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Seventy years ago, in Osmond Laboratory on Penn State's University Park campus, Erwin W. Müller, Evan Pugh Research Professor of Â鶹ÒùÔºics, 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 would allow them to view atoms.

Bahadur made the adjustments. Müller looked at the resulting image and exclaimed, ""

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 Â鶹ÒùÔºics, 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.

It took four years from his invention of the field ion microscope in 1951 for Müller to increase the tool's image resolution to such a degree that individual atoms could be viewed. That achievement in 1955 came 147 years after John Dalton first proposed that all matter was composed of tiny indivisible particles called atoms. For about 25 years after the field ion microscope was invented, it was the only microscope able to achieve atomic resolution.

"Seeing" an atom didn't slow Müller's work, though.

In the 1960s, John A. Panitz joined Müller's lab as a doctoral student studying atom probe technologies. Together, Panitz and Müller invented the atom probe field ion microscope in 1967. Müller and Panitz also worked with S. Brooks McLane and Gerry Leroy Fowler, the lab's electronics technician and lead technician, respectively. The group advanced Müller's original invention so that the new atom probe field ion microscope could not only view individual atoms, but also could determine their chemical nature.

Later, Panitz, who graduated from Penn State and is now professor emeritus of physics at the University of New Mexico, would go on to invent the 10-centimeter atom probe and the imaging atom probe, which is considered by many to be the precursor to modern commercialized atom probes.

These commercialized atom probes—including the local electrode atom probe, the —rapidly advanced in the late 1990s and early 2000s, when these instruments enabled the study materials that are essential for semiconductors and other technologies.

Müller's success brought him many accolades, including election to the National Academy of Engineering and the National Academy of Sciences. Had Müller not died unexpectedly in 1977, many believed he would have gone on to win the Nobel Prize in Â鶹ÒùÔºics. The same year, President Jimmy Carter awarded Müller the National Medal of Science posthumously.

"Remembering the legacy of Müller's work is important here at Penn State," said Terrones, whose lab occupies what once was Müller's field emission laboratory. "His and other discoveries in physics propagate and aid in the development of technology today in biology, materials science and medicine."