Seventy years ago, in Osmond Laboratory on Penn State's University Park campus, Erwin W. Müller, Evan Pugh Research Professor of Physics, became the first person to "see" an atom. In doing so, Müller cemented his legacy, not only at Penn State, but also as a pioneer in the world of physics and beyond.
Originally from Germany, Müller joined the faculty at Penn State in 1951, when it was called the Pennsylvania State College. His lab, the field emission laboratory, was originally in the sub-basement of Osmond and later moved to the second floor of Osmond in 1954.
For 20 years, Müller's research had focused on developing the technology to increase the resolution of images collected from microscopes. First, he invented the field emission microscope in 1936, which was used to study the surfaces of needle tips and nearly reached atomic resolution. He followed that in 1951 by developing the field ion microscope, the tool with which he "saw" tungsten atoms in 1955.
In 1955, Kanwar Bahadur, Müller's graduate student at the time who earned a doctorate from Penn State, experimented with using liquid nitrogen to cool the tungsten tip of the field ion microscope to try and increase the resolution to finally achieve atomic resolution. Bahadur and Müller hoped that a few fine adjustments would allow them to view atoms.
Bahadur made the adjustments. Müller looked at the resulting image and exclaimed, "Atoms, ja, atoms!"
What Müller saw was not just a photograph of atoms that look like the illustrations in chemistry textbooks, hence the quotes around the word "see." The field ion microscope worked by taking a sharp metal tip - made of tungsten for these first images - and placing it an ultra-high glass vacuum chamber. The chamber was then backfilled with helium gas, and the tip was cooled with liquid nitrogen. Once cooled, a positive voltage was applied to the tip and tungsten ions were repelled from the tip. Collecting these ions via a phosphor screen resulted in a magnified image at an atomic resolution of individual atoms.
"Nowadays, to be able to 'see' atoms is remembered as a major achievement in the field of microscopy," said Mauricio Terrones, George A. and Margaret M. Downsbrough Head of the Department of Physics, Evan Pugh University Professor, and professor of chemistry and of materials science and engineering. "Müller's work helped jumpstart a revolution in resolution. Since 1955, atomic resolution imaging has advanced to not only being able to visualize individual atoms, but also to perform electron microscopy to reveal the crystal structure of materials at the atomic scale; atomic spectroscopy to determine atomic-bonding and elemental compositions of materials; and surface reconstruction to visualize how atoms interact in 3D."
When students of atomic imaging techniques, such as atom probe tomography (APT), learn the history of the field, they often start with Müller's invention of the field ion microscope and his later discoveries.
"Once I became involved in the field of APT, the name Müller was something I had to know," explained Oscar Lopez, a previous postdoctoral researcher in Terrones' lab. "People in the field have a real respect for Müller and his legacy."
Today, Müller's advancement of atomic resolution imaging can be seen across research at Penn State and beyond.
"Our chemistry research works to place atoms in precise locations within nanostructured materials and uses atomic resolution imaging and electron microscopy to visualize these atoms and materials," said Raymond Schaak, DuPont Professor of Materials Chemistry and associate department head for research, explaining that his research aims to improve catalytic reactions in clean energy, as well as fuel and solar cells.
Müller's work helps to inspire technologies that weren't even imagined in his time, like smartphones and new generations of computers and televisions, according to Danielle Reifsnyder Hickey, assistant professor of chemistry and materials science and engineering, who now works to discover and characterize new materials that allow these electronics to work faster and better.
"Using aberration-corrected transmission electron microscopy, which allows for imaging at atomic resolution, my lab contributes to creating powerful new technologies that can be used every day," she said.
Schaak and Hickey both conduct their research at Penn State's Materials Research Institute, which hosts the Materials Characterization Laboratory. From undergraduates to faculty studying chemistry, physics and more, the researchers using the facility continue to push the boundaries and applications of atomic and nanoscale resolution imaging, Terrones said.
