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Custom modifications of equipment are an honored tradition
of the research laboratory. In a recent paper, two materials scientists at NIST
describe how a relatively simple mod of a standard scanning electron microscope
(SEM) enables a roughly 10-fold improvement in its ability to measure the
crystal structure of nanoparticles and extremely thin films. By altering the
sample position, they are able to determine crystal structure of particles as
small as 10 nm. The technique, they say, should be applicable to a wide range
of work, from crime scene forensics to environmental monitoring to process
control in nanomanufacturing.
The technique is a new way of doing electron diffraction
with an SEM. In standard SEM-based electron diffraction, the researcher
analyses patterns that are formed by electrons that bounce back after striking
atoms in the sample. If the sample is a crystalline material, with a regular
pattern to the arrangement of atoms, these diffracted electrons form a pattern
of lines that reveals the particular crystal structure or “phase” and
orientation of the material.
The information, say NIST’s Robert Keller and Roy Geiss, can
be critical. “A common example is titanium dioxide, which can exist in a
couple of different crystal phases. That difference significantly affects how
the material behaves chemically, how reactive it is. You need to add
crystallographic identification to the chemical composition to completely
characterize the material.”
SEMs often are outfitted with an instrument for just this
task, a device called an EBSD (electron back-scatter diffraction) detector. The
problem, they say, is that below a certain size, the usual setup just doesn’t
work. “You can determine the crystal structure of an isolated particle
down to a size of about 100 to 120 nm, but below that the crystals are so small
that you’re getting information about the sample holder instead.” A
somewhat more exotic instrument, the transmission electron microscope (TEM),
does much better, they say, but samples below about 50 nm in size show very
limited diffraction patterns because the higher-powered electron beam of the
TEM just blasts through them.
The novel tweak developed by Keller and Geiss combines a
little of each. They moved the SEM sample holder closer to the beam source and
adjust the angles so that instead of imaging electrons bouncing back from the
sample, the EBSD detector is actually seeing electrons that scatter forward
through the sample in a manner similar to a TEM. (They also came up with a
unique method of holding samples to make this work.)
They have shown that their technique produces reliable
crystal phase information for nanoparticles as small as 10 nm across, as well
as for single crystalline grains as small as 15 nm in an ultrathin film.
Electron diffraction in an SEM, says Keller, “in
general represents the only approach capable of measuring the atomic structure,
defect content, or crystallographic phase of single nanoparticles. This is a
critical need in cases of extremely limited sampling of unknown particles. This
work pushes electron diffraction to a new frontier by providing spatial
resolution that rivals that possible in a TEM, and makes it available to anyone
with an SEM. And that’s an ubiquitous tool in virtually all fields that require
characterization of solids.”
Typical applications, the researchers say, include
pinpointing ammunition sources from gunshot residue at crime scenes;
determining the processing history of confiscated drugs; accurate
characterization of nanoparticles for health, safety, and environmental impact
studies; and optimizing grain structure in high-performance electronics based
on thin films; and process and quality control in nanomanufacturing.
