Aberration-corrected microscopes bring 2020 vision to the nano-world

04/19/04

New electron microscopes could hold keys to scientific mysteries, future advances in technology

Advances in electron microscopy at Lehigh University are promising to shed light on the atoms of the nano-world that play a disproportionate role in the efficiency and safety of everyday materials.

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

The new instruments will give scientists an ability they have long sought: to simultaneously image and determine the chemical identity of individual atoms in crystalline materials.

Lehigh has purchased a new, JEOL 2200FS transmission electron microscope (TEM) fitted with an aberration-correction device.

A separate aberration-correction device will be added to a VG HB 603 scanning transmission electron microscope (STEM) that Lehigh bought 10 years ago.

Both pieces of equipment are expected to arrive in April and to be installed this summer.

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

Aberration-corrected microscopes achieve improved resolution by correcting distortions in the lenses that focus the electron beam on the specimen. 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, continuously measures and corrects for the "over-focus" in 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 Prof. Chris Kiely, director of the Nanoscale Characterization Laboratory in Lehigh's Center for Advanced Materials and Nanotechnology.

"Our current HB-603 STEM can tell us whether or not nanoparticles are alloyed," says Kiely. "But it doesn't tell us whether an alloy nanoparticle is homogenous in composition or whether, for example, we have a shell of palladium on a gold-rich core. The aberration-corrected microscope will give us the improved resolution that we need in order to determine this kind of effect."

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 a 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 at Lehigh and a principal investigator on the microscope proposal. "Once we know those things, 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 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 aberration-corrected JEOL TEM will be digital and remotely controllable - users at distant laboratories will be able to drive the instrument via a special website.

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.

Lehigh's VG HB 603 STEM is one of only four 300kV STEMs ever built and 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.

Source: Eurekalert & others

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