Haozhe "Harry" Wang, assistant professor of electrical and computer engineering (ECE) at Duke University and an expert in developing new methods for manufacturing materials, continues to push the boundaries in MXene research.
MXenes (pronounced max-eens) are a novel 2D class of metallic materials boasting high electrical conductivity and strong photothermal capabilities, meaning they can convert light to heat. That heat makes water evaporate, which allows the MXene to shrink, curl, bend, or twist when exposed to light.
Although already useful in applications like energy storage, desalination, and soft robotics, there is still much to explore about the potential of MXenes. Wang received an NSF CAREER award this spring to optimize the fabrication process of MXenes and characterize those improvements.
One challenge with the manufacturing of MXenes is that they are only a few atoms thick, and defects are amplified on that scale. Using a technique called plasma-enabled atomic layer etching (plasma-ALE), Wang and members of his lab, including postdoctoral associate Xingjian Hu, can remove or replace individual surface functional groups — akin to atomic surgery.
New quantitative analysis from the Wang lab published in Matter found that the plasma-ALE process improves MXene conductivity by 80 percent and bends up to 165 degrees, which outperforms similar 2D materials.
The Wang lab’s goal is to develop MXene materials to enable more flexible, programmable, and resilient soft robotics, all controlled with nothing but light.
Here is an exclusive Tech Briefs interview, edited for length and clarity, with Wang.
Tech Briefs: How far have MXenes come in the 15 years since their discovery?
Wang: Some of the very first applications included pretty high conductivity and also EMI shielding — very high-yield shielding effectiveness. And there are a lot of demonstrations on, for example, using it for energy storage via supercapacitors and also applications in MXene electrodes in lithium batteries. Because it's very conductive, it can be used in many applications.
Tech Briefs: What was the biggest technical challenge you faced while developing plasma-ALE?
Wang: MXenes are quite different [than graphene] because you have to use a chemical process to exfoliate MXenes and the chemical process induces contamination. That gave me a thought: Is it possible to make MXene more conductive? Therefore, we needed to find a way to engineer the surface terminations.
I happened to do some work while a postdoc at Caltech. There’s a new technique called atomic layer etching, particularly thermal atomic layer etching. With this technique it is possible to engineer the first few layers of atoms on the material. That's how I got the idea that maybe it's possible to use plasma-ALE to engineer the MXene.
So, to finally answer your question, the challenge was figuring out what the best recipe was. Even though plasma-ALE is quite a well-developed process, we needed to know what combination we should use. In the end, we used oxygen together with argon.
We also tried a lot of other gases, such as combining with hydrogen. At the beginning, when we first had this idea, we thought maybe we could use a fluorine-based gas, such as SF6; maybe that would make it better. But we are happy that our combination helps improve the conductivity.
Tech Briefs: Do you have any set plans for further research, work, etc.?
Wang: Yes, we have different directions for further research. Number one, we hope to build more complex systems. Currently the bending direction is fully controlled by the aspect ratio. I can’t go into much more detail than that.
Another direction is, basically, we want to be more creative and innovative, for example, to make this phenomenon enable more applications in the soft robotics area.
Transcript
00:00:03 Meet vaccines. Futuristic materials just a few atoms thick yet full of potential. Discovered less than 15 years ago, vaccines are already making waves in sensors, energy storage, and soft robotics. They bend when exposed to light, acting like tiny artificial muscles. No batteries, no motors required. Maxines are made of highly conductive
00:00:26 transition metals which let electrons move freely through stacked 2D layers. When exposed to light, they absorb and hold onto the energy, converting it to heat. This heat makes water evaporate, causing shrinkage, but not always evenly, thanks to their layered structure and surface chemistry. That uneven shrinkage makes them curl,
00:00:47 bend, or twist. All controllable by design. At Duke, electrical and computer engineering professor Harry Wong and his team are pushing vaccines even further. With innovative engineering, they can precisely swap out atoms on the surface, like performing atomic level surgery to greatly enhance their efficiency. Their goal is to develop Maxine
00:01:09 materials to enable more flexible, programmable, and resilient soft robotics, all controlled with nothing but light.