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Nanogold does not glitter, but its future looks bright

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28.04.2004

At the nano-level, gold acquires a new shine, a new set of properties and a host of potential new applications

All that glitters is not gold, goes the old adage.

But the shrinking frontiers of science require a qualifier: Gold itself does not always glitter.

In fact, if gold is created in small enough chunks, it turns red, blue, yellow and other colors, says Chris Kiely, who directs the new Nanocharacterization Laboratory in Lehigh’s Center for Advanced Materials and Nanotechnology.

Kiely, a professor of materials science and engineering, explores the properties of "nanogold," or gold particles so tiny - containing only hundreds or even tens of atoms - that they must be measured in nanometers. (One nm is equal to one one-billionth of a meter.)

As is true with other materials, gold in "nano" form exhibits different properties from bulk gold.

"As everyone knows," says Kiely, "normal bulk gold is shiny, it is gold in color, it is inert, and it conducts electricity.

"If, however, you shrink gold down to a nanoparticle, its properties change dramatically. Its color changes, it becomes a very good catalyst, and is no longer a metal - instead it turns into a semiconductor."

Kiely seeks not only to identify the properties of nano-materials but also to find new uses for them and new ways of assembling them into usable structures.

Together with Martin Harmer, director of the CAMN, Kiely takes small numbers of gold atoms, sometimes combining them with atoms of other elements, and seeks to form them into nanoparticles of very well defined shapes and sizes. Their research is supported by a grant from the National Science Foundation.

Much as a child puts together Lego toys, the researchers using Au nanoparticles as building blocks have assembled one-, two- and three-dimensional nanostructures, including 1-D nanowires, 2-D nanofilms and 3-D supercrystals.

Kiely and Harmer have learned that they can tailor the properties of their nanoparticles assemblies by varying the size and elemental composition of the particles.

By heating these nanoparticle arrays at different rates, they can also introduce instability into the structures, since the smaller nanoparticles have a tendency to melt first. They have succeeded in causing a string of nanoparticles to melt into a nanowire that is 10 times thinner than any wire made using the standard microelectronic process called electron beam lithography.

"We have learned that the speed at which we heat and destabilize the nanoparticles is crucial," says Kiely. "If you want to make nanowires, you have to heat very, very quickly. If you go too slowly, the result is a globby mess."

Kiely and Harmer, who is renowned for his work in the sintering (heating) of ceramics, are also assembling an array of gold nanoparticles into a non-metallic super-crystal that behaves like a semiconductor. By altering the size and separation of the nanoparticles that make up the supercrystal, they can control its overall conductivity.

Kiely, Harmer and other nanotechnology researchers at Lehigh have an advantage over their peers at other universities. The electron microscopy facilities at Lehigh are among the best anywhere and are ideally suited for the analysis of materials at the nanoscale. In fact, for 34 years, each June, Lehigh has hosted the world’s most comprehensive microscopy short courses.

Recently, Lehigh’s microscopy facilities received a state-of-the-art boost. The university purchased a new JEOL transmission electron microscope (TEM) fitted with an aberration-correction device, along with a separate aberration-correction device that will be added to a scanning transmission electron microscope (STEM) that Lehigh bought 10 years ago.

The acquisitions make Lehigh one of a handful of universities in the world to possess an aberration-corrected electron microscope and the only school with two. Both pieces of equipment are expected to arrive later in April and will be installed over the summer.

Aberration-corrected microscopes achieve their improved resolution by correcting any distortions in the lenses that focus the electron beam onto the specimen that is being examined. The outer extremities of electron lenses tend to focus more strongly than their centers, limiting the beam diameter to 1 or 2 nm, or about the width of five to six atoms.

An aberration corrector, aided by a sophisticated feedback mechanism, continuously measures and corrects for the "over-focus" in the objective lens. The resulting electron beam measures 0.1 nm in width, which is less than the diameter of most atoms.

The new instruments will give Lehigh researchers an ability that scientists have long sought: to simultaneously image and determine the chemical identity of individual atoms in crystalline materials.

And they will help Kiely shed light on nano-gold.

"The current HB-603 STEM can tell us whether or not our nanoparticles are alloyed," says Kiely. "But it doesn’t tell us whether an alloy nanoparticle is homogenous in composition or whether, for example, we have a shell of palladium on a gold-rich core. The aberration-corrected microscope will give us the improved resolution that we need in order to determine this kind of effect."

Kiely will also use the new microscopes in a collaboration he has undertaken with scientists at the University of Wales in Cardiff. The group is attempting to identify chemical reactions that nano-gold can catalyze.

Scientists in Japan discovered 10 years ago that gold displays fantastic catalytic abilities when it is shrunk to 3 to 5 nm in size. If the gold particles are any bigger or smaller than this, the element resumes its inertness.

One such reaction is the conversion of carbon-monoxide (CO) to carbon-dioxide (CO2). Nanogold catalyzes this at room temperature and with 100-percent efficiency. A potential application is to aid firefighters, who now wear protective masks containing copper-manganese-oxide. That material’s effectiveness at getting rid of CO, however, lasts only 15 minutes, while nanogold protects for several hours.

Kiely will not soon run out of nano-projects to tackle.

Because almost any material can be made into nanoparticle form, he says, it should theoretically be possible to assemble combinations of very different nanoparticles with a variety of desired properties into 2- and 3-D structures that have engineered colors, degrees of magnetism, conductivity, semiconductivity, insulating properties and more.

"There’s no limit to the number and variety of self-assembled nano-materials you can make."


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Kurt Pfitzer | Source: EurekAlert!
Further information: www.lehigh.edu/

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