Ultra-fast gas flow through an atomic-scale aperture in the novel 2D membrane. Image: N Hassani & M N-Amal, Shahid Rajee University.
Ultra-fast gas flow through an atomic-scale aperture in the novel 2D membrane. Image: N Hassani & M N-Amal, Shahid Rajee University.

Two recent studies involving researchers at the University of Pennsylvania demonstrate how to fabricate materials with single atom-sized pores that can be used for liquid and gas filtration. This regime of 'zero dimensional' pores has a broad range of future applications, from water and gas purification to energy harvesting.

The first study, reported in a paper in ACS Nano and led by graduate student Jothi Priyanka Thiruraman, postdoc Paul Masih Das and professor Marija Drndic at the University of Pennsylvania, demonstrates the ionic transport properties of these pores, which show promise for applications in water purification and desalination. They could also be used to create artificial pores that mimic ion channels in biology.

The second study, reported in a paper in Science Advances, demonstrates how helium gas flows through these pores. This work was conducted by experimentalists at the University of Pennsylvania and in Radha Boya’s group at the University of Manchester in the UK, with theoretical modelling by researchers at Shahid Rajaee University in Iran and the University of Antwerp in Belgium.

Researchers in the Drndic lab have expertise in making atomically thin materials and devices that are dotted with nanopores. Wanting to scale these pores down even further, Thiruraman and Masih Das previously developed a method for making 'angstrom-size' pores, which are small enough to only allow single atoms and small molecules to pass through.

While this work was instrumental in demonstrating that these types of pores could be made, fabricating devices for use outside of controlled, experimental settings remained a challenge. “There’s a lot of device physics between finding something in a lab and creating a usable membrane,” says Drndic.

To get from fundamental discovery to a working device, Thiruraman and Masih Das synthesized materials with angstrom-sized pores while making systematic changes to their previous method to try to create a more resilient material. Their method involves first growing a monolayer of tungsten disulphide by chemical vapor deposition. Then, they transfer this 2D material to a transmission electron microscope grid and expose it to either a focused electron or ion beam, which punches out single atoms from the monolayer to leave behind tiny, atom-sized pores.

Using a systematic approach to testing and modifying this fabrication process, the researchers were able to refine their method and develop a prototype that could be tested in more 'real-world' conditions than was previously possible. “Instead of just studying the material inside of an electron microscope, how do you make an actual device? That’s something that took us a long time to figure out,” says Masih Das. “We used our knowledge to make devices that you can measure ionic or gas transport on, and that was the big difficulty.”

“Being able to reach that atomic scale experimentally, and to have the imaging of that structure with precision so you can be more confident it’s a pore of that size and shape, was a challenge,” adds Drndic. “That came with the advancement of the technology as well as our own methodology, and what is novel here is to integrate this into a device that you can actually take out, transport across the ocean to Manchester if you wish and measure.”

Prototypes in hand, the researchers ran experiments using salt water to see how effective the material was at removing salt ions from the water, reporting their findings in the ACS Nano paper. “When you shrink the system to a single atom, we see that it’s independent of the salt water that you are putting in, so the hole does not seem to distinguish between what ion is going through,” says Thiruraman. “For salt ions, we are able to see with just a single hole a very standard saturated current level because the hole is so small, and its size dominates the conduction behavior.”

Then, to see if the material could also filter gases, they collaborated with researchers at the University of Manchester who had previously developed a way to measure gas transport in nanoscale devices. This work, reported in the Science Advances paper, shows that their device can also be used to move helium atoms through atomic apertures and is the first-ever measurement of its kind.

“We were very surprised to get any results at all because the holes are tiny,” Thiruraman says. “No one’s ever measured something like this, so just getting a helium atom to pass and detect through atomic apertures was very cool.”

The opportunities for potential applications are wide-ranging, from water desalination to energy harvesting to measuring small molecules such as hormones and pharmaceuticals. “But of course, at this fundamental level, we’re trying to see how these materials are robust at the atomic scale,” says Drndic.

The researchers are now interested in continuing their fundamental investigations into this material to better understand if the pores change over time and if layering individual sheets could change the material’s properties. They also want to explore how the geometric shapes of the pores themselves influence transport mechanisms and device properties.

This story is adapted from material from the University of Pennsylvania, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.