Seeing the small picture: X-ray nanoprobe pushes
observation to ever-smaller frontiers
Try to picture putting some atoms under a microscope. Even if
you could pick them up, put them on a slide and get them to stay
still, you still could not see them with even the most powerful
optical microscope.
The reason? The wavelength of visible light measures hundreds of
nanometers, the span of thousands of atoms. To zoom in all the way
to the atomic level, scientists from all over the world use the
high-energy X-rays produced by Argonne's Advanced Photon Source
(APS).
The quest to image tiny structures and their environments
presents a complex challenge and valuable scientific opportunity.
The molecular processes involved in the functions of our bodies at
the cellular level, as well as the chemical and physical properties
that characterize materials, depend on compositional, structural
and electronic properties at the micro-, nano- and atomic
scale.
The frontier of materials science has for some time pushed into
the nanoscale, where objects are measured in billionths of a meter.
Researchers continually strive to better understand nature at
ever-smaller length scales and to more precisely manipulate systems
to benefit humanity and the environment. From the next generation
of superconductors to solar cells to cancer treatments, the ability
to image tiny structural features has the potential to make a big
impact.
To enrich our understanding of the nanoscale properties of
complex systems, materials and devices, scientists come to Argonne
to employ the laboratory's new hard X-ray
nanoprobe, which is jointly operated by Argonne's Center for Nanoscale Materials and
Advanced Photon Source. This system can currently resolve
structures as small as 30 nanometers - a distance roughly
equivalent to the width of 100 atoms and less than 1/1000th the
diameter of an average human hair, and received one of the 2009
R&D 100 awards, which recognize the top 100 inventions
of the previous year.
While other forms of imaging - electron microscopy, for example
- can reveal even smaller details close to a sample's surface, the
high-energy X-rays generated by the APS can penetrate into a
material's bulk to reveal buried structures and interfaces. "X-ray
imaging gives scientists a unique window into complex systems,
allowing us to see their structure, composition and dynamics," said
Argonne nanoscientist Jörg Maser, who runs the nanoprobe.
The nanoprobe works much like an optical microscope, but uses
X-rays instead of visible light. These brilliant X-rays are
tailored to the requirements of individual experiments by a series
of X-ray mirrors and crystal optics in the nanoprobe beamline. In
the final step, a Fresnel zone plate focuses this "conditioned"
beam on the specimen.
Unlike refractive lenses used in an optical microscope, Fresnel
zone plates focus X-rays using diffraction. In principle, this
approach could allow scientists to one day focus X-rays to spot
sizes smaller than 10 nanometers. "The smaller the spot to which we
can focus our beam, the smaller the structures we can observe,"
Maser said.
In most of the scattering experiments performed to date,
scientists have been able to determine only the intensity of the
X-ray that hits the detector. However, by using more sophisticated
X-ray techniques - such as coherent diffraction - scientists can
extract not only the intensity of the X-rays, but also their phase.
"The name of the game is 'How do you determine the phase of your
wavefront'," said Argonne materials scientist George Srajer. "This
allows us to fully exploit the information carried from the sample
by the X-rays. Amplitude and phase information go
hand-in-hand."
By taking advantage of the phase information contained in
coherent X-rays, Argonne's researchers can more accurately resolve
the structure of their specimens, even without advanced X-ray
optics like zone plates. Synchrotron radiation sources like the APS
are built expressly to provide X-rays with proper coherence. "In
the end, what we are really trying to create is an
ultra-high-resolution image in real space, as we would see if we
could just take a picture of the sample with a camera," Maser
added. "That is possible if we can determine both the amplitude and
the phase, which requires coherent X-rays."
By shining hard X-rays instead of visible light onto their small
samples, Argonne's scientists can also study biological cells and
tissues. In one experiment, Argonne researchers are using the APS'
X-rays to image blood vessels as they form and branch out. This
process, known as angiogenesis, occurs as one of the most important
steps in the healing of wounds. However, cancerous tumors can also
perform angiogenesis, which allows cancer cells to grow and spread.
With the unique ability to observe angiogenesis at the subcellular
level, Argonne's scientists help to discover ways to inhibit the
growth of blood vessels in cancerous tissues. "It's almost like
having Superman for a doctor - using hard X-rays to find cures for
problems far more severe than just broken bones," Srajer said.
In order to do these types of biological experiments, Argonne's
scientists require a device that can detect the presence of small
amounts of particular compounds in highly dilute solutions. By
using the nanoprobe or other APS microprobes, researchers can study
trace metal distributions in cells at ever finer spatial
resolution. These high-resolution tools provide Argonne researchers
with the capacity to study the cellular processes important in
normal physiological function and in disease.
The different types of information revealed by X-ray optics also
allow researchers at the APS to investigate material processes as
they occur. These experiments, known as in situ studies,
give scientists a deeper understanding of material properties than
they can glean from disconnected structures. The real benefit of
in situ studies comes from the ability to modify materials
while they are being observed.
The basic scientific explorations carried out at the APS hold
the potential to spawn a new generation of products and inventions
that will improve our lives and stimulate the economy. In one
in situ experiment, researchers exposed parallel layers of
silicon to a small, well-defined stress, which caused a tiny
displacement of the atoms in the material.
The mismatch created regions through which electrons could pass
more smoothly, like water pouring through a crack in a seal. Unlike
visible light, the X-rays produced by the APS enabled the
scientists to see the small displacements. This information, Maser
said, could lead to the production of enhanced semiconductors for a
new generation of microprocessors.
The combination of the world's finest X-ray tools and
sophisticated imaging techniques has allowed scientists and
engineers who use Argonne's research facilities to reach a deeper
understanding of the small-scale processes and interactions that
surround us. The new discoveries Argonne scientists make every day
foster advances in basic knowledge and the development of
breakthrough technologies that improve our health, our economy and
our environment.
Learn more about Argonne's Center for Nanoscale Materials:
http://www.cnm.anl.gov/about.html
Follow Argonne online:
http://twitter.com/argonne
http://www.flickr.com/photos/argonne/
http://www.youtube.com/user/ArgonneNationalLab
