Beyond alchemy and the Wright brothers: Nanosecrets of everyday things
Nanoparticle arrays of platinum crystals on a silicon oxide surface reveal the important role in catalysis played by the interface between metal and substrate. Click here for more photos.
I t's their nanostructure that makes many
crucial materials useful, and chemical
processes essential to everyday life routinely
do their work on the nanoscale. There's a lot
more to nanoscience than building itty-bitty
For 15 years, ever since K. Eric Drexler's Engines of
Creation launched the nanocraze, the field has been
plagued by sci-fi notions of tiny robotic "molecular
assemblers" running around shoving atoms together.
But as buckyball pioneer Richard Smalley remarks,
molecular assemblers have long existed: "We call
Catalysts are "helper" substances that promote
chemical reactions without themselves being
consumed. Nature's catalysts, enzymes, assemble
only specific end products. Industrial catalysts are
rarely so precise.
Gabor Somorjai of Berkeley
Lab's Materials Sciences
Division, a professor of
chemistry at UC Berkeley,
notes that "you can
increase the octane rating
of gasoline remarkably" by
catalyzing its hydrocarbon
precursor over platinum,
"but there are at least
seven or eight directions
the reaction can go."
What's wanted are
chemical structures called
branched isomers, but the
reaction can also can
aromatics, which are
molecules-or, by breaking
carbon-carbon bonds, it
can produce gases, "the last thing you want."
Industrial catalysts are often tiny particles of metal supported on an oxide surface,
operating in a high-temperature stream of reactants. The smaller the particle, the larger
the ratio of its surface to its volume; the more catalyst surface that's exposed, the more
efficient the reaction.
"As long ago as the 1920s, industry began depositing metal particles on a support surface
by precipitating salts from solution," says Somorjai. "The trouble is that these particles
vary in size, and their spacing is random."
Size and spacing are both vital to Somorjai's goal of creating industrial catalysts with the
efficiency and precision of enzymes. In collaboration with Erik Anderson and the
Nanowriter facility (see page 38), Somorjai positioned single crystals of platinum 15 to 20
nanometers high just 100 nanometers apart, on a silicon oxide surface only half a
Tests in a high-pressure reaction cell showed that the nanoparticle array was far more
selective and efficient in catalyzing reactions than platinum alone. But there was a
Between each test, Somorjai and his coworkers cleaned the catalytic array, "shaving off
any organic 'dirt' by bombarding the wafer with a beam of ionized neon." The beam
planed the platinum clusters down, atom by atom, until their surface area was less than
half of what it had been.
Remarkably, the array remained 20 to 30 times more active than platinum alone. "It
wasn't just the metal that promoted catalysis. The interface between the metal and the
oxide was much more important that anyone had previously thought"-a discovery that
may lead to efficient "high-tech" catalysts.
Skimming the surface
Somorjai examined his nanocrystal arrays with an atomic-force microscope, which "feels"
its way across a surface. The shallows of the material world are transparent to other
experimental techniques too. One is to penetrate a thin sample with a focused beam of
electrons and construct an image-a kind of electronic shadow- from the interference
patterns of the scattered electrons.
Often these transmission electron microscope images have a "bubble-raft" appearance, in
which ordered arrays of little round blobs encounter other arrays oriented differently.
Each blob represents a column of atoms; seen from a different angle, the spacing and
orientation of the columns gives a different picture, although at some angles the atoms
are too close together to resolve. An important challenge has been to find out how "tall"
each column is-a challenge recently met at the National Center for Electron Microscopy
(NCEM) by advances in computation, improved microscope hardware, and sophisticated
Under the direction of Michael O'Keefe and Christian Kisielowski, NCEM's One-Ångstrom
Microscope (OÅM) has achieved the country's highest resolution-better than 0.8
angstrom (less than a tenth of a nanometer)-resolution so good, says Kisielowski, that
"the OÅM can resolve different projections in many crystalline materials to get to the
The OÅM makes use of "phase-scrambled" information well beyond the in-phase image
components that normally define a microscope's resolution. The additional information can
be deciphered by combining different images of the same sample with a computer.
