Nanoshells May Be Key To Next Wave Of Light-Based Technology

Rice University researchers, led by Naomi Halas, professor of electrical and computer engineering, developed metal nanoshells--particles with an insulating core coated by a thin shell of gold--the 'malted milk balls' of the nanoscale world.

Nanoshells can absorb or scatter light at virtually any wavelength in the visible or infrared ranges depending upon the dimensions of the particle's core and shell.

Metal nanoshells may be embedded in solid-state materials and films, such as plastics or glasses, or attached directly to surfaces for special-purpose coatings. New products could include energy efficient paints and windows, coatings for cars, airplanes or buildings, solar collection materials, or even fabrics. Metal nanoshells may also prove quite valuable in chemical and biosensors and in optical switches. They have been used to enhance signals in Raman spectroscopy, a chemical monitoring technique extensively used in industry.

Their ability to absorb light is so great that if you put just one thousandth of an ounce of nanoshells into a quart of water, the water would be completely opaque. The nanoshells currently range in size from 50 to 1,000 nanometers in diameter; one nanometer is one-billionth of a meter.

A key is that Halas and her team can control which wavelength of light is absorbed or scattered by the nanoshells by controlling the particle size and the ratio of the particle's core and shell. The particles can be made to absorb or scatter visible light, but more important, they can absorb or scatter light in the infrared. This range, particularly around the one micron wavelength, is of critical importance in many technological applications. Very few materials absorb light at this wavelength, and those that do are often quite expensive, difficult to produce, and contain hazardous or toxic elements.

"The nanoshells act as an amazingly versatile optical component on the nanometer scale: they may provide a whole new approach to optical materials and components," Halas says.

Halas and graduate students Richard Averitt and Steve Oldenburg are studying the fabrication, properties and uses for these particles, and they have filed for a patent on this technology.

Averitt has studied the optical properties of naturally occurring nanoshells--gold-terminated gold sulfide nanocrystals. He has explained their growth process and how their structure corresponds to their optical properties.

During the past year, Oldenburg has developed a method of assembling composite metal nanoshells with varying diameters and shell thicknesses that consist of glass cores and a gold shell.

"We are pursuing strategies to get us out to the five micron range in the infrared," Halas says, "and we're pretty confident that we can do that by refining our current method. Using other techniques, I think we might be able to get out to 10 microns and beyond, into the far infrared."

Since the 15th century, craftsmen have embedded nanometer-sized gold particles into glass to produce the brilliant red of stained glass windows. The remarkable optical properties of gold nanoparticles have been used in many technological applications ranging from optical filters to chemical sensors and biological markers.

In 1951 it was discovered theoretically that one could change the color of small gold particles in a highly controlled manner by changing their shape from solid metal spheres to thin metal shells. Up until now, however, it has proven very difficult to make these particles. Halas and her co-workers have developed a highly reproducible method to fabricate metal nanoshells.

In a recent article in Chemical Physics Letters, the team outlined the fabrication process of these nanoparticles and showed how their structure corresponds to their optical properties.

This work is funded by the Office of Naval Research, the National Science Foundation, NASA and the Robert A. Welch Foundation.

For more information about Naomi Halas's research see: http://www-ece.rice.edu/~halas/.