New aberration-corrected microscopes will improve electronic vision to 20:20

Just as the Hubble space telescope is discovering giant galaxies at the edges of the universe, a revolution in electron microscopy at Lehigh promises to shed light on the atoms of the nano-world that play a disproportionate role in the efficiency and safety of everyday materials.

Next spring, with support from the National Science Foundation, Lehigh will become the first and only university in the world to have two aberration-corrected electron microscopes.

The NSF funding will enable the university to acquire a new, aberration-corrected JEOL 2200FS transmission electron microscope (TEM) and an aberration corrector for its existing VG HB 603 scanning transmission electron microscope (STEM). JEOL USA Inc. makes electronic optical equipment and instrumentation for high-end scientific and industrial research and development.

In an electron microscope, atoms in the specimen are identified by a series of characteristic X-rays that they emit when hit by an electron beam. Reducing the electron beam size reduces the area from which these X-rays are emitted.

The new instruments will give scientists an ability they have long sought: to simultaneously locate and identify individual atoms in crystalline materials.

The microscopes achieve this improved resolution by correcting an imbalance in the lenses that focus the electron beam on the specimen being examined. The outer extremities of the lenses tend to focus more strongly than their centers, limiting the beam width to 1 or 2 nanometers, or about the width of five to six atoms. (One nm is equal to one one-billionth of a meter.)

An aberration corrector, aided by a sophisticated feedback mechanism, measures the amount of "over-focus" and adjusts the outermost of the lenses, known as the objective lens.

The resulting beam measures .1 nm in width, or about half the width of an atom.

"This is like fitting a microscope with a new pair of reading glasses, giving it 20:20 vision," says Chris Kiely, professor of materials science and engineering and director of the Nanoscale Characterization Laboratory in the Center for Advanced Materials and Nanotechnology.

The behavior of the atoms and molecules in a material, particularly in the interfaces separating one material from another, often determine the bulk, or large-scale properties of the material.

An improved understanding of these microscopic behaviors has led to advances in materials used in semiconductor chips, airplane wings, VCRs, cell phones and many other modern devices.

It has also given metallurgists new diagnostic capabilities. The Titanic, scientists now believe, may have been doomed while it was being built, when a handful of sulfur atoms slipped unseen into the grains of iron in the ship's hull, rendering it brittle.

"Electron microscopy helps us understand the microstructure and microchemistry of materials," says David Williams, vice provost for research and one of the principal investigators on the microscope proposal. "Once we know those things, then we can learn how to control the physical, mechanical, electronic and chemical properties of a material."

The aberration-corrected microscopes will offer hitherto unobtainable insights into the nature of a variety of phenomena. These include the segregation of impurity atoms that controls brittle fracture of steels in nuclear reactors, the chemistry of catalytic nanoparticles used for to oxidate carbon monoxide and remove organic pollutants from groundwater, and the microstructure of new ion-containing polymers that could provide protection against chemical warfare.

The arrival of the aberration-corrected microscope will culminate a six-year acquisition endeavor that began in 1997, when NSF gave funding to a proposal to bring a new TEM to Lehigh.

"Soon after placing the original order, technology started to advance very rapidly," says Williams. "We didn't want to be our new microscope to be out of date as soon as it arrived. So over the past few years we have been working closely with JEOL to replace the original instrument and bring a fully digital, remotely controlled, multi-purpose instrument to Lehigh."

Unlike other aberration-corrected TEMs, which are used primarily for imaging, the new microscope will be fitted with an omega filter - designed to sharpen electron diffraction patterns and images from thicker specimens - and a top-of-the-range chemical analysis system. As this microscope can be remotely controlled, users from other universities will be able to drive the instrument via a special website.

The VG HB 603 STEM being fitted with the aberration corrector is housed in the department of materials science and engineering and is one of only four 300kV STEMs ever built. It has proven to be the most sensitive instrument in the world for chemical analysis.

The aberration corrector will improve the VG HB603's resolution for chemical analysis from 1.5 to less than 0.5 nanometers and will permit it to detect the presence of a single impurity atom in the analyzed region of the specimen.

"This is extremely important, for example, in trying to understand the embrittlement of steel," says Williams. "This instrument will allow us to see how impurity atoms such as phosphorus segregate to and interact with atomic-level defects in the metal."

The new instruments will play a key role in the Lehigh Microscopy School, largest short courses of their kind, which attract 150-200 participants from academia and industry to Lehigh every June.

"The new discoveries that will inevitably be made using these new microscopes will undoubtedly raise the research profile of Lehigh University," says Williams. "Our goal is to keep Lehigh as one of the leading electron microscopy centers in the world and now, with the help of the NSF, we will have an unparalleled suite of instruments to offer students and faculty colleagues."

by Carol Kiely