Researchers strike gold

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.)
Building blocks
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.
A microscopy advantage
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 be installed over the summer.
--Kurt Pfitzer