The Nelson W. Taylor Lecture in Materials honors the memory of Professor Nelson Taylor, who headed Penn State’s Department of Ceramics from 1933 -1943 and established the College of Earth and Mineral Sciences as a major center for ceramics research. The event brings an eminent engineer or scientist to Penn State to lecture on a topic at the cutting edge of materials science. In addition, three Penn State faculty members are invited to give talks on related topics. This year’s topic, Atom Probe Tomography, involves new techniques and technologies for looking at atoms.
“Defects in Two Dimensional Crystals: An Ultra-High Resolution Aberration-Corrected Electron Microscopy Study” -- Nasim Alem, assistant professor of materials science and engineering
The ability to control and engineer defects and grain boundaries in atomically thin crystalline materials is the basis for growing specific structures for future electronic and photonic devices, according to Nasim Alem. But until the recent advances in electron microscopy, particularly over the past decade, atomic scale defects could not be seen. Alem showed the Taylor Lecture audience examples of her work with graphene, transition metal dichalcogenides and their heterostructures, all considered as promising material for electronic devices and energy storage. High resolution aberration-corrected scanning/transmission electron microscopes, such as the Titan S/TEM at Penn State, are able to clearly show the edge structures of such 2D crystals, called zigzag and armchair, that can modify the properties of the material. By looking at the grain boundaries and defects in two dimensional crystals under the TEM, under electrical and thermal conditions, she was able to show atoms around the defects and edges shuffling, giving rise to new defect structures. This atomic shuffling can alter the properties of materials, and the control of these movements can have profound effects on devices, such as electronic circuits. In collaboration with Penn State colleagues, she is working on understanding heterostructures – layers of two-dimensional materials stacked one on top of another – which show new properties not found in the individual materials. These atomic layer materials are on the cutting edge of materials science.
“Properties of Oxide Interfaces: Benefits of Aberration Corrected TEM/STEM for Interface Analysis” – Bernd Kabius, senior scientist, Materials Research Institute
The monochromators and the two aberration correctors of the Titan enable a resolution of up to 0.6 angstrom in TEM and STEM mode (scanning mode), which allows for resolving most interatomic distances. Aberration correction improves sensitivity for elemental and chemical analysis by a factor of up to 100. In addition, this high resolution can be attained at low acceleration voltage, which is important for the investigation of radiation sensitive materials such as soft matter and very thin two-dimensional films.
There are three types of interface typically studied in TEM, said Kabius: solid/solid, solid/liquid, and solid/gaseous. The solid/solid interface has been the object of many investigations because of the impact of device physics in microelectronics and the interest in quantum effects at the nanoscale. There will always be interest in solid/solid interfaces, Kabius said, but the future is in the other two interfaces. At present we can observe a paradigm change in TEM studies. To date, most of the investigations are performed “post mortem,” and the dynamic processes leading to that state had to be inferred. Now advances in experimental equipment enable performing experiments inside the TEM and observing these dynamic processes, such as catalytic reactions, in situ. For this reason, time resolution is the new frontier. Most of the in situ studies to date have been limited to video frequency. Faster CCD cameras have shifted that limit to milliseconds, and pulsed electron emitters enable a time resolution of up to femtoseconds. Achieving this time resolution for a variety of processes while controlling temperature, pressure, and other parameters will be the most challenging task for future TEM studies.
Researchers at Penn State have a strong interest in perovskites, a class of materials with an enormous range of properties. With the Titan, it is possible to generate atomically resolved elemental maps of these materials, and understand novel physical phenomena occurring at these interfaces, Kabius concluded.
“Understanding Material Properties One Atom at a Time: Scanning Tunneling Microscopy of High Temperature Superconductors” – Eric Hudson, associate professor of physics
If there was ever any doubt that an electron is a wave, the images Eric Hudson showed the Taylor Lecture audience put that question to rest. Using a scanning tunneling microscope, he made images of electrons in materials that showed waves as clearly as a stone thrown in a puddle. An STM uses a thin wire cut so sharply that at its tip is only a single atom. The tip is placed close to the surface to be imaged and a current is put through the wire. The electrons tunnel across the intervening space quantum mechanically. The strength of the current is related to the distance from the sample, making a topological picture of the surface.
Combining the STM with a refrigeration system allows the instrument to study high temperature superconductivity, a phenomenon in which a material’s electrical resistance falls to zero.
“We have no idea why high temperature (around 140K or minus 207°F) superconductivity works, Hudson said. “We are bound to discover brand new properties.”
2015 Taylor Lecture “Atomic-Scale Tomography: An Achievable Vision” – Thomas Kelly
Suzanne Mohney, professor of materials science and engineering, and electrical engineering and Chair of the Intercollege Graduate Degree Program in Materials Science and Engineering, introduced the 2015 Taylor Lecturer - Thomas Kelly. Dr. Kelly is the Division Vice President for Innovation and New Technologies, CAMECA Instruments, Inc. and former professor of materials science and engineering at the University of Wisconsin-Madison where he invented and formed a company to commercialize the Local Electron Atom Probe, or LEAP®, an atom probe tomography microscope.
Before getting into the technical portion of his talk, Kelly looked into the history of microscopy, starting with the seminal work of Erwin Mueller, the Penn State physicist who was the first person to see the atom. Mueller led the work on the atom probe, first developing the Field Electron Emission Microscope in Germany in the early 1930s. He then created the Field Ion Microscope, and in October 1955, in Osborne Laboratory at Penn State, first created images of individual atoms. In the summer of 1967, Mueller and his Ph.D. student, John Panitz, invented the Atom-Probe Field Ion microscope, the progenitor of all atom-probe microscopes.
Kelly then moved almost seven centuries backward in time to discuss how the inability to see the bacterium that caused the Black Death led to the belief that it was heavenly retribution or the result of poisoned wells. It was many hundreds of years before people could see that it was bacteria on fleas and fleas on rats that were the source of plague. Now we can see the interior of a 10-micron bacterium using the TEM. Large areas of human knowledge are tied to the ability to see things for the first time in the microscopes and telescopes we use, he said.
As devices such as transistors become smaller and smaller, we need to be able to see every atom in the device in three dimensions. If it is possible, this will lead to new science, he predicted. He called this ability “tomography on the atomic scale.” He compared this to knowing the location and composition of every block in a building. What are the building blocks of nature, he asked? Atoms, of course. We need to be able to image a billion atoms. Kelly calls this the Atom Project.
Currently, their best instrument is capable of mapping the location of around 60 percent of the atoms in a sample. Like Mueller, his technique uses a field evaporation process and time of flight mass spectroscopy to identify the elements and their location. To get to 100 percent, they need new detector technology and to get rid of distortions. In order to fix aberration, they will bring in additional information by adding a scanning transmission electron microscope (STEM) to the Atom Probe.
The other new approach, which he hopes to have accomplished within a year, is to add superconducting detectors in the same system. If they can do all this, they then hope to have a structure/properties microscope that will allow them to see all the building blocks of matter.
All photos credit: Andrea Kohler