Scientists at the U.S. Department
of Energy's (DOE) Brookhaven National Laboratory, in collaboration with
researchers from Hitachi High Technologies Corp., have demonstrated a new scanning
electron microscope capable of selectively imaging single atoms on the top surface
of a specimen while a second, simultaneous imaging signal shows atoms throughout
the sample's depth. This new tool, located at Brookhaven Lab's Center for Functional
Nanomaterials (CFN), will greatly expand scientists' ability to understand and
ultimately control chemical reactions, such as those of catalysts in energy-conversion
devices.
Uranium single atoms (circled) and small crystallites on a carbon support imaged simultaneously using a scanning probe to produce forward scattering through the sample (top) and backward scattering emerging from the surface (bottom). Center panel shows superimposition of the two in red (bulk) and green (surface). Atoms not seen in the lower image are on the bottom surface of the support.
A paper describing the work will be published online September 20, 2009, in
Nature Materials, along with a commentary article highlighting the development.
“Our knowledge of the role of individual atoms in nanotechnology and
energy-related research is strongly influenced by our ability to visualize them,
not only in bulk but also on the surface, which is where the interactions of
chemical reactions take place,” said Brookhaven physicist Yimei Zhu, lead
author on the paper. “This new microscope and the method we developed
allow us, for the first time, to directly look at atoms on the top surface and
in the bulk of a sample simultaneously to reveal their atomic arrangement and
bonding states. This information will help us identify the active sites and
functions of materials at nanoscale dimensions for a wide range of applications,
such as converting waste heat or chemical energy to electricity.”
Like all scanning electron microscopes, the new tool probes a sample with an
electron beam focused to a tiny spot and detects so-called secondary electrons
emitted by the sample to reveal its surface structure and topography. Though
this technique has been a workhorse of surface imaging in industrial and academic
laboratories for decades, its resolution has left much to be desired because
of imperfect focusing due to lens aberrations.
Using a newly developed spherical aberration corrector, the new tool corrects
these distortions to create a smaller probe with significantly increased brightness.
“The effect is similar to using a larger aperture lens on a camera,”
said biophysicst Joseph Wall, a longtime expert in electron microscopy at Brookhaven
Lab and a co-author on the paper. “It allows you to gather information
from a larger angle and focus on a smaller spot.”
The new device also employs specialized electron optics to channel the emitted
secondary electrons to the detector. The result is a fourfold improvement in
resolution to below one tenth of a nanometer — and thus, the ability to
image single atoms.
Additional detectors, located below the sample, detect electrons transmitted
through the sample, revealing details about the entire structure at the exact
instant the “shutter” snapped to record each pixel of the surface
image. This simultaneous imaging allows the scientists to correlate information
in the two images to understand precisely what is happening on the surface and
throughout the sample at the same time.
“Having information about the surface structure and the bulk sample at
the same time will allow researchers to better determine how the surface and
bulk atoms work together, for example in a catalytic reaction,” said Zhu.
The improved resolution and combined imaging capabilities will also reveal features
such as small variations in composition or the locations of impurities that
could have large effects on function.
“An essential component of this study was selection of a test specimen,
isolated uranium atoms on a thin carbon substrate, where the images could be
interpreted quantitatively to rule out other possible interpretations,”
Wall commented.
Because of its extreme sensitivity, the new microscope must be kept isolated
from a range of environmental effects such as variations in temperature, mechanical
vibrations, and electromagnetic fields. Even the slightest waft of air could
cause distortions in the images.
Fortunately the CFN was built with these needs in mind. Temperatures are regulated
to within three-hundredths of a degree Fahrenheit over a 24-hour period; shock-absorber-like
slabs isolate the room from the rumble of passing trucks and distant slamming
doors; layers of heavy doors keep even subtle vibrations out; and air-cooling
panels replace typical ceiling vents to eliminate airflow.
“The building is really a mechanical-engineering masterpiece,”
said Zhu. “This microscope wouldn't work at all without these sophisticated
systems.” Development of the microscope was funded by the Office of Basic
Energy Sciences within the DOE Office of Science.