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Revealing the infinitesimal

[continued from page 1]

The electron microscopy facilities at Lehigh are comparable to those of a national lab. The university’s annual Microscopy School draws researchers from major government, industry and academic labs around the world. Lehigh was the first university to acquire two aberration-corrected transmission electron microscopes, which determine the chemical identity of atoms in crystals with a focused beam of 0.1 nm.

Materials scientists in the CAMN have developed software (see p. 5) that more efficiently analyzes data gathered from electron microscopes, improves signal-to-noise ratio, and achieves greater resolution of small concentrations of elements. Scientists also use a technique called High-Angle Annular Dark Field imaging to enhance resolution of crystal materials.

“This technique makes it possible to see individual metal atoms or atom clusters dispersed over oxides,” says Christopher Kiely, director of the CAMN’s Nanocharacterization Laboratory. “It turns out that these clusters or atoms often play a critical role in material behaviors.”

“The enhanced resolution of the aberration-corrected microscopes enables us to see what atoms really do,” says Harmer. “Coupled with our improved computational simulation of nanostructure devices, these microscopes have shown us that the scientific assumptions on which our knowledge rests are fragmentary or even fictitious. This leads to completely new thinking about interfaces.”

A computer rendition shows light glowing inside a cylindrical carbon nanotube.
MC3T3 bone-precursor cells growing on nontransparent bioglass.

Nonvolatile memory
To design and fabricate electronic equipment, says Marvin White, one must first understand how electrons and holes move through nanostructures. The control of these charge-carriers and their transport phenomena is vital to the non-volatile storage of data, in which information is preserved without the use of an active power supply.

White has spent 40 years investigating nonvolatile semiconductor memory – the memory inside a thumb drive or an iPod, for example, which relies on the storage of electrostatic charge in the form of negative charge (electrons) and positive charge (holes).

“We model and study the transport phenomena of charge carriers both along the surface, covered by nanolayers of thin films, and perpendicular to this surface,” says White. “The carriers move along the surface in devices that switch and amplify information. In devices that store information, they move perpendicular to the surfaces by a process called ‘tunneling’ and are trapped in these thin films.

“We make our own devices, and we model the transport phenomena observed or created by other researchers. Our partners include Intel, Micron Semiconductor, IBM, AMD and Northrop Grumman.”

The phenomena that White and his students study often occur at what he calls the realm of the sub-nano, with features a few nm or smaller.

“The structures that switch, amplify and store information have dimensions on the order of 10 or 20 angstroms or less. One structure, a silicon-dioxide film grown with atomic layer deposition, is just 4 angstroms thick.” An angstrom is 0.1 nm.

Advanced tools, says White, are critical to his research. Electron beam lithography enables his group to create and investigate the properties of nonvolatile memory and scaled semiconductor devices. Angle-resolved photoelectron spectroscopy (ARXPS) helps determine a structure’s thickness by locating and identifying atoms at or near its surface. Fabricating these nanostructures requires integrating more than 100 process steps in the lab.

Conceptual changes
Nanotechnology research, says Martin Harmer, can reorder the familiar landscape of scientific knowledge. “We have recently discovered that interfaces are not accurately described as two-dimensional,” says Harmer, a professor of materials science and engineering. “The traditional idea was that an interface was a region of misfit between two well-defined crystals. The new idea is that an interface is a third region with its own identity and criteria for stability. It is in fact a new state of matter, neither amorphous nor crystalline, at the interface between crystalline grains.

“We did not appreciate that internal interfaces would behave in this fashion until we acquired the tools to examine them, namely, aberration-corrected microscopy.”

Harmer has coined the term “complexions” to describe these interfaces. He received the Robert B. Sosman Award from the American Ceramic Society in 2008 in part due to this fundamental discovery.

In another project, Harmer and Kiely have been synthesizing nanoparticles of iron oxide that enhance contrasts in an MRI, and characterizing the particles with TEM. Harmer and Kiely and their collaborators, who include physicists, chemists, mechanical engineers and biomedical researchers, hope to use the particles to improve the diagnosis and targeted treatment of breast cancer. The team, led by Winston Soboyejo of Princeton, includes researchers from Duke and Louisiana State Universities, as well as Makerere University in Uganda and Cheikh Anta Diop University in Senegal.

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