By Angela Leroux-LindseyPrint
April 2, 2013
Kisslinger uses a focused-ion beam tool to prepare a sample of organic solar cell material for imaging under an electron microscope.
Who says bigger is better? At the Center for Functional Nanomaterials (CFN) at Brookhaven National Laboratory, the opposite is true. Using state-of-the-art transmission electron microscopes (TEMs), researchers take images of specimens that can be as small as a few billionths of a meter in size—so small that their atomic structure and chemistry are revealed. Materials scientists use this information to develop new nanotechnologies and pursue answers to fundamental energy challenges.
But knowledge of a material at the atomic scale isn't easy to come by. In order to yield results, researchers need a well-prepared nanospecimen.
These miniscule bits of material differ widely, and could be composed of photovoltaic material, ceramics, silicon, organic tissue, or one of any number of materials, each yielding unique data. The only thing all of these nanospecimens have in common is the likelihood that Kim Kisslinger, a technical associate at Brookhaven, prepared the sample to be picture-perfect no matter what the specifications.
"Samples vary tremendously in composition, form, and structure," Kisslinger said. "They're almost all unique, and come from a variety of sources. Whether it's from a scientist here at Brookhaven Lab, institutions around the country, or even labs as far away as South Africa, the material I get depends on the scientific question the user is trying to answer."
Preparing these tiny samples for TEM imaging is complex because these specialized electron microscopes require nanosamples that meet strict criteria: They must be clean, thin, artifact-free, and as uniform as possible. Kisslinger is knowledgeable in a variety of mechanical and non-mechanical methods for preparing specimens, each specific to the material being explored and to the information researchers hope to glean from its analysis.
So how, exactly, is it possible to create a sample that's so small it's invisible to the naked eye? Let's say a researcher needs a chunk of silicon refined to a size appropriate for TEM imaging. First, Kisslinger would break it down to about three millimeters square, then carefully cut off a one-millimeter sample.
One millimeter sounds small, but it's still ten thousand times bigger than most nanosamples. To further scale down the sample, Kisslinger would then use a tripod polisher, a device that positions the sample above a circular abrasive film that spins—just like an LP on a record player. Contact with the polishing disc thins the silicon piece until it's very smooth.
The abrasive films used in tripod polishing, called diamond lapping films, have grits made of diamonds—something like diamond-encrusted "sandpaper." Just like conventional sandpaper, the smaller the grits, the finer the polishing ability. Kisslinger starts with lapping films that have diamond grits sized at about 30 microns, and works down to grits as small as .1 microns. In the final stages of preparation, Kisslinger further thins the sample using a paste-like product with diamond grits clocking in at .02 microns, which is about the diameter of a strand of DNA.
At this point, an ideal sample measures just a few tens of nanometers, and is "electron-transparent," or capable of allowing a beam of electrons to pass through it. The TEM captures images of the nanosample based on patterns made by this electron beam as it interacts with the structure.
Kisslinger also prepares TEM samples using sophisticated non-mechanical methods like focused-ion beam milling (FIB), a complex and nuanced technique that uses powerful electromagnetic lenses to focus a high-voltage ion beam onto a sample. This ion beam, in combination with a built-in micromanipulation device, can be controlled with extreme accuracy and allows the user to mill a section of material with almost nanometer precision. Kisslinger does the majority of his sample prep using Brookhaven's FIB.
A recent example is a bulk piece of organic solar cell material that needed to be thinned for analysis. Using the FIB, he directed an ion beam at the sample to mill it down to about a micron. After this initial reduction, he used a less powerful but more precise ion beam to mill an even thinner sample—and so on, until the solar cell sample was thin enough for analysis under the TEM's powerful lenses.
In addition to preparing most of the TEM samples used at CFN, Kisslinger manages the JEOL-1400 electron microscope, which is specialized for biological material and other soft tissues. Understanding these natural materials on an atomic scale may lead to new insights on light-harvesting and other energy sources. Kisslinger also manages the TEM preparation labs at CFN. His ability to train new users and maintain a consistently organized and productive lab environment are further examples of how his talent and reliability anchor the materials science group at Brookhaven.
"Having an associate with such a wealth of knowledge, and the ability to apply it to get high-quality results quickly, is critical to achieving good scientific results," said Eric Stach, leader of the Electron Microscopy Group at CFN. "Kim is the best at what he does."
2013-3750 | INT/EXT | Media & Communications Office