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ORNL-NIST team explores nanoscale objects and processes with microwave microscopy

When lots of energy hits an atom, it can knock off electrons, making the atom extremely chemically reactive and initiating further destruction. That’s why radiation is so dangerous. It’s also why high-resolution imaging techniques that use energetic electron beams and X-rays can alter, even obliterate, the samples they explore. For example, monitoring battery dynamics using electron microscopy can introduce artifacts that interfere with electrochemical processes. Another case in point: Employing X-ray spectroscopy to see inside a living cell annihilates that cell.

Now, researchers at the Department of Energy’s Oak Ridge National Laboratory and the National Institute of Standards and Technology have demonstrated a nondestructive way to observe nanoscale objects and processes in conditions simulating their normal operating environments. They start with an “environmental chamber” to encapsulate a sample in a liquid. The chamber has a window made of an ultrathin membrane (8 to 50 billionths of a meter, or nanometers, thick). The tip of a scanning probe microscope moves across the membrane, injecting microwaves into the chamber. The device records where the microwave signal was transmitted versus impeded and creates a high-resolution map of the sample.

Because the injected microwaves are 100 million times weaker than those of a home microwave oven, and they oscillate in opposite directions several billions of times each second so potentially destructive chemical reactions cannot proceed, the ORNL-NIST technique produces only negligible heat and does not destroy the sample. The scientists report their novel approach of combining ultrathin membranes with microwaves and a scanning probe–called scanning microwave impedance microscopy, or sMIM–in the journal ACS Nano.

“Our imaging is nondestructive and free from the damage frequently caused to samples, such a living cells or electrochemical processes, by imaging with X-ray or electron beams,” said first author Alexander Tselev. With colleagues Anton Ievlev and Sergei Kalinin at the Center for Nanophase Materials Sciences, a DOE Office of Science User Facility at ORNL, he performed high-resolution microwave imaging and analysis. “Its spatial resolution is better than what is achievable with optical microscopes for similar in-liquid samples. The paradigm can become instrumental for gaining important insights into electrochemical phenomena, living objects and other nanoscale systems existing in fluids.”

For example, microwave microscopy may provide a noninvasive way to explore important surface phenomena occurring on the scale of billionths of a meter, such as the formation of a thin coating that protects and stabilizes a new battery’s electrode but cannibalizes its electrolyte to make the coating. Microwave microscopy, which allows scientists to watch processes as they’re happening without stopping them cold, makes it possible to characterize ongoing chemical reactions at different stages.

“At NIST, we developed environmental chambers with ultra-thin membranes to perform electron microscopy and other analytical techniques in liquids,” said senior author Andrei Kolmakov. He and colleague Jeyavel Velmurugan at NIST’s Center for Nanoscale Science and Technology made chambers to enclose objects and processes in liquid environments and performed preliminary characterizations to identify biologically interesting cells. “Conversations between the ORNL and NIST scientists resulted in the idea to try nondestructive microwaves so the environmental chambers could be used for broader studies. There are very few groups in the world that can image with high resolution using microwaves, and CNMS is among them. The design of the experiment and the adjustment of the technology for imaging required ORNL expertise.”

The ORNL and NIST researchers combined existing technologies in new ways and came up with a unique approach that may prove useful in medical diagnostics, forensics and materials research.

“For the first time, we are able to image through a very thin membrane,” Tselev said. “Microwaves and scanning probe microscopy allowed that.”