One atom can be very powerful. No one knows that better than David A. Muller, Applied and Engineering Physics. Muller has spent his career inventing and refining ways to see down to the atomic level. “I try to understand how stuff fits together and how it works, atom by atom,” he says. “A lot of computer devices are down to atomic dimensions in size where a single atom out of place can cause them to fail. And for a lot of other modern technologies, whether they work or fail comes down to a single layer of atoms at the interface of two materials. So if you want to understand whether a turbine blade is strong, it’s often not the bulk material that’s important, but just that last layer of atoms where two pieces join up. That’s where it comes apart.”
The EMPAD, Getting More Information Faster about Materials Scanned
Muller’s expertise is designing and using very powerful electron microscopes to study the world at the atomic scale. One of his latest successes is a breakthrough detector for electron microscopes. For almost a decade he has been collaborating with Sol M. Gruner, Physics, and members of their research groups to design and build the device. Known as an electron microscope pixel array detector (EMPAD), the new detector allows the researchers to measure every scattered electron that goes through the microscope, as compared to earlier detectors that could only measure a small fraction of the electrons. That means scientists can now gather much more data at the atomic level about scanned materials.
The EMPAD is much faster than previous detectors. Using an array of electron-sensitive pixels, it reads the signals of variable intensities sent by electrons passing through a sample under a scanning transmission electron microscope. It can detect everything from a single electron to intense beams containing a million electrons, then map their position and momentum, drawing an image of the sample at the atomic level on a computer screen.
“The detector lets us measure almost every physical property about the material that we would want to measure,” Muller says. “We can look at strain states, tilts, rotations, and we can measure electric and magnetic fields at very high spatial resolution. Over all, we’ve been able to improve the resolution of the electron microscope by at least a factor of three, so we can clearly see individual atoms and missing atoms. We can now pull out information at below half an angstrom, which is smaller than the smallest bond length in nature.”
In the end, the EMPAD might prove most valuable for imaging materials sensitive to radiation damage, such as biological samples, Muller says. “Whenever you image through an electron microscope, every electron that goes through the material is going to do some damage to it. For biological samples especially, we want to get the most information we can from that one scattering event. With the new detector, we have a higher dose efficiency than with any other method. This could be very valuable for solving the structure of proteins, molecules, and cells.”
“If you want to understand whether a turbine blade is strong, it’s often not the bulk material that’s important, but just that last layer of atoms where two pieces join up.”
Cornell’s Center for Technology Licensing has licensed the EMPAD to FEI (a division of Thermo Fisher Scientific), the largest manufacturer of electron microscopes in the world. FEI will produce the detectors for new and retrofitted electron microscopes. According to Muller the manufacturer has already received orders for the units and plans to ship them by the end of the year.
A Comprehensive Microscope Facility
When Muller came to the Cornell in 2003, the campus was down to one working electron microscope. Since then, Muller and his colleague Lena F. Kourkoutis, Applied and Engineering Physics, have built a comprehensive microscope facility as part of the Cornell Center for Materials Research (CCMR). “We take pride in making sure we have state-of-the-art instruments,” Muller says. “We probably get around 200 on-campus users of these machines each year. For the Cornell materials groups, our instruments are really important because whenever you make a new material you want to know what it looks like. For nanostructured systems, electron microscopes are often the best way to answer that question.”
Muller is also the codirector of the Kavli Institute at Cornell, which focuses on the development and utilization of tools for exploring the nanoscale world. “Kavli lets us innovate and develop new technologies, and that gives us our competitive edge,” says Muller. “We develop the new tools at Kavli, then when they’re ready for general use, CCMR gives us the long-term facility.”
Pioneering New Materials
With their array of microscopes and other instrumentation, Muller and his group are often called upon to collaborate with researchers on a wide range of scientific problems, from computer transistors to electric vehicle fuel cells. “A lot of our work is motivated by technological problems and by the need to develop new technology,” he says. “Some of the materials we’ve developed for transistors, for instance, are pretty much in everyone’s computers and cell phones now. The smallest parts of these devices today are only a few atoms across.”
When the computer industry needed a new material to take the place of silicon oxide (glass) in transistors, Muller, along with Darrell Schlom, Materials Science and Engineering, developed and pioneered the use of thin layers of hafnium oxide. The change became necessary because transistors had shrunk so small that the layer of glass needed to be no more than five atoms thick. At that thickness, glass no longer has the same properties as it has in bulk form, Muller explains. It no longer behaves the way the computer industry needs it to behave. Hafnium oxide, on the other hand, does.
Muller has also been collaborating for more than a decade with General Motors and Honda on fuel cell electric vehicles. “For fuel cell and electric vehicles to be competitive with gasoline-power vehicles, many of the challenges to get there are nanoscience problems,” Muller says. “The catalyst particles in the fuel cell, for instance, are only a few dozen atoms across. How they survive 10 years of cycling is what determines the efficiency and lifetime of the vehicle. With our microscopes, we’re able to look at them and understand what happens to these catalysts under realistic conditions.”
These types of questions drive Muller’s quest to develop better electron microscopes. “If I’m looking at something that’s only five atoms across, I want to know what one particular atom at the interface is doing, compared to the atom next to it,” he says. “I have to be able to measure everything about that atom at atomic resolution: where it is, what type of atom it is, how its bonded to its neighbors, whether it’s conducting or insulating. The advances we’ve made to electron microscopes lead us to the answers.”