Sniffing Out the Foundational Science of Sensors

Estimated reading time: 9 minutes

2D materials are a special form of nanomaterial that are only a few atoms thick. They have so much surface area compared to their volume that they provide a lot of space for gas molecules to interact and are able to hold a large number of them. But 2D materials act so differently from their normal "bulk" counterparts that scientists don't have a good grasp on how they grow. Without this understanding, manufacturers can't consistently produce high-quality versions of them.

To tackle this problem, ORNL scientists explored a better way to grow 2D material gallium selenideExternal link (GaSe). As they grew the material in a container filled with argon gas, they found that by changing the temperature and flow of the gas, they could switch back and forth between laying down and taking away atoms. But just discovering how to change back and forth between the two states didn't tell them what was actually happening on the chemical level.

"To visualize what we were doing in the lab, we needed high-resolution, state-of-the-art facilities as well as in-situ diagnosis tools," said Tolga Aytug, an ORNL scientist. To get that level of precision, the team turned to the Center for Nanophase Materials Sciences, an Office of Science user facility at ORNL. The tools there helped them see how the processes they used to grow the material affected its structure and properties. Based on that information, they refined their methods to get the characteristics they wanted.

In the future, scientists may be able to combine various 2D materials into thin, versatile sensors. "The beauty of 2D materials is that you can stack the different layers together to make some artificial material," said Kai Xiao, an ORNL scientist. These artificial materials would be able to detect a variety of different chemicals instead of just a single one.

Metal-Organic Frameworks

The metal ions and carbon-based connectors of MOFs form open, cage-like structures. A MOF only a few inches wide has an amazing 2.5 acres of surface area. That provides plenty of space for molecules to interact with.

As a result, MOFs can sense minute levels of chemicals. Scientists control which chemicals they want a MOF to detect by changing the size of its spaces, its shape, or how its parts link to each other.

"For a MOF-based sensor to work, it has to be very selective and very sensitive," said Praveen Thallapally, a scientist at DOE's Pacific Northwest National Laboratory (PNNL).

One benefit specific to MOFs is their ability to accommodate new molecules by changing their structures. PNNL scientists found a MOF with a zinc base could capture cobalt and copper. When these metals exited the molecule, the MOF returned to its original structure. This means after a chemical attaches to a MOF and triggers a sensor, someone could reset and reuse the sensor without needing to replace the MOF.

Much of the ongoing research into MOFs focuses on how to discover and build them. MOFs' traditional starting materials are rigid and difficult to work with. In contrast, polymers (flexible chains of molecules) are easier to control. However, they usually bunch together in dense, disorganized clumps. To draw on the advantages of each, scientists from the University of California, San Diego found a way to use polymers to build MOFs. Using both allows researchers to combine MOFs' consistency and large surface area with polymers' ease of use. The researchers used the hybrid materials to create thin films, which are typically used in sensors.

The next breakthrough in MOF research may come from computer modeling. Using trial and error to figure out which structure will interact best with a specific chemical could take years and be very expensive. In contrast, powerful computer models using machine learning allow scientists to find just the right material in a few days.

PNNL scientists searching for a MOF that could select between xenon and kryptonExternal link collaborated with the National Energy Research Scientific Computing Center, an Office of Science user facility at DOE's Lawrence Berkeley National Laboratory. After searching through more than 120,000 options, their computer model pointed to a calcium-based material that excelled at this task. 

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