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
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
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.
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
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
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
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.