Other new image-processing techniques allow individual atoms in a column of atoms to be
counted, provided the atoms are heavy enough. As a result of these advances, says
Kisielowski, "theory and experiment are merging. Once upon a time theorists were on their
own if they designed materials on a computer. Nowadays, the atom clusters they
calculate are similar in size to those we investigate, and there is a reliable way to check
Now almost every atom in a nanocluster lying on a substrate can be accounted for. "We
can, for example, locate columns of silicon atoms at a gate oxide interface-the most
important interface in integrated circuit technology-with the unprecedented precision of
about a hundredth of an angstrom."
The new methods are generally applicable and can visualize interfaces of different
crystal, as well as lattice, defects. "Dislocations determine the plasticity of solids . . . for
the first time their core structure can be examined directly in all sorts of solids," says
Kisielowski. "A 20-year argument about different core models can be resolved by simply
having an image with truly atomic resolution."
The magnetic phenomenon of exchange bias-or "pinning"-has been known for almost half
a century, but scientists at the Advanced Light Source (ALS) opened the door to a new
and better understanding of the phenomenon by examining layered magnetic materials,
each layer only a few nanometers thick, with the PEEM2 photoemission electron
Instead of penetrating the sample
with an electron beam,
microscopes and spectrometers
stimulate the material to emit its
own electrons-whose energy and
spin convey information about
elemental compositions and
electronic and magnetic structures.
"A modern read head uses
magnetic layers of very thin films
with different properties," explains
Andreas Scholl of the ALS. The
magnetic structure at the
interfaces of ferro- and
antiferromagnetic layers determines
the function of the device. "As the
head passes over the hard disk,
these layers sense the orientation
of the domains on the disk and
cause the head's electrical
resistance to change in response."
The electronic spins in the
magnetic domains of ferromagnets
are parallel, and they all point in the same direction; ferromagnetic domains change
orientation in the presence of an external magnetic field. The electronic spins in
antiferromagnets are also parallel but point in opposite directions; on average, they are
insensitive to external fields.
By pinning one ferromagnetic layer in a read head to an underlying antiferromagnetic
layer, the other is free to respond to changing fields as the head skims over the disk.
When the two ferromagnetic layers have the same orientation, they exhibit less electrical
resistance than when they are opposed. Thus magnetic data on the disk is translated
into variations in electric current.
To examine pinning on the nanoscale, a research team led by IBM scientists used the
PEEM2 microscope on ALS beamline 18.104.22.168. When an x-ray beam hits the sample,
electrons are ejected and focused into an image with 20-nanometer resolution.
Since X-rays of different energies stimulate
electrons characteristic of different elements,
different layers can be viewed by tuning the
energy of the beam. Magnetic domains are
revealed by polarizing the beam: circular
polarization produces images of ferromagnetic
domains, and linear polarization shows
PEEM2 images were the first to clearly show the
alignment of magnetic domains in an
antiferromagnetic thin film. When the
researchers compared transmission electron
microscope images of the same sample, they
saw that the magnetic domains corresponded
exactly with the orientation of the material's
Soon afterward, PEEM2 produced perfectly registered images of a ferromagnetic layer
three nanometers thick overlying an antiferromagnetic layer. These showed precise
correspondence between the spin orientations of the adjacent materials, domain by
domain: pinning aligns the magnetic structure of both layers.
To control the orientation of the coupled ferromagnetic layer in read heads,
manufacturers set a bias by annealing the multilayer devices in a magnetic field-taking
advantage of the fact that magnetic materials lose their magnetism above a critical
temperature but regain it when they cool.
To their surprise, the
researchers found that their
layered test samples,
although not annealed,
already possessed a bias
within each local domain.
"Apparently exchange bias is
an intrinsic property of the
interface," says Frithjof
Nolting, a member of the
research team from
The discovery, he says,
"opens the door to new
investigations, which may
affect the way devices
based on the exchange bias
magnetic materials used to
To further these
researches, ALS scientists
and their international
colleagues are even now racing to build a third-generation PEEM that will eliminate
spherical and chromatic aberration, achieving the finest possible resolution, an order of
magnitude better than PEEM2's.
