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New Production Methods for Two-Dimensional Materials Emerge


Research in 2D materials takes a turn towards manufacturing and away from just characterization

While graphene has been marking its tenth year as the wonder material, its two-dimensional (2D) cousins, like molybdenum disulfide MoS2, have barely half that amount of time behind them in the research labs. As a result, many of the developments we’ve heard about for these 2D materials has mainly been about the characterization of their properties.


Now that some of these materials have been in the hands of researchers for nearly a decade, the more recent research is beginning to show an interest on how to manufacture these materials more efficiently and effectively.


Exposing More Surface Area of MoS2

Researchers at Rice University, led by James Tour, whose work we highlighted in our graphene for fuel cells story, have developed a simple method for producing flexible films made from MoS2 that orients the material on its sides. In other words, they made the material in such a way that exposes the maximum amount of its edges. 


When the material is oriented in this manner, the MoS2 serves as an effective catalyst in the hydrogen evolution reaction (HER), a process used in fuel cells to pull hydrogen from water.


“So much of chemistry occurs at the edges of materials,” said Tour in a press release. “A two-dimensional material is like a sheet of paper: a large plane with very little edge. But our material is highly porous. What we see in the images are short, 5- to 6-nanometer planes and a lot of edge, as though the material had bore holes drilled all the way through.”


Tour added: “Its performance as a HER generator is as good as any molybdenum disulfide structure that has ever been seen, and it’s really easy to make.”


The key to the process is something called anodization, which is a electrochemical process used for thickening metal parts by adding a natural oxide layer. Tour also believes this process could serve as a platform for a range of applications. 


“We see anodization as a route to materials for multiple platforms in the next generation of alternative energy devices,” Tour said. “These could be fuel cells, supercapacitors and batteries. And we’ve demonstrated two of those three are possible with this new material.”


Making 2D Materials Flexible by Sliding It Between Substrates


In order to produce many 2D materials, they have to be made on still, inflexible substrates that can endure the high temperatures required during processing. If you want the ultrathin materials to be flexible, they have to be transferred onto a flexible substrate. The problem is that making that transfer between substrates often ruins the 2D material.


Researchers at North Carolina State University (NCSU) have developed a method for transferring MoS2 films onto any substrate without causing any damage to the 2D material in the process. The researchers believe that this comparatively easy process could expand the material’s use in flexible electronics.


"The ultimate goal is to use these atomic-layer semiconducting thin films to create devices that are extremely flexible, but to do that we need to transfer the thin films from the substrate we used to make it to a flexible substrate," said Linyou Cao, assistant professor of materials science and engineering at NC State, said in a press release. "You can't make the thin film on a flexible substrate because flexible substrates can't withstand the high temperatures you need to make the thin film."


In contrast to other transfer methods, this latest technique depends on the mechanical properties of MoS2 instead of chemicals that are used in conventional methods to etch away the underlying substrate.


In the NCSU method, room-temperature water, a tissue, and pair of tweezers are the only tools required to execute the transfer. The key to the technique is that MoS2 repels water. With that in mind, all you need to do is place the MoS2 onto an initial substrate that attracts water.


The way this all comes together is that a drop of water is placed over the MoS2 and then poking at the edge of the film so that the water begins to wedge itself between the MoS2 and the substrate. This serves to float the MoS2 across the surface of the substrate, which in this case is sapphire. Then, the MoS2 can be dried off with a tissue, picked up with tweezers, and transferred over to a flexible substrate. The entire process takes just a couple of minutes as opposed to the hours needed to achieve the same end using a chemical etching processes.


Bottom-Up Manufacturing for Graphene


One of the fundamental differences between manufacturing on the macroscale and the nanoscale is the distinction between so-called “top-down” manufacturing and “bottom-up” manufacturing. 


One way to visualize this difference is to think of top-down manufacturing as starting with a piece of silicon and carving out microscopic transistor patterns into it through some etching process, like lithography. On the other hand, bottom-up manufacturing would involve some kind of self-assembly process in which the patterns would grow out of the material.


Researchers at the University of California Los Angeles (UCLA) and Tohoku University in Japan have developed a bottom-up approach for the production of graphene nanoribbons in which they self assemble exactly into the desired form. 


The researchers were drawn to this development because they needed to develop a way produce graphene nanoribbons that have the zigzag edges that give the material a strong magnetic property


“To make devices out of graphene, we need to control its geometric and electronic structures,” said Paul Weiss of UCLA in a press release. “Making zigzag edges does both of these simultaneously, as there are some special properties of graphene nanoribbons with zigzag edges. Having these in hand will enable us to test theoretical predictions about them, such as magnetic properties.”


When the researchers tried making the graphene nanoribbons into these zigzag shapes using traditional lithographic techniques, it resulted in material that had too many defects to be used. 


While there have been other attempts to create graphene nanoribbons using self-assembly techniques to overcome these limitations of lithography, the end results were bundles of ribbons that required another process to untangle them and position them in a device.


“Previous strategies in bottom-up molecular assemblies used inert substrates, such as gold or silver, to give molecules a lot of freedom to diffuse and react on the surface,” said Patrick Han of Tohoku University in the press release. “But this also means that the way these molecules assemble is completely determined by the intermolecular forces and by the molecular chemistry. Our method opens the possibility for self-assembling single-graphene devices at desired locations, because of the length and the direction control.”


How You Make 2D Materials Determine Its Properties


In collaborative research between Rice University, Oak Ridge National Laboratory, Vanderbilt University, and Pennsylvania State University, a team has developed a novel method for producing hybrid layered 2D structures that combine different 2D materials into one. Their technique, they report, provides a high degree of control on how the resulting devices perform.


The technique comes as we have begun to see the emergence of different types of transistors being produced entirely from layered two-dimensional (2-D) materials featuring the dichalcogenides, tungsten diselenide (WSe2) and molybdenum disulfide (MoS2). 


In research published in the journal Nature Materials, the researchers demonstrated that by altering the temperature the materials are exposed to during the chemical vapor deposition (CVD) process used in making the material, they could yield two types of properties. They could either get an in-plane monolayer composite, which has a small but stable band gap, or a stacked layered hybrid, which exhibits enhanced photoluminescence. 


At high temperatures, the researchers got vertically stacked bilayers of MoS2 and WSe2, with the tungsten on top. At lower temperatures, the two 2-D materials grew side by side.


 “With the advent of 2D layered materials, people are trying to build artificial structures using graphene and now dichalcogenides as building blocks,” said Pulickel Ajayan of Rice University in a press release. “We show that depending on the conditions, we can combine two dichalcogenides to grow either in-plane hybrid or in stacks.”


“What’s even more interesting is that the layered structure has a particular lock-in stacking order,” said Wu Zhou of Oak Ridge National Laboratory in the news release. “When you stack 2D materials by transferring layers, there’s no way to control their orientation to one another. That impacts their electronic properties. In this paper, we demonstrate that in a certain window, we can get a particular stacking order during growth, with a particular orientation.”


Ajayan likens the development to something he has called “pixel engineering” since you are manipulating atomically thin semiconductors in production so that their potential uses in optoelectronics are almost limitless.