Researchers at Purdue University are using a rare
type of electron microscope to see how structures
like carbon nanotubes form at the atomic level, information
that will be crucial for nanotechnology to find practical
applications in computing, electronics and other
The new transmission electron microscope has been
modified so that researchers can watch how atoms
come together to form nanostructures as gases flow
into a chamber in the presence of a metal catalyst.
This is the same method used to make nanotubes in
research labs and electronic devices in the semiconductor
"Before we can consistently manufacture nanostructures
that have the same specifications and qualities,
we have to learn precisely how atoms interact and
come together to form these structures," said Eric
Stach, an associate professor of materials engineering
who operates the microscope at the Birck Nanotechnology
$4 million FEI Titan microscope is equipped with
an "environmental cell," in
which gases such as acetylene or butane, which
contain carbon, are passed over nanoparticles of
metal, such as iron or nickel. The metal particles
act as a catalyst for breaking down the gases and
releasing carbon atoms during the reaction, which
takes place in the cell at temperatures sometimes
reaching more than 1,000 degrees Celsius, or more
than 1,800 degrees Fahrenheit.
"What's unusual about this instrument is the ability
to take high-resolution pictures while you flow gases
over a sample," said Stach, noting that fewer than
five such microscopes exist in the world.
are accelerated under high voltage and manipulated
with a series of "magnetic lenses" that
focus electrons through thin sections of materials
being studied. The electrons bounce off the atoms
in the material, and this scattering process can
be reconstructed to form an image.
"The transmission electron microscope allows you
take pictures of the internal structure of a material," Stach
The instrument, which has 14 main lenses and another
50 smaller lenses, is housed in a specially shielded
facility in the Discovery Park lab to block electromagnetic
interference from sources such as powerlines and
radio transmitters. The 10-foot-tall, 3 1/2-ton microscope
sits on its own concrete slab, separated from the
building's foundation to isolate it from vibration.
Pictures are formed with a resolution of 2 angstroms,
which is fine enough to allow imaging of atomic arrangements
in the sample.
"Researchers have done these sorts of experiments
with other microscopes that have environmental cells
but not with this level of resolution," Stach said.
nanotubes — hollow fibers that have promising
future applications in computer chips and electronic
devices — are "grown" using the metal catalyst to
break down a gas. But Stach said the catalytic mechanism
is not fully understood, and that is one area researchers
will pursue using this microscope.
the reaction, carbon forms into nanotubes having
various lengths, diameters and twists, or "chiralities."
"These different nanotubes possess different performance
characteristics," Stach said. "What we really need
to understand is how to get the same performance
and the same characteristics by growing the same
tubes every time."
nanotubes, which were discovered in the early 1990s,
might enable industry to create a new class of
transistors and more powerful, energy-efficient
computers, as well as ultra-thin "nanowires" for
electronic circuits, but their practical application
requires that they be manufactured to specific standards.
"In the lab, a whole bunch of nanotubes are grown,
and then you see which one has good properties," Stach
said. "You cannot yet control how to get the exact
nanotube twice, but in order to move from the laboratory
into creating something that can be engineered and
made into devices, we have to have an understanding
of the process. How do we get the same nanotube every
time? Now we are going to be able to take pictures
that show individual carbon atoms interacting with
the metal catalyst and growing into nanotubes."
Chemical processes to grow materials using a gas
require an environment that's close to normal atmospheric
pressure, but electron microscopes operate in a vacuum
to maintain the flow of electrons. The new instrument
enables researchers to run experiments at close to
normal pressure levels inside the environmental cell,
while critical microscope components run in a vacuum.
"It's tricky because we are trying to recreate real
growth conditions within the microscope, an instrument
that normally operates in a vacuum environment," Stach
The environmental cell is a cube-shaped chamber,
and each side measures about 5 millimeters, or about
one-fifth of an inch.
"The idea is to keep it small so that you can have
high pressure locally, while maintaining vacuum conditions
everywhere else," Stach said.
Purdue researchers are using the microscope in a
joint project with scientists at IBM Corp.'s Thomas
J. Watson Research Center in Yorktown Heights, N.Y.,
to study how to make silicon nanowires for future
"In addition to studying nanometer-scale structures
made of unconventional materials, such as carbon,
we are also trying to learn how to make smaller devices
and structures from conventional silicon," Stach
this size scale, a material's properties change.
For example, if you take a piece of gold and make
it very small, it's not really quite gold anymore
because the electronic structure changes and it
has different properties. These nanomaterials transmit
electricity and light differently than when they
are in bulk form, and these differing properties
could be harnessed to create superior computers and
electronics, but only if we learn precisely how they
form at the atomic level and how to fabricate them
in a uniform way."
The researchers at Birck also are using the new
instrument for work funded by the National Science
Foundation to study the growth of semiconductor materials,
such as silicon, germanium and gallium arsenide.
"It's the same idea as studying the growth process
of carbon nanotubes," Stach said. "We need to know
which atoms are going where. What is the effect of
temperature, pressure, source gas and catalysts in
creating uniform structures that are the same every
Researchers also will use the instrument to help
Purdue researchers study the workings of catalysts,
which are critical for industry in making everything
from gasoline to plastics. As part of that work,
the microscope will be used in projects involving
Purdue's Center for Catalyst Design.
"Ultimately, the goal is to create better catalysts
to make products more efficiently and at lower cost," Stach
Researchers at the interdisciplinary center plan
to use the instrument for a variety of other research,
including studies to learn more about how metals
rust and oxidize at the atomic level, information
that has potentially major economic value for industry,
"Now that the microscope is up and running, we expect
greater interest for more joint research projects
through Birck and Discovery Park with other industrial
and corporate partners," Stach said. "This microscope
is going to be very busy."
Related Web sites:
Birck Nanotechnology Center: http://
Discovery Park: http://purdue.edu/discoverypark