Into the depths
"The surface is an unavoidable defect in the crystalline structure," as science historian
Ernest Braun wrote; the fundamental nanosecrets of solids lie deeper. One way to dig
them out is with transmission electron microscopy.
Metallurgists have long known how to make strong
alloys, even without understanding their
microstructure. First heat the constituents until
they dissolve in one another, then allow the alloy
to "age" for several days; one or more of the
constituents may precipitate out of solution and
form microscopic inclusions in the matrix, forming
a considerably stronger alloy. Precipitation
hardening has been crucial to aviation since the
first powered flight- although the Wright brothers
didn't know it.
With the help of mechanic Charles Taylor, the
Wrights designed and built the Flyer's
12-horsepower engine in their own shop. The
180-pound engine was wrecked after the fourth
flight of the day, but a recent analysis of a
long-lost piece of it showed that the Wrights
made it of precipitation-hardened aluminum-copper-seven years before such alloys were
knowingly used in aviation.
"Not until the last few decades have scientists been able to investigate how metallurgists
achieved these effects," says Uli Dahmen, head of NCEM, "and in fact it's only now that
we're beginning to get a thorough understanding of crystal structures and the interface
between precipitates and the matrix."
The closer the spacing of precipitates, the
harder the alloy. So-called S-phase
inclusions in advanced alloys such as
aluminum 2219, which includes copper and
magnesium, are so small and constitute such
a small percentage of the total composition
that their crystalline structure and
orientation could only be guessed. In 1999 a
team including Dahmen, led by NCEM visitor
Velimir Radmilovic, professor of metallurgy at
the University of Belgrade, found that none of the proposed models was correct.
"To do the study we combined microscopy and computing in a fully quantitative data
analysis, using every pixel in every image," says Dahmen. "Although these precipitates
are only a few nanometers in size, we were able to get a clear understanding of their
unique crystal structure and shape-and how they strengthen the alloy."
Using sophisticated software recently developed by NCEM's Roar Kilaas, various simulated
"ideal" images were repeatedly compared to real images
of 2219 to find the best fit, thus revealing the precipitates' unknown crystal structure.
The brick-shaped precipitates, themselves composed of aluminum, copper, and
magnesium, form at two different angles in the pure aluminum matrix; in one orientation
the edge of the "brick" has regular stepped ledges.
"The apparently odd orientation of the precipitates within the matrix allows them to make
a perfect fit with the lattice structure of the matrix at the interface," says Dahmen.
Because precipitates interrupt the crystalline structure of the matrix material, they
interfere with shearing. The more different orientations their own crystal structure allows,
the better they can halt shearing along the matrix's crystal planes. And the thicker their
dispersion, the shorter any shear that does get started.
As inclusions approach nanoscale, their thermal response also changes dramatically. "The
energy of their interface with the matrix becomes more important than their internal
energy," Dahmen says. Thus a nanosize inclusion may melt at a much higher or much
lower temperature than the bulk material, or it may have a crystal structure that is
simply not stable in particles larger than a few nanometers.
Applying their new knowledge, Radmilovic, Dahmen,
and their colleagues have patented a new alloy of
aluminum, copper, germanium, and silicon for use in
the aerospace and automotive industries. The new
alloy is characterized by an extremely dense
distribution of ultrafine precipitates of two different
compositions, one kind piggybacking on the other.
Harder and more stable than 2219, the new alloy is
also more energy-efficient to manufacture and
"Scientific investigation of nanoscale inclusions in
solids is not always recognized as 'nanoscience,'"
says Dahmen. "That may be because metallurgists
were so successful for so long at developing better materials without the help of a
scientific understanding. But that situation is changing rapidly. By probing the atomic
structure of materials with electron microscopy, we are learning how their properties can
be controlled on the nanoscale. We've progressed well beyond alchemy and the Wright