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When knowledge precedes understanding
The nano world has always been with us, says Bruce Koel. What is new is our attempt to understand and control it to make new materials and devices.
“For example, people who work in catalysis pass crude oil over catalysts to get gasoline,” says Koel. “These catalysts have always been nanoparticles. What we are now trying to do is control their size, shape and features. To us, 2 nm is very different from 4 nm and a cube is very different from a sphere. This level of control is unprecedented.”
|Transport phenomena of electrons and holes is critical to the performance of advanced CMOS devices.|
|Palladium-coated iron nanoparticles in solution.|
|COT researchers use an ensemble of quantum dashes to generate a new type of broadband semiconductor laser.|
Koel’s point is illustrated by Israel Wachs, a professor of chemical engineering and noted catalysis expert. Wachs has learned that tungsten oxide, an acid used to boost octane in gasoline, is 100 times more reactive when attached to titania nanoparticles measuring 1 nm than when attached to titania measuring roughly 5 nm.
Himanshu Jain, professor of materials science and engineering, says “ruby glass” is another example of knowledge preceding understanding. Two millennia ago, without understanding why it happened, the Romans knew that gold dispersed in glass could cause the glass to take on a reddish hue. Not until the 1920s did scientists learn that colloidal nanoparticles of gold were responsible for imparting the red hue.
Jain has assembled an international team of researchers to explore the electrical properties of gold and silver nanoparticles distributed homogeneously in glass.
“The optical properties of gold nanoparticles distributed in glass are reasonably well-understood,” says Jain, who directs Lehigh’s NSF-supported International Materials Institute for New Functionalities in Glass, “but there is no literature on the electrical behavior of the nanocomposite material. Our theory is that we might see nonlinear electric properties and switching.”
Jain and his team hope that particles of silver measuring 8 to 10 nm might facilitate the development of antibacterial glass. They are using the Scienta ESCA 300 to examine the chemical structure of the nanoparticles as they disperse in phosphate glass.
The silicon chip, says Tom Koch, an NAE member and director of Lehigh’s COT, combines economy of scale, precision and sophistication like no other technology. Its 1 billion transistors contain features as small as 45 nm, a size engineers hope to cut in half.
But silicon does not emit or detect lightwaves usable in most telecommunication applications, a shortcoming that would seem to rule out a central role in meeting the escalating demands for speed, size and power in next-generation networks.
Koch, working in an area called silicon photonics, wants to overcome silicon’s optical deficiency by exploiting its superior electronic properties. Researchers at Intel and at COT partner Lightwire Inc. have shown that silicon chips can be used to encode data on a light beam at speeds of 10 billion bits per second and more. It turns out that silicon’s weak interaction with light, if highly concentrated, can support practical optical device structures.
|Koch wants to overcome silicon’s optical deficiency by exploiting its superior electronic properties and the precision with which it can be fabricated.|
“Silicon is not the greatest material for optics,” Koch says. “But you can deposit layers and pattern structures on silicon chips whose features are extremely well-developed and precise. This could enable us to design and fabricate things we cannot do with any other technology. If it works, this could be very cheaply manufactured as well.”
In one project, Koch is improving the ability to guide lightwaves in silicon nanostructures without losing significant quantities of the light signal. He and his group have succeeded in squeezing a remarkable fraction of the light into a sideways slot waveguide only 8 nm thick.
Koch’s partners in this endeavor include MIT, Cal Tech, Stanford, Cornell, Boston University, and the Universities of Rochester and Delaware. Koch, who also collaborates with researchers in Belgium and Singapore, has given 18 invited talks and short courses on silicon photonics around the world.
One of the goals of Koch’s group is to make a laser out of silicon. Into the silicon nanolayer, the team inserts erbium ions that emit light at 1550 nm, in the telecommunications region of the spectrum. By keeping light losses low in the silicon nanostructure and providing nanoscale electrical access to the ions, the group hopes to be able to excite the erbium to emit light.
“A device with this kind of waveguide also offers very good promise for chemical and biosensing applications,” says Koch. “With so much energy concentrated at the nanoscale on the surface, we can functionalize the surface to sense molecules, proteins or bio species. We think we can measure down to the point of a single molecule.”
Koch receives significant funding from NSF, DARPA, and the Defense Department’s Multidisciplinary University Research Initiative Program.
A nano-ensemble for H2
In the search for clean, renewable energy sources, hydrogen has attracted much attention. Before it can be useful, though, scientists must learn how to produce hydrogen cheaply, cleanly and in large quantities. Dmitri Vezenov is developing a “multinetwork” nano-catalyst that self-assembles and uses solar energy to obtain hydrogen from water.
Photocatalytic water splitting typically uses titania as the catalyst, says Vezenov, an assistant professor of chemistry. Titania is used widely as a pigment and also in sunscreens because it absorbs ultraviolet light. It is also used in photovoltaic cells.
Titania has a limitation, however – its bandgap is too large to absorb visible light, which contains the largest amount of solar energy.
Vezenov has enlisted an ensemble of materials to overcome this shortcoming. He uses an organic linker to assemble a shell of cadmium-selenide nanoparticles (4 nm) and gold nanoparticles (20 nm) around a core formed by titania nanoparticles (20 to 40 nm).
“This is a porous catalyst with three networks,” says Vezenov. “The gold nanoparticles conduct electrons, while the titania nanoparticles do the electrochemistry to split water into hydrogen and oxygen. The CdSe nanoparticles, which are quantum dots, absorb the visible light.”
Sunlight plays a critical role in the reaction, Vezenov says, by exciting the titania electrons. As these electrons are “promoted” out of the filled valence band, they are replaced by electrons coaxed from the water, thus enabling the oxygen and hydrogen to split.
“We have generated some self-assembled catalyst films,” says Vezenov. “Because each of the materials linked has its own spectroscopic signature, we observed that they do indeed interact electronically after self-assembly. We are now checking for catalytic activity by looking for changes in the electrochemistry.”