This schematic illustrates the new technique’s atomic scale manipulation of 2D materials, in which thick wafer-scale 2D materials are split into individual monolayers. Image courtesy of the researchers.
This schematic illustrates the new technique’s atomic scale manipulation of 2D materials, in which thick wafer-scale 2D materials are split into individual monolayers. Image courtesy of the researchers.

Since the discovery in 2004 of the single-atom-thick carbon material known as graphene, there has been significant interest in other types of two-dimensional (2D) materials. These 2D materials can be stacked together like Lego bricks to form a range of devices with different functions, including semiconductors. In this way, 2D materials could be used to create ultra-thin, flexible, transparent and wearable electronic devices.

However, separating a bulk crystal material into 2D flakes for use in electronics has proven difficult to do on a commercial scale. The existing process, in which individual flakes are peeled off the bulk crystals with an adhesive tape, is unreliable and time-consuming, requiring many hours to harvest enough material to form a device.

Now, researchers at the Massachusetts Institute of Technology (MIT) have developed a technique to harvest 2-inch diameter wafers of 2D material in just a few minutes. These wafers can then be stacked together to form an electronic device within an hour.

The technique, which is reported in a paper in Science, could open up the possibility of commercializing electronic devices based on a variety of 2D materials, according to Jeehwan Kim, an associate professor in MIT’s Department of Mechanical Engineering, who led the research. The paper's co-first authors are Sanghoon Bae, who was involved in flexible device fabrication, and Jaewoo Shim, who worked on the stacking of the 2D material monolayers. Both are postdocs in Kim's group.

The paper's co-authors also included other students and postdocs from within Kim's group, as well as collaborators at Georgia Tech, the University of Texas, the University of Virginia and Yonsei University in South Korea.

"We have shown that we can do monolayer-by-monolayer isolation of 2D materials at the wafer scale," Kim says. "Secondly, we have demonstrated a way to easily stack up these wafer-scale monolayers of 2D material."

The researchers first grew a thick stack of 2D material on top of a sapphire wafer. They then applied a 600nm-thick nickel film to the top of the stack. Since 2D materials adhere much more strongly to nickel than to sapphire, lifting off this film allowed the researchers to separate the entire stack from the wafer.

What's more, the adhesion between the nickel and the individual layers of 2D material is also greater than that between each of the layers themselves. As a result, when a second nickel film was then added to the bottom of the stack, the researchers were able to peel off individual, single-atom thick monolayers of 2D material.

That is because peeling off the first nickel film generates cracks in the material that propagate right through to the bottom of the stack. Once the first monolayer collected by the nickel film has been transferred to a substrate, the process can be repeated for each layer.

"We use very simple mechanics, and by using this controlled crack propagation concept we are able to isolate monolayer 2D material at the wafer scale," Kim says. This universal technique can be used with a range of different 2D materials, including hexagonal boron nitride, tungsten disulfide and molybdenum disulfide.

In this way, the technique can produce different types of monolayer 2D materials, such as semiconductors, metals and insulators, which can then be stacked together to form the 2D heterostructures needed for an electronic device.

"If you fabricate electronic and photonic devices using 2D materials, the devices will be just a few monolayers thick," Kim says. "They will be extremely flexible, and can be stamped on to anything." The process is fast and low-cost, making it suitable for commercial operations, he adds. The researchers have already used the technique to fabricate arrays of field-effect transistors at the wafer scale, with a thickness of just a few atoms.

"The work has a lot of potential to bring 2D materials and their heterostructures towards real-world applications," says Philip Kim, a professor of physics at Harvard University, who was not involved in the research.

The researchers are now planning to apply the technique to developing a range of electronic devices, including a nonvolatile memory array and flexible devices that can be worn on the skin. They are also interested in applying the technique to develop devices for use in the ‘internet of things’ (IoT).

"All you need to do is grow these thick 2D materials, then isolate them in monolayers and stack them up," Kim says. "So, it is extremely cheap – much cheaper than the existing semiconductor process. This means it will bring laboratory-level 2D materials into manufacturing for commercialization. That makes it perfect for IoT networks, because if you were to use conventional semiconductors for the sensing systems it would be expensive."

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