researchers add that improving materials properties (such as dielectric constant) can increase the emission yield and lower the operating voltage--leading to an "easy-to-integrate nanoscale source of photons for future photonic and optoelectronic devices."-