Posted in | Nanomaterials | Microscopy

Researchers Use Microwave Microscopy to Study Nanoscale Objects and Processes

Electrons are often knocked off when an atom is bombarded with a large amount of energy. This makes the atom chemically reactive, triggering higher destruction. This is the reason why radiation is considered to be hazardous, and why high-resolution imaging methods that utilize X-rays and energetic electron beams, can change or destroy the samples they investigate.

Sergei Kalinin

When electron microscopy is used to track battery dynamics, artifacts will be introduced that tend to impede with electrochemical processes. Another example is when X-ray spectroscopy is used to view the interior of a living cell, that particular cell is destroyed.

Scientists at the Oak Ridge National Laboratory (ORNL) of the Department of Energy and the National Institute of Standards and Technology have developed a new nondestructive technique that integrates ultrathin membranes with a scanning probe, known as scanning microwave impedance microscopy (sMIM), and microwaves. This approach could be used to view nanoscale processes and objects in conditions that simulate their usual operating environments.

The researchers used an environmental chamber, which has a window composed of an ultrathin membrane measuring 8 to 50 billionths of a meter in thickness. This chamber was used to enclose a specimen in a liquid environment, and was injected with microwaves, as the tip of a scanning probe microscope traveled across the ultrathin membrane. Next, the transmission of the microwave signal against the impeded one was recorded by the device, generating a high-resolution map of the specimen. It was discovered that the injected microwaves rotate in reverse directions at many billions of times each second, and are 100 million times weaker compared to a home microwave oven. As a result, destructive chemical reactions do not occur. The method developed by ORNL–NIST research team creates only a negligible amount of heat and does not promote sample destruction. The new method has been reported in the ACS Nano journal.

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. 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.

Alexander Tselev, First Author

Tselev carried out high-resolution microwave analysis and imaging with his colleagues Sergei Kalinin and Anton Ievlev at the Center for Nanophase Materials Sciences, a DOE Office of Science User Facility at ORNL.

Microwave microscopy could offer a noninvasive means to study critical surface phenomena that take place on the scale of billionths of a meter, like the formation of an ultrathin coating that stabilizes and protects the electrode of a new battery, and at the same time destroys the electrolyte to produce the coating. Microwave microscopy can help to define ongoing chemical reactions at various levels of stages, as this technique enables researchers to observe processes as they occur, without actually stopping them.

At NIST, we developed environmental chambers with ultra-thin membranes to perform electron microscopy and other analytical techniques in liquids.

Andrei Kolmakov, Senior Author

Kolmakov along with colleague Jeyavel Velmurugan at NIST’s Center for Nanoscale Science and Technology developed chambers to encapsulate processes and objects in liquid environments and also carried out initial characterizations to detect cells that are biologically interesting.

“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 research team at NIST and ORNL integrated current technologies in innovative ways and eventually developed an exclusive method that can possibly be used in materials research, forensics, and medical diagnostics

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

Alexander Tselev, First Author

The right tool for the job

In order to image crystals and other highly organized materials, methods like X-ray diffraction and neutron scattering can be used. To image living cell membranes, which are less organized materials, or processes like ongoing chemical reactions, the researchers at NIST and ORNL created a perfect tool for the task. As soon as the environmental chamber was integrated with a scanning microwave capability, the researchers went on to study a model system to check whether their approach would work, and to possibly establish a framework to conduct experiments in the future. The sMIM system was utilized to map polystyrene particles that self-assemble into thickly packed structures in a liquid environment.

After attaining this proof-of-principle, the scientists tested their system to see whether it could distinguish between silver oxide and silver during electroplating; silver oxide is an insulator, while silver is an electrical conductor. Scanning electron microscopy and optical microscopy are not great at discriminating between silver oxide and silver. In contrast, microwave microscopy can easily discriminate conductors from insulators. The next step for the team is to find out that artifacts like silver precipitation are not introduced by sMIM when observing nanoscale objects and processes. Such artifacts may be induced by scanning electron microscopy, posing a major problem.

One paper lists 79 chemical reactions induced by electrons in water. Dendrites behave very badly under an electron beam. Whereas sMIM is not the only nondestructive technique, in many cases it may be the only one which can be used.

Alexander Tselev, First Author

Living cells were then imaged by the scientists. As sick and healthy cells have different properties in terms of their ability to store electrical energy, one basis for diagnosis is to do intracellular mapping.

Tomographic imaging—resolution across the depths—is possible with microwaves as well. If you have microwaves, you can go variably in depth and get a lot of information about the living biological cell membrane itself—shape and properties that depend very much on the chemical composition and water content, which in turn depend on whether the cell is healthy or not.

Alexander Tselev, First Author

The team was able to locate properties that separated healthy cells from sick ones.

In the present experiments, the model system helped in viewing close to surfaces.

That doesn’t mean we’ll not be able to see deeper if we redesign the experiment. Microwaves can penetrate very deeply. The depth is basically limited by the contact size between the probe and the environmental cell membrane.

Alexander Tselev, First Author

The next step for the researchers is to improve their system’s spatial resolution and sensitivity. Since the resolution can be enhanced by thinning the environmental chamber walls, the team intend to use hexagonal boron nitride or graphene in the walls of the environmental chamber. These materials measure just one atom thick. Different image-processing algorithms and probes will also be used to enhance the resolution at varied depths.

The paper is titled “Seeing Through Walls at the Nanoscale: Microwave Microscopy of Enclosed Objects and Processes in Liquids.” Experiments were performed at the Center for Nanoscale Science and Technology at NIST, and the CNMS, a DOE Office of Science User Facility at ORNL.

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