NC State Chemist Creates Structure in Amorphous Materials

A chemist at North Carolina State University has made breakthrough discoveries that advance basic understandings of the nature of liquids and glasses at the atomic and molecular levels. Featured in the Sept. 26 issue of Nature, these discoveries could lead to the development of totally new materials with useful optical and electronic properties - as well as applications not yet foreseen.

Liquids and glass have long been understood by scientists to be amorphous, meaning "without structure." Cartoon pictures in textbooks of atomic arrangements frequently represent liquids to be much like gases, a collection of molecules moving around randomly.

Not so, according to Dr. James D. Martin, associate professor of chemistry at NC State. "Just as a symphony is much more than a collection of random notes, the atoms and molecules in a liquid are quite organized - more like those in a crystal than a gas."

With this new understanding of liquid molecular organization comes the ability to reorganize liquids.

Martin and his colleagues have discovered the chemical principles that allow them to essentially write new "symphonic compositions" in amorphous materials. They have designed the compositions and structure of several glasses and liquids, then gone into the laboratory and made them.

Due to this new ability to design such structures, it will be possible to engineer specific optical and electronic properties of glasses and liquids. This amorphous-material engineering creates the materials foundation for future technologies.

What led to this important discovery? Martin specializes in the structure and physical properties of inorganic materials. His work involves engineering crystals to produce materials with desired properties.

Several years ago, Martin noticed that as he designed and synthesized crystals, he also produced a lot of liquid and glassy blobs. He originally dismissed the blobs as trash, but became curious about them because they appeared so frequently. His curiosity led him into the study of the molecular structure of liquids and glasses, an area not well understood by science.

The first hint of the presence of structure in liquids emerged in 1916, as scientists experimented with the X-ray diffraction of liquids. They observed structural features indicating some organization of molecules, but the organization was far less than is necessary for a crystal. Since that initial discovery, there has been significant scientific debate about whether the structure in liquids is crystal-like or random.

Upon melting into a liquid, most solids undergo a very small change in volume, suggesting that the interactions holding molecules together in liquids, glasses and crystals are quite similar.

Despite these clues, scientists still have only a limited knowledge about the structure of liquids and glasses. In a typical freshman chemistry textbook, there are multiple pages on gases and solids, yet only a paragraph or two on liquids.

"That's the mystery. What is the structure of something that's not supposedto have a structure?" Martin said. "If similar bonding interactions hold molecules in liquids, glasses and crystals, then it should be possible to engineer the structure in liquids and glasses just like it's possible to engineer the structure of crystals."

An analogy occurred to him as Martin stared at the crystal models he'd made by gluing tennis balls together, and then watched his children "swim" through big playpens filled with plastic balls. "Picture the balls as molecules," Martin said. "No matter how kids may move around in the playpen, the balls always touch each other in about the same way. And the arrangement of the balls looks very much like my tennis-ball crystal models."

This new understanding of the structure of liquids and glasses suggests the possibility of engineering new liquids and glasses. "If you understand the network's structure, and the chemical bonds within the structure, you can manipulate the structure," said Martin. "And if you change the structure, you change the properties."

In his laboratories at NC State, Martin and graduate student Steve Goettler have proven this by introducing molecules of a different substance into glasses and liquids. The foreign molecules are engineered at the atomic level to "fit" within the liquid's structure and interact with the liquid's own molecules. The presence of the foreign molecules changes the liquid's properties. Different concentrations of the foreign molecules also change the structure, and thus produce more changes in the liquid's properties.

To prove the structural relationships between their amorphous materials and model crystal structures, Martin's research group took their engineered liquids and glasses to Argonne National Laboratory. There they are able to look at the atomic organization of their materials using a glass, liquids and amorphous materials diffractometer (GLAD) instrument at Argonne's national user facility.

Martin's work, funded by the National Science Foundation, opens a new area of scientific research: amorphous materials engineering. He foresees the ability to control the optical and electronic properties of glasses to produce specialized materials that will advance optical computing and communications technologies, among other applications. "This new understanding," he said, "allows us to create the materials that will be the foundation of tomorrow's technology."

At the very least, someone will have to rewrite a lot of chemistry textbooks.