From: Gina Miller (nanogirl@halcyon.com)
Date: Thu Jun 10 1999 - 14:43:37 MDT
Source: Lawrence Berkeley National Laboratory (http://www.lbl.gov)
Date: Posted 6/10/99
One-Angstrom Micrscope Achieves .89 Angstrom Resolution
BERKELEY, CA — Using the One-Ångstrom Microscope (OÅM) at the National
Center for Electron Microscopy (NCEM), researchers at the Department of
Energy's Lawrence Berkeley National Laboratory have made unprecedented
images of columns of carbon atoms in a diamond lattice, only 0.89 angstrom
apart — less than one ten-billionth of a meter.
For the first time, moreover, an electron microscope has been able to
resolve nitrogen atoms in the presence of more massive gallium atoms in
gallium nitride, in columns spaced only 1.13 angstroms apart.
"The ability to make images of light elements such as carbon, nitrogen, and
oxygen in solids at atomic resolution is a very big step forward — and it
was achieved by a technique that can be a routine tool in the future.
Therefore, it is of great interest to science and industry," says Christian
Kisielowski, who with Michael O'Keefe and their colleagues Christian Nelson,
Chengyu Song and Roar Kilaas of Berkeley Lab's Materials Sciences Division
recently announced the record-breaking result.
Many of the most promising solids under investigation today, including
superhard materials, high-temperature superconductors, and semiconductors
with large band-gap energies, incorporate light elements in crystal lattices
at short interatomic distances.
"Seeing small atoms at atomic resolution has always been a challenge,
because they don't strongly scatter the electrons in the microscope's beam,"
says Michael O'Keefe. "When the light atoms are close to heavy ones, it has
been virtually impossible to resolve them. Heavy atoms scatter electrons
much more, and as a result the interference pattern is just too complex to
resolve."
Kisielowski explains that "the OÅM overcomes this difficulty by making a
through-focal series of images — in the case of the gallium nitride, 20
different images, each with the scattered electrons interfering with
different relative phases — and then uses computer processing to unscramble
the electron waves and combine them into a single high-resolution image in
which all electrons are in phase." He adds, "It's a way of going from the
complexity of the lattice images produced by the OÅM to the simplicity of
crystalline structures."
The OÅM had its genesis in the early 1990s, when NCEM's three-story,
million-volt Atomic Resolution Microscope, or ARM, was the world's most
powerful, with a practical resolution of 1.6 angstroms — though Kisielowski
once managed to squeeze out 1.54 angstroms. Then a Japanese-built,
one-and-a-quarter-million-volt machine in Germany achieved 0.95-angstrom
resolution, but at a cost of more than 10 million Deutschmarks.
At about the same time, O'Keefe proposed a way to computer-process
through-focus images to achieve higher resolution from a medium-voltage
microscope, an approach first suggested in the late 1960s. "Such a
microscope can be designed so that its ‘information limit' — the limit to
which it produces phase-scrambled information — lies well beyond its
traditionally defined nominal resolution, with all the transferred
information in phase," he explains. "By combining information from many
images, a single image with resolution approaching the information limit can
be achieved in practice."
Electron beams are the basis of all transmission electron microscopy, and
through-focus methods depend upon beams with all electrons at nearly the
same energy — beams with very little "energy spread." Not until the early
1990s did field-emission beam sources become stable enough for
medium-voltage instruments to operate reliably.
Thus when a group of researchers working in the European Commission's
BRITE-EURAM program set out to build a new generation of high-resolution
electron microscopes using medium voltages, they invited NCEM to be a
partner in the project, based on NCEM's high-resolution expertise and
O'Keefe's theoretical contributions. In 1993, NCEM was able to secure the
funds to acquire a suitable instrument, a Philips CM300.
Although a typical CM300's resolution limit is 1.7 angstroms, O'Keefe laid
out specifications that would optimize the instrument's information limit.
Recent results confirm the OÅM's capacity to produce phase-scrambled
information far beyond 1.7 angstroms. In the case of diamond, Kisielowski
and O'Keefe, working with Y.C. Wang, have shown that the OÅM's information
limit can extend to at least 0.89 angstrom.
And as planned, powerful computer programs used to process the focal-series
images have allowed OÅM to reconstruct images with resolutions near its
information limit.
Meanwhile the ARM, NCEM's "grandfather" microscope, is far from being
outmoded by its diminutive descendant. The OÅM can only produce ultra-high
resolution with samples less than a hundred angstroms thick, which are
prepared by planing away layer after layer of atoms, using a low-angle,
low-energy beam of argon atoms in an "ion mill" — until the samples are
"close to being all surface," O'Keefe jokes.
Kisielowski stresses that "sample preparation is getting to be a bottleneck.
It's a nasty job, and nobody wants to do it, because you don't get to be a
professor that way."
The ARM can use samples that are three times thicker and composed of heavy
atoms, yet still achieve a respectable resolution. A high-voltage microscope
can accommodate larger sample holders, which are required to perform dynamic
experiments such as in-situ straining or heating. It also allows for larger
tilt angles than the OÅM, and, says Kisielowski, "material scientists love
to observe matter from different angles — different projections are the
essence of any tomographic experiment, for example."
The ARM will see wide use for years to come. Today, however, the ultra
high-resolution performance of OÅM is unsurpassed. The 1.13-angstrom
resolution achieved with gallium nitride, allowing images of its nitrogen
atoms as well as its gallium neighbors, stands as an extraordinary
achievement — but also as a challenge to Kisielowski, O'Keefe, and their
colleagues.
Says Kisielowski, "We're aiming to investigate materials with even shorter
bond lengths with the present information limit. We want to have procedures
in place that work reliably and fast to make the experiments available to
our user community as soon as possible … colleagues from other laboratories
have already started to share our excitement by investigating their own
samples with the OÅM."
Uli Dahmen, head of NCEM, shares Kisielowski's enthusiasm. "This achievement
is based on more than six years of team effort in planning, installation and
testing. After all this time, it's a thrill to actually see it work. NCEM
has reached a very important milestone." He adds, "The one angstrom barrier
has been a Holy Grail for electron microscopists worldwide ... The OÅM makes
a truly extraordinary addition to Berkeley Lab's scientific ‘toolbox,' and I
can't wait to see what new discoveries it will bring for our users."
The Berkeley Lab is a U.S. Department of Energy national laboratory located
in Berkeley, California. It conducts unclassified scientific research and is
managed by the University of California.
Editor's Note: The original news release can be found at
http://www.lbl.gov/Science-Articles/Archive/less-one-angstrom.html
Gina "Nanogirl" Miller
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