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www.us-tech.com
Production Supporting Sub-Angstrom Materials
low breakthroughs in the under- standing of materials. These breakthroughs unlock unique functionalities that create new pathways to design future de- vices.
Research at Oak Ridge National Laboratory A
By Jim McMahon
dvancements in nanofab- rication, pharmaceuticals, energy and aerospace fol-
nanomaterials synthesis, nano - fabrication, imaging/microscopy/ characterization, and theory/ modeling/simulation. Amongst the instruments
The Center for Nanophase
Materials Sciences (CNMS) at Oak Ridge National Laboratory (ORNL) is at the forefront of one of the most powerful capabilities for exploring the nature of mate- rials and energy. CNMS empha- sizes discovery of new materials, and the understanding of under- lying physical and chemical in- teractions that enable the cre- ation of nanomaterials. CNMS researchers have ac-
cess to state-of-the-art mi- croscopy instruments in its Ad- vanced Microscopy Laboratory (AML) for a broad range of nanoscience research, including
used for materials research in the ALM are some of the most advanced Transmission Electron Microscopes (TEM) and Scan- ning Transmission Electron Mi- croscopes (STEM). TEMs utilize a technique in
which a beam of electrons is trans- mitted through an ultra-thin spec- imen, interacting with the speci- men as it passes through. An image is formed from the interac- tion of the electrons transmitted through the specimen. The image is magnified and
focused onto an imaging device, such as a fluorescent screen, a layer of photographic film, or to be detected by a sensor such as a CCD camera. TEMs use phase- contrast, and therefore, produce results which need interpreta-
9:47 tion by simulation.
Scanning Transmission Electron Microscopy A type of TEM which has be-
come highly appealing, the STEM, also permits the elec- trons to pass through an ultra- thin specimen, however, the STEM focuses the electron beam into a narrow spot which is scanned over the sample in a raster. The rastering of the beam across the sample makes the STEM suitable for analysis tech- niques, such as mapping, where the signals can be obtained si- multaneously, allowing direct correlation of image and quanti- tative data. By using a STEM equipped
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with a high-angle detector, it is possible to form atomic resolu- tion images where the contrast is directly related to the atomic number (Z-contrast image). The directly interpretable Z-contrast image makes STEM imaging with a high-angle detector preferable to TEM in some appli- cations. For example, oxygen can now be visualized in supercon- ductors and colossal magnetore- sistant manganites where it plays a dominant role in deter- mining properties. Electron energy loss spec-
troscopy (EELS) is a STEM measurement technique made possible with the adaptation of an electron spectrometer. With the addition of EELS, elemental identification is possible, as well as additional capabilities of de- termining electronic structure or chemical bonding of atomic columns. With EELS, the STEM does not just produce an image, it can also do chemical mapping. Instead of just detecting what scatters from the atom, it can show changes in chemical bal- ance and see an incredible amount of detail about the physics interface when two dif- ferent materials come together. “We are using the STEM for
different applications,” said Dr. Andrew R. Lupini, R&D staff member in the Center for Nanophase Materials Sciences at Oak Ridge National Laboratory in Tennessee. “As well as the im- aging, this STEM can do very high-energy resolution spec- troscopy. For example, we can probe properties of materials re-
lating to their bonding and meas- ure their temperature on a very local scale.” With correction of spherical
aberration, the STEM can re- solve more than just seeing the atoms more clearly. The high-en- ergy convergent electron beam in STEM provides local information of the sample, even down to atomic dimensions of smaller than one ten-billionth of a meter.
Sensitivity to Vibration Scanning Transmission Elec -
tron Microscopes are more sensi- tive to ambient conditions, with the addition of field emission guns, imaging filters, and spheri- cal aberration correctors that give higher spatial and energy resolu- tion, and EELS. The ultimate performance of
these extremely sensitive micro- scopes is strongly influenced by factors such as magnetic fields, barometric pressure changes, room- and chilled-water temper- ature variations, grounding problems, and quite significantly by floor and acoustic vibrations. Atomic resolution spec-
troscopy is particularly sensitive to environmental instabilities as a result of its long acquisition times. The serial nature of the image acquisition in STEM makes the instabilities appear as image distortions. STEMs are most sensitive to low frequency vibration, in the range of a few Hertz. These vibrations are chal- lenging to eliminate from the mi- croscope’s environment.
Vibration Isolation Because of its high vibration
isolation efficiencies, particular- ly in the low hertz frequencies, Negative-Stiffness vibration iso- lation was selected by ORNL’s Center for Nanophase Materials Sciences for sub-angstrom mate- rials research with its Nion Her- mes STEM-EELS. Introduced in the mid-1990s
by Minus K Technology, Negative- Stiffness vibration isolation has been widely accepted for vibra- tion-critical applications, largely because of its ability to effectively isolate lower frequencies, both vertically and horizontally. Negative-Stiffness isolators
are unique in that they operate purely in a passive mechanical
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Jan/Feb 2025
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