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
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
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
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
"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
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
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